Introduction Fossil fuels have been the bedrock of modern civilization, powering vehicles, generating electricity, and heating homes. However, our continued reliance on unearthing fuels stored in the crust of our planet is intrinsically unsustainable and the combustion of fossil fuels has released carbon dioxide (CO2) into the atmosphere. It has led to a devastating unintended consequence: climate change. Now, a transition into sustainable energy is imperative. To grasp the challenges facing such a transition, it is first necessary to understand the energy sources and end-uses of the current U.S. energy landscape. This analysis can help us understand the current status and prospects of five emerging key technologies that have the potential to enable a clean energy transition: 1) renewable energy; 2) carbon capture, utilization, and sequestration; 3) energy storage; 4) sustainable hydrogen and fuels; and 5) the electrical grid. Transitioning from Today’s Energy Landscape The current energy landscape in the United States is still dominated by fossil fuels (petroleum, natural gas, and coal) and renewable energy only accounts for 13% of all energy sources. A clean energy transition requires decarbonizing the sources—removing or reducing CO2 emissions. This transition can be achieved by expanding renewable energy deployment while capturing carbon emissions from fossil fuel sources at the same time. The transportation and industrial sectors account for almost three-quarters of all end-use consumption. Therefore, it is possible to realize the largest decarbonization impact by focusing on these end-use sectors. For the transportation sector, further developing and deploying electric vehicles powered by energy storage devices is a crucial step in the clean energy transition. Decarbonizing hard-to-abate industrial sectors like aviation and steel and cement production will require innovation in the sustainable production of hydrogen and its fuels. Apart from these substantial end-use sectors, the electrical grid must be transformed to support tomorrow's future clean energy landscape. Currently, almost two-thirds of the energy sources that go into the production and distribution of electricity are lost through inefficiency. Renewable Energy In some parts of the world, wind and solar are already some of the most cost-effective ways to generate electricity. In the US, however, only one-tenth of the energy is sourced from renewables, meaning that significant growth is necessary in this sector in the coming decades. Yet, efficiency must be improved to enable economical renewable energy generation in geographical areas with less abundant wind and solar resources. Current solar cells are made from silicon, an abundant and cheap material with a solar-to-electricity energy efficiency of only around 20%. There is an emerging technology that can significantly improve this efficiency. A material called perovskite can be paired with silicon to create a solar cell with efficiencies upwards of 30%, a 50% improvement over today’s solar cells. Manufacturing and durability challenges remain to be solved before such cells can be implemented, but there is potential to further boost the share of renewable energy. Carbon Capture, Utilization, and Sequestration Although expanding renewable energy is essential for the energy transition, challenges such as intermittency, transmission, and the geographical dependency of resources mean that it will be extremely difficult to achieve 100% renewable energy. In addition, there are end-use industrial sectors such as aviation and cement and steel manufacturing for which decarbonization remains highly challenging. Carbon capture, utilization, and sequestration (CCUS) will be critical for decarbonizing these sectors. CCUS captures the CO2 generated from fossil fuels, concentrates it, and then either uses it for carbon-neutral fuels or safely stores it so it does not enter the atmosphere. There are broadly two types of carbon capture: point source capture and direct air capture. Carbon capture at point sources, such as power plants and cement plants, can mitigate the carbon footprint of processes using fossil fuels. Direct air capture is more costly, but it allows CO2 to be captured from the atmosphere to achieve negative emissions. Current technologies employ thermochemical methods where chemicals capture CO2, heat it to high temperatures, and release nearly pure emissions. However, this process consumes considerable energy; for example, capturing the CO2 from a natural gas power plant requires roughly a third of the plant's energy. To lower the energy cost of CO2 capture, scientists are exploring novel solutions using engineered amine molecules, highly porous adsorbent materials, or electrochemical methods. In particular, the electrochemical method can target specific molecules and circumvent the energy loss to inactive supports, significantly enhancing energy efficiency. Apart from innovation, there is an urgent need for policies promoting carbon capture. Currently, proper incentives, such as carbon prices or taxes, are lacking in many parts of the world. In addition, the infrastructure for carbon accounting needs improvement so emissions can be accurately managed. Energy Storage Energy storage is an integral part of the clean energy transition in two respects: it allows for the electrification of transportation and enables grid-scale energy storage for load shifting, that is, moving energy consumption from high-demand (peak hours) to low-demand (off-peak hours) periods. The automotive industry is already undergoing a large-scale transformation from internal combustion engine vehicles to electric, aided by falling costs and improving performance of lithium-ion batteries. Electric vehicles account for about 14% of global sales and are quickly growing to become the dominant technology in the passenger vehicle market. However, grid-scale energy storage technologies that are essential for complementing intermittent renewable electricity generation will require lower costs in order of magnitude. Although energy storage systems based on lithium-ion battery technology are in active use, lithium-ion batteries are projected to be too expensive for storage exceeding a few hours. Aqueous (water-based) chemistries that utilize abundant metals have lower costs because they circumvent the use of expensive metals like lithium and cobalt and do not require nonaqueous organic solvents. Zinc-based batteries are the most widely considered, achieving excellent cycle life and safety while lowering costs, but nickel and manganese-based chemistries are also being explored. Sustainable Hydrogen and Fuels Expanding the clean production of hydrogen will be an important strategy for decarbonizing many end-use sectors. Hydrogen can fuel vehicles (and potentially planes) and is also an important chemical for industrial processes. In addition to today’s uses in petroleum refining and ammonia production, it is projected to be integral in decarbonizing steel production and enabling sustainable fuels. However, hydrogen production, storage, transportation, and end-uses are still under active research. Currently, conventional hydrogen (called “gray hydrogen”) is sourced from fossil fuels through processes such as naphtha reforming, natural gas steam reforming, and coal gasification. It has a significant carbon footprint and is not sustainably produced. Two other types of hydrogen are garnering attention: blue hydrogen is produced through carbon capture and green hydrogen is generated from water, using renewable electricity. Yet, production at scale and cost, storage, and transportation remain significant challenges. New catalysts that bypass the use of expensive precious metals such as platinum and palladium are under development, so are improvements for containing, pressurizing, and liquefying hydrogen for storage and transportation. In addition, hydrocarbon fuel synthesis from CO2 and hydrogen is still at an early stage and needs greater technological innovation. Electrical Grid As energy sources shift from fossil fuels to renewable electricity, the load on the electrical grid will intensify. Moreover, as decentralized electricity generation from solar panels grows and electric vehicles as energy storage mechanisms become available, the grid will have to be able to accommodate bidirectional electricity transmission, where end-users send surplus energy back to the grid. Balancing the intermittency of renewable energy will be a significant challenge and calling for additional features such as grid-scale energy storage and electric vehicle charging will add to the complexity. Therefore, a tremendous grid transformation is needed, not only in quantity but also in quality. To enable this evolution, a new control scheme that can manage the complexity of the new grid will be needed. Algorithms leveraging artificial intelligence that can automate grid control with high reliability and security are under development. In addition, high voltage direct current transmission technology that can enable more efficient long-distance transmission will be necessary. Finally, in addition to these technical improvements, a better infrastructure is required. For example, currently, there is a large shortage of electronic components and personnel for the installation of electric vehicle charging stations, and an infrastructure for training professionals and manufacturing is important. Note: This Explainer is based on a presentation Dr. Kim delivered at the 2023 Trans-Pacific Sustainability Dialogue (TPSD), co-organized by the Walter H. Shorenstein Asia-Pacific Research Center at Stanford University and the Ban Ki-moon Foundation For a Better Future. Resources A. I. Osman et al. 2021. Recent Advances in Carbon Capture Storage and Utilisation Technologies: A Review. Environmental Chemistry Letters. 19. 797–849. C. A Hunter et al. 2021. Techno-Economic Analysis of Long-Duration Energy Storage and Flexible Power Generation Technologies to Support High-Variable Renewable Energy Grids. Joule. 5. 2077–2101. E. B. Agyekum et al. 2022. 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W.Chen et al. 2018. A Manganese–Hydrogen Battery With Potential for Grid-Scale Energy Storage. Nature Energy. 3. 428–435. W. Chen et al. 2018. Nickel-Hydrogen Batteries for Large-Scale Energy Storage. PNAS. 115. 11694–11699. X. Y. Chin et al. 2023. Interface Passivation for 31.25%-Efficient Perovskite/Silicon Tandem Solar Cells. Science. 381. 59–63. Ask the Experts Sang Cheol Kim Stanford Energy Postdoctoral Fellow Sang Cheol Kim works with Prof. Steven Chu at Stanford University, where he also completed his PhD in Materials Science and Engineering. His research focuses on electrochemical systems for tackling energy and climate problems. He began his career in clean energy at LG Chem, a major chemical and battery manufacturer. Follow Sang Cheol Kim on Walter H. Shorenstein Asia–Pacific Research Center (Shorenstein APARC) Founded in 1983, Shorenstein APARC addresses critical issues affecting the countries of Asia, their regional and global affairs, and U.S.–Asia relations. As Stanford University’s hub for the interdisciplinary study of contemporary Asia, it produces policy-relevant research and provides education and training to students, scholars, and practitioners. It also strengthens dialogue and cooperation between counterparts in the Asia–Pacific and the United States. Leave your question or comment in the section below: View the discussion thread.