Alternative Road Vehicles , Electric Rail Systems , Short Flights : An Environmental Comparison

*Vrije Universiteit Brussel, ETEC, Pleinlaan 2, B-1050 Brussel In a context of global warming, the energy-efficiency topic has never been as hot as it is now, especially as far as transportation is concerned. Road and rail transportation are both included in what currently is the main attempt to tackle climate change on a worldwide scale: the Kyoto protocol. The achievement of the objectives put forward in this international agreement might be reached thanks to the high energy efficiency of electric drives for both road and rail transportation. International aviation emissions however, have not been included in the targets of the Kyoto protocol. Nevertheless, as in some cases aviation competes with surface transport, the energy efficiencies and/or climate impacts of different alternative drives or transport modes for surface transport should be compared to (mainly short-haul and very short-haul) aviation.


INTRODUCTION
Transportation is one of the main sectors for which a great effort is required if the international community wants to reach the commitments made in the Kyoto protocol.Road and rail transportation are both included in what currently is the main attempt to tackle climate change on a worldwide scale.The achievement of the objectives put forward in this international agreement might be reached thanks to the high energy efficiency of electric drives for both road and rail transportation.However, the Kyoto protocol targets only include emissions originating from domestic flights and don't include international aviation emissions.The Kyoto protocol requires that Annex I countries: "…pursue limitation or reduction of [GHG emissions from aviation], working through the International Civil Aviation Organization (ICAO)…".In 2001, the ICAO endorsed the development of open emissions trading for international aviation.The extension of the European Union's Emission Trading Scheme (EU-ETS) is being evaluated and developed right now by the European Union under great international attention.
The intensity of air travel has never been as high as it currently is [1].Providing unseen exchanging possibilities and comfort to travelling people around the world, the current growth in international air traffic also threatens the potential beneficial achievements of some efforts made in other sectors towards reducing their contribution to global warming and other environmental impacts.
Alternative vehicles, such as battery, hybrid and fuel cell electric cars, as well as rail transportation systems are often praised for their environmental performances as compared to conventional gasoline or diesel vehicles.The energy efficiency of electric drives is high and the possibility of regenerative braking makes them the most energy efficient transport mode over land [2].Many studies compare the energy efficiencies and/or climate impacts of different alternative drives or transport modes for surface transport.However, in some cases, these transport modes compete with, and therefore should be compared to aviation.This is mainly the case for the so-called very short haul flights (less than one In this context and with the aim to improve the energy efficiency and the climate friendliness of its transport system, the Belgian government is evaluating the possibility to prohibit the very short haul flights (of less than 150km/~90miles) within the country.The main operation concerned by this prohibition was a planned 75 km Boeing 737-400 flight between the Brussels South Charleroi Airport (CRL) and the Liège Bierset Airport (LGG) to group passengers before leaving to Morocco, Casablanca Airport (CMN).This planned leg of the flight has been cancelled in the meantime.
In this paper, the environmental relevance of this measure is evaluated by comparing the greenhouse gas emissions due to this type of very short flights with other alternatives for passenger transfer (surface transport) between both airports and cities.As aviation induces some other climate-related effects besides CO 2 emissions [3], a short overview of the other climate influencing aspects (NO x emissions, stratospheric cloud formation, contrail formation, etc.) is provided.However, the focus of the case-study will be on CO 2 emissions.
In this paper, special attention will be paid to the energy efficiencies of the different transport modes for these specific short distances.In this context, the climate impact of the well-to-tank chain will be taken into account.So in the case of the electric drives, the electricity production mix will be analysed as well.
After modelling the different scenarios and taking the technical feasibility into account, the most climatefriendly and energy efficient scenarios will be selected and an evaluation of the yearly CO 2 -equivalent savings will be performed.

AVIATION AND CLIMATE
Anthropogenic radiative forcing (or global warming caused by human activities) is mainly caused by CO 2 emissions [4].Therefore, although some integrated approach over the different greenhouse gases is necessary, it seems sensible to pay special attention to this greenhouse gas.When looking at the different sectors it seems clear that a significant part of the worldwide CO 2 emissions are currently due to transport.Moreover, the share of transport in an expanding total of CO 2 emissions worldwide is even expected to grow in the future (Figure 1).This is one of the main reasons to perform an analysis of the transport sector and to think about how to improve the efficiency and climatefriendliness of this sector.Moreover it should be noted that currently there is no legislation at European Union level to regulate CO 2 emissions from aviation.However, the European Commission is planning to take action in the very near future by bringing the aviation sector within the scope of the European emission trading scheme, the EU-ETS [5].
As mentioned before, on a worldwide scale, CO 2 is by far the emission that impacts climate in the strongest way.Due to the very long residence time of CO 2 in the atmosphere, aircraft CO 2 emissions get well mixed with the CO 2 emissions of other anthropogenic sources.Regarding other (non-CO 2 ) emissions, aviation presents the specific characteristic of emitting a large fraction of its emissions in the stratosphere.Due to the altitude where these emissions take place, they influence climate in a different way as compared to when they would occur on ground level.Subsonic aircraft (virtually all passenger aircraft) tend to decrease the lifetime of CH 4 in the atmosphere due to their NO x emissions.On the other hand, ozone is a very short-lived greenhouse gas.Its production is increased by the emissions of NO x and thus results in a heating effect.Moreover aviation is also responsible for condensation trails (contrails) and possibly also for the formation of cirrus clouds.Figure 2 gives an overview of the different impacts of aviation on climate.The total impact of aviation on the climate is believed to be two-to five-fold that of CO 2 alone [7], [8].The scientific certainties of the different kinds of impacts are provided below the horizontal axis.Radiative forcing from air traffic as estimated by the IPCC [3] is shown in red, while a recently revised estimation for the same fleet is shown in blue (TRADEOFF project).

DEFINITION OF THE SCENARIOS
The CRL and LGG airports are located about 75 km/~45 miles from each other and are directly connected through a highway (82 km) (Figure 3).The cities of Liège and Charleroi are directly linked by intercity trains (82 km track).Currently, none of both airports The World Electric Vehicle Journal, Vol 2, Issue 4 Alternative Road Vehicles, Electric Rail Systems, Short Flights: An Environmental Comparison is connected directly by rail, however both airports are located close to the existing railway track connecting both city centres (2 km for the CRL airport and 3 km for the LGG airport).This makes the addition of some airport railway stations an interesting option for the future (implicitly increasing the accessibility of both airports).Alternative connections thus include: road transport (using different transport modes, such as conventional and electric drives for cars and buses) and rail transport.

COMPARISON OF THE EFFECTIVENESS OF THE DIFFERENT SCENARIOS
Now the scenarios have been defined, the calculation of the CO 2 emissions has to be performed.The fuel or electricity needed for the different scenarios, as well as the according CO 2 emissions are presented in Table 2.The total fuel consumption is obtained by multiplying the covered distance by the consumption and the number of vehicles needed to transfer the passengers.The CO 2 emissions presented in Table 2 are well-towheel emissions.This means they include direct and indirect CO 2 emissions.4 show that the scenarios using buses are causing the lowest amounts of CO 2 emissions.These scenarios are followed by the electric train battery electric vehicle and carpooling scenarios.In turn these are followed by the other personal vehicle scenarios and finally by the reference scenario (very short-haul flight).Currently six weekly return flights are operated from the Charleroi airport to Casablanca.If it is assumed every flight would make a stop-over, six weekly CRL-LGG flights, as well as six weekly LGG-CRL flights would be operated, resulting in 12 flights/week * 52 weeks/year * 3940 kg of CO 2 /flight = 2.458.560kg of CO 2 a year.Replacing these flights by the transport mode with the lowest CO 2 emissions (electric buses) would result in 12 trips/week * 52 weeks/year * 94 kg CO 2 /trip = 58.656kg CO 2 a year.In other words, the electric bus scenario would save 2.399.904kg or 2.400 tonnes of CO 2 emissions on a yearly basis.

CONCLUSIONS
When comparing the different performances concerning CO 2 emissions, it clearly appears that for such a short distance, the most climate-friendly way to  transfer the passengers from one airport to the other is by surface transportation.Amongst the different options for surface transport electric buses appear to be resulting in the lowest quantity of emissions.In general, as can be seen from the performance of buses, trains and carpooling, it can be concluded that grouping passengers before transferring them over land is the best way to minimize CO 2 emissions.Moreover, within the different scenarios it appears that using electric drives (hybrid and battery electric vehicle or bus, electric train) for passenger transport strongly reduces well-to-wheel CO 2 emissions.
The difference between the scenarios resulting in the highest and the lowest CO 2 emissions is equivalent with a saving of 2.400 tonnes of CO 2 emissions on a yearly basis.
It should be noted that the conclusions of this paper give an indication of the climate-change performance of the different transport modes for this specific veryshort distance transfer of passengers.Consequently another analysis should be performed before drawing any conclusion for long-haul flights.In the specific case of long-haul flights the assessed transport modes should take the feasibility (duration of the trip, travelling overseas, etc.) of the transfer into account as well.
The assessment presented here is based on the CO 2 emissions of the different transport modes and although CO 2 is the predominant greenhouse gas, for completeness, the other greenhouse gases could be included in the future.

The
World Electric Vehicle Journal, Vol 2, Issue 4 Alternative Road Vehicles, Electric Rail Systems, Short Flights: An Environmental Comparison hour of flying).

Figure 2 :
Figure 2: Overview of the different ways in which air transport influences radiative forcing ([9] adapted from [10])

Figure 3 :
Figure 3: Map of Belgium indicating the location of the CRL and LGG airports, as well as the rail track (in green) currently connecting the city centres of Charleroi and Liège

Figure 4 :
Figure 4: Overview of the CO2 emissions for the different scenarios

Table 1 :
Overview of the assumptions for the different scenarios

Table 2 :
Overview of the overall energy consumption and CO2 emissions for the different scenarios

Table 2 and
Figure The World Electric Vehicle Journal, Vol 2, Issue 4Alternative Road Vehicles, Electric Rail Systems, Short Flights: An Environmental Comparison