A general framework for the assessment of solar fuel technologies†
The conversion of carbon dioxide and water into fuels in a solar refinery presents a potential solution for reducing greenhouse gas emissions, while providing a sustainable source of fuels and chemicals. Towards realizing such a solar refinery, there are many technological advances that must be met in terms of capturing and sourcing the feedstocks (namely CO2, H2O, and solar energy) and in catalytically converting CO2 and H2O. In the first part of this paper, we review the state-of-the-art in solar energy collection and conversion to solar utilities (heat, electricity, and as a photon source for photo-chemical reactions), CO2 capture and separation technology, and non-biological methods for converting CO2 and H2O to fuels. The two principal methods for CO2 conversion include (1) catalytic conversion using solar-derived hydrogen and (2) direct reduction of CO2 using H2O and solar energy. Both hydrogen production and direct CO2 reduction can be performed electro-catalytically, photo-electrochemically, photo-catalytically, and thermochemically. All four of these methods are discussed. In the second part of this paper, we utilize process modeling to assess the energy efficiency and economic feasibility of a generic solar refinery. The analysis demonstrates that the realization of a solar refinery is contingent upon significant technological improvements in all areas described above (solar energy capture and conversion, CO2 capture, and catalytic conversion processes).