On availability of the optimal regimes in a Knudsen Cs-Ba thermionic converter at high emitter temperatures

Abstract

A thermionic energy converter (TIC) functioning in collisionless regime at high emitter temperatures demonstrates very high output performance. In practice, such a regime has been realized at moderate temperatures (up to 2300 K) when its inter-electrode gap is filled with Cs and Ba vapors. Changing the Ba vapor pressure, one can vary the emitter work function and obtain optimum output performances, moreover maximum output power being available with any emitter material. The higher the emitter temperature, the more pronounced a shift of this maximum is toward higher Ba pressure, and at high temperature a TIC happens to be working in the collision regime before the maximum power point. We have analysed some experimental data in the literature when studying the emission parameters of refractory metals in the presence of the Ba vapor as well as in the Cs-Ba TIC experiments. Through our analysis, as well as results on the TIC optimization, the Ba optimum pressure is evaluated to obtain maximum power at the high emitter temperature for a set of refractory metals. We conclude that the maximum output performance of the Knudsen TIC with W and Re emitters can be reached up to the emitter temperature as high as 2600 K.