Thermal-integration in photoelectrochemistry for fuel and heat co-generation

Abstract

Photoelectrochemical (PEC) devices for hydrogen generation typically use liquid water as reactant and operate under natural sunlight (irradiation of 1 kW m⁻²). However, devices can instead use the vapor in ambient air as reactant, include solar irradiation concentration, and consider the co-generation of low- and high-temperature heat. This expands the possible PEC device designs to include everything from vapor-fed PEC devices to complex designs featuring solar concentration and beam splitting to better utilize the entire solar spectrum. In this work, fundamental heat and mass transfer limits for liquid- and vapor-fed designs using solar concentration are shown. Four representative system configurations are studied, featuring both liquid and vapor supplied PEC devices, solar concentration on the PEC and on a cavity type solar receiver, and beam splitting. The systems produce hydrogen, low temperature heat collected from the PEC, and (in some cases) high temperature heat (at 500 °C) from a solar receiver under concentrated irradiation. Systems are optimized for band gaps and optical cutoffs and compared on an energy and exergy basis. For systems not utilizing solar concentration on the PEC, vapor-fed designs are shown to be optimized at higher band gaps than liquid-fed designs. In systems using concentration, overall exergetic efficiencies are generally lower due to optical losses and the unusable diffuse fraction of radiation, but they have the benefit of producing high-temperature thermal energy as a useful product. Tuning these systems for the optimal balance of both hydrogen and heat may be critical in industrial processes using both products, such as in solar fuels or green ammonia production.