An improved heat exchanger for use in a gas compression/expansion apparatus. The heat exchanger comprises: a tube connected with a compression/expansion space of a compression/expansion cylinder of the apparatus; a central cylinder coaxially disposed in the tube such that a narrow gap for the working gas is formed only between the central cylinder and the tube. The heat exchanger may liberate heat from, or providing heat to, the compression/expansion cylinder through heat transfer between the working gas in the tube and an ambient medium surrounding said tube. The heat exchanger is capable to serve as a low/high temperature heat source for an external heat load.