If you want to use a refrigerant with a zero ozone-depleting and global warming potential, then why not use natural hydrocarbon as a refrigerant, argues Reg Brown. Manufacturers in Europe and Japan already have products available.
Refrigeration has a long and interesting history. The principles were first demonstrated by William Cullen in 1748 using ethyl ether, a volatile solvent. Over the next 100 years various inventors developed the concept of the mechanical vapour compression cycle. The first commercially successful systems (used in brewing and meat packing) were produced by James Harrison in Australia from late 1850s.
Ammonia-based vapour compression systems became available in the 1890s, and by 1914 were widely used for all manner of commercial applications, particularly in the USA, including for air conditioning.
Synthetic refrigerants were not developed until the 1920s when Thomas Midgely synthesised organic molecules with chlorine, fluorine and carbon to create chlorofluorocarbons (CFCs). The benefits of CFCs, relative to the available natural refrigerants such as ammonia, sulphur dioxide and chloroform, were low toxicity and no explosion risk.
CFCs and hydro-chlorofluorocarbons (HCFCs) dominated refrigeration technology for the next 50 years, with R12 (CFC-12) and R22 (HCFC-22) becoming familiar to building services engineers.
If CFCs had not been invented, or their potentially disastrous effect on stratospheric ozone had been realised from the start, then air conditioning might have developed more slowly. However, once it was decided that CFCs should no longer be produced, then the search was on for ozone-friendly alternatives that could be used in the same applications without major changes in technology.
Replacing CFCs and HCFCs didn't seem to be too much of a problem until the dawning realisation of climate change and the global warming potential (GWP) of refrigerants. Some of the alternative refrigerants, such as R134a (a replacement for R12), have a zero ozone depletion potential (ODP), which is good, but a large GWP, which is bad. So why not use simple hydrocarbons such as propane, a good refrigerant with zero ODP and zero GWP?
Europeans tend to accept propane (R290) as a useful refrigerant, as it has minimal environmental effects and does not pose an unacceptable risk to safety - at least in small quantities.
US experts, however, argued that propane plant was a potential bomb and should be kept out of buildings at all costs. Cynics might argue that the US tried to protect its refrigerant manufacturers from cheap alternatives to their products while ignoring the climate change issue.
That said, there are real issues associated with the use of large quantities of propane inside buildings. EN378 Part 1:2000 contains guidelines on maximum quantities for different situations. These are summarised in Table 1. Additional guidance is provided by ACRIB 2001 (Guidelines for the use of hydrocarbon refrigerants in static refrigeration and air conditioning systems) .
These restrictions have tended to inhibit development of larger propane plant. There are only a few manufacturers offering packaged chiller plant with propane, for example. Meanwhile, ammonia is coming back into fashion, and absorption refrigeration systems (using a completely different refrigeration cycle) are being considered for cooling large buildings.
A natural refrigerant with no flammability issues is carbon dioxide (CO2). While CO2 is not particularly well suited to air conditioners, carbon dioxide heat pumps using a modified vapour compression cycle are under development by major manufacturers in Europe and Japan. Some products are already available.
The main advantage of CO2 as a refrigerant in heat pumps is that it is easy to generate high temperatures. This gives the potential for boiler replacement in conventional heating systems and for generating domestic hot water.
In the short term, HFCs such as R410a are likely to maintain a hold on the conventional commercial air conditioning market including split-unit air conditioners and variable refrigerant-flow systems. Hydrocarbon and ammonia plant will be used where they can.
In the longer term, use of HFCs will probably come under increasing environmental pressure in Europe, if not in the USA. Already the recent F-Gas Regulations impose stringent conditions on the maintenance of systems containing refrigerants with a GWP of more than 150. A European directive phasing out R134a for car air conditioners will come into force by 2011.
Whether Europe becomes fluorine-free remains to be seen, but there are already natural refrigerant solutions for most building services applications.
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