Comparative analysis of U-pipe location on the sizing of borehole heat exchangers

The geometry of borehole heat exchangers (BHEs) is important for their sizing. In practice, a U-pipe can be located anywhere inside a borehole. To simplify the BHE modeling, ASHRAE has two standardized borehole geometries (the B type and BC type borehole geometry). In general, the B type is hard to achieve, although it is widely used in current BHE sizing. The BC type is the one that most likely occurs in practice, but there is a lack of detailed studies. This study presents a comparative analysis of the effects of U-pipe locations on BHE sizing. As current models are only feasible for the B type, a model for the BC type is first developed and verified using the well-known Sandbox experiment developed by Beier. Then, a comparative study of the B and BC types is conducted using the borehole depth and investment costs of a real office building. Finally, the factors influencing the borehole depth differences of the B or BC types are discussed, and guidelines for future BHE sizing are suggested.