capable of storing green power for months at a time, is up and running in Finland. 

One obstacle to year-round renewable power is the difficulty of capturing and storing energy when intermittent sources like wind or solar energy are unavailable. Finnish engineers at Polar Night Energy have employed a solution in a simple material: low-grade sand.
The battery they devised involves a tank measuring 13 by 23 feet, filled with 100 metric tons of sand, for 100 kW heating power and 8 MWh capacity. Solar or wind energy is converted into hot air, which keeps the sand at a temperature of 500 degrees Celsius (932 degrees Fahrenheit). For the town of Kankaanpää, that heat warms water that is then pumped to local buildings via a district heating network, serving some 10,000 people.

Unlike expensive lithium batteries, which leave a considerable physical footprint, the sand battery has a minimal environmental footprint. Developers say the design won’t be affected by sand shortages in the glass and concrete industries because the technology uses low-grade sand or sand like materials.