Our system is a highly reversible, gas cycle machine that works as both an engine and a heat pump. It is the first time that a reversible system has been developed both to store and recover electricity using a thermodynamic approach.
The received wisdom is that thermodynamic devices are not very reversible, but that certain thermodynamic processes in isolation can be highly reversible. This led to the direct incorporation of heat exchange to a storage medium within the gas circuit of our heat engine. The key innovation is the reduction to four highly reversible processes within the cycle with integrated heat exchange to the storage medium.
The storage system uses two large containers of mineral particulate. Electricity is used to pump heat from one vessel to the other resulting in the first container cooling to around -160°C and the second container warming to around 500°C. The specially designed heat pump machine can be thermodynamically reversed to operate as an engine and the electricity is recovered by passing the heat from the hot container back through the machine to the cold container, while the machine drives an electrical generator. During the process, the containers return close to their original temperatures.
The innovations that have allowed high efficiency to be realised are primarily concerned with ensuring that each individual process within the system is performed with minimum losses. This results in an electricity-in to electricity-out (round trip efficiency) in the range of 72 to 80%. This compares very favourably with typical pumped-hydro values of 74%.Hide