Analysis of the performance and cooling uniformity of mobile cold storage

Fruits and vegetables still have vital signs after harvest. Their field heat and respiration of biological entities can greatly dominate the quality of fresh products for a better shelf life. This study aims to keep the fruit and vegetable fresh in the first kilometer after picking during cold chain transportation. An integrated equipment system of mobile cold storage and refrigeration was developed for the precooling and storage of fruits and vegetables in fields, in order to eliminate the field heat and respiratory heat after picking. The performance of the equipment was analyzed from three aspects: the thermal resistance of cold storage panels, the coefficient of performance of the integrated refrigeration, and the cooling uniformity inside the cold storage. Among them, the cooling uniformity was also evaluated using temperature heterogeneity and variation coefficients. Two experiments were carried out to verify the performance and cooling uniformity of mobile cold storage. The heat balance method was used to measure the thermal resistance. Notably, the integrated refrigeration machine was also shut down during the experiment. The temperature difference between the inside and outside of the cold storage was maintained by the external heater and axial fan. Temperature measurement points were arranged 100 mm away from the geometric center of the inside and outside of the cold storage. At the same time, there were eight measuring points 100 mm from three sides of the cold storage top angle inside the cold storage, one of which was arranged in the geometric center of the cold storage. As such, the temperature field distribution was measured to calculate the performance and cooling uniformity of the mobile cold storage, according to the flow and heat transfer characteristics. The results showed that the high thermal resistance of the cold storage panels is 3.98 m²·℃/W and the coefficient of performance of the integrated refrigeration machine is 2.07, which were conducive to the energy-saving operation of the equipment. There was a small overall fluctuation (about 0.1%) of the temperature heterogeneity coefficient at each measurement point in the process of equipment operation. The largest overall fluctuation was lower than 1%, only when the integrated refrigeration machine was shut down and defrosted. The temperature heterogeneity coefficient was lower at the upper left corner of the cold storage door (0.08 %), and higher at the upper right corner of the refrigeration unit (0.48 %). The temperature coefficient of variation was 0.034% in the integrated refrigeration machine. Both the temperature heterogeneity and variation coefficients were lower than 0.5 %. Therefore, cold storage with high cooling uniformity can be beneficial to reduce the decay rate of fruits and vegetables in the process of pre-cooling.