Compressed Air Energy Storage Cavern Research Articles

A simplified and unified analytical solution for temperature and pressure variations in compressed air energy storage caverns

Temperature and pressure variations in compressed air energy storage (CAES) caverns are important factors that affect the overall performance of CAES systems. However, current air storage cavern models used in the thermodynamic analysis of CAES systems usually ignore the effect of heat exchange between cavern air and the surrounding environment and thus cannot accurately predict temperature and pressure variations. In this study, a diabatic analytical solution in a simple and unified form and that considers heat exchange is proposed for temperature and pressure variations in CAES caverns. The solution is derived on the basis of assumptions that the air density in the cavern can be represented by a constant average value and that the cavern wall temperature remains constant. The proposed solution is validated with the test data of the Huntorf plant trial test and the results calculated with other solutions. Moreover, the errors of the proposed solution caused by the assumptions are analyzed. Results show that in representative ranges, the errors have a significant positive correlation with the ratio of the injected to the initial cavern air mass and the difference between the injected air temperature and the initial air temperature. The errors also have an insignificant negative correlation with the rock thermal effusivity and the heat transfer coefficient. Finally, the condition under which the proposed solution is applicable with an error less than 20% is defined on the basis of the combination of the ratio of the injected to the initial cavern air mass and the difference between the injected air temperature and the initial air temperature. This simplified and unified solution can be a simple yet adequately accurate tool to be used in the thermodynamic analysis of CAES systems.

Cyclic Thermo-Mechanical Analysis of Wellbore in Underground Compressed Air Energy Storage Cavern

The compressed air energy storage (CAES) method is a viable method of storing surplus energy underground when there is a mismatch between energy generation and demand. Wellbores embedded in rock are an integral part of energy storage structures, and are used for injecting and extracting the compressed air. During injection and production cycles, the storage reservoir and wellbore are subjected to cyclic change in external pressure and temperature, which may cause failure of the wellbore. In this paper, cyclic thermo-mechanical analysis of a horizontal wellbore in an underground CAES cavern is performed using finite element analysis. The rock behavior is simulated using the Mohr-Coulomb constitutive law. The reduction in the yield strength of rock with increase in the number of loading cycles is taken into account in the analysis. Parametric sensitivity studies are carried out to study the effects of dilation and friction angles of rock, the ratio of in situ horizontal and vertical stresses, loading frequency, and the magnitude of the temperature cycles in the cavern on the wellbore performance for different types of rock. The thermo-mechanical cyclic behavior leads to plastic strains that are greater than those obtained by performing mechanical analysis only. Significantly large deformation is generated in rock for large dilation angle and high loading frequencies.

Temperature and pressure variations within compressed air energy storage caverns

In the present work, the thermodynamic response of underground cavern reservoirs to charge/discharge cycles of compressed air energy storage (CAES) plants was studied. During a CAES plant operation, the cyclical air injection and withdrawal produce temperature and pressure fluctuations within the storage cavern. Predictions of these fluctuations are required for proper cavern design and for the selection of appropriate turbo-machinery. Based on the mass and energy conservation equations, numerical and approximate analytical solutions were derived for the air cavern temperature and pressure variations. Sensitivity analyses were conducted to identify the dominant parameters that affect the storage temperature and pressure fluctuations and the required storage volume. The heat transfer at the cavern walls was found to highly affect the air temperature and pressure variations as compared to adiabatic conditions. In essence, heat transfer reduces the temperature and pressure fluctuations during cavern charge and discharge and effectively leads to a higher storage capacity. Additionally, for realistic conditions, in each cycle, few percents of the injected energy are lost by conduction into the rocks. The principal thermal property that governs the heat transfer process is the rock effusivity. To reduce the required storage volume preference must be given to sites of rocks that have the largest thermal effusivity. Lower injected air temperatures also reduce the required storage volume, but increase the cooling costs. The injected temperature can also be used to control the cycle temperature extreme limits. It is evident from the results that the storage pressure ratio has a dominant effect on the required storage volume and should preferably range between 1.2 and 1.8.

Geotechnical evaluation of a conglomerate for compressed air energy storage: the influence of the sedimentary cycle and filling minerals in the rock matrix.

It is necessary for the rock mass that surrounds the excavated storage cavern to maintain its mechanical and hydrological properties in order to keep the stability and air tightness of the storage caverns of the operating compressed air energy storage (CAES) system. A case study for the CAES, which included drilling a borehole of 600m in depth, was carried out on the Paleogene sedimentary rock that consisted mainly of conglomerate in northeast Kyushu, Japan. Elastic wave velocity corresponds to the sedimentary facies and the mineralogy in the rock matrix that are controlled by the sedimentary cycle. The mechanical properties can be inferred from physical properties such as elastic wave velocity. Hydrological properties such as permeability coefficient and pore water pressure are affected by the grain size distribution and the filling minerals in the rock matrix that are controlled by the sedimentary cycle. Carbonaceous shale and sandstone with abundant filling mineral (calcite), which are distributed at the top of the sedimentary cycle, are inferred to be impervious. It is assumed that the groundwater flow is divided into two zones based on the vertical change of pore water pressure and one of these zones overlies the impervious layers and the other underlies the impervious layers. Finally we concluded that sedimentological and mineralogical studies were required to evaluate the mechanical and hydrological properties of stratified sedimentary rocks.We propose a procedure for the geotechnical evaluation of sedimentary rocks that surround the CAES cavern.