Renewable energy generations sources are not reliable(?). Solar can only generated electricity during the day and wind energy works when the wind is blowing (usually early mornings). So there is a need to develop energy storage solutions that will store energy produced during low demand period to be used later.
The energy storage solutions range from Lead acid batteries, Lithium Ion batteries,
flow batteries, fly wheels, super capacitors, Sand Battery Storage and Compressed
Air Energy Storage (CAES) etc.
Lithium Ion batteries is seen as the top solution for energy storage.
Sand Battery Storage uses sand to store energy. Currently experiemented in Finland