A modified three-dimensional numerical model for predicting the short-time-step performance of borehole ground heat exchangers

A modified three-dimensional numerical model for the borehole ground heat exchangers was proposed which took into account the effect of grout thermal capacitance and fluid circulation period inside the U-tube in a short-time-step analysis. The present model was validated by experimental data from others. It was found that the fluid temperature along the U-tube changed abruptly at the interface between the entering and the existing fluid inside the borehole. Before the entering fluid travelled to the bottom of the U-tube, the borehole was only partly-loaded. The difference in the simulated fluid leaving temperatures between the long- and short-time-step approaches could be up to 3 °C when a periodic intermittent load, common during the low-load season, was applied. This implied that a short-time-step model should be used in a long-term dynamic system simulation.