A Sand Battery: Not obviously a great idea.

Friends, a few weeks ago I was asked by several people whether I had heard about the latest ‘sand battery’? I had never heard of any such thing.

Most people asking me had come across this article on the BBC which gushingly described a heat storage device – not a battery – that had been built in Finland.

I quickly checked that I correctly understood the meaning of the word ‘battery’:

And I then concluded that this was a deliberate ruse to make a thermal storage system sound more interesting. No need – I love thermal storage systems!

Genuine Thermal Storage ‘Batteries’ do exist and Rosemary Barnes visited one on her Engineering with Rosie Channel a year or so ago.

In the Stiesdal company’s system that Rosie visited, excess electrical energy is stored as heat in an insulated container – similar to the sand battery. But instead of just using this stored thermal energy to directly heat homes (as the ‘Sand Battery’ does) the heat is used to drive a generator and make electricity. So it functionally, it operates like a conventional chemical battery – storing and releasing electrical energy.

In the video, Stiesdal claim overall efficiency is expected to be ~60% but the discharge function produces ‘waste heat’ that could be used for district heating, bringing overall storage efficiency to ~90%. In other words it can do what the sand battery does AND generate electricity.

But as will become clear, I suspect that efficiency is only for short term storage – a day or two – rather than genuinely seasonal storage. Anyway: back to the sand ‘battery’.

Sand Battery Details: How long can it store energy for?

The BBC report that:

Sand is a very effective medium for storing heat and loses little over time. The developers say that their device could keep sand at 500 °C for several months.[my emphasis]

Colloquially this sounds plausible: we are familiar with ceramic materials retaining heat for a long time. But I am sceptical.

Why? Because there are no perfect thermal insulators, and the rate of heat loss from an object is proportional to the difference between the object and its environment. So for an object at 500 °C, heat loss will likely be a serious problem.

I wondered if there might be a better way to do this, for example by choosing a material with a larger heat capacity. Then the same thermal energy could be stored in a medium at lower temperatures – and hence have lower heat losses. The storage material would also need to be cheap so I wondered about something really cheap: water.

Water? Yes. A given volume of water stores three times as much heat as sand heated to the same temperature. However (without pressurisation) water is limited to being heated to 100 °C.

Alternatively, one could say that sand is so poor at storing energy that it HAS to be heated to high temperatures to make it even half-way useful. But at high temperature it will lose heat faster. And storage times of months seem unlikely to me.


Friends, I made a calculation!

I calculated the storage time of two storage vessels both storing 8 MWh of thermal energy (like the Finnish design), one storing sand at 500 °C and the other storing water at 100 °C.

To understand the scale of this system:

  • A typical domestic hot water cylinder at 60 °C stores around 7 kWh of thermal energy so we are envisaging systems roughly 1,000 times larger than a domestic ‘thermal storage’ system.
  • My house requires approximately 4,500 kWh (4.5 MWh) of heating over a winter, so 8 MWh of thermal storage could store enough energy to heat perhaps two homes over winter.
  • Yes, I said two (2).

I fixed the height of the vessels at 7 m (as in the Finnish design) and varied the diameter of the vessels to store enough substance to store 8 MWh (as in the Finnish design). The water vessel was 3.8 m in diameter and the sand vessel was 2.8 m in diameter.

I assumed both vessels were covered with 300 mm of mineral wool insulation with a thermal conductivity of 0.03 W/m/°C, and then calculated the time constant over which the vessels would lose 50% of their stored heat.

Click on image for a larger version. Graph showing the rate at which two vessels (see text for details) would lose their stored energy.

  • The sand vessel would lose 50% of its heat in 2.1 months after which the water would still have retained 84% of its stored energy.
  • The water vessel would lose 50% of its stored energy after 8.4 months after which time the sand would have lost 94% of its stored energy.


Friends, I hate this kind of news story. 

This ‘sand battery’ can store enough heating over winter for 2 houses – neglecting losses – or more likely one house accounting for losses.

Community Thermal Storage – may well make sense in some contexts. Indeed I have seen other implementations of similar ideas (can’t find the links at the moment) that don’t require very high temperatures and hence high heat losses.

But this story reads like a BBC correspondent swallowed a PR story from Finland and then regurgitated it all over the BBC web site.

Inter-seasonal storage of renewable energy is an important technology that will be required if we want to build a system capable of supplying year-round energy from sustainable, but intermittent, sources.

But there are many alternatives. The Stiesdal implementation returns energy as electricity rather than heat, which is much more useful. The electricity could then be used to run heat pumps which would boost the overall efficiency of the system.

Alternatively energy could be stored as green hydrogen, or as pressurised gas. It is still not clear to me which technology will prove optimal: in the end it will come down to cost.

So it’s a complex situation, but facile stories like this do not help anyone.


71 Responses to “A Sand Battery: Not obviously a great idea.”

  1. Peter S Says:

    Did you take the self insulating properties of sand into account? I’m refering to how the cooler outer layers of sand acts as insulation for the hotter inner core, in contrast to water where convection causes the entire water mass to be in thermal equilibrium. To me that seems to be the big advantage of sand, especially at larger scales since that also scales up the thickness of the insulating sand layers.

  2. protonsforbreakfast Says:

    Peter S,

    “Did you take the self insulating properties of sand into account?”

    No. If you look at the embedded video there is a short discussion of the effect of particle size.

    Small particles – like sand – require high pressures to pass air through the tiny gaps in the sand.
    Larger particles – like say railway ballast – are much easier to pass air through.

    But the time constant of a sand grain is probably a few tens of seconds or maybe a few minutes at most – much less than the time constant of the container as a whole.

    The thermal contact between the storage medium and the heat exchange medium limits the rate at which heat can be extracted and it is not obvious that small particles will be optimal.

    Water – because it can flow – can have excellent heat transfer properties with pipes.

    I am not saying that water is better than sand, just that sand isn’t actually very useful and not obviously better than some alternatives.

    Best wishes


  3. pdrezet Says:

    I would be very grateful for some feedback from the author/comentors on a beta thermal store calculator that we developed for “community use” – It’s a generic tool that deals with sensible heat only systems currently and lets you dial in ToU tarifs and compare overall costs with heatpumps.

  4. Mr James Fletcher Says:

    It appears that your graph doesn’t take account of the lowering of the temperature of the sand as heat is actively taken out of the system, its just a heat loss graph for sand in isolation to any depletion by heat demand, is that right?

    • protonsforbreakfast Says:


      Yes, That’s correct. It’s just simple heat loss through insulation.

      Best wishes


  5. Lore Says:

    500 Celsius degrees can be used for far more (industrial) applications than 100 C water. I guess thats the whole point.

    • protonsforbreakfast Says:

      Yes, indeed.

      But this application is for inter-seasonal storage of heat for domestic heating, and 50% of the stored heat is lost in just a couple of months.

      Presumably the people who have built it appreciate these things, but the article presented it in such a way that it seemed like a panacea. Together with the fact that the giant silo only stored enough heat for two houses, I felt that the article was a bit misleading.

      Anyway. Best wishes, Michael

      • eenjones Says:

        I completely agree with this post and the comment and response here. By way of illustration: I’m considering a similar solution for a district heat network that is delivering high temperature steam (circa 180degC) to an industrial process. The supply is waste heat from another industry; the supply and demand both fluctuate in a similar range but not in sync with each other. So some short term storage has the potential to greatly reduce the use of back-up fossil-fired systems.
        More broadly, this whole article underlines what an unusual substance water is. It has many unusual properties but its high heat capacity is a major factor in the natural environment, its presence greatly moderates the weather temperatures we experience etc.

      • protonsforbreakfast Says:

        Hi. Thanks for taking the time to stop by.

        The whole ‘Sand Battery’ thing is all over the internet like it was some kind of profound breakthrough! But as you say, you need a pretty good reason to use something other than water!

        All the best


  6. Peter Says:

    Many low tech environmentalists love the idea of dirt batteries. Almost no cost, some personal effort and little pollution involved. Industry is not interested in any tech they cannot profit from.

    • protonsforbreakfast Says:


      I can understand people’s motivation, but unless they have a way to insulate the storage material effectively, then the thermal storage is very leaky.

      This can be OK if you are getting the initial heat for ‘free’ or at some very low cost, but any resource which is useful to someone tends not to remain free for very long!

      Best wishes


      Best wishes


  7. Steve Says:

    Hi, I believe the reason why sand is being used is that you can zone the heat within the store. So, hotter in centre for longer term storage, and for repeated use the heat near the surface. You wouldn’t be able to zone with a liquid. Cheers Steve

    • protonsforbreakfast Says:


      Good Evening. I am still unconvinced that this is a good idea.

      By my calculations, when all the sand is at 500 °C, the store holds 8 MWh – enough to heat 2 homes for a winter. If the sand is cooler around the outside, then that means it stores even less energy!

      All the best


  8. Steve Says:

    Thanks. I was interested to see this come out today, which explains the use of sand a bit more. https://youtu.be/G6ZrM-IZlTE

    I am really interested in trying to design and build my own setup to heat my house in replacement for my oil boiler as with some solar panels to a heating coil, and then an air circulated coil in a sand store, I could then potentially add an air to water heat exchanger and connect it to the existing radiators. Just a pipe dream (excuse the pun) at the moment, and I lack any engineering experience to calculate requirements for each step to heat a 4 bed detached but I’m going to keep eating up. Cheers Steve

    • protonsforbreakfast Says:

      Steve, Good evening, and good luck with your adventures. If you do begin an adventures, be sure to bear in mind The Golden Rule of Experimental Physics:

      Do it quick. Then. Do it right.


      All the best


    • cclambie Says:

      Steve, I am really curious how you go with that.
      Some good Youtube videos on it.
      Also I would be interested in collab if you want to work it out together.
      I am thinking about using sand on a 10 house “micro” district heating network. Wondering on the efficiency of using solar from rooftops and wind from grid at night to “keep” the sand hot…

      • protonsforbreakfast Says:

        Hi. Here are 3 thoughts.

        1. If you are thinking about sand: think again. Start using water: it is a better storage medium – stores 3 times as much energy per degree Celsius for the same volume: and much easier to move – and cheaper!

        2. Solar PV is ~ 20% efficient; Solar Water heating is around 60% efficient. So if you use Solar PV, think about using it to run a heat pump (aka and old fridge operating for heating rather than cooling).

        3. Remember the golden rule – make something that works and get some numbers to describe it, and then see how it would scale: good luck!


  9. Steve Says:

    And reading up!!

  10. Gijzzz Says:

    Here in the Netherlands we have a simular project with an entire housing block that is heated by a heat battery. It is also heated by solar panels in the summer months and provides free heat for the houses around all winter. They make use of basalts rock and metall slugs for heat storage. The whole difference between sand/ rocks and water is the fact that water only can contain low temperature heat (max 100 Celsius/ boilingpoint) while sand/ rock can be heated easily to a 1000 Celsius. Also it stores the heat a lot better because it is an heat isolator unlike a fluid like water. Your calculations are all based on behavior of water and fluids and heat conductors but that now is the whole point.
    And lets be honest it is not a new concept stone ovens, stone funaces, all invented ages ago all make use of great thermal storage of stone and sand. Also Think of the isolating properties of sand in the desert. It heats up at the day time enormously , but still it can be there freezing cold in the nighttime in the desert because the heated sand is a super heat isolator. The beauty of these sand/rock battery projects is they seem to work, are low-tec, low maintainance, low-costs, and make use of simple existing technology.(heatpipes and simple heating elements They are also bad news for the oil and gas industries because 80 % of our energy needs are in heat-energy and this might solve the storage problem and CO2 problems.
    (When every house an heat battery in the cellar and solarpanels on the roof).
    Lets wait another year and see what will become of these projects. If your calculations are right we will all know, because all eyes are watching them closely.

    • protonsforbreakfast Says:

      Dear Gijazz, Good Evening.

      First of all, it is important to be numerical and quantitative in assessing this kind of technology. In your comment you have used phrases such as “stores heat a lot better” “heat isolator unlike water” and suggest my calculations are based upon the properties of “heat conductors”.

      Actually, everything is a heat conductor and the rate at which it loses heat depends on its temperature.

      The point of this article was to highlight that even a few basic calculations that anyone can do will show that the Finnish Sand Battery cannot conceivably store heat inter seasonally, and provides only enough heating energy for a single dwelling.

      As you say, let’s wait a year and see.

      Best wishes


      • Tom Smykowski Says:

        Good Evening

        I did the calculations myself in hope to prove you wrong, but I got the same results as shown in this article, so I wont.

        On the other hand I can’t agree with this sentence: “even a few basic calculations that anyone can do will show that the Finnish Sand Battery cannot conceivably store heat inter seasonally, and provides only enough heating energy for a single dwelling.”

        The calculations show only that assuming the same amount energy to be stored, insulating material and width of insulation, energy stored in sand will be leaking quicker due to higher difference in temperatures. But in practice those assumptions need not to be true. For the energy storage that uses sand, insulation may be thicker and may be made from a material that has lower thermal conductivity. For example PIR foam has thermal conductivity of 0,021, so to counter the difference in temperatures (compared to the solution using water) a 800 mm layer of insulation would be needed, which is doable.

        I’m not saying that this is better solution than using water, rather that it is possible to make the sand energy storage store energy over longer periods. And there may be some other factors that might make it preferable to use sand instead of water.


      • protonsforbreakfast Says:

        Tom: Good Morning,

        PIR foam will not withstand temperatures above roughly 150 °C. High temperature insulation is considerably less effective.

        I kept the insulation thickness the same so as to fairly compare the properties of water and sand. Whatever insulation you add for a sand battery you could also add for water (for which PIR insulation is suitable) and the water solution would look even better.

        The point of the article was that this idea – published glibly and uncritically around the world – is actually not obviously a good idea.

        Best wishes.


  11. overlordq Says:

    I can bury a PTC heater in a bucket of sand, I can’t stick one in a barrel of water.

    • protonsforbreakfast Says:

      Dear Overlordq,

      Good Morning. I’m afraid I don’t understand your point.

      You say are unable to stick a heater in a barrel of water: immersion heaters are widely available and commonly used for the purpose of heating cylinders of water. So why can’t you use them?

      Best wishes


  12. Anonymous Says:

    These types of thermal storage is already used for CSP thermal storage, store solar energy during the day and spread its use during the night. A week or so of thermal storage as far as I understand is easily possible. It is not meant for usage for entire winter, but for a few days when heat source is not available. When it becomes available, you hear it again, acting as a bigger to cope with intermittent energy sources like solar during the day or changing wind conditions.

    It is done on relatively large scale at the Noor power station already for night time electric generation, had some in USA too but the last one was not too successful because it had lot of downtime to be very economical and they couldn’t sort out all the engineering, design and construction issues along with operational optimization issues which are prevalent across on types of plants, nukes, thermal, oil and gas, petrochemical, silicon and solar cells etc. Unless you aren’t in it for the long haul, it won’t be successful unless work is put and operations are refined followed by construction of followup storage facilities with more updated designs. Just like aircraft carriers, nuke submarines, aeroplanes have significant changes to designs over long periods of time with many significant deficiencies identified during operation followed by design advancement followed by new facilities with updated design, the same approach is needed in these types of designs too to gather enough empirical data on operational shortcomings which are resolved during followup design updates.

    The next generation of SMR reactors that bill Gates has been pushing will have thermal storage built along with the steam boilers and will store heat in addition to electric generation to cope with fluctuations in power consumption. Frankly the idea is so elementary that I cannot understand why is it not already used with all nuclear and thermal power stations to store heat during periods of low usage or when lots of intermittent renewable energy is available.

    • protonsforbreakfast Says:

      Dear Anonymous, thanks for your really intelligent comments.

      My article was mainly a response to press reports that suggested this was inter seasonal storage. But as you say, storing thermal energy for a day to a week is pretty straightforward in principle. However, being able to dispatch the heat at a predictable rate from a store which cools as it empties requires clever heat exchange which is never cheap or simple (in my experience).

      Best wishes


  13. Anon Says:

    I think it has its application. For example, my workshop was not built to have heating but I get by with a small electric space heater. That being said I’m sure those space heaters don’t consume very much energy, but if all I had to do was build a rocket stove going through a tank filled with sand and heat my shop for the whole day just by burning a few sticks or junk mail (that i’ve turned into fire logs) I’m going to go for that method as often as possible.

  14. SteveB Says:

    I find the whole field of low (ish) energy storage fascinating and the article was very helpful and has cooled my enthusiasm for digging a vast hole in my garden and filling it full of sand and using it to preheat water in the central heating system!

  15. D.M. Says:

    Would like to see the graph model with the sand Volume the same as water. The existing model for the sand is 3/4 the size, diameter wise. If the footprint were the same, giving the sand more ‘self insulation’ then why not compare the time curve/loss with the same volume ( not mass) and same energy input.

    • protonsforbreakfast Says:

      D.M. Hi. Thanks for stopping by. The purpose of this device is to store energy, and so the comparative calculation is done for the storage of a given amount of energy (8 MWh) using either water or sand.

      If you compare equal volumes of water and sand then the time constant of the sand storage will increase by a factor of about 1.5? Why? Because the sand container radius increases by a factor ~1.5. The heat capacity will increase with the volume of the container (i.e. like R^2) but the heat losses will increase like the surface area (i.e. like R) so overall the time constant would increase by a factor of about 1.5.

      All the best: M

  16. Pk Says:

    I have a question,if we able to use that heat energy which is stored in a sand battery to generate electricity by boiling of water in thermal power plant,it could be a good way to produce electricity because this power plant use coal as fuel which is renewable resource.so may be in future if we come with a better technology to store this heat energy for a long period ,can it will be become a great way to produce power.

    • protonsforbreakfast Says:


      Take a look at this video which explains the complexities of generating electricity from stored heat. Basically, one can only recover about one third of the stored heat as electricity.

      All the best: Michael

  17. Janne Says:

    Usable temperature delta for water storage is only about 50 degrees (50-100C), because heating radiators require at least ~40C temperature at winter time. Delta for sand storage is much wider mitigating the lower heat capacity vs. water. Also these heat storages are meant to be recharged during windy days when electricity price in Finland can approach zero.

    Historically water based heat storage has been widely used in Finland for municipal heating. There’s one in Helsinki that has 3.2M m3 of water with operating delta of 50C-90C. Its charge/discharge power is 120MW. The water is stored in a huge cave with no additional insulation, but the surrounding rock itself.

    • protonsforbreakfast Says:

      Jane, thank you for that very informative post.

      3.2 million cubic metres of water is quite an impressive store. And 120 MW is a very high output power for such a low delta T.

      Thanks again. M

  18. Philip Salter Says:

    Thank you for a very enjoyable and informative read. To solve the weaknesses you outlined above, why not transform the “sand battery” idea into a much larger borehole thermal energy storage system?

    The volume (I.e. capacity) could be made larger, albeit within the in situ soil/rock and not pure sand, and the relative surface area reduced, thereby reducing heat losses.

    By circulating the discharge fluid from the outside towards the central hot core of the system through a series of interconnected pipework, it could be made even more efficient – and operate at temperatures far higher than possible with water.

    • protonsforbreakfast Says:

      Philip, Good Evening. YEs, there are a large number of possible storage materials. Here’s a new one I just heard about today. https://www.youtube.com/watch?v=KpYFdUc1ot0

      But the Physics of them all is just the same. To store significant amounts of heat you need a high heat capacity and a big temperature change, and to store it for a long time you need good insulation.

      The aim of the article was just to put some numbers on this story that went around the web with out any kind of critical assessment.

      The general problem with boreholes is the thermal conductivity of the rock/soil/material. At the interface between the re-circulating liquid the stored heat has to flow across quite a small area into the recirculating pipework. This limits the rate at which heat can be withdrawn. One can increase the rate at which heat its withdrawn by having more pipes, but things quickly become complex and expensive.

      All the best


  19. Philip Salter Says:

    Interesting. I’m currently making up some thermally conductive borehole grouts in the lab which when combined with a novel ground treatment strategy may help overcome this contact resistance issue at the heat exchanger interface… Do you know of any modelling software (ideally free) for testing the performance of a storage system when we know the thermal properties of all the different elements?

    • protonsforbreakfast Says:


      No, I’m afraid I don’t know of such software. If it exists it will certainly be complicated.

      Why? Two difficulties occur to me immediately.

      The first is the water fraction in a grout. The thermal conductivity will depend critically on this fraction, but the fraction is likely to change over time. If teh grout is dried, then it will be very susceptible to small cracks opening up between the borehole liner/pipe and the grout.

      The second reason concerns the bulk thermal conductivity of the grout. It will depend on the thermal conductivity of the component materials, but also on the surface area of the component particles and the heat transfer coefficient at the surfaces between each different component particle. It will be tough to make accurate estimates of those quantities.

      But in any case, all the best


  20. JA Says:

    Very interesting discussion from the physics perspective. I undertood that
    the real issue is the fluctuation of the output from the wind and solar power plants
    which causes the price of kWh to go up and down by a large factor including very high peaks in both directions and consequent need to build up high capacity, high cost infrastructure networks.
    A decent system model based on the factual measured fluctuations and availability of
    low cost electricity would reveal the pain points and pf the parameters the required energy storage.
    The cost of electricity could also be negative in some windy days in short and medium terms, not windy months.
    Consequently the use of the heated air in the energy storage to heat the houses fluctuates.

    A system model much beyond the energy storing capacity calculations
    could pose important information of the relevance of the needed heat capacity parameters
    and how different solutions would compare as close to the practice as possible in terms of price.

  21. JA Says:

    I found occasionally some Software tools for energy storage

    Intuitively, (advanced physic guess):
    * Sand can store quickly high peaks of energy, maybe better than water that should not be heated too much since it boils away.
    * Maintenace of the sand storage might be less costly than water once in place? Water vaporises and it might store harmful biological material, rost problems etc.
    * Energy leackage of the storage in few days does not matter if the windy or sunny periods last typically hours or few days, depends on the area.

    Furthermore, what I am reading in internet the price estimate of the commercial energy storage e.g., sodium batteries, gravity etc. is high about 100-300$/kWh while sand heat storage costs about 10$/kWh. Sand might be price efficient for cutting the very sharp up an down peaks of electrcity production
    in favour of distrcit heating, reducing the need for high cost upgrade of electricity network. In my country there are gigantic plans to
    upgrade the electricty network because of the capacity of the wind turbines will go up significantly in the coming years.
    This upgrade will be extremely costly and environmentally questionnable, lots of trees cut.

    If some heavily fluctuating up/down-peaks could be cut using a large battery/energy storage (gravity, etc.) or heat storage (sand, etc.) more locally it would
    lower the need for costly networks.

    In my view the national plans to deploy very high cost electric networks to balance the electricity grid connecting the wind mills
    and hoping that in average they produce stable flow of electricity might not be very clever. Some very low cost, fool proof energy storages should be installed here and there when commercially relevant.

    Food for thought. The time will show how far the Finns will go with their sand box. They have to walk the walk anyway. There is a seed for innovation and opportunity for big money.

    • protonsforbreakfast Says:

      Hi. Thanks for stopping by. I don’t think I disagree with any of your comments.

      And I do agree there will be a great deal of innovation in this area and that there is opportunity for big money.

      I think the really interesting time will come when renewable generation begins to generate surplus electricity on many days of the year. This electricity has no marginal cost. I think at this point even storage technologies with 30% round trip efficiency will be able to make a profit and new industries will develop.

      Best wishes



  22. David D Says:

    Dear Michael,
    Thank you for pointing out the obviousness in comparing a water system to that of sand.

  23. tom Says:

    “By my calculations, when all the sand is at 500 °C, the store holds 8 MWh – enough to heat 2 homes for a winter. If the sand is cooler around the outside, then that means it stores even less energy!”

    I was thinking the make the sand hot with hot air if you blow hot air to the outside of the container the sand around the out side still get hot.

    • protonsforbreakfast Says:


      I can’t quite understand your question/suggestion. The Sand Battery is heated with hot air already.

      All the best


  24. juancassinerio Says:

    For short term storage, this works pretty good, for example, storing solar energy for the night, almost 99% efficiency, cheaper and longer during than lithium ones

    • protonsforbreakfast Says:

      Juan, Good evening.

      Cheaper than Lithium Ion batteries, yes, but only for heat. The sand battery may have a role to play, but it will be a very limited role.

      All the best


      • juancassinerio Says:

        You can turn heat back to energy, solar/wind is 50$ per kwh, carbon/gas is 150$. Turning heat back to electricity is 35% efficient, this means that energy produces by renewables and stores by sand batteries will produce electricity of 150$, quite the same. I think that in some few years it will be far more cheaper. We are right on the turning point, data says, i could be wrong too.

      • protonsforbreakfast Says:

        Juan, I’m afraid I don’t understand your comment.

        Firstly, heat is already energy.

        Secondly, solar and wind are much cheaper than $50/kWh. Did you mean MWh?

        Converting heat to electricity could possibly be 35% efficient, but that is unlikely in this case.

        To achieve that efficiency requires a stable source of heat at a high temperature. Let us suppose that the sand battery could indeed generate electricity at 35% when it is initially at 500 °C. I think this is unlikely, but even if it were true, as the sand battery cooled, the efficiency would be reduced, AND the rate at which electricity could be produced would be reduced. Below around 300 °C, the efficiency would have fallen by about half and the generating power would have fallen similarly.

        So-called ‘low grade’ heat such as is stored by the sand battery is basically not very useful for generating electricity, unless you are truly desperate!

        Best wishes: Michael

      • juancassinerio Says:

        i didnt think about tmperature efficiency, you are right, lets see what happens

  25. Pískové úložiště energie - ekokutil.cz Says:

    […] Konečně princip, jak si schovat nespotřebovanou energii na horší časy, chtělo by se říci. Při bližším pátrání však narazíte na skeptiky, kteří pochybují jak o prvenství, tak i o efektivitě pískové baterie. https://protonsforbreakfast.wordpress.com/2022/07/21/a-sand-battery-not-obviously-a-great-idea/ […]

    [Google Translate: Finally, the principle of how to store unused energy for worse times, one would like to say. However, upon closer inspection, you will come across skeptics who doubt both the primacy and the effectiveness of the sand battery.]

  26. Joyce Says:

    We have 100 watts of solar on our 7×14 trailer. I’m hoping that a 2gallon sand battery could heat the trailer at night while getting 12volt input continually from a real battery. This seems an ideal application. True?

    • protonsforbreakfast Says:

      Joyce, Good afternoon.

      I think 100 watts represents peak output so if on average the panel generates 50 W for 8 hours, then it might generate around 0.4 kWh of heat. This is both a large amount and a small amount.

      It is enough to heat 2 gallons (9 litres) of sand to several hundred degrees celsius – very hot and significantly dangerous! But 0.4 kWh of heat is not a lot when it comes to heating rooms, even small rooms. My guess is it will heat up quite a lot while cooling down quite quickly.

      So if you are of a mind to try it, I would encourage you to have a go – but to be very careful about moving or lifting the sand when hot – and to use very high quality cartridge heaters rated for (say) 1000 °C – it could potentially get very hot!

  27. David Collins Says:

    I too saw the bbc article and was intrigued to understand if I could build an inter seasonal storage for my home.

    I was aware of past project that had attempted to use very soil and water storage but these seemed impractical.

    I too have wondered if a small scale electricity could be a good use of thermal storage but water would not be hot enough.

    In my approach I have considered creating the storage under my house to help off set ground heat loss. Certainly under my garage floor is practical and at the garage is insulated it would benefit from the storage tank heat loss.

    I would have thought that there are better insulation materials than Rockwool for high temperature vessels. Micro pore used in storage heaters which I think was based on space craft re-entry tiles. Or double skin vessel with a vacuum void.

    Your thoughts would be interesting

    Regards David

    • protonsforbreakfast Says:


      Good Morning. So if I have understood correctly, your idea is to construct a void under your garage, insulate it, fill it with sand and a network of steel pipes, and then heat it with excess solar PV in the summer. You would then flow water through this in the winter to heat your home via radiators. Is that correct?

      Well I think that such an installation is possible.

      Let’s suppose you have 1000 kWh of excess solar for which you might be paid 4p/kWh by your electricity company. i.e. it’s worth around £40.

      According to my spreadsheet, to store that 1,000 kWh you would need 5.5 cubic metres heated to 500 °C – that’s a big hot installation. If it were 1 metre deep it would be a pit with a diameter of 3.2 metres – allowing for 0.3 metres of insulation.

      This would loose energy pretty quickly – about 50% would be gone in a month.Only a few percent would be left after 6 months.

      But even neglecting losses, it would be tricky to recover the heat. If you flow water through pipes the water would evaporate and extremely hot steam would emerge. So you would need an intermediate heat exchanger to get water at the right temperature for your central heating.

      So it’s possible, but I am not sure it would be the best use of the money – say £5,000 – that you used to build the installation. Other possible uses are to buy shares in Ripple, add insulation to the house, buy a small domestic electrical battery.

      Do get make to me if you think I have missed anything. All the best: Michael

      Better options

  28. muskoka1030 Says:

    Dear Protonsforbreakfast,

    I have no doubt in the calculations you have done are accurate.

    You give a single temperature vs time for both water and sand. A single temperature might be more suitable for water because of convective mixing. I do not doubt the effect of a higher temperature causing a higher heat loss. Would it be possible to display the DISTRIBUTION of temperature in the cylinder of sand, so it makes it obvious to everyone that the very poor performance you show does actually account for the slower movement of heat by conduction from the core to the perimeter in a packed solid ? Even if the calculations account for these things, it may just be the sand heat storage needs to a be a larger cylindrical diameter with some fraction of the radius acting as insulation to an inner core. This might lead readers to understand just how much larger the storage cylinder would have to be to assure the core retains a usable temperature.

    A cylinder is a geometry that favours water because it has to hold 2/3 of an atmosphere of pressure at a depth of 7m. With sand, a DIY builder can make a storage shape that better suits sand. Take the beehive shape of structures in northern winter roadworks compounds, used to store … sand. Omitting the cost of the pressure vessel, and using a shape better suited to the low-tech storage of sand, maybe the tradeoff is, twice the amount of sand is needed to keep a useful core temperature for a year. Storing twice the amount of sand might be easier than storing water. Building a pile of sand twice the size might be easier than buying/building a water pressure vessel.

    One can spread concrete on a pile of sand, and with a little reinforcing mesh, create a durable structure over which to lay insulation.

    Sand cannot spring a leak as easily so containing it is far more low-tech. Sand cannot freeze. It is not conductive so no special heaters are needed — parts from discarded electric ranges would work. Heat storage in sand does not exclude electricity generation, in fact, the higher temperature difference would tend to make it more efficient at any given level of technology or investment to get a certain amount of electrical energy out of sand.

    Ground energy storage in areas with no aquifers, or deep aquifers, is just using boreholes to make a structure-less sand heat storage system. I think there are a lot of factors your calculations omit, and only more calculations, on appropriate designs for each option, with costing, will give an accurate story.

    I suspect in industrial installations, water may be better. I suspect that DIYers would be far safer working with piles of sand. Water even in only a static-head pressure vessel of 2/3 atm at a 7m depth, at 100C is a serious hazard to life if it springs a leak.

    • protonsforbreakfast Says:

      Dear muskoka1030, Good Evening.

      I have read your comment a couple of times and I can’t quite figure out what your overall point is.

      I wrote the article because news items about the ‘Sand Battery’ at the time reported it appallingly as if it were a panacea for inter-seasonal energy storage. I just thought I would put some numbers to the idea and it turns it it’s not suitable at all for inter-seasonal storage, and not that great for other storage.

      Turning to the points you make.

      1. I used a so=called ‘lumped’ model which assumes that whatever is within the insulation is at a single temperature. I have previously made more detailed thermal models to show the likely temperature gradients inside such enclosures, but they are just more or less what you would expect. I doubt it would alter the time-constant for heat loss significantly.

      2.You then tell me lots of ways in which a sand battery is simple to make. I agree. It’s just doesn’t store much heat per unit volume per °C. You could indeed generate electricity using a heat engine, but not very efficiently.

      3. You then tell me that you could store energy in the ground – if there is no aquifer! There are not many places where that is the case.

      4. You now come to DIY’ers, and perhaps that’s what you have in mind all along. But I can’t imagine what the DIY situation is. For the 7 metre silo considered, it stores 8 MWh if I recall, and so (assuming no losses) you would need to charge it with 1 kW of excess solar PV for 8,000 hours. Assuming you luckily have 8 hours of excess a day this would take 1,000 days – or 3 years! To charge it up in a summer you would need 6 kW of excess solar for 8 hours a day or ~48 kWh of excess solar PV/day. That’s a really really big solar array if all this electricity is ‘left over’.

      If this idea makes sense to you, please try it. But in my opinion there are lots better ways you could spend an equivalent amount of money – primarily insulation.

      Anyway – best wishes for your endeavours. Michael

      • muskoka1030 Says:

        My overall point is, the first calculation on a technology is not the entire story. To put it more simply “its more complicated”, without you being “wrong”.

        1. I have done calculations that were lumped model, but with more points, accounting for thermal conductivity of the material, in a different calculation for underground heat storage. Mineral wool is a good insulator, but (“more complicated”) it is not a “linear” insulator in the sense that the commonly-used naive linear heat loss equations would suggest:


        At 800K, or 527C, it is only about 10x better at insulating than typical values for sand. So compared to 0.3m (1ft) of rock wool, meters of sand are not irrelevant to the computation. For the dimensions you are using, it is quite irritatingly neither dominant nor negligible where an engineering calculation could either rely on it, or neglect it. It is quite conceivable even a single individual could inexpensively make a pile of dry sand large enough that self-insulation becomes the larger factor. But it would have to be a truly large pile. My (different) calculations suggested that insulation makes the difference between year-round and shorter storage, always, so always insulate.

        2. Generating electricity from heat either involves a lot of technology to typically get less than 50% efficiency, or, a lot less technology to get a lot less. The thermal storage system should never be considered an efficient source of electricity. But even stupidly inefficient conversions like the Peltier effect are very effective in solving isolated problems well — the difference between science and engineering. Peltier woodstove fans are twice as good as no fan — after an added hour at -30C, many people evaluate they are worth it. Hikers use Peltier rechargers for their phones, and find them infinitely better than no recharger.

        3. Most of the places I have lived, any holes over 1-2m deep tend to fill with water. While this may be the common experience of many, there is actually quite a lot of the world where the water is much further down — deep aquifers, and other areas where it is not present at all in patches — one farm might be on an aquifer, the next one not. Much of real design of effective systems in engineering relies on the specifics of the area.


        Even in a “wet” area, making a pile of sand with a membrane above and below is automatically above the water table.

        4. I am considering all scales, from DIYers who might get their information from the popular press and blogs, or engineers who do not and design big things.

        You touch on pointlessness. There are many aspects where one could ask, what is the point ?, there are better ways. The original article says they store excess electricity as heat. Facepalm. Was this community planning on using 1970s style resistive electric heat in the houses ? No? Heat pumps = do you want to store X amount of heat at really high temperatures in sand or 3X heat at a lower temperature using a heat pump to a large reservoir of… water? There are just so many other things that can be done with spare electricity it is a complicated problem, and sand is just one very poor option.

        The thermodynamics of this are all wrong too. To get more heat into a sand thermal storage system already at 500C, one has to have heat at 500C, or electricity one has nothing else one can do with it… Ok, in transitional states that last hours, nuclear plants can be over-producing electricity, but if the grid has some hydroelectricity from a reservoir then pumped hydro, pumping water back up into the reservoir is the better solution — existing infrastructure + pumps.

        And heat at 500C — any engineer worth their salt could turn that into electricity with a steam turbine.

        Upshot, “spare heat” at 500C is unheard of in any well-designed system. Spare heat at 100C, that is a different story. All those thermal electric generating plants that use steam as the working fluid (biomass, gas, coal, nuclear) about 50% of the energy is wasted as heat approaching 100C. It has been a long-solved problem that these plants can provide community heating. Adding a huge pool of water is not really needed, because the plants do not have an intermittency problem, they run all the time. Adding a “small” pool of water might smooth the heat output over the daily cycles of electricity output from the plant.

        Storage is about intermittency. The only truly unsolved intermittency problem I see is off-grid. On-grid, there are far better grid-level solutions already, pumped hydro being brain-dead simple (where there are hydro reservoirs), grid-level batteries that entered service in the last decade, and “gravity batteries” which use solids instead of water. And frankly, none of the engineers on those systems are reading the popular press, or blogs for ideas for work. The only people who might misunderstand the true (limited) usefulness of sand (or any) thermal storage system are DIYers. These articles are simply generating clicks and making youtubers money.

        The use of a sand or water thermal storage system would meet a niche need, in an environment with uncommon shortages and excesses, starting with the land needed to have a storage system. If someone starts the conversation saying, I am in a rural area with more than 40 acres of property (or a rural subdivision developer) in a temperate zone, then there is a point in arguing over sand, or water, or ground thermal storage.

        There are too many other solutions for everyone else to start addressing them all. Insulation is generally the best but even that is complicated. Uniformity in insulation is more important than R value in a limited area. A bucket full of holes that is perfectly sealed over 10% of its area still leaks 90% of the water it did before — don’t pay a lot for only a 10% solution. But a gimmick in the back yard is never better.

      • protonsforbreakfast Says:

        Dear muskoka1030, Good Evening.

        I feel like I should respond to your extended comment, but I just don’t know what to say: you seem to be very aware of all of the engineering realities of trying to store heat in different media.

        As I mentioned previously, the original article was written because this original ‘Sand Battery’ Story was all over the media all over the world. And I didn’t read a single world challenging the grand statements made. So I ran some calculations and it didn’t seem to be all that smart an idea. And that’s all I was saying. The piece is titled “..Not obviously a great idea”.

        It may be that in some circumstances, a sand battery of some kind may be useful. But with the possible exception of heat storage for district heating, I can’t really see a niche for this technology.

        Every best wish


  29. cclambie Says:

    @muskoka1030 thanks for your comments. Seems like you know quite a bit about storage and thermal dynamics, physics and chemistry etc.

    Great to see you and Michael dig deep on this topic, so thank you both for the informative debate 🙂

    I am quite interested in thermal storage, seems to make a lot of sense to me.
    EVEN if the storage only lasts 12-16 hours, it could look after most UK homes for their daily needs from a very cheap (in comparison to A/GSHP) storage for 30-40 kWh of heat, approx needs of one house for a day.
    Even using “100% efficient” forms of heating, in off-peak times you are using up that excess wind/ nuclear power at 1/4 of the price of peak times – so the COP of 3.8 for most ASHPs makes them cost comparable – and the CapEx would be much, much lower – so the long term savings are minimal.
    Especially when you consider ASHP maintenance costs.

    My point is that maybe Sand or similar storage medium has it’s uses outside of district heating, for load shifting, just not seasonal storage, like the BBC and Polar Night are suggesting – which is maybe a bit of hype, or a bit misleading at least.

    What do you all think of adding a spiral pipe to a hot water tank (HWT) and then filling it up with sand or gravel (name the medium) and using the immersion heaters in off peak times to heat the medium up.
    Then running water through the spiral pipe with a low energy pump in peak times to heat water/ central heating system boilers?
    Some sort of temperature controlling device to protect from overheating.
    Daily cycle – so worries from long term heat loss.

    Capex – £500-700
    Opex = similar cost to ASHP using peak power
    Could use Solar to top up heat store in the middle of the day too, even in winter.

    Capex arguement
    Hot water tank – £300
    Pipework – £100
    Pump – £100 or existing pump in HWT
    Sand – £50


    • protonsforbreakfast Says:


      Good Morning from the UK. I’ve delayed replying because I was thinking about what was at the heart of your comment.

      And I think you are absolutely right to point out that – at least at a first look – it would seem obvious that thermal storage could play a bigger role. So why doesn’t it?

      Well in fact thermal storage using hot water cylinders is quite common – but that’s just for DHW, not space heating.

      Storage heaters are still on sale and have improved a lot. These devices heat insulated bricks to very high temperatures – I don’t know exactly but I think it is well above 500 °C – and then blow air over the bricks to extract the heat. They heat at night using cheap electricity. Although I know people who are very happy with their storage heaters, it is not regarded very highly. The heaters are not very compact – and after loading with bricks they are ridiculously heavy! The extent of their charging also needs to be set the night before. So although they can heat well on cold days, they can run out of heat in the evening and their heat leakage in mild weather can lead to over-heating. These heaters are cheaper to install than a central heating system or heat pump – and they are purely electrical so there is no plumbing.

      The end-game of storage heaters is the Zero Emission Boiler.I wrote about these here. But I think there are not so many places where this is a better solution than a heat pump.

      So I think the reason domestic thermal storage has not more popular is not because it’s not a good idea, but there have been cheaper alternatives – mainly gas – easily available. Perhaps our recent price shock and the need to stop burning stuff means that storage heaters might make a comeback.

      Best wishes


  30. Ramiro Says:

    Hola, gracias por compartir resultados y conclusiones, vivo en Ushuaia Tierra del Fuego Argentina, estamos muy lejos de alcanzar proyectos experimentales de este tipo, creo que son fascinantes. Creo que parte del cálculo está limitado a que el aporte energético durante el periodo de invierno o de baja energía es 0, cosa que en la realidad no es así, la distribución del encendido de las resistencias calefactoras de manera alternada también se podría utilizar para realizar barreras térmicas internas que disminuyan la pérdida calórica, mi humilde comentario y gracias nuevamente.

    • protonsforbreakfast Says:

      Google Translation: “Hello, thanks for sharing results and conclusions, I live in Ushuaia Tierra del Fuego Argentina, we are very far from reaching experimental projects of this type, I think they are fascinating. I think that part of the calculation is limited to the fact that the energy contribution during the winter or low energy period is 0, which in reality is not the case, the distribution of the switching on of the heating resistances alternately could also be used to carry out internal thermal barriers that decrease heat loss, my humble comment and thanks again.”

      Ramiro: greetings: how remarkable that we can even communicate!

      I cannot really understand your comment exactly, but perhaps it will be clear to one of the readers.

      Best wishes: Michael

  31. Michael Says:


    Let’s look again at the intent of the storage system, it’s not about how much energy it can store, it’s about storing heat over the winter when the access to regeneration is not as plentiful. It’s about recharging when rates are more affordable.

    But given you want to base your opinion on calculations:

    What your calculations don’t show is that water transfers heat (meaning loses heat) faster than almost any other substance on earth. Did you include the specific heat of water and sand into your calculations? The specific heat of water is 1 while sand is 0.29. This means that heat will transfer out of water more than 3 times faster than sand resulting in the interior of your water tank cooling down more than 3 times faster than the interior of the sand.

    Next take the parameters used in your calculations, 500C Sand vs 100C water. You need to use the same starting temps if you really want an apples to apples comp. Plus given your calculations use a larger diameter vessel with water versus sand, the heat transfer time factor is also skewed.

    All your calcs really say is that a medium with a higher temp will likely have faster heat loss. Doesn’t say anything about the specific heat (which is the rate of heat transfer) or the thermal barrier at 1″, 6″, or even 24″ from the edge of the vessel.

    Lastly, and I mean no disrespect, the people that developed this system are engineers and you’re a journalist. If you would like to rebut my comment please do so, but if you don’t show all your calculations then just tell me you used fuzzy math.

    Best Wishes

    PS: That’s some house you live in, my all electric house is only a bit over 900 kWh per year!

    • protonsforbreakfast Says:

      Michael, Good Morning. Thank you for taking the time to comment. However, almost everything you have written is wrong.

      First of all, I am not a journalist: I am a physicist who has been writing this blog since 2008. I am a Fellow of the Institute of Physics and am quite expert in thermal calculations.

      Second, in contrast, almost every other article you have read about sand batteries has been written by journalists – that’s why the factual quality of the stories has been so poor.

      Now to your assertions:

      What your calculations don’t show is that water transfers heat (meaning loses heat) faster than almost any other substance on earth. Did you include the specific heat of water and sand into your calculations? The specific heat of water is 1 while sand is 0.29. This means that heat will transfer out of water more than 3 times faster than sand resulting in the interior of your water tank cooling down more than 3 times faster than the interior of the sand.<

      The heat capacity of water is roughly 3 times that of sand, but that means it is 3 times BETTER at storing heat. In other words if one wants to store a given amount of thermal energy, using water one can store the same thermal energy with either one third the temperature rise – and hence heat losses – or one third the mass at the same temperature rise.

      Next take the parameters used in your calculations, 500C Sand vs 100C water. You need to use the same starting temps if you really want an apples to apples comp. Plus given your calculations use a larger diameter vessel with water versus sand, the heat transfer time factor is also skewed.

      I did compare apples-to-apples. I compared the performance of two thermal stores which store the same amount of energy.

      All your calcs really say is that a medium with a higher temp will likely have faster heat loss. Doesn’t say anything about the specific heat (which is the rate of heat transfer) or the thermal barrier at 1″, 6″, or even 24″ from the edge of the vessel.

      A medium with a higher temperature will indeed have higher heat losses, and sand needs that higher temperature because its heat capacity is so small compared with water. Specific Heat is not the rate of heat transfer as you assert. The specific heat of a substance is the amount of energy required to heat unit mass of a substance by one degree.

      Best wishes


      P.S. The electrical consumption of a house depends on its location and size. But wherever your home is, your electrical consumption 2.5 kWh/day is exceptionally low. I don’t know what you are doing – but well done.

  32. zekegri Says:

    Water yes has limited capacity. Sand over 1,000 C and far better insulation is available. Put the storage tank’s underground to begin and have a vertical greenhouse above to use waste heat. The (basalt rock best) sand can be topped up daily with excess wind, solar and geothermal and many colleges, schools, etc. already have hot water systems easily tied into. Little to no fossil fuels needed!

    • protonsforbreakfast Says:

      Dear Zegreki,

      Good Morning. The original idea was of a simple, cheap, heat storage device. You are proposing something requiring excavation, custom heat storage material, and situated close to colleges and schools.

      In your proposal, you need to find a way to extract heat from tonnes of material at 1000 °C and yet heat water to maybe 40 °C. This is do-able, but not cheap. Also cannot store geothermal energy at 1000 °C. And even delivering heating using and wind and solar at 1000 °C is an engineering challenge. And at 1000 °C, any kind of insulation would be very expensive.

      Heat storage is not a bad idea in itself, but as the cost rises, other possibilities become more attractive.

      Best wishes


      • zekegri Says:

        Good morning, digging a hole is not very expensive really.

        I myself have aerogel blankets that yes are not cheap, but far better insulation than anything other than a vacuum. And ceramic siloxane based coatings that are incredibly good at both waterproofing and insulation.

        The solar and wind do not have to put out 1,000 C but constantly add heat and the systems for extraction are already being done by companies such as Brenmiller.

        I sell basalt volcanic rock fibers that make far better composites for high temperature piping. And we have coatings applied very thin like paint that we have held 2,000 degree F torch on 50 mm away that never burn and a thermal probe on the backside of a 6mm panel never got over 160 degrees F. Very affordably.

      • protonsforbreakfast Says:

        Zegrekei, Good Evening,

        First of all, if you can build this structure and it helps you, then please do so. But I am unconvinced.

        1. Aerogel has a very low thermal conductivity, but not at 1000 °C. Although it is made of silica, it will be basically transparent to IR radiation. At these high temperatures I doubt it will have very good insulating properties. My experience is that effective insulation at these high temperatures will be expensive.

        2, If your thermal store is at 1000 °C, then in order to put energy into that store, electrical wires must be hotter than 1000 °C. If they are cooler than 1000 °C they will remove heat from your store.

        If you have a miracle insulating material and a wonderful storage material that can make this work: great. But this would be counter to my experience.

        If you are going ahead with such an endeavour I would encourage you to start small and see how it scales with size before committing to building a large store. But good luck with your endeavours. If make a marketable product, do let me know and I will be happy to admit I was wrong.

        You may enjoy this video from Engineering with Rosie.

        Best wishes


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