The food retailing sector increasingly uses natural refrigerants

For a long time now, consumers not only look at quality and price when making purchases. In the results of a study conducted throughout Germany by IBH Retail Consultants published in December 2010, 60% of those interviewed indicated that their purchase decisions have also recently started to take account of the extent to which companies implement the meanwhile increasing demands in society at large for a sustainable approach to everyday business practice. Supermarket chains are also increasingly opting for environment-friendly shop concepts with lower emission stores to reduce their carbon footprint.

The refrigeration systems account for a large share of energy consumption in the food retailing sector. Possibilities for taking a sustainable approach include among others the choice of particular refrigerant. "Depending on the local conditions, today it is possible to develop an individual solution with natural refrigerants for every supermarket", confirms Mark Bulmer, member of the Board at eurammon, the European initiative for natural refrigerants. "Natural refrigerants such as ammonia and CO₂ are used for supermarket refrigeration all over the world. In fact, there are two good reasons in their favour: firstly, they have no or only negligible global warming potential. And secondly, supermarket refrigerating systems with natural refrigerants are energy-efficient in operation."

The type of system suitable for a certain supermarket depends among others on the geographic location and the prevailing climatic conditions on site. Outside temperatures warmer than 26°C prevent liquefaction of carbon dioxide because the refrigerant temperature on the high-pressure side is above the critical temperature. Such trans-critical CO₂ solutions therefore tend to be used in moderate climate zones such as Canada, Scandinavia or Central Europe", Bulmer explains. "Ammonia-CO₂ cascade systems on the other hand constitute a suitable possibility for environment-friendly, efficient refrigeration in warmer regions. Ammonia is deemed to be the most energy-efficient refrigerant of them all."

Sustainable freezing and chilling with carbon dioxide

In 2010, SSP Kälteplaner AG developed a completely new refrigerating system for the Migros supermarket in the Tivoli shopping centre in Spreitenbach, Switzerland. The modern refrigerating solution covers all the requirements made of supermarket refrigeration while taking optimum consideration of the general local conditions with regard to capital expenditure and energy demand.

The new system consists of two 150 kilowatt combined units for chilling and a 53 kilowatt booster combined unit for freezing. Altogether eight Bitzer reciprocating compressors are used for chilling with another four Bitzer reciprocating compressors in the booster combined unit. Direct evaporation of the environment-friendly natural refrigerant CO₂ is responsible for refrigeration distribution in chilling and freezing. Both systems operate in the sub-critical range whenever possible. Under high outside temperatures or when waste heat is called for, the combined chilling units operate in the supercritical range with a working pressure of up to 92 bar.

Additional energy savings are achieved by heat recovery. A heat pump uses the waste heat of the system to provide hot process water and heat for the supermarket and for an adjoining restaurant. The remaining residual heat is discharged to the outside via a gas cooler/condenser on the roof.

Changeover to natural refrigerants in South Africa with ammonia and carbon dioxide

At the moment, many supermarket refrigeration systems in South Africa still use refrigerants with a high global warming potential (GWP) and in some cases even a high ozone depletion potential (ODP). The use of natural refrigerants in supermarkets is still relatively unknown in South Africa and has therefore hardly been tested. In view of the constant increase in energy costs of more than 20% p.a. in some cases, a number of South African supermarkets have decided to change over to natural refrigerants.

In 2009, for instance, the GEA Group provided various supermarkets in South Africa with compressors for NH₃-CO₂ cascade systems. Ammonia and a glycol solution are used in the chilling range to keep the dairy and delicatessen cabinets and the refrigerated warehouses at temperatures between 0 and +2°C. The deep-freeze circuit operates on the basis of direct CO₂ evaporation for the frozen food and ice cream displays. In addition, the waste heat from the ammonia system is recovered to save energy in heating process water for the supermarket.

Various Grasso compressors generate the refrigerating capacity of the ammonia circuits in the various supermarkets, reaching levels between 285 and 860 kilowatt. Furthermore, one supermarket uses part of the compressors to feed a cold water storage tank for air-conditioning of the premises. To this end, a glycol loop freezes water balls in a storage tank. Outside peak times, all compressors work with the same suction capacity so that free capacities from the supermarket's refrigerating circuit can be fed to the air-conditioning system.

"Operators no longer have to revert to fluorinated greenhouse gases for supermarket refrigeration", says Mark Bulmer. "Applications with natural refrigerants offer a good alternative. Thanks to intensive research and development in recent years, natural refrigerants permit energy-efficient operation today in many areas. Depending on the service life, the partly higher investment in the systems can be recuperated by lower overheads, thanks to reduced energy costs and less expenditure on refrigerants."

Annex

Ammonia (NH₃)

Carbon dioxide (CO₂)

About eurammon

eurammon is a joint European initiative of companies, institutions and individuals who advocate an increased use of natural refrigerants. As a knowledge pool for the use of natural refrigerants in refrigeration engineering, the initiative sees as its mandate the creation of a platform for information sharing and the promotion of public awareness and acceptance of natural refrigerants. The objective is to promote the use of natural refrigerants in the interest of a healthy environment, and thereby encourage a sustainable approach in refrigeration engineering. eurammon provides comprehensive information about all aspects of natural refrigerants to experts, politicians and the public at large. It serves as a qualified contact for anyone interested in the subject. Users and designers of refrigeration projects can turn to eurammon for specific project experience and extensive information, as well as for advice on all matters of planning, licensing and operating refrigeration plants. The initiative was set up in 1996 and is open to European companies and institutions with a vested interest in natural refrigerants, as well as to individuals e.g. scientists and researchers.

Internet URL: www.eurammon.com

Contacts

To date, most refrigerated display counters and freezer cabinets in supermarkets use fluorinated greenhouse gases as their refrigerants. Leakages lead to refrigerant emissions which contribute to the greenhouse effect. Apart from this, if the refrigerant charge is too low, the plants’ performance drops, and replenishing incurs additional costs. Add to that the fact that the EU F-gas regulation, which came into force in 2007, increased the cost of using FCs and HFCs in refrigeration plants. All this leads to the operators of such facilities having a very strong interest in alternative solutions. At the same time, in light of climate change, energy efficiency and reducing emissions of greenhouse gases are becoming ever more important. The increased use of natural refrigerants for supermarket refrigeration can make a significant contribution to achieving this.

Carbon dioxide cascade system combines low and normal temperature cooling

In Denmark, the use of more than 10 kilograms of fluorinated gases (F-gases) in newly installed refrigeration systems has been prohibited by law since 2007. For this reason the Danish supermarket chain Super Best opted for a carbon dioxide based cascade system at its Copenhagen branch. So refrigeration specialists Knudsen Køling designed a refrigeration system that uses two refrigeration circuits linked by a plate heat exchanger. This allows three different temperature levels to be achieved, two of which are used for cooling and one for heat dissipation.

The first temperature level is needed in frozen food cabinets and the deep freeze chambers in the cellar. At an evaporating temperature of -28°C, the carbon dioxide is used to maintain an ambient temperature of -20°C. The second temperature level is used for refrigerators and cold stores requiring an air temperature of a few degrees above 0°C. This is achieved by supplying the evaporators with carbon dioxide at an evaporating temperature of -10°C. The third and highest level ensures reliable heat dissipation by condensation or gas cooling, depending on the ambient temperature. Overall the cascade system has a refrigeration capacity of 150 kilowatts.

Güntner components are used for the various temperature levels generated by the system and the cold stores are fitted with evaporators. The heat dissipation enables an evaporator to function as a gas cooler at elevated ambient temperatures and hypercritical processing. To guarantee the operational safety of the system even at pressures of around 120 bar, Güntner used 0.7 mm thick stainless steel for the gas cooler and the entire piping network. Thanks to its low-noise design, the gas cooler can even be used in locations with very high noise-control requirements, e.g. residential areas. The injection to the evaporators and the cooling unit is controlled by Danfoss electrical components. There are currently eleven of these combined deep-freezing/normal refrigeration systems installed throughout Europe, as well as 100 deep freezer systems in use in supermarkets.

A pioneering solution using carbon dioxide and hydrocarbons

Another company favouring natural refrigerants is Tesco, the largest supermarket chain in the UK. True to its goal of taking the lead in environmentally sustainable refrigeration solutions in the retail industry, Tesco commissioned Johnson Controls to install a refrigeration system operating on carbon dioxide and the hydrocarbon refrigerant R1270 at its 60,000 square feet store in Shrewsbury. Consequently, the store is the first supermarket in the United Kingdom using solely natural refrigerants for its cooling needs.

To achieve this, Johnson Controls divided the machine rooms into two sections, each comprising a cascade system with R1270 circulating in the refrigerant loop. Carbon dioxide serves as the coolant, which – due to its particularly good heat transfer performance – is used to regulate the display cabinets for both positive and negative temperatures. The hydrocarbon charge, which is located in a separate part of the machine room dedicated to the purpose, is kept to a minimum.

Each of the refrigeration plants is capable of 255 kW of cooling at medium temperature and 45 kW at low temperature. The plants are fitted with desuperheaters, using dry coolers, and a short glycol loop. Depending on the ambient temperature, this circuit cools the compressed gas as close as possible to the condensing temperature of 20°C. The fans on the dry coolers are driven by an inverter drive, managed by a temperature controller. This helps ensure that the plants’ output is continually adapted to the current prevailing conditions, so that the required refrigeration is achieved while keeping energy consumption to a minimum. The operational safety of the entire refrigeration system is guaranteed by a central control and monitoring system that can identify any possible leaks early on.

The refrigeration solution provided by Johnson Controls is based on proven technology which is widely used across the globe, with in excess of 100 supermarkets successfully installed and maintained. The innovative feature of this project is the fact that it uses ground water to cool the refrigerant circuit. This natural heat sink enables the system to work under stable conditions all year round, regardless of high ambient temperatures during the summer months.

Constant cooling even at high ambient temperatures

Drake Foodmarkets took an ambitious step at its Foodland store in Angle Vale in South Australia. Since December 2007 in this Adelaide suburb the supermarket chain has been operating the first supermarket in the southern hemisphere that uses a transcritical carbon dioxide refrigeration system. The project was supported by the Australian government, which paid for the higher design and installation costs, compared to a conventional direct evaporation plant, as part of a program to cut greenhouse gases.

The system, which is filled with about 450 kg of carbon dioxide, uses direct evaporation of the refrigerant. At -10°C it is evaporated at the cooling points for normal refrigeration and at

-35°C at the cooling points for freezing. The gaseous carbon dioxide is compressed by a total of twelve piston compressors – eight single-stage compressors for normal refrigeration and four two-stage compressors for deep freezing. In total, these units have a total capacity of 250 kW.

The greatest challenge for the refrigeration system is the South Australian climate, with temperate winters and hot summers. The high-pressure side had to be designed to cope with ambient temperatures of 40°C and beyond. Such high temperatures prevent condensation at the end of the cooling circuit, since the compressed carbon dioxide exceeds its critical point of 31°C, and is thus unable to change state and condense.

To solve this problem, an evaporative air pre-cooling unit was fitted ahead of the air-cooled condenser. When the ambient temperature rises above 28°C, this unit cools down the air stream which enters the condenser and removes the heat from the compressed carbon dioxide gas. This allows the refrigerant to be cooled to below outside air dry-bulb temperature, raising the system’s efficiency even if the heat rejection process continues at supercritical pressure. In addition, the plant also has a system to control high pressure, which allows for very precisely controlling the subcooling that the liquefied refrigerant undergoes as it exits the condenser.