Design of a combined heat store and heat exchanger for CAES Systems

A combined heat store and heat exchanger unit (HSX) intended for compressed air energy storage (CAES) is presented. The unit is directly charged by the pressurised air emerging from the compression train, which removes the need for a secondary low-pressure air circuit. Salt is used as the thermal storage medium due to its good heat capacity, thermal conductivity, and its ability to accommodate the thermal expansion of the stainless-steel pipes. This paper uses a CAES system (15 MW, 12-h discharge) driven by an offshore wind turbine as a case study. There are not many commercial CAES systems in operation; however, the levelized cost of the heat storage subsystem of a CAES system (i.e. heat store, set of heat exchangers and ancillary low-pressure circuit) ranges between 45 and 48 £/MWh. Findings show that the most cost-effective design for a HSX has a capital cost of ∼£55k. This translates into a levelized cost of storage of ∼31.5 £/MWh. The roundtrip exergy efficiency of this design is 93.7 %. This accounts for heat-exergy and pressure-exergy losses; losses to ambient are not considered. A HSX unit can considerably reduce the overall cost of a CAES system.