Research opportunities to advance solar energy utilization

Relying more on the Sun Improved technologies for harnessing solar energy are not limited to creating more efficient solar cells. The associated hardware of delivering power from solar cells to homes and businesses, and storing this intermittent resource on the grid, offer R&D opportunities. Lewis reviews the status of these areas, as well as solar thermal and solar fuels approaches for harnessing solar energy. Science, this issue p. 10.1126/science.aad1920 BACKGROUND Despite providing a relatively small percentage of total global energy supply, solar energy systems generally receive enthusiastic support from technologists, regulators, politicians, and environmental groups. The energy in sunlight can be converted into electricity, heat, or fuel. Although the costs of solar panels have declined rapidly, technology gaps still exist for achieving cost-effective scalable deployment combined with storage technologies to provide reliable, dispatchable energy. ADVANCES The costs of Si-based solar panels have declined so rapidly that panel costs now make up <30% of the costs of a fully installed solar-electricity system. Research and development (R&D) opportunities hence lie in the development of very high efficiency conversion materials, to advantageously leverage the associated reduction in area-related balance-of-systems costs. Such materials would optimally either leverage or mate with existing, low-cost Si photovoltaic (PV) technology. Ultralightweight, flexible, robust, and efficient materials could also greatly reduce the installation costs and could allow for enhanced automation and inexpensive support structures. The development of cost-effective persistent grid-scale storage to compensate for the intermittency of sunlight is a major area for R&D. Possibilities include new types of batteries and flow batteries, as well as geologic storage of hydrogen, methane, or compressed air. Opportunities also exist to improve the capabilities of concentrated solar power systems that convert sunlight into heat. Improved thermal storage fluids would provide longer-term storage to compensate for cloudy days in areas of high direct insolation. Thermoelectrics, in principle, could replace engines to provide efficient conversion systems that have no moving parts. New thermochemical cycles could allow for the highly efficient, cost-effective conversion of solar heat into fuels by promoting endothermic reactions, such as water splitting, carbon dioxide reduction, or thermochemical conversion of feedstocks, such as methane to high energy-density liquid hydrocarbon fuels that are needed in the transportation sector. Artificial photosynthetic systems that directly produce fuel from sunlight are in the proof-of-concept stage. Such technologies offer the potential to provide renewable hydrogen by solar-driven water splitting or to produce hydrocarbons directly from sunlight, water, and CO2. Key goals for R&D are development of materials that can absorb and convert sunlight efficiently that are seamlessly integrated with catalysts that promote the production of fuel, with the production of O2 from water also required to complete a sustainable, scalable chemical cycle. Systems must simultaneously be efficient, robust, cost-effective, and safe. OUTLOOK Considerable opportunities for cost reduction that can achieve both evolutionary and revolutionary performance improvements are present for all types of solar energy–conversion technologies. Learning by doing and R&D will both be needed to produce an innovation ecosystem that can sustain the historical rate of cost reductions in PVs and concentrated solar thermal technology. Disruptive approaches to storage technologies are needed to compensate for the intermittency of sunlight and allow for development of a full clean-energy system. Solar fuels technology contains abundant opportunities for discovery of new materials and systems that will allow for deployable, cost-effective routes to the direct production of fuels from sunlight. Solar energy–conversion and storage technologies. (A) Nellis Solar Power Station, a 14-MW PV installation at Nellis Air Force Base, NV. (b) Concentrated solar thermal power 392-MW installation at Ivanpah, CA. (C) World’s largest battery (NiCd) storage installation (40 MW for 7 min, 26 MW for 15 min), Fairbanks, AK. (D) Solar fuels demonstration of a photoelectrode evolving hydrogen gas. [Image sources: (A) Nellis Air Force Base PV installation, https://commons.wikimedia. org/wiki/Category:Nellis_Solar_Power_Plant. (B) Ivanpah solar electric generation installation, http://i.ytimg. com/vi/M5yzgfCNpvM/maxresdefault.jpg. (C) Fairbanks battery installation, http://blog.gvea. com/wordpress/? p=1677] Major developments, as well as remaining challenges and the associated research opportunities, are evaluated for three technologically distinct approaches to solar energy utilization: solar electricity, solar thermal, and solar fuels technologies. Much progress has been made, but research opportunities are still present for all approaches. Both evolutionary and revolutionary technology development, involving foundational research, applied research, learning by doing, demonstration projects, and deployment at scale will be needed to continue this technology-innovation ecosystem. Most of the approaches still offer the potential to provide much higher efficiencies, much lower costs, improved scalability, and new functionality, relative to the embodiments of solar energy-conversion systems that have been developed to date.