Extending the Ashrae Tp8 method for vertical borefield design to uniform BHE temperature boundary conditions

Abstract

The Ashrae method is a fast algorithm for calculating the overall length of closed-loop borehole heat exchangers, considering the ground thermal response and the building thermal load profile. The method includes a corrective variable called Temperature Penalty (Tp) to account for the thermal interaction between boreholes. Several authors proposed different approaches for calculating this parameter, but the majority is inaccurate or too complex. Recently, the same Authors suggested a very simple method, called Tp8. The coefficients included in the original formula were optimized against g-functions obtained by spatial superposition of a single source always working at the same heat rate. In this paper, the early Tp8 method is improved by the calculation of new constants based on g-functions calculated for uniform temperature borehole conditions. A large dataset of 300 borefield configurations have been considered for optimization and validation purposes. In this case, all the BHEs are at the same temperature and the overall heat transfer rate of the field is constant in time. Compared with EED software outputs, the results from present Tp8 method show a good accuracy (within 3% for the overall BHE field length) while maintaining a great simplicity in applying the method at engineering design level.