Full decarbonization of the electricity sector is critical to global climate mitigation. Across a wide range of sensitivities, firm low-carbon resources-including nuclear power, bioenergy, and natural gas plants that capture CO 2 -consistently lower the cost of decarbonizing electricity generation. Without these resources, costs rise rapidly as CO 2 limits approach zero. Batteries and demand flexibility do not obviate the value of firm resources. Improving the capabilities and spurring adoption of firm low-carbon technologies are key research and policy goals.

Long-duration energy storage (LDES) is a potential solution to intermittency in renewable energy generation. In this study we have evaluated the role of LDES in decarbonized electricity systems and identified the cost and efficiency performance necessary for LDES to substantially reduce electricity costs and displace firm low-carbon generation. Our findings show that energy storage capacity cost and discharge efficiency are the most important performance parameters. Charge/discharge capacity cost and charge efficiency play secondary roles. Energy capacity costs must be ≤US$20 kWh -1 to reduce electricity costs by ≥10%. With current electricity demand profiles, energy capacity costs must be ≤US$1 kWh -1 to fully displace all modelled firm low-carbon generation technologies. Electrification of end uses in a northern latitude context makes full displacement of firm generation more challenging and requires performance combinations unlikely to be feasible with known LDES technologies. Finally, LDES systems with the greatest impact on electricity cost and firm generation have storage durations exceeding 100 h.

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ith China having the largest fossil fuel CO 2 emissions today and the United States being higher in per capita emissions (see related energy consumption in the first figure), these countries have a strong mutual interest in stabilizing climate and reducing air pollution. Yet even Germany, despite sizable subsidies of renewable energies, gets only a small fraction of energy from them (see the first figure). Historically the fastest growth of low-carbon power occurred during scale-up of national nuclear power programs (see the second figure). Some studies project that a doubling to quadrupling of nuclear energy output is required in the next few decades, along with a large expansion of renewable energy, in order to achieve deep cuts in fossil fuel use while meeting the growing global demand for affordable, reliable energy (1-4). In light of this large-scale energy and emissions picture, climate and nuclear energy experts from China and the United States convened (see Acknowledgments) to consider the potential of increased cooperation in developing advanced nuclear technologies. Barriers to expansion of nuclear energy include high construction costs relative to coal and gas; a long time to build conventional large nuclear plants (about 4 to 7 years in Asia versus 1 or 2 years for coal-fired plants); and public concern about reactor safety, waste disposal, and potential for weapons use. Innovative nuclear technologies can help address some of these issues. A large reduction of cost and construction time, essential to accelerate deployment rates, likely requires mass manufacturing, analogous to ship and airplane construction. Such an approach lends itself to product-type licens

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