Energy Storage: The Sand Battery

The intermittent nature of solar and wind energy — almost feast or famine — means that it some form of economic power storage is required, the balance supply with demand.

Two Finnish developers say that their sand battery could solve the problem of year-round supply, a major issue for green energy — and for cold countries like Finland.

And they do it with 100 tons of builders sand (other sizes are available).

Low grade energy? What’s that — is it like unleaded gas you may ask?

No, far from it — at the other end of the car in fact. Low grade energy is all that heat that comes out of your exhaust pipe and gets wasted. There’s plenty of it around — factories, power stations, aircon systems, but it’s not easy to store and use efficiently. And that’s not something the sand battery solves.

But it is something that needs solving. I’ll come back to that.

Energy is energy isn’t it?

Not quite.

High grade energy is energy which almost completely converts into some other forms. For example electrical energy can be converted into thermal energy. On the other hand low grade energy cannot be converted into other forms very efficiently. But it can be used, in a limited way.

The two Finns have developed a way of storing excess electrical energy, but unfortunately it doesn’t solve the waste heat problem.

Their invention takes high grade electrical energy and converts it to low grade, heat stored in sand.

Cold reality in Finland

Finland is faced with a cold winters and they are likely to get much colder following the shutdown of Russian gas and electricity supplies as a result of the Ukraine war and Finland’s decision to join NATO.

Wind doesn’t blow all the time, and the sun — well that’s only there about half the time on a good day. It’s easy to add solar and wind power to electricity grids but it is the intermittency that is the problem. Highs and lows of power.

“Whenever there’s like this high surge of available green electricity, we want to be able to get it into the storage really quickly,” said Markku Ylönen, one of the two founders of Polar Night Energy who have developed the product. — BBC

Lithium batteries are expensive and the mining of lithium is a dirty business. However, they do store high grade energy.

I have friends with a small farm and they have gone completely off the grid with solar power and lithium batteries — a huge investment. Lithium batteries are good for 3,000 recharge cycles. 10 years maybe, and they will have to re-invest, painfully.

I think sand is forever. But it only stores low grade energy. Sand power would not run the freezers in which my friends store their home-kill beef, goat, lamb and chicken. Yet.

The Finns’ idea in practice

Solar and wind-power energy is used to heat sand in an insulated silo raising the temperature of the sand of up to 600⁰ C. This heat can then be stored for months at a time (say from summer to winter).

The actual heat storage is about 4 meters wide and 7 meters high steel container that has an automated heat storage system and a hundred tons of sand inside. As a material, sand is durable and inexpensive and can store a lot of heat in a small volume at a temperature of about 500–600 degrees Celsius.

The heat storage [ed: unit] has 100 kW of heating power and 8 MWh of energy capacity.

— Polar Night Energy website.

The ways in which the energy can be converted back to electricity are limited. 600⁰ Celsius is not hot enough to efficiently generate high pressure steam for a steam turbine generator (as far as I am aware).

So for now, the sand has to be heat for heating.

Conclusion

This looks like a practical solution for local home heating and delivers another small piece to the jigsaw of green energy solutions.

To my way of thinking, pumped hydro-electric storage is a superior solution, as the stored energy is readily converted back to electricity, but it has to operate on a large scale. Of course, in Finland winter freezing of the water could be a problem, but the Swiss seem to have managed it.

And on the horizon we have high capacity low-cost water batteries.

The green energy jigsaw is becoming ever larger, that’s for sure, and every piece helps.

The future

Reverse Peltier-effect (Seebeck) devices are small scale only and not yet capable of the sort of scale required for electrically powering a home (converting heat directly into electricity using a semi-conductor). But serious work is underway. This is important, as there is so much waste heat we could recover and use.

Maybe in the future we’ll see large-scale practical Seebeck devices which could be embedded in the sand silos and other locations where plentiful low grade heat is available. That could be an ideal solution and bring us much closer to completing that energy jigsaw.

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This story was first published in Medium on 14 July 2022