REFRIGERANT | ENVIRONMENTAL IMPACT REFRIGERANT RETHINK Specifiers need to start assessing a refrigerants embodied carbon as well as its global warming potential, says Klima-Therms Tim Mitchell, who is working with bodies such as LETI and CIBSE to calculate the full carbon impact of HVAC systems Enerblues heat pumps use CO2 as a natural refrigerant and can heat water up to 90C A host of different criteria needs to be assessed and balanced to decide which heat pump or chiller technology is right for a particular application. Complicating the picture further for heat pumps are the refrigerant choices; refrigerants are a particular minefield that must be negotiated before a consultant can go any further in specifying HVAC equipment for new projects. All the most common hydrofluorocarbons (HFCs) are categorised as A1 and so non-toxic and non-flammable. They are readily available, well understood, and relatively cheap so why consider using natural refrigerants at all? There are fundamentally two reasons. The first is application suitability, which has been created by the recent drive to remove the burning of fossil fuels from the built environment. For example, using CO2 as the refrigerant in a heat pump can deliver hot water at boiler-like temperatures and beyond 90C is no problem at much better efficiencies than any HFC. This means that, for modern boilerreplacement projects, it is typically the most applicable option. A typical air source CO2 heat pump might deliver hot water at 75C from air at -4C with a coefficient of performance (CoP) of 2.53 or so, whereas an HFC using the same criteria would give a CoP of around 1.78 and would be at the top of its operating range. However, that is not the end of the story. CO2 systems are peculiar in the HVAC selection ranges because they operate trans-critically, so require a very wide differential between the leaving and return water temperatures. This characteristic means they are especially suited to sanitary or process applications, for which you consume a lot of very hot water. This makes CO2 stand out among the natural refrigerant options it has its own application envelope, whereas there are HFCs that have a more complete operational overlap with ammonia and propane. The second reason to consider natural refrigerants is sustainability, particularly in relation to their ultra-low global warming potential (GWP). For example, ammonias GWP is 0; CO2 has a GWP of 1 (it is the base for the GWP metric); and propanes GWP is 3. This compares with the lowest GWP HFC that we currently see in common use for HVAC applications R32 which has a GWP of 675. Some HFC/HFO blends can offer lower GWPs R513A and R454B have GWPs of 631 and 467 respectively, for example but none get down into the ultra-low category of natural refrigerants, with the exception of pure hydrofluoroolefins (HFOs) which have environmental issues of their own (the energy needed to produce HFO refrigerants can be problematic, and the by-products from the breakdown of some are acidic). There is a third metric that specifiers One reason to consider natural refrigerants is sustainability, particularly in relation to their ultra-low GWP Tim Mitchell says a refrigerants embodied carbon must be considered www.cibsejournal.com February 2021 43 CIBSE Feb21 pp43-44 Natural refrigerants.indd 43 22/01/2021 15:49