Low GWP Refrigerants - Understanding GWP, GHG, ODP and Climate ChangeGary Kardys | April 27, 2017
Part 2 – The Selection and Use of Low GWP Refrigerants
Even if you doubt that human activity impacts climate change, engineers are increasingly tasked with designing products for the global marketplace, and despite loosening regulations in the U.S., most industrial countries are attempting to limit or eliminate high global warming potential (GWP) gases as part of their commitment to the Kyoto Protocol Agreement.
The European Union MAC Directive has mandated that residential air conditioning and refrigeration must use refrigerants of GWP of 150 or less by 2015. The same is true of new automobiles by 2017. In the EU, low GWP refrigerants have a GWP of 150 or less. Medium GWP refrigerants are between 150 and 2,500. High GWP gases have GWP greater than 2,500.
The low global warming potential (GWP) refrigerants emerging today can be grouped into several categories:
- Low GWP hydrofluorocarbon (HFC) refrigerants
- Several HFCs are available with lower GWP, such as R-32 and R-152a. However, HFCs will be phased out over time, so a selection of a natural, hydrocarbon or hydrofluoroolefin refrigerant for new equipment design is advised.
- Natural refrigerants
- Ammonia is produced in many biological processes and has been used as a refrigerant for over 150 years. Ammonium (NH4) has a GWP of zero, but a higher toxicity level (ASHRAE Safety Classification of B2).
- Carbon dioxide (CO2) has a GWP of 1, but a longer lifetime. CO2 has been used as a refrigerant in industrial and commercial applications, but specialized high pressure or transcritical systems are required, and the theoretical cycle efficiency is lower compared to HFCs
- Hydrocarbon refrigerants can be considered natural as well. Methane is produced in nature when organic matter decomposes. Natural gas is essentially methane.
- Propane, isobutane, propylene, ethane and ethene have GWPs of 6 or less, but these hydrocarbon gases are highly flammable, gaining them an ASHRAE safety classifications of A3.
- Hydrofluoroolefins (HFOs)
- HFOs, generically known as R-1234yf (2,3,3,3-Tetrafluoropropene), have low toxicity, low persistence, low GWPs of 6 less and are only slightly flammable nature (ASHRAE A2L).
Figure 2 provides data on the environmental and safety properties of various refrigerants, from banned CFCs and commonly used HFCs, to low GWP HFCs, HFOs, hydrocarbon and natural refrigerants. Properties compared include ozone depleting potential (ODP), global warming potential (GWP), environmental persistence and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) safety classification. The trend in the industry is to utilize the “green,” low GWP refrigerants as retrofits and as the working fluid in new cooling systems. Many additional low GWP refrigerants are available such as azeotropic and zeotropic low GWP blends.
The EPA Significant New Alternatives Policy (SNAP) Program databases can be used to find alternative refrigerants and refrigerant blends with low ODP and GWP values that are acceptable for use in the U.S., along with any specific use restrictions. See the following references:
- EPA SNAP - Acceptable Substitutes in Residential and Light Commercial Air Conditioning and Heat Pumps
- EPA SNAP - Acceptable Substitute Refrigerant Blends
If you are retrofitting existing equipment to replace a refrigerant that is being phased out or banned, you should contact the manufacturer for alternatives that will not impact any viable warranties.
Low GWP is important, but it represents only one of many factors involved in refrigerant selection and safe use. Flammability, toxicity, volatile organic compound (VOC) levels, permissible exposure limits (PELs), occupational exposure limits (OEL), cost, leak tendency and, finally, availability are all important factors.
Natural refrigerants are attractive for their low cost (less than $1 per kilogram). Linde Industrial Gases provides high quality natural hydrocarbon, low GWP refrigerants under the CARE® and TEGAN® brands. Their hydrocarbon refrigerants are at least 97.5 percent pure, with minimal levels of critical impurities, including moisture (typically less than 10 parts per million), unsuitable hydrocarbons (typically less than 0.5 percent) and sulfur, which make them ideal for use in refrigeration systems.
HFO refrigerants have higher cost, approximately $60 per kilogram. This is especially noticeable when compared the cost of low GWP HFCs, which come in between $2 and $7 per kilogram according to “Alternative to High-GWP Hydrofluorocarbons” from IGSD. HFO refrigerants are only available from a few manufacturers, such as Chemours / DuPont Opteon® YF Refrigerant (HFO-1234yf) and Honeywell Soltice® ze Refrigerant (HFO-1234ze).
Another consideration in choosing the correct refrigerant for your application is the chemical compatibility of the refrigerant with cooling system components, such as tubing, a condenser, hoses, impellers, compressor liners and seals (see Figure 3).
The seals in pumps or compressors, valves and fittings can result in loss of your working fluid. If the refrigerant is not compatible with the seals in the existing equipment you plan to retrofit, or if compatible seal materials are not chosen for a system under design, then higher leak rates, additional maintenance and even equipment failure can result. Phelps Industrial Products has several excellent compatibility reference tables, Fluid Compatibility of Elastomers and Fluid Compatibility of Metals, that can help you with this. Linde Industrial Gases also provides a Material Compatibility look-up tool. For instance, carbon dioxide is not compatible with Buna N, Neoprene or Viton, while ammonia is not recommended for use in systems with brass, Monel, nylon(polyamide) or Viton. The refrigerant must have sufficient solubility and limited reactivity with the refrigeration lubricants used.
It is also necessary to consider how the refrigerant will be delivered? What supply options are available for the refrigerant? For retrofits and smaller projects, delivery of the refrigerant in cylinder might be convenient. For large scale industrial and commercial applications, micro-bulk or bulk delivery using a gas tube or cryogenic trailer is more economical. Shipping costs can increase overall refrigerant cost, so the distance to the refrigerant manufacturer or distributor should be part of your decision. Bulk delivery may also require additional costs for on-site refrigerant storage tanks or vessels. On-site gas production can be another alternative for applications consuming very large quantities of gas.
The ASHRAE safety classification of the refrigerant provides an indication of flammability and toxicity (see Figures 6 and 7). The specific toxicity (chronic or acute), health risks and occupational exposure limits (OEL, PEL) of a new low GWP refrigerant should be determined (see TOXNET, OARS OEL Resource Guide, and the OSHA PEL Table). Personnel should read the refrigerant material safety data sheets and receive training in proper handling procedures. Refrigeration systems eventually require maintenance, so a regulation-compliant process should be in place for reclamation, recycling or disposal of any refrigerant wastes.
Systems utilizing flammable low GWP refrigerants (HFO or Hydrocarbon) must be designed to reduce ignition sources and provide venting to prevent the build-up of an explosive leaked gas-air mixture (see “Update — Revisiting Flammable Refrigerants from Underwriters [UL]).
Hydrocarbon refrigerants are flammable, and some are used as combustible gas fuels. HFO refrigerants fall under the 'slightly flammable' category (A2L). HFOs have a higher minimum ignition energy, and a slower burning velocity compared to hydrocarbons. Refrigerant systems should be designed to comply with applicable building and fire codes.
In addition, new equipment designs (higher compression, cascading, secondary loops, etc.) are frequently required make the new low GWP natural refrigerants work efficiently. Refrigerants are working fluids that transfer heat from one area to another through an evaporation and condensation process. Most refrigerators, heat pumps and air conditioners utilize a reverse Carnot cycle. The cycle efficiency can vary depending on the refrigerant and equipment design. Knowledge of thermodynamic properties, such as boiling point, critical temperature, temperature glide (zeotropic blends, dew point - bubble point), specific heat and critical pressure of the refrigerant, are needed to properly design with a new working fluid.
Several databases and software packages are available for refrigerant selection as well as refrigeration and AC system simulation, analysis and design. Danfoss A/S provides several free refrigeration and air conditioning system software packages on their website, Danfoss Cooling. Danfoss A/S also supplies a tool for selecting a low GWP refrigerant for a retrofit. The Popular Refrigeration Software site also provides access to several free downloadable software programs. The NIST Reference Fluid Thermodynamic and Transport Properties Database (REFPROP) can be ordered from the NIST website. REFPROP is a useful tool for chemists or engineers involved in developing or applying new refrigerants. REFLEAK, the NIST Standard Database 73, provides a simulation program, and can estimate leak and recharge composition changes for zeotropic refrigerant mixtures. CYCLE_D, the NIST Standard Reference Database 49, simulates vapor compression refrigeration cycles that use single-compound refrigerants or refrigerant blends. The model can also simulate a basic subcritical or transcritical refrigeration cycle, which could be useful in designing a refrigeration system using an inert, nonflammable and low GWP refrigerant like carbon dioxide.
An their article, "Limited options for low-global-warming-potential refrigerants," published in Nature Communications, McLinden and his colleagues provide a useful chart reproduced in Figure 7. The chart compares the performance factor (COP = heat removed/work) versus volumetric capacity (Q, refrigeration effect/unit volume) of several existing and emerging refrigerants normalized to an R-410A reference point.
R-161, R-1132(E), R-717, R-1270, R-290, R-c270, R-22 and difluoromethanethiol had performance factors above the R-410A baseline, but lower capacity ratings compared to R-410. R-1141 and R-1123 had capacity ratings higher than R-410, but lower performance factors compared to R-410. R-32 had a higher combination of performance and capacity than R-410A. While a low GWP refrigerant, R-32, or difluoromethane, has a GWP of 675, which is orders of magnitude higher than hydrocarbon or natural low GWP refrigerants. R-32 is also considered flammable when using an ASHRAE A2 classification.
Researchers at the National Institute of Standards and Technology (NIST) have developed a new computational screening method to more rapidly find and evaluate candidate low GWP refrigerants (see the "Thermodynamic Analysis for Low-GWP Refrigerants -Possibilities and Tradeoffs for Low-GWP Refrigerants" Seminar). The new NIST method uses the molecular structure of a candidate chemical compound to predict the compound's GWP by combining calculations of a compound's radiative efficiency with atmospheric lifetime. The method also applied additional filters to screen out chemicals based on toxicity, flammability, stability, cost and critical temperature. In a multi-year study, "NIST Quest for Climate-Friendly Refrigerants Finds Complicated Choices", NIST researchers screened 60 million chemicals to find 138 fluids with estimated GWP of 1000 or less. The list was narrowed down to 27 compounds with low toxicity and high efficiency. However, these 27 compounds still have some degree of flammability. Finding new efficient, non-toxic and environmentally friendly refrigerants with low flammability continues to challenge researchers.