Memorandum submitted by Dr Stuart Hillmansen
EXECUTIVE SUMMARY
1. Railways have a key role in the UK transport
system. Unlike other transport modes, the railway is able to use
primary energy from a range of sources. In the UK the railway
is powered using diesel (gas oil) fuel, and using electricity
taken from the national grid. Calculating emissions for a typical
railway journey is therefore complex and the results are very
dependent on the assumptions made. The current state-of-the-art
electric powered trains have very efficient traction systems,
and are able to regenerate energy while the vehicle brakes. However,
older rolling stock is often less efficient, and often cannot
regenerate into the supply. Because of the long design life span
of rolling stock, the full penetration of technology into the
railway can take a long time (40 years approximately). There are
numerous coordinated industry led initiatives that are tackling
energy efficiency. These include, both operational improvements,
and the investigation of the impact of new technologies. In the
UK the recently announced electrification plans will future proof
the railway system, and will enable the railway to benefit from
the increasing decarbonisation of grid electricity. Lightly used
railway lines on the extremities of the railway network may be
a proving ground for novel forms of low cost traction systems.
2. I have been asked to produce a memorandum
on the role of new technologies in reducing emissions from rail
transport. The scope of the emissions considered in this submission
include gaseous and particulate emissions from the primary power
source (power station or on board engine), and does not include
noise and vibration, electromagnetic emissions, or other material
emissions. The focus of the memorandum is on passenger transport,
and will primarily focus on operational emissions. Emissions from
any system need to be assessed in terms of the construction, operation,
and decommissioning phases, in a life cycle analysis manner in
order to make meaningful intermodal comparisons.
3. This memorandum will first review the
fundamental characteristics of rail vehicle transport, and then
it will review the current mix of available traction in the UK,
and then will identify the likely future trends in railway vehicle
propulsion and the current and future activities that are being
implemented to improve the environmental credentials of rail.
4. The rail network in the UK is approximately
40% electrified (in terms of track miles). The majority of electric
routes use 25 kV 50 Hz AC (Alternating Current) overhead
electrification, and the rest is made up of third rail 750 V DC
and some 1500 V DC (Direct Current). Most of the passenger
kms are on the electrified network (approximately 60%).
5. Railway transport is an exceptional form
of mass transit—since it is the only form of mass transit
which is currently able to deploy zero carbon emissions technology
on a wide scale. This is due to the ability of the electrified
network to deliver power directly to a moving train at high efficiency.
The total emissions are therefore directly linked to the up-stream
power generating emissions, which can be zero for hydro, nuclear,
and renewable powered grids.
6. Railway vehicles have a low specific
resistance to motion in comparison to other modes. 1 This is because
the rolling resistance of a steel wheel on a steel rail is very
low. At higher speeds, aerodynamic forces become more important,
and railway vehicles benefit from the convoy formation in which
trailing vehicles travel in the slipstream of the proceeding vehicles.
The overall resistance of a railway vehicle follows a quadratic
curve with increasing speed. Therefore when a train doubles its
speed, the energy dissipated by the resistive forces approximately
goes up by a factor of four. However, these forces are still relatively
low, and once a railway vehicle is at speed, it is able to coast
for a considerable distance before coming to a stop. The kinetic
energy stored in a railway vehicle at cruise is therefore significant.
7. The UK rolling stock consists of a roughly
equal mix of DC, AC and self powered stock (some of the electric
powered stock is dual voltage). There are a number of different
traction systems within these trains. So the power flow from prime
mover (be it the power station or on board engine) to the wheels
undergoes a number of energy conversion processes. It is important
to recognise that in these systems the traction motors, or indeed
the hydraulic drives, are able to provide a retarding force to
assist in braking the train.
8. DC powered systems in the UK are fed
from the power grid via rectifying substations. These substations
are positioned along the track at intervals of about 2-3 kms.
The substations convert 3 phase AC high voltage to low voltage
DC which is then fed to the train using the line side conductor
rail. DC railways are low voltage high current systems, and many
thousands of amps may flow through the third rail just to power
one train. DC fed railway vehicles are usually able to use dynamic
braking, where the motor is used as a generator which then provides
a braking force on the vehicle. The braking effort provided by
the generators is blended with that produced by the friction brakes
to achieve the overall braking deceleration rate. The UK DC railway
network is beginning to implement DC regeneration, whereby the
braking energy from the motors is fed back into the network. This
energy cannot be fed back into the grid due to the nature of the
rectifying substations, but must be used by a vehicle in the vicinity
(in electrical connection) of the braking train. This method is
suitable for dense DC urban networks where the spacing between
the trains and stations is small. Regeneration in these systems
can save a considerable amount of energy and typically incur lower
track access charges for the regenerative capable vehicles (15%).
Future energy savings can be made through the more widespread
implementation of regenerative capable DC powered rail vehicles.
Further benefits are currently being realised through the implementation
of driver training programmes which optimise the station-to-station
journey profile, train regulation and effective timetabling. Line-side
energy storage systems are being deployed in some international
research studies. These systems essentially perform two functions;
they improve the line receptivity to regenerated energy (and are
often positioned at the entrances to station platforms where all
trains brake), and they reduce the total demand on the grid connection
(thus lowering the infrastructure requirements for grid connection).
On less intensive DC systems, such as light rail or tram systems,
regeneration back into the network may suffer from receptivity
problems due to the large distance to the nearest powering train.
In these systems, regenerated energy can be stored on board the
vehicle and later reused during the accelerating phase.
9. AC fed railways are preferred for high
speed and high power applications. In the UK the overhead line
equipment energised at 25 kV 50 Hz AC. The currents
are generally much lower, but the voltage is considerably higher,
thus necessitating excellent insulation and large clearances from
nearby structures. A fundamental advantage of the AC fed systems
is that the grid connections can easily be configured to transmit
power in both directions (as opposed to rectifying substations
in DC systems which only send power in one direction). This means
that braking trains do not need a powering train to absorb the
regenerated energy. AC fed railway vehicles also make use of dynamic
braking where the motors are used as generators. Again the braking
effort is blended with the effort produced by the friction brakes.
Therefore in order to improve the proportion of energy that is
returned to the grid in a braking cycle, the vehicle should be
slowed using a minimal amount of friction braking, and the majority
of the braking effort must be produced by the motors. The most
effective solution is to use distributed traction (applies to
both AC and DC fed railways) where there are many motors along
the length of the train as opposed to a single locomotive at one
end. The UK Government has recently announced a major electrification
plan for the UK (which will use 25 kV 50 Hz AC). It
includes electrification of some key routes, with a number of
other routes still being considered under future plans. This strategy
will go someway towards insulating the railway from future uncertainty
over diesel availability on these routes, and will enable the
railway to benefit from the continuing decarbonisation of grid
electricity.
10. The routes around the UK which are lightly
used are currently served with aging Diesel Multiple Units and
are potential candidates for novel forms of traction when the
vehicle fleet is replaced (2020-2030). Some routes may not be
economic to electrify with current technology, and therefore a
new technology could provide a solution for these routes. The
likely candidate technology for lightly used routes could include;
lower specification of electrification (such as that used for
trolley buses and tram systems), variants of hybrid electric diesel
vehicles with potential for recharging at stations, or depots,
and the use of low carbon synthetic fuels. Some recent work at
the University of Birmingham has investigated diesel hybrids.
Hybrids allow a more efficient operating point for the prime mover,
and allow regenerated braking energy to be stored for later use.
Depending on the route, we found that savings of up to 25% could
be realised in comparison to the non-hybrid case. 2
11. Fuel cells are another technology which
could provide motive power for railway vehicles. Much effort has
been devoted to demonstrators outside the UK and the technical
feasibility of a fuel cell to provide traction for a railway vehicle
has been proved beyond doubt. However, the reliability, cost,
effect on vehicle range, and well-to-wheel efficiencies (compared
to electric fed railways) present some serious challenges. Fuel
cell and hydrogen research and development has target applications
which rarely include rail. However, rail vehicle manufacturers
are adept in translating technology from other sectors to the
railway, and should the technical development of fuel cell powered
systems for transport reach a suitably advanced stage, then it
would be relatively straightforward for a railway vehicle manufacturer
to offer a fuel cell powered vehicle.
12. Energy metering for the railway takes
place at the connection points to the national grid. There are
now efforts to decompose this data further, by metering railway
vehicles individually. The data from individual vehicles and drivers
will enable operational improvements to be quantified and implemented
on a wide scale. Improvements in driver style, auxiliary power
management (turning off equipment over night for example), are
therefore properly measured and therefore able to be managed more
effectively.
13. The mass of UK railway vehicles has
tended to increase in recent years. The reasons for this are quite
complex, however, increasingly the procurement process for new
rolling stock is placing stringent requirements on the mass of
the vehicle, and it is expected that newer vehicles will have
lower masses. Lower mass brings benefits to rail maintenance,
as well as saving traction energy.
14. System wide solution. The UK railway
is very much an integral part of the UK transport system. Most
journeys involve multiple modes and the railway can be effective
in providing the essential backbone of many journeys. Increasing
integration between station access modes and the railway will
lead to reduced overall environmental impacts, for example, having
electric bus services, or tram systems sensibly connected to terminal
stations, and providing electric vehicle charging points at parkway
stations could be a realistic solution given the current efforts
to electrify road transport.
REFERENCES1 What
Price Speed-Revisited. The evolution of transport propulsion efficiency
over the last 50 years. Ingenia. The informative quarterly of
The Royal Academy of Engineering. The Railway Research Group,
Imperial College. (ISSN 1472-9768), 2006, available from http:/
/www.raeng.org.uk/news/publications/ingenia/issue22/Imperial.pdf.
2 DMU Hybrid Concept Evaluation, Birmingham Research
and Development Limited, March 10 2009 http://www.railway.bham.ac.uk/documents/Hybrid_Rail_Report_DMU_V1.pdf
October 2009
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