Vehicle air conditioning 10.1 Introduction

10 Vehicle air conditioning

10.1 Introduction

In this chapter, mobile air conditioning systems are those used in passenger cars, light duty trucks, buses and rail vehicles that are generally based on vapour compression cycles. This chapter also addresses the heat pump mode of the air conditioning cycle, for example systems used for heating battery-driven electric vehicles (BEVs). Annex C (Not-In-Kind Alternatives) provides a brief overview of other refrigeration techniques which might become important for vehicle air conditioning systems in the future (beyond 2020).

Chapter 10 covers the new developments in the field of mobile air conditioning systems since the RTOC 2010 Report. For more details on the system design and the history of refrigerant system development for these vehicles prior to 2010, see that and all preceding RTOC reports.

It should be mentioned that the development in this field is strongly driven by legal guidelines and regulations (probably much stronger than in other fields). So, Annex B provides a partial overview of the legal situation worldwide.

The ozone depletion potential (ODP) and global warming potential (GWP) values of the refrigerants mentioned in this chapter are given in chapter 2 of this report.

10.2 Description of systems and current and future applications of mobile air conditioning systems

HFC-134a systems have become increasingly more leak tight taking benefit in part from the technology developed in the study of R-744 (carbon dioxide) and in part from regional leak reduction programs, e.g., US light-duty vehicle regulations, the Improved Mobile Air Conditioning (I-MAC) program, etc. Correspondingly, hose materials and coupling designs for hoses have also been improved in a relevant way. In addition to that, compared to earlier designs, HFC-134a systems today show improved energy efficiency and lower fuel consumption to meet new regulatory requirements in the USA (USEPA, 2011b) and driven by increased awareness of fuel consumption of mobile air conditioners (MACs) in the EU.

1. Refrigerant cost and recharge

The rise in counterfeiting is probably due to production and use restrictions on some refrigerants in accordance with the 1987 Montreal Protocol and the 1997 Kyoto Protocol and to the higher cost associated with more environmentally friendly refrigerants than what they are replacing (Velders, 2009). Hence, counterfeited HFC-134a is infiltrating the refrigerant bank in Asia, and affecting also other countries.

Today, even if the relative cost of HFC-134a is low, i.e. 5 US$/lb or 11 US$/kg (or even less), counterfeit HFC-134a is still appearing in the automotive market. Counterfeit HFC-134a containing multiple CFC, toxic, or corrosive components is a serious threat destroying equipment and injuring end-users. It also has an increasing opportunity to enter the supply chain. The issue is even more relevant in non-European countries (Coll, 2012).

So, in case that a drop-in or nearly drop-in but more expensive refrigerant will be chosen to replace HFC-134a worldwide, the counterfeit risk will become even more relevant and this, beside all the other impacts, could strongly reduce the expected environmental benefit.

The commercial expected price of HFC-1234yf (Weissler, 2013) is estimated at US$ 40-45/lb or US$ 88-99/kg (retail prices could be 2 to 3 times higher). The expected price of R-445A probably will be lower but there exist questions that have been raised on patent issues. Cost and availability are crucial issues that have to be considered in identifying replacement(s) for HFC-134a, currently widely adopted in car air conditioning systems.

2. Future Trends

This drives to more efficient on-board systems including MACs and to the replacement of high GWP fluid with lower GWP substances.

In this framework the thermal systems and especially the MAC systems have a relevant role and will likely undergo a deep change where the system integration will represent a relevant evolution guideline.

It is expected that the passenger cars and light duty vehicle domain will rapidly move toward downsized turbocharged engines often coupled with different levels of hybridization to make the overall powertrain more efficient and to reduce CO₂ emissions, with effects on MACs as described below. Increased levels of transmission gears are also occurring, moving vehicles from 4/5/6 levels to 7/8/9 levels leading to a reduction of average compressor speeds so implying AC capacity concerns at low speeds in some cases or to the increase of the compressor displacement or finally to the adoption of electrically driven systems.

To answer to the increased demand of capacity and reduction of GHG emission, the car air conditioning systems using HFC-134a have to become increasingly more leak tight and efficient.

The adoption of improved components (compressor and heat exchangers) and the introduction of internal heat exchangers as well as improved control strategies allowed a significant increase of system efficiencies.

New sealing concepts and improved hose materials have also been developed to reduce refrigerant direct emissions and to reduce the frequency of required maintenance or service.

The trend in the MACS service profiles over the last 10 years (2003-2013) indicates that with reduced refrigerant emissions, the system is being serviced later in the vehicle’s life, indicating more years of useful life from the system. With this extended lifetime, service is more likely to be purchased by the vehicle’s second owner as compared to previous surveys of earlier vehicles (Atkinson, 2014).

Stop & Start and Hybrid Vehicles

The diffusion of vehicles able to carry out part of their mission with the combustion engine off (e.g. Stop & Start, extended Stop & Start, and hybrids) asks for new solutions for the air conditioning system to guarantee the summer and winter thermal comfort in all the operational conditions.

The majority of these vehicles will have 12 V to 48 V electric energy sources and only part of them will have higher voltage network (e.g. up to 350 V), while all will have an additional on-board electric energy storage unit with a capacity ranging from 0.2 kWh (low voltage) up to 5 kWh (high voltage).

This implies that only a small portion of the future vehicles will have an electric compressor while a large part will be equipped with mechanically driven compressors as today due to energy balance and cost, so measures will be adopted to maintain the required comfort and guarantee the safety performance (i.e. de-fogging), as for example:

 cooling energy storage unit based on phase change materials

 secondary loop system taking benefit from its thermal inertia to store cooling power

 additional electric compressor, downsizing the belt-driven compressor

The increase of on-board electric power and the diffusion of turbocharged engines together with the need to at least maintain the aerodynamic drag will presumably lead to a low temperature cooling loop integrating the charge air cooler, air conditioning condenser, power electronics and generator in the case of hybrid powertrain.

In synthesis, the evolution will produce a deeper integration of the on-board thermal systems and the air conditioning will become part of it.

Plug-in Hybrids and Battery-driven Electric Vehicles

For Plug-in Hybrids (PHEV) and Battery-driven electric vehicles (BEV), vehicle air conditioning systems for cooling as well as heat pump systems for heating need to have very high energy efficiency to minimize the impact on the vehicle driving range.

MAC systems, when operating as a heat pump with HFC-refrigerants currently used can take benefit of the on-board outdoor heat sources (battery, power electronics, etc.) to enhance their effectiveness in case of very low ambient temperatures partially compensating for their low efficiency and capacity. This can be achieved adopting a secondary loop that by collecting the heat from the on-board electronics can raise the temperature of the heat source. In this framework, dual loop systems (with liquid cooled condensers and liquid heated evaporators) offer the highest flexibility level and at the same time allow the OEM to minimize the refrigerant charge, the leak rate, and the risk of dispersion in case of an accident. However, these secondary loop systems do increase the vehicle mass due to the additional coolant and components which might adversely affect vehicle fuel economy at all times of vehicle usage.

For some applications the MAC system will be also used for battery thermal control as well as power electronics (i.e. cooling and heating).

The need to progressively increase the energy efficiency requires the exploitation of all available waste energies, heat included. The application of heat driven refrigeration systems (e.g. adsorption, absorption, etc.) can be foreseen for the heavy duty vehicle domain (e.g. trucks and coaches) which often operate at near constant speed (highway) and where the weight and packaging constraints are less severe than in the passenger cars field.

The adoption of a Rankine Cycle to convert part of the combustion waste heat of a thermal engine into mechanical or electric energy could allow manufacturers to increase dramatically the vehicle energy efficiency (see section 5.1.6 of this report, and for example Horst et al. 2014 or Hartmann 2014).

These systems are under investigation for diesel heavy duty truck applications in Europe and the US and for gasoline light trucks in the US.

As a working fluid, the Rankine cycle can use water, ethanol or a mixture of both, or an organic fluid like HFC-245fa, hydrocarbons, or carbon dioxide.

The fluid HFC-245fa has good properties for the application but has a GWP of 1050, higher than the threshold of the MAC Directive in Europe.

Studies are ongoing to evaluate the application of fluids with similar properties but with a lower GWP, as for example HCFC-1233zd(E) and HFC-1336mzz(Z) (Kontomaris, 2014).