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 <italic>thermodynamic</italic> efficiency <inline-formula><tex-math notation="LaTeX">$({\eta _{{\rm{sys}}}})$ </tex-math></inline-formula> 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 <inline-formula><tex-math notation="LaTeX">${\eta _{{\rm{sys}}}}$ </tex-math></inline-formula> of a “generalized” PV-EC integrated system. An <italic>M</italic>-cell PV module of <italic>N</italic>-junction bifacial tandem cells is illuminated under S-suns and mounted over ground of albedo <inline-formula><tex-math notation="LaTeX">$R$</tex-math></inline-formula>. There are <inline-formula> <tex-math notation="LaTeX">$K$</tex-math></inline-formula>-EC cells in series, each defined by their reaction potential, exchange current, and Tafel slope. We derive the optimum thermodynamic limit <inline-formula> <tex-math notation="LaTeX">${\eta _{{\rm{sys}}}}({N,M,K,R,S})$</tex-math></inline-formula> for all possible combinations of a PV-EC design. For a setup with optimal-<inline-formula><tex-math notation="LaTeX">$({M,K})$ </tex-math></inline-formula> and large <inline-formula><tex-math notation="LaTeX">$N$</tex-math></inline-formula>, under 1-sun illumination and albedo = 0, the ultimate limit is <inline-formula><tex-math notation="LaTeX"> ${\eta _{{\rm{sys}}}}\sim 52$</tex-math></inline-formula>%. A comparison of our results with experimental results published by various groups worldwide suggests opportunities for further progress toward the corresponding thermodynamic limit.

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