A way to economically store surplus energy generated by wind and solar farms for use during calm/dark conditions and in an environmentally friendly way remains the last great frontier for renewable energy.
The challenge has already been met to a degree with technologies such as molten salt batteries and solar water pumping, i.e. pumping water from a low to high point to be released when required and the energy of the flow converted to electricity via turbines. The problem with using reservoirs for energy storage is the cost involved and the huge amount of space they require.
Another option may be gravel. If you’ve ever walked barefoot on a gravel road on a summer’s day, you’ll understand how hot gravel can get, even after the sun goes down!
UK-based Isentropic says it has designed a system that uses the company’s Isentropic heat pump to create electricity from energy stored in gravel in a thermal form – a thermal battery.
The battery is made up of two large containers of gravel. Electricity is used to power a pump to heat and pressurise air to 500C; which is fed into one of the containers of gravel. As the air gives up its heat to the gravel, expands and leaves the first container; it is much cooler and is fed into the second, dropping temperature of that gravel to -150C. In order to regenerate the electricity, the cycle is simply reversed. The temperature difference is used to run the Isentropic machine as a heat engine to make electricity.
Isentropic says its Pumped Heat Storage Plant could occupy as little as 1/300th of the area of pumped water storage facilities and at a cost of USD $55/kWh – and $10/kWh at scale.