Background article

Supermarkets advocate sustainable refrigeration

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."