A hybrid GSHP operation strategy to mitigate ground temperature variations in ground heat exchangers

GSHP (Ground-source heat pump) systems use the vast heat capacity and less fluctuated temperature of the ground as a heat source, allowing for efficient and long-term heat exchange regardless of weather or seasonal changes. This results in a higher COP (coefficient of performance) compared to commonly used air-source heat pump systems.

To ensure the long-term and stable operation of GSHP systems, appropriate design and operational strategies that consider the characteristics of the ground are necessary. In buildings with significant annual imbalances in heating and cooling loads, dominant loads can cause thermal accumulation in the ground, leading to ground temperature imbalances. Furthermore, when dealing with large loads, multiple ground heat exchangers may be required, leading to thermal interference among them. This interference can cause overheating or overcooling in specific ground heat exchangers, ultimately reducing the heat exchange efficiency. Therefore, various operational strategies have been proposed in previous studies to ensure the long-term stable operation of GSHP systems.

Most previous studies have proposed intermittent operation and the combination of hybrid heat sources based on parallel operation of ground heat exchangers. While these strategies can help mitigate ground temperature imbalances and thermal accumulation to some extent, they may not adequately address thermal accumulation issues under conditions of severe load imbalances and thermal interference. Additionally, specific overheat or overcooling conditions in particular ground heat exchangers cannot be addressed.

Therefore, this study proposes a group/individual operation strategy and compares its effects with the traditional method of using only ground heat exchangers under conditions dominated by cooling loads throughout the year while simultaneously handling the Inverse domestic hot water load.

When only ground heat exchangers were used, it was observed that the ground temperature continuously increased due to the dominant cooling load, and adequate recovery of the ground temperature was not achieved, resulting in a sharp rise in ground temperature. The proposed group/individual operation strategy also showed a trend of ground temperature increase due to the dominant cooling load, but it was confirmed to increase more gradually compared to using only ground heat exchangers.

This is attributed to the central ground heat exchanger, which operates individually to handle the DHW (Domestic hot water) load, continuously extracting ground heat. This allows the ground temperature of the peripheral ground heat exchangers, which handle the dominant cooling load in group operation, to partially recover due to thermal interference.