A directly charged thermal store for compressed air energy storage systems

Abstract

This paper discusses the design of a heat storage unit with integrated heat exchangers (TES + HX), which is intended to work in a Compressed Air Energy Storage (CAES) system. The unit can be charged directly by the system's stream of pressurised air, eliminating the need for additional heat exchangers and reducing the number of heat transfer processes. Silica sand is used as the storage medium due to its high heat capacity, non-corrosivity and its ability to accommodate the thermal expansion of steel pipes.A medium-scale, medium-duration CAES system (250 kW/1MWh) is used as a case study. The heat storage subsystem comprises a packed-bed thermal store, three air-to-air heat exchangers and an ambient pressure air blower. Combined, this subsystem has an approximate cost of £147k and achieves an efficiency of ∼89 %, which translates into a levelized cost of ∼48.5 £/MWh. An integrated TES + HX unit can achieve a levelized cost of ∼35 £/MWh. The unit has an estimated cost of £38.5k and achieves an overall roundtrip exergy efficiency of ∼91.8 %. The integrated TES + HX unit not only offers a significant reduction in the capital cost of a CAES system and an efficiency improvement, but it also allows for a simpler overall architecture.