Where and how is entropy generated in solar energy conversion systems?

Abstract: The hotness of the sun and the coldness of the outer space are inexhaustible thermodynamic resources for human beings. From a thermodynamic point of view, any energy conversion systems that receive energy from the sun and/or dissipate energy to the universe are heat engines with photons as the "working fluid" and can be analyzed using the concept of entropy. While entropy analysis provides a particularly convenient way to understand the efficiency limits, it is typically taught in the context of thermodynamic cycles among quasi-equilibrium states and its generalization to solar energy conversion systems running in a continuous and non-equilibrium fashion is not straightforward. In this educational article, we present a few examples to illustrate how the concept of photon entropy, combined with the radiative transfer equation, can be used to analyze the local entropy generation processes and the efficiency limits of different solar energy conversion systems. We provide explicit calculations for the local and total entropy generation rates for simple emitters and absorbers, as well as photovoltaic cells, which can be readily reproduced by students. We further discuss the connection between the entropy generation and the device efficiency, particularly the exact spectral matching condition that is shared by infinite-junction photovoltaic cells and reversible thermoelectric materials to approach their theoretical efficiency limit.