Getting to 100%: Six strategies for the challenging last 10%

Meeting the last increment of demand always poses challenges, irrespective of whether the resources used to meet it are carbon free. The challenges primarily stem from the infrequent utilization of assets deployed to meet high demand periods, which require very high revenue during those periods to recover capital costs. Achieving 100% carbon-free electricity obviates the use of traditional fossil-fuel-based generation technologies, by themselves, to serve the last increment of demand—which we refer to as the “last 10%.” Here, we survey strategies for overcoming this last 10% challenge, including extending traditional carbon-free energy sources (e.g., wind and solar, other renewable energy, and nuclear), replacing fossil fuels with carbon-free fuels for combustion (e.g., hydrogen- and biomass-based fuels), developing carbon capture and carbon dioxide removal technologies, and deploying multi-day demand-side resources. We qualitatively compare economic factors associated with the low-utilization condition and discuss unique challenges of each option to inform the complex assessments needed to identify a portfolio that could achieve carbon-free electricity. Although many electricity systems are a long way from requiring these last 10% technologies, research and careful consideration are needed soon for the options to be available when electricity systems approach 90% carbon-free electricity.