Solar methanol energy storage

The intermittency of renewable electricity requires the deployment of energy-storage technologies as global energy grids become more sustainably sourced. Upcycling carbon dioxide (CO 2 ) and intermittently generated renewable hydrogen to stored products such as methanol (MeOH) allows the cyclic use of carbon and addresses the challenges of storage energy density, size and transportability as well as responsiveness to energy production and demand better than most storage alternatives. Deploying this storage solution efficiently and at scale requires the optimization of production conditions to ensure predictable and maximum long-term process performance. Key to enabling this solution is the generation of highly productive syngas that is rich in carbon monoxide (CO) via reverse water-gas shift (RWGS) or solid-oxide electrolysis cell technologies. The focus herein is the RWGS reaction as it enables a solar-to-fuel efficiency of around 10% that can be deployable at a commercial scale. The need for a higher-efficiency route to renewable MeOH is discussed, and a comparative technoeconomic analysis of two solar-derived MeOH (solar MeOH) strategies is presented: the solar-CO-rich (based on the solar-RWGS process) and the solar-direct-CO 2 routes. Methanol is a leading candidate for storage of solar-energy-derived renewable electricity as energy-dense liquid fuel, yet there are different approaches to achieving this goal. This Perspective comparatively assesses indirect CO- and direct CO 2 -based solar strategies and identifies the conditions under which the former becomes economically viable.