Maximum thermionic energy conversion efficiency based on the thermodynamics of irreversible processes

An electron gas in a vacuum between two electrodes is treated as a thermoelectric material, with thermopower, electrical conductivity and thermal conductivity. Figure of merit zT values obtained from the analysis are very high compared with solid thermoelectrics due to the absence of parasitic phonon thermal conductivity. Due to the large zT, thermionic conversion can work even in the ignited mode regime, which is very paradoxical as a thermodynamic cycle. In this mode, the electron gas (the working fluid of the cycle) is first heated by electrical power to a temperature T_M much higher than the evaporator temperature T_E, in order to generate positive ions for electron space charge compensation. The electrons are then cooled from T _M to T_C (the condenser temperature) and enough electrical power is generated to compensate for losses inside the cycle and to provide power to the electrical load. The voltage drop required in order to maintain the ignited mode regime is evaluated