Memorandum submitted by Dr Stuart Hillmansen (LCT 40)

1. Executive summary: 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.

 

October 2009

References

1. 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