Energy consumption in transcritical CO₂ refrigeration

New factual data on energy consumption in Danish supermarkets shows that transcritical CO₂ refrigeration systems can be as energy efficient as traditional HFC refrigeration systems. 

Background

In Denmark, there has been a growing interest in CO₂ as a refrigerant during the last 10 years. This is more than likely due to the fact that in 2003 the government introduced taxes on HFC refrigerants. In addition hereto it was decided in 2007 to prohibit the production of refrigeration systems with a charge of more than 10 kg of the HFC greenhouse gases. 

However, there has been some disagreement about whether transcritical CO₂ systems could be a real alternative when energy consumptions are taken in to account. By comparing them in the traditional way with a simple COP, the CO₂ system will look less attractive because it has higher energy consumption when it runs transcritical. But it is worth noting, that there will be relatively few periods during which the refrigeration systems will run transcritical in the Nordic climate. A more valid comparison would therefore be to consider and compare the annual operating hours of the two types of systems. 

Some of the arguments against transcritical CO₂ systems have been that the operation of these systems would be too costly in the warm months of the year and that the transcritical CO₂ systems would only be profitable in the north (Scandinavia and countries with similar climates). However, now, there are actual and specific consumption figures for a number of supermarkets (Netto and Fakta) in Denmark. These figures show that transcritical CO₂ systems can match the energy consumption of conventional refrigeration systems - even during the warm months in Denmark, where energy consumption of a CO₂ system was traditionally thought to be higher than that of conventional systems. On this basis, there is a foundation for annual energy savings with a CO₂ system, as energy consumption during the cold months of the year will be lower.

The new figures support the belief that, for a large part of the climates in northern European countries, it will be profitable to use transcritical CO₂ refrigeration systems. See the illustration to the right for indications of savings compared to systems with R404a. The rule is, the cooler the climate, the greater the possible savings with a transcritical CO₂ system can become. (Yearly average temperature map prepared by http://globalis.gvu.unu.edu) 

The previously mentioned results show that the theoretical calculations on yearly savings can be matched in reality.

Director of Super Køl, Tom Gøtsche, who has been responsible for the refrigeration systems at the Danish supermarket chain Fakta for many years, says that with their second generation of transcritical CO₂ plants they have been able to design a system that uses no more energy than the plants they have running with HFCs. 

These consumption figures were even measured over the summer months, and achieved without the systems being 100% optimally designed, since there is not yet an equally wide range of components for CO₂, as is the case for traditional refrigerants. Therefore Tom Gøtsche also hopes that the transcritical CO₂ systems will be optimized even more when, over time, a wider range of parts and components for CO₂ will become available. Here he refers, among other things; to the big difference in load during the course of the day and that there are still opportunities for optimization at part. load, when the right components become available on the market. 

The very fact, that there is a need for additional components for CO₂ systems, is something which Danfoss is working actively to improve. CO₂ Project Manager at Danfoss Hans Ole Matthiesen says that although Danfoss has the widest range of products for CO₂, it is a part of the Danfoss strategy that this program will be expanded.

Today transcritical CO₂ systems have a higher acquisition cost, but an expected growth in the number of these types of systems and a wider variety of components will naturally result in lower acquisition costs in the future. In Denmark and Norway, HFC taxes have already been introduced, and officials in Sweden are also working on introducing taxes on HFC. This will make CO₂ even more attractive as a refrigerant. As this article has pointed out, there will be potential savings in energy consumption and on the costs of refrigerant (CO₂ is cheaper than HFC). Thus, there are many good arguments for transcritical CO₂ refrigeration systems. It will be interesting to follow the developments in energy consumption of transcritical CO₂systems over the coming years and particularly exciting to see which possibilities there are in the CO₂ systems when they can be fully optimized. 

There is no doubt that we will see many more refrigeration systems with CO₂ in the future. Not only as a result of new laws, but also because they are economically viable. Furthermore, several companies want to be able to promote themselves with a green profile, and cooling with CO₂ is a natural addition to their company image. As an example, the Danish supermarket chain Netto has this year opened two new distribution centres, respectively, 1.3 and 1.5 MW, which are cooled with CO₂. Other international supermarket chains are also working on introducing CO₂ systems, as a part of their green profile. 

According to Per Holm Johansen, International Technical Manager at Netto, the new distribution centres are a clear example that CO₂-based refrigeration systems are fully competitive with other types of refrigeration systems. "There was no experience with CO₂-based systems of this size, which we could use as a point of reference, and we were therefore very anxious when we sent the project out to tender. 

It quickly became clear that the project could be realized without compromising the efficiency or economy of the system", says Per Holm Johansen. 

Automatic control of cooling, heating, ventilation and lighting has for many years been a natural part of Netto's construction concept. It is therefore critical to Per Holm Johansen that the new technology does not use more energy and that at the same time it is: 
• Reliable 
• A sound investment 

Nordic Sales Manager Kristian Breitenbauch from Danfoss Food Retail explains that until now Danfoss has been involved in the construction of more than 1,000 transcritical CO₂refrigeration systems worldwide (nearly 400 systems in the Northern part of Europe), and is an absolute global market leader in components for CO₂ refrigeration systems. Danfoss has among other things delivered electric valves, pressure transmitters and a complete ADAP-KOOL^® electronic control and monitoring system. 

Contact Danfoss to find out how you can optimize your energy consumption with a CO₂refrigeration system.