With the world now focused on the need to reduce carbon emissions and improve sustainability, railways around the globe are busy devising smarter approaches to electrification. Keeping emissions on track never sounded so good.
Rail electrification has long been seen as a means of improving the energy efficiency of passenger train propulsion in relation to the alternative of diesel traction; more recently, it has come into favour as a means of removing air pollution at point of use.
These benefits make it easy to see why this form of traction is favoured: on a like-for-like basis, electric trains are lighter in weight, accelerate more quickly, have lower maintenance costs and consume less energy than diesels.
Moreover, electric traction also benefits from the ongoing efforts to decarbonise grid power generation. In the UK, for example, this improvement has been significant. Coal-based power generation, once the predominant source of energy for the power grid, has been dramatically scaled back in favour of renewables such as wind energy and solar. As a result, by 2018 the average carbon intensity of the electricity supply in mainland Great Britain had fallen to just 270 g/kWh CO2e – approximately half the prevailing level at the start of the decade. In fact, the first week of May 2019 saw the country reach the milestone of its first week without coal-generated electricity since the 1880s.
But while electrification may have distinct benefits once in place, it is far from a low-cost option in terms of upfront capital investment. Installation and commissioning costs vary widely depending on many factors, not least the need to accommodate overhead catenary systems into legacy infrastructure such as overbridges and tunnels. Where height is restricted, the additional cost of lowering the track bed and its foundations to accommodate the increased headroom requirement – or rebuilding those structures completely – can be significant and may also incur financial penalties for line closures during reconstruction.
In the UK there has also been something of a feast or famine approach to electrification strategy over the past 50 years: the length of track completed per year has varied from zero to in excess of 800 single-track kilometres (stk). This compares with an almost constant 200 stk per year in Germany over a similar period. This unpredictability is widely thought to have exacerbated the situation, requiring supply chains to be reconfigured for each major new electrification project. This combination of factors results in a current UK cost of approximately £2.5 million per single track kilometre for electrification as opposed to around £1 million per stk for other major European networks.
Pragmatic and smarter electrification
Yet while significant efforts are being made to reduce installation costs, it is almost inevitable that most mixed networks of urban, intercity and rural routes will include regions for which the case for conventional overhead electrification cannot be made.
“the key enabling technologies for maintaining zero emissions at point of use are those of on-board energy storage and range extenders”
In these circumstances, argues Jon Brown, UK business development manager for Ricardo Rail, a smarter and more pragmatic approach is required: “A key challenge for the rail industry internationally is to look at how we can accommodate gaps in electrification while still maximising the proportion of electric traction. This might be anything from short section-gaps in overhead line through bridges or tunnels to avoid the costs of reconstruction, to longer sections that remain non-electrified.”
The rationale for allowing such longer gaps in electrification can be varied. As examples, it might be for aesthetic considerations around installing a catenary system for a light rail or tram system in a historic city centre, or it could be the lack of commercial justification for the investment required for a rural route with lower traffic volumes and load factors.
“Either way,” continues Brown, “the key enabling technologies for maintaining zero emissions at point of use are those of on-board energy storage and range extenders: these allow electric trains to operate beyond the reach of catenaries.”
Developing zero emissions rail in the Netherlands
The Ricardo Rail team based in Utrecht was asked to investigate exactly this approach to electrification of the Dutch rail network by the northern Netherlands provinces of Fryslân and Groningen, explains Ricardo sustainability consultant, Martijn Wolf. “There is a strong regional imperative to reduce emissions, with the provinces having taken the political decision to aim for zero emissions public transport – both in terms of point of use and generation of the energy used – by 2025. This is quite an ambitious target that cuts across both buses and trains,” he says.
“The region is unusual in that its rail lines are part of only 5 percent of the Dutch network that remains unelectrified,” continues Wolf, “but we had previously conducted a study that demonstrated that full conventional electrification of the lines in this region was not cost-effective due to the comparatively light load factors and train frequencies. From the outset of this study, therefore, we needed to explore alternative approaches.”
The study, which was carried out in collaboration with infrastructure consultancy Arcadis, focused in particular on evaluating different >
“there is a strong regional imperative to reduce emissions, with the provinces having taken the political decision to aim for zero emissions by 2025”
options for range-extender solutions based on hydrogen fuel cells used in the form of a hybrid propulsion system, or for partial catenaries in combination with higher capacity on-board battery energy storage. With recent improvements and the reduced cost of battery systems, the latter was clearly identified as the more attractive solution.
Initially, a proposal was considered to use charging points at station stops, in a similar manner to some battery-operated bus networks. However, as Wolf recalls, there were challenges in this approach: “A train has a much higher charging requirement than that of a bus, due to its size and weight. Using the type of discrete charging points that are available for commercial buses would have been impractical due to the positioning of stations and the length of time that would be required for recharging. Instead, we looked into the options for partial electrification.”
Station-based ‘opportunity’ charging
The team modelled a range of scenarios combining station-based recharging with partial electrification. In the partial electrification zones, trains use the overhead line for both their immediate tractive power needs as well as for battery recharging. This is augmented in the non-electrified sections with station-based recharging points similar to those used for battery bus networks.
The simulation of daily train usage was based on a standard reference train representing a Stadler GTW 2/6 articulated rail car, typical of the worst case (in terms of its likely energy storage requirement) of rolling stock used on the lines in the region. In order to test the feasibility of running battery trains using only station-based opportunity charging, the team calculated the required battery capacity per route to run the specified timetable, including an allowance for the weight of the installed pack. This required fitment of a 535 kWh pack, based on the specified end of life capacity of 420 kWh. Charging was assumed to be carried out at a maximum rate of 4C, meaning that a full recharge cycle could be carried out in a minimum of 15 minutes. To accommodate this high charging rate the team viewed LTO (lithium titanate) as the most effective cell chemistry, a type that the rail industry in general seems to be converging upon. Despite their high cost in comparison with other cells, LTO batteries provide high C rate charging capacity, high power transfer and very long cycle lives which, coupled with exceptional safety characteristics, provides an attractive business case for rail.
“The initial scenario we assessed revolved around solely station-based recharging without any electrification beyond the existing network,” explains Wolf. “However, the analysis of this mode of operation showed that the trains’ on-board batteries would be fully depleted on all but three of the seven routes modelled, so some level of additional power supply for recharging would be required.”
Addition of partial catenary
In a second scenario, the same seven routes were assessed with the inclusion of a 4 km length of overhead electrification – representing approximately 1.5 percent of the non-electrified network covered by the seven routes. Current would be collected via a pantograph sized to accommodate the requirements of 4C battery recharging in addition to immediate traction power needs of the vehicle. The results of this intermediate scenario indicated that the use of the battery-equipped train would be sufficient for all the timetable requirements with the exception of an express service between Groningen and Leeuwarden.
In a further optimised scenario, the length of partial catenary was extended by a further 6.5 km – representing a total of 10.5 km or approximately 4 percent of the network covered by the seven routes. “This final scenario demonstrated sufficient capacity to operate the entire timetable,” concludes Wolf. “Moreover, through the inclusion of this additional length of overhead line, the simulation showed that two station-based recharging facilities could be deleted, thus mitigating part of the infrastructure investment required.”
In addition to the assessment of zero-emissions solutions based on partial catenary and station-based opportunity charging, the Ricardo-Arcadis team also assessed options for hydrogen fuel cell propulsion using a battery-hybrid powertrain solution. Armed with the information generated from the study, the provinces of Fryslân and Groningen and the regional rail operator Arriva can now make an informed choice about the possible roll-out of a battery-powered fleet, with greater insight into the costs and the modifications required to trains and infrastructure.
Optimising the power supply
High-voltage overhead AC electrification systems such as the 25 kV standard used in Great Britain have a high transmission efficiency. The use of regenerative braking processes can therefore be highly effective within these systems as a vehicle returning energy back into the overhead line as it brakes can be geographically remote from another vehicle drawing power for traction.
However, this is not the case for the 750 V DC third-rail electrified routes of London and the South of England, as John Brown explains: “The voltage fall-off on DC systems restricts the usefulness of regenerative braking where energy is fed back into the track. Typically, such energy that is regenerated in braking will have a useful range of around 1.5 km on this type of third-rail system; beyond this, DC voltage drop means that it is merely dissipated and lost.”
Even more crucially for operational efficiency, many traffic pinch points on the third-rail network are restricted by electrical capacity rather than by signalling. This is particularly the case at busy stations or complex junctions where multiple trains might be accelerating simultaneously. “To address this issue and see how we can perhaps co-ordinate local scheduling to gain more effective use of the available capacity, we are exploring the modelling of power flows at such pinch points.” he continues. “To do this we’re using the Ricardo IGNITE software –a physics-based package developed for complete system modelling and simulation of complex systems that has been successfully used in both automotive and water industry applications. This may well provide opportunities to alleviate some of the challenges presented by power-restricted pinch points, while also making more effective use of regenerative braking on this type of DC network.”
Incorporating renewable energy
Elsewhere on the UK rail network, Ricardo is assisting in the Green Valley Lines project to incorporate community renewable energy schemes into the proposed smart electrification of the commuter lines running to and from the Welsh capital, Cardiff. Aside from its benefits in reducing the carbon intensity of rail traction, this approach can be particularly attractive in areas where the power grid is restricted and would otherwise require significant upgrades to supply such electrification schemes.
Working with infrastructure owner Network Rail and the Energy Saving Trust, the Ricardo team is identifying potential sites to install community-owned solar, wind or hydroelectric generators next to railway lines in the region. These sites would directly supply the new overhead lines with low-cost, low-carbon electricity, at the same time providing financial benefits to local communities who would own the generators. The study is also scoping technical solutions for directly connecting renewables to overhead electrified lines and analysing how best to integrate new energy storage technologies to help keep electrification costs down.
Elsewhere, Ricardo is also collaborating on a research project to incorporate local solar PV generation capacity into the electrical distribution system of the DC third-rail network south of London. In this case the objective is both to provide a source of geographically distributed demand to match local solar farms, while also enabling the railway’s own distribution network to act as a shadow grid where connectivity to the main distribution grid is difficult.
Air quality – the big unspoken issue
Given the patchwork nature of electrification in the UK, a significant and increasing cause for concern is the exposure of passengers and staff at major stations and railway depots to pollution such as diesel exhausts and other particulate emissions in the working environment.
In 2012 the World Health Organization’s International Agency for Research on Cancer (IARC) re-classified diesel exhaust emissions and related ambient air pollution as carcinogenic and associated with increased mortality from lung cancer. Employers are responsible for managing the risk from exposure to this and other hazardous substances: this applies to both workers and others who might be affected; in the case of railway stations, this means the travelling public. So, station and train operators who have employees working on site have a legal duty, so far as is reasonably practicable, to manage the risks to the health of their employees and passengers resulting from exposure to hazardous substances as specified by the regulations for the Control of Substances Hazardous to Health.
“It might be concluded that the worst incidences of station air quality would be found in major termini served by diesel fleets,” explains Brown, “but station topology can have a very significant impact too.”
He cites the case of Birmingham’s New Street Station, which, despite having overhead electrification as part of the West Coast Main Line network, is also served by numerous cross-country and local services
which cover non-electrified lines for at least part of their routes and are hence operated using diesel-powered rolling stock. “Many of these services will change direction at New Street and remain at the platform for an extended period,” he explains. “To compound the situation, the station itself is approached through a cutting and has a major retail development above; as such, it is in effect a large underground station.”
Ricardo’s rail and air quality teams are researching the issue of such pollution hotspots, and are examining potential abatement options for the immediate, medium and longer term. And examination of this issue goes beyond consideration of diesel emissions. In the Netherlands, for example, where diesel usage is comparatively low, discussions are currently focusing on non-exhaust sources of PM2.5 emissions from rail, such as those arising from the wheel/rail interface, the action of pantographs on the overhead contact lines, and from friction-based brakes.
Hybridization and last-mile zero emissions operation
While partial electrification schemes coupled with battery-equipped trains as modelled by Ricardo in the Netherlands may provide a long-term means of removing diesel exhaust from hotspots such as stations and depots, a more immediate solution is needed in order to alleviate the worst cases within a shorter time horizon. One innovation being actively explored by Ricardo in the UK is that of the hybridization of the existing diesel-powered fleet.
“decarbonising the railways is clearly a complex challenge for which there are multiple potential technological solutions”
Ricardo is working as part of the HybridFLEX project with UK-based rolling stock owner Porterbrook to provide engineering support with the integration of an MTU hybrid powerpack into a converted Turbostar diesel multiple unit train. This is a first in the UK rail industry. The HybridFLEX concept aims to eliminate diesel operation in built-up areas where there is no electrification of the rail network, through the use of battery operation during the last mile of running in the vicinity of urban areas, as well as during station stops. In addition to reducing diesel emissions in such urban areas, a further benefit will be to significantly reduce noise.
When diesel power is required for propulsion of the train it will be provided by an engine conforming to the latest EU Stage V emissions standards (due to come into force in 2021). With the on-board batteries capturing the energy usually lost when slowing the train through braking, coupled with a more efficient engine and transmission, the overall CO2 emissions will be reduced by as much as 25 percent when the train enters service in 2020. The initial trial period should also confirm the higher performance and capability of the hybrid systems compared to the standard variant of the Turbostar train.
Ricardo’s engineering team is performing all the integration engineering and safety assessments of the bare hybrid powerpack provided by MTU for the existing Turbostar train for conversion into the first HybridFLEX vehicle. Ricardo is also overseeing integration of all the mechanical, electrical and critical control systems, committing teams from its Rail and Automotive divisions to support all the engineering tasks; the teams will also cover the relevant safety, certification and approvals activities to ensure the safe and successful operation of the train during public trials.
“The UK has many diesel vehicles on its network,” explains Olivier André, UK MD of consulting, Ricardo Rail. “This is an opportunity to show they still have a viable long-term future as hybrids, offering reduced NOx, carbon and noise emissions as well as providing the potential for lower operating costs.”
Once the completed trial confirms the expected performance advantages of the hybrid system, Porterbrook will work with existing and future Turbostar customers to evaluate conversion of these fleets to HybridFLEX, accelerating the move away from diesel-only trains in advance of the UK Government’s stated 2040 target.
Innovation focused, technology neutral
Decarbonising the railways is clearly a complex challenge for which there are multiple potential technological solutions; each is appropriate to different passenger network types and locations – from light rail, trams and tram-train networks through to high speed intercity trains, and from the highly electrified lines of Europe to the patchwork electrification of the UK. Inextricably linked to this challenge is the requirement to provide a safe working environment for staff and passengers, as well as value for money for fare payers and governments and regions investing in new infrastructure.
As the work of Ricardo in this area is showing, innovations in simulation technologies, power networks, renewable energy, and air-quality monitoring and control can offer significant advantages when considered in parallel with efforts to decarbonise rail travel. This is only half of the rail picture, however, and in a future feature we will consider the emerging opportunities to further reduce the carbon intensity of rail freight operations.