Improving thermal performance of freezers through the application of phase change materials

Abstract

Many industries require their products to be stored and transported at low temperatures. Chilled and frozen foods require storage temperatures ranging from 14 °C to below -18 °C. Storage and transportation facilities such as cold stores, refrigerated trucks, retail cabinets, and domestic refrigerators/freezers are some of the units used to maintain products at the desired temperatures. Refrigeration systems are used to remove heat loads and control temperature. The major heat load comes from the environment outside of the storage space through insulated walls. Door openings bring warm, moist air into the cold storage space, raising the temperature, and also causing frost to form on the evaporator of the refrigeration system. Defrosting is then required to melt this ice, which is another unavoidable heat load. Phase change materials (PCMs) are capable of absorbing a large amount of heat during melting over a small temperature range. This property can be used to minimize temperature fluctuations. The aims of this project include developing a PCM with attractive properties for use in cold stores, and to test the performance in a domestic freezer. A CFD model of the PCM/freezer system was then created. Eutectic PCMs suffer from large supercooling problems. This means that the PCM must be cooled below its freezing point before freezing occurs. This was the main property of the PCM that needed to be improved, and the results have shown that a suitable PCM has been successfully developed. This PCM is based on the eutectic ammonium chloridewater binary system. By modifying the salt content from the eutectic composition (19.5 wt% salt) to an extra water composition (15 wt% salt), the effective supercooling was reduced from 7.5 °C to 3.8 °C as it allows initial freezing at slightly elevated temperature prior to the eutectic freezing. Further additions of either 0.25 wt% AlF3 or CaC12 reduced Many industries require their products to be stored and transported at low temperatures. Chilled and frozen foods require storage temperatures ranging from 14 °C to below -18 °C. Storage and transportation facilities such as cold stores, refrigerated trucks, retail cabinets, and domestic refrigerators/freezers are some of the units used to maintain products at the desired temperatures. Refrigeration systems are used to remove heat loads and control temperature. The major heat load comes from the environment outside of the storage space through insulated walls. Door openings bring warm, moist air into the cold storage space, raising the temperature, and also causing frost to form on the evaporator of the refrigeration system. Defrosting is then required to melt this ice, which is another unavoidable heat load. Phase change materials (PCMs) are capable of absorbing a large amount of heat during melting over a small temperature range. This property can be used to minimize temperature fluctuations. The aims of this project include developing a PCM with attractive properties for use in cold stores, and to test the performance in a domestic freezer. A CFD model of the PCM/freezer system was then created. Eutectic PCMs suffer from large supercooling problems. This means that the PCM must be cooled below its freezing point before freezing occurs. This was the main property of the PCM that needed to be improved, and the results have shown that a suitable PCM has been successfully developed. This PCM is based on the eutectic ammonium chloridewater binary system. By modifying the salt content from the eutectic composition (19.5 wt% salt) to an extra water composition (15 wt% salt), the effective supercooling was reduced from 7.5 °C to 3.8 °C as it allows initial freezing at slightly elevated temperature prior to the eutectic freezing. Further additions of either 0.25 wt% AlF3 or CaC12 reduced.