Comparison of isothermal and isoflux g-functions for borehole-heat-exchanger fields

• Published in: Journal of physics. Conference series Vol. 1224; no. 1; pp. 12026 - 12035
• Main Authors: Naldi, C, Zanchini, E
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.05.2019
• Subjects: Boreholes; Boundary conditions; Heat; Heat exchangers; Heat flux; Heat transfer; Physics; Temperature
• ISSN: 1742-6588; 1742-6596
• DOI: 10.1088/1742-6596/1224/1/012026

A new numerical approach to determine the g-function of a borehole field with the boundary condition of uniform temperature and time-constant mean heat flux at the surface of the boreholes is presented. The method is employed to compare the g-functions obtained by this boundary condition with those obtained by the usual condition of uniform and constant heat flux, for a single borehole and for a field of six boreholes placed in two lines. Boreholes with length 150 m, diameter 15 cm, buried depth 1.5 m and mutual distance 7.5 m, for the field, are considered. The results show that the difference between the two kinds of g-functions is less than 1.5 % for a single borehole, while it reaches 8.7 % for the borehole field, at high values of time. Finally, we show that the superposition of the effects of the single boreholes yields correctly the g-function of the field in the case of uniform heat flux, but overestimates the g-function in the case of uniform temperature and constant mean heat flux.