Thermionic Refrigeration with Planar and Nonplanar Electrodes - Chances and Limits -

In this paper for the first time a precise theoretical analysis of efficiencies and power densities for nano-scaled thermionic vacuum gap devices is presented. Plane electrodes with nano-gaps will be treated first. With nonplanar electrodes (nanotips) enhanced electron field emission occurs, giving rise to new design options. The theories of field and thermionic emission are combined by calculating the transmission coefficient for the surface potential barrier exactly numerically for thermionic and tunnelling energies. This then is used for current and energy transport determination and derivation of cooling and generator efficiencies. Because of the high work function of metals, efficient thermionic electron emission takes place only at high temperatures. At low temperatures also efficiencies near Carnot are possible but with very low power densities. The energy exchange processes in the cathode during emission are reviewed and an improved model is presented.