The average global temperature is predicted to rise between 1.5 and 4.5K in the next 100 years. The principle cause of global warming is the emission of greenhouse gases into the Earth’s atmosphere.
Refrigeration systems are one of the contributors to the emission of greenhouse gases due to a direct leak of HFC or HCFC refrigerant. These refrigerants have a very high global warming potential (GWP) typically between 1500 and 4000, whereas the GWP of CO2 is 1.
Over the last twenty-five years legislation has prohibited the use of ozone-depleting CFC refrigerants, and these were replaced with HFC refrigerants. In more recent times it has been recognised that GWP is equally damaging to the atmosphere so legislation has provided limitations with regards to the GWP value, the manufacture, and usage of the refrigerants. Current legislation is covered by the F-Gas Regulations.
Carbon dioxide is a very efficient refrigerant (GWP of 1) and is classified as a safety group A1 refrigerant which is non-toxic and non-flammable, therefore can be safely used in a public location like Supermarkets. Carbon dioxide is not corrosive to copper or brass, but due to the very high operating pressure the installation materials need to be specifically designed and selected.
In recent years due to legislation (F-Gas Regulations), and in some Countries high taxation of the use of HFC refrigerants, the use of natural refrigerants within Commercial & Industrial Refrigeration Industry has become more important and many manufacturers have invested in the development of equipment and system designs using carbon dioxide as a complete single-stage or two-stage system.
Trans-critical carbon dioxide systems have become the preferred option for most UK Supermarkets in recent years as the site installation is very similar to a conventional HFC system, without the complication of a cascade design. All the complicated components and controls are contained within the “pack” which is manufactured within a factory by specialist manufacturers.
The Pro’s and Con’s of Co2 design:
Pro’s
- Carbon dioxide is environmentally benign, non-toxic and non-explosive.
- Carbon dioxide refrigerant is far less expensive than synthetic refrigerants.
- Carbon dioxide is non-corrosive with most materials and has good miscibility with compressor lubricants for oil return.
- Carbon dioxide is a very dense refrigerant with a high volumetric efficiency and hence allows the pipe size and compressor swept volume to be much smaller than an equivalent HFC system.
- The effect of pipeline pressure drop is less noticeable with carbon dioxide than HFC refrigerants.
- Carbon dioxide has a much higher heat transfer efficiency, therefore heat exchangers can be smaller, or operate at a lower TD, than an equivalent HFC unit.
- Trans-critical carbon dioxide packs are designed at higher pressures and supplied with pressure relief valves that do not discharge when the power is turned off, or the equipment shuts down for any reason.
- Trans-critical carbon dioxide packs run with a very high discharge temperature therefore the opportunity to offer efficient heat reclaim is far better than for an equivalent HFC system.
- Carbon dioxide is a stable molecule leading to a low potential for decomposition within the refrigeration system.
- There is no impending legislation phasing out carbon dioxide so it can be viewed as a long-term refrigerant and has significant value in reducing companies carbon footprints.
Con’s
- Carbon dioxide operates at a far higher pressure than typical HFC refrigerants, therefore the leak potential increases and the component cost becomes higher.
- Carbon dioxide packs and condensing units need special components, more controls and safety features than an equivalent HFC unit, therefore the component and equipment cost becomes higher.
- Carbon dioxide can change from a gas directly to a solid (triple point) at 4.2bar therefore the commissioning engineer must take more care when charging the system with refrigerant. Also the design of PRV vent lines must allow for the possible freezing of carbon dioxide as it approaches atmospheric pressure.
- Carbon dioxide is very sensitive to water contamination and can form unusual compounds when there is a leak in a water heat exchanger. It is also important to ensure that no moisture is allowed into the system when charging refrigerant or oil.
- When the requirement for gas leak detection is considered we need to compare the room volume with the refrigerant “practical limit” value to establish the maximum gas charge of the system. Carbon dioxide has a “practical limit” of approx. 25% when compared to other HFC refrigerants, which means that leak detection equipment is 4 times more likely to be required.
