Are Rocks The Future Of Solar Power Collection?

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Yves here. Could readers sanity check this idea of using rocks as energy stores? Or rather whether it would scale?

By Brian Westenhaus, the editor of the popular energy technology site New Energy and Fuel. Originally published at OilPrice

  • Soapstone and granite, rocks formed under high heat, are being considered as thermal energy storage (TES) materials for concentrated solar power systems.
  • An analysis of rock samples from Tanzania found that certain soapstone and granite types possess high energy densities and maintain stability at high temperatures.
  • The Craton soapstone showed the best performance as a TES, able to absorb, store, and transmit heat effectively while maintaining good chemical stability and mechanical strength, pointing to promising potential in sustainable energy storage applications.

The next generation of sustainable energy technology might be built from some low-tech materials: rocks and the sun. Using a new approach known as concentrated solar power, heat from the sun is stored then used to dry foods or create electricity.

The School of Materials, Energy, Water and Environmental Sciences (MEWES), Nelson Mandela African Institution of Science and Technology team reporting in ACS Omega has found that certain soapstone and granite samples from Tanzania are well suited for storing this solar heat, featuring high energy densities and stability even at high temperatures.

Energy is often stored in large batteries when not needed, but these can be expensive and require lots of resources to manufacture. A lower-tech alternative is thermal energy storage (TES), which collects energy as heat in a liquid or solid, such as water, oil or rock.

When released, the heat can power a generator to produce electricity. Rocks such as granite and soapstone are specifically formed under high heat and found across the globe, which might make them favorable TES materials.

However, their properties can vary greatly based on where in the world they were formed, possibly making some samples better than others. In Tanzania, the Craton and Usagaran geological belts meet, and both contain granite and soapstone. So, Lilian Deusdedit Kakoko, Yusufu Abeid Chande Jande and Thomas Kivevele from Nelson Mandela African Institution of Science and Technology and Ardhi University wanted to investigate the properties of soapstone and granite found in each of these belts.

The team collected several rock samples from the belts and analyzed them. The granite samples contained a large amount of silicon oxides, which added strength. However, the Craton granite contained other compounds, including muscovite, which are susceptible to dehydration and could make the rock unstable at high temperatures.

Magnesite was found in the soapstone, which conferred a high density and thermal capacity. When heated to temperatures over 1800 degrees Fahrenheit, both soapstone samples and the Usagaran granite had no visible cracks, but the Craton granite fell apart. Additionally, the soapstone was more likely to release its stored heat than the granite.

In all, the Craton soapstone had the best performance as a TES, able to absorb, store and transmit heat effectively while maintaining good chemical stability and mechanical strength. However, the other rocks might be better suited for a lower-energy TES application, such a solar dryer. The researchers say that though further experiments are needed, these samples show good promise in being a sustainable energy storage material.


One isn’t seeing many fundamental research work papers coming out of the developed west. That makes this post’s basic and highly practical work all the more interesting.

Among those looking into geothermal energy harvesting and storage, to this author’s knowledge, this kind of work hasn’t been seen in the scientific press.

The reporting paper is not behind a paywall and gives the alert observers a quick lesson of what should be significant in examining natural heat storage materials.

A look through this team’s paper is quite illuminating! As geothermal finds more market traction this kind of know how is going to be much more important.

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  1. Joe Well

    My uninformed question is if this is so low tech and also effective, why haven’t humans been doing this for millennia?

    1. PlutoniumKun

      The use of rocks as thermal stores is as old as the neolithic. This is how the people of central Asia keep their sleeping areas warm during very cold night – its how people can sleep snugly in a yurt when its intensely cold outside (they sleep on rock structures warmed by fire during the day).

      1. Steve H.

        Some rocks explode while heating, a fact rediscovered every day, and which underlies the research of this post. The work is valuable in itself, and I learned useful things from the paper.

        But the practical application… From NC: In models with high levels of renewable power, the cost of storage can dominate the costs of the whole grid. In California, 80 percent renewable share would require 9.6 terawatts of storage but 100 percent would require 36.3 terawatts.

        This work would seem to fall in that last 20%, meant to continue steam-based electrical generation while the sun don’t shine. Other commentaters are better equipped to address the issues of shutting down generation overnight. I look at the energy to store 600’K, vs 100′ to boil water, and I see loss rates increasing as the thermal gradient increases. It concentrates power at the plant, rather than an egalitarian spread with greater efficiencies.

        Very useful for a solar forge, though. Thanks for the post.

        1. Synoia

          Some rocks explode while heating,…

          True. Typically ones with water content. Dry them out and they become safe. As neolithic man discovered.

          Something about “experience” crosses my mind.

      2. Joe Well

        I guess I just got the solar part in my head and didn’t think about other ways of heating rocks.

        But if Archimedes could make a death ray from mirrors, why didn’t the ancients do solar heat stones? Or did they?

      3. Piotr Berman

        I am not sure about yurts, but I read about Mongolian cooking technique: heat stones in an open fire and then use it for “slow cooking”. One variant is to put pieces of mutton and vegetables into a leather bag, drop some heated stones into the bag and wait. Second variant on YouTube is to remove entrails from a woodchuck and stuff it with herbs and few hot stones, cover it and wait. In either case, it gives you lightweight equipment for slow cooking, and practical in paleolithic. In turn, using stones in that manner allowed to discover metal dripping from some of them when they were heated, and at some point this observation was pursued further… Contemporary yurts have wood stoves with pipes that emit smoke outside, but hot stones could be used in the past. Mind you, -30 C is routine in winter in Mongolian steppe…

        1. Paul Whittaker

          Going back a few decades I took the R2000 course and also picked up from the Saskatchewan conservation house which were even then using a heat ex changer made of layers of 1X2 and plastic sheet. The trombay? (not sure of spelling) was another version with barrels of water stacked to the ceiling behind a glass exterior wall. Another house not far from here had a rock bin with heat from a solar roof pushed through it.

        2. PlutoniumKun

          The landscape of Ireland is littered with what are known as fulact fiadh. They are crescent shaped mounds formed when people set up cooking camps for a long period from the neolithic to medieval times. They would cut a pit into the ground, line it with wattle and mud and fill it with water (or allow it to fill naturally, they are almost always beside a stream). They would then set a fire and heat up rocks. When very hot, the rocks would be thrown into the pit until the water boiled, they they would (presumably) cook. They were likely seasonal hunting camps. The rocks could only be used once as they became brittle, so they were discarded behind the fire, hence the crescent shape would build up. As the article indicates, the builders knew which stones held heat better, usually sandstone.

          I’ve always wondered why they didn’t just cook directly over the fire – most likely they didn’t just prefer boiled meat (they also probably realised that boiled meat is less likely to hold parasites), but that they could also cook other things for long term use, such as haggis type sausages or the type of foods you describe. They probably also used them as saunas. I’ve always suspected that rather than just simple hunting camps as archaeologists usually describe them, they were more like holiday camps for family groups – during early spring, when the weather was miserable and food was low, hunting a deer and then having a family sauna while preparing protein and fat rich food for the trip home would have been a joyful event to brighten up a tough time of the year..

    2. Samuel Conner

      Circa late ’70s/early ’80s a college friend described a thermal storage/retrieval system he had built. A room in the basement full of rocks, a glazed south-facing roof to admit light to a heat-collecting attic, and duct-work and fans to move hot air into the basement thermal store in Summer and from the store to the living area in Winter. He claimed to not need to burn fuel to heat his house through Massachusetts winters.

      I don’t recall numbers, but I have the impression that in terms of cash outlay for materials (the labor was DIY), he expected that the system would nominally pay for itself over a period of about 10 years (but that was a simplistic calculation. Conventional heating would have been less cash up front and I don’t think he included the forgone investment income from the big up-front cash outlay).

      It sounded like a huge amount of work and if labor costs were included, it would probably have been more costly than conventional heating. I think that the use of “thermal mass” in more passive systems (adobe as a building material, for example; stabilized earth construction is similar) may be more common (though not common).

      1. tomk

        I knew of one of those. It developed moisture issues, I heard it wasn’t fun lugging 40 tons of slimy, moldy rocks out of the basement.

    3. Polar Socialist

      Humans have been using thermal properties of rocks for several millennia. Both to keep dwellings cool and warm. Humans have really used concentrated energy generation (or collection, in this case) for about hundred years. And until a few decades ago, coal and oil were so cheap and abundant that there was chance for anything else to replace them.

      Before that, energy was generated or collected on the spot it was used. And people have been really clever about it, especially when it comes to using masonry to retain stable temperatures within a well defined, albeit small region.

      My French friends live in an old stone house, and in the winter they live in the southern end while during the summer heat they “migrate” to the northern end of the house – they don’t have air conditioning, they are taking advantage of the way the masonry is trying to retain the temperature by acting as a heat sink or heat source depending on the ambient temperature.

      This kind of “passive solar” energy has been used succesfully in China ( and in Europe (

      And don’t get me started on sauna, the most clever way humanity has ever used the ability of rocks to store and release heat!

      1. Henry Moon Pie

        Adobe houses work the same way. We had an interior adobe wall in the house we built to act as a flywheel in New Mexico’s high altitude where temperatures often varied 40 degrees F from daytime to nighttime.

        Spending a winter in a stone inn in Istria demonstrated a bad side to all that mass. Once it gets cold, I takes a lot of energy to heat it back up.

    4. Ignacio

      Your question is IMO well pointed in the sense that there are now simple technologies that allow for thermal storage at household and industrial level and IMO are underused. Water has an excellent capacity to be used as heat reservoir in homes, whether you heat it directly or via heat exchangers you can then use it when the sun has gone down for heating and showers. In industrial applications you can have expanded applications. Simpler than the rock thing and already available.

      1. Polar Socialist

        The coolest (pardon the pun) application I’ve encountered regarding the use of water as heat storage is using a tank of water as a brake for windmill: the water heats up due to the friction and even light wind can provide enough warm water for the needs of a farm.

      2. Joe Well

        I’m pessimistic since solar water heaters are already in wide use in parts of Latin America, and yet in the US I never hear them mentioned. If our society won’t do something so simple and obvious, why would they do something much more complicated?

        1. albrt

          Twenty years ago in Arizona a lot of the solar companies were pushing water heater systems as an entry level product, because they have the best return on investment. It never seemed to catch on. I suspect hot water is such a small part of the household budget that it just didn’t move the needle for most people. People were much more attracted to solar panels because they could power all the computers, TVs, and other gadgets people cared about.

          1. Laura in So Cal

            My parents had a roof top solar water heating system when they built their house in the late 1970s. It worked great for a year or two until an uncharacteristic cold snap caused a hard freeze that shattered all the glass panels. My Dad decided not to rebuild the system since the economics didn’t support it.

            1. Joe Well

              I never would have thought of that. I wonder why that doesn’t happen to the electric panels.

              1. some guy

                I will guess that the freezing water shattered the glass by expanding against it.

                If electric panels contain no water, then they have nothing that sharply expands upon freezing, hence nothing to pressure-shatter themselves with.

          2. Ignacio

            The problem with thermal solar modules is their relatively complex building and maintenance, compared with photovoltaic. But now you can couple photovoltaic with heat exchangers and obtain hot water. This latest combination is still expensive but it might be pushed to be more affordable.

            Problem in houses is that energy demand is not that high and any investment takes too many years to be recovered. Much better if you can make solar communities sharing part of the system but we have been educated to be individualistic.

            1. PlutoniumKun

              Water heating modules are extremely popular in Ireland and very effective, although gradually being displaced by cheaper photovoltaics. The lack of very hot days and very cold snaps makes them much easier to operate and cheaper to build. In long summer days they are highly effective – a friend of mine has so much free hot water from his rooftop set up he happily washes his car with it.

              1. jochen

                Solar collectors are/were popular in Germany. Due to the electrovoltaic installations having become so popular they are not that visible anymore. And people probably install solar cells rather than heat collectors, now.

                The house in which we rent (six flats, of which two are under the roof) was build ca 20 years ago and has two collectors on the roof. As tenants we don’t know much about them and if they get and need maintenance. They seem to be the the vacuum tube type. They have a reflector behind the tubes and the inner tube with coolant is inside a vacuum tube for insulation. These are more effective than the standard type and can supply 150°C.

                As for the heat storage in the article. A few months there was an article, possibly at BBC news about using sand in a large silo as a cheap heat storage. So these things are thought about.

                In a previous house, in which I lived, the owners had an electric storage heating system installed. That was the early 90s. It used night tariff electricity and heated a large ceramics/stone block inside to a pretty high temperature. My guess is a bit over 300°C. The house had floor heating. And a standard wall mounted radiator in the bathroom, for extra heat if wanted.

                1. Joe Well

                  Why couldn’t they have both kinds of units? Is roof space really that scarce? And why wouldn’t they be on the ground next to the building?

                  1. jochen

                    Technically that wouldn’t be a problem. But if it was done for economic reasons then the whole roof area is used because solar cells make more money. At least they used to when high tariffs were guaranteed for the power sold to the utility.

                    And yes, solar heat collectors can be placed on the ground level. My sister in law has that. It came with the house, which has a roof in east-west orientation. About 2m² of collecors are placed next to the house just outside the boiler room in the basement. Another advantage is that the coolant line to the hot water tank is very short. The house is from the 60s and this was retrofitted by the previous owners.

            2. PlutoniumKun

              And I forgot to mention the simplest use of all, which I have in my own apartment – electric storage heaters using stone as a thermal store – its heated using cheap electricity at night and stays warm all day. These are a very efficient and cheap form of electricity storage in smaller grids, which is why they’ve been encouraged in Ireland since the 1960’s.

    5. some guy

      Well, they have, but at smaller scales.

      antique soapstone foot warmers . Here is a bunch of images. You would heat up the foot warmer and then take it with you in a long cold coach ride to keep your feet warm.;_ylt=AwrE.4UsfKBk2qIic45XNyoA;_ylu=Y29sbwNiZjEEcG9zAzEEdnRpZANMT0NVSTA1OENfMQRzZWMDc2M-?p=soapstone+foot+warmers&fr=sfp

      Somewhere in the Lo Tech Magazine site I remember having seen an article about ” heat storing brick kilns” which directed the exhaust from the fuel burning to make bricks through a mazeway of thermal-storage firebrick material. Then a batch of bricks could be made using air pulled ” the other way” through the heated firebrick mazeway to feed the combustion of far less fuel than otherwise, because the feeding air was already so heated up by the heat from the firebrick mazeway, that far less fuel could be used to bake the next batch of brick. And that air went through another mazeway at the “other end” of the system. So the brick bakers could go back and forth in terms of direction of airflow into the burn-chamber for baking the bricks and then recovering most of the exhausted heat.

      So why not scale these methods up? Since humanity has indeed been using rock-thermal storage at the individual household and small enterprise level for centuries at least, and maybe even millennia, thereby proving the basic concept?

    6. Jay

      Because it’s a really bad idea for reasons the article skips over. Yes, you can warm up a rock by leaving in bright sunlight. But what then? Are you going to heat houses by carrying tons of rock to them every night and then out in the morning? No. And more relevantly today, how are you going to get substantial quantities of rock hot enough to generate steam and power a turbine?

      1. Joe Well

        I’m assuming the rocks would be on the roof, which are flat in most of the world, and also in many apartment buildings. Single family homes are usually very wasteful and bad for the environment and are becoming less popular in the US anyway.

      2. Joe Well

        Check out the comments by Plutonium Kun and others about how these are used in Ireland and Germany, and have been for decades. This has been very educational for me.

        We really are living in the [pun ] stone ages [ / pun ] in the US when it comes to this stuff.

  2. PlutoniumKun

    There have been studies of various types of rock as thermal storage goes back many years. This study is interesting, but not particularly new.

    Simple thermal storage systems are one of many very promising ways of storing excess energy from a variety of sources, but it seems to have fallen out of favour recently as other low to medium tech methods such as compressed air are closer to being fully viable. CSP systems usually use molten salt as a storage system to balance out loads over a 24-48 hour cycle. Using thermal storage would probably be more suitable for areas which require a longer time period – i.e. areas with more unpredictable sun.

    They are not, incidentally, a replacement or alternative for batteries. Battery storage is vital for shorter time periods- usually 2-3 hours. Thermal systems would be for longer term balancing. The article appears to be looking at systems where instead of generating electricity the rocks would be directly heated by a CSP system with the power then drawn off later. This would be very simple if it could be made to work, but I suspect it would only be a niche application.

  3. Phil R

    it’s easy to heat a rock sample to 1800 degrees F in a laboratory situation. I would like to know how they plan on heating rocks in situ at scale.

    Also, we already have this. it’s called geothermal. Only Mother Nature provides the heat.

  4. TomDority

    Yes – it is a good thermal storage system – Sand is also being used for storage.
    Seasonal storage (using summer heat for winter use) makes good sense and, IMO, the localized use of multiple systems of various energy production and storage avenues with some clawing of passive gains is more efficient than massive distribution models. Distribute the means of production and storage.
    Unfortunately….the modest costs of building into new construction or slightly higher costs of retrofit are now beyond the reach of many due to the asset price inflation, financialization and it debt bondage it creates. The economic system prevents progress and the economic system is legislated through congress.
    Conservation of energy, efficient energy use, minimization of use, minimizing embedded energy…call it what you want, but reducing use reduces storage needs and both reductions reduce costs……
    But, as I see it, the Free market has never been Free…it’s the shinning example of how gullible folks can be…. so tax the special the FIRE sector and make it hard to be a con man in this way.

    1. digi_owl

      As Hudson got me thinking recently, free market mean freedom for the usurers to ply their trade. As it frees them from oversight by “tyrant” (aka populist) sovereign that are liable to declare debts null.

      Basically all the modern economic language can be traced back to a certain period of England, when the merchant class found themselves in a position of power vis a vis the king. And it has twisted the meaning of worlds like democracy, tyrant and freedom to benefit the merchants/usurers.

  5. The Rev Kev

    Seems that finding rocks that have the right combination of materials may be problematical as there would be only so many deposits worldwide that are accessible and that can be mined. If true, then as PK says in the previous comment, it would be only enough to supply what would amount to a niche application. It could be useful in places where there is natural heat rising from the ground which could be diverted to heat those rocks – think Iceland & Yellowstone – but you might as well just tap that heat direct as it is constant. So unless I see this technology being used on an industrial scale, I would not be one to invest in it.

    1. Susan the other

      But if we consider that quarries might be greener than lumber mills and wall board fabrication then stone as a building material is both en excellent passive solar material and it’s wider use could eliminate the over-production of other less friendly materials. And it is available everywhere.

  6. Jack

    Interested readers might take a look at “trombe walls”, and Vermont soapstone stove company.

  7. Tom Pfotzer

    Water is excellent as thermal storage medium. It’s specific heat capacity is about 4, while rock is about 2.

    Specific heat capacity is a material’s ability to absorb heat while not getting hot (rising in temp).

    Here are some comparisons of various common materials, and a bit more explanation of specific heat.

    Collecting thermal energy is relatively simple; the technique uses inexpensive, common materials (aluminum, glass) and has few moving parts (a few valves, and a small pump).

    Using a water-based heat bank, which would likely be a well-insulated tank of water, and then using a heat pump to move that heat to the desired location (via HVAC system, for ex) is a well-explored, fully-technically-developed alternative to fossil fuel heating.

    That’s what ground-source heat-pumps do. The earth is a constant 55 degrees, so ground water that collects in a bore hole has a lot of heat in it. It has much more heat in it than winter air does (a gas, with low energy density, at 20 or 30 degrees F, for ex)

    Water has these advantages over rock:

    a. higher specific heat capacity
    b. You can move the water, and the heat it contains. Just pump it.
    c. Water is cheap, safe and ubiquitous
    d. You can use the water for other purposes, like a fish tank

    Moving heat is relatively cheap; acquiring the heat is the expensive part.

    So collecting and storing thermal energy in a high specific-heat capacity material is quite a good idea. Using water is a great candidate for heat storage. For those applications which require higher temperature levels, other materials like the minerals the article mentions have great potential.

    Note that the equation for thermal transfer (heat loss) is:

    (surface area of the hot-body times delta-T) divided by the heat-flow-resistance factor of any thermal barrier that’s present.

    Delta-T is the difference in temperature between two bodies (water tank and surrounding air, for ex).

    Heat-flow resistance would be provided, for ex. by insulation. Higher resistance values (“R-Value) is better.

    If you can store a lot of heat in a material without it rising in temperature, then you’ll lose less heat from radiation (an undesirable transfer-out).

    1. juno mas

      So here is an Energy Conservation Optimized (ECO Home that implements many of the concepts discussed above.
      I designed it in the late 1980’s and it was built in 1990. The house combines PV, high insulation (Rvalue), thermal mass (south facing kitchen tile), hydronic (water-based) heating system, and combines natural day-lighting and efficient night-lighting. The refrigerator was initially propane powered but is now ultra-efficient electric. All of these building techniques were taken from the likes of Steve Baer and others developing passive solar design in the 70’s and 80’s. Nothing new under the Sun!

    2. Piotr Berman

      A downside of using heated water for storing energy is that Carnot cycle requires high temperature for efficiency, and that would mean high pressure in a water based system, so there was research on molten salts and lead. Lead can be heated to amazingly high temperature with low pressure of its vapor, so you can heat lead at nights with a nuclear reactor that using lead as coolant liquid, and use the energy during days when the electricity demand is higher. That would make nuclear power plant that operates the reactor at constant rate, most efficiently, and produces the electricity in a manner adapted to daily demand cycle.

      A variant would use lead with some bismuth, so it can be easily melted after you stop the reactor but later you want to use it again. That has some problems (some bismuth is transmuted into polonium).

      The same could be applied into high temperature solar systems that use mirrors, store the super-heated liquid like lead or lead/bismuth for electricity production at night. In this case, no polonium is created.

      1. Tom Pfotzer


        I had in mind HVAC applications (lo temps) .vs. industrial applications or generation of electricity (need higher temps).

        Raising 100 gals of water from 50 degrees F to 150 degrees F takes about 82K BTUs, equivalent to about 1 gallon of propane.

        My 3K sq ft house uses about 3 gals of propane per day in the winter. 100 gals water takes up 3′ x 4′ x 2′ of space. That’s about half a chest freezer’s volume. I’d need three of these tanks to store enough heat to get me through an average winter day.

        I would use a heat pump to move the heat from the solar-heated water tank into the forced-air ducting that is installed in my house already.

        There’s no question this technique would work. The solar collectors take up space, but I have a lot of space; I live in a rural setting.

        I can make the tanks oversize, and the collector array oversize, and not have to buy propane for heating – either for water or living space – ever again.

        I spend about $2500 a year on propane. The payback period for this investment would be somewhere around 10 years or less, and the investment would be tax-deductible.

        I will be making this investment within a few years’ time.

        Note: nearly all the materials for the solar collectors are fully recyclable. They have very long (decades) service lives. The main fail point is the heat pump, with a service life of 15-20 yrs, and that pump’s materials are … recyclable.

          1. Tom Pfotzer

            Very good idea, and thanks for pointing it out (again).

            Sorry I didn’t see it first time around.

            Very interesting article, thanks for the link.

        1. Cristobal

          I know that I am a little late to the game, but I will put in my two cents. Having lived in houses with forced air heat I detest it. Waking up in the middle of the night when the blower kicks in ($$$$) is unpleaasant, and the heat is not so good – too dry, uneven, and noisy. I have thought a lot about a system very similar to what you are describing (maybe the storage tank well insulated and burried) but the main diffference in what I have imagined is the use of a radiant floor – hot water tubes (warm, really) installed below the floor with what might be a pretty complicated automatic thermostat-valve system to avoid hot spots and even things out. Even radiators. Good luck with yours however you do it, and let us all know.

    1. digi_owl

      One such in recent news was from a town in Finland, involving a sand filled silo hooked up top the town’s district heating network.

    2. Gordon

      Here is a link that describes that experiment.

      One advantage is that any sand, not just the sharp sand builders use which is in short supply.

      In the Finnish experiment it was linked to a district heating system which are common in some parts of Europe – but not the UK as far as I know.

      Strikes me a sand/district heating system would be ideal for utilising cheap off-peak or surplus renewable power.

  8. Bruce F

    Yves asks if you can use rocks to store heat, and if the idea scales.
    As other commenters have mentioned, its an old technology. About 5 years ago I built a masonry heater in the farmhouse, clad with rocks picked from our fields (we pick a multple tons every spring to keep them from damaging farm implements).
    Building the heater was a lot of work, but the result is impressive.

    So, I’d say it’s possible to do, with significant effort. At this point I’m not sure how this would scale.

  9. Heat guy

    The issue is how fast is the heat transfer to the storage medium.
    Molten Salts are used for the power towers that use heliostats, as they produce 1000°C. The heat is then stored in liquid form making it really fast to extract the energy to make steam and drive a turbine and make electricity as it’s a liquid.
    Rocks on the other hand while they can store large amounts of heat or cold are slow to release their energy making them a poor choice for electrical generation.
    For house heating it can work, but unlikely that you can store enough heat/energy for a winter. Because you’re not going to be generating really high temp heat as 500°c heat buried in a tank at your house is kinda dangerous.
    And storing low temp with thermal panels at maybe 150°f just isn’t all that much heat. And then for your house what are you using to produce that heat? A single 4×8’ good quality thermal panel can produce in perfect conditions maybe 10-15 kWh of energy. About the same as 1/3 gallon of propane. So you’ll need a lot of collectors and cubic feet of rock/sand to store heat. It’s been tried for years and doesn’t pencil out.

    1. heresy101

      This is just another fossil fuel attempt to replace PV and batteries with a Rock Hopium since Hydrogen Hopium will not be energy efficient and will fail like Toyota’s hydrogen car.

      Thankfully, a new battery is just about ready for mass production – the sodium battery. It is about 40% cheaper than lithium but less energy dense so it will be ideal for stationary ie home applications. There isn’t a shortage of sodium either given that 2/3 of the earth is covered with it. The stored electricity can run a heat pump or ground source heat pump in colder climates. Warren Buffett’s BYD and a couple of other Chinese auto makers are going to bring out sodium powered EVs at the end of this year.

      Rocks are a idea that will never pencil out. Geothermal is a different story and some new man-made geothermal projects are just beginning to work.

      1. Synoia

        A sodium battery?

        I seen to recall that Liquid sodium and water are nor a good mix.

        Water H20, Sodium S
        Mix – H2so4 Sulfuric Acid. Which is a water loving acid very destructive to skin.

            1. heresy101

              Sodium metal (Na+) appears to do what you say, but sodium battery manufacturers are not throwing sodium metal into water to make a battery! They are using anodes and cathodes for the circuit with a polymer liquid electrolyte to circulate the ions. The liquid is proprietary so it’s composition is not known.

              CATL, the worlds largest battery manufacturer, is soon to manufacture sodium batteries that a have a 15 minute recharge from 20% to 80% recharge cycle. I’m sure this will occur without exploding!!

              1. Piotr Berman

                Sodium is nearly as reactive as lithium… so testing is required. Lithium batteries do not explode if charged properly and not exposed to high temperature, but charging can be improper and temperature too high.

                In NYC there were many fires caused by el-cheapo lithium batteries like those used for bikes by delivery people. Thinking about it, sodium batteries that can be quickly recharged could be good alternative to lithium on bicycles if cheaper and safer. The fire incidents made it difficult to charge bicycles legally in multi-unit buildings. Now, bicycles propelled by the energy of hot rocks….

                1. c_heale

                  Sodium is more reactive than lithium (BSc in Chemistry). Sodium is also less toxic than lithium.

                  The danger with batteries depends on the chemicals present in the battery.

                  1. chris

                    And the casing materials used for the battery. Lithium batteries aren’t bad, but when the ceramic casing cracks they can start to bulge or experience other issues. I imagine it will be the same with Sodium.

  10. John

    The town of Okotoks in Canada uses a swimming pool as a heat sink. Basically, the heat from the sun is used to heat the pool using glycol radiators, the heat is stored in the pool, then a heat pump is used to heat the entire sports complex. (there is some supplemental heat from the ice plant for the Hockey Rink also captured)

    Up the street the Community of Drake Landing stores heat in an underground heat sink through the summer, then extracted to heat the community through the winter. This is all possible, but not done primarily because it is more expensive than Natural Gas

  11. Henry Moon Pie

    This kind of thinking will be of more value in the future than all the fusion hopium or “green” energy solutions being offered in the “climate space.” As the Energy Return On Investment continues to decline for fossil fuels because the easiest stuff to extract is already gone, we’re left with solar energy, and we’ll need updated technologies like this not to keep us flying to Paris when we’d like but to keep from freezing in the cold or becoming heat exhausted in the summer.

    Nate Haugens has produced a very good 30-minute animated video about how we arrived at this place and what changes will be necessary in our lifestyles for humanity to survived in the future. One thing I’d never heard before was Haugens’s point that we are presently converting oil into dopamine. That’s very true in this consumerist society where you are what you buy and where you travel.

  12. Wukchumni

    I had this nightmare that granite was the new energy source and Yosemite was mined until there was featureless flat land everywhere, with a superhighway going through what is now Yosemite Valley.

    It was even worse in Sequoia, aside from the groves, everything else was mined.

  13. Colin

    Hi Yves,
    As a moderately generous benefactor to your website, could I trouble you to please view a music video I made about my cat? It’s called Abby Rolling. If you don’t like it or share it, no problem as I’ll continue to patronize this website because the information you disseminate is just that important!

  14. Colin K. Ng

    Speaking of rocks, here’s a music video I made about my cat that rocks and rolls

  15. Oguk

    I read years ago about soapstone woodstoves, basically, which were scandiavian in origin. The idea was that though the firebox was dangerously hot, the soapstone distributed and retained the heat. The proposed design was something like the old central chimney design of 18th century New England houses. Seemed reasonable to try.

  16. New Okie

    I’ve been looking into using a gravel based heat sink for residential heat storage. A few thoughts:

    Residential vs Industrial:

    Residential and industrial applications are fairly different. It’s not practical in residential applications to heat the rocks up to temperatures higher than about 160 F or so (Usually somewhere in the piping will be PEX tubing and the highest temperature rating I’ve seen on PEX tubing was for 180 F. And you need some amount of temperature differential to drive the heat exchange. So if you had 180 F water the heat bank wouldn’t get hotter than maybe 160 F or so.)

    Additionally, the hotter the heat bank the more insulation that’s required to prevent unreasonable amounts of heat loss. Economies of scale obviously play a role here–three or even six feet of insulation is reasonable when your heat bank is the size of an office building because the surface area to mass ratio goes down. It’s not cost-effective when it’s maybe a 10 foot cube.

    On top of this if the rocks were heated to much higher temperatures–perhaps 500 F or so?–then in the event of an earthquake I would be concerned with any of those hot rocks which escaped the enclosure and touched something flammable.

    Heat Loss:

    While storing renewable energy as heat can be cost effective, it comes with a caveat: There will be heat loss. For example I calculated the heat loss for a 1000 cubic foot gravel heat bank. This one is a cylinder–other shapes will give you slightly different results. But assuming this one’s storage temperature is 135 F and the water must be at least 80 F to be used for space heating, there’s about 960 kBTU stored in it when it’s “full”. Of that, it’ll lose about 52 kBTU per day to the environment in the winter. So it works for short-term storage–maybe for five or so days. It doesn’t work so well beyond that, however.

    Obviously heat loss can improve a bit as the storage system scales up because, as I mentioned before, the surface area to mass ratio improves, which helps in-and-of-itself but also allows insulation to provide a bit more bang for the buck. However without seeing the math I’d still be skeptical of the economics of an industrial scale system that was going to store heat for a whole season.

    Rocks vs Water:

    I actually think there are compelling reasons to use rocks instead of water for heat sinks. Water requires more care than rocks do. Small cracks in a gravel bed heat sink are inconsequential–the gravel won’t leak out. Gravel doesn’t grow mold or algae and require regular use of potentially hazardous biocides. And finally, gravel can be contained with fairly durable means (ie a gabion wall or a foundation wall) whereas water requires a tank, and the tanks generally have a shorter service life than a gravel bed heat sink whose service life is basically however long it takes for PEX to decay (maybe 50 years from what I read). And as far as I can tell, the initial cost of these systems is fairly similar.

    On an industrial scale I think the practicalities would be similar, with the additional plus that you can heat rocks to a much higher temperature than water both because industrial applications can use molten salt (or…something like that?) for the heat transfer fluid and because, as mentioned above, it can make economic sense to insulate large heat batteries more.

    Final Thoughts:

    So this article, as other have said, isn’t ground-breaking as far as I can tell. It’s basically saying that some kinds of rocks can transfer heat a little bit better than other kinds of rocks, and as far as I’m aware that just translates to being able to spend less money on heat transfer pipes. Which can play a big role in the economics of the whole thing. But I don’t think it’s anything that people didn’t know the broad strokes of already. Still, I’m a big fan of gravel bed heat sinks as a way to capture renewable energy–at least in climates that aren’t too close to the poles (because that means that solar, at least, is not really available in the winter when you need the heat the most, so something like hydrogen or powdered iron that can be generated in the summer and used in the winter might make more sense, depending on the economics…which I don’t know much about).

    I’m not sure how much it mattered that one of the rocks broke apart under high heat while the other did not. Are they really suggesting that they would go to the effort of removing giant slabs of the rock rather than just using gravel or sand and compensating by increasing heat exchange pipes? I don’t know, maybe there’s some industrial application where that makes sense but I would have thought that gravel would be cheaper than large slabs of rock which, by definition, are more fragile and require a good bit of effort to machine into the exact shape required (whereas gravel fills a space more-or-less like a liquid). But maybe it’s just that if you start off with gravel and heat it until it breaks you wind up with sand, and that might create an engineering challenge?

  17. Tom Pfotzer

    Of that, it’ll lose about 52 kBTU per day to the environment in the winter. So it works for short-term storage–maybe for five or so days. It doesn’t work so well beyond that, however. “

    That depends upon delta-T, and the amount of insulation you use, right? If water’s specific heat index is twice that of water, your losses using the water method would be half that of rock, assuming the same amount of insulation, correct?

    For industrial apps that need high temps, I agree that rocks would be better.

    For household use, I’d probably go with water. I can get a 1000 polyethylene tank for about $1000. If I protect it from UV light, I think it’d last indefinitely. What would degrade it?

    Either solution will work, but in a home setting, I think I’d go with water, but your rock solution would be a close second. And thanks for the commentary.

    As far as algae and other biological agents in water, the water would be in a closed system, protected from light. No biologicals are going to grow in such an environment.

    === Separately…directed toward Polar Scientist above…

    If one were to use solar-electric panels, instead of solar-heat panels or your wind-mill idea, then the electricity could be used to power nichrome-wire resistance heaters immersed in the tank. Those heaters are almost 100% efficient (converts all elec to heat). They don’t corrode; I’ve been using stock-tank heaters (nichrome) for years, and they show no corrosion.

    That solution would work well in both rural and suburban settings with decent sun exposure.

    That solar-electric method confers a lot of flexibility. Use the elec power to gen heat, run the heat pump, run the house, charge the car battery, or sell back to the utility (least econ return to homeowner).

    1. Tom Pfotzer

      Oops. Should read: “If water’s specific heat index is twice that of rock”.

  18. Curt Chatelain

    Piotr Berman is 100% correct about Mongolians cooking with stones. A full description of “Khorkhog” is available on Wikipedia.

    I live in Mongolia and looking for a heat storage system that will enable me to accumulate and store solar heat for the purpose of heating greenhouses on my farm. Solving this need with enable Mongolians to grow vegetables during winter and reduce the amount of imports.

    All ideas and referrals are welcome. Thanks

    1. Tom Pfotzer

      Curt: what is your heat collection plan? Is the thermal mass inside your greenhouse? How much sun do you get during winter (full sun, hi altitude?) Do you have drums, barrels etc. for storing water?

      How will the heat get from thermal mass to your veg? Convection from the thermal mass, or are you piping warm water into the root zone? What crops? Hardy winter greens (like Bok Choi) or what? Veg varieties differ a lot for heat and light req’mts.

      How is the heat to be conveyed to the thermal mass (the stone)? Direct sun exposure, or some sort of collection device?

      Give us more details so we can make suggestions. Tell us what light intensity, materials, and equipment are available / affordable, and what your greenhouse type is (high tunnel, earth-bermed, etc.)

      If you have full design flexibility, then use a bermed (dug into hill) wall with major thermal mass (water, stone) painted black. Greenhouse film/glazing faces sun for max thermal gain. Use adequate (a lot) of ventilation tools to exhaust heat that isn’t getting stored in to thermal mass.

      Consider a removable cover (blanket) that can be put on / deployed at night (max thermal loss).

      Those are a few ideas. Give us more detail so we can help.

      1. Curt Chatelain

        Hi Tom:
        Thanks for responding. I shall do my best to answer your questions:
        Currently, we have two summer [plastic] greenhouses. We experimented last winter with one 55 gallon barrel filled with water inside one greenhouse. As expected a noticeable difference in temperature between the two greenhouses was recorded.
        There are 230 – 260 clear blue sky sunny days in Mongolia. It’s high latitude (1,400 m) also means that during the summer it gets a lot of sunlight. The yearly total is 2600 – 3330 hours of sunlight a year!
        Our plan is to construct/modify a solar [Chinese design] greenhouse. This design has a large brick thermal backwall, a low plastic cover with an adjustable thermal blanket cover. This combined with water barrels and other strategies common to other cold climate countries can be modified to suit Mongolia. For us experimentation is the name of the game.
        The reason for my query to this blog is to create a thermal collection/storage system that maintains a warm temperature in the soil during winter. Convection features such as, thermal walls, water barrels and a blanket cover during winter nights are great. However, we are willing to push past these “knowns” and seek systems that can create more heat during winter.
        A bermed greenhouse is not a practical solution. Such hillside locations are available but will limit our flexibility to expand our business in many locations throughout Mongolia.
        Of course, leafy greens are suitable for growing during the winter. We want to increase our growing season and the types of vegetables grown in winter.
        We have access to equipment via China. For us it is important to know what to purchase and how the construct/assemble the heating collection/storage system.

        1. Tom Pfotzer


          I am familiar with the Chinese design greenhouse, with thermal cover and heat bank. I am currently experimenting with a greenhouse that uses several of those concepts, and pushes the performance envelope a bit further.

          Here are a few ideas I’m experimenting with:

          a. Put the water tanks under the plant’s root zone. I am using foam-crete and plastic liners to build water troughs that hold water, and serve as benches to grow plants upon. The variant to that idea is to run pipes thru thermal mass under the plants, and grow in the soil over those pipes. I like the trough idea because it gets the plants up in the air, where the humans are (less stoop labor). The troughs could ultimately serve as fish habitat if they’re big enough

          b. Use a thermal mass heater to provide boost heat during esp. cold days or cloudy spells. If you have wood available, that’s a viable option. You may not.

          c. Use supplementary outside-the-greenhouse thermal collectors. You may be able to build these yourself, if not, there’s plenty of Chinese mfg’rs that can do it for you

          d. Consider hydroponics if you haven’t already. Much faster crop cycles and better plant nutrition than growing in soil. Nutrient tanks can double as thermal storage

          e. Use more insulation. The Chinese gh designs I saw only had limited-thickness insulation on the top (where all the heat leaves from) because they roll it up (weight and bulk restrictions). Find a way to add more insulation on the top.

          f. Plants are excellent solar collectors. During transpiration, they convert incoming light to heat (both sensible and latent) and load that heat into the air column above the plants. Consider using heat pumps to pull that heat out of the air column and put it into your (water-based) heat sink. Then you don’t have to vent all that valuable heat into the atmosphere during the day when it gets too hot and humid in the greenhouse

          If you wish, find me on the internet, get in touch, and we can talk more.

  19. redleg

    The author might know something about heat capacity, but almost nothing about rocks.
    Soapstone is a layman’s term for a soft , nearly featureless rock that can be carved. In terms of petrology, it could be nearly anything. Clay minerals (smectite, illite, montmorillonite), metamprphic minerals (talc, chlorite, lizardite, graphite, etc.), evaporites (gypsum, anhydrite, halite, etc), and carbonates (calcite, aragonite, dolomite) can all be found in “soapstone” but react very differently from each other when heated.
    Same thing with granite. What are the minerals? How big are the crystals? How weathered are they? Does it have a fabric or fractures? Etc. So many variables for one rock type, not even getting into whether the granite is actually granite, a very specific kind of igneous rock; or “granite”, a layman’s term for any crystalline igneous rock that has more than one color of non-metallic crystal.
    The article might be describing something potentially useful, but the rock descriptions are useless to anyone with a geologocal or geotechnical background.

  20. chris

    It’s a great idea but not exactly plug and play. I’ve dealt with home owners who accidentally created similar systems when they installed a massive slab of black granite in their kitchen for their island, in full view of the sun, in a custom home outside of Tucson. They needed to install a cooling system for the granite slab because of all the solar heat gain.

    Also, consider Trombe walls.

  21. Tom Finn

    Having worked in the Solar Energy Field in the late 70’s (Zomeworks/Steve Baer a passive solar manufactory with Baer one of Solar’s pioneers). I’ve watched the resurgence of solar out of the ashes which Reagan left.
    The unspoken question in this article is one of Centralized Power vs Individual. Centralized serves the community but requires storage to even out distribution. Individual/homeowner also may require storage, i.e. batteries for nighttime, which can be circumvented by tying into grid where possible (a good thing) but what is overlooked these days is passive/active heating and cooling which are readily available but do require some activity on the part of the homeowner and design considerations by architects and developers which is nigh nonexistent.
    Rocks as a storage medium (for the individual) for heat or as a heat sink for cooling are, following water a viable and sometimes preferable choice.
    Rocks in the super heated applications required by Centralized energy generation May or may not be optimal and like much of what I see with solar the research may be redundant or over ruled by what was done pre Raygun.
    The numerous comments about exploding rocks, common sense where sweat lodge participants are concerned, is simply silly hand waving.

  22. William Verick

    The traditional bed used by North China peasants is called a kang. It’s a platform made of clay with a flue running from the foot of the kang to the head. At the foot is a small stove and at the head is the chimney flue connecting to the kang and going up the chimney. In the winter a fire is built in the stove, which heats the clay and keeps the bed warm almost like an electric blanket. The heated clay probably also keeps the house warmer.

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