Abstract
In this paper, the performance of multiple boreholes (multi-BHEs) field is evaluated by considering the groundwater flow. Optimization strategies are presented to mitigate thermal anomalies in the BHEs field. This study shows that groundwater flow greatly improves the heat transfer but causes thermal anomalies downstream. To overcome this problem, a heat transfer model is established for multi-boreholes based on temperature field superposition and moving finite line source model (MFLS). The MFLS multi-boreholes model considers the axial effect and groundwater flow and produces results in agreement with the field tested data of a 4 × 4 boreholes field. Using a dynamic annual load pattern, the long-term performance of the 4 × 4 boreholes field is analyzed. Three dynamic diurnal cooling load models are proposed to evaluate the temperature changes in the underground. The intermittent load model could reduce the local temperature anomalies in downstream tubes. The optimization model for cooling cases for multi-BHEs is elaborated to keep the outlet temperature as low as possible and minimize the extreme temperature anomalies, and by this, ultimately improve the system performance. Furthermore, the temperature variations and thermal anomalies downstream of multi-BHEs are investigated by evaluating the arrangement optimization and load optimization. The results show that the optimization could mitigate thermal anomalies downstream and reduce the rate of temperature imbalance of the BHEs field.
Highlights
In China, with the implementation of energy conservation and emissions reduction policies, the vertical ground source heat pump (GSHP) system is widely used because of its high efficiency, environmental friendliness, and low running cost
In order to estimate the heat transfer, many analytical or numerical models for designing and analyzing have been developed: the infinite line source model (ILS) [1] and cylindrical heat source model [2] were widely used for the thermal analysis of Borehole heat exchangers (BHEs) because of their simplicity and high speed in computation
The soil temperature downstream is greatly larger than that of the upstream. This would lead to larger ground temperature differences in the multi-BHEs field and the ground field could not exert its overall heat exchanger efficiency
Summary
In China, with the implementation of energy conservation and emissions reduction policies, the vertical ground source heat pump (GSHP) system is widely used because of its high efficiency, environmental friendliness, and low running cost. A new analytical model is presented for simulation of ground thermal effects from vertical borehole heat exchangers (BHEs) by Jaime A and Peter Bayer [28]. It accounts for long-term changes in land use and groundwater flow. When operating a BHE field over years, the interference among the individual boreholes and thermal anomalies downstream will increase the imbalance of the BHEs ground temperature. With the aid of a dynamic annual load pattern, the temperature variation of typical points for ten years using the finite and moving finite line source models were analyzed to reveal the long-term ground temperature considering the groundwater flow.
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