Pumped hydroelectric storage (PHS) projects generally involve an upper and lower reservoir. Some projects use a river as the lower reservoir; others have used massive lakes or even an ocean. Another interesting concept being considered is to locate one or both reservoirs below ground (sub-surface). While a project utilizing sub-surface reservoirs has yet to be completed, these types of projects are attractive due to their perceived site availability and their potential for reduced environmental impacts.
Abandoned mines, caverns, and man-made storage reservoirs have all been proposed as potential project reservoir options, and there are examples of several projects under initial phases of development. The underground excavation or materials-handling costs, construction risk, and time required for underground excavation and construction could make the economics of such a project difficult, so most developers are looking to utilize existing subsurface structures or minimize/offset underground costs through the sale of excavated materials (ore, aggregate, etc.). The discussion below examines three projects at various stages of development.
The Elmhurst Quarry Pumped Storage Project (EQPS) is a conceptual underground pumped storage project that would utilize an abandoned mine and quarry for the both upper and lower reservoir. The project would be located in the City of Elmhurst, Illinois within 20 miles of downtown Chicago. EQPS is being developed by DuPage County, Illinois, and appears to be an attractive project due to its flood control capabilities, renewable generation opportunities and potentially low environmental impacts, and to its proximity to electrical transmission and a large load center. The project would divert and gravity-feed water from an above-ground source into an underground powerhouse, where it would travel through the pump-turbine. The power generated would be delivered to the power grid during peak demand periods. Once through the turbines, the water would then be temporarily stored in abandoned mine caverns before being pumped back to its original source using lower-cost (off-peak) power. The EQPS has an initial design capacity of between 50 MW and 250 MW with an estimated 708.5 GWh of energy storage potential.
Riverbank Wisacasset Energy Center (RWEC) is a proposed 1,000-MW pumped hydroelectric storage facility located 2,200 feet underground in Wisacasset, Maine. The RWEC project would divert water into its underground shaft down 2,000 vertical feet to drop into a powerhouse containing four 250-MW pump-turbines. Similar to the EQPS, the water passing through the pump-turbines would be stored in large underground reservoirs (caverns) before being pumped up using low-cost, off-peak power.
Gravity Power-Grid-Scale Electricity Storage System – Gravity Power, LLC, is also proposing to develop a grid-scale electricity storage system. The company’s Gravity Power Module (GPM) uses the established principles of pumped storage combined with a large piston that is suspended in a deep, water-filled shaft. Once the shaft is initially filled with water, no additional water is required. As the piston drops, it forces water down the storage shaft, up the return pipe and through the turbine, and spins a pump-turbine motor/generator to produce electricity. To store energy, power purchased off-peak drives the pump-turbine in reverse, spinning the pump to force water down the return pipe and into the shaft, lifting the piston.