> The sand itself will also be sustainably sourced – it’ll consist of crushed soapstone, which is a manufacturing byproduct of another local industry.

The requirement is not really construction grade sand but any substance with a largish thermal mass. Sand/dirt/crushed rock, etc. whatever you have right on your doorstep. There have been plenty of prototypes using such various materials as thermal mass.

All you need is a big container, some insulation to keep the heat in and lots of mass in whatever form. And some pipes to get heat in and out via e.g. water. The bigger the container, the smaller its surface area is relative to its volume. So these things can be quite efficient. If you make them large enough, you can store enough energy in the summer to last for months during the winter.

Here's a story about a Dutch retiree who built a prototype using basalt as the thermal mass in his backyard: https://deepresource.wordpress.com/2020/03/08/cesar-seasonal...

  Claimed performance:

  Storage volume: 200-250 kWh/m3

  Storage cost: 2 cent/kWh

  Storage leak: 80% energy still present after 6 months

Awesome and very simple. There is a small community of eco housing that uses this system, in Boekel. They use solar panels in summer to heat the battery which lasts all winter.

On a diy scale, there is also this system for passive greenhouse called a climate battery which uses the same principle, storing heat underground and using it in winter. With almost zero energy we can use it to grow bananas in a greenhouse in the Netherlands.

The article points out that it doesn't actually use what people usually think of as sand:

"The sand itself will also be sustainably sourced – it’ll consist of crushed soapstone, which is a manufacturing byproduct of another local industry."

And this is key to the finances...

Waste material costs over €100/ton to dispose of across most of the EU. So therefore this material costs less than nothing!

That's why the project doesn't use water (which has a 4x higher heat capacity and can be pumped allowing the heat exchangers to be far smaller and eliminating the 'temperature decreases as it discharges' problem).

For construction you need a special kind of sand (still pretty common on average). For this battery you can use sand unusable for construction.

Sand used for construction should not have salt content.

For energy storage, any sand is sufficient.

It is still inexpensive, they just steal a lot