Abstract
Deep-borehole heat exchangers (DBHE) are generally coaxial pipes installed in deep boreholes and has become an alternative approach to utilize geothermal energy. Since the performance of the DBHE system can be affected by several parameters, it is important to optimize the design of parameters for the DBHE. In this paper, based on the analytical method, we carried out the sensitivity analysis of DBHE design parameters, including outer pipe diameter, inner pipe diameter, flow rate, outer pipe materials, grout materials, and borehole depth during continuous operation for 4 months. The sensitivity analysis results indicate that the heat extraction rate can be significantly affected by outer pipe diameter, borehole depth, and flow rate. The effects of grout materials, inner pipe diameter and outer pipe materials are of second-order. Finally, an optimization method based on the lowest Average Energy Cost index was proposed to optimize these DBHE design parameters under different geological conditions. Given the cost in this study, a combination scheme of all the optimal parameters is given for different depth wells under different geological conditions.
Highlights
Based on the differences in burial depth, utilization mode, and storage medium, geothermal energy is usually divided into three types: shallow geothermal energy (0–200 m), medium–deep hydrothermal energy (200–3000 m), and hot dry rocks energy (> 3000 m) (Wang 2015)
Optimization method In this study, we used the index of Average Energy Cost to optimize the deep-borehole heat exchangers (DBHE) design, which includes the outer pipe diameter, inner pipe diameter, flow rate, outer pipe materials, grout materials, and DBHE depth
The heat extraction rate is more sensitive to the increase of the outer pipe diameter when the pipe is smaller and does not linearly increase with outer pipe diameter
Summary
Based on the differences in burial depth, utilization mode, and storage medium, geothermal energy is usually divided into three types: shallow geothermal energy (0–200 m), medium–deep hydrothermal energy (200–3000 m), and hot dry rocks energy (> 3000 m) (Wang 2015). The China Geothermal Energy Development Report released in August 2018 shows that the shallow geothermal energy is the main method used in China for geothermal heating, which has been rapidly developed. The extent of hydrothermal heating is increasing steadily. The development in the utilization of geothermal resources has some difficulties due to the large areas demanding of shallow geothermal energy and the uneven distribution of hydrothermal energy (Kong et al 2014). When utilizing the hydrothermal energy, reinjection of geothermal wastewater must be carried out for maintaining the pressure of geothermal reservoirs (Rybach 2003). Deep-borehole heat exchangers (DBHE) have become an alternative approach to utilize geothermal
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