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CO2 at the forefrontUse of R744 to refrigerate Danish supermarket to satisfaction ofoperator

Supermarket in Denmark

Line of Busi-ness:

Commercial Refrigeration

Application: Supermarket Cooling

Country / City: Denmark / Koge

Fluid: CO2

Product: Gas cooler / Condenser GVH

In the early 1970’s, Mario J. Molina discoveredthat chlorofluorocarbons deplete the ozonelayer, which is essential to our survival here onearth. At first, this discovery was pretty muchdisregarded, but today our awareness level ismuch higher. These so-called CFCs meet withworld-wide disapproval. For a long time, halo-genated hydrocarbons (HHC) were considereda clean alternative. They have little or no effecton ozone depletion, but many experts feel thatthey do have a considerable greenhouse effectand contribute to global warming. This correla-tion let states such as Austria and Denmark togo beyond the steps that were already plannedby the European F-Gas Regulation over thenext few years.

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This background led to the development of oneof the most advanced refrigeration plants in theEuropean Union - in Copenhagen, Denmark.

Legal situation in Denmark and Austria

In Denmark, this came into force at the be-ginning of 2007: for new installations, a maxi-mum of 10 kg of fluorinated hydrocarbons arepermitted in refrigeration systems. For Austria,a quantity limit for HHC was discussed dur-ing 2007, but no directive materialised. Plantswith refrigerating capacities that require larg-er quantities can be filled with natural refriger-ants such as, for example, with the ammonia(R717) which is still unusual for small capaci-ties, propane (R290) which is technically pos-sible, but flammable or the still exotic-appear-ing carbon dioxide (R744). Neither for HHC istthere a limit.

Since March 1, 2001 in Denmark, whether Fgases or natural refrigerants will be used ina plant is an important factor for economicrefrigeration, because since then, the DanishState levies a tax of 13.50 € per CO2 equiva-lent ton that is used in a refrigeration plant.This CO2 equivalent is referred to as the GlobalWarming Potential or GWP and expresses thesubstance’s contribution to the greenhouse ef-fect. The GWP of CO2 is used as the referenceand is given the value of 1 by definition.

Example: A plant is to be filled with 250 kg ofR404A refrigerant. R404A has a GWP value of3,800. This means that one ton of R404A hasthe same greenhouse effect as 3,800 tons ofCO2.

Therefore: 0,25 tons•13,50 €/(ton CO2)•3.800tons CO2=12.825 €

Therefore, 51,30 € in tax must be paid for eachkilogram of R404A.

These are naturally substantial sums that haveto be considered when deciding which refrig-erant should be used. In the given example,the responsible technician from the KnudsenKøling Company clearly decided in favour ofCO2 as the refrigerant.

Knudsen Køling

Knudsen Køling is a medium-sized companylocated in Køge, Denmark. As well as refrigerat-ing plant construction, the company also offersa complete range of refrigeration and freezingunits. The refrigeration units are manufacturedin Køge, where the company is headquartered.The evaporators needed when CO2 is the re-frigerant used are outfitted with electronic ex-pansion valves and integrated in stainless steelrefrigeration units.

Refrigeration unit in the Super Best supermarket

Getting into position in time to meet thechallenges of the next years

Knudsen Køling paved the way for the futurein a timely manner. While some were pres-sured into finishing a planned refrigerationplant within 2006 in order to still be able tofill it with HHC, the team at Knudsen alreadysuccessfully moved on to the alternative, CO2.Many technical questions had to be clarifiedbefore the project could get underway, for ex-ample:

• How can the high pressure of CO2 be

coped with?• What is the best design for the piping

so that the oil return functions proper-ly?

• How should the high pressure be con-trolled?

• What about the high compressiontemperatures?

At Knudsen Køling, the answers to these ques-tions were not left to chance. The plant in Køge,location of the company's headquarters, wasbuilt up for testing and the optimal operating

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point for the plant during transcritical opera-tion was determined. Therefore, it is now pos-sible to operate the plant at all points in time atthe most energetic, economic operating point.

Plant refrigerant: CO2

The system

Graphical representation of the refrigeration circuit

In principle, the system is made up of two re-frigeration circuits that are thermally coupledby a plate heat exchanger. This results in threetemperature levels, of which two are used andthe third, the highest, provides reliable heatdissipation:

• Evaporation at -28 °C. This temper-ature level is needed in deep-freezeequipment and in the deep-freeze

cells in a building’s basement. The airtemperature in this area is -20 °C.

• Condensation and evaporation in aplate heat exchanger at -10 °C. At a-10 °C evaporation temperature, theevaporators in cold rooms and refrig-eration units will bring the air temper-ature into the positive range.

• Condensation or gas refrigeration, de-pending on the ambient temperature.

S-GVH gas cooler condenser with fanswired in pairs to the repair switch

On the high pressure side of this cascade, thecompressor rack, made up of nine compres-sors, provides regulated refrigerating capacityup to a total of 100 kW. Add to that a compres-sor rack with 4 compressors and a total refrig-erating capacity of 50 kW for the deep-freezelevel. Frequently, rack refrigeration plants aredesigned for evaporation temperatures of -15°C and -35 °C for the deep-freeze level. How-ever, designs with higher evaporation temper-atures have many disadvantages for the oper-ator which are quite noticeable during oper-ation. Due to the minimal difference betweenthe ambient temperature and the evaporationtemperature, the air will not be greatly dehu-midified.

For this reason, the coolers do not frost overas much and the defrosting intervals are re-duced. An increase in the evaporation temper-ature of only 1 Kelvin also saves about 4 % ofthe compressor power due to the higher COP.The operating costs of a refrigerating plant ex-ceed the investment costs in a relatively shortperiod of time (1.5 to 4 years, depending on

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the design). For this reason, especially in timesof increasing competition, the economical op-eration of refrigeration plants is an importantfactor in successfully running a supermarket.

The components

Compressors from Bock Company were cho-sen for the compressors in both cascade sys-tem levels. To control the evaporation injectionand the cooling point, electronic componentsfrom Danfoss Company were installed. Pipingwas designed completely in stainless steel toprovide adequate compression strength, be-cause, especially in a CO2 circuit, extremelyhigh pressures can arise on the pressure sideof the refrigeration plant. Güntner expertisewas also used for the cooling chambers andhigh-efficiency GDF model evaporators wereselected for normal cooling and GHF modelswere used for deep-freezing.

Compressor rack

The heat dissipation system: the GüntnerCO2 gas cooler/condenser - a special Günt-ner solution

Güntner CO2 gas cooler/condenser

For heat dissipation, the plant design relied onGüntner’s long standing experience and com-petence. The condensers that function as gascoolers under high ambient temperatures andsupercritical process control are especially de-signed for the stresses and strains that canarise when CO2 is used as the refrigerant. Inaddition to the gas cooler inlet temperature,which corresponds to the temperature at thecondenser end for short pipelines, the chal-lenge is also to ensure operating safety in theplant at operating pressures of 120 bar. Youare not necessarily on the safe side using con-ventional copper pipes.

A Knudsen Køling refrigeration unit

For this reason, when CO2 is used, Güntner re-lies on stainless steel, which is manufacturedwith a wall thickness of 0.7 mm according topressure systems safety regulations. Before itleaves the factory, every CO2 gas cooler is pres-surized to a test pressure of 172 bar to guaran-tee that there is no leakage. The test is record-ed and the results are included in the deliv-ery. Due to the extremely quiet and energy-sav-ing design of the gas cooler and the fans, it isalmost noiseless and therefore well suited tobeing installed in highly-populated areas withstrict noise control regulations.

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Gas cooler condenser in stainless steel

Güntner takes its responsibility towards theenvironment and our society very seriously.Efficient system solutions with long-term op-erating safety ensure the conservation of re-sources, health and quality of life and are an in-tegral part of our vision. In this way, the Copen-hagen supermarket achieved an ideal situa-tion: Operators and consumers can enjoy reli-ably cooled products.

Pressure display

Only an expert glance at the pressure displaylets you know that something is different …