As renewable penetration grows, the need for energy storage grows as well. However, existing storage technologies may not be able to meet that need. A Department of Energy (DOE) grant program aims to address that problem by trading efficiency for low cost. Existing storage technologies fall short in terms of duration. Lithium-ion batteries, which have captured about 90% of the energy storage market, can economically run for about four hours. Pumped hydro storage, which accounts for the most energy storage today in terms of capacity, can generate power by releasing water from storage reservoirs for up to 20 hours under certain conditions.