Heat extraction by deep coaxial borehole heat exchanger for clean space heating near Beijing, China: Field test, model comparison and operation pattern evaluation

• Published in: Renewable energy Vol. 199; pp. 803 - 815
• Main Authors: huajun, Wang, Yishuo, Xu, Yukun, Sun, Sumin, Zhao
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.11.2022
• Subjects: Deep coaxial borehole heat exchanger; Heat extraction; Intermittent operation; Long term performance; Thermal influence radius; Intermittent operation; Heat extraction; Deep coaxial borehole heat exchanger; Thermal influence radius; Long term performance
• ISSN: 0960-1481; 1879-0682
• DOI: 10.1016/j.renene.2022.09.017

Deep coaxial borehole heat exchangers (CBHEs) are being widely used in clean space heating of buildings in winter. Here we report a new field test on a 1800m deep CBHE in the southern Daxing uplift, Beijing, a representative geological region in North China. Under the economic flow rate of 20–25 m3/h, the steady heat extraction power is 237.24–256.54 kW. Further, numerical modelling and simulation calculations are performed to compare the applicability of the Eskilson-Claesson model and Al-Khoury model for different operation patterns and then analyze the operation performance of deep CBHE during the whole heating season. For typical flow rates, compared with the continuous pattern, the intermittent pattern can increase the heat extraction power per unit depth by 25.4–31.0% and reduce the thermal influence radius by 1.46–1.89 m due to a better ground temperature recovery around the borehole. In addition, a new contribution factor (CF) is introduced to evaluate the temperature contribution of different ground layers during the heat extraction, which can reveal the influences of flow conditions, geothermal gradient, thermal properties, and even groundwater flow. The present results support that deep CBHEs have more obvious advantages for space heating of buildings with intermittent heating requirements.