Near Isothermal edit
Near isothermal compression (and expansion) is a process in which air is compressed in very close proximity to a large incompressible thermal mass such as a heat absorbing and releasing structure (HARS) or a water spray. A HARS is usually made up of a series of parallel fins. As the air is compressed the heat of compression is rapidly transfer to the thermal mass, so the gas temperature is stabilised. An external cooling circuit is then used to maintain the temperature of the thermal mass. The isothermal efficiency (Z)[1] is a measure of were the process lies between an adiabatic and isothermal process. If the efficiency is 0% then it is totally adiabatic with an efficiency of 100% it is totally isothermal. Typically with a near isothermal process an efficiency of 90-95% can be expected.
Near Isothermal edit
A number of methods of near isothermal compression are being developed. Fluid Mechanics has a system with a heat absorbing and releasing structure (HARS) attached to a reciprocating piston[2]. Light Sail inject a water spray into a reciprocating cylinder[3]. SustainX use an air water foam mix inside a compressor. All these systems ensure that the air is compressed with high thermal diffusivity compared to the speed of compression. Typically these compressors can run at speeds up to 1000 rpm. To ensure high thermal diffusivity the average distance a gas molecule is from a heat absorbing surface is about 0.5mm. These near isothermal compressors can also be used as near isothermal expanders and are being developed to improve the round trip efficiency of CASE.