A small town in Finland is about to ditch fossil fuels in its heating network thanks to a sand-filled energy storage tank the size of a house. Finnish startup Polar Night Energy recently turned on the so-called sand battery in the municipality of Pornainen, an hour north of Helsinki.  The machine, which uses dirt to store excess renewable energy as heat, will warm the homes and businesses in the town of 5,000 people. It is expected to replace natural gas and oil in Pornainen’s district heatin...