Immediate opportunity for large greenhouse gas emissions reductions with new mobile air conditioning refrigerants

Abstract

Vehicle air conditioning, or “mobile air conditioning” (MAC), has a significant impact on the Earth's climate. HFC-134a, the predominant refrigerant in today's MAC, has a global warming potential (GWP) of 1430. MAC is responsible for a third of global HFC emissions and is the single largest user of HFC-134a. Fortunately, three technically and economically feasible low-GWP refrigerant alternatives exist, making MAC an immediate opportunity for non-carbon dioxide greenhouse gas emission reductions worldwide. This article presents cost-effective refrigerant alternatives and their potential for reducing anthropogenic greenhouse gas emissions. It will also cover policies that have been enacted to date to encourage the switch.

Keywords:

• mobile air conditioning
• 1234yf
• HFC-134a
• CO₂
• HFC-152a
• LCCP

1 How HFCS came to be used in vehicle air conditioning and why they are now unsustainable?

The auto industry is facing difficult refrigerant choices, but history shows that refrigerant choice was rarely ever easy. Before chlorofluorocarbons (CFCs) were invented, refrigerants were often toxic, flammable, or both. When mechanical refrigeration became available in the 1920s, entrepreneurs rapidly commercialized systems with a wide variety of refrigerants, including carbon dioxide (CO₂), water, ammonia, isobutene, sulfur dioxide, and methyl chloride. Mechanical refrigeration was vastly superior to ice, which could be contaminated and did not always assure safe temperatures for food refrigeration. However, leaks of the most common refrigerants of the 1920s – sulfur dioxide and ammonia – typically required rapid evacuation of homes and buildings. People who came into contact with those substances suffered from vomiting, burning eyes, and painful breathing. Accidents with sulfur dioxide and ammonia rarely resulted in death, but accidents with methyl chloride refrigerant were frequently fatal (Andersen and Sarma 2002).

In 1928, Thomas Midgley, working with Albert Henne and Robert McNary at General Motors Research Laboratory, invented CFCs (Kauffman 1989). CFCs proved to be non-flammable, non-explosive, non-corrosive, low toxicity, and odorless chemicals with vapor pressures and heats of vaporization that made them excellent refrigerants. General Motors patented the family of CFCs and formed a joint stock company with DuPont to manufacture and market them (Andersen and Sarma 2002). For the next several decades, CFC sales grew as they became widely used for many different applications.

Then, in 1974, Mario Molina and F. Sherwood Rowland – building on the work of many scientists including Paul Crutzen who shared their 1995 Nobel Prize – warned that CFCs deplete the stratospheric ozone layer that protects life on earth from harmful ultraviolet radiation (Rowland 1997). In the next two decades the Molina-Rowland hypothesis was scientifically verified; the 1987 Montreal Protocol was signed, ratified, and entered into force; and CFC, hydrochlorofluorocarbon (HCFC) and other ozone depleting substance (ODS) production was scheduled to be halted (Andersen and Sarma 2002; Andersen et al. 2007).

When the Montreal Protocol was signed in 1987, there was an urgent need to identify CFC alternatives in order to comply with the phase-down deadlines. Fortunately, many replacements already existed. HFC-134a had been identified decades earlier and had been patented in the 1970s (Andersen and Sarma 2002). With zero ozone depletion potential (ODP), eight times less global warming potential (GWP), low toxicity, and no flammability, HFC-134a was an ideal replacement for CFC-12 in mobile air conditioning (MAC). The fluorocarbon chemical industry and their refrigerant customers moved quickly to commercialize HFC-134a. Hydrocarbon and CO₂ refrigerants were also proposed for vehicle air conditioning, but the typical leak rates and service venting practices would have been unsafe, and no one knew how quickly technology could be implemented to mitigate toxicity and flammability. By the time Gustav Lorentzen and coworkers filed for their first modern patent for CO₂ systems in 1989 (granted in 1993), this technology was too late to capture the market for the CFC phase-out (Lorentzen and Pettersen 1993). Still, the switch from CFC-12 (GWP 10,720) to HFC-134a (GWP 1430) in MAC dramatically reduced CFC emissions worldwide, making a significant contribution to climate as well as ozone layer protection (Velders et al. 2007).

Although HFC-134a was the unequivocal choice to replace CFC-12 in 1990, it is unsustainable now for several reasons. First, with a GWP over 1400, HFC-134a is still a very potent greenhouse gas, and MAC is its single largest application. Second, HFC-134a emission rates from MAC are unfortunately still very high in many markets. Not all countries mandate recovery and recycling of refrigerant and those that do have often failed to enforce HFC-134a recovery and recycling laws. Sometimes loopholes exist as well. In the United States, for example, although servicing technicians are required to recover refrigerant for recycling, they are not required to fix refrigerant leaks before re-charging systems with HFC-134a. Without fixing leaks, refrigerant leaks back out to the atmosphere. Additionally, the law only applies to professional servicing technicians. Individuals who recharge their air conditioners themselves lack recovery and recycling equipment, and therefore vent refrigerant to the atmosphere. Third, vehicle sales are increasing worldwide, particularly in emerging markets such as China and India. Air conditioning, once considered a luxury, is now standard in most vehicles sold worldwide, further increasing the demand for HFC-134a refrigerant. The percentage of consumers who choose to purchase a vehicle without air conditioning has dropped to under 2% in the United States and under 10% in Europe; similar trends are evident in developing markets worldwide. According to the Intergovernmental Panel on Climate Change (IPCC), HFC-134a emissions from MAC will exceed 175,000 tonnes by 2015 – equivalent to more than 250 million tonnes of CO₂ based on a GWP of 1430. (IPCC and TEAP 2005; p. 300). Global HFC emissions in 2050 from all applications are projected to be 5.5–8.8 Gt CO₂-eq yr⁻¹ (Velders et al. 2009).

Fortunately, three proven alternatives exist. Advances in vehicle air conditioning technology have made previously impractical refrigerants, such as HFC-152a and CO₂ feasible. At the same time, new, low-GWP refrigerants have been developed that satisfy the rigorous performance standards for vehicle air conditioning. These alternative refrigerants can provide substantial greenhouse gas savings, especially if they are implemented with due consideration for life-cycle climate performance (LCCP).

2 Policies encourage switch to low GWP refrigerant: the importance of life-cycle climate performance for the climate and consumer

Several governments have created or are considering policies to encourage vehicle manufacturers to switch to low-GWP refrigerants; however, switching to a low-GWP refrigerant does not guarantee reduction in overall GHG emissions. Vehicle air conditioning contributes to greenhouse gas emissions in two ways: direct refrigerant emissions and fuel consumption. Air conditioning fuel consumption is quite substantial, consuming up to 20% of overall motor fuel in hot climates such as those found in India (Rugh et al. 2004). If a new refrigerant results in more fuel-intensive air conditioning, it could negate the greenhouse gas benefits.

LCCP is important for both the climate and the consumer. Policies that encourage LCCP result in greater greenhouse gas emission reductions. (See Box 1). Additionally, policies that encourage greater air conditioning fuel efficiency, a key element in life-cycle performance, result in cost savings to the consumer. For example, a study conducted by the US Department of Energy's National Renewable Energy Laboratory (NREL) found that if MAC used 30% less energy, it would save the average US driver 42 liters (11 gallons) of fuel per year, and up to (106 liters) 28 gallons in warmer states such as Hawaii (Rugh et al. 2004). At a cost of $0.66 per liter ($2.50 per gallon), this translates to annual consumer savings of $27.50 to $70; assuming a vehicle lasts 10 years, total consumer cost savings range from $270 to $700. SAE International's Cooperative Research Program demonstrated in 2006 that efficiency gains of this magnitude were easily achievable with currently-available technology (United States Environmental Protection Agency 2008).

The following sections provide an overview of refrigerant alternatives, explain the status of MAC rules and regulations worldwide, and compare the extent to which they encourage LCCP.

3 Refrigerant alternatives

Alternative refrigerants that have been considered for MAC include hydrocarbons, HFC-152a, CO₂, and HFC-1234yf (also known as HFO-1234yf). Hydrocarbons are efficient refrigerants with zero ODP and low GWP ∼3 to 5. However, hydrocarbons are highly flammable, easily ignited by sparks, and are not considered a viable option by any vehicle manufacturer, particularly because lower-flammability refrigerants are available.

3.1 HFC-152a

HFC-152a is a technically and economically viable replacement for HFC-134a in mobile air conditioners. Like hydrocarbons, HFC-152a is a more efficient refrigerant than HFC-134a. It has zero ODP and a low GWP of 124. However, it is moderately flammable, with an ASHRAE 1 refrigerant classification of A2 (lowest toxicity, moderate flammability). Although it is difficult to ignite HFC-152a with high-temperature surfaces or sparks, it can be ignited by open flame or fused wires. This presents a potential safety concern in the case of vehicle crashes or in the case of refrigerant leakage into the passenger cabin.

One solution for overcoming this challenge is to use HFC-152a in a secondary loop system. As explained by the Intergovernmental Panel on Climate Change, “A secondary loop system would overcome the concern of leakage into the passenger cabin by allowing the refrigerant to be contained under the hood where it is completely separated from the passenger compartment. The refrigerant cools a chiller (liquid–liquid heat exchanger), which in turn chills a water–glycol mixture that is pumped into the passenger cabin heat exchanger for cooling.” (IPCC and TEAP 2005; p. 309).

Secondary loop systems are an engineering challenge because they introduce additional weight, complexity, and maintenance issues to the MAC system. Indeed, this added complexity also led many engineers to presume that the secondary fluid loop was environmentally undesirable: it was hypothesized that the refrigerant-to-fluid heat exchange, subsequent fluid-to-air heat exchange, added weight, and energy needed to pump chilled antifreeze would result in an overall loss of energy efficiency. Engineers also presumed that cooling performance would be unacceptable because the AC would have to cool the fluid before the chilled fluid could cool the passengers. These presumptions were invalidated in July 2003 when Volvo, Delphi, and EPA demonstrated that the fuel efficiency, cooling comfort, and economy of such a system in a mid-sized sport utility vehicle equaled or surpassed existing single-loop HFC-134a systems. (IPCC and TEAP 2005; p. 308–309).

In 2007, the United States Department of Energy's NREL estimated that AC fuel use for an HFC-152a secondary loop system on an Opel Astra could be 21% lower than a baseline primary loop HFC-134a system by taking advantage of the hither thermal ballast available in secondary loop systems. An added benefit of a secondary loop system is that it enables controls that exploit vehicle inertia to cool the secondary loop fluid and components during deceleration or when the engine is operating at high energy efficiency (and lowest tailpipe emissions). In addition, the secondary loop allows extended idle stop operation in hot weather by drawing on the thermal ballast of that cooled secondary fluid and components. Idle stop requires additional components beyond those of the secondary loop AC system. NREL has estimated that secondary loop HFC-152a systems with capacity control and extended idle stop vehicle operation can save up to 9.8 billion liters (2.6 billion gallons) of fuel per year for the United States alone if implemented in every vehicle, eliminating 23.2 million tonnes of CO₂-equivalent (CO₂-eq) per year in greenhouse gases.

The IPCC estimated that the added cost of an HFC-152a system would be 25 to 48 US dollars. (IPCC and TEAP 2005; p. 308–310).

3.2 CO₂

With a GWP of 1 by definition, CO₂ has the lowest direct GWP of all MAC refrigerant alternatives. It has no impact on ozone depletion and is non-flammable. However, CO₂ systems operate at a much higher pressure. Additionally, although CO₂ is a natural refrigerant, it has acute toxicity risks. As explained by the United States Environmental Protection Agency's significant new alternatives program: “Individuals exposed to CO₂ concentrations as low as 4–5% over a few minutes reported headache, uncomfortable breathing, and dizziness. Significant performance degradation (e.g. reaction time) was noted in pilots exposed to 5% CO₂. Individuals exposed to 6% CO₂ for periods as short as 2 min had hearing and visual disturbances, and significant reasoning and performance decrements have been observed in healthy young adults after exposures of 5 min to 7.5% CO₂.” (Federal Register 2006). Accidental CO₂-system charge releases can result in passenger-cabin concentrations exceeding the limits recommended by the US Occupational Safety and Health Administration (3% for 15 min); as a result, some regulatory authorities such as the US Environmental Protection Agency have proposed exposure limits for CO₂ used in MAC. (Federal Register 2006, 2009).

While CO₂ systems perform well and have high energy efficiency in mild to moderate climates, cooling performance and efficiency degrades in hotter and more humid climates such as those found in the southern United States, India, and China. CO₂ is also the most expensive alternative. The IPCC estimates that the additional cost of a CO₂ air conditioning system is 48 to 180 US dollars (IPCC and TEAP 2005; p. 308–310).

Until recently, CO₂ was the refrigerant of choice for Germany's automotive association Verband der Automobilindustrie (VDA), representing major automobile manufactures such as Audi, BMW, Daimler, and Volkswagen. However, in April 2009, German carmakers stopped all development plans for next-generation refrigerants (“Germans will not respect MAC Directive” 2009). More recently, VDA announced its willingness to work toward a common agreement among all vehicle manufacturers on a standard refrigerant, stating that “a standard refrigerant to be used worldwide is the only appropriate response to the challenge of achieving the greatest economic and ecological benefit.” (“Call for a Global Standard Refrigerant” 2009).

3.3 HFC-1234yf

HFC-1234yf (also called HFO-1234yf), a relatively new chemical, is a mildly flammable, ozone-safe, low GWP refrigerant. Introduced by DuPont and Honeywell in 2007, HFC-1234yf refrigerant has quickly become the leading contender to replace HFC-134a in vehicle air conditioning for several reasons: With a GWP of only 4, it has one of the lowest GWPs of any alternative refrigerant. Technically, its properties are very similar to the current refrigerant, meaning that neither the air conditioning technology nor the production lines would need to undergo significant re-design. HFC-1234yf does not present the pressure or acute toxicity risks of CO₂, nor does it present the flammability risks of hydrocarbons or HFC-152a (see Table 1: flammability properties). For example, whereas propane has a lower flammability limit (LFL) of 2.2% and HFC-152a has a LFL of 3.9%, concentrations of HFC-1234yf have to reach 6.5% by volume in air before it can ignite. Additionally, whereas propane has a minimum ignition energy (MIE) of 0.25 mJ and HFC-152a has a MIE of 0.38 mJ, it would take much more energy – over 1000 mJ – to ignite 1234yf. Practically, this means that flammability risks for HFC-1234yf are easier to contain. An independent risk analysis conducted by gradient found that the overall risk of vehicle occupants (or former occupants) being injured by HFO-1234yf ignition was 9 × 10⁻¹⁴. By comparison, the risk of an airbag-related fatality associated with a vehicle collision is 2 × 10⁻¹⁰ (Gradient 2008, 2009). Comprehensive risk assessments conducted by SAE International have determined that HFC-1234yf poses the lowest overall safety risks of any other refrigerant alternative (SAE International 2009). Since SAE International's cooperative research project members include major automobile manufacturers from Europe, Asia, and US, accounting for 70% of all new vehicle sales worldwide, these findings can be expected to have a major influence on the selection of a new refrigerant. Although HFC-1234yf is expected to be a more expensive refrigerant than HFC-134a, it would be the least costly alternative, because other options such as CO₂ or HFC-152a require total system re-design.

Table 1. HFC-1234yf flammability properties (Minor 2008).

	LFL^a (vol%)	UFL^a (vol%)	Δ (vol%)	MIE (mJ)	BV^c (cm/s)
Propane	2.2	10.0	7.8	0.25	46
R152a	3.9	16.9	13.0	0.38	23
R32	14.4	29.3	14.9	30–100^b	6.7
Ammonia	15	28	13	100–300^b	7.2
1234yf	6.5	12.3	5.8%	>1,000^b	1.5
^aFlame limits measured at 21 C, ASTM 681–01.
^bTests conducted in 12-l flask to minimize wall quenching effects.
^cBurning velocity ISO 817 (1234yf burning velocity measured by AIST, Japan).

In the United States and in Europe, use of new chemicals such as HFC-1234yf requires regulatory approval. In Europe, chemicals must be registered in accordance with the regulation on registration, evaluation, authorization and restriction of chemicals (REACH), and HFC-1234yf has been registered. In the United States, the environmental protection agency's significant new alternatives program reviews alternative refrigerants for environmental acceptability. This program has proposed listing HFC-1234yf as acceptable for use in vehicle air conditioning with use conditions to mitigate risks associated with exposure to HFC-1234yf and its decomposition products.

3.4 European union

In 2006, the European Commission issued Directive 2006/40/EC (commonly known as the “MAC Directive”), requiring vehicle air conditioners to use low-GWP refrigerants. Starting 1 January 2011, all new vehicle types sold in the EU must have an air conditioning refrigerant with a GWP equal to or less than 150. Starting 1 January 2017, all vehicles sold in the EU must have a refrigerant with a GWP of 150 or less.

This law only applies to the refrigerant; LCCP is not addressed. However, in 2008, the Commission reported that they were considering complementary efforts to improve air conditioning efficiency. The commission held a “public consultation on future regulations addressing reduction of CO₂ emissions of light-duty vehicles by more efficient MAC equipment” in the spring of 2008. 2 The consultation addressed several topics, including potential test procedures and safety regulations. As of October 2009, the commission had analyzed the results and reported that a legislative proposal is forthcoming.

3.5 The US state of California

The US state of California was the first authority to incorporate important elements of LCCP in their regulations. California's vehicle greenhouse gas standards – approved by the US EPA on 30 June 2009 after a protracted legal battle – provides incentives for manufacturers to use alternative refrigerants, reduce refrigerant leakage, and improve MAC energy efficiency. The California regulation limits the grams of CO₂-eq that a vehicle emits per mile, and it applies to 2009 and subsequent model years. The formula they proposed to determine the CO₂ emissions per mile is:

where AC allowances is the credit for reducing direct (leakage) and indirect (fuel use) emissions from MAC. The California proposal estimates that on average, vehicle air conditioning emits 6 g CO₂-eq per mile (3.75 g/km) for AC-direct (refrigerant leakage) and 17 g CO₂-eq per mile (10.5 g/km) for AC-indirect (fuel use). (California Code of Regulations 2006). Actions to reduce direct and indirect emissions that would receive credit are described in Table 2. California's work in this area is now reflected in new US national regulations.

Table 2. Credit for reducing MAC greenhouse gas emissions under California regulation.

Direct AC emissions, Maximum credit = 9 g CO2-eq per mile for low GWP refrigerant option, 6 g otherwise.	Indirect AC emissions, Maximum credit = 11 CO2-eq/mile
• Qualified “Low-Leak AC System”*	• Qualified “AC system with reduced indirect emissions”
• Refrigerant with GWP of 150 or less	• Refrigerant with GWP of 150 or less
• Alternative technologies if reductions are demonstrated to be equal or greater than measures listed above	• Alternative technologies if reductions are demonstrated to be equal or greater than measures listed above
Source: California Air Resources Board.*Additional details available from California Air Resources Board: http://www.arb.ca.gov.
See also section 3.6.1. “Credit for reducing refrigerant GHG emissions”.

California also implemented a regulation that requires disclosure of vehicle GHG and smog emissions on a window label (Environmental Performance Label). This regulation, in effect for model year 2009 and beyond, also seeks to improve LCCP. Manufacturers can apply for MAC credits to improve GHG emission ratings. Credits can be earned for reducing refrigerant leakage, using a low-GWP refrigerant, or improving MAC efficiency. For the future, California is already moving forward with its low emission vehicle III program (LEV III). It is anticipated that the new standards will include additional requirements (as opposed to credits) for MAC efficiency, refrigerant GWP and refrigerant containment, in order to guarantee the emission reductions associated with MAC. Other refrigerant measures for the in-use fleet of vehicles are also being designed and implemented, but those are beyond the scope of the present article.

3.6 United States

In 2009, the United States took important steps to regulate greenhouse gas emissions. Activities pertinent to vehicle air conditioning include the US EPA's endangerment finding on greenhouse gasses, the approval of California's request to regulate vehicle greenhouse gas emissions, and newly-announced national auto standards that will impose the first-ever US greenhouse gas emissions standards on cars and trucks.

The “endangerment finding” is a formal step that must be taken before the US EPA regulates a pollutant. On 12 April, 2009, EPA Administrator Lisa P. Jackson signed the proposed endangerment and cause or contribute findings for greenhouse gases under the clean air act, which found that greenhouse gases threaten the public health and welfare of current and future generations, and that emissions of CO₂, CH₄, N₂O, and HFCs from motor vehicles contribute to the atmospheric concentrations of these key greenhouse gases and hence to the threat of climate change. An endangerment finding forms the basis for regulating GHG emissions (United States Environmental Protection Agency 2009).

On 30 June, 2009, the US EPA took another important step in regulating GHG emissions by approving the state of California's request for permission to regulate vehicle greenhouse gas emissions. Under the United States Clean Air Act, the US EPA can allow California to adopt its own emission standards for motor vehicles due to the seriousness of the state's air pollution challenges. In December 2005, California requested the right to control greenhouse gas emissions from motor vehicles. The request was denied by then-EPA Administrator Stephen Johnson on 6 March, 2008. However, when President Obama took office, he instructed the US EPA to reconsider the waiver, which was then approved.

Finally, in May 2009, President Obama announced new national auto standards that will accelerate increases in auto fuel economy and impose the first-ever national greenhouse gas emission standard on cars and trucks. (“President Obama Announces National Fuel Efficiency Policy” 2009). Cars and light trucks will have to achieve a corporate average fuel economy (CAFE) standard of 35.5 miles per gallon (∼16 km per liter) by 2016, and emit not more than an average of 250 g of CO₂ per mile (∼156 g/km). These controls include incentives for vehicle manufacturers to reduce GHG emissions associated with vehicle AC.

On 15 September, 2009, the US EPA and the US Department of Transportation's National Highway Traffic and Safety Administration (NHTSA) followed up on the President's announcement, releasing proposed rules for public comment. 3 Like California's regulation, the US EPA proposed to give vehicle manufactures credit for MAC improvements that reduce GHG emissions. However, the Federal rules give more credit for reducing refrigerant-related GHG emissions and less credit for improving energy efficiency: cars can earn up to 13.8 g CO₂-eq per mile (8.6 g/km) for switching to a low-GWP refrigerant (the number increases to 17.2 g of CO₂-eq per mile for trucks), and up to 5.7 g of CO₂-eq per mile (∼3.5 g/km) for improving AC energy efficiency.

3.6.1 Credit for reducing refrigerant GHG emissions

Under the US EPA's proposed rules, the formula used to determine credit for reducing refrigerant GHG emissions is:

where

	Max Credit is 12.6 and 15.7 g CO2-eq per mile CO2-eq for cars and trucks, respectively. These become 13.8 and 17.2 g CO2-eq per mile for cars and trucks if alternative refrigerants are used.
	Leak Score is the leakage score of the AC system. This can be calculated using SAE 4 International's J2727 standard for estimating refrigerant emissions. The minimum score which is deemed feasible is fixed at 8.3 and 10.4 g/year for cars and trucks, respectively.
	Avg Impact is the average impact of AC leakage, which is 16.6 and 20.7 g/year for cars and trucks, respectively.
	GWP Refrigerant is the GWP for direct radiative forcing of the refrigerant as defined by EPA (or IPCC).

For example, credits for the lowest-leak HFC-134a car (currently 8.3 g/yr) would be calculated as follows:

Vehicles using a low-GWP refrigerant would earn more credit. For example, credits for a car that used HFC-1234yf (GWP = 4) and leaked 16.6 g/year (the current average) would be calculated as follows:

3.6.2 Credit for improving MAC energy efficiency

The US EPA acknowledges that improvements in MAC energy efficiency are critical, particularly because “while leakage may disappear as a significant source of GHG emissions if a shift toward alternate refrigerants develops, no parallel factor exists in the case of efficiency improvements.” (Federal Register, Vol. 74, No. 186, p. 49529). Although EPA would prefer a performance-based test procedure, it is proposing a design-based “menu” approach for now. Vehicle manufacturers could earn up to 5.7 g of CO₂-eq per mile (∼3.6 g/km) by implementing the technologies in Table 3 (note that although the total of the individual technology credit values may exceed 5.7 g of CO₂-eq per mile, synergies among the technologies mean that the values are not additive, and thus AC efficiency credit could not exceed 5.7 g of CO₂-eq per mile.). By 2014, the US EPA hopes to have a performance-based test in place to determine whether a vehicle's air conditioning system qualifies for credits.

Table 3. Credits for improving vehicle air conditioning efficiency.

Technology description	Estimated reduction in AC CO2 emissions (%)	Credit (g/CO2-eq per mile)
Reduced reheat, with externally-controlled, variable-displacement compressor	30	1.7
Reduced reheat, with externally-controlled, fixed-displacement or pneumatic variable-displacement compressor	20	1.1
Default to recirculated air whenever ambient temperature is greater than 75°F	30	1.7
Blower motor and cooling fan controls which limit waste energy (e.g. pulse width modulated power controller)	15	0.9
Electronic expansion valve	20	1.1
Improved evaporators and condensers (with system analysis on each component indicating a COP improvement greater than 10%, when compared to previous design)	20	1.1
Oil separator	10	0.6

Box 1. How life-cycle climate change performance is calculated for mobile air conditioning.

Life-cycle climate change performance provides a holistic approach in estimating all direct and indirect greenhouse gas contributions emitted during the lifetime of a refrigerant and a mobile air conditioning system. Direct emissions are an aggregate of the following leakage categories:

• Regular emissions, which refer to the refrigerant leaks or permeation from the AC system.

• Irregular emissions due to accidents, stone hits, product defects, etc.

• Service emissions from maintenance and repair.

• End-of-life emissions during dismantling or disposal of the vehicle at its end of life.

• Leakage during refrigerant production and transportation, and

• Atmospheric reaction products associated with the atmospheric breakdown of the refrigerants.

Indirect emissions result from the energy consumption due to MAC manufacturing, and operation, and end-of-life and is an aggregate of the following CO2 emissions categories:

• Fuel use emissions associated with manufacturing and end-of-life recycling processes of refrigerant.

• Fuel use emissions associated with manufacturing and end-of-life recycling processes of each component of the AC system.

• Fuel use emissions associated with the AC operation (such as those associated with the operation of the compressor and engine cooling fan) during the lifetime of the vehicle, and

• Emissions associated with the additional fuel consumption due to the AC mass transportation onboard the vehicle throughout the lifetime of the vehicle.

To date, the most comprehensive LCCP model for MAC systems is the global refrigerants energy and environmental mobile air conditioning life-cycle climate change performance (GREEN-MAC-LCCP) model, a transparent, globally peer-reviewed tool that is available free to the public at www.epa.gov/mac. The GREEN MAC LCCP has determined that HFC-1234yf systems would lead to the lowest overall GHG emissions compared to alternatives CO2 and secondary-loop HFC-152a.

4 Conclusion

MAC has a significant and growing impact on the earth's climate, but new technologies and refrigerants are available that can deliver significant and cost-effective greenhouse gas emission reductions. Advances in vehicle air conditioning technology have made previously impractical refrigerants feasible, and chemical companies have developed new low-GWP refrigerants that can dramatically reduce the climate impacts of refrigerant emissions. Of these, HFC-1234yf has emerged as a leading candidate. As alternatives are implemented, policymakers can optimize consumer savings and climate benefits by promoting LCCP. For the climate, a low-GWP refrigerant alone will not guarantee greenhouse gas savings; fuel consumption must also be taken into account. For the consumer, life-cycle policies that result in improved air conditioning fuel efficiency can save a typical vehicle owner hundreds of dollars over the life of their vehicle. Air conditioning systems designed for low leak rates can save additional money with improved reliability and less frequent maintenance. Several governments have created policies to encourage vehicle manufacturers to switch to low-GWP refrigerants; however, they have only recently started to take air conditioning fuel consumption into account. The US state of California took the lead developing the first comprehensive regulation addressing both AC fuel efficiency and refrigerant greenhouse gas impacts, and the US federal government is following suit. If a global transition to a low-GWP refrigerant is successful, avoided annual refrigerant greenhouse gas emissions would be up to 250 million tonnes of CO₂-eq by 2015; if coupled with fuel efficiency increases; this figure will be even greater.

Notes

1. ASHRAE, American society of heating, refrigeration, and air conditioning engineers.

2. Results of the consultation were published on-line at: http://ec.europa.eu/enterprise/automotive/environment/mac/consultation/contributions.htm

3. These rules are available on-line at: http://edocket.access.gpo.gov/2009/pdf/E9-22516.pdf. Comments are being accepted until November 27, 2009.

4. Formerly the society of automotive engineers.