Study On Energy-saving Optimization Of Cold Storage Based On The Freezing Process Of Food

Recently, with the advance of the standard of living, people rely more and more on frozen foods. As a result, the industry of cold storage has gained rapid development, with the increasing power consumption year by year. Under the environment of "energy conservation and emission reduction", there has been growing attention on the energy-saving research of cold storage. Food processing in cold storage can be divided into several categories, including cooling process, refrigeration and freezing process. Different types of food processing have different features and process requirement of their own. However, most research focused on the refrigeration system itself, by not taking full account of the differences between different types of food processing. Meanwhile static optimization methods were employed in most existing research, which were not applicable for food cooling and freezing in batch operation mode. In this paper, research on optimization of energy saving of food freezing was conducted. Based on the modeling of freezing process of single food product and distribution of air parameters in cold storage, dynamic optimization on food freezing was conducted and a multistage freezing system was presented and verified.For the packaged food frozen in cold storage, a model of freezing process for single product was established. First, enthalpy method was used to model the heat transfer inside food. The heat transfer in air layer between food and package material was treated as free convection and radiation. CFD method was employed to simulate the air temperature around food, as well as the surface heat transfer coefficient. Finally, an iteration approach was presented to combine all parts together. Freezing tests on pork cuts packaged in different packageing material were conducted to verify the model presented in this paper. The results show that the model above can predict the freezing process of packaged food compared to traditional treatments on air layer, including heat conduction or ignored.Based on the modeling of single food freezing, flow field and temperature field in freezer were simulated to study the effects of freezing condition on food freezing. First surface heat transfer coefficients of food under different air parameters were calculated according to numerical experiments conducted in wind tunnel, which were then fitted in dimensionless correlations. Then the distribution of flow field and temperature field in freezer with empty load and full load were modeled, as well as the distribution of freezing condition. Finally, sensitivity and nonuniformity analysis were carried out on food freezing under different outlet velocities of air cooler.Evaluation indexes on process requirement of food freezing was presented. Then the dynamic models of food freezing in freezer were established, including air in freezer, packaged food, envelop enclosure and refrigeration system. Power consumption of food freezing was taken as objective function, with the freezing time and freezing speed as constraints of process requirement respectively. Constraints were introduced into objective function in form of penalty function. Iterative dynamic programming was used to solve the problem with evaporation temperature of refrigeration system and air volume of air cooler as controlled variables. Results show that under the constraint of freezing time, there exists an optimal combination of evaporation temperature and air volume of air cooler, which ensures the least power consumption of freezing process. More energy saving can be obtained under constraint of freezing speed than freezing time.Based on the results of theoretical optimization above, multistage freezing system was presented. Evaporation temperature and air volume of air cooler were fixed at single stage and changed in different stages. First, under the constraint of freezing time the optimal operation modes were obtained with different stage numbers, which show that energy saving of multistage freezing under the constraint of freezing time increases with the number of stages. Then effects of intial temperature profile of phase change was studied. Under the constraint of freezing speed, the the optimal operation modes were obtained with different allowed freezing time and multistage freezing system was presented. Finally, the calculation method of coincidence factor of multistage freezing system was proposed.For an existing cold storage with multiple freezers, the practical operation condition was modeled and compared with test results to verify the models of storage and refrigeration system. With monthly changed meteorological parameters and production of frozen food, annual simulation on operation condition was carried out for traditional mode and multistage. The results show that freezing time in multistage operation mode changes with production of frozen food, which will increase the number of freezer used simultaneously. The multistage operation mode can conserve energy by means of raising the COP of refrigeration system and reducing the energy consumption and heat load of air cooler.