Since 2004, the automotive industry evaluated a number of fluorocarbon refrigerants as alternatives to a conventional HFC-134a systems, in addition to continuing the evaluation of trans-critical CO2 systems. The evaluations have been undertaken through a range of co-operative research programmes, such as SAE-CRPs and the B-COOL project, funded by the European Union.
It is necessary for any new refrigerant and 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 HFC-134a systems, 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 the development of electric vehicles that might require battery cooling and the provision of additional passenger heating. Heat pump systems are being considered to provide heating and cooling, but demisting capability needs to be considered. However the system becomes more complex, as do environmental and performance comparisons.
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.
The phase-out of HFC 134a in new vehicles will maintain comparable energy consumption due to the availability of fluorocarbon refrigerant HFO-1234yf, but considerably reduce refrigerant emissions due to its low GWP. In comparison, HFC-134a has a GWP of 1300 (Third Assessment Report value referenced in the F-Gas Regulation).
HFO 1234yf has very similar thermophysical properties to HFC 134a allowing the use of modified HFC 134a air-conditioning systems with comparable energy efficiency in warm and hot climates, in contrast to CO2 (R-744). The energy efficiency of the transcritical CO2 cycle is very sensitive to ambient temperature: the energy efficiency of HFO based refrigerants is therefore expected to be better in warm climates than CO2 as a consequence. With the lower GWP offered by use of HFOs, the overall carbon footprint of vehicle air-conditioning will be dominated by the energy efficiency of the system1 . The CRP-1234 consortium has thoroughly investigated the safe use of HFO-1234yf for car air-conditioning. See also EFCTC position on HFOs and HFO Fact sheet.
Car companies and component manufacturers 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 had to be designed and tested. Several companies initially made announcements on the introduction of trans-critical CO2 systems. However the challenging technical and reliability issues, and the poor energy performance in warm and hot climates in comparison to HFC-134a and a new low GWP fluorocarbon refrigerant HFO 1234yf has resulted car companies selecting HFO 1234yf to replace HFC 134a.
By 2011 car companies in Europe, USA and Japan have announced the use of HFO 1234yf due to its performance and environmental performance in comparison to HFC 134a and trans-critical CO2.
The car companies and air-conditioning manufacturers have undertaken comparisons of a number of types of systems and refrigerants. 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.
Factors that influence the comparison are the GWP and leakage rate for the refrigerant (direct effect), and energy efficiency in a range climatic conditions (energy effect). The efficiency of trans-critical CO2 systems decreases much more rapidly in hot conditions. In its review of Options for Future Mobile Air Conditioning Systems the TEAP report comments ‘With GWPs less than 150 energy use dominates.’2
The LCCP (Life Cycle Climate Performance) is used an objective comparison for refrigerants, for a range of climatic conditions and drive cycles. A well established LCCP comparison tool is the GREEN-MAC-LCCP Model™ (available at http://www.epa.gov/cpd/mac/compare.htm) , and guidance is available in a standard SAE J2766 Life Cycle Analysis to Estimate the CO2 - Equivalent Emissions from MAC Operation.
Even allowing for its GWP of 1300, comparisons have shown that the LCCP of a trans-critical CO2 systems is better than that of the HFC-134a baseline only in cold climates3. HFO 1234yf, with a GWP of 4, is better than transcritical CO2 in a very wide range of climates, and globally significantly better than trans-critical CO2.