An improved method for vertical geothermal borefield design using the Temperature Penalty approach

The Ashrae method (Kavanaugh and Rafferty) is one of the few engineering models that allows a system Borehole Heat Exchangers (BHE) to be quickly designed starting from the knowledge of the building thermal energy requirements. The method is based on Infinite Source solutions from ground dynamic response to a series of three heat pulses, representing the building thermal history from the short to the long period. The key parameter of the Ashrae procedure (recently adopted also as an Italian Standard) is the evaluation of the Temperature Penalty correction Tp, which takes into account the thermal interactions of neighbour boreholes in the long term period. In this paper a new method is addressed to the calculation of the Tp parameter and it refers to a physically based approach of mutual interactions among the BHEs. The improved method has been conceived for maintaining the simplicity of the original Ashrae scheme while enabling a more accurate estimation of the Temperature Penalty values and hence a more reliable BHE field design data. The validation of the proposed procedure and the estimation of the constants involved in the new Tp8 method is based on the "exact" calculation of the Tp values starting from FLS generated g-functions, able to describe the ground response of a large number of BHE configurations, including square, rectangular, inline, L-shaped, open rectangles. It is demonstrated that for the set BHE configurations here considered (about 120) the average deviation of the Ashrae Tp values (with respect to the FLS benchmark) is above 46% with a typical underestimating behaviour which reflects in underestimating the BHE field overall length. On the other hand the proposed method yields Tp percentage deviations well centered around the benchmark line and with an average deviation of 18%. The new method, with respect to benchmark set and in heating mode operations, is able to yield the design BHE length within 5% with respect to the reference solutions.