Other mobile air-conditioning systems

In 2008, emissions of the HFC refrigerant R 134a by the following applications totalled:

trucks 151 t, buses 94 t, agricultural machinery 60 t, rail vehicles 14 t and air-conditioning units in ships 23 t [Schwarz 2010].

Buses (long-distance coaches, interurban buses, city buses)

In Germany today, 100% of all new long-distance coaches delivered are equipped with air-conditioning. In the case of interurban buses, which are used for regional services on weekdays and for tourist traffic at weekends, about 73% of vehicles are delivered with air-conditioning systems. To create incentives to use public transport, city buses are also increasingly being equipped with air-conditioning. Whereas in 1993 only 5% of new city buses had air-conditioning, the figure rose to 40% in 2002 and reached 64% by 2008. The refrigerant used is almost exclusively R 134a [Schwarz 2004; Schwarz 2010; Schwarz 2010a].

For years now, air-conditioning systems in buses have continued to use the vapour-compression process. In most cases they use standard refrigeration components. Different concepts are employed for air-conditioning these vehicles: whereas long-distance coaches are usually equipped with more or less complex split air-conditioning systems¹⁷

The refrigeration capacity is between 12 and 30 kW (split systems) or between 18 and 24 kW (roof-mounted compact systems) [Mayer 1998]. The systems are designed in such a way that in the cooling mode they can maintain a temperature difference of 2-3 Kelvin below ambient temperature in the case of city and interurban buses, or 5-8 Kelvin in the case of long-distance coaches. Controllable fresh-air operation (up to 100%) is generally standard. In addition, nearly all systems are fitted with water-cooled heating registers, which permits heating of the vehicle as well [Mayer 1997].

, in the case of city and interurban buses there has from the start been a trend to equipping them with simpler and hence cheaper roof-mounted compact systems. With the exception of the evaporator, these combine all components necessary for air-conditioning in a single unit. The compressor, which is frequently power-controlled, receives its power direct from the vehicle engine via a V-belt and an electromagnetic clutch [Mayer 1997]. Since the introduction of the refrigerant R 134a (from about 1995 onwards), the systems have generally used fixed pipes, the only flexible hose connection being to the compressor.

The average annual leakage rate of new bus air-conditioning systems is 13.3% of refrigerant for long-distance coaches and 13.7% for scheduled-service buses, according to a study on behalf of the European Commission [Schwarz 2007b].

In the bus sector there are two possibilities for minimising emissions and reducing costs. One is to use a liquid chilling package that can also be operated with natural refrigerants (e.g.

propane, CO₂), the other is to use the natural refrigerant CO₂ (R 744) in the usual bus

conditioner types (roof-mounted system) [Mayer 1998; Kirsamer 1999: in Schwarz, Leisewitz 1999; Sonnekalb 2002].

Compact air-conditioning system with secondary circuit (liquid chilling package)

The heart of the liquid chilling package is the so-called “energy station” at the back of the bus, a compact assembly consisting of a plate evaporator, standard condenser, all necessary valve groups, the water equalising tank, and the auxiliary heating system. In the passenger compartment, water heat exchangers are used instead of evaporators [Mayer 1998].

Unlike the roof-mounted system driven by the rear engine, with more than 10 kg of circulating refrigerant, the primary circuit needs only a few kilograms of refrigerant. In this way the refrigerant charge was reduced from 12 kg to 7 kg in a quarter of all long-distance coaches [Schwarz 2004]. The small assembly as supplied to the bus manufacturer is fully pre-assembled, tested and filled with refrigerant. As well as the marked reduction in refrigerant charge, low leakage rates, better control convenience and reduced maintenance result in considerable cost savings for the bus manufacturer combined with improvements in performance. The result is even more favourable if the system is installed outside the passenger compartment and natural refrigerants such as propane or CO₂ are used in the primary circuit.

Roof-mounted air-conditioning system with CO₂

In 1996 the first bus worldwide with an air-conditioning system based on CO₂ went into public transport in Bad Hersfeld. One year later a second bus was put into regular service. The long-term test in city traffic – more than 2,000 operating hours – showed that the CO₂ air-conditioning system developed by Konvekta AG, with commercially available open compressors, worked well and was very reliable. Comparative measurements on a city bus that was identical apart from the refrigerant used (with the necessary modifications) revealed no differences with regard to refrigeration capacity, efficiency and dynamic behaviour.

Konvekta AG, one of the market leaders in the bus air-conditioner sector, continued to optimise the CO₂ system. Today several city buses are equipped with CO₂ air-conditioning systems. Since mid 2009 the BVG has been running a city bus with a CO₂ system, and six more buses went into service in mid 2010 [BVG 2010]. Saar-Pfalz-Bus, a subsidiary of Deutsche Bahn, has one vehicle in service with a CO₂ system, and it is due to be joined by five more [BReg 2010]. Instead of 10 kg of R 134a, the system needs only 5-6 kg of CO₂ as refrigerant. By converting from R 134a to CO₂ it is possible to save about 30% of the direct and indirect greenhouse gas emissions. A reduction in the cost of CO₂ systems can be expected if the air-conditioning systems are installed by the bus manufacturers themselves, in which case the cost would be about 20% above the price for present-day R 134a systems. As a result of the reduction in operating and maintenance costs, the payback period for a CO₂ system works out at only 4 to 5 years [Sonnekalb 2002; Sonnekalb 2003; Konvekta 2010].

A concept for a compact roof-mounted system with a 24-volt electrically powered R 744 refrigerant compressor has also been proposed for city buses [Rindsfüßer 2008]. All refrigeration components are included in the assembly, the air management system is

in city services. The system is hermetically sealed to a very large extent, which reduces maintenance work. To date the concept has not been introduced, partly because of the lack of suitable R 744 components.

Combination of cooling and heating

A new possibility for buses is to use an air-conditioning system with a heat-pump function, which improves the efficiency compared with conventional heating systems using burners or electric heaters. Tests have already been carried out and documented [Sonnekalb 2009]. In summer 2010 a city bus will be equipped with a CO₂ air-conditioning system with heat-pump function [Konvekta 2010].

Truck cabs and other drivers’ cabs

The technical development of air-conditioners for trucks cabs or drivers’ cabs on utility vehicles (such as construction or agricultural vehicles) is closely connected with developments in the car and bus sectors. When phasing out CFCs, the manufacturers developed, with a slight time-lag, modified solutions for specialised applications in trucks and utility vehicles, using the same technology as for cars. The means that in the case of trucks and utility vehicles any phase-out of R 134a and introduction of CO₂ systems, which is technically possible, is not likely to take place until this has been done for cars, once the necessary technologies have proved successful and the components are available on the market at affordable prices.

With the growing demands on drivers with regard to concentration and reaction capacity, the percentage of new vehicles fitted with air-conditioning systems has increased in recent years.

In 2008, 45% of small commercial vehicles (< 1.5 t), 43% for medium-sized and 85% of large commercial vehicles (> 7.5 t) were equipped with air-conditioning [Schwarz 2010; Schwarz 2010a]. The refrigerant charges are between 700 g and 1,450 g, i.e. an average of around 1 kg, and are thus slightly larger than in cars [Schwarz 2004; Schwarz 2007a]. The average annual leakage rate, taking account of irregular emissions, is put at 11% of the refrigerant charge [Schwarz 2007a].

Rail vehicles

Air-conditioning in the railway sector has a long tradition. Until the 1990s, mainly long-distance trains in Germany were equipped with conditioning systems. Today air-conditioning is usual in local trains, suburban trains and new trams even in regions with a temperate climate. In underground trains in Germany, only the driver’s cab has been air-conditioned to date. The local transport sector uses air-conditioning systems that have proved successful in buses. Today’s air-conditioners are preferably executed as compact systems and combine a number of functions: cooling, ventilation, heating and - in high-speed trains - pressure protection. The units and/or components can be located in the roof, under the floor or inside the vehicle structure [Adolph 1998].

The cooling or heating capacity of the individual system depends on the ambient thermal conditions and the requirements for passenger comfort, the insulation of the vehicle, the

electrical consumer load in the air-conditioned zone, such as lighting, fan motors or control devices [Adolph 1998].

In Central Europe the refrigeration capacity needed for about 70 passengers per railway carriage is about 25-35 kW, of which about 10 kW is required for dehumidification. Drivers’

cabs and power cars need a refrigeration capacity of about 5-10 kW because of the higher fresh-air rate required, the large windscreens used in many cases, and the waste heat from numerous additional electrical control devices [Adolph 1998]. Heat insulation is generally good in trains, whereas the level of insulation in buses and trams has fallen because of the light-weight design and large windows.

After the phase-out of the CFC refrigerants R 12 (in Germany) and R 22 (southern Europe), which were used in the early phases of railway air-conditioning, only R 134a is now used in Germany. In southern Europe the HFC blend R 407C¹⁸

Air-conditioning systems in railway passenger cars of local or long-distance trains and ICE (high-speed) trailers contain an average of 18 kg of refrigerant, commuter trains 10 kg and driver’s cabs and power cars 2.2 kg [Schwarz 2004]. After 1995 the total quantity of R 134a in railway air-conditioning systems displayed a nearly tenfold increase to reach 224 t in 2008 [Schwarz 2010]. Hermetic or semi-hermetic systems are generally used for air-conditioning in trains. The annual emission rate of new railcar air-conditioning systems is estimated at 5%.

Units with open compressors driven directly by the diesel engine, known as diesel-driven multiple units (DMU), have an annual emission rate of 10% [Schwarz 2007]. The emission rates of converted older systems are about 25% higher [Schwarz 2004]. Emissions from railcars are on the increase: in 1995 they stood at 2 t, and by 2008 they had reached 14 t [Schwarz 2010]. Deutsche Bahn AG has a total of 10,500 passenger compartment air-conditioners, 95% of which use the refrigerant R 134a [BReg 2010].

is also used.

In 1989 the Danish railways installed an indirect cooling system with a primary refrigerant circuit and a secondary water circuit in 150 passenger cars of the IC/3 “Rubber Nose” series.

Instead of 12 kg of refrigerant for a single direct circuit, this needs only 6 kg [Schwarz 2007].

An innovative solution is found in the trains of the ICE 3 series of Deutsche Bahn AG, which are cooled with cold air, a technology used for aircraft. In summer 2003 problems were encountered during operation of the first generation of AC systems, which were then modified and improved. The trains of the second ICE 3 series were equipped with different cold-air systems, which work without any technical problems and are very service-friendly. A total of 504 air-cooled systems for passenger cell air-conditioning are in service, i.e. 5% of all systems [BReg 2010]. DB has yet to conduct an overall review of the air-cooled systems.

However, calculations indicate that the air-cooled systems can make sense from an energy point of view, for example in high-speed trains operating in middle latitudes [Aigner 2007, Liebherr 2010]. One advantage for servicing is that there is no handling of R 134a. On the

other hand, the latest trains in this series have gone back to conventional R 134a air-conditioning systems [Wüst 2010]. However, the latest air-cooled systems optimised for trains, on show at Innotrans 2010, indicate that air-cooled systems continue to be available for future applications [Liebherr 2010].

In summer 2010 there were a number of failures in air-conditioning systems on ICE 2 trains.

These were R 134a systems. DB has not yet taken any decisions on future air-conditioning systems for the new trains of the ICX series; the railway company is calling for an environmentally sound, non-flammable and non-toxic refrigerant that is approved for the lifetime of the vehicle [BReg 2010].

When choosing a new refrigerant for rail vehicles, it is necessary to satisfy a wide range of requirements and carry out trials to ensure that these are met, e.g. with regard to fire precautions, maintenance, operating costs, environmental impacts or availability [BReg 2010]. Moreover, railway air-conditioning systems have to be developed separately, or at least adapted, for the various climate zones and types of trains.

CO₂ air-conditioning systems could be an alternative to R 134a systems for use in trains. In this context it is important to target a lower overall contribution to the greenhouse effect (TEWI), something which is basically feasible from a technical point of view with CO₂. Most components needed to build CO₂ air-conditioning systems for railcars are now available on the market. The estimated lead time for the first new trains with CO₂ systems is in the region of 2-3 years.

Deutsche Bahn AG is planning to equip a diesel trainset with a prototype CO₂ system based on CO₂ air-conditioning technology for buses (see section on buses). Test operation is to start in 2011 [Konvekta 2010; BReg 2010]. The CO₂ system is about 20% more expensive than an R 134a system. Thanks to savings on maintenance and energy consumption, the payback period for a CO₂ air-conditioning system is about 5 years [Konvekta 2010].

Prototypes of compact electrically driven CO₂ air-conditioning systems for electric and diesel-electric rail vehicles such as trams and local trains have undergone long-term tests (10,000 hours) in a climate test rig, which means that first vehicles could now be equipped for test operation [Konvekta 2010; Presetschnik 2008 and 2010].

An electrically driven CO₂ under-floor unit has been undergoing tests in the Czech Republic since 2007 [Presetschnik 2008].

In general, the energy consumption of railway air-conditioning systems needs to be determined on a project-specific basis; it depends on the individual system concept and on the operating and ambient conditions. Measurements of the annual energy consumption of a CO₂ prototype system for passenger cars revealed that consumption was 52% lower than for a conventional R 134a system and 12% lower than for an optimised R 134a system [Aigner 2007; Morgenstern, Ebinger 2008].

A new means of optimising heat management in trains is offered by a combination of the functions cooling in summer and heating in winter with the refrigerant CO₂. Tests have been

Hafner 2010]. Computer simulations indicate that by implementing heat-pump heating with CO₂ as refrigerant, energy consumption could be reduced by up to 78% compared with electric heating, depending on the climatic situation [Hafner 2010].

Ships

All 353 sea-going ships (cargo vessels) under the German flag with a gross registered tonnage (GRT) of more than 100 are air-conditioned, as are 16 passenger ships and 1 cruise liner.

After brief tests with the refrigerant R 407C, conversion to R 134a took place from 1996 onwards. The HCFC R-22 was still used up to the end of 1998. Average refrigerant charges are 100 kg in new cargo vessels, 250 kg in passenger ships and 500 kg in naval vessels.

Cruise liners each have an air-conditioning system with about 1,000 kg of refrigerant [Schwarz 2004].

By contrast, the more than 1,300 cargo vessels and tankers in the inland waterway sector are basically not air-conditioned. They use household appliances to cool provisions on board.

However, new passenger vessels are air-conditioned throughout, using R 134a since 1997.

Cabins ships need an average of about 250 kg and day cruisers about 100 kg of refrigerant [Schwarz 2004].

As a rule they use water chillers, i.e. water and not refrigerant circulates through long pipes [Schwarz 2004]. Ship air-conditioning systems are relatively prone to leaks. Systems with direct evaporation of the refrigerant (as in cargo vessels) emit 40% per year, indirect systems (as in passenger ships) 20% of the refrigerant charge [Schwarz 2007].

Conclusions

Since the technical development of air-conditioning systems for trucks and utility vehicle cabs is closely connected with developments in the car sector, the reader is referred to the conclusions in Chapter 3.3.7.1 (Car air-conditioning systems) for the basic technical means of reducing greenhouse gas emissions in this sector. Phasing-out of the refrigerant R 134a should be targeted in the above mentioned sectors as well. The use of flammable refrigerants such as hydrocarbons, HFC-152a or new flammable refrigerants in the hydrofluoroalkene group is difficult to imagine because of the large refrigerant charges required in these applications by comparison with car air-conditioners. At most, systems with a secondary circuit might be an exception here. But then these could contain hydrocarbons instead, and the use of R-152a or other fluorinated refrigerants would not be necessary. In view of this and other aspects it would be desirable - as in the car sector – to give preference to the climate-neutral refrigerant CO2.

After years of trials with test vehicles, several city buses have been in daily service since 2010 with CO2-based air-conditioning systems. The systems used in the rail sector are similar to those in the bus sector. Long-term tests with CO2 systems have proved successful. A start could be made on their commercial introduction. In view of the special importance of rail vehicles for local public transport systems and the fact that rail vehicles are extremely durable goods with difficult constructional issues and special

sector in particular. The air-conditioning requirements should be calculated as precisely as possible, since mobile air-conditioning systems need energy and also increase the weight to be transported. For example, air-conditioning of the passenger compartments in underground trains could largely be dispensed with in Germany.

The manufacturers concerned draw attention to the fact that timely creation of a clear framework by the state authorities could stimulate the desired innovations. For this reason the EU regulations which at present impose bans relating to cars and small commercial vehicles only, should be supplemented by rules and deadlines for all other vehicles.