The Effect of Radiation Shields on Power and Efficiency of an Internally Heated AMTEC Cell

Radiant heat transfer between the coated solid electrolyte surface and a condenser in an AMTEC cell can be reduced by the use of radiation shields. The shields do not deliver or remove any heat from the system, but only increase the “resistance” of the heat flow path. By reducing heat transfer, radiation shields serve not only to minimize parasitic heat loss, but also to reduce thermal stresses in the electrolyte below a critical level. A radiation shield also increases the impedance to atom flow and reduces power density. The pressure drops across gas flow impedances in the Knudsen regime are proportional to flow rate and since the effect on voltage of pressure is logarithmic through the Nernst equation, the loss of power by insertion of a shield may be acceptably small for some practical circumstances. In this paper, we present calculations of radiation transfer and power loss for a model AMTEC that consists of an internally heated, infinitely long electrolyte cylinder surrounded by a coaxial radiation shield and a reflecting condenser. Photon and atom emission and reflection are assumed to be diffuse, and all surfaces are gray bodies. The effect of uniformly distributed pores or perforations in a shield is considered.