A description is given of the studies and field investigations undertaken to develop the design of caverns to be excavated in hard rock for large central energy storage facilities. Two schemes are considered including a 1000 MW capacity compressed air energy storage (CAES) and a 2000 MW capacity underground pumped hydro (UPH) requiring the excavation of approximately 900,000 yd (690,000 m3) and 8,000,000 yd3 (6,100,000 m) respectively. Material presented includes the site selection methodology, site investigations, the design approach including construct ion and geotechnical considerations, and the construct ion cost estimates. Geotechnical considerations discussed include the selection of the cavern arrangement and cross sect ion through evaluation of precedent experience, empirical assessment of the rock mass and preliminary strength/stress analysis.

INTRODUCTION

This paper has been prepared to present the findings of a major study, sponsored and financed jointly by the U.S. Department of Energy and the Electric Power Research Institute (EPRI), into the technical, economic and environmental feasibility of underground energy storage. The program, which was undertaken under the general direction of the Potomac Electric Power Company of Washington, D.C., was concerned with two energy storage concepts, underground pumped hydro (UPH) storage and compressed air energy storage (CAES). Both of these concepts call for the provision of large caverns excavated in rock at depths ranging up to 5000 ft (1500 m) below ground level. Energy storage is required by electric power utilities to allow surplus power available at times of low power demand (usually at night or during the weekend) to be stored for subsequent use during periods of high power demand - usually during weekdays.

This function, currently filled by "conventional" surface-located hydro pumped plants, not only promotes the most efficient use of "base load" coal and nuclear plants, but also displaces the high fuel cost gas turbines often used to meet peak loads. Although both the CAES and UPH concepts utilize rock caverns, their basic mode of operation is entirely different. For CAES, surplus electrical energy from the power system is fed to a motor to drive an air compressor; the compressed air is stored in the underground cavern until times of peak demand when it is released and fired with oil fuel to drive a gas turbine and hence generate power, now using the motor as a generator. Typically, the most economic pressures at which to store the air lie between 600 and 1100 psi (4.8 to 7.6 Mpa). At 1000 psi (6.9 Mpa) the volume of air required to provide 10,000 MW hours of storage (1000 MW for 10 hrs) amounts to approximately 900,000 m (690,000m3). For UPH, the operating concept is essentially the same as for "conventional" surface-located pumped storage: to store energy, water is pumped from a lower reservoir, here located at depth below ground surface in a rock cavern, to an upper reservoir located at ground level. To release energy the water is returned to the lower reservoir through a power plant located at or near the lower reservoir.

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