Investigation on the Effects of Different Time Resolutions in the Design and Simulation of Bhe Fields

The correct design of a field of Borehole Heat Exchangers (BHE) requires the knowledge of ground thermal properties, heat pump performance and building heating and cooling demand. The sequence of heat pulses from (to) the ground by the heat pump can be described according to different time steps, from hours to months and even years. The monthly time step approach is often the preferred design choice which involves recursive calculations (temporal superposition techniques) and the availability of precalculated temperature response factors (or g-functions) for given BHE field geometries. Such a complex computing task is usually performed thanks to commercial codes in order to fulfil a carrier fluid temperature at the end of a given time horizon, typically 10 or 25 years. In this paper the monthly design approach (EED code and TecGeo proprietary code) is compared with the three thermal pulse approach (modified ASHRAE Method Tp8) and it is demonstrated that for a representative series of case studies the three pulse calculation, easy to be performed at engineering level, is able to provide the correct BHE field overall length with 8% accuracy with respect to the reference monthly calculations.