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.

Calculation

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.

Conclusion

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.

 

13 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

    Michael

  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:

      James,

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

      Best wishes

      Michael

  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

        Michael

  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:

      Peter,

      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

      Michael

      Best wishes

      Michael

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