A Computational Study of Optically Concentrating, Solar-Fuels Generators from Annual Thermal- and Fuel-Production Efficiency Perspectives

Abstract

The commercial deployment of wireless photoelectrochemical cells (PECs) may provide a viable means to close the anthropogenic carbon cycle associated with the global transportation sector. The growing body of research on PECs has largely focused on developing and integrating the materials necessary for robust, efficient solar-fuel production on the laboratory benchtop. While these efforts are a prerequisite for the commercialization of PECs, deployed PECs will have to contend with extreme heat, cold, and insolation variations in the outdoor environment. They will also have to operate efficiently throughout their lifetime and in multiple locations. This paper reports a computational framework for estimating the hourly profiles of time-varying temperature and solar-to-hydrogen efficiency that optically concentrating, wireless PECs will attain over the course of a typical year. It is found that annual weighted average solar-to-hydrogen efficiencies in excess of 9 to 11% can be achieved in extremely cloudy and sunny locations, respectively. Additionally, typical PECs will likely incur damage due to overheating or freezing; measures to protect PECs from extreme heat and cold are outlined. The findings also help to bring into focus issues regarding real-world deployment of energy-generation technologies and methodologies towards tackling them. The development of solar-fuel-producing photoelectrochemical cells (PECs) could be a catalyst for decarbonizing the transportation sector, which is critical if we are to reduce anthropogenic global greenhouse-gas emissions. This is because the global transportation sector derived more than 99% of its primary energy from hydrocarbon combustion in 2010 1 and accounted for 22% of anthropogenic greenhouse-gas emissions in 2012.2 These emissions are predicted to grow more than 40% by 2040, 1 enabled by the enormous supply of remaining fossil-fuel reserves that can be refined into transportation fuels.3 Thus, despite probable future increases in the cost of oil and advances in battery technology, it is likely that chemical fuels will directly power the global transportation sector for many years to come.1 For a transition to renewable, PEC-generated fuel to be successful, however, PECs must be deployed on a very large geographical scale due to the diffuse nature of sunlight and the large global energy demand, as well as function for many years. 4,5 PECs convert solar energy into an electric potential using a photovoltaic (PV) light absorber, which then produces fuel via electrochemical reactions at an anode and cathode immersed in an electrolyte. PECs that produce H 2 and O 2 as products consume H 2 O as a reactant. In those that utilize acidic electrolyte, the water-splitting reaction can be understood using three simplified reaction steps. In the first step, photons above the band gap energy of the PV cell must be absorbed to create electron holes (h o ) and electrons (e − ), where n J is the number of junctions in the PV cell,In step 2, the electron holes oxidize the H 2 O at the a...

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