Standard Thermal: Energy Storage 500x Cheaper Than Batteries
115 comments
·August 25, 2025adverbly
Cthulhu_
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...
Aurornis
> 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.
werdnapk
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.
mrgaro
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.
Ekaros
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.
teeray
> 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
shrubble
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.
jfengel
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?
abeppu
... and similarly doesn't it mean the pile is slow to absorb heat when your PV installation is trying to dump energy into it?
progbits
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.
voakbasda
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.
nyeah
It means the heat stays near the hot pipes for quite a while.
profsummergig
> 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.
wcoenen
> 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.
werdnapk
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.
adverbly
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.
vasco
Had the same thought, we'd have to put a thermometer inside a 10ft cube full of dirt for science.
ilaksh
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.
teiferer
> 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.
0xbadcafebee
[delayed]
stogot
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?
Ciantic
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.
metalman
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.
nyeah
But it's easy to build & easy to read about, so "why isn't it everywhere already" still feels like an open question.
DangitBobby
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.”
Ekaros
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.
parpfish
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)
AndrewDucker
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.
PaulHoule
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.
bjoli
That is obviously a scam. Not a chance in hell.
I am giving that one a 0% chance of long term success.
staticlink
And this is why gravity is considered a weak force.
javcasas
Sit down. Now stand up. Congratulations, you just beat the gravity force generated by a whole f*cking planet.
colechristensen
And the strong force holding two protons together in an atom is on the order of 10 pounds.
javcasas
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.
empyrrhicist
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.
Cthulhu_
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.
adrco
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 !
raincole
Pumping water up is a super old idea, but as far as I know you'll need some natural terrain to build it efficiently.
vitro
Someone has already been thinking along the same lines: https://www.swissinfo.ch/eng/business/energy-vault_revolutio...
cromulent
EnergyVault are building these GESS (Gravity Energy Storage System) arrays right now.
elil17
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.
Symmetry
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.
olejorgenb
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.
capitainenemo
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).
Ekaros
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.
eplawless
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.
carlos_rpn
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.
Aurornis
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.
nyeah
Yeah, when baseline efficiency is zero then there's probably room for improvement.
yodelshady
More efficient, but much more expensive. I'm sick of people handwaving $100 per kWh. That is two orders of magnitude off where it needs to be to do anything more than virtue signal.
Meanwhile multiple grids are now paying renewable to curtail, because guess what, the variability is correlated (it's the exact same damn mathematics we used to fuck up the entire global economy in 2008, which is why I'm so surprised people are handwaving that too, but whatever). If you want to minimise cost without relying on gas to save you on dark still days, you want a cheap use for the surplus, round-trip be damned.
Panzer04
100$/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.
adgjlsfhk1
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
stinos
Another thing they don't seem to mention is environmental impact (if there even is any worth mentioning, not sure).
jmpman
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?
smartmic
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.
profsummergig
I do like how well and concisely they've explained not only the technology, but the exact use case, on their landing page.
dwallin
Seems like a case where directly going from sunlight to heat would be a better approach for this, instead of converting to electricity first.
orev
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.
quickthrowman
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.
zdragnar
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.
arnoooooo
Why use photovoltaic panels instead of direct solar thermal ?
thelastgallon
Dupe: Building Ultra Cheap Energy Storage for Solar PV https://news.ycombinator.com/item?id=44998441
gigel82
> 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.
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!