Performance of secondary condensation defrosting cold storage refrigeration system

An optimal performance can greatly contribute to the energy saving of refrigeration systems in cold-chain logistics. Since lots of agricultural products need to be kept at a temperature below 0 ℃, cold storage can be expected to store food products over a long time without spoiling. However, a great challenge is the defrosting energy consumption of low-temperature cold storage. Two problems can occur during defrosting. One is that the ever-increasing temperature can cause food preservation in cold storage. Another, it takes lots of energy in the defrosting period. In this study, a new defrosting approach was proposed for cold storage refrigeration, which was called secondary condensation defrosting. The specific defrosting cycle was summarized as follows: Firstly, a certain proportion refrigerant was obtained to reduce the pressure from the condenser outlet. Secondly, the defrosting refrigerant flew into the first evaporator, where the maining refrigerant was transferred the heat into it. Thirdly, the defrosting refrigerant came into the frosty evaporator and then released the heat to melt the frost layer, finally the refrigerant was condensed again (called secondary condensation defrosting). The secondary condensation defrosting decreased the temperature in the cold storage for energy-saving in the defrosting refrigerant, compared with the hot-gas defrosting. The thermodynamic model was established for the secondary condensation defrosting. The effectiveness and energy saving of the model were also verified by the experiments. A comparative analysis was performed on three indexes: evaporator heat transfer temperature difference, evaporation temperature, and condensation temperature. The results show that: 1) The better performance was achieved in the secondary condensation defrosting of cold storage when the evaporator heat transfer temperature difference between 7-9 ℃. 2) The energy consumption of the secondary condensation defrosting cycle increased significantly with the decrease in evaporation temperature. But more energy saving was obtained, compared with the hot-gas bypass defrosting. The optimal energy consumption was 0.055-0.080 kW·h/m³ in the secondary condensation defrosting cycle of cold storage, where the condensation and evaporation temperature were 34-38 ℃, and −20-−10 ℃, respectively. The energy saving was 12.7%-15.8%, compared with hot-gas bypass defrosting. 3) The secondary condensation defrosting cycle was more energy efficient when the condensation temperature increased gradually. There was an increase in the energy consumption of two defrosting cycles. Once the evaporation and condensation temperatures were −15 ℃ and 34-38 ℃, respectively, the energy consumption of the secondary condensation defrosting cycle of cold storage was 0.0675-0.0785 kW·h/m³, and the energy saving was 12.3%-16.5%, compared with the hot gas bypass defrosting. Anyway, better energy saving can be expected in the secondary condensation defrosting cycle. The finding can provide a strong reference for the energy saving and efficient operation of low-temperature cold storage during defrosting.