Long-term thermal storage is something I've been fascinated with the last year or so.
Heat loss inside of dirt is so incredibly slow it's hard to wrap your head around. One fact that I find helps is the fact that after an entire winter of extremely cold temperatures, you only need to go down 10 ft or so before you hit the average annual temperature. 4 months of winter buffered by 10 ft of ground!
Obviously there is incredible potential to this even if you just keep the energy as heat. The amount of electricity we use on heating and air conditioning is huge. If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling.
Really excited by companies looking into this and wish them the best of luck!
> Heat loss inside of dirt is so incredibly slow it's hard to wrap your head around. One fact that I find helps is the fact that after an entire winter of extremely cold temperatures, you only need to go down 10 ft or so before you hit the average annual temperature. 4 months of winter buffered by 10 ft of ground!
That’s not entirely insulation. Some of the heat flows upward toward the surface during winter and some warmth flows downward during summer.
> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling.
Geothermal heating and cooling already exists. It’s semi-popular in some areas. It can be expensive to install depending on your geology and the energy savings might not compensate for that cost for many years. Modern heat pumps are very efficient even if the other side is exposed to normal outdoor air, so digging deep into the earth and risking leaks in the underground system isn’t an easy win.
the parent was not talking about geothermal, but storage which would come as interesting to store winter cold for the summer, I have no idea whether it would work.
Based on some guesses and uninformed searches if a house spends 200MJ on cooling and there is a 20 C delta between winter temperatures and desired cooling temperature and assuming a specific heat capacity of ~800 J/(kg*K) you would need 12.5 tons of rock as battery which would be around 6~8 m³ which sound very small.
I am sure that there are hundreds of complex factors at plays (eg rain water and aquifers reheating the battery during spring) but it came out to be a far smaller number than I would have guessed.
Again, dirt isn't very insulative. It has a high thermal storage capacity, but if you pumped heat into it, it would dissipate to all compass directions and downwards, and you couldn't retrieve it later.
If that wasn't true, you'd need to keep moving the underground passageways of buried passive cooling systems.
I think you missed a 0. I've never heard of an AC unit for a house that's 400W. 4000W and above is more common.
Google:
> An AC unit's electricity usage varies by type, with window units using around 500–1,500 watts and central air systems using 3,000–5,000 watts, though usage can range from 2,000 to over 6,000 kWh annually for central units
Also, how much you use it during the year can vary hugely from 0 (when I lived near the coast) to like 10 hours a day for months in hot or cold places. There's not a standard, but 55kwh for a year means you live someplace that doesn't really need AC / heating.
Underground heat storage isn't new nor anything startuppy though, we're well beyond the "companies looking into it" stage. This page [0] mentions it's been around commercially since the 90's and experimentally since the (19)30's, and interest started in the 70's.
But depending on your definition of this, it's been around for hundreds if not thousands of years. People used to cut ice out of frozen lakes and store it in underground basements for year-round cooling. And in arid climates they have windcatchers [1] and other techniques where they store the nighttime cool for usage during the day, or these [2] to store or even create ice, all without using electricity.
The problem here is the density. Air conditioners are rated in tons of cooling, with even the small ACs rated at at least 1 ton a day. So even a small house can easily melt through its weight in ice during summer.
10ft below ground is enough to take advantage of geothermal heat. You don't have to go "very far" to reach warmer soil in winter because the soil PLUS the snow on top is pretty much just insulating the deeper ground from the cold air.
Start getting into permafrost though where the cold is more constant and that cold layer gets deeper.
10ft is definitely not enough for practical use. In order to heat a rural house with a heatpump connected to geothermal you need in order of 200-300ft deep hole, at least here in Finland.
For ground source heat pumps you have two approaches. Either you have deep hole. Or you have a large field. In later case not as much depth is needed, but you do need much larger area.
At a certain depth the temperature curve is 6 months delayed from the surface. So it's getting to its warmest during the winter, and coldest during the summer. At a deeper depth it's pretty constant year round.
With regards to wrapping your head around heat loss- this winter some work was done on our property while there was snow on the ground. A bunch of snow got covered in dirt. In the spring, maybe three weeks after all the snow on the ground had melted, I moved the pile and was surprised to find all the snow still frozen. It had been under maybe 18 inches of dirt. I was pretty surprised to see it.
We (USA) could have 80% of our Northern homes off fossil fuel and electric heat for less cost if we were a little more forward thinking and willing to work together.
But after nearly two decades they're decommissioning because the one-off components needed too much NRE to refurbish. If we all adopted this it'd be cheaper than what we pay today and zero greenhouse gas emissions. It'd finally make living in the temperate climates more climate-friendly than the warmer latitudes.
Yeah, district heating/cooling would make us so much more efficient. It wouldn't even take a whole lot of space or land to implement. You could stick it right next to a community water tower.
Except that won't heat your house at night during a week-long blizzard. The "heat pumps and solar" plan requires natural gas peakers be built for maximum load of the grid, which we're doing even without heat pumps so far, and then we just run the peakers more partially because utilities are cost-plus regulated.
The article is talking of the revival of [solar {...} thermal] storage precisely because solar panels are so cheap and heat pumps are reasonable.
The {...} is the counter intuitive step of solar -> electric -> heat storage for six months -> electricity for later.
The secret sauce is increased heat deltas, not just heating dirt directly using sunny daytime temperatures but really cranking up the heat of an underground mass to 500C + (IIRC - I skimmed the article some hours ago).
Not the somewhat recently popular specific household commercial gadgets for cooling and heating a home, no.
Just thermodynamic pumping of heat, from the article:
Electricity from the solar arrays flows to heating elements in the earthen mound, building up heat. The storage temperature is 600 °C or higher. The outer mass of the mound, plus a favorable volume-to-area ratio, insulates and minimizes heat loss. Pipes embedded in the mound carry fluid that delivers heat to users.
and at industrial scale, using bleeding edge HVAC technological advances, it's all about the creation, storage, and pumping of heat.
One can move heat from one place to another by moving a fluid. This is not a heat pump, even if the fluid is pumped.
A heat pump is a specific kind of thermodynamic device that moves heat energy from lower to higher temperature.
The system that is described in the OP link does not use a heat pump. Electrical energy is used to make heat, but it does so with a resistive heater, not a heat pump.
This is called PAHS, passive annual heat storage and has been tried in some alternative energy housing.
You put pvc pipes into a hill of dirt that is covered by a plastic sheet or other waterproof membrane; during hot summer months you use a small fan to put heat into the pile; during winter the heat moves from the dirt to the house.
This is definitely not what the OP is talking about.
He is talking about electrically heating very large amounts of dirt to temperatures of 600C or more. Your PVC tubing approach is talking about 50 times smaller swings.
This reminds me of the interesting articles over the last few years about the heat from the London metro system over the last century saturating the ground so the tubes have become extremely hot.
Does the slow heat transfer interfere with attempting to use that heat?
I can imagine that there's a lot of total energy in the dirt 10 feet down. But once you've tapped the energy near your well, how long does it take to replenish? How long until the immediate vicinity reaches equilibrium with the surface?
You don't need to store it in the dirt. You use the dirt as insulation, and store in something like molten salt or whatever which can be pumped up to surface and has good thermal conductivity to then extract the heat. At least that's my understanding of all these systems.
I really can't help but think there might be a fundamental problem here. If heat loss in dirt is so incredibly slow, and we use a heat pump to extract that heat, then wouldn't it be equally slow to replace the heat extracted? (absent heat injection, i.e. reversing the heat pump's operation in summer)
Has anyone looked at the subsurface ground temperatures after days, weeks, months, even years of heat pump operation?
I do seem to remember seeing one article on the subject showing that after one winter the subsurface temperature had declined enough to materially affect the heat pump's COP. But the timescale didn't extend to multiple years.
In one of eight cases studied where heat flow is unidirectional (cooling load only) over a 20 year timescale the authors find:
"the mean ground temperature ... increased from 21.87 °C to 26.18 °C, ... . This significant rise could have a potential impact on the performance of the system in the later years of its operational life."
The other 7 cases showed weaker or negligible long term variation.
An additional graph shows COP variation over 15 years and the worst case shows a decline of perhaps 10% (just eyeballing it).
Surprisingly, some cases showed a long term improvement in heating COP - presumably the injection of summer heat into the soil made for warmer soil than just sunshine and natural diffusion?
Oh! You're right, I hadn't realized.
But... wouldn't it be more efficient with a heat pump and exploit the 2-4x COP?
Ok, maybe I get it now: heat pump wouldn't work at the high temps they are achieving.
Thx.
Dirt has an r-value of between 0.25 and 1 per inch according to some quick googling, so 10ft would be maybe R-60.
Surprisingly, that's only equivalent to about 10" of polyiso rigid foam.
What this project is really taking advantage of is the super cheap thermal mass. Dirt has about a quarter of the specific heat of water, but it is, literally, dirt cheap, and much easier to keep in place than a liquid.
You can also have a temperature cycle of 600C with dirt. Water is limited to 50-70C so the dirt loses a factor of 4 in specific heat, but wins back a factor of 10 due to delta T.
You are confusing the concepts of insulation and reservoirs.
When 5 of the 6 semiaxes of possible heat flow have no temperature gradient, the temperature becomes much more stable.
Insulation would be if a 10 ft radius dirt ball maintained a stable temperature all year round - which would surprise me, although dirt does have some insulative value. However, wet dirt is really not very insulative - try sleeping on the ground sometime.
The thermal time constant of a 3d object scales as the square of the linear dimensions of the object, so time constants measured in years are quite achievable.
The dirt here will not be wet in most of its volume -- the initial charging will bake out all volatiles. The hottest part of the pile will become something like dry brick. It's like storing energy in ziggurats.
> Is this because of geothermal energy leaking upwards
No. The heat energy comes from the sun. Power flux from geothermal is measured in milliwatts per square meter, while the sun can provide more than a kilowatt during the day. So real geothermal heating is negligible at the surface. That's why the temperature a few feet down equals the average annual temperature at the surface.
The only reason people call this "geothermal" is because marketing people realized that this sounds more impressive than "ground source heat pump". It really should not be called "geothermal", because that's something very different. Real geothermal involves extremely deep drilling (not feasible for residential use) or unusual geology.
> Air source heat pumps are the most common models, while other types include ground source heat pumps, water source heat pumps and exhaust air heat pumps.
Heat pump > Types:
- SAHP: Solar-assisted heat pump; w/ PV
- acronym for a heat pump with TPV thermophotovoltaic heat to electricity:
- acronym for a heat pump with thermoelectric heat to electricity:
FWIU archimedes spiral turbines power some irrigation pumps in Holland at least. Is there an advantage to double/helical archimedes spirals in heat pumps if/as there is in agricultural irrigation?
Noiseless double-helical Achimedes spiral wind turbine on a pivot like a pinwheel: Liam F1 average output with 5m/s wind: 1500 kWh/yr (4.11 kWh/day);
Weight: ~100 kg / ~220 lbs;
Diameter: 1.5 m / 4.92 ft
What about CO2 and heat pumps? Would a CO2 heat pump make sense?
There's also buoyancy. The pyramid builders may have used buoyancy in a column of heated bubbly water to avoid gravity, in constructing the pyramids as a solar thermohydrodynamic system with water pressure.
Yes, the you can put "thick" insulation over top of any buried plumbing and the exposed bottom will gain geothermal heat from the below and it can prevent freezing.
Its a bit of both, but its primarily due to the high insulation.
There are 2 gradients: The surface gradient is what I mentioned about and its quite steep(only a few meters to drop tens of degrees). After that, you reach approximately the average annual surface temperature, but do continue to get small drops due to the geothermal gradient. The geothermal gradient is relatively shallow - you need to go down a thousand meters to see tens of degrees drop.
> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling.
This is literally what ground-loop heatpumps are doing. The ground loop is used as an energy source in winter, and since water is always at 0C, the heat pump efficiency can always be around 500%. And vice versa in summer.
Yes, the article addresses this explicitly. The key criterion to make it viabile is steady long-term energy consumption for the winter months. Otherwise the cost to rapidly extract the heat gets too high and wrecks the economics.
The application here is big, slow annual oscillations. Slow charge, slow discharge.
I'm sorry, but you write this as if that's nothing. Making a 10 foot hole is a massive amount of energy being spent. It's a massive amount of weight as 1 cubic yard of dirt is roughly one ton. In 10 cubic feet, that's roughly 3.5 tons. I say this as someone that moved 6 cubic feet of dirt by myself with a shovel and a wheelbarrow.
So to think of 10 feet of dirt as a slow insulator would have to be one of the worst insulators out there.
the reason that thermal storage is becoming very atractive is that electrical power from PV, wind, and other sources has become increadably cheap, and there is litteraly no where to put it, so prices go negative now, which is a new thing.
so 500x cheaper may be an understatement, considering the nature of how cheap, mature and availible the technology to build a thermal storage battery is, any municicipal civil engineering team can build one from off the shelf and localy sourced materials, and the basic battery "housing" could be a re purposed industrial building, cheap does not begin to describe it.
> Two economists are walking down the street. One of them says “Look, there’s a twenty-dollar bill on the sidewalk!” The other economist says “No there’s not. If there was, someone would have picked it up already.”
In this case, the economics of solar has been changing rapidly in recent years, so the caveat won't apply. Worst case, you can bend over and get a closer look.
Yeah, presumably the Software Engineer laughs at the two moronic "experts" and picks up the religious literature or strip club ad masquerading as cash.
Combination of very cheap periodic power and suitable infrastructure to supply heat energy is more recent phenomena. Supply that is very cheap power and demand that is capability to use that energy later need to match.
500x should be conceptualized as the bare minimum, not evidence of massive low-hanging fruit.
A battery that cycles daily makes revenue on its capacity about 350 times in a year. A seasonal energy store makes revenue on its capacity about once in a year.
A battery arbitrages between the most expensive and least expensive energy generators in the system. A seasonal energy store arbitrages between seasonal price averages.
A battery smoothing out solar production is operating on the difference between how much sun there is in the day, and how much sun there is at night. A seasonal energy store in the same role averages between summer and winter.
A factor 500 cheaper plus a significant quantity of solar energy production is about where you'd expect this kind of thermal storage to start making economic sense.
If we're looking at bare minimum cost then batteries are 10x cheaper than batteries.
And being capable of seasonal storage doesn't stop you from using it for daily storage. It's less efficient than batteries, but you can overcome that.
Let's say you can make a 24 hour power source with $10M in solar panels and $20M in batteries, including the other equipment and costs. $30M total. If we need twice as much solar for thermal storage, but the storage only costs $1M, then that's $21M for an equivalent system.
What stops systems like that from being built right now? I was under the impression that batteries were most of the cost if you want them to last more than a few hours.
Yes! I created a comment here in similar aspect but the thing that was bugging me a lot was what to do about night time though
Imo, the current system of solar energy etc. might be more carbon emissions than even coal because of the way of using batteries and how they are extracted and the whole process and batteries limitations and not solar panels themselves (I think)
Like, please pardon me but I think that there might be batteries that overall are cheap/economical/less carbon emissions and they can store energy for a night cycle right?
Then using those batteries in your system and I don't know the price point, but I am definitely sure that they might be orders of magnitude cheaper
So in your 21M$ example lets say that we can add such night time cheap batteries to counter solar panels not generating light at night and use this 1M$ to generate energy when its either rain or solar production is less or when winters come
I still feel like, this might be more economical than the current state of batteries + solar panels.
Okay, I just realized that the night time issue can be solved by the grid but still if that's the case, then why have batteries in the first case if lets say every community builds something like this (if this is economical) and people could just pull up energy from it 100% solar in rainy times etc.
Like maybe my mind is perfectionist or I genuinely don't know about green energy but to me I wish to know if there is a way that we can transition (almost) 100% to the green (solar) without the drawbacks that I saw in the michael's moore documentary in the sense that it produces more carbon emissions net overall theoretically (which hurts my heart :< but the logic was sound imo)
There aren’t grid-scale batteries that can handle hundreds of thousands of cycles at an affordable price. If we crack that problem, solar and wind for everything will immediately be the only technology worth deploying for energy in 99% of areas.
I meant ‘bare minimum’ in the sense of what's needed for the system to be profitable, not in the sense of ignoring costs.
The problem with using this approach for daily cycled loads is that it relies on passive heat transfer to distribute heat through substantial regions of dirt. This simply doesn't work for daily storage.
You can overbuild, but then your energy losses are going to be immense, because you never saturate or drain the bulk of the material, and are just losing energy to it.
You could build faster cycling systems instead, and active systems especially can cycle reasonably fast, but then your dominant costs no longer reduce down to a pile of dirt with a few rods stuck into it.
You would need more rods if you're cycling more of it at once... but you can add 25x as many rods and it'll still be 20x cheaper than a battery, right?
I don't think losses would be immense. You'll spend a few months warming up the neighboring dirt, but after that the amount of heat escaping per day will look about the same as the seasonal system.
I'd assume the design as proposed isn't overbuilt, so we'd need to go from, say, 4 months cycle times to 8 hour cycle times, which is maybe 350x.
While that's not going to increase the cost by 350x directly, it is going to change the character of the pile from a bunch of dirt to a bunch of dirty pipes. This makes a lot of the simplifying assumptions no longer work; like you can no longer ignore the heat losses through the rods, or the lower thermal mass of the rods.
And to be clear, you can do this. There are faster-cycling thermal storage solutions out there. It's just not implied from the claim that these solutions would be so much better than batteries.
The heat right next to the rods is easier to access. With less dirt per rod, you can also get more watts per rod, so the number of rods doesn't need to scale directly with the cycle time.
One thing they neglect to mention (which is by no means a deal-breaker) is that you waste a good portion (about half) of the electricity in the process of charging and discharging the pile of dirt. Chemical batteries are much more efficient in this regard.
Solar prices are coming down quite fast, I don't think a factor of two is going to be a killer here if the storage is cheap and long-lasting enough. Some people are already considering over-provisioning solar panels relative to available transformers/grid connections so that they can maintain output on cloudier days. "What do we do with all the extra power when the belly of the Duck Curve [1] hits the ground" is a problem lots of people are thinking about.
The cost-prohibitive portion of this, which is greatly glossed over in the article and which I showed in my other reply - is that the steam generator required to recover this heat as electricity, is a massive part of the capex - more than half of the entire system end-to-end, including the solar and dirt storage. That makes the economy of it far less viable even with nearly free solar, which we're still quite far from.
This isn't true for high latitude areas, where the overprovisioning that would be needed for solar to work in winter would be massive. This is one of the markets he discusses.
This scheme, if it works, makes PV work great in Alaska.
Yeah, but then people should start to actual incorporate the full cost of these kind of things in the total cost of solar power when comparing it to other sources.
Or hydrogen for combined cycle power plants, which have a LHV efficiency in excess of 60%.
This scheme is probably superior though, with lower capex and working at smaller scale, especially if one doesn't have deep salt formations to solution mine for hydrogen storage caverns.
> There is an efficiency penalty converting back to electricity; round-trip efficiency is 40%-45%, but sometimes the steady supply of electricity is worth it.
One thing they also mention is how incredibly cheap storage of natural gas is.
https://en.wikipedia.org/wiki/Power-to-gas#Efficiency The efficiency of power to gas is not great, but it's about the same as this thermal storage method, with probably much longer lifetimes,easier transportation and more general utility (the natural gas could for example be converted to methanol using the holy grail catalyst that was in the news recently).
The power to gas is also carbon neutral, even negative depending on what you decide to do with the natural gas (if you don't burn it for power but use it for industrial chemistry, you get some sequestration out of it).
The problem is the cost of getting the carbon and hydrogen to make the natural gas. If you're making a synthetic fuel, you might as well make methanol, which can be stored as a liquid, and is about as easy to make from syngas as methane.
Yeah, did mention the methanol option as an additional step once the natural gas is created. But is the cost of getting the carbon and hydrogen that high? In a P2G system you'd be electrolysing water (plentiful) and combining it with carbon dioxide (also relatively plentiful, although it can also be linked to an existing fuel burning plant for better efficiency).
There are existing production systems using this approach right now with surplus energy from renewables described in the wikipedia article.
Getting it from existing fuel burning plants defeats the whole purpose.
Direct air capture is out, so it'll have to be recovered from the combustion of the synfuel. Using the Allam cycle has been explored to do this (you also have to store the oxygen from electrolysis for later use in this oxyfuel combustion cycle) but it ends up being more expensive than just burning hydrogen, if there's reasonable geology for hydrogen storage.
So, if this thermal storage scheme is cheaper than hydrogen, as it appears it will be, then these alternative synfuel schemes are ruled out.
More efficient, but much more expensive. I'm sick of people handwaving $100 per kWh. That is two orders of magnitude off where it needs to be to do anything more than virtue signal.
Meanwhile multiple grids are now paying renewable to curtail, because guess what, the variability is correlated (it's the exact same damn mathematics we used to fuck up the entire global economy in 2008, which is why I'm so surprised people are handwaving that too, but whatever). If you want to minimise cost without relying on gas to save you on dark still days, you want a cheap use for the surplus, round-trip be damned.
100$/kwh on a battery that does 1000 cycles is 10c/kwh, 5000 cycles ("Claimed" lifepo4 these days), that's 2c per kwh. These aren't that unreasonable, albeit one would need to account for cost of capital and so on increasing these effective numbers.
Batteries are already economical in most grids where they can arbitrage daily prices of 0-10c during the day to 10-30c during the night, with the occasional outlier event contributing dollars per kwh.
They will never load-shift across seasons, agreed, but for daily loadshifting they are already economical, and being 90%+ efficient (and very simple/easy to deploy and scale) is part of why they're popular. It opens up power shifting opportunities that aren't just daytime solar too.
you're undercounting cycles for batteries. batteries are quoted for until 80% capacity is left which makes sense for mobile applications, but for grid storage, a battery that's 80% degraded is still useful. as such, you probably get 15-20k cycles before it's worth recycling
Kilowatt hour of capacity and kilowatt hour delivered are two very different numbers. Sources rarely distinguish, and you're almost certainly confusing them if you think batteries have to get down to $1.
Correlated errors are a problem in all sorts of places. Most statistics assume everything is independent; super important to verify that before drawing conclusions.
I didn't have time to read the whole thing so I don't know if they mention it, but another article about about using sand as heat storage pointed out one of the advantages is that the material isn't toxic, unlike chemical batteries.
They do mention it, but it’s downplayed relative to how much of a problem it can be.
In a situation where you have a lot of energy generation that would go to waste, storing it in a system with low round trip efficiency could be better than losing it.
For planned installations where the generation cost is nontrivial (like a solar install) then increasing the generation to compensate for poor battery efficiency isn’t as easy of a decision.
However, for long term storage, the "cost of inefficiency" is swamped by the amortized capex of the batteries. The goal is to minimize capex, not maximize efficiency. This becomes even more true as the cost of the input energy declines.
I visited a pumped storage facility a while back that stored electricity by pumping water uphill to store it and then draining it past a turbine to reclaim it. Ever since I’ve been intrigued by using gravity instead of batteries.
For home use, it seems like you could rig up some heavy stones on pulleys to do the same thing could be fun because you’d get to physically see your batteries filling up. Back of the envelope calculations suggest that an array of ten 10-ton concrete blocks lifted 10m in the air could power a house for a day (ignoring generator inefficiencies)
I like the picture, but the the size of the construction is enormous, especially if you're considering a tank for some kind of pumped hydro. Hydroelectric power is practical because a dam in a strategic location can back up much more than 1000x of its volume in water. If you had to build all those walls forget about it.
I am giving that one a 0% chance of long term success.
Edit: no seriously. Do some back of the napkin maths. The amount of energy stored is too small. Way too small. And then the infrastructure to haul hige blocks of concrete around.
It's the same scam selling ideas that sound good to people who don't understand what a "joule" as Pavegen and the other systems that generate energy from footfall or passing cars. Mechanical energy is pretty "low grade" as energy goes.
It 100% works, but it's a system that has very specific applications and doesn't scale up well. And the best systems use a magical property of some fairly heavy materials called "being liquid" to simplify the logistics of getting millions of tonnes of weight to the lifting mechanism.
7000 * 25t / 2.5t/m^3 = 70,000m^3 of water. So about 1% of the water storage of Dinorwig, which runs dry in about 6 hours of use.
Now, I know reservoirs are ecologically pretty iffy, expensive and obviously geographically sensitive, so you can't slap them around everywhere. But all these mechanical schemes have big "look what they need to mimic a fraction of our power" vibes!
I could imagine that steel-on-steel block movements could actually be quite efficient and effective in limited scenarios, but logistically it just seems like a lot of squeeze for not a lot of juice considering how much power is required to be stored for utility-scale projects. I would like to say that that they're just delusional people truly hoping it'll work, but I think there's a core of hard-nosed scammers who smell money for a shiny PPT and a plausible-to-non-engineers Wile E. Coyote/Troll Physics contraption with big numbers in the brochure: 7000 blocks! 25 tonnes per block! Megajoules! Efficiencies! Scale! Repurposed coal mining infrastructure! They even have AI in the spiel now: https://www.energyvault.com/solutions/software
Or maybe it'll work and I'll look stupid in 30 years when there are huge fields of hundreds of kilometre-deep boreholes with 100 kilotonne masses moving up and down in them. But somehow it seems quite unlikely on a practical level considering the cost of boring gigantic holes that you'd have to do to make it scale. Onsies-twosie installations in a few mines here and there may work for lucky outlying towns, but they aren't civilisational scale solutions.
Gpt says that would require about 275 million steps on a magic rigid weightless stairway. Roughly 4.1 million calories. So at Phelps level energy expenditures you are still talking over a year of climbing every day.
You'd need to go to about 47km for the end of the stairs to reach escape velocity I think? Past geo. Was using 20% efficiency. Still something off there.
You are not hiking much in the mountains, are you? 1000m of elevation gain per day are no problem for a slightly out of shape sit-all-day programmer. Not sure how high up you want me to go, but given a high enough mountain (and a thick jacket and supplemental oxygen) and most people here can do that in a few weeks or months.
Every time I see again the idea of moving big concrete blocks for storing energy, I remember the time I made the calculations, and estimated around USD100K of infrastructure to store the same amount of energy as a nissan leaf.
Gravity storage is cool when nature has already made most of the work, I/E pumped hydro where nature has already built this huge canyon with a river in the middle just waiting for someone to put a dam at the end.
Hoisting 100 tons of stuff high into the air, and then efficiently converting that into the high RPM needed to drive a generator seems like it would take a truly staggering construction effort. Suspending that amount of weight high above your house also has some... interesting potential failure modes.
I don't even think the gravity battery thing is viable for individual residential power storage at all. I was just wondering why you'd assume that the 100 ton weight would be placed directly above your house given the obvious problems with that approach, and the obvious way to avoid those problems.
The comment I was replying to literally said "For home use", and a heavy object 10m in the air does not have to be directly above something to be meaningfully (and dangerously) above something.
It's a silly scenario anyway, but I was doing a bit of guesswork about typical "home" lot sizes.
Yeah I understand it's for home use. I am imagining a tower in the back yard or something. It would be closed so that nobody can walk under the weight. Or it could be internal to the house like an elevator shaft.
Anyway I agree it's silly, definitely not a realistic idea
Right - if a tower in the back yard falls down it can still hit your house, since it isn't guaranteed to neatly collapse straight down. Worst case, it may tip over from the base and directly smash stuff up to its height away (and 10m is pretty far).
I have trees in my back yard I'm kind of worried about, which is why this immediately came to mind.
Gravity based with weights is generally considered not cost effective; others already did the math that your 100 ton proposal can still only store a fraction of what a consumer grade battery pack can do, but on top of inefficiency there's space and maintenance requirements. It works in situations where e.g. trains go uphill empty and full downhill, but generally it doesn't work.
Water based systems work better because water is easy to move, plentiful, and there's natural basins to pump into / flow out of that can contain billions of liters.
You need a lot of weight. IMO for home use the risks heavily outweigh the benefits for anything outside of a hobbyist project... that's just a lot of potential energy in a system that can go wrong. Weights falling quickly, pullies and cables under tension.
The same is true for batteries of course, but at the very least there are protections and checks for failures in most consumer accessible home solutions (and decades of engineering at this point). Worst case you at least have smoke detectors... not sure if there's a "cable is wearing thin and might snap and decapitate you" warning system.
And environmentally, that tends to be pretty bad for a long time. Looks more peaceful than a fracking site, but it's still pretty bad.
There is no magic solution. I'm happy to see all those efforts, but am missing a mention of saving energy. In the age of record-setting data centers for AI training, that's not a popular aspect to mention. Though at least we get higher res more realistic artificial cat videos out of it.
> an array of ten 10-ton concrete blocks lifted 10m in the air could power a house for a day
No, that's only 2.7 kWh. Most homes use 10-20 kWh/day. A battery of that size is easily under $1k. Good luck building your ridiculous concrete block system for that.
Batteries are really good. Gravity, not so much. It only works when you can lift & store a tremendous amount of stuff "for free" because nature has done most of the work, e.g. in valleys, mountains, aquifers, caves, etc. If you have to build the whole thing it will never be viable.
Batteries are good, but you can’t repair a broken battery with odds and ends you find at the hardware store. If you’re living off grid space isn’t really an issue.
People have been able to get their Tesla Model 3 batteries (75kWh iirc) replaced with used battery packs for around $5k, so quite close to what you’re asking.
There are some videos on Youtube discussing the hypotheticals of this. They're never really very positive about the feasibility. Neither on a small scale nor on a country-wide scale.
If you'd want to store 1kWh at 10m height, assuming no loss at all from heat, friction, etc, you'd need about 4 of those blocks block weighing 10 tons (according to ChatGPT). So you'd need a lot of those blocks to power a house for a day, unless you're very efficient.
Please don't cite ChatGPT as a source or as a caveat, instead show the actual math, which should only be about high school level; kinetic energy formula ½mv² = e,
In perfect conditions assuming no loss through drag, you're looking at the kinetic energy formula which is ½mv² = E (in joules).
E = 1 kWh = 3,600 kilojoules, velocity v at 10 meters is 14 m/s, so we need to calculate m for v = 14 and E = 3600k, which is just under 36735 kg. "about four of those blocks" is "about" correct.
I'm surprised that they use the PV power for heating coils instead of using heat pumps. I'm sure they've run the numbers and considered it too expensive or too much maintenance hassle.
However, if that's the case you would think that you can cut out the PV step as well and use direct heat from the sun to heat the dirt, by running water hoses though the dirt and through solar water heaters. Should be cheaper and more efficient than the sun -> PV -> heat coils cycle.
Pumping heat from 300K to 900K is not a big gain over heating -- the entire thing is premised on using extremely cheap intermittent electricity during the summer, and your savings are capped at 30%.
I just read Project Hail Mary which suggests a use of solar thermal energy and I thought "ha too bad solar PV is so cheap in the real world this would almost certainly never happen" and yet, solar thermal energy plus heat pumps does seem extremely simple and cheap for this company
A solar water heater is just a black box with a tube running through it. Many houses here have one, because they're much cheaper than PV panels to generate similar kW heat. But a sibling comment pointed out they want to go up to 900 °K, you can't do that with those, of course.
I think in most places the economics have shifted to PV now.
Despite the simplicity of a solar water heater the even greater simplicity is often part of the argument for PV being better, but the higher grade of energy that you only optionally have to use for heat and relative price changes are the key driver
In addition to being at 600 C, this scheme also crucially revolves around keeping capex as low as possible. Resistive heaters have lower capex than heat pumps. In a system with low numbers of charge/discharge cycles the efficiency drops in importance compared to capex.
Seems like a great concept. Hope they are commercially successful.
It reminded me about another geothermal energy idea: dig about 3 or so miles straight down and harvest the heat that is there already. I guess that's a lot harder than making a dirt pile. But maybe it could become practical if there was enough commercial effort and large scale manufacturing of the equipment.
Kind of brings it around full bore though. Why do that kind of project when you can just harvest actual fuel like oil or gas?
I think this stuff can become practical with more scale and wide manufacturing of equipment and development of efficient techniques. But it requires you to do a lot of upfront work based on principal rather than the bottom line.
So anyway again great idea because it eliminates a lot of challenges and costs that come with concepts like "Journey to the Center of the Earth" etc.
> another geothermal energy idea: dig about 3 or so miles straight down and harvest the heat that is there already
Deep geothermal ought to work. Deep drilling is hard, but it's been done. Eavor-Deep got down to where they got 250C water. [1] That was back in 2023. Not much new since.
The problem seems to be that when you drill into really hot rock, most drilling techniques run into trouble. Rock becomes plastic and clogs things up. The drilling tools have problems with the heat. Progress continues, slowly.
There's these guys, trying to drill with microwaves: [1]
On September 4, they're going to do a public demo and try to drill a 100 meter hole.
The other issue is that the steam loses energy as it's pumped over a long distance. Iceland and a few other locations are big users since they are on fault zones with hot rocks relatively close to the surface. Even if deep drilling is demonstrated, it's unclear if geothermal power will become geographically independent.
Forget climate change, the best reason is national security. Russia's war plummeted Europe into gas shortages and price hikes. Many countries would love to not be dependent on them. And since WW2, all war requires vast quantities of oil; maybe drones will reduce that a bit, but you still need to move stuff (and people) around a lot. So you need a reliable source of energy both in peacetime and wartime.
For the US, the best reason is sustainable energy. Gas, oil and coal are not renewable, so you eventually need to adapt a new form of energy. Just transporting it is problematic, with most communities rejecting pipelines. In the meantime you're polluting your local environment and putting workers at risk. Whereas if your energy plan is largely "the sun shines", "the wind blows", and "dirt holds heat", that is ridiculously more sustainable.
The biggest problem we have is we demand too much energy. AI has made this problem way worse. Nuclear is the only thing that's going to fill the gaping chasm of demand.
Sure, if that's what convinces people to do the right thing, so be it. Though the continuation of that thinking tends to cause behavior that's not very friendly w.r.t. climate or environment.
If you want heat, why bother converting sunlight to electricity first? You lose 80%. Is it that much more expensive to use mirrors to concentrate sunlight and capture near-100% of the energy as heat?
Because with a heliostat you have to move the energy as heat and that isn't as efficient and definitely not as easy or cheap as moving it as electricity.
Do the numbers. Making the top of the heliostat isn't what matters. What matters is making the inside of a pile of dirt that hot.
At what scale does this become efficient? I may have 1000 sqft to dedicate to this type of system on my lot. Feels like that’s at least an order of magnitude too small to maintain the energy through the seasons. Could we build one of these slightly larger systems for every square mile (~1000 homes), or does this only work at a 10,000 home scale? The article is showing a pile of dirt on the ground. Could this just be an area of the subsurface which is heated, or does ground water become too much of a problem?
There are some companies making heat batteries that you can install inside your house. They won't last very long but it's a great way to store excess energy from your rooftop solar and use it for heating and warm water. Also it allows you to timeshift when you draw power from the grid (i.e. when it's cheap rather than when you need hot water).
Larger scale batteries can store enough energy for seasonal storage. The larger the size, the better the insulation can keep the heat losses to a minimum. Basically you have a smaller surface area relative to the volume and mass. But even with a small unit, you can keep it hot for quite long.
Stuff like this is easier in areas that are already on some sort of district heating or have some kind of water based central heating. For those systems it's pretty much plug and play. You don't really need to modify the houses.
I think Helsinki has a few larger scale units already operational and a few more under planning / construction. I think the largest one will store 90ghw of heat. Which is quite a lot.
The beauty with thermal storage is that almost any kind of mass with enough heat capacity works. Water, rocks, sand, etc. All fine.
How would you move the solar energy into the piles of dirt? You’d need something like an array of mirrors focusing the rays, which has definitely been done already but has drawbacks. Electricity can easily be moved to where it’s needed.
You could heat up a metal heat exchanger that you circulate a working fluid through. Probably easier to just convert sunlight to electricity to heat via resistive heating, less maintenance.
At home, it's suitable in warm climates but is more challenging in snowy / very cold regions. Generally speaking, converting to electricity then using an electric water heater is more efficient because there's much less insulating, heat loss, and piping that can leak and cause water damage.
If you look at "evacuated solar" panels this is as close as you could get, and they don't get anywhere near the temperature you need to generate consistent steam. Concentrated solar is closer, but cost impracticalities make it unattractive here as well.
This is a blog article outlining a rough concept idea. As others have commented, many questions remain unanswered, and speculation about isolated physical properties and technical ideas is unhelpful.
For it to be worth spending more time and effort on, I would need a closed system thermodynamic calculation. The technical term for this is a "heat balance diagram". This is the first thing any technical consultant would request.
Overall it's a good idea but I'm skeptical that random "dirt" will reliably withstand the temperatures required for
> the stored heat creates steam on demand for the turbines instead of burning coal
but mostly-quartz sand without any calcite and crushed granite surely can, and those are widely available for barely more than the cost of driving a truck to your construction site, because those are the main ingredients in concrete. (As he says in his white paper https://findingspress.org/article/141340-thermal-energy-stor..., US$20–50 per tonne.)
If your random dirt is good enough I'd think you'd want to trench into the existing dirt to lay pipe, the way most ground-source heat pumps are done, rather than hiring a bulldozer. But I haven't done it; maybe the cost structure is different than what I imagine.
I wonder why he says
> Surrounding the earthen mound will be high-density, low-profile solar arrays.
What's stopping you from putting the solar arrays on top of the earthen mound (or sandpile) too?
Sand or feolite thermal "batteries" on the daily rather than seasonal timescale are eminently economical for even household thermal storage for heating. With TCES energy storage, which I expect to be barely more expensive at the household scale, you can get cooling too. TCES also scales down to small systems and long storage times in a way that sensible and phase-change thermal energy storage can't, while also bettering them on density.
Incidentally, energy superabundance is apparently becoming a reality in the PRC, and CO₂ emissions are falling:
https://fortune.com/2025/08/14/data-centers-china-grid-us-in... “#AI experts [#Rui-Ma] return from #China stunned: The U.S. grid is so weak, the race may already be over. (...) In the U.S., surging AI demand is colliding with a fragile power grid, the kind of extreme bottleneck that Goldman Sachs warns could severely choke the industry’s growth. ¶ In China, Ma continued, it’s considered a “solved problem.”” because “China has an oversupply of electricty [sic]”. “In China, renewables are framed as a cornerstone of the economy because they make sense economically and strategically, not because they carry moral weight. Coal use isn’t cast as a sign of villainy, as it would be among some circles in the U.S. It’s simply seen as outdated.” #energy #USA
https://xcancel.com/ruima/status/1955040979259650267 #Rui-Ma’s account of her trip to #China to go to the World #AI Conference (#WAIC) and see investors and renewable #energy. "We met with companies ranging from Baidu, Alibaba, and Tencent (BAT) to unicorns, as well as young startups founded less than a year ago. (...) Energy is considered a solved problem. The Chinese government’s investment in sustainable energy — from advanced hydropower to next-generation nuclear — means that, relative to many other markets, electricity supply is secure and inexpensive. Everywhere we went, people treated energy availability as a given. This is a stark contrast to the U.S., where AI growth is increasingly tied to debates over data center power consumption and grid limitations."
https://xcancel.com/ruima/status/1955372325970514161 #Rui-Ma on #energy in #China: “China: Electricity generation jumped from roughly ~5.6 PWh (2015) to ~10.1 PWh (2024), which is +80% in less than a decade. And 2024 alone grew ~7%, accounting for ~HALF of the world’s increase. China now produces ~1/3 of global electricity. That’s what “supply went up a lot” looks like, so yeah even if coal as a % of total went down, the total will still increase but the increase is QUICKLY PLATEAUING (...) Multiple independent analyses now show China’s CO₂ fell year‑on‑year across the last 12 months (and in Q1’25), driven by the clean‑power surge rather than an economic slump.”
Another minor note as I read the rest of the page:
> the resistor material itself is $1-$2/kilowatt. (...) There are only a few materials that are even acceptable as resistors.
This depends strongly on your target temperature and conditions. Nichrome may cost US$2/kW but galvanized barbed wire doesn't. It can kill you with the fumes, and in oxidizing conditions it won't last long above 200°, but zinc fumes inside a dirt pile won't hurt anybody, and you can probably maintain reducing conditions by mixing some humus into your dirt initially and keeping the pile dry. If process heat or climate control is your objective, you don't need temperatures high enough to oxidize iron rapidly even in an oxidizing environment.
(Incidentally, reducing conditions will destroy nichrome at the temperatures people use nichrome for. Maintaining oxidizing conditions inside your dirt pile over the years is going to be a lot more challenging, I suspect.)
I suspect keeping the pile dry is the reason for not just trenching in existing soil. Dryness is essential for maintaining temperature over 100°.
My intuition is that you could probably scale the seasonal-sensible-TES-in-dirt approach further down from the few megawatts he's talking about if you can use real insulation instead of dirt. But insulation is expensive at this scale, even things like perlite and vermiculite.
Spitballing, suppose we want to withdraw 100kW energy for 4 months (1TJ) and can deal with a time constant of 8 months, so leakage is ballpark 50kW. Suppose we have 1J/g/K sand and ΔT = 200K. Then we need 5300 tonnes of sand; at US$20/kg that's US$100k, a dollar a watt, cheaper than power from a coal plant but not really competitive. A larger ΔT can save you but might shorten the life of your barbed wire. But at those low temperatures you pretty much can use any old dirt, US$5300 of it by Vernon's embankment figure.
But that's about 2000m³, whose minimum insulated surface area would be as a 7.8m sphere with 800m² of surface area. 50kW of leakage over that area would be 70W/m², or 0.3W/m²/K, or 3K·m²/W.
There's not enough comparison with the conventional ground-source heat pump here. There's not enough modeling of the expected system dynamics. I don't have a physics argument against it (right now at least) but I think that the author is trying way too hard to sell me on the idea of energy storage and not hard enough on why this proposal can work. And I don't think it's just me. Anyone reading the pitch for an energy storage startup in 2025 is probably aware of the basic goals, and more importantly is fatigued and suspicious after watching several dozen clever ideas go nowhere.
Surely you can write a short model of the system at the level of undergraduate thermo. If you have a pile of dirt this big (say about a thousand times the size of a spherical cow) with these pipes running through it, then at a storage temperature T your capacity is X, your leakage is Y, and your recovery rate is Z. Fill in the blanks.
GSHP/ASHP can't get to steam-generating temperatures on the hot side unfortunately - you're looking at a "heat concentrator" which doesn't really exist in the form of a commercially available heat pump today - possibly not even in the theoretical.
Sorry, that comparison to heat pumps wasn't meant to imply that heat pumps would be used in this system. Rather, the focus was on the pipes that transfer heat from the system to the ground and back. That component remains basically the same — the heat pump components of a GSHP are above ground. My thought was that experience installing GSHP would allow you to bracket out the cost of the interred heat exchanger and make a comparison to the title proposal.
My lingering question is how do you service a leaky pipe surrounded by dirt that’s 600C? Do you just have to forfeit a good chunk of your energy savings for the year?
Or is there like a practical maintenance window each year at the end of the winter when you’d do this?
That's not a lingering question. It's the question.
This is a steam boiler. Those are well understood. They have well understood problems. Leaky boiler tubes. Crud in the tubes. Cleaning. The problem here is that you can't easily turn the heat source off.
It's possible to build a long-life boiler for a heat source you can't fully turn off. Every nuclear reactor has one. Heavy stainless steel tubes, precision welding, distilled water.
Works fine, but not cheap.
This paper is very hand-wavey about the details of getting the energy out. They're all about the side that puts the energy in, which is the easy part.
I always wondered if, instead of using solar panels in the deserts, we could use very long and black pipes running water, heated by the sun. Then the heat is moved to the ground for storage, and once there is enough heat, we use a turbine to generate electricity.
I don't think you ever get enough of a temperature difference just by having a passive black pipe in the sun, to do any useful work besides potentially keeping someone warm in the winter. You could do useful work if you concentrate the energy from the sun somehow, like with mirrors.
Heat pumps do magic by changing the pressure at which a working fluid changes phase, so you can boil the fluid over here, have it absorb an enormous amount of energy then compress it back to a fluid elsewhere and push that heat back out -- this works pretty well because you're just moving the heat and only pushing the temperature on the "hot" side up a relatively small amount. I don't think, for instance, you could make an oven with heat pumps.
To do useful work you need a _substantial_ energy gradient -- it's hard to live in the sun even though its got lots of free energy floating around. The sun is very useful to the earth because the energy it provides is so much more energetic than the ambient environment.
Edited to add:
There are discussions of using exotic working fluids like compressed CO2 -- that'd allow you to manage the phase change maybe to a region where you could concentrate the energy in the fluid then expand it elsewhere at "room temperature" temperatures -- but I think things like compressed (to a _fluid_) CO2 are really hard to work with.
It's done a bunch, all over the place: https://en.wikipedia.org/wiki/Solar_thermal_energy. Most commonly people use them either for batch pre-heating of hot water or for full heating. I looked in to getting something like that for the hydronic heat in my house because currently it's propane-fueled and the deliveries are quite expensive in the winter.
Does the article describe how the heat gets from the mound to the houses or buildings it plans to heat, or factor in the cost of that?
Naively, I'd assume that would like 90% of the cost.
I know that physics is under no obligation to be intuitive, but it's also surprising to me that it's so easy to heat and keep dirt this temperature (600C / 1100F) throughout Winter, and I didn't see how that piece worked either, though I'm willing to assume that part is figured out and factored in.
Apparently the insulation value (R value) of dirt and soil is between .25 and .8 per inch (depending on moisture content). That wouldn’t be great if it was a material like fiberglass, but since it’s dirt cheap (ba-dum-tssh) and easy to pile up in large quantities with little to no ongoing maintenance in this kind of context, it matters.
A 10 ft pile of dirt (assuming 10 ft between heat exchanging pipes and the outside air) has an R value of 24 to 96, which is extremely significant.
I expect there would still be notable losses trying to keep it at 1100F indefinitely, but 10 ft of dirt will have insulation values approximating many feet of fiberglass insulation.
You’d want a very large mass to heat however, scaling matters a lot. You’d want the ratio of surface area to mass to be as small as possible, and that means as large a volume with as thermally dense a material as possible inside. Surface areas increases by the square, while volume increases by the cube.
Also, no matter what you do, you would eventually cook whatever was at the surface or underground, so don’t do this where you want trees - or where there are underground coal seams
> Pipes run through the pile, and fluid flowing through them removes heat to supply the customer.
Dirt keeps a constant temperature year round quite close to the surface that’s a ~60 degree difference between summer and winter in many areas. So 600c would just be a tradeoff between depth, heat loss, and thermal efficiency. However, what they aren’t saying is electricity > heat > electricity is quite lossy and even just using the heat directly is far less efficient than a winter heat pump.
That’s just for heat to thermal and quite optimistic not end to end. “Conversion back into electricity is 40%-45%”
More realistic end to end numbers are likely in the 30% range which means summer electricity needs to be vastly less valuable than winter energy before you nominally break even and start repaying the investment. Further you instantly lose all the electricity required to heat the mound up to working temperatures. IE: If you can only operate between 550C and 650C then going from 20C to 550C needs to happen before you can extract any energy and you don’t get that investment back. On the other hand if you’re a chemical plant that needs 200C things start looking a lot better.
Interesting because I've been thinking about mechanical energy storage recently. I feel like these concepts hold a lot of promise on a small scale, per-house; coupled with solar panels. Although mechanical batteries they are not as efficient as electric batteries (and lose some energy), they can be both cheap to make and durable; these characteristics are much more important than raw efficiency when dealing with a single house.
this works in principle. heating dirt to store energy is cheap, the material costs almost nothing and the physics are solid. if the output needed is heat then it can beat batteries by a mile on cost.
the problem is scale. the dirt is free but heaters, piping, controls, permits, and contractors are not. balance of system costs creep up fast and thats where most cheap energy ideas collapse.
the market fit is narrow too. industrial heat or maybe district heating could work. coal plant conversion sounds good in headlines but takes forever to line up politics and utilities. daily cycling wont compete with batteries, only long slow seasonal storage makes sense.
execution decides if this survives. if they can keep real projects near the claimed cost then it has a shot, otherwise it stays as a cool demo.
The biggest problem is it has to be located near things that can use the heat.
Why not sell something in tbis vein to households and then let them use cheap daytime electricity to charge it up and and then heat their homes at night.
California has thousands of abandoned (orphaned) oil and gas wells, with more than 5,500 identified as "likely orphaned" in 2023, and an additional 70,000 economically marginal and idle wells that could become orphaned in the future.
Kern County, California has 75% of the state oil wells, and the largest solar farms. Even after the 70,000 wells are idled, Kern County will continue to produce enough oil to meet all internal demands in California, although the state no longer has the refining capacity or anywhere to lay off the oil.
Interesting how this turns the standard cogeneration plant strategy on its head. Instead of creating chilled water overnight and using that for cooling buildings, heat up the center of a large mass and use that for heating buildings or making steam to run turbines.
Ground-source heat pumps are really common in the nordic countries.
Could an PV system energise an existing GSHP steel bore and warm up the earth and rock a bit around the bore? This heat would then be tapped in the winter.
Exowatt requires rethinking the solar energy system entirely. This is meant to be bolted onto an existing solar panel field that is mostly selling energy to the grid, but gets curtailed sometimes
Direct solar thermal that would produce 600 C would require expensive tracking concentrators. These lose a lot of energy to reradiation, don't use diffuse sunlight, and now you have very hot fluid you have to convey to your storage mass. With PV, you can just send electricity to resistive heaters in the storage mass.
Another question I don't see answered in the article: is there any risk for existing life by heating a huge amount of dirt? Will at some point surface and possibly influence local weather / thermal winds? Or should I just get my tin hat off?
If the underlying stored heat leaks to the surface then it may indeed melt the snow and you'll have "snow-free" parking, but that won't be the end of the story, as all that resulted water will most likely turn your parking lot into an ice rink.
snow-free means you're losing heat to the environment vs keeping it contained. I would imagine if properly designed you could stand on top of the completed dirt pile without feeling any temperature delta with the air.
It begs the question - why use solar if the storage mechanic is sufficient to cover peak/off-peak storage of any surplus electricity in a significant way? Hydro Quebec has significant ($500M+/yr) electricity surpluses on a given year in the form of underutilized hydro during spring/fall months where energy demand is very low and export possibility is limited. These seem like ideal candidates for this vs something like a (relatively) small-scale solar setup. The slow-charge slow-release is also a considerable factor here since most battery energy storage solutions are designed for charge/discharge events multiple times per day to arbitrage energy rates - their ROI (which is currently very aggressive, sometimes less than 4-5 years) is a function of how many charge-cheap/discharge-expensive cycles they can perform per year. That is something that this doesn't seem equipped to do which is a negative multiplier on the ROI these systems can yield. If you can charge/discharge a battery 500 times per year, and the cost is 500x more, this is only at parity with a much slower charge/discharge schedule (charge during the summer, discharge during the winter - or vice versa). The other reality here is the steam turbine required to generate electricity (assuming electricity storage and not heat storage model) is a significant capex - roughly $300-400/kW (and some considerable OPEX in the form of maintenance and regulation/compliance). In this reality it only makes sense to retrofit an existing sunken-cost turbine and grid transmission system vs purchasing and co-locating a generating turbine along with the storage solution.
For reference, point-in-time energy market rates usually swing by 2x-3x per day - meaning if you charged during the cheapest market rate and discharged during peak you'd net about 2.5x return on that cycle) - even more so during extreme temperature events like heat waves or cold freezes - those are ultimately what you're riding here in terms of validating the system's viability from a financial perspective. If you reduce that scale from hours to months, and if draw-down speed is slow (ie: you can't sustain 50MW of steam with 500,000 tons of dirt even at 600'C) then you're looking at even more complicated returns.
By my simple, assumption-laden math, a 50MW "system" (capable of providing up to 50MWe peak output and requiring a requisite (assuming since it's not mentioned in the article - that at 200'C a 1,000,000 ton dirt pile would only be able to sustain 40MW of thermal output/20MW of electrical output and 240MW thermal/120MW electrical output at 600'C) would be:
PV system (20MW system would require ~30 days of charging to provide 50MWe output for 1 day, ~1200MWhe), alternatively, per day, you could discharge 50MWe for ~48 minutes.
1,000,000tons of dirt storage at 600C should hold a theoretical ~28 days of 50MW electrical supply.
(also worth noting, getting the dirt pile heat up to "steam" temp would likely eat up a considerable number of months charging, which is also capex)
$1,000,000 for dirt
$5,000,000 for balance of system (heater elements and wiring + ASME tubing - as an aside this seems very opportunistic for 20MW of heaters and tubing to supply 100MWt of steam)
$12,000,000 for Solar Panels ($0.60/w bulk)
$8,000,000 for Solar Supporting systems and installation (assuming heaters can run on DC power and no inverters are required and there is no grid tie, minimal permitting and simplified ground install)
$25,000,000 for a 50MW steam generator turbine and transformer yard, provisioning etc
land use: ~25 acres for dirt pile, ~100 acres for solar, 10 acres for steam/aux, call it $300,000 assuming US averages for cleared land.
----------
Assuming north-eastern US (~20% solar efficiency with subzero winters where you also have high off-solar peak demands)
If you only charge/discharge this twice per year you're looking at some pretty paltry economics - you could only really fill about 18k MWh of thermal energy during half of the year for a ~7,400MWhe discharge - $592,000 gross electricity revenue per discharge cycle at an opportunistic 7-day "peak" market rate of $80/mWh which is about $1,184,000/yr gross margin. If you did it once per day (40MWhe per day at peak average intra-day market rate -$68/mWh) you're looking at ~$2,720/day or $992,800/yr gross margin.
$51M capex would be difficult to justify margins of only $1.1M/yr, and that's before any operating costs of which there would be several.
If you just sold the same solar at market rate (~$36/MWhe) throughout the year you'd net out at $1,261,440. Capex would be ~$40M and grid-tie solar is very cost effective in OPEX.
Likewise, if you just connected the system to the grid and skipped solar altogether (powering the heaters with grid energy like battery storage would): 50MW in for 12 hours on cheap time-of-day rates (typically overnight ~$18-20/MWh) and sold for 5 hours during daily peak rates ($55) you'd cut your capex considerably without the solar component and you'd be able to net, even with round-trip energy efficiency around 41%, (600MWe in @ $11,400, 248MWe out @ $13,640 = $2,240/day ~$817,600/yr gross margin) for a capex of $31.3M.
So in the end, the best solution seems to be collocating this on an existing coal/gas plant, where the capex is already sunk in the transformers, grid interconnect, steam turbine, land and permits and you're only adding the earth battery - you could run the model with the above margins with a capex of only $6-7M, which is very viable and even more favorable than the economics of spinning up a new gas/coal plant.
The economics of battery energy storage (BES) systems are much better known (ROIs of <4 years in extreme-swing energy markets doing intra-day peak arbitrage is very possible) since your round-trip efficiency is closer to 91%. A 250MWh BES plant with 1-hour charge/discharge window would be~ $40M installed and could arb twice per day - at 2x (low end averages - buying at $26 and selling at $52 twice per day = $14,285 cost for $26,000 revenue) $11,715 margin per day, $4,275,975/yr on $41M capex is still better economics than all the above models except those where the steam generator and grid infrastructure is already sunk.
Imagine 1,000,000 Drake Landing installations per year in Canada, pre-heating with the excess electricity. In 30 years Canada would need zero fossil fuels for buildings.
which... is only 13% of their GHG emissions? Oh we're fucked. The planet's so fucked.
ive been obsessed with passive heating recently. one thing that is really interesting that i only heard of until recently is pumping heat into the ground beneath a dwelling… far enough depth into the dirt so that by the time the heat rises and reaches the house the seasons have changed and its cold outside. the ultimate passive thermal
You can do that with normal pipes buried 5+ft underground that pull air into the greenhouse - the earth's temperature regulates the cold in the cooler months and the heat in the warmer months. There are similar calculations for how much piping you need to sustain a certain volume of air at earth temperature (typically 15'C in most places). This is shallow geothermal.
This seems really cool if this can be done at scale
See, I was a huge solar fan/renewable fan except but then I watched the infamous michael moore video and I had sort of concluded that either we need to focus on nuclear since I have this only grudge against the documentary that it didn't depict nuclear energy.
But overall, the conclusion of that documentary was that solar might not be the right approach given its storage mechanism and that batteries might be net negative overall the way they might be used right now
And that it might lead to more net negative emission even compared to Coal etc. (or comparable)
Seeing this, I feel like this can atleast be a very huge boost into not using batteries and building systems that only require land with less maintanence overall.
Like, I can imagine a solar farm with solar panels and communities can build them in vacant land and then they build the system proposed in the article and then basically have more solar panels than the consumption or just enough to store them into such a system and then lets say that a stormy weather comes or rainy seasons comes or something or winter comes and then we might be able to push the energy from the reservoir.
This might be more efficient compared to using batteries or having more solar panels in the first place and "wasting" excess solar energy in the summers if lets say that we overprepare ourselves with a lot more solar panels (which seems to be the current approach) without using batteries.
Now to be fair, the author does propose that he likes batteries. I wonder if I can change that but I myself can be wrong too (I usually am), and so maybe we can discuss it!
I wonder, how this system without batteries might work though.. like how are we gonna prepare for night without batteries completely, like maybe we can have a system where we can shift to the grid in the night?
Now although I will be honest, I feel like using the grid at night doesn't make sense but I am assuming here that michael's moore's video is right (which it is) and that the current way that batteries are used is more carbon emissions in the context of solar right now
Then this system that I was talking about doesn't include batteries and such might be atleast definitely better for the environment overall.
But that being said, I am still not a big fan of solar compared to nuclear. I feel like nuclear is one of the best things.
I wonder if we can actually have a nuclear reactor which can create energy and then send it to such communties and then they can store it in their ground and (its efficiency seems to be 50%) which might be actually good considering how atleast to me nuclear genuinely feels like unlimited energy and thus I was genuinely thinking of a cheap way to store energy, this isn't definitely portable but I feel like that communities can build it economically and so maybe countries with nuclear energy can setup lines with other countries and all that nuclear energy straight up goes to the community which stores it in such land and then they might actually use solar too and rely on this too and just store a ton of energy as much as it makes sense economically and then maybe combine this with solar and then use that nuclear energy at winters or if it turns out to be economical even at nights.
Now why I am saying this? Because this feels like a completely clean method. Now although I like nuclear energy a lot, I feel like a lot of countries can't access nuclear energy as they can't build it because if they can, they would've made bombs and its hard and they can't rely on other countries which have unlocked it for their power because that's a geopolitical disadvantage
but just using it for storing energy at scale might make true sense since it isn't that big of a priority and even if some nuclear power lets say denies them, then maybe they can go back or go to other nuclear power without their infrastucture being completely reliant or essentially melting because of some other countries decision and they still have autonomy which can make it a lot more viable
Overall a net win win situation imo. Makes me a little bit more optimistic.
> But that being said, I am still not a big fan of solar compared to nuclear.
This technology, if it works, is a bullet in the head for any remaining chance for nuclear. It provides heat at a cost competitive with natural gas, and natural gas is what destroyed the nuclear renaissance here in the US.
I'm sure you meant it to be edgy and offensive but it's quite flat in substance - you can't reasonably reduce the population and any means you'd employ to do so would be equally destructive to the environment - you can't change the capitalistic paradigm of energy generation and consumption, without some effort and resources drastically beyond what you've demonstrated here, and you can't ignore economy of scale that is demonstrated particularly with energy generation to insinuate that these solutions must be collocated on premise for them to be viable.
> Go ahead, flag this message so it will disappear and no other persons will be ever know it existed. That's the HN way.
Commentary like this is a reason to flag posts in and of itself.
But the rest of your post presents far more rhetoric and conspiratorial thinking (including bringing in entirely unrelated policies that nobody ITT is arguing for) than analysis, evidence or even logical reasoning.
> 'Battery storage' has no scalable reason to exist as a topic except for a millionaire's survival enclave. Natural disasters span days and weeks NOT hours.
An experiment to try: fully charge your cell phone or some other similar device, then turn it off and leave it off. How long into the future do you expect you'll be able to turn it on again without another recharge? You might be surprised.
A fascinating energy storage startup just emerged from stealth mode. The concept involves DC coupled PV feeding resistive heaters buried in dirt, providing heat at 600 C for a capex of $.10/kWh-th of storage capacity. Storage is seasonal, from summer to winter.
I'm not saying it's bad, but it's clearly not a replacement for batteries, given its applicability is quite limited.
The question is about conversion is, is it still cheap if you add a powerplant (i.e. converting heat into electricity) and have to maintain it (moving parts, in contrast to batteries).
As I understand it, this is meant as a supplement to batteries, so that batteries can do most of the intraday load shifting while thermal can store energy over longer durations.
Some yes. But there's a flaw in your reasoning: It ignores the concept called "rate of change". Because of this concept there will be more energy put into the pile than escapes, leaving some to be extracted and not just "escaping".
Long-term thermal storage is something I've been fascinated with the last year or so.
Heat loss inside of dirt is so incredibly slow it's hard to wrap your head around. One fact that I find helps is the fact that after an entire winter of extremely cold temperatures, you only need to go down 10 ft or so before you hit the average annual temperature. 4 months of winter buffered by 10 ft of ground!
Obviously there is incredible potential to this even if you just keep the energy as heat. The amount of electricity we use on heating and air conditioning is huge. If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling.
Really excited by companies looking into this and wish them the best of luck!
> Heat loss inside of dirt is so incredibly slow it's hard to wrap your head around. One fact that I find helps is the fact that after an entire winter of extremely cold temperatures, you only need to go down 10 ft or so before you hit the average annual temperature. 4 months of winter buffered by 10 ft of ground!
That’s not entirely insulation. Some of the heat flows upward toward the surface during winter and some warmth flows downward during summer.
> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling.
Geothermal heating and cooling already exists. It’s semi-popular in some areas. It can be expensive to install depending on your geology and the energy savings might not compensate for that cost for many years. Modern heat pumps are very efficient even if the other side is exposed to normal outdoor air, so digging deep into the earth and risking leaks in the underground system isn’t an easy win.
the parent was not talking about geothermal, but storage which would come as interesting to store winter cold for the summer, I have no idea whether it would work.
Based on some guesses and uninformed searches if a house spends 200MJ on cooling and there is a 20 C delta between winter temperatures and desired cooling temperature and assuming a specific heat capacity of ~800 J/(kg*K) you would need 12.5 tons of rock as battery which would be around 6~8 m³ which sound very small.
I am sure that there are hundreds of complex factors at plays (eg rain water and aquifers reheating the battery during spring) but it came out to be a far smaller number than I would have guessed.
Again, dirt isn't very insulative. It has a high thermal storage capacity, but if you pumped heat into it, it would dissipate to all compass directions and downwards, and you couldn't retrieve it later.
If that wasn't true, you'd need to keep moving the underground passageways of buried passive cooling systems.
That’s because 200 MJ is a small amount of cooling. It’s only about 55 kWh.
I considered 400W 6 hours a day for 20 days because it is about what we I use.
I think you missed a 0. I've never heard of an AC unit for a house that's 400W. 4000W and above is more common.
Google:
> An AC unit's electricity usage varies by type, with window units using around 500–1,500 watts and central air systems using 3,000–5,000 watts, though usage can range from 2,000 to over 6,000 kWh annually for central units
Also, how much you use it during the year can vary hugely from 0 (when I lived near the coast) to like 10 hours a day for months in hot or cold places. There's not a standard, but 55kwh for a year means you live someplace that doesn't really need AC / heating.
Which climate zone?
In this example it would be for the whole year, yes?
Anywhere in the South, Southwest, West, or many places in the Midwest will be doing that each month for 6 months out of the year at least.
Underground heat storage isn't new nor anything startuppy though, we're well beyond the "companies looking into it" stage. This page [0] mentions it's been around commercially since the 90's and experimentally since the (19)30's, and interest started in the 70's.
But depending on your definition of this, it's been around for hundreds if not thousands of years. People used to cut ice out of frozen lakes and store it in underground basements for year-round cooling. And in arid climates they have windcatchers [1] and other techniques where they store the nighttime cool for usage during the day, or these [2] to store or even create ice, all without using electricity.
[0] https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag...
[1] https://en.wikipedia.org/wiki/Windcatcher
[2] https://en.wikipedia.org/wiki/Yakhch%C4%81l
The problem here is the density. Air conditioners are rated in tons of cooling, with even the small ACs rated at at least 1 ton a day. So even a small house can easily melt through its weight in ice during summer.
10ft below ground is enough to take advantage of geothermal heat. You don't have to go "very far" to reach warmer soil in winter because the soil PLUS the snow on top is pretty much just insulating the deeper ground from the cold air.
Start getting into permafrost though where the cold is more constant and that cold layer gets deeper.
10ft is definitely not enough for practical use. In order to heat a rural house with a heatpump connected to geothermal you need in order of 200-300ft deep hole, at least here in Finland.
For ground source heat pumps you have two approaches. Either you have deep hole. Or you have a large field. In later case not as much depth is needed, but you do need much larger area.
Good point and very true!
At a certain depth the temperature curve is 6 months delayed from the surface. So it's getting to its warmest during the winter, and coldest during the summer. At a deeper depth it's pretty constant year round.
With regards to wrapping your head around heat loss- this winter some work was done on our property while there was snow on the ground. A bunch of snow got covered in dirt. In the spring, maybe three weeks after all the snow on the ground had melted, I moved the pile and was surprised to find all the snow still frozen. It had been under maybe 18 inches of dirt. I was pretty surprised to see it.
It's already done and proven to work further North than anywhere in the USA:
https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community
We (USA) could have 80% of our Northern homes off fossil fuel and electric heat for less cost if we were a little more forward thinking and willing to work together.
But after nearly two decades they're decommissioning because the one-off components needed too much NRE to refurbish. If we all adopted this it'd be cheaper than what we pay today and zero greenhouse gas emissions. It'd finally make living in the temperate climates more climate-friendly than the warmer latitudes.
Yeah, district heating/cooling would make us so much more efficient. It wouldn't even take a whole lot of space or land to implement. You could stick it right next to a community water tower.
> if we were a little more forward thinking and willing to work together
it's really depressing to read this and deep down immediately know: well so that's never going to happen then.
Uhh, I think a big reason why solar thermal is more meh these days is that solar panels are so cheap and heat pumps are reasonable
Except that won't heat your house at night during a week-long blizzard. The "heat pumps and solar" plan requires natural gas peakers be built for maximum load of the grid, which we're doing even without heat pumps so far, and then we just run the peakers more partially because utilities are cost-plus regulated.
The article is talking of the revival of [solar {...} thermal] storage precisely because solar panels are so cheap and heat pumps are reasonable.
The {...} is the counter intuitive step of solar -> electric -> heat storage for six months -> electricity for later.
The secret sauce is increased heat deltas, not just heating dirt directly using sunny daytime temperatures but really cranking up the heat of an underground mass to 500C + (IIRC - I skimmed the article some hours ago).
This idea has nothing to do with heat pumps.
Not the somewhat recently popular specific household commercial gadgets for cooling and heating a home, no.
Just thermodynamic pumping of heat, from the article:
and at industrial scale, using bleeding edge HVAC technological advances, it's all about the creation, storage, and pumping of heat.Resistive heating isn't pumping heat. It's turning zero entropy energy into heat. An ideal heat pump, on the other hand, creates no entropy.
Operating a digger to move dirt isn't pumping heat either, if we're listing project components that aren't pumping heat.
I'm guessing you caught but ignored the part that is moving heat?
One can move heat from one place to another by moving a fluid. This is not a heat pump, even if the fluid is pumped.
A heat pump is a specific kind of thermodynamic device that moves heat energy from lower to higher temperature.
The system that is described in the OP link does not use a heat pump. Electrical energy is used to make heat, but it does so with a resistive heater, not a heat pump.
This is called PAHS, passive annual heat storage and has been tried in some alternative energy housing.
You put pvc pipes into a hill of dirt that is covered by a plastic sheet or other waterproof membrane; during hot summer months you use a small fan to put heat into the pile; during winter the heat moves from the dirt to the house.
This is definitely not what the OP is talking about.
He is talking about electrically heating very large amounts of dirt to temperatures of 600C or more. Your PVC tubing approach is talking about 50 times smaller swings.
This reminds me of the interesting articles over the last few years about the heat from the London metro system over the last century saturating the ground so the tubes have become extremely hot.
Does the slow heat transfer interfere with attempting to use that heat?
I can imagine that there's a lot of total energy in the dirt 10 feet down. But once you've tapped the energy near your well, how long does it take to replenish? How long until the immediate vicinity reaches equilibrium with the surface?
Slow heat transfer (and the absolute need to minimize capex) drives the system to operate on very long (seasonal) time scales.
... and similarly doesn't it mean the pile is slow to absorb heat when your PV installation is trying to dump energy into it?
You don't need to store it in the dirt. You use the dirt as insulation, and store in something like molten salt or whatever which can be pumped up to surface and has good thermal conductivity to then extract the heat. At least that's my understanding of all these systems.
Read the article.
He is talking about storing the heat in the dirt and he gives good economic reasons for that.
The point of the article is that storing in the dirt is much simpler and cheaper than molten salt.
Deliberate exchange of heat would be done with internal radiators designed to maximize the transfer.
Environmental exchange would be limited to the interface between the storage tank and the surrounding soil.
It should be orders of magninitude more efficient to transfer energy intentionally than what would be lost to the environment.
It means the heat stays near the hot pipes for quite a while.
I really can't help but think there might be a fundamental problem here. If heat loss in dirt is so incredibly slow, and we use a heat pump to extract that heat, then wouldn't it be equally slow to replace the heat extracted? (absent heat injection, i.e. reversing the heat pump's operation in summer)
Has anyone looked at the subsurface ground temperatures after days, weeks, months, even years of heat pump operation?
I do seem to remember seeing one article on the subject showing that after one winter the subsurface temperature had declined enough to materially affect the heat pump's COP. But the timescale didn't extend to multiple years.
edit: found this one: https://pangea.stanford.edu/ERE/db/GeoConf/papers/SGW/2024/M...
In one of eight cases studied where heat flow is unidirectional (cooling load only) over a 20 year timescale the authors find:
The other 7 cases showed weaker or negligible long term variation.An additional graph shows COP variation over 15 years and the worst case shows a decline of perhaps 10% (just eyeballing it).
Surprisingly, some cases showed a long term improvement in heating COP - presumably the injection of summer heat into the soil made for warmer soil than just sunshine and natural diffusion?
So my takeaway: "it depends." :-)
There are no heat pumps involved in the OP idea.
Oh! You're right, I hadn't realized. But... wouldn't it be more efficient with a heat pump and exploit the 2-4x COP? Ok, maybe I get it now: heat pump wouldn't work at the high temps they are achieving. Thx.
Why are you optimizing for efficiency rather than overall cost?
Dirt has an r-value of between 0.25 and 1 per inch according to some quick googling, so 10ft would be maybe R-60.
Surprisingly, that's only equivalent to about 10" of polyiso rigid foam.
What this project is really taking advantage of is the super cheap thermal mass. Dirt has about a quarter of the specific heat of water, but it is, literally, dirt cheap, and much easier to keep in place than a liquid.
You can also have a temperature cycle of 600C with dirt. Water is limited to 50-70C so the dirt loses a factor of 4 in specific heat, but wins back a factor of 10 due to delta T.
The net is dirt wins by a factor of 2.5.
The thermal conductivity of rocks (and, I presume, soils) also typically declines as temperature increases, falling by a factor of 2 or 3 at 600 C.
You are confusing the concepts of insulation and reservoirs.
When 5 of the 6 semiaxes of possible heat flow have no temperature gradient, the temperature becomes much more stable.
Insulation would be if a 10 ft radius dirt ball maintained a stable temperature all year round - which would surprise me, although dirt does have some insulative value. However, wet dirt is really not very insulative - try sleeping on the ground sometime.
The thermal time constant of a 3d object scales as the square of the linear dimensions of the object, so time constants measured in years are quite achievable.
The dirt here will not be wet in most of its volume -- the initial charging will bake out all volatiles. The hottest part of the pile will become something like dry brick. It's like storing energy in ziggurats.
Tech Ingredients did a video on this last week. https://youtu.be/s-41UF02vrU?feature=shared
> you only need to go down 10 ft or so before you hit the average annual temperature
Is this because of geothermal energy leaking upwards? If so, it's not the dirt, it's the geothermal energy.
> Is this because of geothermal energy leaking upwards
No. The heat energy comes from the sun. Power flux from geothermal is measured in milliwatts per square meter, while the sun can provide more than a kilowatt during the day. So real geothermal heating is negligible at the surface. That's why the temperature a few feet down equals the average annual temperature at the surface.
The only reason people call this "geothermal" is because marketing people realized that this sounds more impressive than "ground source heat pump". It really should not be called "geothermal", because that's something very different. Real geothermal involves extremely deep drilling (not feasible for residential use) or unusual geology.
Geothermal energy: https://en.wikipedia.org/wiki/Geothermal_energy
Geothermal heating > Extraction (GCHE, GHX) || Ground source heat pump (GSHP) https://en.wikipedia.org/wiki/Geothermal_heating
GSHP: Ground source heat pump: https://en.wikipedia.org/wiki/Ground_source_heat_pump
Heat pump: https://en.wikipedia.org/wiki/Heat_pump #Types :
> Air source heat pumps are the most common models, while other types include ground source heat pumps, water source heat pumps and exhaust air heat pumps.
Heat pump > Types:
- SAHP: Solar-assisted heat pump; w/ PV
- acronym for a heat pump with TPV thermophotovoltaic heat to electricity:
- acronym for a heat pump with thermoelectric heat to electricity:
- TAHP: Thermoacoustic heat pump
- ECHP: Electrocaloric heat pump
Electrocaloric effect > Electrocaloric cooling device studies: https://en.wikipedia.org/wiki/Electrocaloric_effect#Electroc...
GCHE, GHX: Ground-coupled heat exchanger: https://en.wikipedia.org/wiki/Ground-coupled_heat_exchanger
Acronyms! From https://www.google.com/search?q=Ground-coupled+heat+exchange... :
HGHE: Horizontal Ground Heat Exchanger: a GCHE installed horizontally e.g. in trenches
VGHE: Vertical Ground Heat Exchanger: GCHE installed vertically e.g. in boreholes or piles.
PGHE: Pile Ground Heat Exchanger: A specific type of GCHE that is integrated into the structural foundation piles of a building.
Solar chimney or Thermal chimney: https://en.wikipedia.org/wiki/Solar_chimney
OTEC: Ocean Thermal Energy Conversion: https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversio... and the ecological salinity gradient:
FWIU archimedes spiral turbines power some irrigation pumps in Holland at least. Is there an advantage to double/helical archimedes spirals in heat pumps if/as there is in agricultural irrigation?
Screw turbine: https://en.wikipedia.org/wiki/Screw_turbine
Noiseless double-helical Achimedes spiral wind turbine on a pivot like a pinwheel: Liam F1 average output with 5m/s wind: 1500 kWh/yr (4.11 kWh/day); Weight: ~100 kg / ~220 lbs; Diameter: 1.5 m / 4.92 ft
What about CO2 and heat pumps? Would a CO2 heat pump make sense?
Absorption Heat pump (AHP) https://en.wikipedia.org/wiki/Absorption_heat_pump
Adsorption Heat pump (AHP)
CO2-Sorption Heat Pump: a Adsorption Heat pump (AHP) that uses CO2 as the adsorbate.
NISH: Nano-Ionic Sorption Heat Pump; with e.g. sustainable hydrogels
Is it better to just recover waste heat from other processes; in a different loop?
LDES heat pump
Supercritical CO2 heat pump
Aerogels don't require supercritical drying anymore,
There's also buoyancy. The pyramid builders may have used buoyancy in a column of heated bubbly water to avoid gravity, in constructing the pyramids as a solar thermohydrodynamic system with water pressure.
Yes, the you can put "thick" insulation over top of any buried plumbing and the exposed bottom will gain geothermal heat from the below and it can prevent freezing.
Its a bit of both, but its primarily due to the high insulation.
There are 2 gradients: The surface gradient is what I mentioned about and its quite steep(only a few meters to drop tens of degrees). After that, you reach approximately the average annual surface temperature, but do continue to get small drops due to the geothermal gradient. The geothermal gradient is relatively shallow - you need to go down a thousand meters to see tens of degrees drop.
Had the same thought, we'd have to put a thermometer inside a 10ft cube full of dirt for science.
> a 10ft cube full of dirt
suspended in air
> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling.
This is literally what ground-loop heatpumps are doing. The ground loop is used as an energy source in winter, and since water is always at 0C, the heat pump efficiency can always be around 500%. And vice versa in summer.
Wouldn't the thermal conductivity/insolution that makes this so appealing be a liability when you want to extract useful heat to use?
Yes, the article addresses this explicitly. The key criterion to make it viabile is steady long-term energy consumption for the winter months. Otherwise the cost to rapidly extract the heat gets too high and wrecks the economics.
The application here is big, slow annual oscillations. Slow charge, slow discharge.
> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later
We already do, in a way: septic tanks
> 4 months of winter buffered by 10 ft of ground!
I'm sorry, but you write this as if that's nothing. Making a 10 foot hole is a massive amount of energy being spent. It's a massive amount of weight as 1 cubic yard of dirt is roughly one ton. In 10 cubic feet, that's roughly 3.5 tons. I say this as someone that moved 6 cubic feet of dirt by myself with a shovel and a wheelbarrow.
So to think of 10 feet of dirt as a slow insulator would have to be one of the worst insulators out there.
Finland has an operational "sand battery", which primary purpose is heating. That was discussed in HN few months ago [1].
When it comes to this article, I doubt the 500x cheaper statement, we would see these already everywhere if that were the case.
[1]: https://news.ycombinator.com/item?id=44295132
the reason that thermal storage is becoming very atractive is that electrical power from PV, wind, and other sources has become increadably cheap, and there is litteraly no where to put it, so prices go negative now, which is a new thing. so 500x cheaper may be an understatement, considering the nature of how cheap, mature and availible the technology to build a thermal storage battery is, any municicipal civil engineering team can build one from off the shelf and localy sourced materials, and the basic battery "housing" could be a re purposed industrial building, cheap does not begin to describe it.
But it's easy to build & easy to read about, so "why isn't it everywhere already" still feels like an open question.
It's a good question. But still...
> Two economists are walking down the street. One of them says “Look, there’s a twenty-dollar bill on the sidewalk!” The other economist says “No there’s not. If there was, someone would have picked it up already.”
If it's a relatively busy sidewalk and you know the object has been there for several hours, that gets to be a pretty good argument.
In this case, the economics of solar has been changing rapidly in recent years, so the caveat won't apply. Worst case, you can bend over and get a closer look.
Yeah, presumably the Software Engineer laughs at the two moronic "experts" and picks up the religious literature or strip club ad masquerading as cash.
Yeah, valid point.
Combination of very cheap periodic power and suitable infrastructure to supply heat energy is more recent phenomena. Supply that is very cheap power and demand that is capability to use that energy later need to match.
500x should be conceptualized as the bare minimum, not evidence of massive low-hanging fruit.
A battery that cycles daily makes revenue on its capacity about 350 times in a year. A seasonal energy store makes revenue on its capacity about once in a year.
A battery arbitrages between the most expensive and least expensive energy generators in the system. A seasonal energy store arbitrages between seasonal price averages.
A battery smoothing out solar production is operating on the difference between how much sun there is in the day, and how much sun there is at night. A seasonal energy store in the same role averages between summer and winter.
A factor 500 cheaper plus a significant quantity of solar energy production is about where you'd expect this kind of thermal storage to start making economic sense.
If we're looking at bare minimum cost then batteries are 10x cheaper than batteries.
And being capable of seasonal storage doesn't stop you from using it for daily storage. It's less efficient than batteries, but you can overcome that.
Let's say you can make a 24 hour power source with $10M in solar panels and $20M in batteries, including the other equipment and costs. $30M total. If we need twice as much solar for thermal storage, but the storage only costs $1M, then that's $21M for an equivalent system.
What stops systems like that from being built right now? I was under the impression that batteries were most of the cost if you want them to last more than a few hours.
Yes! I created a comment here in similar aspect but the thing that was bugging me a lot was what to do about night time though
Imo, the current system of solar energy etc. might be more carbon emissions than even coal because of the way of using batteries and how they are extracted and the whole process and batteries limitations and not solar panels themselves (I think)
Like, please pardon me but I think that there might be batteries that overall are cheap/economical/less carbon emissions and they can store energy for a night cycle right? Then using those batteries in your system and I don't know the price point, but I am definitely sure that they might be orders of magnitude cheaper
So in your 21M$ example lets say that we can add such night time cheap batteries to counter solar panels not generating light at night and use this 1M$ to generate energy when its either rain or solar production is less or when winters come
I still feel like, this might be more economical than the current state of batteries + solar panels.
Okay, I just realized that the night time issue can be solved by the grid but still if that's the case, then why have batteries in the first case if lets say every community builds something like this (if this is economical) and people could just pull up energy from it 100% solar in rainy times etc.
Like maybe my mind is perfectionist or I genuinely don't know about green energy but to me I wish to know if there is a way that we can transition (almost) 100% to the green (solar) without the drawbacks that I saw in the michael's moore documentary in the sense that it produces more carbon emissions net overall theoretically (which hurts my heart :< but the logic was sound imo)
There aren’t grid-scale batteries that can handle hundreds of thousands of cycles at an affordable price. If we crack that problem, solar and wind for everything will immediately be the only technology worth deploying for energy in 99% of areas.
Nuclear energy sort of doesn't require batteries if they supply energy provided directly to grid man.
Solar takes more space than nuclear, the batteries are a huge huge mess. Its still a huge ecological issue that we can't forgive or brush it off man
I meant ‘bare minimum’ in the sense of what's needed for the system to be profitable, not in the sense of ignoring costs.
The problem with using this approach for daily cycled loads is that it relies on passive heat transfer to distribute heat through substantial regions of dirt. This simply doesn't work for daily storage.
You can overbuild, but then your energy losses are going to be immense, because you never saturate or drain the bulk of the material, and are just losing energy to it.
You could build faster cycling systems instead, and active systems especially can cycle reasonably fast, but then your dominant costs no longer reduce down to a pile of dirt with a few rods stuck into it.
You would need more rods if you're cycling more of it at once... but you can add 25x as many rods and it'll still be 20x cheaper than a battery, right?
I don't think losses would be immense. You'll spend a few months warming up the neighboring dirt, but after that the amount of heat escaping per day will look about the same as the seasonal system.
I'd assume the design as proposed isn't overbuilt, so we'd need to go from, say, 4 months cycle times to 8 hour cycle times, which is maybe 350x.
While that's not going to increase the cost by 350x directly, it is going to change the character of the pile from a bunch of dirt to a bunch of dirty pipes. This makes a lot of the simplifying assumptions no longer work; like you can no longer ignore the heat losses through the rods, or the lower thermal mass of the rods.
And to be clear, you can do this. There are faster-cycling thermal storage solutions out there. It's just not implied from the claim that these solutions would be so much better than batteries.
The heat right next to the rods is easier to access. With less dirt per rod, you can also get more watts per rod, so the number of rods doesn't need to scale directly with the cycle time.
Yes, fair.
One thing they neglect to mention (which is by no means a deal-breaker) is that you waste a good portion (about half) of the electricity in the process of charging and discharging the pile of dirt. Chemical batteries are much more efficient in this regard.
Solar prices are coming down quite fast, I don't think a factor of two is going to be a killer here if the storage is cheap and long-lasting enough. Some people are already considering over-provisioning solar panels relative to available transformers/grid connections so that they can maintain output on cloudier days. "What do we do with all the extra power when the belly of the Duck Curve [1] hits the ground" is a problem lots of people are thinking about.
[1]https://en.wikipedia.org/wiki/Duck_curve
The cost-prohibitive portion of this, which is greatly glossed over in the article and which I showed in my other reply - is that the steam generator required to recover this heat as electricity, is a massive part of the capex - more than half of the entire system end-to-end, including the solar and dirt storage. That makes the economy of it far less viable even with nearly free solar, which we're still quite far from.
This isn't true for high latitude areas, where the overprovisioning that would be needed for solar to work in winter would be massive. This is one of the markets he discusses.
This scheme, if it works, makes PV work great in Alaska.
Yeah, but then people should start to actual incorporate the full cost of these kind of things in the total cost of solar power when comparing it to other sources.
I think generating hydrogen for fuel cells seems prima facie a reasonable approach?
That has similar efficiency compared to this but probably looks much worse in terms of capex
Or hydrogen for combined cycle power plants, which have a LHV efficiency in excess of 60%.
This scheme is probably superior though, with lower capex and working at smaller scale, especially if one doesn't have deep salt formations to solution mine for hydrogen storage caverns.
They mention it:
> There is an efficiency penalty converting back to electricity; round-trip efficiency is 40%-45%, but sometimes the steady supply of electricity is worth it.
One thing they also mention is how incredibly cheap storage of natural gas is. https://en.wikipedia.org/wiki/Power-to-gas#Efficiency The efficiency of power to gas is not great, but it's about the same as this thermal storage method, with probably much longer lifetimes,easier transportation and more general utility (the natural gas could for example be converted to methanol using the holy grail catalyst that was in the news recently).
The power to gas is also carbon neutral, even negative depending on what you decide to do with the natural gas (if you don't burn it for power but use it for industrial chemistry, you get some sequestration out of it).
The problem is the cost of getting the carbon and hydrogen to make the natural gas. If you're making a synthetic fuel, you might as well make methanol, which can be stored as a liquid, and is about as easy to make from syngas as methane.
Yeah, did mention the methanol option as an additional step once the natural gas is created. But is the cost of getting the carbon and hydrogen that high? In a P2G system you'd be electrolysing water (plentiful) and combining it with carbon dioxide (also relatively plentiful, although it can also be linked to an existing fuel burning plant for better efficiency). There are existing production systems using this approach right now with surplus energy from renewables described in the wikipedia article.
Getting it from existing fuel burning plants defeats the whole purpose.
Direct air capture is out, so it'll have to be recovered from the combustion of the synfuel. Using the Allam cycle has been explored to do this (you also have to store the oxygen from electrolysis for later use in this oxyfuel combustion cycle) but it ends up being more expensive than just burning hydrogen, if there's reasonable geology for hydrogen storage.
So, if this thermal storage scheme is cheaper than hydrogen, as it appears it will be, then these alternative synfuel schemes are ruled out.
More efficient, but much more expensive. I'm sick of people handwaving $100 per kWh. That is two orders of magnitude off where it needs to be to do anything more than virtue signal.
Meanwhile multiple grids are now paying renewable to curtail, because guess what, the variability is correlated (it's the exact same damn mathematics we used to fuck up the entire global economy in 2008, which is why I'm so surprised people are handwaving that too, but whatever). If you want to minimise cost without relying on gas to save you on dark still days, you want a cheap use for the surplus, round-trip be damned.
100$/kwh on a battery that does 1000 cycles is 10c/kwh, 5000 cycles ("Claimed" lifepo4 these days), that's 2c per kwh. These aren't that unreasonable, albeit one would need to account for cost of capital and so on increasing these effective numbers.
Batteries are already economical in most grids where they can arbitrage daily prices of 0-10c during the day to 10-30c during the night, with the occasional outlier event contributing dollars per kwh.
They will never load-shift across seasons, agreed, but for daily loadshifting they are already economical, and being 90%+ efficient (and very simple/easy to deploy and scale) is part of why they're popular. It opens up power shifting opportunities that aren't just daytime solar too.
you're undercounting cycles for batteries. batteries are quoted for until 80% capacity is left which makes sense for mobile applications, but for grid storage, a battery that's 80% degraded is still useful. as such, you probably get 15-20k cycles before it's worth recycling
Kilowatt hour of capacity and kilowatt hour delivered are two very different numbers. Sources rarely distinguish, and you're almost certainly confusing them if you think batteries have to get down to $1.
Correlated errors are a problem in all sorts of places. Most statistics assume everything is independent; super important to verify that before drawing conclusions.
I didn't have time to read the whole thing so I don't know if they mention it, but another article about about using sand as heat storage pointed out one of the advantages is that the material isn't toxic, unlike chemical batteries.
There were reports last year, IIRC, of "sand shortages". Presumably a logistics infrastructure problem that could be relatively easily overcome?
That's of "sharp sand".
Sand batteries don't need sharp sand.
I remember those reports.
I wonder if it has to be the same kind of sand, or could be some that we neither have another use for, nor would damage any ecosystem (too much).
They do mention it, but it’s downplayed relative to how much of a problem it can be.
In a situation where you have a lot of energy generation that would go to waste, storing it in a system with low round trip efficiency could be better than losing it.
For planned installations where the generation cost is nontrivial (like a solar install) then increasing the generation to compensate for poor battery efficiency isn’t as easy of a decision.
Yeah, when baseline efficiency is zero then there's probably room for improvement.
But chemical batteries cost a lot more and don't have lifespans of hundreds or thousands of years in seasonal storage scenarios.
And when electricity is in essence too cheap like with solar and wind it can be, losing half in efficiency actually doesn't matter too much.
> But chemical batteries cost a lot more and don't have lifespans of hundreds or thousands of years in seasonal storage scenarios.
Practically speaking, you're probably not going to get 1000s of years out of any storage method. There's just too much stuff that breaks down.
Heck - a lot of historic dams are in the low hundreds of years old and are experiencing serious problems.
IMO, the shorter lifespan of batteries isn't that big of a downside as long as the "bad" batteries can be mined for raw materials eventually.
We're talking about the storage layer, dirt/sand won't break down.
However, for long term storage, the "cost of inefficiency" is swamped by the amortized capex of the batteries. The goal is to minimize capex, not maximize efficiency. This becomes even more true as the cost of the input energy declines.
Another thing they don't seem to mention is environmental impact (if there even is any worth mentioning, not sure).
I visited a pumped storage facility a while back that stored electricity by pumping water uphill to store it and then draining it past a turbine to reclaim it. Ever since I’ve been intrigued by using gravity instead of batteries.
For home use, it seems like you could rig up some heavy stones on pulleys to do the same thing could be fun because you’d get to physically see your batteries filling up. Back of the envelope calculations suggest that an array of ten 10-ton concrete blocks lifted 10m in the air could power a house for a day (ignoring generator inefficiencies)
An AA battery contains approximately the same as 1 ton raised 1m. (About 3Wh)
A Tesla Powerwall contains about 13.5kwH (about 4,000 times as much)
So you can either raise 100 tons 10m above your house, or you can have 1/13 of a Tesla Powerwall.
There is a company that claims they can store energy by lifting and lowering heavy blocks with a crane
https://www.energyvault.com/products/g-vault-gravity-energy-...
I like the picture, but the the size of the construction is enormous, especially if you're considering a tank for some kind of pumped hydro. Hydroelectric power is practical because a dam in a strategic location can back up much more than 1000x of its volume in water. If you had to build all those walls forget about it.
That is obviously a scam. Not a chance in hell.
I am giving that one a 0% chance of long term success.
Edit: no seriously. Do some back of the napkin maths. The amount of energy stored is too small. Way too small. And then the infrastructure to haul hige blocks of concrete around.
It's the same scam selling ideas that sound good to people who don't understand what a "joule" as Pavegen and the other systems that generate energy from footfall or passing cars. Mechanical energy is pretty "low grade" as energy goes.
It 100% works, but it's a system that has very specific applications and doesn't scale up well. And the best systems use a magical property of some fairly heavy materials called "being liquid" to simplify the logistics of getting millions of tonnes of weight to the lifting mechanism.
One of them is moving almost seven thousand 25-tonne concrete blocks around. I can't see how it would ever compete with something like compressed air.
It just seems so awfully wasteful.
If someone can find some real costs I would be more than happy.
7000 * 25t / 2.5t/m^3 = 70,000m^3 of water. So about 1% of the water storage of Dinorwig, which runs dry in about 6 hours of use.
Now, I know reservoirs are ecologically pretty iffy, expensive and obviously geographically sensitive, so you can't slap them around everywhere. But all these mechanical schemes have big "look what they need to mimic a fraction of our power" vibes!
I could imagine that steel-on-steel block movements could actually be quite efficient and effective in limited scenarios, but logistically it just seems like a lot of squeeze for not a lot of juice considering how much power is required to be stored for utility-scale projects. I would like to say that that they're just delusional people truly hoping it'll work, but I think there's a core of hard-nosed scammers who smell money for a shiny PPT and a plausible-to-non-engineers Wile E. Coyote/Troll Physics contraption with big numbers in the brochure: 7000 blocks! 25 tonnes per block! Megajoules! Efficiencies! Scale! Repurposed coal mining infrastructure! They even have AI in the spiel now: https://www.energyvault.com/solutions/software
Or maybe it'll work and I'll look stupid in 30 years when there are huge fields of hundreds of kilometre-deep boreholes with 100 kilotonne masses moving up and down in them. But somehow it seems quite unlikely on a practical level considering the cost of boring gigantic holes that you'd have to do to make it scale. Onsies-twosie installations in a few mines here and there may work for lucky outlying towns, but they aren't civilisational scale solutions.
Oh, but only at about 145 meters. Dinorvig has a fall of over 300m.
And this is why gravity is considered a weak force.
Sit down. Now stand up. Congratulations, you just beat the gravity force generated by a whole f*cking planet.
Now try to escape it.
What limits me is the lack of solid matter to push against, not lack of strength in my muscles.
Gpt says that would require about 275 million steps on a magic rigid weightless stairway. Roughly 4.1 million calories. So at Phelps level energy expenditures you are still talking over a year of climbing every day.
GPT not so good at elementary physics evidently.
Assuming you don't have to carry your food... 4.1 million calories would be around 0.5 tons of olive oil.
Well, 100kg raised 10000m is only about 2350 food calories, from a purely physics perspective.
You'd need to go to about 47km for the end of the stairs to reach escape velocity I think? Past geo. Was using 20% efficiency. Still something off there.
ok, but what do you say?
You are not hiking much in the mountains, are you? 1000m of elevation gain per day are no problem for a slightly out of shape sit-all-day programmer. Not sure how high up you want me to go, but given a high enough mountain (and a thick jacket and supplemental oxygen) and most people here can do that in a few weeks or months.
That doesn't really escape the gravity well, does it?
It would if there was a mountain that kept going up, and you had oxygen to breathe.
Now if it were a magnet..
And the strong force holding two protons together in an atom is on the order of 10 pounds.
I wondered when anybody would bring nuclear fission into this discussion.
Every time I see again the idea of moving big concrete blocks for storing energy, I remember the time I made the calculations, and estimated around USD100K of infrastructure to store the same amount of energy as a nissan leaf.
I did the same calculation. Gravity energy storage is a joke. Came to similar conclusions when running numbers on hydrogen-powered vehicles.
Pumped hydro storage and flywheels are cool but ultimately battery storage, distributed everywhere, will win.
Gravity storage is cool when nature has already made most of the work, I/E pumped hydro where nature has already built this huge canyon with a river in the middle just waiting for someone to put a dam at the end.
It is really perplexing when people come up with an energy storage method that is even more expensive than just straight up batteries.
Hoisting 100 tons of stuff high into the air, and then efficiently converting that into the high RPM needed to drive a generator seems like it would take a truly staggering construction effort. Suspending that amount of weight high above your house also has some... interesting potential failure modes.
Why would you put it above your house? Just construct a sort of battery tower nearby.
Why not just build a water tower? Easier to manage, it's a proven technology and it has well-known ancillary uses beyond energy storage.
I don't even think the gravity battery thing is viable for individual residential power storage at all. I was just wondering why you'd assume that the 100 ton weight would be placed directly above your house given the obvious problems with that approach, and the obvious way to avoid those problems.
The comment I was replying to literally said "For home use", and a heavy object 10m in the air does not have to be directly above something to be meaningfully (and dangerously) above something.
It's a silly scenario anyway, but I was doing a bit of guesswork about typical "home" lot sizes.
Yeah I understand it's for home use. I am imagining a tower in the back yard or something. It would be closed so that nobody can walk under the weight. Or it could be internal to the house like an elevator shaft.
Anyway I agree it's silly, definitely not a realistic idea
Right - if a tower in the back yard falls down it can still hit your house, since it isn't guaranteed to neatly collapse straight down. Worst case, it may tip over from the base and directly smash stuff up to its height away (and 10m is pretty far).
I have trees in my back yard I'm kind of worried about, which is why this immediately came to mind.
Just suspend your house, become the battery.
Gravity based with weights is generally considered not cost effective; others already did the math that your 100 ton proposal can still only store a fraction of what a consumer grade battery pack can do, but on top of inefficiency there's space and maintenance requirements. It works in situations where e.g. trains go uphill empty and full downhill, but generally it doesn't work.
Water based systems work better because water is easy to move, plentiful, and there's natural basins to pump into / flow out of that can contain billions of liters.
You need a lot of weight. IMO for home use the risks heavily outweigh the benefits for anything outside of a hobbyist project... that's just a lot of potential energy in a system that can go wrong. Weights falling quickly, pullies and cables under tension.
The same is true for batteries of course, but at the very least there are protections and checks for failures in most consumer accessible home solutions (and decades of engineering at this point). Worst case you at least have smoke detectors... not sure if there's a "cable is wearing thin and might snap and decapitate you" warning system.
Pumping water up is a super old idea, but as far as I know you'll need some natural terrain to build it efficiently.
And environmentally, that tends to be pretty bad for a long time. Looks more peaceful than a fracking site, but it's still pretty bad.
There is no magic solution. I'm happy to see all those efforts, but am missing a mention of saving energy. In the age of record-setting data centers for AI training, that's not a popular aspect to mention. Though at least we get higher res more realistic artificial cat videos out of it.
It's only bad if it's placed on and disrupts an existing river.
But PHES can be placed far from any river, even in a desert.
https://www.whitepinepumpedstorage.com/
I suspect this project may not happen, what with batteries getting so cheap.
You just need some change in elevation over moderately short distances. You don't even need rivers.
Someone has already been thinking along the same lines: https://www.swissinfo.ch/eng/business/energy-vault_revolutio...
This type of plant is generally used for emergency power to balance the grid, whilst other plant come online.
Relevant video on a pumping water on the roof + turbine system : https://youtu.be/CMR9z9Xr8GM It's quite far from powering your whole house tho !
EnergyVault are building these GESS (Gravity Energy Storage System) arrays right now.
https://www.energyvault.com/projects/cn-rudong
Some guy on Youtube tried this. I think the conclusion was, not worth it.
https://www.youtube.com/watch?v=l19AMYd0oks
Stones are typically not very dense. Iron or lead or 2-4x denser than typical stones, and so will net you better energy density.
Go all the way: depleted uranium
> an array of ten 10-ton concrete blocks lifted 10m in the air could power a house for a day
No, that's only 2.7 kWh. Most homes use 10-20 kWh/day. A battery of that size is easily under $1k. Good luck building your ridiculous concrete block system for that.
Batteries are really good. Gravity, not so much. It only works when you can lift & store a tremendous amount of stuff "for free" because nature has done most of the work, e.g. in valleys, mountains, aquifers, caves, etc. If you have to build the whole thing it will never be viable.
Batteries are good, but you can’t repair a broken battery with odds and ends you find at the hardware store. If you’re living off grid space isn’t really an issue.
I am not sure you want to repair something holding one ton blocks in the air with odds and ends.
I’d love to buy a 20 kWh battery for under $1k. Can you give me a link to what you’re thinking of?
Sorry my wording was ambiguous; when I said "that size" I meant 2.7 kWh, which is what the hypothetical concrete blocks might provide.
Nevertheless, you can get a 16 kWh battery (which is enough for most days of a typical house) for only £2k, which is kind of insane really: https://www.fogstar.co.uk/products/fogstar-energy-16kwh-48v-...
People have been able to get their Tesla Model 3 batteries (75kWh iirc) replaced with used battery packs for around $5k, so quite close to what you’re asking.
Do they get much for the old used battery? the materials are still lithium etc are still inside it, they're just no longer in the right places..
where can I get a 20kWh battery for 1k USD?
here in Switzerland 1kWh is 1k CHF.
Gravity is weakest force of nature. Any strong force battery idea ?
You already have some decent amount of weight in the house itself, so I'd look into raising it all or parts of it
Of course it's probably not the simplest engineering effort...
There are some videos on Youtube discussing the hypotheticals of this. They're never really very positive about the feasibility. Neither on a small scale nor on a country-wide scale.
If you'd want to store 1kWh at 10m height, assuming no loss at all from heat, friction, etc, you'd need about 4 of those blocks block weighing 10 tons (according to ChatGPT). So you'd need a lot of those blocks to power a house for a day, unless you're very efficient.
Please don't cite ChatGPT as a source or as a caveat, instead show the actual math, which should only be about high school level; kinetic energy formula ½mv² = e,
In perfect conditions assuming no loss through drag, you're looking at the kinetic energy formula which is ½mv² = E (in joules).
E = 1 kWh = 3,600 kilojoules, velocity v at 10 meters is 14 m/s, so we need to calculate m for v = 14 and E = 3600k, which is just under 36735 kg. "about four of those blocks" is "about" correct.
Simpler to use the potential energy formula, surely.
E = mgh
m = E/gh
m = 3.6 * 10^6 J / (9.8 m/s^2 * 10m) = 3.6735 * 10^4 kg
I'm surprised that they use the PV power for heating coils instead of using heat pumps. I'm sure they've run the numbers and considered it too expensive or too much maintenance hassle.
However, if that's the case you would think that you can cut out the PV step as well and use direct heat from the sun to heat the dirt, by running water hoses though the dirt and through solar water heaters. Should be cheaper and more efficient than the sun -> PV -> heat coils cycle.
Read the article. He is talking about 600C heat. There aren't any heat pumps that
a) give you significant gain when you already have ΔT that large (because 900K / 300K is a big ratio)
b) have a working fluid that avoids decomposition at 900K
Ah that explains it, thanks, I missed that.
Pumping heat from 300K to 900K is not a big gain over heating -- the entire thing is premised on using extremely cheap intermittent electricity during the summer, and your savings are capped at 30%.
I just read Project Hail Mary which suggests a use of solar thermal energy and I thought "ha too bad solar PV is so cheap in the real world this would almost certainly never happen" and yet, solar thermal energy plus heat pumps does seem extremely simple and cheap for this company
Does this become much trickier because they're trying to heat to 600c?
Solar water heaters are expensive, complicated, fragile, and generally immobile.
Solar panels and heating elements are cheap, simple and easily replaceable.
A solar water heater is just a black box with a tube running through it. Many houses here have one, because they're much cheaper than PV panels to generate similar kW heat. But a sibling comment pointed out they want to go up to 900 °K, you can't do that with those, of course.
I think in most places the economics have shifted to PV now.
Despite the simplicity of a solar water heater the even greater simplicity is often part of the argument for PV being better, but the higher grade of energy that you only optionally have to use for heat and relative price changes are the key driver
In addition to being at 600 C, this scheme also crucially revolves around keeping capex as low as possible. Resistive heaters have lower capex than heat pumps. In a system with low numbers of charge/discharge cycles the efficiency drops in importance compared to capex.
Seems like a great concept. Hope they are commercially successful.
It reminded me about another geothermal energy idea: dig about 3 or so miles straight down and harvest the heat that is there already. I guess that's a lot harder than making a dirt pile. But maybe it could become practical if there was enough commercial effort and large scale manufacturing of the equipment.
Kind of brings it around full bore though. Why do that kind of project when you can just harvest actual fuel like oil or gas?
I think this stuff can become practical with more scale and wide manufacturing of equipment and development of efficient techniques. But it requires you to do a lot of upfront work based on principal rather than the bottom line.
So anyway again great idea because it eliminates a lot of challenges and costs that come with concepts like "Journey to the Center of the Earth" etc.
> another geothermal energy idea: dig about 3 or so miles straight down and harvest the heat that is there already
Deep geothermal ought to work. Deep drilling is hard, but it's been done. Eavor-Deep got down to where they got 250C water. [1] That was back in 2023. Not much new since. The problem seems to be that when you drill into really hot rock, most drilling techniques run into trouble. Rock becomes plastic and clogs things up. The drilling tools have problems with the heat. Progress continues, slowly.
There's these guys, trying to drill with microwaves: [1] On September 4, they're going to do a public demo and try to drill a 100 meter hole.
[1] https://eavor.com/eavor-deep/
[2] https://www.quaise.com/
The other issue is that the steam loses energy as it's pumped over a long distance. Iceland and a few other locations are big users since they are on fault zones with hot rocks relatively close to the surface. Even if deep drilling is demonstrated, it's unclear if geothermal power will become geographically independent.
> Why do that kind of project when you can just harvest actual fuel like oil or gas?
How can that still be a question in this day and age? Unless somebody doesn't "believe" in climate change caused by greenhouse gas emissions.
Forget climate change, the best reason is national security. Russia's war plummeted Europe into gas shortages and price hikes. Many countries would love to not be dependent on them. And since WW2, all war requires vast quantities of oil; maybe drones will reduce that a bit, but you still need to move stuff (and people) around a lot. So you need a reliable source of energy both in peacetime and wartime.
For the US, the best reason is sustainable energy. Gas, oil and coal are not renewable, so you eventually need to adapt a new form of energy. Just transporting it is problematic, with most communities rejecting pipelines. In the meantime you're polluting your local environment and putting workers at risk. Whereas if your energy plan is largely "the sun shines", "the wind blows", and "dirt holds heat", that is ridiculously more sustainable.
The biggest problem we have is we demand too much energy. AI has made this problem way worse. Nuclear is the only thing that's going to fill the gaping chasm of demand.
> Forget climate change,
And that's why we are in trouble.
> the best reason is national security.
Sure, if that's what convinces people to do the right thing, so be it. Though the continuation of that thinking tends to cause behavior that's not very friendly w.r.t. climate or environment.
I think Austin Vernon has spent some time investigating geothermal which is likely why they've arrived at stored energy in dirt
Geothermal already does the “harvest energy within the earth” but it’s closer to the surface. What are the challenges with digging 3 miles down?
If you want heat, why bother converting sunlight to electricity first? You lose 80%. Is it that much more expensive to use mirrors to concentrate sunlight and capture near-100% of the energy as heat?
Because with a heliostat you have to move the energy as heat and that isn't as efficient and definitely not as easy or cheap as moving it as electricity.
Do the numbers. Making the top of the heliostat isn't what matters. What matters is making the inside of a pile of dirt that hot.
They are pitching co-located PV, and they already have a system to move the heat inside the pile of dirt!
Mirrors require heliostats.
PV panels are dirt cheap and dead simple. They're also portable. Wire a bunch of panels to a heating element and dunk it in sand/water/etc.
And PV panels work with diffused sunlight; high concentration solar only works with direct sunlight.
If you 'bother' with converting it to electricity first you can sell the electricity for profit and only convert the excess to heat.
I don't understand how, but apparently solar photovoltaic panels are indeed cheaper than thermal solar collectors now.
I do like how well and concisely they've explained not only the technology, but the exact use case, on their landing page.
At what scale does this become efficient? I may have 1000 sqft to dedicate to this type of system on my lot. Feels like that’s at least an order of magnitude too small to maintain the energy through the seasons. Could we build one of these slightly larger systems for every square mile (~1000 homes), or does this only work at a 10,000 home scale? The article is showing a pile of dirt on the ground. Could this just be an area of the subsurface which is heated, or does ground water become too much of a problem?
There are some companies making heat batteries that you can install inside your house. They won't last very long but it's a great way to store excess energy from your rooftop solar and use it for heating and warm water. Also it allows you to timeshift when you draw power from the grid (i.e. when it's cheap rather than when you need hot water).
Larger scale batteries can store enough energy for seasonal storage. The larger the size, the better the insulation can keep the heat losses to a minimum. Basically you have a smaller surface area relative to the volume and mass. But even with a small unit, you can keep it hot for quite long.
Stuff like this is easier in areas that are already on some sort of district heating or have some kind of water based central heating. For those systems it's pretty much plug and play. You don't really need to modify the houses.
I think Helsinki has a few larger scale units already operational and a few more under planning / construction. I think the largest one will store 90ghw of heat. Which is quite a lot.
The beauty with thermal storage is that almost any kind of mass with enough heat capacity works. Water, rocks, sand, etc. All fine.
You can scale this down to be reasonable for homes, yes, but they're specifically trying to make it usable for large industrial processes: https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag...
Seems like a case where directly going from sunlight to heat would be a better approach for this, instead of converting to electricity first.
How would you move the solar energy into the piles of dirt? You’d need something like an array of mirrors focusing the rays, which has definitely been done already but has drawbacks. Electricity can easily be moved to where it’s needed.
You could heat up a metal heat exchanger that you circulate a working fluid through. Probably easier to just convert sunlight to electricity to heat via resistive heating, less maintenance.
It's been done, but not without controversy. https://en.wikipedia.org/wiki/Ivanpah_Solar_Power_Facility
At home, it's suitable in warm climates but is more challenging in snowy / very cold regions. Generally speaking, converting to electricity then using an electric water heater is more efficient because there's much less insulating, heat loss, and piping that can leak and cause water damage.
If you look at "evacuated solar" panels this is as close as you could get, and they don't get anywhere near the temperature you need to generate consistent steam. Concentrated solar is closer, but cost impracticalities make it unattractive here as well.
This is a blog article outlining a rough concept idea. As others have commented, many questions remain unanswered, and speculation about isolated physical properties and technical ideas is unhelpful.
For it to be worth spending more time and effort on, I would need a closed system thermodynamic calculation. The technical term for this is a "heat balance diagram". This is the first thing any technical consultant would request.
Dupe: Building Ultra Cheap Energy Storage for Solar PV https://news.ycombinator.com/item?id=44998441
Oh hey, this is the Energy Superabundance guy.
Overall it's a good idea but I'm skeptical that random "dirt" will reliably withstand the temperatures required for
> the stored heat creates steam on demand for the turbines instead of burning coal
but mostly-quartz sand without any calcite and crushed granite surely can, and those are widely available for barely more than the cost of driving a truck to your construction site, because those are the main ingredients in concrete. (As he says in his white paper https://findingspress.org/article/141340-thermal-energy-stor..., US$20–50 per tonne.)
From perusing https://www.eia.gov/analysis/studies/powerplants/capitalcost... I'm also skeptical that the turbines (especially steam turbines rather than gas turbines) will have a low enough cost to make this worthwhile.
If your random dirt is good enough I'd think you'd want to trench into the existing dirt to lay pipe, the way most ground-source heat pumps are done, rather than hiring a bulldozer. But I haven't done it; maybe the cost structure is different than what I imagine.
I wonder why he says
> Surrounding the earthen mound will be high-density, low-profile solar arrays.
What's stopping you from putting the solar arrays on top of the earthen mound (or sandpile) too?
Sand or feolite thermal "batteries" on the daily rather than seasonal timescale are eminently economical for even household thermal storage for heating. With TCES energy storage, which I expect to be barely more expensive at the household scale, you can get cooling too. TCES also scales down to small systems and long storage times in a way that sensible and phase-change thermal energy storage can't, while also bettering them on density.
Incidentally, energy superabundance is apparently becoming a reality in the PRC, and CO₂ emissions are falling:
https://fortune.com/2025/08/14/data-centers-china-grid-us-in... “#AI experts [#Rui-Ma] return from #China stunned: The U.S. grid is so weak, the race may already be over. (...) In the U.S., surging AI demand is colliding with a fragile power grid, the kind of extreme bottleneck that Goldman Sachs warns could severely choke the industry’s growth. ¶ In China, Ma continued, it’s considered a “solved problem.”” because “China has an oversupply of electricty [sic]”. “In China, renewables are framed as a cornerstone of the economy because they make sense economically and strategically, not because they carry moral weight. Coal use isn’t cast as a sign of villainy, as it would be among some circles in the U.S. It’s simply seen as outdated.” #energy #USA
https://xcancel.com/ruima/status/1955040979259650267 #Rui-Ma’s account of her trip to #China to go to the World #AI Conference (#WAIC) and see investors and renewable #energy. "We met with companies ranging from Baidu, Alibaba, and Tencent (BAT) to unicorns, as well as young startups founded less than a year ago. (...) Energy is considered a solved problem. The Chinese government’s investment in sustainable energy — from advanced hydropower to next-generation nuclear — means that, relative to many other markets, electricity supply is secure and inexpensive. Everywhere we went, people treated energy availability as a given. This is a stark contrast to the U.S., where AI growth is increasingly tied to debates over data center power consumption and grid limitations."
https://xcancel.com/ruima/status/1955372325970514161 #Rui-Ma on #energy in #China: “China: Electricity generation jumped from roughly ~5.6 PWh (2015) to ~10.1 PWh (2024), which is +80% in less than a decade. And 2024 alone grew ~7%, accounting for ~HALF of the world’s increase. China now produces ~1/3 of global electricity. That’s what “supply went up a lot” looks like, so yeah even if coal as a % of total went down, the total will still increase but the increase is QUICKLY PLATEAUING (...) Multiple independent analyses now show China’s CO₂ fell year‑on‑year across the last 12 months (and in Q1’25), driven by the clean‑power surge rather than an economic slump.”
Another minor note as I read the rest of the page:
> the resistor material itself is $1-$2/kilowatt. (...) There are only a few materials that are even acceptable as resistors.
This depends strongly on your target temperature and conditions. Nichrome may cost US$2/kW but galvanized barbed wire doesn't. It can kill you with the fumes, and in oxidizing conditions it won't last long above 200°, but zinc fumes inside a dirt pile won't hurt anybody, and you can probably maintain reducing conditions by mixing some humus into your dirt initially and keeping the pile dry. If process heat or climate control is your objective, you don't need temperatures high enough to oxidize iron rapidly even in an oxidizing environment.
(Incidentally, reducing conditions will destroy nichrome at the temperatures people use nichrome for. Maintaining oxidizing conditions inside your dirt pile over the years is going to be a lot more challenging, I suspect.)
I suspect keeping the pile dry is the reason for not just trenching in existing soil. Dryness is essential for maintaining temperature over 100°.
My intuition is that you could probably scale the seasonal-sensible-TES-in-dirt approach further down from the few megawatts he's talking about if you can use real insulation instead of dirt. But insulation is expensive at this scale, even things like perlite and vermiculite.
Spitballing, suppose we want to withdraw 100kW energy for 4 months (1TJ) and can deal with a time constant of 8 months, so leakage is ballpark 50kW. Suppose we have 1J/g/K sand and ΔT = 200K. Then we need 5300 tonnes of sand; at US$20/kg that's US$100k, a dollar a watt, cheaper than power from a coal plant but not really competitive. A larger ΔT can save you but might shorten the life of your barbed wire. But at those low temperatures you pretty much can use any old dirt, US$5300 of it by Vernon's embankment figure.
But that's about 2000m³, whose minimum insulated surface area would be as a 7.8m sphere with 800m² of surface area. 50kW of leakage over that area would be 70W/m², or 0.3W/m²/K, or 3K·m²/W.
There's not enough comparison with the conventional ground-source heat pump here. There's not enough modeling of the expected system dynamics. I don't have a physics argument against it (right now at least) but I think that the author is trying way too hard to sell me on the idea of energy storage and not hard enough on why this proposal can work. And I don't think it's just me. Anyone reading the pitch for an energy storage startup in 2025 is probably aware of the basic goals, and more importantly is fatigued and suspicious after watching several dozen clever ideas go nowhere.
Surely you can write a short model of the system at the level of undergraduate thermo. If you have a pile of dirt this big (say about a thousand times the size of a spherical cow) with these pipes running through it, then at a storage temperature T your capacity is X, your leakage is Y, and your recovery rate is Z. Fill in the blanks.
GSHP/ASHP can't get to steam-generating temperatures on the hot side unfortunately - you're looking at a "heat concentrator" which doesn't really exist in the form of a commercially available heat pump today - possibly not even in the theoretical.
Heat pumps are a nonstarter here, because the goal here is minimizing capex, not maximizing efficiency.
Sorry, that comparison to heat pumps wasn't meant to imply that heat pumps would be used in this system. Rather, the focus was on the pipes that transfer heat from the system to the ground and back. That component remains basically the same — the heat pump components of a GSHP are above ground. My thought was that experience installing GSHP would allow you to bracket out the cost of the interred heat exchanger and make a comparison to the title proposal.
My lingering question is how do you service a leaky pipe surrounded by dirt that’s 600C? Do you just have to forfeit a good chunk of your energy savings for the year?
Or is there like a practical maintenance window each year at the end of the winter when you’d do this?
That's not a lingering question. It's the question.
This is a steam boiler. Those are well understood. They have well understood problems. Leaky boiler tubes. Crud in the tubes. Cleaning. The problem here is that you can't easily turn the heat source off.
It's possible to build a long-life boiler for a heat source you can't fully turn off. Every nuclear reactor has one. Heavy stainless steel tubes, precision welding, distilled water. Works fine, but not cheap.
This paper is very hand-wavey about the details of getting the energy out. They're all about the side that puts the energy in, which is the easy part.
Then we talk round-trip efficiency.
I always wondered if, instead of using solar panels in the deserts, we could use very long and black pipes running water, heated by the sun. Then the heat is moved to the ground for storage, and once there is enough heat, we use a turbine to generate electricity.
I don't think you ever get enough of a temperature difference just by having a passive black pipe in the sun, to do any useful work besides potentially keeping someone warm in the winter. You could do useful work if you concentrate the energy from the sun somehow, like with mirrors.
Heat pumps do magic by changing the pressure at which a working fluid changes phase, so you can boil the fluid over here, have it absorb an enormous amount of energy then compress it back to a fluid elsewhere and push that heat back out -- this works pretty well because you're just moving the heat and only pushing the temperature on the "hot" side up a relatively small amount. I don't think, for instance, you could make an oven with heat pumps.
To do useful work you need a _substantial_ energy gradient -- it's hard to live in the sun even though its got lots of free energy floating around. The sun is very useful to the earth because the energy it provides is so much more energetic than the ambient environment.
Edited to add:
There are discussions of using exotic working fluids like compressed CO2 -- that'd allow you to manage the phase change maybe to a region where you could concentrate the energy in the fluid then expand it elsewhere at "room temperature" temperatures -- but I think things like compressed (to a _fluid_) CO2 are really hard to work with.
It's done a bunch, all over the place: https://en.wikipedia.org/wiki/Solar_thermal_energy. Most commonly people use them either for batch pre-heating of hot water or for full heating. I looked in to getting something like that for the hydronic heat in my house because currently it's propane-fueled and the deliveries are quite expensive in the winter.
>the heat is moved to the ground for storage
..or into a giant sand-filled barge which transports the thermal energy up north for the winter.
I have more questions than answers.
Does the article describe how the heat gets from the mound to the houses or buildings it plans to heat, or factor in the cost of that?
Naively, I'd assume that would like 90% of the cost.
I know that physics is under no obligation to be intuitive, but it's also surprising to me that it's so easy to heat and keep dirt this temperature (600C / 1100F) throughout Winter, and I didn't see how that piece worked either, though I'm willing to assume that part is figured out and factored in.
Apparently the insulation value (R value) of dirt and soil is between .25 and .8 per inch (depending on moisture content). That wouldn’t be great if it was a material like fiberglass, but since it’s dirt cheap (ba-dum-tssh) and easy to pile up in large quantities with little to no ongoing maintenance in this kind of context, it matters.
A 10 ft pile of dirt (assuming 10 ft between heat exchanging pipes and the outside air) has an R value of 24 to 96, which is extremely significant.
I expect there would still be notable losses trying to keep it at 1100F indefinitely, but 10 ft of dirt will have insulation values approximating many feet of fiberglass insulation.
You’d want a very large mass to heat however, scaling matters a lot. You’d want the ratio of surface area to mass to be as small as possible, and that means as large a volume with as thermally dense a material as possible inside. Surface areas increases by the square, while volume increases by the cube.
Also, no matter what you do, you would eventually cook whatever was at the surface or underground, so don’t do this where you want trees - or where there are underground coal seams
I think there have been about 5 different contexts over the years where I've been surprised by how good an insulator a pile of dirt is.
> Pipes run through the pile, and fluid flowing through them removes heat to supply the customer.
Dirt keeps a constant temperature year round quite close to the surface that’s a ~60 degree difference between summer and winter in many areas. So 600c would just be a tradeoff between depth, heat loss, and thermal efficiency. However, what they aren’t saying is electricity > heat > electricity is quite lossy and even just using the heat directly is far less efficient than a winter heat pump.
They mention end-to-end efficiency of 40-45%...
That’s just for heat to thermal and quite optimistic not end to end. “Conversion back into electricity is 40%-45%”
More realistic end to end numbers are likely in the 30% range which means summer electricity needs to be vastly less valuable than winter energy before you nominally break even and start repaying the investment. Further you instantly lose all the electricity required to heat the mound up to working temperatures. IE: If you can only operate between 550C and 650C then going from 20C to 550C needs to happen before you can extract any energy and you don’t get that investment back. On the other hand if you’re a chemical plant that needs 200C things start looking a lot better.
they mention that demand source should be close by to reduce losses in transportation
Interesting because I've been thinking about mechanical energy storage recently. I feel like these concepts hold a lot of promise on a small scale, per-house; coupled with solar panels. Although mechanical batteries they are not as efficient as electric batteries (and lose some energy), they can be both cheap to make and durable; these characteristics are much more important than raw efficiency when dealing with a single house.
this works in principle. heating dirt to store energy is cheap, the material costs almost nothing and the physics are solid. if the output needed is heat then it can beat batteries by a mile on cost.
the problem is scale. the dirt is free but heaters, piping, controls, permits, and contractors are not. balance of system costs creep up fast and thats where most cheap energy ideas collapse.
the market fit is narrow too. industrial heat or maybe district heating could work. coal plant conversion sounds good in headlines but takes forever to line up politics and utilities. daily cycling wont compete with batteries, only long slow seasonal storage makes sense.
execution decides if this survives. if they can keep real projects near the claimed cost then it has a shot, otherwise it stays as a cool demo.
The biggest problem is it has to be located near things that can use the heat.
Why not sell something in tbis vein to households and then let them use cheap daytime electricity to charge it up and and then heat their homes at night.
(Or it could store “cold” during the summer)
Electricity is easy to move. Heat isn’t.
California is converting thousands of abandoned oil wells to thermal storage for electrical generation.
https://eepower.com/news/engineers-repurpose-oil-wells-as-so...
California has thousands of abandoned (orphaned) oil and gas wells, with more than 5,500 identified as "likely orphaned" in 2023, and an additional 70,000 economically marginal and idle wells that could become orphaned in the future.
Kern County, California has 75% of the state oil wells, and the largest solar farms. Even after the 70,000 wells are idled, Kern County will continue to produce enough oil to meet all internal demands in California, although the state no longer has the refining capacity or anywhere to lay off the oil.
https://www.latimes.com/environment/story/2025-08-06/major-c...
Best landing website! Keep pushing it
Interesting how this turns the standard cogeneration plant strategy on its head. Instead of creating chilled water overnight and using that for cooling buildings, heat up the center of a large mass and use that for heating buildings or making steam to run turbines.
Wasn't this covered yesterday on HN? [1]
[1] https://news.ycombinator.com/item?id=44998441
Ground-source heat pumps are really common in the nordic countries.
Could an PV system energise an existing GSHP steel bore and warm up the earth and rock a bit around the bore? This heat would then be tapped in the winter.
There are simpler solutions using thermal solar panels which don't require converting solar energy into electricity first:
https://www.sciencedirect.com/science/article/pii/S266711312...
https://youtu.be/OdyrF96q2TQ?si=GT7ar0yoS6jR0mZe
How hot do they get the piles of dirt? They are making steam from it?
600'C at the high end seems to be the cited number. Steam or other heat extraction method such as air.
Wow that is insane. Steel is brown-red at that temp.
Oxidization also rapidly accelerates at these temperatures so durability of the greeting element is doing to be a significant challenge.
There are plenty of slag heaps (spoil tips) near coal plants, wonder how it could work with those? I guess the specific heat capacity is greater..
My basement without any heating/cooling. External/Basement temp in C: -10/12, 40/24
How does this compare with Exowatt? Assume fresnel lenses are more efficient.
Exowatt requires rethinking the solar energy system entirely. This is meant to be bolted onto an existing solar panel field that is mostly selling energy to the grid, but gets curtailed sometimes
Why use photovoltaic panels instead of direct solar thermal ?
Direct solar thermal that would produce 600 C would require expensive tracking concentrators. These lose a lot of energy to reradiation, don't use diffuse sunlight, and now you have very hot fluid you have to convey to your storage mass. With PV, you can just send electricity to resistive heaters in the storage mass.
Another question I don't see answered in the article: is there any risk for existing life by heating a huge amount of dirt? Will at some point surface and possibly influence local weather / thermal winds? Or should I just get my tin hat off?
Is there bigger harm to env than building parking lot? We've got plenty of these.
Why not building it under already wasted dead space like parking lot and have snow-free parking lot as extra bonus.
If the underlying stored heat leaks to the surface then it may indeed melt the snow and you'll have "snow-free" parking, but that won't be the end of the story, as all that resulted water will most likely turn your parking lot into an ice rink.
snow-free means you're losing heat to the environment vs keeping it contained. I would imagine if properly designed you could stand on top of the completed dirt pile without feeling any temperature delta with the air.
When did it become fashionable to say "PV" instead of photovoltaic, without ever saying what that stands for?
It begs the question - why use solar if the storage mechanic is sufficient to cover peak/off-peak storage of any surplus electricity in a significant way? Hydro Quebec has significant ($500M+/yr) electricity surpluses on a given year in the form of underutilized hydro during spring/fall months where energy demand is very low and export possibility is limited. These seem like ideal candidates for this vs something like a (relatively) small-scale solar setup. The slow-charge slow-release is also a considerable factor here since most battery energy storage solutions are designed for charge/discharge events multiple times per day to arbitrage energy rates - their ROI (which is currently very aggressive, sometimes less than 4-5 years) is a function of how many charge-cheap/discharge-expensive cycles they can perform per year. That is something that this doesn't seem equipped to do which is a negative multiplier on the ROI these systems can yield. If you can charge/discharge a battery 500 times per year, and the cost is 500x more, this is only at parity with a much slower charge/discharge schedule (charge during the summer, discharge during the winter - or vice versa). The other reality here is the steam turbine required to generate electricity (assuming electricity storage and not heat storage model) is a significant capex - roughly $300-400/kW (and some considerable OPEX in the form of maintenance and regulation/compliance). In this reality it only makes sense to retrofit an existing sunken-cost turbine and grid transmission system vs purchasing and co-locating a generating turbine along with the storage solution.
For reference, point-in-time energy market rates usually swing by 2x-3x per day - meaning if you charged during the cheapest market rate and discharged during peak you'd net about 2.5x return on that cycle) - even more so during extreme temperature events like heat waves or cold freezes - those are ultimately what you're riding here in terms of validating the system's viability from a financial perspective. If you reduce that scale from hours to months, and if draw-down speed is slow (ie: you can't sustain 50MW of steam with 500,000 tons of dirt even at 600'C) then you're looking at even more complicated returns.
By my simple, assumption-laden math, a 50MW "system" (capable of providing up to 50MWe peak output and requiring a requisite (assuming since it's not mentioned in the article - that at 200'C a 1,000,000 ton dirt pile would only be able to sustain 40MW of thermal output/20MW of electrical output and 240MW thermal/120MW electrical output at 600'C) would be:
PV system (20MW system would require ~30 days of charging to provide 50MWe output for 1 day, ~1200MWhe), alternatively, per day, you could discharge 50MWe for ~48 minutes. 1,000,000tons of dirt storage at 600C should hold a theoretical ~28 days of 50MW electrical supply. (also worth noting, getting the dirt pile heat up to "steam" temp would likely eat up a considerable number of months charging, which is also capex)
$1,000,000 for dirt
$5,000,000 for balance of system (heater elements and wiring + ASME tubing - as an aside this seems very opportunistic for 20MW of heaters and tubing to supply 100MWt of steam)
$12,000,000 for Solar Panels ($0.60/w bulk)
$8,000,000 for Solar Supporting systems and installation (assuming heaters can run on DC power and no inverters are required and there is no grid tie, minimal permitting and simplified ground install)
$25,000,000 for a 50MW steam generator turbine and transformer yard, provisioning etc
land use: ~25 acres for dirt pile, ~100 acres for solar, 10 acres for steam/aux, call it $300,000 assuming US averages for cleared land.
----------
Assuming north-eastern US (~20% solar efficiency with subzero winters where you also have high off-solar peak demands)
If you only charge/discharge this twice per year you're looking at some pretty paltry economics - you could only really fill about 18k MWh of thermal energy during half of the year for a ~7,400MWhe discharge - $592,000 gross electricity revenue per discharge cycle at an opportunistic 7-day "peak" market rate of $80/mWh which is about $1,184,000/yr gross margin. If you did it once per day (40MWhe per day at peak average intra-day market rate -$68/mWh) you're looking at ~$2,720/day or $992,800/yr gross margin.
$51M capex would be difficult to justify margins of only $1.1M/yr, and that's before any operating costs of which there would be several.
If you just sold the same solar at market rate (~$36/MWhe) throughout the year you'd net out at $1,261,440. Capex would be ~$40M and grid-tie solar is very cost effective in OPEX.
Likewise, if you just connected the system to the grid and skipped solar altogether (powering the heaters with grid energy like battery storage would): 50MW in for 12 hours on cheap time-of-day rates (typically overnight ~$18-20/MWh) and sold for 5 hours during daily peak rates ($55) you'd cut your capex considerably without the solar component and you'd be able to net, even with round-trip energy efficiency around 41%, (600MWe in @ $11,400, 248MWe out @ $13,640 = $2,240/day ~$817,600/yr gross margin) for a capex of $31.3M.
So in the end, the best solution seems to be collocating this on an existing coal/gas plant, where the capex is already sunk in the transformers, grid interconnect, steam turbine, land and permits and you're only adding the earth battery - you could run the model with the above margins with a capex of only $6-7M, which is very viable and even more favorable than the economics of spinning up a new gas/coal plant.
The economics of battery energy storage (BES) systems are much better known (ROIs of <4 years in extreme-swing energy markets doing intra-day peak arbitrage is very possible) since your round-trip efficiency is closer to 91%. A 250MWh BES plant with 1-hour charge/discharge window would be~ $40M installed and could arb twice per day - at 2x (low end averages - buying at $26 and selling at $52 twice per day = $14,285 cost for $26,000 revenue) $11,715 margin per day, $4,275,975/yr on $41M capex is still better economics than all the above models except those where the steam generator and grid infrastructure is already sunk.
If you had more centralized heating systems (like https://en.wikipedia.org/wiki/Seattle_Steam_Company or https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community ) then put the extra heat in there for use in winter.
Imagine 1,000,000 Drake Landing installations per year in Canada, pre-heating with the excess electricity. In 30 years Canada would need zero fossil fuels for buildings.
which... is only 13% of their GHG emissions? Oh we're fucked. The planet's so fucked.
ive been obsessed with passive heating recently. one thing that is really interesting that i only heard of until recently is pumping heat into the ground beneath a dwelling… far enough depth into the dirt so that by the time the heat rises and reaches the house the seasons have changed and its cold outside. the ultimate passive thermal
... and then build a 4 season greenhouse nearby...?
You can do that with normal pipes buried 5+ft underground that pull air into the greenhouse - the earth's temperature regulates the cold in the cooler months and the heat in the warmer months. There are similar calculations for how much piping you need to sustain a certain volume of air at earth temperature (typically 15'C in most places). This is shallow geothermal.
This seems really cool if this can be done at scale
See, I was a huge solar fan/renewable fan except but then I watched the infamous michael moore video and I had sort of concluded that either we need to focus on nuclear since I have this only grudge against the documentary that it didn't depict nuclear energy.
But overall, the conclusion of that documentary was that solar might not be the right approach given its storage mechanism and that batteries might be net negative overall the way they might be used right now
And that it might lead to more net negative emission even compared to Coal etc. (or comparable)
Seeing this, I feel like this can atleast be a very huge boost into not using batteries and building systems that only require land with less maintanence overall. Like, I can imagine a solar farm with solar panels and communities can build them in vacant land and then they build the system proposed in the article and then basically have more solar panels than the consumption or just enough to store them into such a system and then lets say that a stormy weather comes or rainy seasons comes or something or winter comes and then we might be able to push the energy from the reservoir.
This might be more efficient compared to using batteries or having more solar panels in the first place and "wasting" excess solar energy in the summers if lets say that we overprepare ourselves with a lot more solar panels (which seems to be the current approach) without using batteries.
Now to be fair, the author does propose that he likes batteries. I wonder if I can change that but I myself can be wrong too (I usually am), and so maybe we can discuss it! I wonder, how this system without batteries might work though.. like how are we gonna prepare for night without batteries completely, like maybe we can have a system where we can shift to the grid in the night?
Now although I will be honest, I feel like using the grid at night doesn't make sense but I am assuming here that michael's moore's video is right (which it is) and that the current way that batteries are used is more carbon emissions in the context of solar right now
Then this system that I was talking about doesn't include batteries and such might be atleast definitely better for the environment overall.
But that being said, I am still not a big fan of solar compared to nuclear. I feel like nuclear is one of the best things.
I wonder if we can actually have a nuclear reactor which can create energy and then send it to such communties and then they can store it in their ground and (its efficiency seems to be 50%) which might be actually good considering how atleast to me nuclear genuinely feels like unlimited energy and thus I was genuinely thinking of a cheap way to store energy, this isn't definitely portable but I feel like that communities can build it economically and so maybe countries with nuclear energy can setup lines with other countries and all that nuclear energy straight up goes to the community which stores it in such land and then they might actually use solar too and rely on this too and just store a ton of energy as much as it makes sense economically and then maybe combine this with solar and then use that nuclear energy at winters or if it turns out to be economical even at nights.
Now why I am saying this? Because this feels like a completely clean method. Now although I like nuclear energy a lot, I feel like a lot of countries can't access nuclear energy as they can't build it because if they can, they would've made bombs and its hard and they can't rely on other countries which have unlocked it for their power because that's a geopolitical disadvantage
but just using it for storing energy at scale might make true sense since it isn't that big of a priority and even if some nuclear power lets say denies them, then maybe they can go back or go to other nuclear power without their infrastucture being completely reliant or essentially melting because of some other countries decision and they still have autonomy which can make it a lot more viable
Overall a net win win situation imo. Makes me a little bit more optimistic.
> But that being said, I am still not a big fan of solar compared to nuclear.
This technology, if it works, is a bullet in the head for any remaining chance for nuclear. It provides heat at a cost competitive with natural gas, and natural gas is what destroyed the nuclear renaissance here in the US.
[flagged]
I'm sure you meant it to be edgy and offensive but it's quite flat in substance - you can't reasonably reduce the population and any means you'd employ to do so would be equally destructive to the environment - you can't change the capitalistic paradigm of energy generation and consumption, without some effort and resources drastically beyond what you've demonstrated here, and you can't ignore economy of scale that is demonstrated particularly with energy generation to insinuate that these solutions must be collocated on premise for them to be viable.
> Go ahead, flag this message so it will disappear and no other persons will be ever know it existed. That's the HN way.
Commentary like this is a reason to flag posts in and of itself.
But the rest of your post presents far more rhetoric and conspiratorial thinking (including bringing in entirely unrelated policies that nobody ITT is arguing for) than analysis, evidence or even logical reasoning.
> 'Battery storage' has no scalable reason to exist as a topic except for a millionaire's survival enclave. Natural disasters span days and weeks NOT hours.
An experiment to try: fully charge your cell phone or some other similar device, then turn it off and leave it off. How long into the future do you expect you'll be able to turn it on again without another recharge? You might be surprised.
A fascinating energy storage startup just emerged from stealth mode. The concept involves DC coupled PV feeding resistive heaters buried in dirt, providing heat at 600 C for a capex of $.10/kWh-th of storage capacity. Storage is seasonal, from summer to winter.
EDIT: dupe, darn it.
These are cool ideas but there's always an asterisk.
The issue here is: the "stored energy" isn't electricity, but heat. Converting heat into electricity is quite wasteful.
It's not really an asterisk if it's right in the title and the name of the company, is it?
They’re using the stored heat energy as heat, not converting it back to electricity.
And if it’s very cheap, does it matter if the conversion is wasteful?
I'm not saying it's bad, but it's clearly not a replacement for batteries, given its applicability is quite limited.
The question is about conversion is, is it still cheap if you add a powerplant (i.e. converting heat into electricity) and have to maintain it (moving parts, in contrast to batteries).
As I understand it, this is meant as a supplement to batteries, so that batteries can do most of the intraday load shifting while thermal can store energy over longer durations.
It's explicitly not a replacement for batteries. Did you not read the article?
It's a crucial complement for batteries. Batteries suck for seasonal storage.
> Our system can store the summer excess production for winter thermal demand.
This concept appears immediately flawed. Heat will definitely escape the "dirt pile" at some point between summer and winter.
Some yes. But there's a flaw in your reasoning: It ignores the concept called "rate of change". Because of this concept there will be more energy put into the pile than escapes, leaving some to be extracted and not just "escaping".
Honestly not bad if there's enough earth between the heater element and the air - you'd only lose about 1% per month with a thick dirt wall.