Fuelless CAES is an energy storage concept that removes the natural gas combustion from conventional diabatic CAES. There are two main variations, dubbed either (Advanced) Adiabatic CAES ((A)A-CAES) or isothermal CAES. In adiabatic CAES the heat generated by the compression of air (the charging process) is stored in a Thermal Energy Store (TES) which is separate from the ambient temperature high pressure air store. On discharge the high pressure air is reheated using this stored heat and then expanded. Without the stored heat the process has an unacceptably low efficiency. Isothermal CAES aims for an isothermal compression process (or at least as close to it as possible) so that the temperature achieved during the compression is not far above ambient. Presently this process generally involves a water spray in the compression chamber of a specially designed reciprocating compressor/expander unit. The water droplets absorb the heat of compression and their high specific heat capacity causes the temperature increase to be much smaller. This warm water is then stored and on discharge is re-injected as a mist into the reciprocating machine which now acts as an expander. The expected efficiency of these systems is debated – for adiabatic CAES academic literature gives efficiencies of 60-75% while the companies working on isothermal CAES, Lightsail Energy, General Compression and SustainX, have a similar goal of above 60% for their first generation of machines and believe that 75% is achievable in the long term. The largest planned demonstration is a 290 MW adiabatic CAES project based in Germany called project ADELE, while SustainX isothermal prototype is 1.5 MW.
Figure: A schematic diagram of an adiabatic CAES system. C, HE, TES and E represent compressors, heat exchangers, thermal energy stores and expanders respectively.
Fuelless CAES systems are unproven technologies, with the expertise on A-CAES spread across a number of different universities and small tech companies – some of which are in the UK – and the expertise on isothermal CAES based in the US. It is an unproven technology with promise for use in renewables integration, energy management, peak shaving and grid reserves.