Simple upper bound efficiencies for endoreversible conversion of thermal radiation

Simple upper bound formulae for the efficiency of converting the energy of thermal radiation into mechanical work are derived. They are upper bounds as they refer to a large class of systems. They are simple as they are functions of two parameters only, namely the temperatures of the energy sources. The original contributions are as follow: (1) Two sorts of "cold" thermal reservoirs are considered. (2) An endoreversible thermal engine is used here to convert radiation energy into work. A number of existing theories are particular cases of the present approach.