This article discusses a novel battery, a heat pump that uses gravel (small rocks) to store heat. When renewable energy sources are producing surplus energy, the surplus could be used to heat the gravel, and when there’s a shortage of energy, the system can draw heat from the gravel.
The beauty of such a system is that it doesn’t require a specific type of terrain, like pumped hydro. And it’s more efficient. Between 20% and 28% of the heat is lost in storing and then retrieving the heat, which compares favourably with the 26% energy loss for storage using pumped hydro. Costs amortized over 25 years are estimated to be about $50 per megawatt-hour, while pumped hydro is about $65 per megawatt-hour. It’s a battery with no dangerous chemicals.
(Tree Hugger: 25 June 2014)