The second life of oil wells (2 photos)
The American company Geo2Watts has proposed an unusual way to utilize idle oil and gas wells: converting them into underground heat storage batteries. 
The idea is particularly interesting for solar energy. On sunny days, power plants often generate more electricity than the grid can handle, so some of their generation must be limited. The Borehole Battery system uses this excess to operate a high-temperature heat pump, which heats water to 200°C. The accumulated heat is stored in a sealed borehole and can be converted back into electricity when needed.
If at least some of these abandoned wells are converted into energy storage facilities, gigawatts of long-term storage could be generated without building new facilities, while simultaneously giving old oil infrastructure a new lease of life. 
