For renewable energy to be viable at grid scale, it must be able to store energy for later use. Isentropic Energy says they can do it with pumped heat. Use the heat differential between two tanks of compressed gas and gravel to power turbines. Surplus renewable energy heats one tank.
At first, the compressed gas is made to pass through one of the stores. This heats up the gravel up to 500 degrees Celsius and during heat exchange, the temperature of the gas almost drops to ambient condition. In the second step, this gas is made to expand to its original pressure and as a result the temperature drops to –160 degree[s] Celsius. It is then passed through the other store and it exchanges heat with the gravel by direct contact. In this process, the gas is warmed back to its original temperature.