Role of Long-Duration Energy Storage in Variable Renewable Electricity Systems

Reliable and affordable electricity systems based on variable energy sources, such as wind and solar may depend on the ability to store large quantities of low-cost energy over long timescales. Here, we use 39 years of hourly U.S. weather data, and a macro-scale energy model to evaluate capacities and dispatch in least cost, 100% reliable electricity systems with wind and solar generation supported by long-duration storage (LDS; 10 h or greater) and battery storage. We find that the introduction of LDS lowers total system costs relative to wind-solar-battery systems, and that system costs are twice as sensitive to reductions in LDS costs as to reductions in battery costs. In least-cost systems, batteries are used primarily for intra-day storage and LDS is used primarily for inter-season and multi-year storage. Moreover, dependence on LDS increases when the system is optimized over more years. LDS technologies could improve the affordability of renewable electricity. Laws in several U.S. states now require the adoption of zero-carbon electricity systems based primarily on renewable technologies, such as wind and solar. Long-term, large-capacity energy storage may ease reliability and affordability challenges of systems based on these naturally variable generation resources. Long-duration storage technologies (10 h or greater) have very different cost structures compared with Li-ion battery storage. Using a multi-decadal weather dataset, our results reveal that long-duration storage can fill unique roles, like seasonal and even multi-year storage, making it valuable to least-cost electricity systems. Indeed, we find that variable renewable power systems are much more sensitive to reductions in long-duration storage costs than to equal reductions in battery costs. Long-term modeling horizons, typically not used by utilities and regulators, are necessary to capture the role and value of long-term storage, informing technology investments and policy. Laws in several U.S. states mandate zero-carbon electricity systems based primarily on renewable technologies, such as wind and solar. Long-term, large-capacity energy storage, such as those that might be provided by power-to-gas-to-power systems, may improve reliability and affordability of systems based on variable non-dispatchable generation. Long-term storage can reduce costs of wind-solar-battery electricity systems at current technology costs by filling seasonal and multi-year storage functional roles. Innovation in long-term storage technology could further improve the affordability of reliable renewable electricity.