Modifications to ASHRAE's sizing method for vertical ground heat exchangers

Abstract

The current article proposes modifications to the ASHRAE sizing equation for vertical ground heat exchangers. The proposed method uses the same three pulse approach as the current sizing equation but uses g-functions to calculate the effective ground thermal resistances. One key feature of the iterative methodology is the ability to calculate new g-functions as the geometry morphs during the solution process. Long-term g-functions are evaluated analytically using the finite line source (FLS) solution over borehole segments while so-called short-term g-functions are calculated based on a hybrid analytical/numerical method to account for the borehole thermal capacity. The current article examines three aspects of the proposed methodology. First, it is shown that the time-consuming evaluation of the full g-function curve, typically obtained by temporal superposition, is not necessarily required. Second, the optimum number of borehole segments to obtain an accurate bore field length with reasonable calculation time is examined. The selection of a convergence criteria and its impact on calculation time is also discussed. The excellent agreement between results obtained with the proposed alternative method and the ones obtained from other design software tools confirms the validity of the proposed method. Finally, it is shown that short-term effects can have a relatively significant effect on the calculation of the required borehole length.