Thermodynamic Limit of Solar to Fuel Conversion for Generalized Photovoltaic–Electrochemical Systems

Daily and seasonal variability of the solar irradiation poses a major hurdle to the widespread adoption of photovoltaic (PV) systems. An integrated photovoltaic-electrochemical (EC)storage system offers a solution, but the thermodynamic efficiency (η
_sys
) of the “ideal” integrated system and the optimum configuration needed to realize the limit is known only for a few simple cases. Moreover, these limits are often derived through complex numerical simulations. In this paper, we show that a simple, conceptually transparent and physically intuitive analytical formula can precisely describe the η
_sys
of a “generalized” PV-EC integrated system. AnM-cell PV module of N-junction bifacial tandem cells is illuminated under S-suns and mounted over ground of albedo R. There are K-EC cells in series, each defined by their reaction potential, exchange current, and Tafel slope. We derive the optimum thermodynamic limit η
_sys
(N, M, K, R, S) for all possible combinations of a PV-EC design. For a setup with optimal-(M, K) and large N, under 1-sun illumination and albedo = 0, the ultimate limit is η
_sys
~ 52%. A comparison of our results with experimental results published by various groups worldwide suggests opportunities for further progress toward the corresponding thermodynamic limit.