Building ultra cheap energy storage for solar PV
71 comments
·August 23, 2025zackify
Batteries are getting crazy inexpensive. You can get 16kwh for $1850 now https://www.youtube.com/watch?v=7bShGUPU3TQ
y-curious
I'm not knowledgeable enough to know whether or not this will cause a perma-fire in my home. I wouldn't even know where to begin to determine whether this is a death trap or a great deal. I'm leaning towards the former
nine_k
Lithium-ion: fire hazard, capable of thermal runaway and pack both fuel and oxidizer, napalm-style.
LiFePO4: safe, unless you crack them and ignite the liquid with a blowtorch, or something.
benjiro
Lithium-ion: produces its own oxygen, the reason why you get thermal runaways. LiFePO4: Does not produce its own oxygen, any fire (can happen) is limited to the surrounding air.
A trend in home batteries, that we see now (because of killer competition on the Chinese Market / export), is LiFePO4 batteries that have a included fire extinguisher module.
This adds another layer of security beyond:
* LiFePO4 (not oxygen producing)
* Most batteries are encased in a iron protective layer, this reduces the risk of punctures. And also act as a fire suppressor as any fire has a hard time escaping / limited amount of oxygen access.
* Depending on the batteries, they can be installed in a enclosed rack.
* The now often fire extinguisher module in a lot of pre-made batteries.
The chance of a fire from a LiFePO4 install is so small, that your more likely to get a fire from any other part of your house (probably your laptop or smartphone lol ).
roarcher
...if you want to take your chances with cheap junk in a safety-critical application. I watched about 3 minutes of that video and the number of issues that guy ran into was disturbing. You'd have to be nuts to put that thing in your house.
3eb7988a1663
I am in no position to install a battery system, but if I were, I would definitely build a little cinder-block shed to hold the array. Offset that a good distance from standing structures and instant piece of mind.
null
jacquesm
Please do not give that guy more exposure than he already has, his advice is going to get someone killed one of these days. He knows just enough to be dangerous.
owenversteeg
I would strongly disagree. Inexpensive batteries are everywhere now - this is the work of market forces, not some guy on Youtube - and he's one of a handful of people who you can trust to do an honest dissection of a battery. You don't have to be a rocket scientist to check for a handful of basic safety features and characteristics, which is more or less what he does.
rootusrootus
Given how many years of experience we have with Will Prowse, it would be helpful if you could elaborate a little on why we should trust your opinion more.
Squeeeez
As my grandma used to say, regarding speaking a foreign language: "enough to get you beat up, not enough to get them to stop"
Squeeeez
Why the downvotes?
Conversation instead of anonymous bashing would be appreciated.
Unless you have no good arguments, so I dare you, random aggressive strangers, I triple dare you!
jacquesm
I've done a lot of spreadsheets on this kind of solution, but on a slightly smaller scale: a single family home. While it will work for a while it is not enough to meaningfully offset the seasonal cycle which is the thing that needs solving. Storing energy for a few days up to two weeks is (relatively) easy, storing it cheaply for up to 6 months is very hard unless you are willing to invest massively offsetting much of your savings. The pile of rock required to heat an average home for a couple of weeks handily outweighs the house itself. And that's without a double conversion, it is used and stays as heat, the idea was to moderate the leakage upwards as a source of heat by blowing air in a controlled manner rather than to convert it again.
So I really hope these guys will succeed where I can't even get it to work on paper, sometimes scale really is a requirement to make something work and this could very well be one of those.
nine_k
Depending on your case. Where I live (NYC, a barebones Great Depression-era building) my electric bills skyrocket (3-5x) in the summer due to the house being heated by the sunlight. The roof is white, but it helps moderately. Heating in the winter takes much less energy, especially if we factor in all the electric devices inside that also release some heat.
Storing a half-day's worth of electricity, and powering stuff from a solar panel at daytime, would likely let my apartment stop consuming external electric power in the summer.
Assuming my apartment consumes no more than 15 kWh during the hottest days (according to the meter), a moderate 8 kWh battery, and $1000 worth of solar panels, would suffice, given the room and a permission to mount the solar panels somewhere.
blitzar
You could potentially skip the storage by cooling aggressively on the solar and then not cooling when it is dark.
myself248
Isn't it cheaper to just buy more panels, enough to meet your winter needs? Then who cares if they sit mostly idle for most of summer; energy not used is harmless.
Then your storage model becomes "a cloudy week" rather than "a whole season", and the storage scale changes significantly.
PaulKeeble
This is certainly the solution in somewhere like Australia where the winter production is about half of the summer production. It gets more problematic in places further north/south where its 5x or worse and then you need quite a bit of land to produce enough power and battery storage than can go multiple days at least.
The difference between net zero across the year and 95% unlikely to need the grid on solar and batteries is a staggering difference, about 10x the panels and battery storage. But the equation changes drastically depending on local weather patterns and the solar irradiance difference from summer to winter.
nine_k
How about keeping an ICE-powered generator for those several days in a year when the sky is stubbornly cloudy and dark? Could make sense for remote houses.
BTW keeping your house connected to the mains costs $10-20 / mo, even if you consume nothing. Connection cost is one-time, but likely several thousand.
jacquesm
No, (1) you need too much space and I'm pretty much maxed out already (50 panels), during the wintermonths (2) the angle is really bad, (3) the days are shorter and (4) you're overcast a lot of the time. So that simply will not work. But in the summer I have so much excess I get penalized for it, which is ridiculous.
toomuchtodo
Once homeowners have done everything they can (as you have, in your example), the remaining optimization is to drive down storage costs (LFP is ~$52/kWh in China at utility scale) until seasonal storage is solved for (which is likely a combination of electrical transmission [to be able to shift low carbon energy from areas that are constantly plentiful] and longer term storage like ammonia or similar fuels created and transported [pumped around] with low carbon energy). Of course, if you have the space, keep putting solar PV up until it's no longer an issue when paired with heat pumps.
Gys
> the angle is really bad
Optimize the angle for the Winter. The Summer will do well enough anyways.
buckle8017
The panels themselves are cheap and getting cheaper.
However the mounting systems, solar charger controllers, and inverters are mostly not.
The bulk of the cost of panels now isn't the panels themselves, but all the supporting infrastructure.
The seasonal variation can be as high as seven to one, so the total install has to be very very cheap to support a 700-1000% over build.
dv_dt
The last time this was discussed, someone mentioned you only needed as much inverter capacity that you wanted to process. So in winter all panels were hooked up but in summer some were disconnected or bypassed somehow?
Though if you are on a grid that allows it, you might want to send summer power back into the grid for credits
XorNot
Panels are surface area limited. The maximum my house and land can support is the 10kW I already have.
Honestly I'm pretty skeptical of residential rooftop solar - it's not even close to the solution, it's just a cost optimization we shouldn't actually need.
hvb2
In almost all cases you lack the area for doing that. And that's not even considering exposure, as in winter most north facing panels on a roof won't get any direct sun because it's lower on the horizon
algo_trader
> I've done a lot of spreadsheets on this kind of solution
As an investor - what RoIC do you want to see when doing initial analysis
(for example, $10M capex per system, with 10,000 systems TAM )
Veedrac
> The pile of rock required to heat an average home for a couple of weeks handily outweighs the house itself.
Maybe I'm failing to follow the intended argument here, but I do not see what is expensive about this. Houses are hollow; they do not weigh all that much. Dirt is cheap, especially when sourced locally.
jacquesm
The only place where you could conceivably store this without having another conversion or lossy transport loop is under the house. It's not exactly trivial to excavate that much space under an existing structure without having the whole thing come down. I think I've figured that part out but it would still be legally quite shaky and it might destroy the resale value of the house unless it can be made compact enough that it would fit within a standard basement. Insulation is the key problem to solve here.
Veedrac
The article never frames this as off-grid storage for individual homeowners. They say
> A typical site is a factory, power plant, or town with a large earthen mound at the edge. The mound might be the size of a house for a smaller factory, and up to many football fields for a large power plant. Surrounding the earthen mound will be high-density, low-profile solar arrays.
I agree that trying to give every suburban house its own rock pile would not be very practical.
madaxe_again
That depends on how hot you are prepared to get your rock.
I’m also off grid, also suffer in the winter (although a spot of hydro power is going to alleviate that somewhat), and have also thought about thermal storage - I actually calculated that a 20ft container filled with sand could give us thousands of kWh, as long as the sand could get up to ~600C.
At that temperature you of course have to think about making really damned sure no water gets in there, but it makes the volume of material more manageable. Would still work with resistive heating, energy out would be trickier but not impossible to engineer something that wouldn’t explode.
iamthemonster
Absolutely superb. The 'hot bricks in a box' concept of very high temperature thermal energy storage has been really coming into its own with the fall of solar PV costs, and it's quite suitable for industrial consumers of medium temperature heat. There's also a heated granite storage silo in Scandinavia that's recently gone into operation. The rate of heat transfer in the heat storage material is a classic constraint.
I think this niche of 'very long duration, very constant slow rate of discharge' is clever, and it would suit industrial heat consumers but could also suit district heating for buildings in a climate that's predictably in need of heating all winter long (Canada for example).
They seem to have a decent grasp of the fundamentals, both of the technology and how to commercially carve a niche. I wish them well, and thank you for the post.
ZeroGravitas
They skipped a few real competitors that'll shrink their market:
1. In places with underground water reservoirs, these can be used for heat storage very similar to this. It's location dependant but I think there's European district heating networks doing this already.
2. Heat pumps for process steam are gaining ground. The temperature at which heat pumps lose competitivness is slowly rising over time. They mention storage at 600C. Heat pumps win under 80C and are up to 160C and aiming for 200C though at those temps cheap gas will probably win for now. Heat pumps can also recycle process heat and cool and heat at the same time.
3. Wind doesn't get mentioned. Wind doesn't pair as neatly with batteries as solar does but in places with seasonal storage issues the batteries can be used for solar and wind during summer and wind plus whatever (biogas/nuclear/hydro) in winter. They need to worry about both centralised big batteries and customer sited ones.
In general I'm supportive of the idea, but similar to the Nordic sand battery, when they start talking about generating electricity from the stored heat I take that as a signal that they've realised the alternatives above combine to really squeeze the market they have left and undermines my faith in the rest of the product.
conradev
Yeah, Form Energy exists to basically disprove this:
Batteries will still be too expensive to arbitrage these seasonal differences effectively. Thermal electricity storage is orders of magnitude cheaper for storage, but worse at daily cycling
ZeroGravitas
Form's basic idea is that low round trip efficiency can work as long as it's cheaper enough.
They've always had $20/KWh as a notional price but there's already talk of Sodium batteries in China getting near that price point, while having nearly double the round trip efficiency.
ziofill
missed opportunity to call it “dirt cheap”
SlowTao
See if they are looking for marketing people. If so, you're hired!
mastax
Im shocked, and a little incredulous, thats its feasible to heat soil to 600C.
throwaway81523
This is confusing, why PV to heat the dirt? Black paint will do it from sunlight directly. Or circulating hot liquid. Way cheaper and more efficient than PV at collecting heat from the sun.
Animats
Because they're trying to go way up on temperature. The idea is to get dirt up to 600°C. It's easy to get warm water from solar, and solar water heaters have been around for decades, if not centuries. But that's well below 100°C. If you start from electricity, you can reach any temperature up to plasma. If you're just storing warm water, you can't get much of the energy back. E = (T2-T1)/T2, remember.
They've made no progress on getting the energy out of their heated dirt. They want to make a hot dirt powered boiler - run pipes through the dirt, put in water, and get steam. Not hot water, superheated steam. That's the hard part, and it will have to be custom.
Small steam turbines are available. Siemens has a whole range.[1] They start around 750KW, at the high end of automotive scale. Siemens can sell you a matching generator. When these guys can power one of those, it's real.
Right now, it's three guys. They need VC funding and a really good boiler engineer. The goal should be a working prototype at about 500KW scale. That would be a big enough prototype to get some meaningful efficiency measurements.
(Maintenance will be tough. You can't turn the heat source off.)
[1] file:///home/john/Downloads/SE-Brochure-Dresser-Rand-Steam-Turbines-2021-pdf_Original_20file.pdf
theptip
Water will be limited to 100C, and tanks are going to be way more expensive to maintain.
You don’t get much long-term storage with your black paint as it’s just going to heat the surface. The point here is to heat up a big enough pile of dirt that you can draw out power months later.
fabbbbb
Liquid doesn’t need to mean water? Traditional solar thermal systems use special liquids that can get way more hot than 100C.
cyberax
You can get to around 200C with oil-based media. But oil goes bad pretty quickly, and it leaches tons of crap out of everything.
Above that temperature, you need molten salts or liquid metals that are extremely corrosive. Your other option is to use gaseous media instead.
hackernudes
My understanding is this is for excess PV capacity. But I think they should address this! Their plan is to heat deep inside a big pile of dirt using resistive heating elements and then pump the heat out in pipes. Maybe it's hard or expensive to add solar heating into the system? But why couldn't they pump it in? Maybe they don't have extra space because the dirt mound is covered in PV panels.
owenversteeg
The sibling comment by theptip explains this well for this specific case, but the other sibling comment still seems confused, so I will explain more broadly: productive uses of heat are all about the temperature. The boundaries vary, but "high-grade" heat is roughly 300-500C, medium-grade is 100-300C, and low-grade <100C. Up to a certain point [0], heat is easier to use the hotter it is.
High-grade heat can be easily turned into electricity with a turbine, or reused in an industrial process (the entire point of a nuclear reactor is to create heat in this temperature range!) Medium-grade heat can still be used for some processes or used to generate electricity, but the electricity generation will be less efficient. Low-grade heat is under 100C and is a lot harder to use. You cannot economically generate electricity from it, or use it for most industrial processes, so use cases often focus on district heating.
The problem with these low-grade-heat district heating schemes, or more broadly any use of low-grade heat, is the economics. Let's take your idea. The efficiency from sunlight to heat is indeed high (much higher than PV panel -> resistive element) but the heat generated is all low-grade heat.
So what's the root cause here, why is low grade heat usually not economic to use? It comes back to two main causes: 1) efficiency, and 2) storage. Most power is generated from turbines that use heat - a type of heat engine. Carnot efficiency is the maximum theoretical efficiency of a heat engine. Carnot efficiency is η = 1 – Tcold/Thot where temperatures are absolute temperatures (Kelvin/Rankine.) In other words, 7.7% for 50C->25C (298K->323K), 37% for 200C->25C, and 61% for 500C->25C. Note that this is _theoretical_ maximum efficiency; real world efficiency varies quite a bit - from ~1/2 to ~1/10th of Carnot efficiency at peak depending on your heat engine. The second, storage costs, are even more important. You need to insulate your warm object to keep it warm, and if your heat is low-grade, then it is spread out across a huge volume.
Back to your idea, your "paint the dirt black" idea will generate far more heat, but very low-grade and entirely non-economic to use. You will have a somewhat warm pile of dirt, but nobody really wants a somewhat warm pile of dirt. This is the same reason why you see people, logically, tearing down their high-efficiency solar water heaters to install low-efficiency solar panels.
"Use solar panels to resistively heat the dirt", on the other hand, is less efficient and generates far less heat - but it generates high-grade heat. This startup proposes to eventually sell that heat to power plants, to generate electricity directly; and if you're doing that, the temperature of the heat is critical. As you can see from the Carnot efficiency, a power plant couldn't economically do anything with a warm pile of dirt, a solar water heater, or other similar technologies. But they _can_ do something with a source of high-grade heat - namely, they can run the turbines that currently run on fossil fuels. In other words, you can solve the seasonal solar curve problem and have constant electricity production year-round, even in northerly climes.
[0] Above - very roughly - 500C, it is harder to use the waste heat efficiently, because the engineering gets a lot harder, but the theoretical maximum efficiency is higher. That's one reason why there are a lot of efforts to try to build nuclear reactors working at these higher temperatures (see: molten salt reactors.)
simne
Excellent explanation. I could add, typical Thermal Electric Station (Gas or Coal) could have significantly larger working temperature than Nuclear, because more strict safety measures for Nuclear on nearly same level technologies.
For example, Nuc typically don't cross 400C, when Gas/Coal could work on 450C or even more.
Plus, non-Nuc could use some more exotic technologies, like MHD (magnetohydrodynamic) generator, to provide much more efficiency of power generation, or CO2 turbine to make installation much smaller. (Theoretically, somebody could build Nuclear MHD device, but it will not pass safety restrictions).
buckle8017
There's very few materials cheaper than dirt.
Solid state solar panels should be more reliable than any hydronic system.
Though I suppose their heat recovery system is probably hydronic.
scblock
Energy storage yes, electricity storage no. Very limited utility and questionable use of solar PV, which has about a 20% conversion efficiency of solar energy to electricity. Seems like a lot of extra steps to make heat.
3eb7988a1663
One of the biggest uses of residential energy is heating/cooling. Especially in winter months where PV generation is significantly lower (I have seen numbers of 2-5x summer generation), having "free" heat could be quite valuable.
Now, do the economics shake out for storing heat to deliver in winter? No idea, but the idea is not far fetched. Outside of bitcoin mining, there are few uses which can soak up the increasing glut of midday PV generation. Anything that can seasonally store that (even at terrible efficiencies) is valuable. What is going to be the most economic option? No idea. My personal bet is iron-air batteries, but there are a lot of contenders in the space that are competing for widespread adoption.
gaoryrt
Why go throught the extra solar -> electricity -> resistive heating -> soil instead of directly doing solar -> heating a metal rod -> soil?
Cutting out the conversion steps should be more efficient.
3eb7988a1663
If the electricity is free, efficiency does not matter that much. The capex of the storage system might matter a lot more. In which case, heating up a big mass might be more economically feasible than battery installations.
nabla9
> The purpose of Standard Thermal is to make energy from solar PV available 24/7/365 at a price that is competitive with US natural gas.
They compare to natural gas. Not the cheapest alternative.
>Pipes run through the pile, and fluid flowing through them removes heat to supply the customer
Just basic district level geothermal heat pumps do the same. You don't need to heat the soil. Just drill down and install pipes. Earth generates heat. What is the cost difference now?
hvb2
> Just basic district level geothermal heat pumps do the same.
This doesn't with everywhere though? Because of geology?
buckle8017
I've found that lots of people get viscerally mad when it's pointed out that solar pv is not reliable year long and that the batteries to make it seasonally reliable are wildly expensive.
I wonder if they have had issues trying to solve a problem so many people insist doesn't exist.
jacquesm
> I've found that lots of people get viscerally mad when it's pointed out that solar pv is not reliable year long
Solar PV is very reliable year long. It is the day-night cycle that requires batteries, for seasonal offset they do not work and probably never will.
In the off-season they still work just fine, they just make much less power. Which you can offset to some degree by a good balance between wind and solar at the grid scale (not at the private dwelling scale, unless you live rurally).
I'm on PV during the day time all year round in a densely built up region in Western Europe. In the summer we overproduce a ridiculous amount that we can't consume ourselves (though I'm getting better at finding good uses for it), in the winter it is touch-and-go during the day. Netmetering is enough to give me a negative electricity bill which goes towards paying whatever NG we still buy (which isn't a lot, we spent a good bit of money on insulating this house as much as we could without rebuilding it).
> I wonder if they have had issues trying to solve a problem so many people insist doesn't exist.
Nobody who knows anything about PV is going to insist the problem does not exist.
buckle8017
> Solar PV is very reliable year long. It is the day-night cycle that requires batteries, for seasonal offset they do not work and probably never will.
You acknowledged that you're wrong in the very same paragraph.
I suspect you're misunderstanding what the word reliable means in this context.
jeffbee
Maybe they are just angry because you're being dishonest. That would be natural and expected. Do you try this often?
I have a home in the southwest that is off grid and runs on solar plus lifepo3 batteries. It has been 5+ years now. My cost per kwh is below $0.008 as of today including all capital and maintenance. These numbers get a bit complicated, for example I run the AC much colder than I would if I was paying more for it. I have extra fridges and freezers I probably wouldn't if I had to pay higher per kwh. I "throw away" a lot of power too that I am not counting when the batteries fill up.
I have about 40kwh of storage. The batteries are in steel boxes and there are some basic precautions to take with them but lifepo3 has a very manageable risk profile quite different from lipo. Batteries and solar equipment continue to get cheaper, the same system I have is now 50% cheaper today then when I bought it, including tariffs.
The link really discusses more of a single neighborhood or medium industrial site possible type of technology. Really just a huge very hot pile of sand and steam turbine or propane cell generation. On a kwh basis it is probably not competitive with solar+battery unless your use case involved a lot of direct use of hot water or heating something.