Sub-Surface Pumped Hydroelectric Storage

Executive Summary

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 reservoirshave 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 riskExposure to or the existence of potential danger, harm, or loss related to something of value., 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 generationThe manner in which electricity is generated. The electricity that flows through California opportunities and potentially low environmental impacts, and to its proximity to electrical1. An adjective meaning “pertaining to electricity”. Electrical Engineer. 2. Related to, pertaining to or associated with electricity but not having its properties or characteristics. transmissionAn interconnected group of lines and associated equipment for the movement or transfer of electric energy between points of supply and points at which it is transformed for delivery to customers or is delivered to other electric systems. and a large loadAn end-use device or an end-use customer receiving electric powerThe rate at which energy is generated, converted, transmitted, distributed or delivered. and using electric energy from the electrical system (grid). Note: The term load is sometimes treated as a synonym for demand, which is the measure of power that a load receives or... 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 demand1. The rate at which electricAn adjective meaning “needing electricity to operate” such as electric motor or wire. IEEE: Containing, producing , arising from, actuated by or carrying electricity. energy1. Energy is the potential of a physical system to perform work. (A common unit of work is foot-pound—the amount of energy needed to lift one pound up a distance of one foot.) Energy exists in several forms such as electromagnetic radiation ... is delivered to or by a system or part of a system, generally expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time. 2. The rate at which energy is being used by... 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 capacityThe rate at which equipment can either generate, convert or transfer energy. 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-peakThose hours or other periods defined by NAESB business practices, contract, agreements, or guides as periods of lower electrical demand. 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.