Air conditioning has become standard equipment on most vehicles, enhancing traveling comfort and safety. Modern systems feature integrated cooling, heating, demisting and defrosting, air filtering, and humidity control.

MAC (Mobile air conditioning) improves:

• passenger comfort and driver alertness when driving in high ambient temperatures
• driver's visibility and passengers' security by de-misting and de-fogging windows during rainy or humid weather.

Energy-efficient air conditioning is essential for energy efficient vehicles and new concepts, like hermetic refrigerant cooling systems, are being developed. Keeping windows closed by using air conditioning during hot weather also saves energy, while preserving driver alertness.

There are a wide range of factors that affect the global warming contribution of a car, many of which result from how the car is used in practice. The current design of mobile air-conditioning systems require a non-flammable, low toxicity refrigerant and HFC-134a is the refrigerant of choice for passenger cars, trucks, trains, and buses. State of the art system design has resulted in improved efficiency, reduced refrigerant charge and minimum refrigerant leakage, dramatically improving the environmental performance of mobile air-conditioning.Car manufacturers are taken for the future direction for car air-conditioning.

Car manufacturers continue to evaluate and improve HFC-134a systems and alternatives, but considerable work is required to ensure that robust and objective decisions are taken for the future direction for car air-conditioning.

The MAC debate

The EU Commission DG Environment organized in 2003 a conference "Options to Reduce Greenhouse Gas Emissions due to Mobile Air Conditioning" (Brussels, 10-11 February 2003)

EFCTC issued a position paper and a response to the consultation.

Position paper: EFCTC Response to DG Environment Consultation Paper on MAC (03/2003)

Position paper: EFCTC position on European Commission consultation paper on mobile air conditioners (MAC) (02/2003)

Further information on the EU MAC Regulation on the “Regulatory developments”

Research continues to further improve HFC-134a MAC technologies, in order to define and implement so-called “enhanced HFC-134a” systems. (See the Press release: Fluorocarbons (HFCs) in Mobile Air Conditioners (MAC): a full assessment of performances and emissions data confirm their sustainability, April 22-24, Washington DC, USA) 

The use of an HFC (HFC-134a) in car air-conditioning is well established, with MAC installation in around 80% of cars in USA and Japan for many years. In the EU the penetration of a/c systems in cars was previously low, about 10%, but will reach 80% in the next few years.

The availability of HFC-134a for car MAC systems enabled a rapid switch from CFCs (CFC 12) using MAC systems of essentially the same design and high level of safety. Other refrigerants did not offer this possibility. Based on these factors, it is clear that car companies made the appropriate choice when switching from CFC 12.

The use of HFC-134a has contributed to an enormous reduction in the environmental impact of individual cars with MAC systems:

• Eliminating emissions of ozone depleting CFCs
• Reducing the global warming of the car MAC system by about 92%, based on the elimination of CFC emissions
• Reducing the global warming of the car in total by about 45%, based on the elimination of CFC emissions

Reduced Refrigerant Emissions

During the switch to HFC-134a major efforts were made to reduce refrigerant loss to atmosphere, which are today considerably less than those for an equivalent CFC 12 system in 1990. Improvements have been made in leakage rates from the system through better assembly, inspection, and component design. Reduction in refrigerant loss has also been achieved through better recovery and recycle procedures adopted by service engineers. The end-of-life vehicle directive requires recovery of HFC-134a when the vehicle is scrapped.

Impact of increasing MAC installation rates

The overall contribution of emissions from HFC-134a car MAC systems to global warming, even at an 80% market penetration rate, is less than the contribution of CFC 12 systems at a 10% market penetration rate, which was the approximate rate for the EU in 1990.

Car companies are evaluating a range of options for future MAC systems to improve the overall performance. These include enhanced HFC-134a systems, and trans-critical CO₂ systems.

In addition improvements in thermal management of the passenger compartment are being investigated. The objective is to reduce the thermal load, so that less energy is required for the MAC system, potentially a smaller MAC system, and reduced fuel consumption due to the MAC system.
Like other car component developments performance, reliability and cost need to be established.

Enhanced HFC-134a MAC Systems

Car companies and air-conditioning designers are working to improve the overall performance of HFC-134a system. Reducing refrigerant loss is being achieved through reduced leakage from improved component design, assembly and inspection, and also from reducing the refrigerant charge. Such measures, with the end-of-life vehicle directive could potentially reduce refrigerant loss to approximately 2 to 3% of the overall global warming of a car (from about 4% currently in the EU). To put this in context, the refrigerant emissions (direct effect) from an enhanced HFC-134a system would be similar to the fuel used from always carrying the weight of a full tank of fuel.

It may be possible to cost effectively improve the efficiency of HFC-134a systems using enhanced component designs. The impact of an improved efficiency HFC-134a system (30% less energy) on TEWI has been discussed in a recent report . The combination of reduced refrigerant loss and improved efficiency could significantly reduce the global warming contribution from an enhanced HFC-134a air-conditioning system.

Completely Redesigned MAC Systems

The automotive industry is actively evaluating a number of alternatives to a conventional HFC-134a system. It is necessary for any new system to meet requirements for efficiency, environmental impact, reliability, safety and cost. Extensive testing and evaluation is necessary to ensure that overall performance, safety and reliability are fully characterized prior to any decisions being taken to introduce new technology. This is not a simple task for the automotive industry given that the air-conditioning system has to perform in a wide range of climatic conditions. Environmental performance, in comparison to a HFC-134a system, can vary considerably for different climatic conditions. The automotive industry is a global industry, and overall global environmental performance may be different to regional environmental performance due to differences in average climatic conditions.

In addition, car manufacturers need to consider other technical developments that may impact on the MAC system. For example high efficiency diesel engines produce very little waste heat (which is normally used for heating the passenger compartment). There are a number of options being considered for providing additional heating. A small auxiliary diesel heater could be used with a conventional MAC system. Alternatively a heat pump system could be used providing heating and cooling, but demisting capability needs to be considered. However environmental and performance comparisons become more complex.

Trans-Critical CO₂ System

Car companies and component manufacturers have invested considerable research and development activity assessing the use of CO₂ as the refrigerant for car MAC systems. The extremely high system pressures and the specific technical requirements for the efficient operation of a trans-critical CO₂ system dictate that a completely new system has had to be designed and tested.

Several companies have made announcements on the introduction of trans-critical CO₂ systems, however there appear to be a number of technical and reliability issues that are yet to be completely resolved. In addition it is well accepted that trans-critical CO₂ systems will be significantly more expensive than HFC-134a systems. One application where trans-critical CO₂ systems may have advantages is for heat pumps for the heating mode. This adds additional complexity with the issues of demisting in the heating mode and icing of the evaporator (in heat pump mode) needing to be considered. At this stage it is not clear whether a heat pump or auxiliary heater is preferred.

Environmental Comparison of HFC-134a and trans-critical CO₂

The car companies and air-conditioning manufacturers have undertaken comparisons of a number of types of system including standard HFC-134a, enhanced HFC-134a and trans-critical CO₂. Much of this work has been carried out in consortia such as the Alternative Refrigerant Cooperative Research Project, which allows a more rapid assessment of new technologies.

Even after extensive work, there remains a range of views as to the relative performance for HFC-134a and trans-critical CO₂. Two key factors that directly influence the comparison are the leakage rate for HFC-134a (direct effect), and the climatic conditions (energy effect). The efficiency of trans-critical CO₂ systems decreases much more rapidly in hot conditions.
In 2002 it was reported ‘that for North America, where relatively more miles are driven annually in a warmer climate, the energy savings of an HFC-134a based system compared to CO₂ and the associated reduced indirect warming more than offset the direct warming of refrigerant emissions, making HFC-134a the best LCCP [Life Cycle Climate Performance] alternative’.

Since then, enhanced HFC-134a systems have been developed, which have a significantly better LCCP in warm and hot climates, which is where the majority of the global population of car air conditioners are located.

The way fordward

Car manufacturers continue to evaluate and improve HFC-134a systems and alternatives, but considerable work is required to ensure that robust and objective decisions are taken for the future direction for car air-conditioning.

An Improved Mobile Air Conditioning Cooperative Research Program ("I-MAC" CRP) has been set up by the Society of Automotive Engineers (SAE), with the goal of identifying advanced cooling technologies that can decrease the environmental impact of HFC-134a in MAC, by reducing refrigerant leakage and providing greater system energy efficiency.

The SAE I-MAC will seek to identify advanced systems and components that can:

• Reduce direct system refrigerant leakage by at least 50 percent
• Improve system efficiency by at least 30 percent
• Reduce system cooling loads by at least 30 percent
• Reduce service refrigerant losses by at least 50 percent.

The estimated cost for the two-year project is $3 million, contributed by the U.S. EPA, car and equipment manufacturers and refrigerant suppliers.