Influences of air thermal energy utilization system with fan-coil units on the thermal and humid environment of multi-span tunnel in cold wave

Multi-span plastic tunnel greenhouse has widely been used in the cultivation of vegetables, fruits, and flowers. However, it is easily suffered from freezing damage to crops, particularly from cold waves in winter. In this study, an air thermal energy utilization system was developed with the fan-coil units to effectively control the thermal and humid environment in the shed. A field experimented was also performed on a two-span plastic tunnel in Wenzhou city of China during the cold wave from January 8 to January 13, 2021. The system was composed of fan-coil units, water reservoir, circulating water pump, supply and return water pipes, as well as control devices. The circulation of water in the system was utilized to collect and then store the surplus heat in the shed during the day, while finally released at night to heating the two-span plastic tunnel. In the collected phase, once the air temperature in the shed reached 15 ℃ on the day, or it was 5 ℃ higher than the water temperature in the water reservoir, the system was started until the air temperature dropped to 10 ℃, or it was 2 ℃ lower than the water temperature. In the released phase, once the outside air temperature was below 2 ℃ at night, or it was 4 ℃ lower than the water temperature, the system was started to operate until the air temperature reached 5 ℃, or it was only 2 ℃ higher than the water temperature. The collected and released performance of the system was evaluated using the collection and discharge heat, as well as heat flow, according to the change of water temperature in the experiment. The difference between water and air temperature was taken as the main influencing factor to analyze the heat flow in the collected and released phases. An exergy analysis of condensation dehumidification was also made during this time. The results showed that the system ensured the temperature inside the shed was 5.2-7.8 ℃ to protect crops from freezing damage, higher than that outside the shed in cold waves. The heat collection was 390.6-693 MJ, while the heat release was 361.2-609 MJ. The Coefficient of Performance (COP) of the system was 4.4-7.2, indicating quite remarkable energy saving. When the difference in water temperature increased by 1 ℃ during the operation of the system, the heat collection flow rate increased by 0.82 kW, while the heat release flow rate increased by 0.58 kW, indicating a larger heat transfer rate per unit time. In addition, the moisture absorption coefficient of the system was about 1.70 in the heat collected phase, due mainly to the presence of condensation and dehumidification. Meanwhile, the exergy efficiency tended to increase rapidly, up to 82.8%, as the temperature difference between water and air increased, indicating high energy utilization performance in the process of condensation and dehumidification. Nevertheless, the condensation and dehumidification gradually weakened until stopped, and concurrently the exergy efficiency decreased significantly, with the increase of water temperature. Consequently, the air thermal energy utilization system with fan-coil units can be expected to serve a safe, low carbon, and controllable heating technology. The finding can provide a potential application to ensure the safe production of multi-span plastic tunnels in winter.