THERMAL MECHANICAL ANALYSES OF COMPRESSED AIR ENERGY STORAGE PILE FOUNDATIONS

Abstract

Nowadays, renewable energy has become a widely accepted energy source due to the growing interest in environmental issues such as air pollution and greenhouse gas emissions. However, it highly depends on the day and night cycle and has an intermittent nature. Thus, ways of storing the extra energy generated by renewable energy have been developed including pumped hydroelectric, compressed air, solar batteries, and thermal energy storage. An energy storage pile foundation system is being developed through a multidisciplinary research project. The main idea of the proposed system is to store extra energy in hollowed pile foundations by utilizing compressed air energy storage technology. In previous studies, structural responses of the energy storage pile foundations under internal air pressure were evaluated. These studies were based on an assumption of completely cooling air down to the ambient temperature before entering the pile foundation for storage. This assumption may require additional resources due to the efficiency of the cooling process. Another option is to keep the storage air temperature higher than the ambient temperature, which can be achieved by controlling the cooling process. As a result, thermal mechanical loading will be induced to the pile foundation. However, the stress states in the pile section that originated from the thermal loading are different from the stress states that originated from the internal air pressure. Therefore, it is necessary to investigate the structural response of the pile foundation under the combined air pressure and thermal mechanical loading. This thesis focuses on analytical investigations of structural responses of the energy storage pile foundations under combined loadings from the internal air pressure and temperature changes. To conduct this study, two steps of analytical research work were performed. In the first step, non-steady state thermal analyses were conducted to identify the temperature distributions inside the pile section. In the second step, the temperature distribution, obtained from the first step, was used as thermal loadings forstatic thermal mechanical analyses to evaluate the structural responses of the pile. The analyses were performed using general finite element software, Abaqus CAE. Several pile designs with different parameters were studied. These parameters include the geometry of the piles, spacing between them, level of cooling, the thermal expansion coefficient of the pile, and inner diameter of the pile. From the analytical results, it has been found that the maximum tensile stress originated from the internal air pressure can be reduced by introducing an appropriate level of thermal mechanical loading. The appropriate level can be achieved by adjusting the storage temperature. Moreover, by limiting the days of continuous usage of the energy storage pile, the thermal mechanical loading can eliminate the state where the entire concrete section is under pure tension, which has been a critical issue for the pile under the internal air pressure. Design recommendations regarding the optimum storage temperature and the number of days of continuous usage were made as follows: the appropriate range of storage temperature lies between 31.5°C to 34°C; it is recommended to pause the air storage one day for every 5-14 days depending on the pile section sizes to avoid the entire pile section transiting to a pure tension state.