Pumped Thermal Energy Storage (PTES) is a new idea for a method to store energy, exploiting the high energy density of sensible heat contained in solids. The process stores energy as sensible heat and cold in both a high temperature and low temperature vessel. The principle idea is to take electrical energy from the grid, using it to pump heat from the cold vessel to the hot vessel, propagating a cold thermal front through the low temperature storage vessel and a hot thermal front through the high temperature storage vessel. The charging process is analogous to a heat pump, and can achieve coefficients of performance (COP) many times unity — by pumping heat from cold to hot it is possible to move more heat energy than the electrical energy input that is inputted. The discharging process is then a heat engine, using the difference in temperature between the hot and cold storage tanks to generate work. This allows a warm front to propagate through the cold storage and a cool front through the hot storage, bringing the system temperatures closer together. Even though the conversion of heat to electrical energy is relatively inefficient in absolute terms, the fact that the heat pump can have a high COP means that preliminary calculations suggest an efficiency in excess of 70% is achievable. The Figure shows a schematic diagram of a PTES system.
Figure: Schematic of a PTES system
The PTES idea has several advantageous aspects. It should have a favourable volumetric energy density compared with many other types of energy storage (with bricks at 1000oC the energy density is about 600 kWh/m3 (water at 360 m height has 1 kWh/tonne, Li-Ion batteries have about 200 kWh/tonne)), it doesn’t require any favourable geography and percentage energy losses from heat losses are reduced as the system size gets bigger (this is because heat losses depend on surface area whereas capacity depends on volume). However, it is a new technology in research and development, and currently only prototype devices about which information is commercially sensitive and thus scarce exist. The main company trying to develop this technology into a grid energy storage solution is Isentropic Ltd, a UK based company and there is some interest from Italian oil and gas company Saipem . Isentropic claim that they can achieve a levelised cost of storage of $35/MWh.