Underground Pumped Hydroelectric Storage (UPHS) is a technique for supplying electric power to meet peak load requirements. Presently there are not any existing UPHS plants, either operating or under construction. Studies indicate that the UPHS concept is technically feasible and economically viable. This paper describes conceptual design parameters, plant layout, turbomachinery options, site-selection considerations, geological requirements, economics, and environmental concerns. Most elements of UPHS technology are sufficiently developed to enable plant construction. Advances in high head turbomachinery design and geotechnical excavation techniques are expected to enhance the economic attractiveness of UPHS.
Underground Pumped Hydroelectric Storage (UPHS) is a technique for supplying electric.power to meet peak load requirements of electric utility systems. Its incorporates reversible pump-turbines or single function pumps and turbines in tandemand an underground reservoir mined from hard rock. During night and weekend off'-peak periods, low-cost power would be used to pump water from the underground reservoir to a surface reservoir located 1000 to 2000 meters higher (Fig. 1). During the subsequent daytime peak-load Periods the surface water would supply hydroelectric power over an 8 to 10 hour period, by refilling the lower reservoir through the turbines. This would eliminate the consumption of petroleum fuels by conventional peaking gas turbines. UPHS plants are not limited by the topographic siting difficulties of conventional pumped plants. A well designed UPHS plant could have a smaller adverse impact on the environmental than a conventional peaking plant. The underground pumped hydroelectric storage concept evolved because of the dwindling number of conventional pumped storage sites, decreasing public acceptance of large reservoirs, increased capacities at higher operating heads, and the feasibility of locating UPHS plants nearer load centers to reduce transmission capital and operating costs. The authors are not aware of any existing UPHS plant, either operating or under construction. The USSR has completed a feasibility study for a plant near Minsk and preliminary designs have also been developed for plants in the eastern and midwestern USA. A typical conceptual UPHS plant would have a capacity of 1000 to 3000 MW, an operating head of 1000 to 2000 meters, and a reservoir storage capability of 8 to 10 hours generation at full load. An upper reservoir capacity of about eight million cubic meters is contemplated. Its containment embankments could be economically constructed out of rock excavated from the subsurface shafts and caverns. Geologic conditions must be favorable for mining out the requisite volumes for the lower reservoir, powerhouse, access shafts, and interconnecting waterway tunnels. A competent and impervious geologic formation is required. Candidate rock types include granite/granodiorite, quartzite, massive gneiss, dolomite, limestone, and impermeable sandstone. TURBOMCHINERY The various types of pump-turbines considered for high-head pumped storage are briefly described (Blomquist, Frigo, and Tam, 1979; Rodrique, 1979). Single Stage Francis-Type Reversible Pump-Turbine (Fig. 2) This unit is widely used in the USA because of extensive design and operating experience in this country, least equipment cost, high unit capacity and speed regulation/load following capability with adjustable wicket gates.
