Improving the Ashrae method for vertical geothermal borefield design

Description

tThe design of a borehole heat exchanger (BHE) field coupled to a geothermal heat pump requires a suit-able dynamic model able to take into account the intrinsic time varying behaviour of the building heatload profile and ground thermal response. In this paper a new method is proposed for the calculationof the Temperature Penalty, the key parameter of the Ashrae dynamic method originally developed byKavanaugh and Rafferty. The proposed method is conceived for maintaining the simplicity of the originalAshrae model while enabling a much more accurate design of the BHE field. The validation of the newprocedure is made with reference to a comprehensive set of 240 BHE configurations, described in termsof the corresponding temperature response factors (i.e. g-functions). It is demonstrated that the Ashraetemperature penalty values typically underestimate the corresponding true values with an average devi-ation of more than 40%. On the other hand the proposed method is able to provide temperature penaltyvalues well centered around the benchmark line and with an average deviation of less than 10%; it isfinally demonstrated that the present procedure is also much more accurate than other existing modelsand simpler to apply in engineering design.

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