I’m no expert for sure. I’m surprised they can be as much as 70% efficient, I would have thought the heat losses would be much greater. But clearly it doesn’t work like that.
Some of the diagrams I've seen in the past show that as the air comes out of the compressor its passed through a gravel bed before being allowed into the general mine. This picks up a lot of heat and then when the system is run in reverse the incoming air is pre-heated by the gravel bed prior to going into the turbine.
