According to the World Energy Council (WEC, 2011), transport sector global energy consumption in 2010 was almost 2,200 million tons of oil equivalent (MTOE), representing about 19% of world energy resources.
Currently, more than 96% of total energy supply to transport comes from oil (WEF, 2011); in 2010, around 60% of oil production worldwide was consumed by the transport sector (IEA, 2012).
In particular, transport accounted for 38% of total energy consumption in the UK in 2011 (DECC, 2012), and for 27.8% in the U.S. the same year (EERE, 2012).
Carbon emissions are closely related to energy consumption in the transport sector and in 2010 it accounted for about 23% of global levels of CO2 emissions (WEC, 211).
Economic development and population growth are increasing the energy consumption of transport (WEC, 2011); however dependence on oil supplies, inefficient use of resources and associated CO2 emissions make the growth of this sector a completely unsustainable process (WEF, 2011).
It is necessary to evaluate the energy efficiency of transport sector and improve its processes through technology and practice in order to achieve world sustainable development goals.
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The purpose of this report is to evaluate and compare air and rail transport, in terms of their relative use of energy and their CO2 emissions, and also consider the potential strategies to improve the energy efficiency and reduce CO2 emissions from these modes of transport. Air transport is the fastest-growing mode of transport (WEF, 2011) and it is considered to be, after diesel engine cars, the most contaminating one (Chapman, 2007) and rail transport is commonly referred to as the cleanest alternative.
It is convenient to clarify some concepts in order to have a more precise comprehension of how energy use and energy efficiency are measured in the transport sector, before the analysis of air and rail transport.
Energy efficiency is defined as "the relationship between the energy consumed and the output produced by that energy" (EEDO, 2012: 19). When efficiency is higher, more products or services can be produced with the same amount energy. This information is useful when comparing similar technologies or processes; however, air and rail transport are different technologies that consume energy in different ways to produce the same service, so it is more practical to compare them in terms of energy intensity (EERE, 2012a), which is essentially the inverse of energy efficiency and in transport is the amount of energy required to move one passenger over 1 kilometer, or passenger-km (NRC, 2011). For freight transport energy intensity would be energy per ton-km, but this report will focus on passenger transport to reduce the number of variables in intensity calculation and simplify the comparison.
Use of energy
As illustrated in Fig 1., road transport is responsible for the majority of total transport energy consumption in 2010, around 76%, while air transport accounted for about 10% of the total and rail transport for 3% approximately (WEF, 2011).
Fig. 1. 2010 Transport energy consumption by mode (total ~2,200 MTOE) (WEF, 2011)
These percentages represent the absolute values of air and rail transport energy use with respect to total consumption. In order to compare them with respect to each other, it is necessary to analyze first where they take energy from and how each mode of transport invest the energy to be able to move and transport people, in this case, from one location to another.
Currently, rail transport energy supply comes mainly from diesel (88%) and electricity (12%) (IEA, 2008). Due to the low resistance of rail vehicles on railways (steel on steel) and high efficiencies of electric and diesel engines, diesel engine efficiency is around 45% (Beggs, 2009), rail transport presents a potential advantage over other modes of transport, but the determinant aspects on rail passenger transport efficiency are the services on board, the technology, the speed of the train and the occupancy (Fraser J., et al 1995).
Aerodynamic trains can be very energy efficient compared to previous model, however, when their speed increases over 200 km/h, energy consumption also increases significantly due to air drag (Beggs, 2009).
As passenger trains can weight up to 90 tons, the energy efficiency of an empty train is almost the same as if it is full. Intensity increases [kJ/p-km] as number of passenger decrease; therefore energy efficiency is very related to occupancy rates (Fraser J., et al 1995).
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Fuel accounts for 20% of modern aircraft total operating costs; therefore fuel consumption reduction is a priority for aircraft and engine manufacturers to increase energy efficiency (Kahn S., et al 2007).
Similarly to trains, aircrafts need to use energy to overcome the air drag force, but, unlike trains, planes also consume energy standing up (Mackay 2008).
Studying the relative energy consumption per seat, for a London to Edinburgh journey with different occupancy rates, Kemp (2004) as cited by Beggs (2009), found that rail transport is indeed more efficient than air transport. However, this statement is not true when it applies to rail vehicles travelling at 350 km/h or more, where it consumes slightly more energy than the aircraft, as illustrated in Fig. 2.
Fig. 2. Energy consumed by various modes of transport from London to Edinburgh (Beggs, 2011).
In the transport sector CO2 emissions are closely related to energy consumption. CO2 emissions from air transport will vary depending on technology, occupancy rat and route (DTF, 2011), although aircrafts produce other greenhouse gases apart from CO2 such as water , ozone and nitrousâ€¨oxides (Mackay 2008).
The following figure, Fig 3., presents CO2 emissions per passenger-km and per mode of transport in Europe. Rail transport is the second less polluting mode of transport and according to the European Environment Agency (EEA, 2011), shifting from diesel to electric trains has decreased the carbon emissions of rail transport by about 27 % from 1995 to 2009. CO2 emissions of air transport have also decreased by 29% due to aircraft technologies improvements and higher occupancy rates.
Fig. 3. Specific CO2 emissions per passenger-km and per mode of transport in Europe, 1995-2009 (EEA, 2011)
Energy efficiencies of air transport can be improved by reducing fuel consumption through aircraft technology improvements, infrastructure improvements, operations improvements and use of biofuels (WEF, 2011). Reducing weight and drag are some of the objectives, although according to Mackay (2008: 35) "no redesign of a planeâ€¨is going to radically improve its efficiency".
Regarding rail transport, again, it is important to reduce weight and aerodynamic resistance improving trains infrastructure to reduce energy consumption and carbon emissions. Also, higher efficiency propulsion system and better regenerative brake mechanisms are some of the potential improvements (Kahn et al, 2007).
This report intended to present a general view of the relationship between transport sector and global energy, focusing on a comparison between air and rail transport modes in terms of their use of energy and their CO2 emissions, based mainly on global official energy agencies and organizations.
It was found that rail transport appears to use energy more efficiency than air transport, as well as lower CO2 emissions. However, rail vehicles speed and occupancy rates are determinant aspects when evaluating energy efficiency and carbon emissions.
In order to evaluate appropriately the energy efficiency of modes of transport and present reliable results, it is necessary to consider all the energy system inputs and outputs.
With appropriate practices and technology developments it is possible to achieve higher transport efficiencies and decrease the environmental impact of the transport sector.