Additionally, a portable ultra-small hydropower generator, one meter long, has been developed. It can generate power in a stream with a one-meter height difference, producing 5W with a flow rate of 10 liters per second.
In Finland, sand batteries are also in use. These store electricity generated from solar and wind power as heat in sand. The insulated tank is 4 meters wide and 7 meters high, containing 100 tons of sand. This heat is supplied to the surrounding area for building heating and warm swimming pools. Sand heated to over 500°C can store energy for several months. The lifespan is several decades. Any dry sand without flammable waste can be used, making this feasible in Japan.
In Finland, it is calculated that to supply heat to a district of 35,000 people, a storage tank 25 meters high and 40 meters in diameter filled with sand would be required. The sand battery's structure is simple, consisting of pipes, valves, fans, and electric heating elements, and the construction costs are low.
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