One of the biggest challenges for solar and wind powers has been 24/7 non-stop of supply. The grid needs to be powering even when the sun has set and wind has stopped. To solve this energy storage needs to be installed. High-tech solutions like Li-Ion battery, compressed air storage, melted salt, etc, have been proposed. Now, a California based company, Advanced Rail Energy Storage (ARES), comes out with a energy storage solution using old technology - gravity and heavy train.
The idea is simple! When solar farms or windmills are producing excess power, then it will be used to push a train loaded with heavy weight up the hill. When power is needed again, like in the night or hot sunny day when demand is high, the train will be rolling down the hill, by gravity, and regenerating power back to the grid. The charge/discharge efficiency can now reach 80%.
Battery poses great environmental issues when disposing. Re-pumping water back to dam, a commonly used storage method by utility company, with excess power also is expensive and wasteful, and building dams also cause environmental damage. ARES's solution is clean and produces no harm to the environment even at the end of its life.Also, railway can easily last for 40 years.
The other advantage of ARES's technology is scalable, up to gigawatt storage.