Finland's town of Pornainen has switched on a thermal battery that uses crushed rock — also known as sand — to provide plentiful, cheap renewable energy.

It might sound too good to be true, but the battery works using simple physics. 

As TechCrunch detailed, electricity heats the sand, which is stored in an insulated silo. That thermal energy can then be accessed for various functions, including as a heat source for buildings. 

Pornainen's sand battery was made by Finnish startup Polar Night Energy. The 2,000 tons of sand is able to store 1,000 megawatt-hours of heat for weeks at a time. 

According to TechCrunch, that's enough for a full week's worth of heating in the dead of winter. The sand is remarkably efficient at maintaining its heat, losing only 10% to 15% from storage to recovery.

And price-wise, it's hard to imagine a cheaper energy storage system. The pulverized soapstone inside Pornainen's sand battery was discarded from a fireplace maker; it was "basically trash," TechCrunch wrote. 

The storage silo was relatively easy and cheap to build, and the power it provides is much cheaper than that from other alternative batteries. 

A smaller prototype cost approximately $25 per kilowatt-hour, TechCrunch explained, and it's likely that the current model is even more efficient. For comparison, lithium-ion batteries cost roughly $115 per kilowatt-hour.

The sand battery will also dramatically lower Pornainen's toxic planet-warming emissions by reducing its dependence on burning oil or gas to provide heat for the town. 

It's a planet-friendly decision in line with Finland's renewable-heavy energy portfolio, which consists of 43% renewables and 26% nuclear power. The country's electricity prices also happen to be the cheapest in Europe, according to European Union data. 

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TechCrunch shared that other startups are looking into thermal batteries, particularly as the renewable energy that serves as their electricity source has become more abundant and affordable. 

For example, Scottish company Sunamp is building a thermal battery that uses a sodium compound typically found in salt-and-vinegar potato chips. 

Several companies are also experimenting with using bricks and graphite blocks as small yet mighty thermal batteries to store heat in a building's own materials.

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