A self-consistent computational model for a thermionic energy convertor*

Abstract

Summary form only given. High grade heat sources used to drive mechanical conversion systems suffer from losses of 30-40% due to friction and fluid losses, as well as venting as much as 40% of the energy as waste heat. System complexity and cost is also driven by conversion mechanisms. Direct conversion of part or all of the heat to electrical energy could be part of an overall efficiency improvement strategy. The direct conversion mechanism should not only be efficient, but also inexpensive, compact, and low maintenance. Potential applications range from high temperature small engine designs to nuclear power direct converters. Thermionic energy converters (TECs) can meet many of these criteria. A TEC comprises a small vacuum gap (~10 microns) between a thermionic emitting electrode and a collector, sometimes with a fill gas to neutralize some of the space charge. TECs can operate at high temperatures approaching the adiabatic combustion temperature of most hydrocarbon fuels or the core temperature of a nuclear reactor, and hence have a high Carnot efficiency. They have no moving parts, and use electrons in a vacuum or low pressure gap as the working fluid, eliminating friction and working fluid losses and associated complicated parts and maintenance. Current TEC performance is constrained by the space charge limited current within the gap. The transmitted current can be increased using neutralizing plasma, but the current required to ionize the background gas reduces the power output of the TEC by about 50%. In this work, a one-dimensional model of a TEC using argon gas is developed. The model includes self consistent space charge effects, kinetic effects, impact ionization of the background gas, and an external circuit including the external load and impact on the electron transport in the gap via the surface charge. The model is implemented in the one dimensional particle-in-cell code, XPDP1 and compared with theoretical predictions