On substances that deplete the ozone layer
Refrigerants 2.1 Introduction
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- Navigate this page:
- Refrigerant progression
- Unsaturated hydrofluorochemicals
- Recent assessment on current and future refrigerants and GWP
- Flammable refrigerants and Safety classification
- Climate impact metrics for refrigerants
- ODP and GWP data for regulatory and reporting purposes
- Selection of refrigerant
- HCFC-22 replacements
2 Refrigerants2.1 IntroductionSince the 2010 assessment report 21 refrigerants obtained standardized designations and safety classifications. One new molecule, HCFC-1233zd(E), can be found in this group of new refrigerants; it is an unsaturated HCFC (HCFO) with potential to replace HCFC-123. Approximately a quarter of the new refrigerants are blends which are replacements for HCFC-22. Twelve of the new refrigerants are blends of high global warming potential (GWP) saturated HFCs and low GWP unsaturated HFCs (HFOs), and seven of these blends are with class 2L flammability. The search for new refrigerants addresses both climate and ozone concerns, in particular by reducing the GWP of the refrigerant and replacing ozone-depleting substances. The search for new refrigerants addresses both climate concerns and ozone depletion concerns, in particular by reducing the GWP of the refrigerants and replacing ozone-depleting substances, such as HCFC-22.
The historic progression of refrigerants encompasses four phases based on defining selection criteria (Calm, 1997; Calm, 2012):
Facing regulatory pressures to eliminate refrigerants with high GWPs, the major refrigerant manufacturers have pursued unsaturated fluorochemicals. They are chemicals consisting of two or more carbon atoms with at least one double bond between two or more of them as well as fluorine, hydrogen, and possibly also chlorine or other halogens. Unsaturated fluorocarbons also are identified as fluoro-alkenes or fluoro-olefins. Double carbon-carbon bond(s) make(s) the compounds more reactive. The result is short atmospheric lifetimes and, thereby, very low ODPs and GWPs. The unsaturated HFC (also identified as hydrofluoro-alkene or hydrofluoro-olefin, HFO) family is a focal example with varying extents of halogenation, in part, as a trade-off between flammability associated with a lower degree of stability and high GWP associated with a higher degree of stability. Chemical producers are pursuing alternatives for the most widely used low-, medium-, and high-pressure refrigerants. In the naming of unsaturated halocarbons the prefix for unsaturated HCFC, and unsaturated HFC can alternatively be chosen to be HCFO and HFO respectively, where the O replaces the carbon C (ISO 817:2014). In this report the prefix used is the default HCFC and HFC respectively. The extent of long- term acceptability of unsaturated HFCs or more broadly unsaturated hydro-halochemicals is uncertain, though a number of initial studies looking at the trifluoroacetic acid (TFA) build up resulting from the use of unsaturated HFCs in mobile air-conditioning indicate manageable environmental impacts (Kajihara, 2010; Luecken, 2010; Papasavva, 2009).
There are a number of studies providing assessments of a broad number of alternative refrigerants. For example, the AHRI Low-GWP Alternative Refrigerants Evaluation Program (AHRI/AREP) phase 1 evaluated more than 30 new refrigerants (AHRI, 2013), most of them blends and most of them based on unsaturated HFCs mixed with traditional HFCs. The refrigerant categories and corresponding 100 year GWP values are given in Table 2-1 (calculated based on the GWP values of Table 2-7). Table 2-1: 100 year GWPs for AHRI/AREP alternative refrigerants
This table illustrates the relationship between the pressure and the flammability of a fluid, although it can be argued that not all the proposed R-410A replacements have high enough capacity to be considered replacements for R-410A. There is a clear trend that the higher the pressure the higher the minimum GWP that is needed for pure compounds or blends to be non-flammable. The above also gives a good estimate of which levels of GWP are to be expected for various capacity/pressure levels for a given flammability class. Another recent study (McLinden, 2014) started with a database of 100 000 000 chemicals, screening the more than 56 000 small molecules and finding non ideal. The conclusion is that the perfect cheap, energy efficient, non-toxic, non-flammable, and broadly applicable refrigerant with negligible GWP does not exist, and is highly unlikely to come into existence.
ISO 817 (ISO 817:2014) and ASHRAE 34 (ASHRAE 34-2013) assigns a safety class for each refrigerant, for instance “A2L”. The first letter of the safety class describes the toxicity, A for lower level of toxicity and B for higher level of toxicity, while the second part is the flammability of the refrigerant: 1 (no flame propagation), 2L (lower flammability), 2 (flammable), and 3 (higher flammability) (Table 2-2; see the annex to this chapter for addition details). Table 2-2: Safety classifications
As mentioned above, there is a trade-off between GWP and flammability. A low GWP can be achieved by having the refrigerant break down quickly in the atmosphere, however this also means that the refrigerant breaks down easier when subjected to ignition sources. For many applications, flammability is a new aspect to take into account when designing systems. Although safety standards are not strictly mandatory in most countries, they are one source where the industry looks for guidance for handling the flammability. For instance, ISO 5149 (ISO 5149:2014) Part 1 describes limits to charge amount depending on system type, system location, and accessibility by people unaccustomed with the safety procedures relating to the system. Local legislation will also have an influence, for instance national legislation and building codes. The requirements for flammable refrigerants are very similar across the different flammability classes, the different flammability properties result in different risks (probabilities) and consequences; therefore varying refrigerant charges and thereby system designs for each refrigerant. Further discussion is included in the annex to this chapter. It is still not clear for all applications whether flammability will be accepted for each level of cooling or heating capacity needed, and how the various markets will split-up with regard to the classes of flammability. The acceptance of flammable refrigerants and the appropriate updates of standards and legislation is clearly a contemporary challenge for the refrigeration air conditioning, and heat-pump industry.
The most popular metric for indicating the climate impact of refrigerant emissions is 100 year GWP, which is the integrated radiative forcing over a 100 year time horizon of 1 kg of refrigerant emitted to the atmosphere. Some parties advocate use of GWP values for shorter or longer integration time horizons or various instantaneous or sustained metrics such as the global temperature potential (GTP) (Shine, 2005; Hodnebrog, 2013; see §8.7 of (IPCC, 2014) for detailed discussions of instantaneous GTP and sustained GTP). In this chapter both 20 year GWP and 100 year GWP values are listed. The advantage of 20 year GWP over 100 year GWP is that a 20 year time horizon seems more relevant when discussing the climate change in the coming decades and it is also better at differentiating between substances with short life times. The advantage of using 100 year GWP is that it is more commonly used and the popularity of a metric is important to the efficiency of communicating the climate impact. Also the 100 year GWP is better at differentiating between substances with longer lifetimes, although this seldom applies to currently used non-ODS refrigerants. An example is PFC-116 and HFC-143a; the difference in 20 year GWP is less than 20% while the difference in 100 year GWP is more than a factor of 2. As long lived substances can be used in small amounts in blends with short lived substances to create lower GWP blends, using 20 year GWP may lead to decisions favouring the shorter term at the cost of the longer term. A classification of 100 year GWP levels which is a slight modification of the proposal in the previous RTOC report (UNEP, 2011) is given in Table 2-3. A complication of naming a substance as low GWP or high GWP is that the climate impact is the product of the GWP and the amount (in kg) emitted, so even though CO2 is classified in Table 2-3 as ultra-low GWP, the amount emitted by the human civilization is large enough to give a significant climate impact. Table 2-3 categorizes using “less than” or “more than” and this enables a refrigerant such as HFC-1234yf to have ultra-low, very low, and Low GWP at the same time. Likewise a refrigerant such as HFC-23 has ultra-high, very high, and high GWP. The purpose of this feature is to allow simple references to all refrigerants below or above a given limits. Table 2-3: Classification of 100 year GWP levels
For gases with lifetimes of a century or more the uncertainties are of the order of ±20% and ±30% for 20- and 100-year horizons. For shorter lived gases the uncertainties in GWPs will be larger (IPCC, 2014). This is one reason why care must be taken when comparing the GWP of refrigerants. For substances with very low GWP the picture is further complicated since, for historical reasons, GWPs for hydrocarbons are indirect GWPs, which include breakdown products such as CO2, as opposed to the GWPs for HCFCs, HFCs etc., which do not cover breakdown products. When comparing carbon emissions associated with the use of refrigerants, care must be taken to also include a comparison of energy efficiency of the refrigerant in the given application. Emissions from energy production may have a greater climate impact than the refrigerant emissions only. For further discussion see chapter 11.
The ODP and GWP data presented in this chapter are based on international scientific assessments and reflect the latest consensus determinations on potential impacts. However, the reduction requirements and allocations under the Montreal Protocol, emission reductions and reporting pursuant to the Kyoto Protocol, and provisions in many national regulations pursuant to them use older, adopted values. Table 2-4 compares the latest consensus ODP data (WMO, 2014) and (WMO, 2011) to the “regulatory” ODPs used in the Montreal Protocol (UNEP, 2012). Table 2-5 similarly contrasts the latest consensus GWPs, for a 100 year integration, with those used for reporting and emission reductions under the Kyoto Protocol from IPCC (1995). Table 2-4: Scientific and regulatory ODPs for selected BFC, BCFC, CFC, and HCFC refrigerants
Table 2-5: Scientific and regulatory GWPs for 100 year integration for selected refrigerants
There are several factors that should be considered when selecting an alternative refrigerant for refrigeration, air conditioning and heat pump systems and applications. These include:
The selection of a refrigerant for a given application will be a compromise of the above criteria. Other than zero ODP, the rest of the parameters will to be traded-off against one another to arrive at the optimum for each type of system and application. In particular the carbon emissions include both the “direct” and “indirect” contribution of the product over its life time. A number of approaches have been documented in the literature including: Total Equivalent Warming Impact (TEWI), Life Cycle Climate Performance (LCCP), Multilateral Fund Climate Impact Indicator (MCII) and other methods. For new refrigerants for existing systems, there are a number of criteria that should be achieved in order for a suitable refrigerant to be selected. In summary, with respect to the refrigerant being replaced, these are:
There are a number of other parameters that should be considered. However, in practical terms it is unlikely that any of the commercially available refrigerants can meet all of the above criteria and therefore some compromise should be anticipated. A refrigerant that is capable of replacing an existing refrigerant without changing any major components, including the oil, is sometimes referred to as a drop-in refrigerant. The term drop-in is somewhat misleading, as safety standards require the system to be upgraded to the latest safety requirements for any change of refrigerant type, and the trade-offs mentioned above may affect the performance and durability of the system.
The challenge of reducing the use of HCFC-22 has led to the development of refrigerants specifically designed to replace HCFC-22 in existing equipment or replace HCFC-22 in new systems without the need for significant design changes. A main differentiator between the HCFC-22 replacements is whether the refrigerant is miscible with the mineral oil often used with HCFC-22. The refrigerants which are more miscible with mineral oil typically contain a minute amount (< 4%) of hydrocarbon to increase the miscibility and help the oil return to the compressor. These refrigerants typically have safety classification A1 and include: R-417A, R-417B, R-417C, R-422A, R-422B, R-422C, R-422D, R-422E, R-424A, R-425A, R-428A, R-434A, R-438A, and R-442A. The refrigerants that are designed to fit the thermodynamic properties of HCFC-22 refrigerants, but not to be used with the mineral oil often used with HCFC-22 includes: R-404A, R-407A, R-407B, R-407C, R-407D, R-407E, R-407F, R-421A, R-421B, R-427A, R-444B, and R-507A. While they typically have the safety classification A1, some of the more recent HCFC-22-like blends (specifically R-444B) have the safety classification A2L. When changing refrigerant in a system there is always recommended to check gaskets, elastomeric seals, and other materials for compatibility with the new refrigerant, and there is a risk that exposure to HCFC-22 has changed the properties of polymer materials to the point where they will not function correctly or exasperate leakage after a refrigerant change. There may also be a need for other system modifications to reach an acceptable energy efficiency and capacity. See also the discussion in 2.1.7 on a general discussion about replacing one refrigerant with another in existing systems. For new systems, there are several other options for replacing HCFC-22 with system designs adapted to e.g. R-717, R-744, HCs, HFCs, or other newly developed molecules. Directory: meeting meeting -> Program description meeting -> Iea shcp/pvps working Group on pv/t solar Systems meeting -> Office of the collector: jagatsinghpur (st & sc development Section) meeting -> World meteorological organization meeting -> WG/hwsor action item summary (Current as of 28 Feb 2013) meeting -> Review of the past hurricane season meeting -> Review of the past hurricane season reports of hurricanes, tropical storms, tropical disturbances and related flooding during meeting -> Town of seabrook island meeting -> World meteorological organization Download 1.63 Mb. Share with your friends:
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