A system for recycling heat or energy of a working medium of a heat engine for producing mechanical work is described. The system may comprise a first heat exchanger (204) for transferring heat from a working medium output from an energy extraction device (202) to a heating agent to vaporise the heating agent; a second heat exchanger (240) for transferring further heat to the vaporised heating agent; a compressor (231) coupled to the second heat exchanger (240) arranged to compress the further-heated heating agent; and a third heat exchanger (211) for transferring heat from the compressed heating agent to the working medium. A heat pump is also described.