3 ms·

Isn't the round trip efficiency of compressed air terrible? You lose a lot of energy as heat when you compress air which you then have to resupply when expandin

• pfdietz 4y ago
  There is adiabatic compressed air, where the heat of compression is transfered to a thermal store, cooling the compressed air before it's injected into the storage cavern. Then, on discharge, the heat is recovered from the thermal store and used to reheat the air. This illustrates, btw, that compressed air doesn't actually storage energy! The energy content of a gas is the kinetic energy of its molecules (and some small contribution from the potential energy of molecules at the moments they bounce off each other, but that's typically minor). The kinetic energy of the molecules is a function of temperature, but not of pressure. What the air is doing is acting as a store of reduced entropy, which can later be exploited to convert that separate heat back to work at high efficiency. However, one could also do this by PTES, Pumped Thermal Energy Storage, where a reversible thermal cycle produces heat and cold, which are stored separately, then brought back together to recover the work. This is like adiabatic CAES, but instead of storing the cooled compressed air, it is expanded through a turbine and the cold of the resulting expanded gas is stored (say in a tank of mild cryogenic hexane).