Compressed Air Energy storage (CAES) in underground caverns and Underground Pumped Hydro Storage (UPHS) are systems which can be used for electrical power generation during peak demand periods. Key features for economical use of such systems involve the structural and leakage stabilities of the storage caverns over operational lifetimes of some 30 years. A goal of this paper is to establish a consistent set of stability and design criteria for caverns in hard rock, which are both practical to the designer and applicable to numerical modeling and failure probability assessment techniques. To formulate stability criteria, the phenomenological modes of potential instability are identified for the physical situations and "quantified" in terms of appropriate constitutive laws of material behavior.
Compressed Air Energy storage (CAES) in underground caverns and Underground Pumped Hydro Storage (UPHS) are two methods for storing energy that can be used for generation of electrical power during peak demand period. In the CAES concept, air is compressed and stored in underground caverns during off-peak demand periods, and withdrawn and used with surface turbine systems to generate electrical power during peak demand periods. In the UPHS concept, water is pumped from underground caverns to an upper reservoir during off-peak demand periods. During periods of peak demand, electrical energy is produced when the water in the. surface reservoir is permitted to flow down a penstock shaft, through a water-power turbine system, and into an underground reservoir. For these systems to be economical, the structural and leakage stabilities of the storage caverns must be maintained over an operational lifetime of some 30 years. The objectives of this paper are to examine stability and design criteria for CAES and UPHS caverns in hard rock in terms of performance criteria and acceptability limits, and to present some numerical modeling results which illustrate the mechanical and thermal responses of the caverns during construction and operation.
The underground layout of a compensated CAES system in hardrock consists of a collection of parallel storage caverns, each of which is connected at either end by inclined entry's to common (air and water) crosscut tunnels. The crosscut (air) tunnels are connected to the surface compression/generation equipment by an air shaft and to a surface water reservoir by a water inlet/outlet shaft. Water from the surface reservoir provides a compensating head for maintenance of constant air pressure (with varying volume) in the storage caverns. This is designated a "wet" system because of the rise and fall of water in the caverns during the withdrawal (generation cycle) and injection (compression cycle) of compressed air. To prevent air ejection through the water shaft, the base of the shaft and any intermediate water storage caverns must be situated at greater depth than that of the air storage caverns. Because of turbine machinery limitations and pressure losses during the generation cycle, a maximum air pressure of about 7 to 7.5 MPa is required, dictating a facility depth of 715 to 765 m.
