Research On Operating Performance Of Hybrid Ground-coupled Heat Pump System With Heat Compensation Unit

The ground-coupled heat pump (GCHP) has been widely applied in cold regions.Most of them suffered from the soil thermal imbalance due to the large heat extractionin winter and little heat injection in summer, which leads to the serious decline ofsystem performance and even the failure of system operation. Increasing the boreholedepth and space of ground-coupled heat exchanger just slows down deterioration of soil.Adding the auxiliary unit, such as the solar collector, boiler, air-source heat pump,seems solve this problem essentially, but the application shows the real performance ispoor. Thus, the research focused on a new solution which could keep soil thermalbalance efficiently and environment-friendly.The main work and contributions of this research were as follows:(1) The principle of heat compensation unit (HCUT) combined air-sourcethermosyphon and heat pump was proposed, which could efficiently collect the heatfrom the air storing them into the soil during non-heating season. The prototype ofHCUT was developed, which had four different working modes: heat compensation inthermosyphon mode, heat compensation in heat pump mode, direct heating mode, anddirect domestic hot water mode.(2) Based on the energy conservation law, the soil heat compensation theory wasproposed, which included reducing heat extraction, increasing heat injection andcompensating heat efficiently. Consequently, the hybrid GCHP with heat compensation(HCUT-GCHP) was built by combining the HCUT with the traditional GCHP. Themodel of HCUT, GCHP units and pump was proposed and the dynamic system modelwas built in TRNSYS. Through simulation, the design method and optimal operatingstrategy for HCUT-GCHP system were put forward.(3) The long-term operation performance of HCUT-GCHP system was analyzedbased on simulation. The results showed that this system could keep the soil thermalbalance well and achieve good heating performance. Compared with traditional “boiler+split air-conditioner” system, the energy saving rate of HCUT-GCHP system was higherthan15%~20%. In contrast to “solar collector+GCHP” system, HCUT-GCHP could bemore economical during30-years’ operation.(4) A demonstration project of HCUT-GCHP system was built in Hezhi railway station in Shanxi province. The testing data indicated that HCUT operated well inpractical application.The research reflected that the hybrid GCHP system with heat compensation byHCUT would be an alternative efficient and clean heating approach in cold regions.