The novel use of phase change materials in refrigerated display cabinets for energy conservation

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonEach day the world's attention increases on how to minimize the energy consumption in order to reduce energy cost and carbon emissions as well as conserve the energy resources we have. Open-type vertical refrigerated display cabinets take the largest refrigeration load in a supermarket refrigeration system. One technique that has been used to improve the efficiency of such refrigeration equipment is to employ thermal energy storage inside. This approach will lead to improve the overall efficiency and also reduce the required cooling of the equipment due to the reduction of food temperature variation. This research project details the effect of phase change materials (PCMs) integration on the thermal performance of an open type multi-deck display cabinet in terms of energy savings, food product temperature improvements, cabinet air temperature and comparisons with conventional units. This work is divided into two parts of experiment and theoretical analysis. The experimental part included a series of tests that were carried out to determine that effect of incorporation of PCM through two types of display cabinets depending on their availability with nearly same design. Two integration procedures of PCMHEs with different PCM types were employed for each cabinet depending on its design. The test results showed that the energy saving of the cabinets with PCM significantly improved at climate class 3. In terms of product and cabinet air temperatures, the results also showed considerable benefit from reducing the maximum value of air temperature. Moreover, the defrost period was found to be the most affected parameter for the Norpe cabinet with PCM. It was approximately 5 min longer than the basic cabinet, which represented 70% of the original defrost time resulting in energy savings. It is worth noting that savings are a function of the ambient temperature, relative humidity, operational settings of the cabinet and PCM freezing point. The theoretical study involves a 2D CFD model established for the Norpe display cabinet. The model is used to investigate the effect of adding PCM-HE on the energy consumption of display cabinet, thermal performance and cabinet air temperatures in similar method and same test conditions. Validation was carried out by comparing experimental results. Calculations are carried out for this cabinet first without PCM integration, then with a container filled with pure water as a PCM. It has been shown that an enhancement of thermal system performance and reduction in the cabinet air and product temperatures when the cabinet is off and increasing defrost, on and off periods. It is also found that with PCM, the energy saving potential of the cabinet is significantly improved. In addition, in the CDF results, the defrost and compressor off intervals are the most affected factors for the modified cabinet. They are increased by approximately 98% and 50%, respectively, compared to those with the basic cabinet. Furthermore, a significant decrease (27%) in the number of starts/stops of the compressor is observed for the modified cabinet. Also optimum PCM container width was found to be 16 mm such that the required air flow pattern is still achieved. The effect of different conditions; ambient air and fan pressure jump, on the PCM effectiveness is also considered in this study.Department of Environment, Food and Rural Affairs (Defra: AFM280