Performance evaluation and optimization of the cooling system of a hybrid thermionic-photovoltaic converter

Hybrid thermionic-photovoltaic (TIPV) converters are efficient and clean solutions for the direct conversion of thermal energy to electricity, taking advantage of both the photovoltaic and thermionic phenomena. An important hurdle for their efficient operation is the overheating of the PV cell integrated within the TIPV anode, due to partial conversion of the emitted electron and photon fluxes to thermal heat. This obstacle needs to be overcome with an efficient, yet practical, cooler. In this work, a copper plate heat spreader is experimentally tested for TIPV cathode temperatures up to 1450 degrees C, whilst its performance is also assessed using a validated CFD model for temperatures up to similar to 2000 degrees C. A multi-parametric analysis is conducted testing two coolants: i) a water/ethylene glycol mixture at various temperatures (-5-40 degrees C) and mass flow rates (0.05-0.4 kg.s(-1)), and, ii) cryogenic liquid nitrogen at a temperature of -196 degrees C and mass flow rate of 0.074 kg.s(-1). Numerical results reveal that with water/ethylene mixture the PV can withstand heat fluxes up to 360 W.cm(-2), without its temperature exceeding 100 degrees C. For higher thermal fluxes (360-600 W.cm(-2)), cryogenic liquid nitrogen is found to prevent the PV overheating and, therefore, is an attractive coolant; however, it poses safety concerns due to its possible boiling. Finally, two additional cooling system designs are proposed, a heat sink with straight fins and another with copper pipes, which offer higher heat transfer areas, but are more difficult to manufacture, than the copper plate heat spreader.