Evaluation of thermal short-circuiting and influence on thermal response test for borehole heat exchanger

• The heat transfer between the two legs of the pipe inside BHE was investigated.
• Expression of short-circuiting thermal resistance was presented.
• Fluid temperature was analyzed by 3-D model considering short-circuiting effect.
• Experimental analysis of TRT taking the short-circuiting effect into account.

The fluid extracts or rejects heat with subsurface by downward leg of pipe (DLP) and upward leg of pipe (ULP) inside the vertical borehole heat exchanger (BHE). As the borehole diameter is only 0.11 m to 0.2 m, the temperature difference between DLP and ULP inevitably leads to thermal short-circuiting. In order to discuss how different geometrical characteristics influence on short-circuiting, the heat transfer between the two legs was investigated by a 2-D model, and then a best-fit expression of short-circuiting thermal resistance was presented in dimensionless form. A 3-D equivalent rectangular numerical model was established to evaluate the fluid temperature variations along the pipe, how the flow velocity and grout conductivity and borehole depth influence on the outlet temperature and average heat flux per unit length and short-circuiting loss rate were analyzed. By comparing the arithmetic average fluid temperature and integral average fluid temperature, it was found that the lager short-circuiting loss rate would lead to greater error for effective subsurface conductivity estimation. The experiment done in NanJing, China also validated that the smaller flow velocity and larger borehole depth would bring about the smaller measured effective subsurface conductivity during TRT.