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Refrigerant evolution and natural selection

Andy Pearson, chairman of the IOR Technical Committee explains the latest refrigerant guidance

The Technical Committee has been working on non-partisan guidance on the effects of refrigerant choice and system design on the carbon footprint of a refrigerating system. Two major effects have been considered in detail: the direct global warming potential of the refrigerant selected and the climate change effect of energy use by the system. 

Strategies include designing more efficient systems; minimising sources of leakage through the selection of more robust system components; reducing the quantity of refrigerant needed to operate the system; and substituting lower GWP refrigerants such as ammonia, hydrocarbons, carbon dioxide, some low-GWP HFCs and hydrofluoroolefins (HFOs), which are a subset of hydrofluorocarbons with a very short atmospheric life.

There are many situations where the direct substitution of a HFC with a “natural” refrigerant is not reasonably practicable, either due to toxicity or flammability or high pressure. In such cases it would be possible to minimise HFC charge, or even eliminate it, by using a secondary fluid, but this would make the system more expensive and less efficient. 

However, rather than promoting a total ban on HFCs, it would be more appropriate to advocate responsible use.

A ‘responsible refrigerant policy’ would place a high emphasis on the elimination of leak sources, the efficiency of the overall system and the life cycle cost of ownership. The government could make a significant step in this direction by introducing either regulation or incentives in these three areas to encourage significant improvement in the behaviour of manufacturers and users.

Low GWP HFCs (those with a global warming potential lower than 150) are being investigated by chemical companies, but none has yet been commercialised, although there are prospects for creating useful blends combining low GWP HFCs with other compounds.

The Technical Committee sees little probability of HFOs being adopted for use in commercial or industrial refrigeration in the near future. R-1234 is unusual in this chemical family because it is of low flammability and low toxicity – other members of the same group are significantly more hazardous. But it seems highly unlikely that anything else will be commercialised in the next 5 years. 

Energy efficiency and refrigerant choice

Efficiency is not only dependent on choice of refrigerant but also on good design, selection of an appropriate system and good maintenance. The highest efficiency refrigerant is unlikely to improve system efficiency by more than 10 per cent over that of an optimised system using an HFC refrigerant. 

Some natural refrigerants such as carbon dioxide may result in reduced system efficiency unless they are integrated into a heat recovery system. At current levels of knowledge and expertise, practicality often leads to a choice of HFCs for good efficiency to be realistically achieved and maintained. In some applications, natural refrigerants could be used but they are not suitable for all uses. 

The practicality element will change as costs and design changes associated with HFC use evolve, and more components, experience and skills for natural refrigerants are developed. This is expected to reduce the applications that require HFCs, but not eliminate them, in the foreseeable future. Highly practical HFC applications include split ac systems, and in particular VRF combined cooling/heating systems.

Arbitrary constraints on how designers can specify systems, such as bans on the use of specific refrigerants, should not be imposed. Such constraints could result in less efficient systems being installed and will not drive the industry towards lowest GHG emissions.

To obtain ‘good efficiency,’ attention needs to be given to the following steps:

  1. Avoid refrigeration/reduce the cooling load - there is no point designing an efficient system if the load was unnecessary
  2. Get the overall system design right (e.g. best cycle)
  3. Get the control philosophy right (avoid head pressure control, avoid partly loaded compressors, avoid fixed speed pumps and fans etc)
  4. Optimise individual components for efficiency
  5. Operate and maintain the plant for best efficiency.

Mistakes in any of the above can change the efficiency of a plant by large amounts (20 to 50 per cent). Where does refrigerant selection fit in? It can either be thought of as a system design issue or a “component optimisation”. The impact of the refrigerant on efficiency is likely to be less than 5 per cent.

The key message is that refrigerant choice matters, but many other design issues matter much more.