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Extending the ASHRAE method to a 25-year horizon through the Tp8 model for temperature penalty accurate estimation

Fossa, Marco
Morchio, Stefano
Memme, Samuele
Priarone, Antonella
Parenti, Mattia
The accurate design of Borehole Heat Exchangers (BHE) fields in ground-coupled heat pump (GCHP) systems is crucial for ensuring long-term performance. Traditional sizing methods, such as the ASHRAE method as modified by ASHRAE-Tp8 version, consider the annual building heating and cooling demand over a 10-year time horizon by applying the temporal superposition of 3 aggregated thermal pulses of different durations. The present paper aims to clarify how the ASHRAE-Tp8 method could be adapted to be employed over a 25-year plant operation horizon. Temperature penalty estimations are compared with "real" precalculated temperature response factors (g-functions) through the minimization of a suitable objective function. Optimized constants for the present new ASHRAE-Tp8 method are derived, enabling its easy adaptation for the 25-year time period. Comparisons with EED and GLHEPRO commercial software results demonstrate the reliability of this improved method, with borefield length estimations accurate within 7% and 6% respectively. The results reported in the present paper lead to easily inferring the error in terms of overall length and borehole depth that would be obtained by employing the design process proper of the 10-year reference period when the 25-year time horizon is considered. The methodology is in general demonstrated to be applicable to different time frames.