Numerical assessment on the thermal imbalance of multiple ground heat exchangers connected in parallel

Thermal imbalance is a common problem in ground source heat pump (GSHP) systems, especially those with large scale multiple ground heat exchangers (GHEs). It usually happens due to the imbalance between the accumulated heat extracted from and injected into the soil and can lead to heat or cold accumulation, cooling or heating performance deterioration and even system failure. In most projects, the GHEs are not independent of each other, but are connected in parallel as an entirety. Therefore, the interconnections among each GHE within the borehole field should be considered. In this study, a numerical approach considering models of the heat pump and the water separator/collector was established for the thermal imbalance evaluation of multiple GHEs connected in parallel, and it was validated by an in-situ test of a single GHE. The necessity of introducing such models was discussed, and the effects of load ratios, borehole layouts, and scales were also investigated. Results show that when both the heat pump and the water separator/collector were considered, the temperature distribution of the borehole field was always uniform during the five-year operation with the highest temperature unevenness value of 0.37 degrees C, proving that the self-adjusting mechanism of the GHE load (i.e., the GHE load distribution of the borehole field is able to be optimized automatically according to the heat transfer condition of each GHE) is beneficial to maintaining the thermal balance of the borehole field. However, when the interconnections among the GHEs are ignored, the thermal imbalance of the borehole field is overestimated, in particular, under conditions with large load ratios. With the increase in the field scale, the temperature unevenness value of the borehole field first increases and then decreases whether in square or regular hexagon layouts. The numerical approach in this study is helpful to correctly evaluate the thermal imbalance of the borehole field.