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.

The BBC’s ‘Sand Battery’ story.

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.