Effect of depth and fluid flow rate on estimate for borehole thermal resistance of single U-pipe borehole heat exchanger

Changxing Zhang, Xinjie Wang, Pengkun Sun, Xiangqiang Kong and Shicai Sun

Renewable Energy, 2020, vol. 147, issue P1, 2399-2408

Abstract: Accurate estimates for ground thermal parameters and borehole thermal resistance are important to improve the design of borehole heat exchangers (BHEs) in ground-coupled heat pump systems(GCHPs). In order to improve the estimating accuracy of borehole thermal resistance, this paper presents a simple analytical method for evaluating the actual averaged –over-the –depth mean fluid temperature (MFT) in the U-pipe of BHE to calculate borehole thermal resistance Rb. Furthermore, the effects of borehole depth and volumetric flow rate on the calculating RMSE distribution between borehole thermal resistance Rb and effective borehole thermal resistance Rb∗ are investigated. The conclusion shows that the relative deviation between the two borehole thermal resistances corresponding to the volumetric flow rate 1.5e-4m3/s increases from 4.2% to 29.7% when borehole depth changes from 50 m to 200 m. Finally, the borehole depths corresponding to different volumetric flow rate are optimized to find the boundary line where Rb are nearly equal to Rb∗ in the operating time, and the impacts of grout thermal conductivity and heat rate per unit depth of BHE on the boundary line are quantitatively analyzed. Volumetric flow rate has more effect on Rb with the higher grout thermal conductivity, the relative error between the two Rb corresponding to V = 1.5e-4m3/s and V = 3e-4 m3/s is 10.8% for kg = 2.3 W/(m.°C). The effect of heat rate per unit depth of BHE on Rb is very limited, the relative error between the two Rb corresponding to q1=50 W/m and q1=80 W/m is 5.4% under V = 1.5e-4m3/s, and it is only 3.7% under V = 3e-4 m3/s.

Keywords: Vertical temperature distribution; Mean fluid temperature; Borehole thermal resistance; Borehole depth; Volumetric flow rate (search for similar items in EconPapers)
Date: 2020
References: View references in EconPapers View complete reference list from CitEc
Citations: View citations in EconPapers (6)

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Persistent link: https://EconPapers.repec.org/RePEc:eee:renene:v:147:y:2020:i:p1:p:2399-2408

DOI: 10.1016/j.renene.2019.10.036

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