Improvements on the American Society of Heating, Refrigeration, and Air-Conditioning Engineers Handbook equations for sizing borehole ground heat exchangers

There is much room for improvement in the method of ground heat exchangers design presented in the ASHRAE handbook. This article reports on an alternative equation for sizing ground heat exchangers, anchored to recently developed quasi-three dimensional and full-scale line source models. The theoretically complete models not only simplify the form of the design equation but improve the ASHRAE equations in calculating the short-term thermal resistance and tackling the effect of the variation in fluid temperature along U-shaped pipes (including the effect of flow rate). The authors provide a set of charts (G-charts), allowing engineers to conveniently calculate short- and long-term ground thermal resistances. The new equation always yields less conservative design lengths than the ASHRAE equations. The new equation is validated by in situ data, which are carefully monitored from four ground-coupled heat pump systems in the world, confirming that less conservative designs are more close to actual borehole depths, at least, in the three of the four systems. Moreover, the current article presents a sensitivity analysis and Monte Carlo simulation to explore the sensitivity and robustness of the new equation.