Topical Article: The Future of Refrigerants for Heat Pump Applications


Industry and other sectors are currently looking for solutions to decarbonize their processes, including heating, which is mainly based on fossil fuel boilers. Moving to the electrification of heat by implementing heat pump technologies to provide the demanded load can be complex. Choosing the right working fluid, synthetic or natural refrigerant, with less harm to the environment (low GWP) and humans (Safety issues) is under debate globally and is needed urgently before 2030. Some measures have been taken to solve this problem by introducing short-term synthetic refrigerants such as R32 in Europe and R454B in the US with low GWP values. This article summarizes the debate of which refrigerants are suitable for usage in heating and cooling systems and their effects on the surroundings.

Thermal factors affect the coefficient of performance and volumetric efficiency of the system, while chemical properties have important effects on the environment and on the selection of the refrigerant itself. Based on the triangle of elements [Refrigerant Selections], adding chlorine atoms to the chemical structure of the refrigerant would increase the Ozone Depletion Potential values, increasing hydrogen atoms would elevate the flammability rates of the refrigerant, and more fluorine atoms would raise the Global Warming Potentials index. For example, the new refrigerant R-1336mzz  with the chemical formula of (C4H2F6), which is not flammable and of low GWP value, is based on Butene, also known as Butylene, with the chemical formula of (C4H8), which is highly flammable, by replacing six hydrogen atoms with fluorine atoms.

Another issue that affects the selection of a suitable synthetic refrigerant is Per- and PolyFluoroAlkyl Substances and its sub-groups. The Organisation for Economic Co-operation and Development (OECD) [OECD 39] identified over 4700 substances as PFAS in their risk management report published in 2018. In the same report, they define new groups that fulfill the common definition of PFASs, which include hydrofluorocarbons and hydrofluoroolefins refrigerants. The result of the decomposition is a TriFluoroacetic Acid which is yet another sub-group of PFAS. The European FluoroCarbons Technical Committee, which provides an up to date information about applications, safety, health and environmental effects for HFCs, HCFCs, and HFOs, does not classify these gases to meet the criteria of Persistent, Bioaccumulative, and Toxic substances, and it claims that these gases pose no harm to the environment.

The GWP100 values for synthetic refrigerants and natural refrigerants were adopted as a metric to implement the multi-gas approach embedded in the United Nations Framework Convention on Climate Change and made operational in the 1997 Kyoto Protocol. Not all agree with the move to the new index because the presented data over 20 years could be misleading to the public as well as policymakers in terms of which refrigerants are truly climate-friendly and sustainable. It is also important to evaluate other indexes, like the Total Equivalent Warming Impact and the Life Cycle Climate Performance.

Another issue that affects the selection of a suitable synthetic refrigerant is Per- and PolyFluoroAlkyl Substances and their sub-groups. In the same report, they define new groups that fulfill the common definition of PFASs, which include hydrofluorocarbons and hydrofluoroolefins refrigerants. Some of these refrigerants break down in the atmosphere because their chemical formula contains double bonds, which makes their molecules less stable.

Bassam E. Badran, Researcher, Royal Institute of Technology, KTH

The text has been shortened by the HPC team
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