Nihon Kikai Gakkai ronbunshu (May 2024)
Heat extraction characteristics of ground heat exchangers using the Earth Retaining Wall under the Pre-installation method
Abstract
Ground heat is a renewable energy source that is less affected by atmospheric temperature and maintains a nearly constant temperature throughout the year. Therefore, it can serve as a heat source for hot water and air conditioning systems, resulting in a significant reduction in carbon dioxide emissions compared to conventional systems. In Japan, the recognition of the use of ground heat is still low, and the prices of ground heat exchangers and geothermal heat pumps tend to be high. In addition, the excavation cost is very high, so it is very important to reduce the construction cost. One cost-saving method is to install a ground heat exchanger in the core material of the earth retaining wall, which has been shown to reduce construction costs by more than 50%. In this study, a numerical program was developed to clarify the performance and characteristics of a ground heat exchanger attached to an H-beam in a “Pre-installation method using the earth retaining wall”. Then, the factors influencing the heat extraction performance of two U-Tube heat exchangers connected in series, four U-Tube heat exchangers connected in series and two W-Tube heat exchangers connected in series were investigated. Firstly, the validity of the calculation program was confirmed by comparing the numerical results with the experimental thermal response results. Secondly, numerical simulations were performed using the calculation program, and the following results were obtained. Under the conditions defined in this study, we found that the rate of heat extraction increases as the flow rate of the heat extraction medium increases in all ground heat exchangers. However, it becomes difficult to increase the heat extraction when the flow of the heat extraction medium reaches a turbulent area. All three models confirmed that the H-beam expanded the area where the formation temperature changes and increased the temperature of the heat extraction medium. This led to an enhanced heat extraction performance of the ground heat exchanger. Finally, we compared the heat extraction performance of several ground heat exchangers using the 'coefficient of heat extraction or rejection'. The results showed that the four U-Tube model had the highest performance, followed by the two W-Tube model, and the two U-Tube model had the lowest performance, although the differences were small.
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