Experimental and computational investigation of multi U-tube boreholes

• Effects of number of U-tubes on HTR value and cost of a borehole are investigated.
• Performance increases considerably for 2 and 3U-tube but negligible for 4 and 5U.
• 2U-tube BHE is the best solution for countries like Turkey where pipe prices are high.
• If the cost of pipes decreases, then 3 or even 5U–tube can be the optimal solution.

In ground source heat pump (GSHP) applications, borehole drilling cost constitutes an important part of the investment cost and it can be reduced by improving borehole performance. In vertical GSHP applications, usually double-U tube configurations are used to improve the heat transfer rate per unit length of a borehole, (unit HTR value). To determine the optimal number of U-tubes which maximizes the commercial and engineering benefits of multi U-tube applications, cost and performance analyses of multi U-tube boreholes are crucial. In this study, a triple U-tube is used in a borehole of 50 m depth. Time variation of unit HTR value of the borehole is experimentally measured when single, double and triple U-tubes are in operation separately. Furthermore a computational model is calibrated by fitting the computational results to the experimental ones, and effects of using four and five U-tubes in a borehole are computationally investigated. The relations between number of U tubes and time variation of unit HTR value of a borehole as well as investment cost are analyzed. Long term borehole performance predictions are made and compared for multi U-tube applications. Both experimental and computational results showed that performance improvements are remarkable for 2U-tube and 3U-tube configurations while it is nearly insignificant for 4U and 5U ones. If the investment cost per thermal power is considered, 2U-tube configuration is the optimal one if the prices of polyethylene pipes are relatively high, like in Turkey. When the cost of pipes decreases, then 3U-tube or even 4U–tube configuration can be the cheapest solution.