Saving energy in supermarkets, doors the big win

The recent increasing cost of energy has had a major impact on supermarkets operating costs[1]. The cost of energy to run supermarket refrigeration systems is a major part of their operating cost. Refrigeration systems consume the greatest proportion of the electrical energy in a supermarket with 35–50% of all electrical energy used to power the refrigerated cabinets.

Supermarkets have a number of options to reduce the energy they consume. Previous studies such as the International Institute of Refrigeration (IIR) ‘Technological options for retail refrigeration[2] reviewed the potential energy and carbon savings from 68 different technologies that could be applied to the refrigeration system in a supermarket. For a case study site, the technologies that had the shortest payback time and shortest time to be able to be retrofitted were all technologies that reduced air infiltration in open fronted display cabinets. These included air deflectors, strip curtains and doors. Doors had by far the greatest impact in reducing energy consumption and carbon emissions.

The impact of applying doors depends largely on the integrity of the door and the frequency of door openings. The type of door can also have an impact, with theoretical modelling studies showing that sliding doors can reduce energy consumption more than hinged doors[3]. The ultimate energy savings very much depend on how well the door is fitted and sealed. Often retrofit doors have gaps at the edges which allow air infiltration. Orlandi et al (2013) found in a theoretical study that a 7 mm wide gap between doors on a 3-door chilled cabinet in climate class 3 (25°C, 60% RH) was responsible for 23% of the heat extraction rate (HER) overnight (doors closed and lights off) and 15% of the HER during the day (including the standard EN23953 door opening sequence).

Adding doors to an open fronted cabinet also has other benefits. The range in temperature within the cabinet is less and so there is potential to be able to raise evaporating temperatures. For example, in a like for like test with and without doors a study found that the average evaporating temperature could be raised by 4.4°C while the range in test pack temperatures reduced from 8.4°C without doors to 3.8°C with doors[4].

Adding doors to a cabinet almost inevitably saves energy. However, any air gap around the door, the insulation of the door itself and the number of door openings all affect the effectiveness of the door. Savings achieved are understandably varied with savings of 25-80% reported in the literature[5]. As well as technical differences between doors, there are often issues with consumer usage. Consumers leave doors open or door return mechanisms fail or are poorly installed/maintained. This can result in doors remaining open and much of the benefit of the doors being lost.

Door opening impacts are part of the standard testing procedure for display cabinets (EN23953). It is therefore possible to quantify the energy savings that can be achieved from different door configuration and options. Also, by using a programmable door opening mechanism and varied ambient conditions it is possible to simulate real life usage and to identify the real savings in a supermarket. With the increasing cost of energy, it is now much more cost effective to apply doors to open fronted cabinets. The additional savings that can be achieved though applying better fitted and insulated doors can be quantified and the cost-benefit for varied options validated. This is exactly the type of testing we can carry out at RD&T to provide independent evidence on the best technological options available. If you would like to discuss how to reduce energy demand by applying doors, please contact Judith Evans (j.a.evans@rdandt.co.uk) or Alan Foster (alan.foster@rdandt.co.uk) at RD&T.

Finally, as it is nearly Christmas, we would like to wish all our customers and contacts a very merry and energy efficient Christmas! With all best wishes for 2023 from all at RD&T.

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