Growing interest in carbon dioxide as ideal refrigerant

Carbon dioxide is being viewed as an attractive alternative refrigerant for many applications in the face of new restrictions on the potent greenhouse gases hydrofluorocarbons (HFCs). Companies such as Nestlé and Iceland are already using CO2 refrigeration systems in the UK.

The most widely used refrigerant remains HCFC-22. But, because of its ozone-depleting potential, its use in most new systems will be banned in the EC from the beginning of next year. Existing systems, some with lifetimes of over 30 years, can be maintained with virgin HCFC-22 until 2010 and with recycled material until 2015 - dates which may yet be brought forward.

Hydrocarbon refrigerants are suitable for markets such as domestic refrigeration, water cooling and building air-conditioning - but applications beyond these are limited by perceived flammability dangers. Another option is ammonia, but controls on its use may become more stringent, some observers believe, because of toxicity concerns.

Meanwhile, the new generation of HFC refrigerants, designed by the chemical industry to replace HCFC-22, faces restrictions under the Government's climate change strategy (see pp 24-26 ). Earlier this year, Environment Minister Michael Meacher said they were "not a sustainable technology in the longer term, and should only be used elsewhere when there is no safe, practicable or environmentally acceptable alternative."

Increasingly, the refrigeration industry sees CO2 as the best long-term alternative to HCFC-22. It is generally regarded as non-toxic, is non-flammable, does not deplete the ozone layer, has a low global warming potential, and is much cheaper than HFCs.

According to RAC Magazine, a great deal of development work has been carried out on CO2 systems over the past five years. "The secrecy (not too strong a word for it) is because of the huge commercial advantage that the companies concerned would have in producing a commercially attractive proposition."

The major barrier to the adoption of CO2 is that it is not a drop-in replacement and systems cannot be retrofitted to use it. Instead, new components and technology are needed to cope with the much higher pressure at which such systems operate.

The International Institute of Refrigeration says CO2 can compete with common technology in automotive and transport air conditioning as well as in some heat pumps and industrial drying processes. The less efficient CO2 cycle can be compensated for by taking advantage of particular operating conditions - requiring less powerful compressors, for example - in some applications.

In November, the Institute of Refrigeration held a seminar to discuss how CO2 on its own or in combination with other refrigerants "may now provide permanent solutions in many cases."

The consensus was that CO2 systems were on their way in the UK - the only question being how soon. Robert Heap of Cambridge Refrigeration Technology said they are ten years away because "there's a big difference between having compressors and heat exchangers available and their being produced on a commercial scale and bought in large quantities."

But the chair of the Institute's technical committee, Dr S Pearson, said it would be "more like ten months. It's practical to use CO2 now. There can be both cost and efficiency advantages - just a few years ago this would have been thought of as heretical."

CO2 is already used widely in some countries. In Sweden, for example, York Refrigeration has more than 50 CO2 systems in operation, mostly in supermarkets and cold stores.

Delegates heard how Nestlé plans to replace its low-temperature HCFC-22 refrigeration plants worldwide with cascade systems using CO2 and ammonia. The company is a major user of such systems as part of its production and distribution of frozen foods, pet foods, ice cream and freeze-dried beverages.

Until the early 1990s, its first choice replacement for low-temperature industrial installations was ammonia. But toxicity concerns for traditional systems, which use a large quantity of ammonia, made permit applications too time-consuming, said specialist engineering manager Adrian Page.

Preliminary investigations into replacing the firm's only HCFC-22 plant in the UK, built in the 1960s, identified CO2 in cascade with a small ammonia unit as an interesting option because of the small size of the plant and pipework. This suggested that a new plant could be built next to the existing one while it continued to run - avoiding an expensive maintenance shutdown.

In the mid-1990s, Nestlé's head office decided that HCFC-22 had no long-term future. It examined a range of alternatives. An HFC system was considered, said Mr Page, but there were concerns about the effect of contaminants at low temperature. The company was not convinced at the time that "their use would be regulation-free for the required 25-plus year life of the plants."

The company built a cascade system using CO2 and a small charge of ammonia. The advantages of this option, said Mr Page, were immediately apparent. The refrigerant pressure and density at the low temperature resulted in a small plant with low cost and ease of installation.

The much smaller and more efficient compressors have a bigger effect than the loss of thermodynamic efficiency caused by the additional heat transfer step and account for the lower than expected energy consumption compared with the all-ammonia option.

Installation costs were £4.5 million, with maintenance costs of £75,000 per annum. This compares with equivalent costs of £5.4 million and £90,000 for an all-ammonia system.

Iceland announced plans to eliminate HFCs from its food halls and storage and distribution centres in 1998 (ENDS Report 285, pp 32-33 ). Last year, it completed the installation of a store in Harlesden, north-west London, which uses only "natural" refrigerants. The low-temperature display cases and bulk store room used a direct-expansion CO2 system. Hydrocarbon technology was used to cool a central water system for the chilled cases, air conditioning and chilled storage room.

The system has now been running for 12 months at "a high level of stability and reliability," Iceland says. Capital costs for the prototype were "significantly" higher than for an HFC installation, but the company believes that costs could be brought into line once over-cautious spare capacity is removed. Nevertheless, components are likely to carry a premium unless produced specifically for the refrigeration industry.

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