Solar fuel processing: Comparative mini-review on research, technology development, and scaling

Solar energy provides an unprecedented potential as a renewable and sustainable energy resource and will substantially reshape our future energy economy. It is not only useful in producing electricity but also (high-temperature) heat and fuel, both required for non-electrifiable energy services. Fuels are particularly valuable as they are energy dense and storable, and they can also act as a feedstock for the chemical industry. Technical pathways for the processing of solar fuels include thermal pathways (e.g. solar thermochemistry), photo pathways (e.g. photoelectrochemistry), and combinations thereof. A review of theoretical limits indicates that all technical solar fuel processing pathways have the potential for competitive solar-to-fuel efficiencies (>10 %) but require very different operating conditions (e.g. temperature levels or oxygen partial pressures), making them complementary and highly versatile for process integration. Progress in photoelectrochemical devices and solar thermochemical reactors over the last 50 + years are summarized, showing encouraging trends in terms of performance, technological viability, and scaling.