Innovations in traction
The UK’s railway currently runs over 50% of passenger train and over 95% of freight train mileage on diesel power. If its railways are to decarbonise, this must change. There is no debate about that, but plenty of debate about how. In practice the trains will all probably be driven by electric motors. The debate is how that electricity gets from generation source to the train.
Conventionally there are three main methods proposed: conventional electrification via overhead wires or third rails; power to charging stations to recharge battery powered trains; and electrolysis of water to form hydrogen which is then transported to the trains and reconverted to electricity in a fuel cell. Also, environmentally friendly internal combustion engines powered by an environmentally friendly fuel (hydrogen, ammonia, or biodiesel) might be possible – see later in the article.
All the options have disadvantages: conventional electrification is costly; batteries are costly in whole life terms and have limited range; and the green hydrogen cycle is incredibly inefficient, although hydrogen might be manufactured cheaply from surplus wind or solar power. For freight, the only practical option is conventional electrification with, perhaps, batteries for last mile operation to avoid wiring complex yards or industrial premises.
All the options require electrical power, placing demands on the national grid which, it is fairly clear, has major issues in providing enough capacity as demand for electricity rises.
IMechE seminar
The Institution of Mechanical Engineers Railway Division often holds topical seminars and in March 2024, it discussed Innovation in Traction, covering these and many other issues.
Setting the scene, Rich Fisher from the Great British Railway Transition Team (GBRTT) outlined many issues and scenarios. In 2020, Network Rail’s Traction Decarbonisation Network Strategy (TDNS) proposed comprehensive electrification with minor and niche roles for battery and hydrogen trains, respectively. GBRTT suggest that lower cost options with a significant role for battery trains would bridge the gap between electrified sections of railway and extend electrification’s benefits.
Having examined several scenarios, a targeted electrification option would cover key freight routes, the remainder of the original Great Western scheme, and the line to Hull electrified, in addition to work already in progress. Lower Capex cost options lead to increased Opex due to continued reliance on diesel traction. That said, GBRTT expects rail to need carbon offsetting and alternative fuels beyond 2050.
Given that range-extending battery/electric trains will need to charge their batteries from the existing electrification system and electric freight locos will also draw more power, some power supply upgrades will be necessary. After all that there will still be a few diesel vehicles around by 2050 and there might possibly be a role for a small number of hydrogen or some other independently powered trains.
The same but changed
Neil Ovenden from Rail Partners summarised how passenger traffic had recovered, but changed since the pandemic, and outlined some of the challenges to procuring future trains proposed by Rich Fisher and others. Firstly, the procurement process had become more complex compared with the pre-2020 situation when TOC owning groups invited tenders as part of their franchise bids. TOCs operating under National Rail Contracts are effectively in the public sector, so TOC-led procurements now have to be carried out to the requirement of the Utilities Contract Regulations 2016. Operators procuring trains also now have to follow the Strategic Outline, Outline and Final Business Case process before inviting tenders (not open access TOCs).
Design requirements are changing too. The leisure market is increasingly important and, far from merely demanding the maximum possible number of seats per coach, leisure customers also want space for bikes, buggies, and luggage. Wider doors and level boarding access are also increasingly expected and not just by people with disabilities. That said, a large proportion of platforms do not currently comply with the required height/offset to readily deliver level access and will require work. The third challenge for TOCs, and back to this seminar’s topic, is what traction power type(s) to specify when the extent and capability of future electrification is still uncertain.
There is clearly a short/medium term demand for ‘off the wires’ power which needs to be addressed while also reducing the use of diesel power and emissions. Some of this might be addressed by fleet cascades, but there are many uncertainties about demand for legacy fleets, and it is costly to upgrade them to modern requirements. Also, there can be an uncertain timeframe to amortise upgrade costs.
Customer requirements change a little over time but the basics of reliable/punctual, clean, and uncrowded trains remain the top three. Traction battery and rapid recharge facilities, better internal air quality and temperature management, the ability to upgrade on-train IT rapidly and frequently, easy cleaning, and comfortable seats are also likely to be important requirements.
Scientific overview
Professor Stuart Hillmansen from the University of Birmingham introduced an overview of the science applicable to various energy sources and how they might be applied to typical railway duty cycles. It’s often said that rail is a very small contributor to carbon emissions. For context, an Airbus A380 consumes about 11,000 litres of fuel per hour, and the entire UK passenger diesel fleet consumes about the same amount of fuel as eight A380 aircraft.
In terms of options for decarbonising those diesel trains (by far the most capable self-powered option), batteries add weight and lack range, and whilst hydrogen fuel cells can provide range, they also need batteries – think hydrogen fuel cells as a range extender for battery trains. None of these options provide the capability of an electric train that can draw typically 7.5MW from the overhead line.
It is also important to consider how energy is used. The objective is to deliver the desired run time using the least energy. For frequent stop services, accelerating hard plus reasonable top speed and some coasting before braking hard, uses the least energy, and allows some make up time if running late.
Safety standards
Kieron Lyons from the Health and Safety Executive discussed some of the safety risks from using hydrogen as a fuel. HSE’s Science Division has been carrying out hydrogen safety research as part of the EU HyTunnel project (https://hytunnel.net/), focussing on jet fires and ignition delay effects especially in tunnels.
For testing, a close to full size tunnel was built at HSE’s 550-acre Buxton site. HSE aimed to assess the effect of a credible failure of a hydrogen storage system releasing hundreds of grams of hydrogen per second. Illustrated by some truly scary videos of the tests, it was concluded that hydrogen release in tunnels is a big risk. Ignition of the escaping gas from an overpressure valve is bad, but later ignition after the hydrogen has mixed with the atmosphere is even worse. That said, they have established that a hydrogen jet fired directly onto a concrete tunnel wall is unlikely to cause structural failure.
Neil Dinmore and Darren Fitzgerald from RSSB discussed fire safety standards for alternative-powered rolling stock. Given that a standard is generally intended to be the collection of relevant requirements determined from industry experience there had been a great deal of research, and 15 RSSB projects were cited. Standards need to cover the electricity supply, hydrogen supply, and storage/handling risks both on and off train. RSSB has commissioned modelling, day in the life reviews, and risk assessments for loss of containment of hydrogen.
Batteries have been the subject of research project T1272 ‘Compatibility and optimisation considerations for rolling stock traction batteries and battery charging’. Fire safety of batteries and how to extinguish fires that may occur is also under review, especially after a fire on one of the vehicles involved in the Carmont accident in 2020, which started some 45 minutes after the collision. With much bigger batteries fitted to self-powered and battery electric bi-mode trains there is a significant fire risk although choosing the right battery technology minimises that risk.
Alternative fuels
Dr Dawei Wu from the University of Birmingham discussed the potential for ammonia as an alternative fuel in rail. His team’s work focusses on safely and efficiently using ammonia in internal combustion engines, without releasing the gas into the atmosphere. A slide showing volumetric energy density vs specific energy density showed clearly how good petrol and diesel are. Ammonia has approximately three times the volumetric energy density of hydrogen (including storage containment), but about one third that of diesel fuel. The direct combustion of neat ammonia presents challenges due to its high ignition energy, narrow flammability limit, slow burning velocity, and NOx and N2O emissions when air is used as an oxidant.
Issues affecting the use of ammonia in internal combustion engines were discussed, including the pressure and spray pattern from injectors, use of fuel enhancers, the benefits of lean mixture burning, low oxygen dilution/exhaust gas recirculation and humification. Ammonia fuel engines are not new – they were used during World War 2 – and are seen as a promising solution for large bore internal combustion engines for the marine and power generation sectors. That said, solid oxide fuel cells (SOFC), a relatively new technology, are adaptable to use ammonia as well as hydrogen. The SOFC is an electrochemical conversion device which operates at high temperature (600°C to 800°C) and needs no expensive platinum group catalyst metals. It might also be possible to retrofit existing high power rail diesel engines to run on ammonia.
Cutting costs
Richard Stainton, Network Rail’s electrification expert, returned to the subject of OLE and delivering better electrification more cheaply. The cost of an electrification scheme is roughly one third civils works (tunnels, stations, bridges), one third electrification (OLE, traction power) and one third everything else (signalling immunisation, fencing/de-vegetation, mobilisation, authorisation [NTSN etc.], TOC compensation).
Key issues discussed included:
Over bridge parapets: Until recently, standards mandated a minimum height of 1.8 m. Delivering this work to the required strength can lead to the carriageway width being reduced below acceptable limits and/or wind loading might require bridge strengthening. In extreme situations the bridge might need to be replaced. New Network Rail Standard NR/L2/ELP/27717 covering bridge electrical risk assessment allows risk assessment based on the likelihood of mischievous acts and spend proportionate to the risk.
OLE bridge design: Many bridges have been reconstructed during more recent OLE schemes to achieve clearances, but the cost and disruption has been significant. No doubt the bridge asset manager is delighted to have a new bridge on someone else’s budget, but is it expensive to prematurely replace an asset with possibly decades more life. Methods of minimising clearances have included voltage-controlled clearances (surge arrestors/earth connection) and better understanding of real world catenary uplift from passing pantographs.
Over 30,000 surge arrestor tests have been successfully carried out at and in conjunction with the University of Southampton with clearance gaps down to 20mm. A good example is the small clearances provided under the Cardiff Intersection bridge, a steel bridge carrying a rail line over the main line (Issue 190, May/June 2021).
Insulated pantograph horns: The horn – the curved run off area either side of the contract trip on a train’s pantograph – is typically conducting. This is sometimes an issue close to, for example, legacy station canopies. Simply converting this to a non-conducting material reduces arcing risk. This has now been thoroughly tested and proven.
Ice loading: The design of the OLE has to allow for the formation of ice which impacts on the strength of masts, catenary wire droppers and gauge. The origin of the requirement – a 9.5mm thick coating of ice around the conductor – was not known, but following extensive research was eventually traced to a requirement for electric telegraph wires from 1886, the last recorded time the river Thames froze over! More recent modelling by the US military has shown that the risk of significant ice build-up is negligible. Icicles in tunnels is a much bigger risk.
The number of OLE masts required has been the subject of extensive work to understand the effect of wind on the conductors leading to approval to increase span length from 65 metres to 95 metres. Network Rail has also worked with University of Birmingham and Deutsche Bahn to improve understanding of wind loading and benchmark each other’s OLE mast and foundation designs.
The provision of traction power has also been reviewed, given that the architecture used until recently was developed over 50 years ago. The new architecture provides for modern protection technology and modern traction packages and features fibre optic comms and IEC 61850 safe and secure remote control. This delivers benefits of reduced switchgear, reduced land requirements, reduced construction time, improved speed of protection and real time traction information.
Frequency converters
Joshua Fanshaw, and Eliot Clark from Siemens Mobility discussed battery charging and solid state frequency converters (SFC)
Germany was an early convert to AC electrification, long before diodes, thyristors, or transistors were invented. To the untutored eye, a traction motor for one of these early electrification schemes looked like a DC motor but ran on AC. To get the right performance, the solution was to run on AC at one third of the mains frequency, 162/3 Hz. To achieve this, the supply to the railway was distributed as a railway dedicated two phase, 110kV 162/3 Hz railway. Traction at 15kV could be drawn from the 110kV supply at various transformer substations. To provide the dedicated supply, large rotating frequency converters were required, the predecessor of the SFC.
50Hz to 50Hz SFCs have begun to feature in UK’s electrification schemes and offer several advantages. They take the three-phase supply and convert it to single phase which eliminates the issue of unbalanced loads on the individual phases of the mains supply, opportunity for paralleling, no need for neutral sections, a controlled constant output, and low fault currents (especially compared to high power auto transformer supplies).
A variant of the SFC was described as the Rail Charging Converter (RCC). This is intended to be connectable to the UK 11kV grid network where connection points are much more extensive than are available with the 400kV/ 275kV / 132kV networks. If used on a low-density intermittent electrification scheme it could be sited where needed and not according to where the power is available. It avoids difficult, expensive areas, such as tunnels, bridges, and level crossings, avoids long power cables from out-of-town supply points running through limited clearance areas, and perhaps avoids long lead times for large scale connection points. The RCC provides a scalable 2.5MW output at a nominal 25kV from a 11kV to 3kV input voltage. A trial installation is to be tested at Porterbrook’s Long Marston site in 2025.
3,000VDC for metros?
The last speaker, Professor emeritus Felix Schmid from the University of Birmingham, postulated an opportunity for further energy efficiency on metros by proposing a 3,000VDC supply, potentially on third and fourth rails. He stated that DC is usually deemed appropriate for urban rail where services are frequent. Tunnels must be much bigger for 25kVAC electrification and it is not suitable for street running, even where it is not prohibited by law. Moreover, it makes sense for metro trains, which accelerate and brake frequently, not to include the mass of a 25kV transformer and its protection systems.
Alternating current at low voltage suffers large volt drop due to relatively high resistance, the skin effect and inductance. Presently, trams use 600VDC or 750VDC with OHLE, and metros 750VDC with third rail, whereas other metros opt for 1,500VDC, mostly under OHLE. The London Underground third and fourth rail system uses a floating earth arrangement (except where it shares tracks with other operators’ third rail trains) where either rail can be at 750V relative to earth, but is usually biased approximately +500V/-250V. Direct current systems suffer quite high volt drop necessitating frequent substations, but the 3,000 VDC proposal increases the spacing and each pantograph/pick up shoe has to carry less current. A third and fourth rail split roughly +1,500V/-1,500V was proposed but, whether this would be with a floating earth system, is not yet determined.
Whilst not in use anywhere, there might be advantages for fully segregated metros (e.g., fully underground or on viaduct with platform screen doors). These are:
- More suited to large city metros where trains are becoming heavier.
- Limitations of 1,500VDC OHLE are becoming apparent, particularly in terms of pantograph wear.
- Voltage drops require shorter spacing of substations and more capable substations.
- 1,500VDC 3rd rail electrification is now being accepted for fully segregated metros, notably in China.
- Technical developments make 3,000VDC entirely feasible.
In short, having more power available, fewer sub-stations, halving current, reduced voltage drop and transmission losses look attractive, but the approach requires a full scientific and technical analysis.
Conclusion
Your writer attended the conference with a firm view that electrification is still the way for heavily trafficked passenger and freight lines. Was his mind changed? In short, no, although he recognises that branch lines might justify other solutions and that complete system decarbonisation might be achieved a lot later than 2050. It is accepted though that hydrogen or ammonia might be suitable for other transport modes where a tethered power supply is impractical or impossible. One cannot, for example, imagine OLE on the open seas!
