Abstract
The boundary element method was introduced to calculate the thermal resistance between pipe outside surface and the borehole inner surface of vertical U-shaped pipe underground heat exchanger. The effects of U-tube diameter, borehole diameter and tube spacing on the thermal resistance were also discussed. The results showed that it took only 10 min to complete a simulation of an underground heat pump running continually for 1000 h, which took least 3 h to complete the same cycle through finite volume method. When the thermal conductivity λ of the backfill material was 1.8 W/m2·K, the thermal resistance of the U-shaped pipe decreased with the increase in the tube spacing, it decreased with the increase in the U-tube diameter, however, it increased when the borehole diameter increased.
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