Abstract:
Interest in Compressed Air Energy Storage (CAES) is rising. CAES facilities are designed to deliver full-power capacity in a very short time period, which implies high gas-production rates and multiple yearly pressure cycles. A decrease in gas temperature during a pressure drop depends upon gas pressure, withdrawal rate and cavern size. Thermal tensile stresses, resulting from gas cooling, may generate fractures at the wall and roof of a salt cavern. However, thermal stresses do not penetrate deep into the rock mass. These fractures are perpendicular to the cavern wall. The distance between two parallel fractures becomes larger when fractures penetrate deeper in the rock mass, as some fractures stop growing. These conclusions can be supported by field observations, closed-form solutions and numerical computations based on fracture mechanics.
Cavern Thermodynamics
Salt caverns are deep cavities (from 200 m to 3000 m) that are connected to the ground level through a cased and cemented well. One to several strings are set in the well to allow injection or withdrawal of fluids into or from the cavern. These caverns are created by solution mining in both bedded and domal salt formations; they range in volume from 5000 m3 to 5,000,000 m3. When solution mining is completed, the cavern can be filled with crude oil, LPG, natural gas, hydrogen, compressed air, etc. In this paper, we mainly are interested in the thermal and mechanical effects of gas withdrawal (and injection) in a Compressed Air Energy Storage (CAES).
Energy Balance
Gas temperature can be considered almost uniform throughout the entire volume of a cavern. Gas effectively is stirred by thermal convection as the geothermal gradient (Ggeo = 1.6–1.8°C/100 m is typical) is larger than the dry adiabatic gradient (Gad = g/Cp, g = 10 m/s2, CP = 2345 J/kg-°C = for natural gas; CP = 1000 J/kg-°C for air), below which no convection appears (Bérest et al., 2012). In a typical gas cavern, the Grashof and Prandtl numbers are so large that convection is turbulent. The actual thermal gradient, G, in the cavern is small. Fosse and Røvang, 1998, measured G = 0.39°C/100 m and 0.50°C/100 m in two gas caverns at Etzel in Germany. Kneer et al., 2002, computed that the temperature difference between the top and the bottom of a 200-m high cavern was 0.5°C. An exception was found when cold brine was left at cavern bottom, hindering onset of thermal convection (Crotogino et al., 2001, Klafki et al., 2003, Skaug et al., 2010).
