Making A Do-It-Yourself Sand Battery

Storing energy can be done in many ways, with the chemical storage method of a battery being one of the most common. Another option is a thermal battery, which basically means making something hot, and later extracting that heat again. In this video by [Robert Murray-Smith] the basic concept of a thermal battery that uses sand is demonstrated.

By running a current through a resistive wire that’s been buried inside a container with sand, the sand is heated up to about 200 °C. As [Robert] points out, the maximum temperature of the sand can be a 1000 °C or more. Because sand doesn’t boil like water, the total amount of energy stored in sand is correspondingly higher.

Extracting the thermal energy can be done rather inefficiently using the demonstrated Peltier element. A Stirling engine, or steam generator and turbine, would get a lot more energy out. Either way, the thermal battery itself is made using just plain sand, which makes it an attractive DIY target to tinker with.

87 thoughts on “Making A Do-It-Yourself Sand Battery

      1. Coming in at ~ 0.8 J/(g*K), one needs a 5x temperature increase to store the same amount of energy as in water (gram per gram), but this also means that most of the total stored energy can be extracted as higher grade heat (with more favorable Carnot efficiency).

          1. A small spherical `boiler’ with water trapped permanently inside could be heated above the critical temperature of water to have super-heated steam inside (the wall would have to be strong enough to withstand that pressure). This `heat nugget’ would store heat both from the specific heat as well as the latent heat of water. A collection of such elements could be used like pebbles, to replace the sand with great storage capacity.

          1. How is it ‘self insulating’?

            I’m building a solar-powered sand battery (for heating) in a 60L steel barrel and it’s hit over 500C – I am scared to have it too close to the house.

            I was considering a 200L barrel filled with sand, aluminium ventilation pipes and the 60L barrel with its element inside that, both inside a rockwool-lined box with temperature probe to turn the solar off if it exceeds 550C

    1. The specific heat of sand is 0.83kJ /g K, so it is 5x less than water.

      However, that is only part of the story.
      Water can only be heated to 100°C, Sand be used 10x hotter, so you can easily make up for this.

      There are other things to consider as well.

      1. Water can be heated way past 100°C if it’s contained in a pressure vessel, that’s how steam engines work. They can only extract energy in the vapor phase, which effectively becomes the difference in temperature between when the steam enters the engine and when it is exhausted, ideally juuust above it’s condensation point.

        The biggest issue with steam is that you have to get it out of the engine before it condenses, which means you either throw away all the energy you put into the phase change from water or come up with a complex way to reheat the cold steam.

        A Stirling engine could probably work with hot water, but since they were never widely adopted, I’m assuming there was some kind of inefficiency in using them that I’m not familiar with

        1. The main problem Stirling engines have is that when they’re competing with steam engines, gas turbines, internal combustion, and electric motors, one of those four usually has a decisive advantage. Stirling engines (and the related Ericsson cycle engines) still have an edge when you need to extract work from a tiny temperature difference though and would make sense here. Plus, you can use the engine as a heat pump to warm up the sand.

          1. That’s an important contribution. IIRC, creating heat through resistance takes roughly three times the energy compared to just moving the equivalent amount of heat from one location to another. If a Sterling engine can run as a pump to heat the sand at better efficiency than resistive heat, that’s a win.

          2. The stirling engine has the main problem that all the energy has to be put in through a heat exchanger, and the waste heat on the cold side out through another heat exchanger. These heat exchangers are much more expensive and voluminous than a steam pipe or a fuel and a exhaust pipe or a cable, which carry the energy into the other mentioned machines (and eventually a good part of the waste heat out)

        2. Some steam engines are or at least were low pressure steam, so the water temperature for them isn’t all that high if actually over boiling at all. Not that I’d advise anybody to actually try to build one of those now…

          Stirling engine can be pretty efficient, but with how they work steam wins out historically in my opinion entirely because of how controllable the output power is – you can turn a valve to let more/less flow of steam out, fresh water in and control the heat. With the Stirling engine the only control you have at all (at least in all the versions I’ve ever seen) is the heat input. So now when the Stirling would likely be connected to an electric generator and some form of energy store before that electric is then used its not so important – but historically a steam locomotive that can be directly and easily stopped and started..

        3. Water also leaks easily since it’s a liquid, is quite dangerous under pressure, and gives off its heat rather quickly. Sand on the other hand is easily held in a container, even with holes, and it holds heat for hours after it was heated. You might note that even though according to your criteria water is “better”, molten sodium is being used commercially for the same reasons that sand is.

      2. so what about using a solar panel to heat the sand have a hot water coil in that to pre-heat your domestic hot water before it goes to Hot water heart ( oild fired or electric ) would this make it more efficient would that make a difference if the water going in lets say 42 degree’s and you can even raise it to lets say to 60- 65 degree’s would this not be a solution as they always say a 1/3 of our energy goes to heat domestic hot water ?? Just say may be a valid use I couple of solar panels that can drive a 12/24 volt hot water heater in the sand and a copper coil in there to circulate the water.

        1. It leaves the issue of the water boiling in the pipe if the sand is over 100C. It would work if you ran the water through say top down through a coil, but you wouldn’t have much temperature control. It also leaves the issue that the pipes would conduct the heat out of the battery when not in use.

          1. I’ve been considering these problems as well. I have decided to go with several direct contact solar heating elements for heating the sand and I was wondering instead of running water through a loop to extrsct the heat could we not do some kind of direct heat conduction through a solid copper bar from the sand battery to a water tank to get heat into the water tank, this would bypass pressure problems altogether it would be slow heat transfer I guess but my application is underfloor heating so only 40 celsius required to circulate to the floors.

        2. Ok, I know this is an old post and I’m no engineer but maybe someone here could help me.
          I want to use a sand battery to heat a greenhouse and a 10,000 gal Tilapia fish tank without digging deep, long trenches.
          The greenhouse is A 20 ft wide and 80 ft long poly tunnel. The pond will be 4 ft wide 4 ft deep 60 ft long and be placed in the center.
          My idea is to dig 2 ft deep install concrete wall footers then dig an additional 3 ft deep inside the footer. This will give me a 3’x3’x60′ base to lay sand. This will have coiled tubing throughout its length. 4 ft wall block wall will then be built and a 4 in. Concrete food laid (capping the sand battery). I might need to insulate between the sand and Concrete floor.
          I want to use the North wall as a heat the water (or other liquid (???)) much like the solar water heaters people use on their house. This will heat the fish pond as well as store heat in the sand. Then at night use the pond water to keep the greenhouse warm the first part or the night and as things get cold in the am hours pump the water from the sand battery through the panels and use them to radiate the heat back into the greenhouse until morning.
          The excavated dirt would be used on the sides of the pond both raising it above grade and insulating it.
          Would this work or would there be to much heat loss for such a small battery? I’m thinking the water battery combo would work together to keep things warn in 30* F winters.
          Any suggestions?

          1. Six inches of depth is needed per month of recovered heat. In other words, November, December, January, February would require 2 feet of earth.

            Hire a trenching machine and cut down to 3 feet and you can store the heat you need plus some extra. Hiring someone allows you to not dig it yourself.

          2. Hey Shane. You sounded like you were serious on making something work here. I’m interested on how you got on. I have a remote manufacturing plant with power restrictions, where I need to heat 2000L per day to 90degrees. I am currently designing a solar/wind charged IBC based sand battery system which can be daisy chained with multiple units. The hot air would be run through a converted commercial boiler coil exchanging the heat into the water. No pressure, open system, no risk of explosions..
            If anyone has made something work, I would be interested to hear of any pitfalls.

            My inspiration comes from these awesome Finish dudes.

      3. Sorry…I know this is an old thread, but at 830j/kg deg K, and let’s say, 4 metric tons of sand to heat to 400deg, exactly how much generated energy will be required, and over what period of time. I’m no expert, but that is going to take a very long time with your average 4kw solar set up?

        1. Harvesting dividends from fluctuations in the cost of daily energy price may prove possible with a thermal storage. Energh sources like nuclear, wind and solar fluctuates, and price policies aim to focus our energy use to comply with peaks in energy production which is likely to differ from My personal energy peak time. Obviously heating is in fucus here. So: Buy, when heat is low price, and store in thermal storage to use when needed.

    2. Idea is you can accumulate energy in sand via heat from excess wind or solar generation, then tap that “heat sump” to do things like provide supplemental heat for a building, hot water, or power generation via steam or other tech. There was research into using molten salt to store this type of energy too, but for many, sand is free.

      1. Long ago Scientific American had a story on a solar power plant that used focused energy at 1200C to crack a compound (in the presence of a catalyst) into two constituent salts. The molten salts would need to be kept above something like 300C, but when recombined the reaction would produce 800C. So the plant would generate electricity on demand, and store excess energy in the form of two molten salts; and at night it would combine them to continue producing energy until the next day (and to keep the salts in the molten state).

        I’m pretty sure this works great in the desert areas of the world, where there are no clouds nor neighbors to worry about the large tanks of molten salts rupturing. I just have to wonder about the durability of the equipment to contain and circulate molten salts for years on end without corroding or seizing.

        1. They spent a fortune building one in SCal. You can see it from the highway that runs to Vegas.

          It’s bankrupt and idle. Can’t generate enough power to cover it’s maintenance costs. SoCal Edison (and their partners) will sell it to you, cheap. You get the debts with it though.

    3. Pardon me but what about how long sand holds heat without having a power source to maintain it. So if you subtract that amount of energy from the equation doesn’t sand win and the water boil off and the sand still remains so now you have the added expence of piping in water.Seams like water is pretty unsustainable because its finite only so much water. I’m not an engineer but I do have common since please tell me if I’m wrong and why. Somone mention pressure vessels well you don’t need all that for the sand thats another expence and we already have steam turbine power plants. Adding water to sand woudn’t the water boil away wasting energy and the sand never reaching 10x of water if you keep on adding water. I’m sorry but hasn’t all this been done with water and steam before? Don’t you just get to the point where the heat from the sand just turns the water to steam. Whats the point.

      1. Holding heat for any substance is almost entirely down to external factors like quality of insulation surrounding it and the temperature differential between it and the outside world. There is a little bit of the substances own material properties in there – the how well does it transfer heat through itself for instance, and just how much energy is it holding.

        And in some cases there will be a deliberate desire to generate steam, adding water to the hot sand in its pressure vessel would be a good thing – its a good way to get mechanical energy out of thermal energy stored in the sand, as steam engines/turbine are really well developed and efficient technologies. And the water doesn’t have to be lost, it is possible to condense and recycle it there if you wish to – but water is so abundant on this planet I’m not sure its worth the effort most of the time.

  1. I used to be a fan of Robert Murray Smith’s channel, and to a point I still am (I still subscribe, but rarely view). The main issue I have is that he touches on topics and does a tiny bit of experimentation, and then… moves on! For example in his wind turbine experiments, he measured open circuit voltage and said “See, it works!” and discuss the theoretical amount of power that could be extracted, but stopped there. I suppose for some people, seeing an open voltage reading is good enough, but it doesn’t satisfy me at all. I’d love to see it doing real work.

    Just as in this video- putting the working into showing it works, but a *Peltier* junction for energy extraction? I realize that just about anything else is vastly more complex, and it proves that it’s a working concept, but so many of his experiments never reach beyond the low hanging fruit. I think he’s missing out on a lot of potential.

    Pun intended.

    1. I agree. As I said in the Minecraft coal-fired thermoelectric generator article comments, the TEG is INCREDIBLY hard to get working well. Heating up some sand and hand waving your way through converting that heat to usable energy is disingenuous or uninformed.

    2. Yeah, unfortunately he’s not too far away from a scam artist…. As you followed him earlier I believe you have seen his tentative with the carbon based battery that never came of the ground because the effective capacity of that thing is very small. Many have asked for the numbers of his measurements but he never delivered anything related to the capacity because he knew it was bad.
      He was too deep into it to stop it. People where buying loads of stuff from his ‘lab’ to do experimentation on their own.
      At the time he also got some of is friends on board and they even tried to sell the ‘technology’ to another company, but those guys where smart enough to ask for a certification from an independent lab before jumping on the wagon.

      In terms of entertaining people looking out for experimentation he’s got very good ideas and can bring it in a very understandable manner. I hope he stays in that area and doesn’t over-promise anymore.

    3. A generator with an open circuit becomes essentially a tachometer. Would tell you to use the shortest lowest pitch blades possible for highest voltage.

      He seriously didn’t know to use a load resistor?
      That’s just fail. Like F on assignment in middle school.

  2. Put the sand in a bucket on a cable connected to a motor/generator and you have a “gravity battery”. Better than 80% efficiency, 50 year service life and costs $0.17/kWh.

    1. The biggest issue with a gravity battery is that lifting stuff stores a surprisingly small amount of energy

      Raising a metric ton of material ten meters stores about 27 watt hours of energy. That’s enough to brightly light a room with a couple of mid-sized LED bulbs for an hour or completely recharge a drained laptop, and that’s not a trivial thing – but you still have a one-ton weight and a thirty foot tower involved.

      Gravity storage isn’t really super practical unless you have a LOT of mass to lift or a conveniently placed mineshaft handy.

      1. Or are just using it to shift load/supply peaks a small amount – for instance if there is a huge renewable peak and every tall building in the area has elevator that can regen on the way down you might as well drag them all the way up – a few hundred, maybe few thousand elevators up potentially a few hundred meters, but probably more like 30 meter adds up hugely. It is still only going to be stored for a short time, maybe helps carry that reliable evening breeze and sunshine energy on towards morning, but its still energy stored effectively at no cost.

        1. Lifts don’t work like that, though. They’re counterweighted, so the motor only has to lift/lower the weight of the people in the car (plus overcome friction in the system). A lift at the top of its shaft has no more potential energy in the system than one at the bottom.

          1. There are regen lifts out there as well, counterweights are common, and were entirely required to be nearly as heavy as the lift itself when motor power and efficiencies were lower. But now some places do infact run regen systems with little or no counterweight – as its possible. Some place though I can’t remember where trialed some sort of automatic weight changing system too to further improve the energy storage potential.

      2. But a cubic meter of sand (5 tons 3ftx3ftx3ft) in a bucket inside a 2 story house (10 meters) would run your laptop for 5 hours or your cell phone for a couple of weeks (in an emergency) and you could build it with off-the-self parts. Put a 10 meter shaft in your basement to double that. [Not so good in an earthquake.]

        A cubic meter of hot sand not so much.

        1. Lifting 5 tons of sand on an emergency is harder than to keep a small powerbank charged all the time to use during said emergency.

          If you lift that bucket, you will probably have the weight spinning a generator. Strapping a bike to that generator would be more practical.

          1. I don’t think it would be that hard, a 5ton chain hoist should do it, although it wouldn’t be quick, or quiet. Although I think the assumption is you would have your 5 ton gravity battery already “charged”/lifted so it was ready to go in an ’emergency’.

        2. “Oh.. Hi Honey. Um… You’re home early..”

          “What am I doing? Oh.. nothing. Just ripping out a couple of our closets to build an elevator shaft for this 5 ton bucket of sand so I can store as much energy as an old car battery”

          “Wait… Where are you going?”

          1. To be fair, she was storing shoes in the closets until she decided engineering was easy.

            Give her credit for going outside her comfort zone, don’t break her spirit.

        1. online in 1972 and IIRC it is still operational today. And 1,143 MW for 8 hours is impressive. I do wonder how efficient it is today relative to battery packs. But would they last 50 years is the question.

          1. Back of the envelope calculation would be that lithium batteries have about 270Wh/kg, so for the 9144MWh this provides you would need about 37 tons of batteries (plus the wiring, connections and circuitry)

  3. If you match the thickness and material type of your walls such that the time for the peak heat of the day to travel into the interior is offset by 12 hours you will have a far more stable interior thermal environment as the structure itself acts as the battery. You can apply the same principle to the yearly cycle of temperatures too. This is a key part of a low technology passive house design.

  4. If somehow you have way more energy around than you are using(does that describe anyone at all?) dump it into the hot water heater. Then directly use hot water. Minima conversion steps and losses. There is no practical use for hot sand for most people.

    1. Since the vast, *vast*, majority of electricity comes directly from generation and not storage, basically all grid-based electrical systems have more energy around them than what they are using. Without storage, the options are either adjusting generation to meet demand (not using all the energy you have available) or brownouts.

      Now, in most cases (fossil fuels and nuclear), that energy that’s available and not being used is actually stored in a more useful form than hot sand. However for renewables, the case of having more energy than needed and not being able to store the energy source (sunlight, wind) is quite common.

      I suspect that the choice of hot sand rather than hot water comes down to being able to store energy at a higher temperature (which allows for more efficient and/or different uses) more safely. While it would be easier to use hot water directly, there are plenty of use cases in which a temperature greater than boiling is needed, and 200C sand is easier to deal with that worrying about a steam explosion.

  5. I followed that guy for years and years, and while he seems like a genuinely nice guy; he’s reminds of that outfit from Aus, Green and Gold I think. Solar scammers.
    The guy is a legit chemist IIRC, however he’s had so many ‘companies’ and world shattering advances that all seem to end up shuttered after hyping products on his various e-stores.
    Never would give proper details on volts/amps/watts/time on his various inventions. Always replied with ‘buy some and you can experiment yourself’.

    1. Yes, you are completely right. I used slightly different words in my other comment but what you say is exactly the same.
      He’s got a lot of potential to entertain and so on but unless the viewer is somewhat literate in physics and can see through it, that guy is setting unrealistic expectations and sells his stuff like that.

      1. Regular paraffin wax. It’s the heat of fusion that powers it, not the heat capacity.

        It’s what is used in all the different scammy ‘keep your coffee hot for hours’ cup projects. Too much wax, not enough coffee to be useful.

  6. Building heating is a huge carbon hit in Canada and expensive to convert to heat pump, and little or no solar production in winter to run it. The Finnish sand batt stores heat for months, then uses the heat to heat homes & businesses. I’m here looking for help designing a scalable sand heat store for heat pump or bio-char kiln input and hot air recovery later. Self-discharge of course would be dependant on insulation integrity. Separate stainless tubing bundles for input/output in a barrel? Could start small for greenhouse application.

    1. From my reading these heat store systems in the real world do follow the expected math rather well, and so work rather better at larger scales than small, and once you get small enough in practical terms it becomes rather less useful (the internal volume vs surface area to loose heat, the lack of thermal mass to volume taken up as you need so much insulation etc). That said I don’t know how small your ‘could start small’ would really be, I understand much of Canda gets really damn cold for a prolonged period so you may well be thinking big enough to become really practical in the real world. Once you start getting into really small scale battery start to look much much better – as long as they are inside the area they are heating, as generally battery don’t like cold.

      Sounds like you are going to want to put a great many tubes into your sandbox for the air paths – both the hot air recovery and heating from kiln exhaust sound like air as the heat transfer medium. Which means you are going to be effectively building a steam boiler but filled with sand – actually a good thing for the ‘scalable’ factor you wanted – create whatever size unit you like and all you have to do is plumb more in series or parallel to expand the system.

      I would expect not the most efficient way to heat the sand, but if there are enough tubes and your exhaust is hot enough it could work – I suspect it may be better off heating water directly with the kiln – good for immediate use and can then run a heat pump from that so the sand can actually get hotter than the exhaust gas, or just heating the sand with the good ol’ resistance wire. But without a great deal more detail on the plan and no real understanding of the demands on the system hard to really give any good answer.

  7. Is it possible to use thermal oil as heat transport that lead the heat in the sand battery via copper pipes to a water buffer tank. just as it happens with a solar collector. So the problem is that you don’t let it rise above 90 degrees in the buffer tank. How to fix that?

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