the sand up to around 500 degrees celsius. crucially, the sand can keep this heat without loss, potentially for months. here in kankaanpaa, the heat in the sand battery is then used to help power the district heating system, keeping homes, offices and indeed swimming pools warm all year round. heat is the biggest end use for energy all over the world. it s also critically important for survival in a country like finland, where the winter is long, cold and dark. and the recent move by russia to cut off supplies of gas and electricity in response to finland joining nato has caused concern. as the world has shifted to green energy,