Thermal performance of a tight borehole heat exchanger

Abstract

A major attribute that affects a borehole heat exchanger's performance is its thermal resistance. Internal and borehole thermal resistances, among other factors, depend on the location of pipes in relation to each other and the borehole wall. The internal borehole resistance defines the short-circuit effect between the pipes, and the borehole thermal resistance is an indication of heat exchange ability with the ground. Effective borehole resistance depends on both abovementioned thermal resistance components, and describes the thermal efficiency of the BHE. It is found that the temperatures of the loop fluid and borehole wall respectively are sensitive to pipe separation and borehole diameter. This study confirms that proximity of the pipes to the borehole wall is more important than the pipe separation in reducing the total borehole resistance (i.e. temperature difference between loop fluid and borehole wall). Hence, a tight borehole design, with little spacing between the down-hole pipes, and the borehole wall, is proposed here. In such a design, a narrow borehole diameter is used with no pipe spacers. Through numerical modelling and thermal response testing, this study shows that a tight borehole heat exchanger could provide an acceptable thermal performance while optimizing the drilling and grouting volumes.