Operational characteristics of cross-seasonal partitioned heat storage in a PV/T coupled ground source heat pump system for flower greenhouses

The ground source heat pump (GSHP) system has been widely used in flower greenhouses. However, its long-term operation can cause the soil thermal imbalance, leading to a decline in the heating performance of heat pump units. In this study, a photovoltaic/thermal-couple ground source heat pump (PV/T-GSHP) system was proposed with a cross-season zonal heat storage mode. The coordinated scheduling was developed for the different sections of the buried pipe network in the various periods of thermal storage. Multiple cross-season zonal heat-storage modes were designed and then simulated over a 20-year cycle. Finally, a systematic analysis was conducted for the no thermal storage, year-round full-zone, and various cross-season zonal heat storage modes. The operational characteristics were obtained after simulation. Firstly, the annual requirement of soil thermal storage was predicted to simulate system operation under a no thermal storage mode. Subsequently, the installation area was determined for the PV/T modules, according to the national standards. A 20-year simulation was then conducted on the year-round full-zone thermal storage mode. Some indicators, including soil temperature, heat pump coefficient of performance (COP), and the coefficient of performance of the system (COP_sys), were compared against the no thermal storage baseline. The results confirmed that the PV/T components effectively resolved the serious imbalance of soil thermal and the degradation of the heat pump's heating performance, while simultaneously enhancing the overall system performance. Furthermore, it was verified that an increase in the soil temperature was beneficial for the heat pump's coefficient of performance for heating (COP_h). Multiple operational modes were established and simulated over 20-year periods, assuming that the cross-season zonal heat storage mode was used to regulate soil temperature. Comparative analysis between the year-round full-zone and various cross-season zonal heat storage modes was conducted to examine the soil temperature, COP_h, coefficient of performance for cooling (COP_c), mean annual coefficient of performance (COP_m), mean annual coefficient of performance of the system (COP_msys), solar fraction (S_f), and operating costs. The results demonstrated that the cross-season zonal heat storage modes further enhanced the operational performance of PV/T-GSHP systems. The optimal heat storage mode was ultimately identified as follows: The entire buried pipe field was provided with the heating and cooling sources throughout the year. Full-zone thermal storage was implemented during the cooling season, while the cross-season zonal heat storage-utilizing quarter of the total buried pipe area, was applied during the transitional seasons (spring and autumn) and the heating season. This configuration fully met the cooling demands, leading to a decrease in the COP_c but prioritizing the improvement of COP_h and COP_msys. The COP_m and the COP_msys increased by 7.7% and 50.1%, respectively, after 20 years of simulated operation under this optimal heat storage mode, compared with the no thermal storage mode. Compared with the year-round full-zone thermal storage mode, the COP_msys and S_f increased by 9.2% and 7.8%, respectively. The total operating costs were reduced by 38.5% and 4.5%, compared with the no thermal and the year-round full-zone thermal storage mode, respectively. The payback period for the initial investment in PV/T was shortened by 7.6%, compared with the year-round full-zone thermal storage mode. Energy consumption analysis demonstrated that when the net electricity consumption (total consumption minus PV/T generation) was converted into equivalent coal consumption, the optimal heat storage mode reduced the coal consumption by 50.5% and 6.8%, respectively, compared with the no thermal and the year-round full-zone thermal storage mode. This finding can provide a valuable reference for the more efficient and energy-saving operation of the PV/T-GSHP systems in the flower greenhouses. Additionally, the initial investment was reduced to further shorten the payback period in the PV/T system. Subsequent research was recommended to optimize the installation area, tilt angle of the PV/T modules, and the circulating water flow rate.