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 genera

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.