Comparison of 10- and 25-year horizon designs for vertical borehole heat exchangers in geothermal heat pump applications

The precise design of Borehole Heat Exchanger (BHE) fields in ground-coupled heat pump (GCHP) systems is essential for maintaining optimal long-term performance. Conventional sizing methodologies, such as the ASHRAE method, typically account for a 10-year operational horizon. This study seeks to expand the applicability of the ASHRAE-Tp8 method to a 25-year operational period and to compare the design results related to different time horizon strategies. To this aim, the dimensionless temperature penalties have been compared against reference results obtained from g-functions pertaining to actual borehole field geometries. New optimized constants have been derived specifically for the ASHRAE-Tp8 method, facilitating its adaptation to the longer time frame of 25 years. The accuracy and reliability of this enhanced method have been extensively validated (at a 25-year time horizon, the average error is 3.8 % respect to the reference code). The findings of this study underscore the potential errors in total borefield length and borehole depth that could arise when applying the conventional 10-year design methodology to a 25-year horizon. On the other hand, the present study leads to deducing that the drilling costs due to a necessary higher borehole depth for a correct design, proper to a 25-year horizon, relatively increase (overall length can increase up to +16.8 %). The proposed methodology demonstrates wide applicability, providing a robust framework adaptable to various operational time frames.