Thermodynamic limit of solar to fuel conversion for generalized photovoltaic-electrochemical system

Variability of the energy output throughout the day/night poses a major hurdle to the widespread adoption of photovoltaic systems. An integrated photovoltaic (PV) and electrochemical (EC)-storage system offers a solution, but the thermodynamic efficiency ({\eta}_sys) of the integrated system and the optimum configuration needed to realize the limit are known only for a few simple cases, derived though complex numerical simulation. In this paper, we show that a simple, conceptually-transparent, analytical formula can precisely describe the {\eta}_sys of a 'generalized' PV-EC integrated system. An M-cell module of N-junction bifacial tandem cells is illuminated under S-suns 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 of {\eta}_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 for {\eta}_sys ~ 52%. The analysis will unify the configuration-specific results published by diverse groups worldwide and define the opportunities for further progress towards the corresponding thermodynamic limit.