Coupled heating of ground-coupled heat pump system with heat compensation unit : Performance improvement and borehole reduction

The heat compensation unit with thermosyphon has been developed to eliminate the annual soil thermal imbalance of ground-coupled heat pump in heating-dominant buildings. But the issues on heating capacity deficiency at peak heating loads and on the high borehole investment are still unsolved in this non-coupled system. In this paper, a coupled operation of the heat compensation unit and ground-coupled heat pump system is proposed. That is, the heat compensation unit reheats the borehole outlet fluid and improves the temperature of the fluid entering the evaporator of heat pump during the heating season. By doing so, it can enhance the heating capacity at peak loads and reduce the number of boreholes required. The heat compensation required by the heat compensation unit in the non-heating season is also reduced, increasing the system efficiency. To demonstrate the effectiveness of the coupled system against the non-coupled one, the system models are built in TRNSYS to analyze the system reliability, efficiency and economy. Results show that, while the number of boreholes is reduced to 40%, the coupled system can maintain the soil thermal balance and meet the indoor heating demand. For the coupled system with different numbers of boreholes (40–100%), the seasonal average heating COPs of the ground-coupled heat pump and heat compensation unit are 4.39–4.70 and 3.67–3.80, respectively. The hourly heating capacity of the ground-coupled heat pump is also increased by 19–65%. The annual system COP of the coupled system is about 2.48–2.61, which is higher than that of 1.82–2.45 for non-coupled one. Compared to the conventional Boiler assisted ground-coupled heat pump system, the payback period of the coupled system is only 1 year.