Design of underground heat exchanger is crucial to application of ground source heat pump system. This paper presents the method and self-developed program for designing vertical U-pipe underground heat exchanger using numerical simulation. The program's accuracy was validated by comparison of outlet temperature between field test and simulation: the biases are not more than 1 centi-degree in most cases. This paper conducted a 30-years dynamic simulation using annual hourly loads as inputs and recommended a scheme of underground heat exchanger. This scheme ensures that the hours during which heat pump inlet temperature exceeds 32 centi-degree is not more than 18 in summer. And also the inlet temperature in winter is higher than 5 centi-degree. The authors indicate that heat rate per meter of borehole is insufficient for designing underground heat exchanger, and hourly simulation is essential.
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