Compressed Air Energy Storage (CAES) is one type of commercialized multi-scale electrical energy storage technology. A large-scale CAES plant can provide a power output of over 100 MW from a single unit. A schematic diagram of CAES plant is shown in the figure below.
A CAES system is composed of ﬁve major components:
- A motor/generator unit combined with power conditioning devices (including power electronic converters/inverters) and clutch mechanisms which provide alternate engagement to the compressors or the turbines.
- A multi-stage air compressor unit operating with intercoolers and after-coolers is used to achieve economic air compression process.
- An underground cavern (for large-scale CAES) or an over ground container (for small-scale) is for storing a certain amount of compressed air.
- A turbine train is composed of both high-pressure and low-pressure turbines.
- There are some other devices for control and auxiliaries, such as fuel storage and heat recuperator (recovery) units.
The working process of the CAES system can be considered as decoupling the compression and expansion cycles of a conventional gas turbine into two separate processes. During the periods of low power demand, the surplus electricity drives a reversible motor/generator unit in turn to run a chain of compressors for injecting air into a storage vessel, which is either an underground cavern or over ground tanks. The compression process normally uses intercoolers and after-coolers to reduce the working temperature of the injected air thus improving the compression efficiency and minimising thermal stress on the system component walls. The energy is stored in the form of high pressure air (typically 40-80 bar for underground storage, up to 300 bar for over ground storage). When the power generation cannot meet the load demand, the stored compressed air is released and heated by a heat source which can be the heat come from combustion of fossil fuel or the heat recovered from the compression process. The compressed air energy is finally captured by the turbines. The waste heat from the system exhaust can be recycled by a recuperator (heat recovery) unit.
Technology breakthrough – CAES:
- to avoid involvement of fossil fuel – to improve the round trip efficiency
- to gain a clear picture of national storage resources
- to study the methodology of engineering storage
- to map the storage with the renewable power generation locations
- to maximise renewable energy penetration