Long-duration energy storage for reliable renewable electricity: The realistic possibilities

Several American states mandate zero-carbon electricity systems based primarily on renewable technologies such as wind and solar power. Reliable and affordable electricity systems based on these variable resources may depend on the ability to store large quantities of low-cost energy over long timescales. Long-duration storage technologies (that is, those that provide from 10 to hundreds of hours of storage) have much cheaper energy storage capital costs than lithium-ion batteries. Long-duration storage plays unique roles, such as seasonal and multi-year storage, that increase the affordability of electricity from variable renewable energy. We compare realistic options for seasonal energy storage, including underground hydrogen, pumped hydro, pumped thermal, and compressed air systems. To make 100 percent renewable electricity reliable and more affordable, such long-duration storage technologies can be employed.