A prospective on energy and environmental science

This Editorial commemorates the first decade of Energy & Environmental Science (EES), reviewing its remarkable success as well as outlook for the future. In the past ten years, EES filled a void by fostering research and development in energy and environmental science. The success of EES can be attributed to several factors: at initiation, EES was the only high-profile journal to rapidly publish only the highest quality energy-related research; a growing global community of vibrant energy researchers who initiated Gordon Research Conferences, international meetings, etc.; energy research becoming important in the eyes of the public, due to high oil prices and environmental issues; a generation of scientists, especially junior researchers and students, eager to embrace the challenge of a transition to net-zero emissions energy systems; increased funding for energy research internationally as well as in the U.S. at the National Science Foundation and Department of Energy. EES became the preferred option to publish high-quality, high-impact energy research, with success reflected by a record-setting impact factor for a full-featured journal.

A survey of topics emphasized in EES reveals a breath of disciplines and technologies, including solar cells, fuel cells, batteries, solar fuels, photovoltaics, biofuels, carbon capture and storage, and energy systems modeling. Much progress has been made, with major advances in many areas. This Editorial will focus on several currently unsolved challenges and opportunities to facilitate a transition to a net-zero CO₂ emissions energy system, while allowing for development, economic growth and global prosperity.

Energy transitions are a 50-to-100 year effort. 150 years ago, no one would have predicted nuclear power, solar power, light-emitting diode (LED) lighting, or Li-ion batteries, and many would have insisted that the highest-impact technology for improving transportation services would involve faster horses. General areas can nevertheless be identified in which advanced knowledge fosters innovation. These fields include chemistry, biology, and materials science, from which essentially all of the new energy technologies have emerged over the past century.

Materials for energy efficiency

It is cheaper and less polluting to save energy than to make energy. The development of LED lighting technology is clearly a success story for materials science, especially in growth of monolithic III–V semiconductor devices. Other frontiers in energy efficiency include controlling the thermal envelope of buildings, especially in retrofits of existing buildings. Both passive and active materials, including cost-effective phase-change materials, are ripe for development. Active optical and thermal management materials and window glazings, that adjust both their thermal and optical properties (by adjusting transmission/reflection in the visible and infrared) in response to the temperature and insolation over both short and long periods, are also promising.

Materials in extreme environments

Realizing commercially deployable nuclear fusion and/or advanced nuclear fission power plants requires the development of materials that are robust under extreme conditions. What are these materials and how can they be certified for performance in extreme radiation and thermal environments without having to demonstrate 40–60 years of operation in a prototype power plant?

Solar thermal plants require materials for both optical and thermal management at illumination intensities as great as 1000 suns and at temperatures in excess of 1500 C. Higher heat capacity thermal storage fluids would extend storage times and energy densities of solar thermal systems, allowing them to operate on 24 hour cycles, providing dispatchable, reliable power to compensate for the local intermittency of sunlight.

Solar paint

Solar electricity systems that can be painted onto a roof or wall would render current solar panel technology obsolete. No law of physics dictates that semiconducting particles cannot exhibit mobilities and charge-carrier lifetimes comparable to those of single crystals. Control over the surface and interconnections of the particles could allow such behavior to be realized. A “bottom-coat” of a conducting layer would be painted onto the desired area, followed a “color coat” of the semiconducting particles. Then a transparent conducting “clear coat” would be applied. The system might be thermally cured in sunlight or annealed by passing current through the leads. The system could also capture and utilize, in an integrated fashion, the low-grade heat to provide hot water and low-grade steam for residential and/or commercial use.

High-efficiency Si-based tandem solar cells

Monolithically formed tandem junction, Si-based solar panels offer an opportunity to raise efficiencies, and thus lower the cost of solar electricity. Tandem junctions should be prepared with tailored perovskites or analogous thin-film materials using processes compatible with Si panel manufacturing techniques. Low-cost tandem cells with efficiencies in excess of the single-junction Shockley–Queisser limit would leverage the sunk costs of Si-based photovoltaic panel manufacturing facilities. Research focused on fundamental failure mechanisms and methods for obtaining extended operating lifetimes is as important as the demonstration of record-setting efficiencies.

Self-assembled, high performance structural materials for wind turbines

Increases in rotor diameter provide more electricity from a wind turbine and thus increase capacity and lower levelized costs. In many regions the rotor diameter is constrained by the maximum cargo length that can be transported on highways. On-site processed and/or self-assembled wind turbine blades would enhance cost-effective, rapid deployment. Opportunities are also present to develop substitutes for the rare-earth elements, specifically neodymium, used as supermagnets in the motors and generators, as well as to implement alternative technologies such as magnet-free motors controlled by modern software and power electronics.

Cost-effective conversion of electricity into fuels

Three biogeochemical cycles can store sufficient energy in their reservoir species to satisfy all human energy needs: the water/H₂ cycle; the CO₂/hydrocarbon cycle; and the N₂/NH₃ cycle. Efficient, cost-effective, scalable catalytic processes already exist for coupling any one of these cycles to any of the other cycles. Steam reforming of CH₄ produces H₂; Fischer–Tropsch catalysis in conjunction with the reverse water–gas shift reaction converts H₂ and CO₂ into hydrocarbon liquid fuels; and the Haber–Bosch process converts H₂ and N₂ into NH₃. Hence an opportunity exists to cost-effectively convert carbon-free electricity into any one of these fuels, from which all of the others can be readily produced by known thermochemical processes. Of all of the reagents in these cycles, water has by far the highest mass flux to the earth's surface. Thus water is the preferable reagent for fuel production, as embodied in the electrolytic production of H₂ and O₂ from H₂O. Current electrolysis systems are however capital-intensive, involving the use of precious metal electrocatalysts and/or complex chemical process control systems. Radically new catalysts, materials and processes to effect water electrolysis are important opportunities for research and development (R&D). Low temperature thermochemical cycles are another potential approach to substantial reductions in the cost of converting electricity into fuels. Such conversions would provide a bridge between the stationary and transportation energy sectors, as well compensation for the variability of electricity from renewables such as wind and/or solar energy. Hydrogen could also potentially be used as part of a power-to-gas-to-power approach for energy storage, especially for weekly, monthly, and seasonal filling of gaps between supply and demand.

Solar fuels

The direct conversion of sunlight into chemical fuels, in the form of artificial photosynthesis, is a complementary approach to the conversion of carbon-free electricity to fuels. The system must be scalable, cost-effective, robust, and produce a readily collectable, distributable, and usable/convertible fuel. Opportunities are associated with designs and materials that take advantage of the synergistic integration of earth-abundant light absorbers and catalysts in a cost-advantaged fashion relative to discrete components that produce electricity and then separately convert electricity into fuels.

Advanced biofuels

Biofuels constitute yet another option to produce carbon-neutral liquid transportation fuels. 40% of current global transportation fuel consumption is used in ships, long-distance trucks, and aircraft. Advanced biofuels should produce a useful fuel or a convenient fuel precursor, and should have nearly no net life-cycle CO₂ emissions. Such biofuels should be grown on existing cultivatable land, to avoid primary and secondary carbon debts involved with land use conversion. Further, the processes must avoid trading land used for growing food into land used to produce fuel. Opportunities are present in systems biology approaches as well as possibly breeding and other technologies to improve the photosynthetic yield as well as to direct the products towards desirable fuels or fuel precursors.

Structural materials: steel and cement

As urbanization accelerates, especially in developing nations, per capita demand for structural materials, e.g. steel and cement, is constant or increasing. Current production methods use large energy inputs and produce substantial quantities of CO₂. Hence R&D opportunities exist for the development of alternative production processes and/or new materials to replace cement and steel. Methods are urgently needed to validate the long-term performance of new materials using short term testing.

Enabling negative emissions

To meet aggressive CO₂ stabilization targets by the mid- or late-21st century, current integrated assessment models invoke large amounts of negative emissions, i.e. CO₂ capture from the atmosphere and underground storage. Deploying biological capture and storage at large scale could impact land use, hence R&D opportunities in bioenergy with carbon capture and storage (BECCS) include improving the photosynthetic net primary productivity of crops. R&D opportunities for chemical capture (CCS) technology include cost-effective sorbents that selectively capture CO₂ from ambient, humid air, while not requiring large energy inputs to desorb and concentrate the CO₂.

Fate and transport of subsurface CO₂

BECCS and conventional CCS require an understanding of the fate and transport of subsurface CO₂, especially in aquifers that can sequester large amounts of CO₂. Each reservoir is unique geologically, and technically successful sequestration involves globally averaged leak rates for millennia of <0.1% per year. R&D opportunities include development of transport models for a specific reservoir being considered for use, as well as development of tracers, sensors and measurement techniques for monitoring subsurface CO₂.

Solving the inversion problem for attribution of CO₂ emissions

COP21 in Paris resolved to achieve net-zero CO₂ emissions by the latter part of the 21st century. To date, all robust global agreements have associated validation and verification methods and protocols. R&D opportunities involve methods for quantifying sources and sinks of emissions based on the spatial and temporal measurements of atmospheric CO₂ concentrations by a satellite. Other approaches could involve use of “all source” data including point sensors; sensors on aircraft, ships, and/or trucks; commercial data on fossil fuel transport, consumption and conversion; vehicle miles driven, etc., to construct a robust estimate of a country's CO₂ emissions. These data sources and analyses should all be independent of unilateral (unverified) declarations of CO₂ emissions specified to occur every five years in the COP21 agreement. Similar challenges and opportunities are associated with monitoring other greenhouse gases, including methane, and aerosols.

Ocean acidification chemistry and ecology

By Le Chatelier's principle, increases in the atmospheric CO₂ concentration will produce increases in the oceanic CO₂ concentration. Consistently, the pH of the oceans is lower now than in the past 4 million years, and probably than in the past 20 million years. Moreover, calcium carbonate dissolution occurs when the pH of water is lowered in vitro. Opportunities exist to advance the understanding of chemical and biological changes in the oceans as a result of increased CO₂ uptake. The rate of ecosystem change, ecological impacts in deep ocean waters vs in coral reefs, and complexities associated with thermal and physical water transport, temperature changes and salinity changes due to melting ice, as well as other key properties of the oceans, are R&D frontiers for a variety of scientific disciplines.

In conclusion, there are many R&D opportunities to evaluate the impacts of and to develop mitigation technologies for anthropogenic CO₂ emissions both in the air and in the water, as well as to attribute such emissions to regions and nations. R&D opportunities in currently under-represented areas include energy efficiency, structural materials for development, and the fate and transport of subsurface CO₂, as well as mitigation technologies including carbon-neutral fuel and electricity production, transmission and storage. Hopefully leading-edge, fundamental, high-profile research advances will occur in the next decade in at least some of these fields, with EES continuing to play a leading role as a forum to publish, critique, and put into perspective these advances at the intersection of energy and environmental science.

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