Optimization and exergy analysis of fan-coil units-heat pump combined heat collection system

Many environmental factors have posed an important impact on crop growth in a greenhouse. Among them, the temperature is often the dominated factor in the greenhouse production. Water is also suitable for the medium of heat transfer or storage. Most research has been focused on the active heat collection and release system using water circulation and heat storage for nighttime warming in the greenhouse. A Fan-coil Units-Heat Pump Combined Heat Collection System (FUHPS) has been developed, where a heat pump has been added to the fan-coil units and heat storage pool for the heat collection (TSFU). A systematic investigation has been made to explore the performance under three modes of heat collection in different sizes of horticultural facilities. However, the water temperature of the storage tank cannot be raised by more than 12°C from the beginning to the end of the heat collection process in the field test. The reason was that the water temperature of the storage tank was not high enough to cause a small temperature difference between the water and gas during the heat release. As such, there was a relatively small Coefficient of Performance (COP) of heat release. Therefore, it is necessary to improve the heat release COP of the system. The initial water temperature of heat release can be expected to effectively improve the heat release performance of the TSFU. Furthermore, the heat release performance of the FUHPS with the same heat release mode can also be used to increase the initial water temperature of heat release. It is probable to reduce the actual water storage capacity of the heat storage pool. This study aims to improve the heat release performance of the FUHPS, and then further optimize the heat collection system. The actual water storage capacity was firstly calculated at the target water temperature. Secondly, an analysis was made to clarify the impact of water storage capacity on the heat release performance of the system. Thirdly, the exergy analysis was carried out under two kinds of heat collection modes and one kind of heat release mode, in order to determine the specific location and main reasons for the loss of exergy. Finally, optimization was proposed for each component of the FUHPS. The results show that the heat release power and COP of the optimized system were 27.1 kW and 6.2, respectively, which increased by 33.5% and 37.8% than before. The overall performance coefficient was also improved after optimization. The exergy analysis demonstrated that an excellent energy utilization quality was achieved in this case, indicating the highest exergy efficiency of the water pump. Specifically, the exergy efficiencies of the heat-collecting device and fan-coil units were 89.3%, 87.8%, and 60.1% under the fan-coil units’ heat collection mode, combined heat collection mode of fan-coil units+heat pump, and heat-releasing mode, respectively. In addition, some consideration was made for the irreversible loss caused by heat transfer temperature difference. Nevertheless, the lowest exergy efficiency was obtained in the heat pump unit, which was the key point of the energy-saving transformation of the system. This finding can provide a new idea to optimize and improve the performance of the active heat collection and release technology. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.