Experimental and computational investigation of multi U-tube boreholes

• In ground source heat pump (GSHP) applications, borehole drilling cost constitutes an important part ofthe 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 aborehole, (unit HTR value). To determine the optimal number of U-tubes which maximizes the commercialand engineering benefits of multi U-tube applications, cost and performance analyses of multi U-tubeboreholes are crucial. In this study, a triple U-tube is used in a borehole of 50 m depth. Time variation ofunit HTR value of the borehole is experimentally measured when single, double and triple U-tubes are inoperation separately. Furthermore a computational model is calibrated by fitting the computationalresults to the experimental ones, and effects of using four and five U-tubes in a borehole are computationallyinvestigated. The relations between number of U tubes and time variation of unit HTR value ofa borehole as well as investment cost are analyzed. Long term borehole performance predictions aremade and compared for multi U-tube applications. Both experimental and computational results showedthat performance improvements are remarkable for 2U-tube and 3U-tube configurations while it isnearly insignificant for 4U and 5U ones. If the investment cost per thermal power is considered, 2U-tubeconfiguration 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 cheapestsolution.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Energiteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Energy Engineering (hsv//eng)

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