Sustainable Retail Refrigeration

Carbon emissions from the retail segment of the food cold chain are relatively high compared to other parts of the food cold chain. Studies have also shown that food temperature is less well controlled at the retail and consumer end of the cold chain. There is therefore considerable potential to optimize performance of refrigerated display cabinets and the refrigeration systems that are used to operate them to reduce carbon emissions and to improve food temperature control.

Sustainable Retail Refrigeration draws together world experts on retail refrigeration. In a single resource, the authors cover the latest technologies and best current knowledge in the field. With increasing concerns about energy use and global warming gasses, retailers are increasingly being called to account for their actions.

Sustainable Retail Refrigeration is a valuable reference to manufacturers, managers and policy makers, incorporating both a design and an operational perspective.

• Language: English
• Publisher: Wiley
• Release date: Oct 27, 2015
• ISBN: 9781118927403

Book preview

1

Overview of Retail Display in Food Retailing

Alan M. Foster and Judith A. Evans

Department of Urban Engineering, London South Bank University, London, UK

1.1 History

In the first half of the 20th century, retailers operated from small premises, serving only their local community. Few products were displayed as they are today, with many selected by an assistant from behind the counter. Most food was not pre-packaged but was instead measured and wrapped to the customers’ requirements by the shopkeeper. Only fresh foods that could be grown locally were available; these had to be purchased and used on a daily basis. Shopping was a daily process, with meat being bought from the butcher and milk delivered every morning.

After the Second World War there was a greater level of consumer choice, especially with regard to food. Retail trends from the US were becoming popular in Europe, particularly the trend for self-service. Customers wanted to see and choose from an ever-growing range of foods. Helped by the advent of the car, increased road networks and domestic refrigeration, larger stores (supermarkets) developed to serve this thirst for choice. The increasing penetration of domestic refrigerators into the home, in particular, extended the periods between shopping trips. This allowed larger, less regular shopping to be carried out, often weekly or fortnightly. For example, in 1970, over 40% of the UK population did not have a fridge, whereas by 1980 almost all households owned a domestic refrigerator (DECADE, 1997). Combined with changes to the family structure, where more women went out to work and mobility of labour was simpler, householders began to shop less regularly. This resulted in a move from shopping in small outlets to ‘one stop shopping’ in larger supermarkets. Less regular shopping was also driven by the demise of daily deliveries after the Second World War, which led to consumers needing to store food, and an increase in domestic refrigeration sales. For example, until 1980 doorstep milk delivery was common. However, by 2000 doorstep milk delivery had almost disappeared as consumers had refrigerators, and milk that was cheaper than the doorstep delivery could be bought in the supermarket.

After the Second World War there was also a huge expansion in home building. Houses built up until the 1960s commonly had larders to keep food chilled. However, after the Parker Morris report of 1961 there was a greater emphasis placed upon living and circulation space, and larders were often not included in homes. Homes were also better heated from around this time, and so there was less opportunity to store food without some form of refrigeration. Research shows that in 1970 internal household temperatures in the UK had a mean of 12°C, whereas by 2004 the mean had risen to 18°C (Fawcett, 2005).

The advent of chlorofluorocarbons (CFCs) introduced in the 1930s allowed the expansion of refrigeration within retail. This was because CFCs were considered much safer than the previous natural refrigerants (ammonia, carbon dioxide, propane and sulphur dioxide) and therefore more suited to a retail environment. R502, R22 and R12 were the common refrigerants used, until it was found that these refrigerants depleted the ozone layer. These refrigerants were replaced by intermediate HCFC (hydroclorfluorocarbons) and then ozone-friendly hydrofluorocarbon (HFC) refrigerants (e.g. R134a and R404A). These refrigerants are now considered harmful to the environment due to their impact on global warming, called their global warming potential (GWP). These refrigerants can warm the globe thousands of times more than the same quantity of carbon dioxide (the main global warming gas). For this reason much work has recently been carried out on making sure these refrigerants do not escape from the refrigeration system. Some countries (such as Denmark) have placed a high tax on these refrigerants. Chapter 7 (Current and Future Carbon-saving Options for Retail Refrigeration) discusses these refrigerants in more detail.

The post-war period was also a period of great technological growth. Consumers began to own televisions, and the power of advertising grew. Frozen food sales grew in this period partially because of the power of media advertising. As supermarkets displayed more frozen food, the sales of freezers in the home also expanded. Frozen food manufacturers were probably key in this development, and were not just responsible for the greater uptake in frozen foods but also the technological infrastructure surrounding them (Cox et al., 1999). This in turn generated a cycle of improved technology and development of further frozen goods.

Birds Eye in particular was instrumental in developing display cabinets. Towards the end of the Second World War they were aware that to expand their business they needed to have higher levels of sales than they achieved in their own stores. In 1957 Birds Eye persuaded two manufacturers to design and market ‘open-top’ refrigerated display cabinets for retail use. Birds Eye agreed only to supply to those retailers who installed such cabinets. They later developed a policy of leasing cabinets to their more important retail customers on the proviso that the equipment was used only for stocking Birds Eye products or other foods that were not direct rivals of Birds Eye. At the same time, Birds Eye heavily marketed their products and gave customers inducements to buy.

With the success of Birds Eye, new frozen food companies entered the market and support infrastructure was developed to deliver and stock these items. As the infrastructure grew, so did new frozen food developments, and that in turn led to expansions and improvements in infrastructure. This in the end led to shorter shelf-life meals (e.g. ready prepared meals) that could only be successfully retailed with a highly evolved manufacturing, storage and delivery infrastructure.

Over the years, the design of cabinets has tended to develop incrementally. The basic method of maintaining food at the correct temperature has changed little over the past 30–40 years. However, incremental changes have been made to components (for example to improve their efficiency), temperature control has been improved, energy consumption has been reduced, refrigerants have been changed, and cabinet features have been modified. Energy reduction has increased in importance over the past 10 years, with manufacturers changing to LED lighting, DC fans, and increased use of doors on cabinets. The application of energy labelling for commercial cabinets, which is likely to occur in 2015, means this trend is likely to continue.

1.2 Retail refrigeration and the food cold chain

1.2.1 Temperature

Very little information is available on temperature control throughout the whole cold chain, and generally data are only available for each section of the cold chain. The exception to this is a survey carried out by Derens et al. (2006) which monitored the temperature of yoghurts and meat products throughout the French cold chain. The results clearly show that temperature control becomes progressively worse as the cold chain progresses from production to the consumer (Fig. 1.1). In manufacture, transport, warehouse and distribution, the food was found to be maintained below 6°C for yoghurts and 4°C for meat for at least 86% of the time. In warehouses only 0.5% of food was outside of these temperature levels. Once the food entered the supermarket the number of samples below 4°C or 6°C was reduced to 70%. This was further reduced to 16% during transport to the home and to 34% in the home.

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Figure 1.1 Temperatures throughout the French cold chain (from Derens et al., 2006).

Reproduced with permission from EDP Sciences

1.2.2 Emissions

Overall the cold chain is believed to be responsible for approximately 2.4% of global greenhouse gas emissions through direct and indirect effects. The food chain is responsible for greenhouse gas emissions through direct (refrigerant emissions) and indirect (energy consumption) effects. In the developed world, emissions post farm gate are thought to be responsible for approximately half the total food chain emissions (Fig. 1.2) (Garnett, 2011). Overall emissions post farm gate, from each section of the cold chain are reasonably evenly distributed, but vary if just refrigeration processes are examined.

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Figure 1.2 Emissions in the food chain (Garnett, 2011).

Reproduced with permission from Elsevier

1.2.2.1 Indirect emissions

There are few data covering refrigeration energy usage or emissions in the whole food cold chain. Data on energy from the UK Market Transformation Programme (MTP, 2006) indicate that within commercial refrigeration, retail display cabinets use most energy (Fig. 1.3). The exception to this is a study on the chicken supply chain that shows that in the case of chicken, catering is a large energy user (MTP, 2005) (Fig. 1.4). Data from Australia (Estrada-Flores and Platt, 2007) indicate that indirect emissions are greatest from retail and domestic refrigeration (Fig. 1.5). It should be noted that both of these datasets exclude significant areas of the food cold chain. In the case of the MTP (2006) data there is no information on industrial refrigeration (food processing and storage or transport) or domestic refrigeration, and in the Australian study, transport and commercial catering refrigeration are excluded.

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Figure 1.3 Energy used in commercial refrigeration in the UK (MTP, 2006). DEFRA, under the terms of Open Government Licence 3.0

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Figure 1.4 Energy used in the UK chicken supply chain (MTP, 2005). DEFRA, under the terms of Open Government Licence 3.0

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Figure 1.5 Energy used in the cold chain in Australia (excludes transport). From Estrada-Flores and Platt (2007),

reproduced with permission from S. Estrada-Flores

Retail food stores and supermarkets are energy-intensive commercial buildings and the majority of their energy use is refrigeration. In the US in 2003, 119 trillion BTU (35 billion kWh) was used in refrigeration in commercial buildings used for selling of food: 57% of the total energy use for these buildings (EIA, 2012). Westphalen et al. (1996) estimated that there was the potential to save 53 trillion BTU (16 billion kWh) of refrigeration energy in supermarkets. For this reason, much effort has been expended over the years by retailers and refrigerated equipment manufacturers to reduce energy use.

Chapter 7 describes current and future carbon-saving options for retail refrigeration.

1.2.2.2 Direct emissions

The relative impact of direct emissions from refrigerants compared with the effect of indirect emission from energy usage varies with country. In countries where there is a high level of renewable energy or nuclear energy, the emissions associated with energy generation are low. Therefore the relative effect of refrigerant leakage is high. This can influence policy and actions to reduce emissions country by country.

Information on refrigerant emissions is mainly available from supermarkets where emissions are considered to be greatest. In 2003, UNEP estimated that leakage across all refrigeration systems was 7–10%, whereas Clodic and Palandre (2004) estimated the figure to be closer to 17%. Data covering more than one sector of the food cold chain have been reported by several authors (Heap, 2001; RAC, 2005; MTP, 2008) (Tables 1.1, 1.2 and 1.3). Bivens and Gage (2004) reported leakage figures for different countries (Table 1.4) and systems (Table 1.5). They also demonstrated that there is a large variability in emissions as shown by data from supermarkets in Sweden and the US (Figs 1.6 and 1.7). Rhiemeier et al. (2009) reported consistent leakage rates for retail multi-compressor refrigeration systems of between 5% and 10% in Germany, and 8% for supermarkets in the US. In the Netherlands, where the STEK programme has been in operation since 1992, average emission rates of only 3% are reported, although the reliability of the data is questioned by Anderson (2005).

Table 1.1 Food chain refrigerant emissions estimated by Heap (2001)

Table 1.2 Food chain refrigerant emissions estimated by RAC (2005)

Source: Reproduced with permission from RAC Magazine, EMAP

Table 1.3 Food chain refrigerant emissions reported by MTP (2008)

Source: DEFRA, under the terms of Open Government Licence 3.0

Table 1.4 Emissions by country (from Bivens and Gage, 2004)

Source: Reproduced with permission from D. Bivens

Table 1.5 EU emissions in 2010, business-as-usual scenario (from March 1998) (from Bivens and Gage, 2004)

Source: Reproduced with permission from D. Bivens

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Figure 1.6 Leakage from a Swedish supermarket (from Bivens and Gage, 2004).

Reproduced with permission from D. Bivens

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Figure 1.7 Leakage from US supermarkets related to charge size (from Bivens and Gage, 2004).

Reproduced with permission from D. Bivens

Natural refrigerants have much lower global warming potential (GWP) and therefore the ability to reduce direct emissions. Chapter 9 describes the Use of Natural Refrigerants in Supermarkets. The chapter describes five classes of natural refrigerants, ammonia, carbon dioxide, hydrocarbons (HCs), water and air. The only two classes that are currently being deployed in supermarkets to replace HFCs are CO2 and HCs.

High GWP refrigerants have also traditionally been used as foam blowing agents. Data on environmental impact are scarce. Alternative blowing agents such as CO2, water and hydrocarbons (pentane, cyclopentane) are available and are commonly used today.

1.3 Types of store

Refrigeration usually accounts for the major share of the energy used in supermarkets. The proportion of energy used for refrigeration in stores varies according to the type of store and its size.

Retail stores are often characterized into types (Tassou et al., 2010):

Hypermarkets – 5000 m² to over 10,000 m² sales area

Superstores – 1400 m² to 5000 m²

Supermarkets (mid-range stores) – 280 m² to 1400 m²

Convenience stores including forecourts of less than 280 m²

Convenience stores are smaller and more local to the community, often in town centres. They may not have much parking and many customers will visit for only enough shopping that they can carry, perhaps even just a carton of milk. They are often open long hours, seven days a week. Supermarkets are often on the edge of town and will be accessed by car. They may be visited weekly. They will be larger than the convenience store and hence sell a wider range of products. Superstores and hypermarkets are larger still and sell many more items than just groceries. They may also contain other shops, restaurants and cafés.

Chapter 8 (Design of Supermarket Refrigeration Systems) describes the different types of refrigeration and HVAC specifically to cater for the food retail section of these different store types.

1.4 Purpose of retail display

The purpose of retail display is to display product to customers such that they will purchase it. Good display will present the product in its most attractive format. For non-food product this is more straightforward as the temperature of the product does not need to be controlled.

With perishable food, its temperature is of prime importance. Fruit and vegetables will generally be displayed below 8°C. Chilled food, such as dairy, cooked meats and ready meals, will be displayed below 5°C, and fresh meat, poultry and fish below 4°C. Frozen food will be below −18°C, but can increase to −15°C during a defrost.

Storing food cold is not difficult or particularly energy intensive, as long as it is kept in a well-insulated box. Duiven and Binard (2002) estimated that cold stores use between 30 and 50 kWh m-2 year-1. The process of displaying cold food in a warm environment creates problems, leading to high energy usage, temperature deviations and increased maintenance. Arteconi et al. (2009) reported approximately 130 MWh/month just for food refrigeration for a 10,000 m² typical supermarket situated in central-northern Italy. This equates to 156 kWh m-2 year-1. This is three times the energy use of a cold store. Similar energy figures for supermarket stores are given for the US (Energy Star, 2003) and Sweden (Olsson et al., 1998).

1.5 Types of cabinet

There are generally 2 ways to display food.

Open (no barrier between the customer and the product) – these have been the common method of displaying chilled food for many years. This method is also used for both vertical and horizontal display of chilled and frozen foods.

Closed cabinets (door or lid between customer and product) – this is the common method to display frozen food in vertical cabinets, but it becoming more common for display of chilled food.

1.5.1 Open-fronted vertical display

The advantage of open-fronted vertical cabinets, and the reason why they are the preference for high-value product in supermarkets, is that there is no barrier between the customer and the product. Customers can browse and handle the products without opening the doors. Customers can be drawn to products that they were not considering buying (impulse buying), which may not be the case if the food is behind a door.

The disadvantage is that open-fronted cabinets use considerably more energy than closed cabinets. They also entrain more moisture, requiring more defrosting. The barrier between the cold product and the food is maintained by an air curtain. This air curtain is not perfect and allows warm air to infiltrate into the cabinet. Approximately 70% of the refrigeration load on these types of cabinets comes from entrainment through the air curtain. Chapter 7 describes the design of an open curtain in detail. With this entrainment comes moisture. When moist air passes over a cold evaporator coil, the moisture in the air will condense and freeze onto the evaporator. This will require regular defrosting. If the cabinet is not defrosted regularly enough, the air curtain velocity drops, reducing the effectiveness of the air curtain, causing more entrainment. This can lead to a cabinet that performs very badly (poor temperature control). The air curtain is also very sensitive to outside influences. This can be from draughts from store doors opening, nearby ventilation outlets, or even customer movements. A disrupted air curtain can increase energy consumption and temperature deviations.

Chapter 4 (Airflow Optimization in Retail Cabinets and the Use of CFD Modelling to Design Cabinets) shows how important optimal airflow is to these types of cabinets. It is time-consuming and costly to optimize a cabinet’s airflow by trial and error in a test room. Computational fluid dynamics (CFD) offers a more efficient way of optimizing the airflow, although this should be carried out in parallel with experimental verification. Chapter 4 demonstrates the use of CFD along with experimental flow visualization and measurement techniques to study the airflow pattern of these cabinets. Flow visualization techniques, mainly particle image velocimetry (PIV) and laser Doppler velocimetry (LDV), are used to achieve a better understanding of the flow pattern and characteristics, as well as CFD code validation.

1.5.2 Closed display

Due to concerns about energy consumption, chilled closed vertical cabinets are becoming more popular. Fricke and Becker (2010) compared two stores where they received either a new set of open-fronted or closed cabinets. They found that the energy consumption of the open-fronted cases was 30% more than the closed cabinets, and there was no change in sales. It is important that the doors are well maintained and that their seals are effective. Heaters are often required to prevent condensation on the glass surface, but can be minimized by anti-sweat heater controls.

Chapter 7 describes these cabinets in more detail. It explains that closed cabinets have a lower refrigeration duty (around 50%) due to less ambient air entrainment and reduction of radiation.

1.5.3 Food display

Chilled food can be displayed either wrapped or unwrapped. Produce tends to be unwrapped, whereas processed food and meat tends to be wrapped. An exception to this is meats, fish and cheese in the delicatessen area where the food is generally displayed unwrapped.

Wrapped food is much easier to display from a food safety and quality point of view because the wrapping stops the food from drying, keeps it clean, and if the food is packaged in modified atmosphere packaging (MAP) the shelf life can also be increased. Unwrapped food is often considered better from a marketing point of view, as it is more appealing to the customer.

Chapter 5 (Display of Unwrapped Foods) discusses cabinets for unwrapped product in detail.

1.5.4 Refrigeration systems

Display cabinets can either be locally (integral) or centrally refrigerated (remote). Large supermarkets have predominantly used central refrigeration systems as this can be more efficient (figures on the increased efficiency are scarce but are probably in the region of 20%) than providing a separate refrigeration system for each cabinet. Control and monitoring systems can also be centralized. The main disadvantage of a centralized system is the long pipe runs carrying refrigerant. Therefore leakage rates for centralized systems have traditionally been high. Typical leakage rates of 15–20% of the charge per year (A.D. Little Inc., 2002) were not uncommon a few years ago. However, as the problem of leakage has received greater exposure, supermarkets have made improvements and leakage rates have fallen. As the refrigerants have generally high GWP, this has a large impact on the environment, as well as the cost of the refrigerant. Velders et al. (2009) estimate global HFC emissions in 2050 equivalent to 9–19% (CO2-eq. basis) of projected global CO2 emissions. Centralized systems also allow the waste heat to be dealt with, rather than allowing it to heat the store. Chapter 11, Maintenance and Long-term Operation of Supermarkets and Minimizing Refrigerant Leakage, has information on leakage rates and methods to reduce them.

Integral systems tend to be used in small stores, where there are not many cabinets. They are also common as additional cabinets in large stores, for example in the restaurant area or end of aisles, where impulse buys are located. This is because they can be placed anywhere as long as there is a source of electrical power, as opposed to centralized cabinets which require refrigerant to be piped to the cabinet.

The safety implications of a centralized plant running a hydrocarbon (HC) refrigerant are too great. HC refrigerants are classed as A3 (EN 378-1:2008) which means lower toxicity (A) and higher flammability (3). EN 378-1:2008 states the maximum charge allowable for different space designations. For a supermarket with a direct expansion refrigeration system using A3 refrigerant, maximum charge is restricted to 1.5 kg (this can be lower for small spaces), whether a remote or integral system.

With integral systems, any leak is likely to be restricted to one cabinet with a limited refrigerant charge, and therefore safety is greatly increased. Large integral cabinets may use a split system such that each system contains less than the maximum allowable charge (1.5 kg). HC refrigerants have benefits with regards to increased efficiency (COP) and low GWP in case of leakage. They are also generally factory-assembled and tested, so leaks are identified in the factory where repairs can be made. Spatz and Yana Motta (2004) showed comparable or slightly better efficiency of propane (R290) (<5%) than a medium temperature R22 system. However, the biggest benefit is the greatly reduced GWP of R290, which is 3, compared with GWP for R404A, which is 3700 (100 year) (UNEP, 2010).

Secondary systems allow a remote refrigeration system, but instead of long pipes runs of high GWP refrigerants with their potential for leakage, the cooling from the central refrigeration system is transferred to the cabinets via a more benign heat transfer fluid (e.g. brine or most recently CO2).

1.6 Cabinet performance

It is important to know how refrigerated equipment performs. For refrigerated cabinets, the purchaser should be interested in the temperature performance in a worst-case scenario (e.g. summer ambient) and the energy consumption of the cabinet. This information can be used to prove that temperature legislation covering the product is being enforced. There are different international standards, energy performance thresholds and legislation applicable in different countries. Chapter 3, Retail Display Testing Standards and Legislation, discusses these standards and legislation in detail.

Performance of cabinets varies considerably. Even similar cabinets may perform differently due to often quite subtle differences in construction. Work carried out by Evans and Swain (2010) demonstrated that the positions of minimum and maximum temperature can vary considerably. In the study, 319 cabinets were tested according to the EN441 or EN23953 test standards. Positions of maximum and minimum temperature within different cabinet types varied, but generally maximum temperatures were in open or exposed (to ambient) areas of the cabinet and minimum temperatures in the least exposed areas. Temperature range (minimum to maximum) in the cabinets examined was significantly greater in frozen than chilled cabinets. The range in temperature in freezers varied from a mean of 15.2 K in well freezers to 19.5 K in chest freezers. Reducing this range would have significant effects on reducing energy consumption. Chilled cabinets with glass doors had the lowest mean temperature range (5.1°C).

Evans and Swain (2010) also demonstrated that energy use, average temperature and temperature range varied between and within cabinet types. Table 1.6 shows energy consumed as total energy consumption (TEC) divided by the total display area of the cabinet (TDA). TDA is a standard methodology used to present the area of a cabinet that is visible to a consumer. Temperature control was also found to vary between cabinet types, with some cabinet formats having overall lower temperatures and less variation between the minimum and maximum temperatures in the cabinet.

Table 1.6 Energy and temperature performance of cabinets under test conditions (EN23953 or EN441)

Source: Data from Evans and Swain (2010)

Note: Values with a superscript with the same letter have no significant difference between them (P < 0.05).

1.7 Store ventilation and air conditioning

Retail display cabinets have a large effect on the ventilation and air conditioning of retail stores. Cabinets with centralized refrigeration systems take a large amount of heat and moisture out of the store. ASHRAE (1995) stated that in a typical store the heat removed by the refrigerated display equipment was 56 kW, 19% of this being latent heat (this value varies with the number of refrigerators). This means that air conditioning is often not required unless ambient temperatures are very high. However, the cabinet refrigeration system is a far less efficient way to air condition than the heating ventilation and air conditioning (HVAC) system, due to the lower coefficient of performance (COP) of the cabinet refrigeration (Fricke and Sharma, 2011).

The aisles where the display cabinets are located can get very cold and uncomfortable, and customers experience the ‘cold feet effect’ (Foster and Quarini, 2001). Getting heat to these aisles without affecting the cabinets can be quite complex. Stores with integral cabinets have the opposite problem as the cabinets add heat into the store. Chapter 10 describes the integration of heating, cooling (including air conditioning and humidity control, heat recovery) and energy generation in supermarkets. The chapter shows how energy savings can be achieved not only with development of display cabinets and refrigeration systems, but also with careful integration with the HVAC plant, heat recovery, combined heat and power generation and tri-generation.

The refrigeration system rejects heat from the condensers continuously throughout the year. This heat can be recovered and used for applications like space heating. Combined heat and power (CHP) systems can be considered due to the ability to provide electrical energy and ambient heating by using the waste heat from the electrical generation. Renewable energy is becoming more prevalent, with many supermarkets installing photovoltaic cells and wind turbines. Methods to fully integrate heating, cooling and renewable energy efficiently is an ongoing area of research and development.

1.8 Design and optimization

Computer modelling is an effective way to investigate new ideas and concepts without the expense of trial and error investigations. Computational fluid dynamics (CFD) modelling has been used to a wide extent, mainly to model the airflow in display cabinets as described in Chapter 4, but also for the supermarket ventilation system (Foster and Quarini, 2001).

1.9 Future trends

It seems likely that supermarkets will play a growing role in supplying food to the people of the world in the foreseeable future. The developing world is demanding the same level of choice from their retailers that the developed world has enjoyed in the last half century. Whereas supermarkets have probably reached almost their full market penetration in the developed world, there is rapid growth in the developing world. It is argued that the spread is accelerating as it moves around the world. Latin America achieved in a decade what took five decades in the US, and store growth in China was three times as fast in 2003 as in Brazil and Argentina a decade earlier (Reardon et al., 2003).

This accelerating expansion of supermarkets is not only driven by rising incomes, urbanisation, motorisation and increasing female employment, the process is taking place at increasing speed because of the active promotion of supermarket formats in the context of international retail expansion

(Humphrey, 2007)

It is clear that supermarkets will need to supply food in a more green and sustainable way than is currently practised. For example, in some countries there is already a strong move towards not using energy-inefficient open-fronted cabinets to display food. Perhaps the next generation of shoppers who have grown up buying everything they need from the internet will not want to go to a supermarket to purchase their food. Many high street shops have found it very difficult to compete with internet shopping. Will supermarkets face the same challenges in the future? Will internet shopping become the major player in buying food? Another possible outcome is that the trend in small independent markets where food is sourced locally will become more common. Supermarkets and their refrigeration systems will need to adapt to whatever future trend becomes reality.

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2

Operation, Design and Performance of Retail Display Cabinets

Onrawee Laguerre

Irstea UR Génie des procédés frigorifiques (Refrigeration Process Engineering Research Unit), Antony, France

2.1 Introduction

Several field studies have shown that the display cabinet is a critical link in the cold chain. A survey carried out by Cemagref and ANIA (2004) in France on three chilled foods (yoghurt, ready-to-eat meals and meat products) showed that 8% of products presented in refrigerated display cabinets were subjected to temperature abuse (more than 2°C higher than the recommended preservation temperature). This study also shows that the mean product temperature in display cabinets is 3.44°C (standard deviation = 1.77°C) and the mean residence time is 3.82 days. Willocx et al. (1994) carried out a survey on processed vegetables in Belgian retail display cabinets. This study showed that temperature differences of more than 5°C were measured on the shelves. These authors observed that the temperature in one position increased towards the end of the day by 4°C and towards the end of the week by almost 7°C. Evans et al. (2007) observed that the majority of high-temperature packs (97%) were located at the front and the largest numbers (60%) of them were at the front base.

Good design and control of operating conditions in supermarkets should take place in order to improve the product temperature inside the display cabinets. The objective of this chapter is to give information on different types of display cabinet, their operation, the influence of operating conditions, and the refrigerant leakage, which has financial and environmental impacts.

2.2 Different types of display cabinet

Display cabinets can be distinguished by their shape: vertical and horizontal (Fig. 2.1). These two types can be used for chilled or frozen foods.

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Figure 2.1 (a) Vertical and (b) horizontal display cabinets

Two refrigeration systems are used:

Integral (or plug-in or self-contained) cabinets for which the refrigeration system is included in the product housing. They are mainly used in stores and in supermarkets for occasional extra sales. The advantage is a relatively small refrigerant charge and very leak-tight circuits. The main disadvantage is that the heat rejection from the compressors is directly into the ambient surroundings.

Centralized refrigeration units. The refrigeration units are located separately from the sales area of the supermarket. Thus, the heat rejection and the noise from the compressors do not take place in the sales area. This system forms the majority in use.

The vertical open multi-deck is the most used because it allows the customer almost unrestricted access to the product. It represents roughly 60% of the refrigerated cases in grocery stores and supermarkets. To minimize overall losses and maintain appropriate product temperatures, the design of the airflow over the shelves and at the front is important. The vertical multi-deck with glass door/s is often used to keep frozen food. A door on the cabinet significantly reduces the heat gain, thus the efficiency is improved. However, this efficiency depends also on the frequency of door opening by customers. There are several configurations of combined cabinet: open top/open bottom, open top/glass lid bottom, glass door top/open bottom, glass door top/glass lid bottom. Roll-in cabinets are often used for products with a high turnover rate, such as milk and butter, since it allows easy loading.

The horizontal display cabinets can be open, with glass lid and serve-over counter. In principle, the higher density of cold air promotes stagnation inside the display cabinet. As a consequence, they do not suffer significant cool air loss and warm air ingress. Thus, the overall efficiency can be fairly high, even when open. In practice, the air current in supermarket (e.g. air-conditioning, customer movement) can affect this stagnation. Water condensation on the walls and on products is often observed in horizontal cabinets.

2.3 Display cabinet operation

Like other refrigerating equipment, the cooling system of display cabinets is composed of an evaporator (cold wall), compressor, condenser and expansion valve. The operation of this system is presented in several books (Stoecker, 1998; ASHRAE, 2002; Meunier et al., 2005).

The open vertical display cabinet is presented in detail (Fig. 2.2) because it is highly used. Air flows downward from the discharge air grille (front top) to the return air grille (front bottom). This airflow, termed the cold air curtain, provides not only cooling capacity but also insulation from ambient air. Air also flows horizontally from the rear to the front through a perforated plate. The air flows through the evaporator where it is cooled and then is circulated upward. This air circulation is provided by fans, allowing uniformity of air temperature and effective food protection. The air velocity can be adjusted due to the product characteristics to reduce the product weight loss due to water evaporation. The air velocity in the curtain may vary from 0.1 to about 1.0 m/s (Laguerre et al., 2012a). The air temperature difference between the inside and outside can attain 30°C (for example, 2°C inside, 32°C outside), particularly in grocery stores without air conditioning during the summer, which leads to high energy consumption of display cabinets.

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