Vertical Display Cabinets in Supermarkets - Energy Efficiency and the Influence of Air Flows

Doktorsavhandling, 2002

This thesis concerns investigations of the performance of vertical refrigerated display cabinets. Such cabinets are in widespread use and account for a considerable proportion of the energy used in shops. The open front is important for the shop-owner, who wants the chilled items in the cabinet to be well exposed and easily accessible. The purpose of the work has been to analyse the energy flows in display cabinets and subsequently analysing the possibilities to reducing the cabinets' energy requirements while retaining or improving their performance. The work has involved controlled investigations in a climate chamber in order to determine how the ambient climatic conditions and other factors affect energy use. The results show that the energy requirement of a display cabinet is linearly proportional to the difference in specific enthalpy between that of the ambient air and that of the cold air in the cabinet. This means that the cooling power requirement for any arbitrary climate can be determined from measurements of the energy requirements of two different ambient climates. An analysis of energy flows shows that infiltration provides the greatest inward heat leakage in a display cabinet. Radiation and illumination also have a relatively substantial effect in comparison with other possible heat inputs, as they act on the most poorly cooled products in the cabinet. The overall energy requirement of a display cabinet is determined by the ambient climate and the temperature of the warmest item in the cabinet, which therefore in turn determines the mean temperature level in the entire cabinet. Results from experimental measurements and CFD calculations show that the performance of an open vertical display cabinet with cold air blown in through the back is affected by how it is loaded and by how the perforations in the back-plate are arranged. The conclusion is that cold air input through the back-plate is not the best method of cooling the foodstuff packages, as the cold air is supplied where the need for cooling is least. The air curtain is required to provide good enclosure, and also to cool the foodstuffs at the front of the shelves, all with the least input of energy. The results from the modelling show that it is not possible to achieve the same efficiency and function with air curtains having the same height/width ratio but with different heights. If the air curtain is to cool items at the front of shelves, while at the same time being stable, a shorter air curtain is more efficient than a longer air curtain for the same height/width ratio. The overall conclusion is therefore that a single long air curtain should be replaced by a number of shorter air curtains, and that a thick air curtain is more efficient than a thin air curtain.