Mott MacDonald Technical Principal Chris Tindall assesses the positives and negatives of a range of technologies promoted as an alternative to traditional LRT electrification.
While conventional overhead (OLE) or third-rail electrification are still the go-to solutions for most of the tram and light rail industry, recent years have seen progress in a number of alternative traction power technologies for rolling stock.
Significant drivers for such developments have been the desire to reduce infrastructure costs, to provide more aesthetically-pleasing tramway installations within our urban environments with the use of overhead-free alignment, and the ability to economically re-introduce passenger services to existing sections of non-electrified alignment.
In terms of modern tramways, early alternative solutions involved either onboard traction batteries (typically in the form of Nickel-Metal Hydride cells), or onboard supercapacitors. These technologies established a new form of technology, generally termed ‘Onboard Energy Storage Systems’, or OESS.
Other alternative traction sources in the form of ground-level power supply systems have been developed by Alstom and Ansaldo STS (now part of the Hitachi rail group), but this approach has not been adopted more widely within the vehicle supply market.
Whilst these early OESS systems provided opportunities for sections of tramway to be non-electrified, they had limitations in terms of ultimate range, power and, in the case of batteries, recharging time. However, since these applications, technological developments have led to the widespread personal, commercial, and industrial use of lithium-based batteries. This has resulted in their use becoming technically and economically feasible for application to commercial passenger transportation.
The suitability of lithium batteries within a tramway environment is dependent upon the chosen battery chemistry, as there are a large number available, with differing capabilities in terms of performance, safety, and durability.
Key factors in the selection of an appropriate lithium battery chemistry for a tram or light rail solution are: the ability to provide the required performance, alongside ensuring safety and resistance to thermal runaway (a failure mode whereby chemical reactions within the cell result in uncontrolled and continued elevation of cell temperature, generally resulting in cell failure, rupture and/or explosion); delivering a lifespan which is operationally viable; and delivering all of the above at a cost which provides meaningful savings over the equivalent cost of conventional electrification.
Lithium battery chemistries such as Nickel Manganese Cobalt (NMC) are widely used throughout the consumer road electric vehicle (EV) market. However, aspects related to stability, durability and cost have resulted in the majority of new battery-based OESS solutions in the rail market using either Lithium Iron Phosphate (LiFePo) or Lithium Titanate (LTO) chemistries. Both have good safety performance in terms of resistance to thermal runaway, acceptable durability interms of overall charge/discharge cycle life, and a cost that is feasible for use on trams and LRVs.
Supercapacitors (and lithium battery/supercapacitor hybrids) are still offered by some vehicle suppliers for tramway applications, but the overwhelming majority of current OESS solutions are based upon lithium batteries.
Whilst the capability of each OESS solution is dependent upon its specific capacity, and the environment and conditions in which it is used, the development of lithium battery OESS has allowed previously unachievable lengths of non-electrified alignment to be negotiated in passenger service using purely onboard stored energy. Additional benefits include the potential to recover energy during electro-dynamic braking and store this within the batteries for use during off-wire traction phases.
Existing battery OESS technology is not generally suitable for use across an entire network as the sole mode of traction power delivery, due to the time required for charging and the high currents required from the power supply system. High current charging points can be implemented to boost charge levels, butsuch charging points are typically located at termini due to the longer dwell periods, allowing a greater level of recharging. Considering current technology, battery applications are more suitable to a bi-mode application where OESS power is used to negotiate key non-electrified route sections, with conventional electrification used across the remainder of the network.
Supercapacitor OESS solutions can be used as the sole traction mode, as they can rapidly charge during typical dwell times at each stop. These systems have potential for non-electrified networks, with charging points installed at each stop. The downside is that, whilst supercapacitors are very quick to recharge, they also discharge quickly, and are therefore not typically suitable for applications with longer inter-stop distances, or where significant gradients exist. While hybrid solutions which combine both lithium batteries and supercapacitors do exist, and address some of the above issues, they impart other challenges surrounding the complexity of installation, cost, and their impact on the vehicle’s maximum axle load.
With regards to battery OESS sizing, it is worth noting that only a portion of the total battery capacity is used for normal service – typically from around 95% down to 55%. The remainder is used to accommodate factors such as ‘worst case’ operational conditions, emergencies, and future reductions in battery capacity due to degradation. All these need to be considered when establishing the correct battery capacity for the application.
While the carbon impact associated with the extraction of raw materials for battery manufacture is not insignificant, analysis by Mott MacDonald has indicated that this is likely to be comparatively lower than that of conventional overhead electrification, largely offset by the carbon impact of catenary pole concrete foundations and larger traction substations.
The carbon impact of OESS solutions also has the potential to be improved further via the implementation of second-life applications for used batteries, once they’re unable to provide the level of performance required for passenger service (for example in land-side storage applications).
Moreover, OESS solutions offer opportunities to reduce depot and maintenance workshop infrastructure, improving occupational safety via the use of non-electrified layouts – reducing construction costs. The implementation of battery OESS does create requirements for additional storage space for spare battery modules, however, and additional equipment and working practices to ensure safe isolation, grounding, and access within the OESS modules for maintenance purposes.
So, while there are many benefits, there are also a number of limitations and disadvantages of this type of technology. The capability of an OESS solution to deliver the traction, operational and durability performance required is linked to the route, operational requirements, and ambient conditions in which it is being used. To avoid over- or under-sizing of a planned installation, detailed modelling and simulation is needed to determine the energy required in the worst anticipated conditions, including emergency recovery scenarios.
If any of these parameters change in the future, the impact upon the OESS needs to be carefully modelled to avoid repeated excessive depletion of the batteries. This would be likely to result in premature ageing of the assets, and hence more frequent replacement. In the very worst case, it could also result in the vehicle being unable to deliver the off-wire range and performance required.
This limitation can make changes to operational practices, routes and diagrams more onerous in comparison with conventional electrification where essentially, if overhead electrification exists, the vehicle can operate regardless of changes.
Hydrogen Fuel Cells
To date, the implementation of Hydrogen Fuel Cells (HFC) as a source of alternative traction power on rail vehicles has primarily occurred within the heavy rail market, with suppliers such as Alstom and Stadler producing hydrogen-powered variants of their existing vehicles.
Within the tram and light rail market, development of hydrogen solutions has been much more limited, with a small number of hydrogen-equipped vehicles being manufactured by CRRC, short tourist lines in Oranjestad and Dubai and current development projects by South Korea’s Hyundai Rotem and Germany’s HeiterBlick.
HFC applications typically involve the fitment of additional equipment in the form of the fuel cells themselves, hydrogen storage tanks, DC-DC converters, energy storage batteries (to recapture excess and/or recovered energy), thermal management equipment, and other sundry items. HFC can be used both as the sole traction mode to provide power across an entire non-electrified network or can be used as part of a bi-mode application similar to that previously described for battery OESS.
The vehicle-mounted tanks are filled from dedicated filling stations, either on-depot or at key network locations. Production and delivery of hydrogen can be achieved via a number of different methods; however, the most sustainable approach would likely involve the use of renewable energy to conduct hydrogen production via electrolysis. While this can provide a low/zero carbon approach, unless production can be realistically achieved on-depot (as has already been achieved in some HFC bus applications), then the transportation and distribution of the hydrogen also needs to be considered from a cost, operational and environmental perspective.
As with battery OESS, initial assessments indicate that most HFC applications could result in a reduced carbon impact compared with conventional electrification. This is, however, dependent upon the scale of its use, the amount of OLE it would replace, and how the associated hydrogen would be manufactured and distributed.
Unlike OESS, HFC solutions have increased operational flexibility, as a vehicle can potentially (dependent upon onboard storage tank size) be filled with hydrogen at the start of service, then remain in use for the remainder of the day, or until refilling can be scheduled. This results in short- and long-term changes to service, operations and timetables being more easily achieved than with a battery solution, based upon existing technology.
Whilst there are undoubted benefits to HFC applications, feedback from discussions with manufacturers suggests that the vehicle market generally considers battery- or supercapacitor-based OESS as more suitable technology options for the tram and light rail mode of operation. Reasons for this are likely to be the generally limited inter-stop distances, terminus dwell times to allow re-charging, and constraints related to vehicle weight and available space for mounting equipment.
Whilst the rolling stock market currently seems to have a somewhat suppressed appetite for HFC solutions in tram and light rail applications, there are clear benefits where stops are more widely spaced – for example in tram-train type applications. For tram-train, HFC could feasibly be used as a low carbon replacement for the segment occupied by diesel/electric tram-trains, and hence delivering significant off-wire operation and inter-stop distances. This could currently only be achieved with OESS technology via the fitment of excessively large, and heavy, battery packs.
Whilst we have outlined distinct advantages and disadvantages for OESS and HFC applications, some common challenges exist. A primary issue with both technologies on tramway infrastructure is their weight and the impact this has upon both overall vehicle weight and maximum axle loads. Tramway infrastructure has historically been designed to maximum axle loads in the region of 10-12t, which modern rolling stock can generally achieve, even when crush laden with passengers.
The introduction of OESS or HFC as either the sole traction power source, or as part of a bi-mode solution, can have significant effects upon both overall weight and the ability of the vehicle to comply with the maximum axle load limits of the infrastructure. It is therefore critical that this issue is considered throughout the vehicle procurement process.
Additional challenges exist due to the innovative nature and complexity of these technologies, the lack of historic service proven reliability, and the impact this could have upon the overall reliability of the vehicle. Whilst there are now some examples of battery OESS in high-density passenger service, and industry standards are now being developed and used, HFC is still in its comparative infancy within the tram and light rail industry. With either technology, it is critical that accurate and appropriate specification requirements are defined, and that detailed design scrutiny is applied throughout the procurement process, from tender evaluation and design review, to ultimate testing and commissioning.
The final common issue is cost, both from an initial capital procurement perspective, and also from a long-term lifecycle perspective. Both OESS and HFC solutions have increased capital costs with the initial procurement of the vehicles/spares, but also operational costs associated with the maintenance of equipment and periodic overhaul/replacement during the vehicle’s life.
These need to be considered alongside the potential cost savings which may be realised due to not installing conventional overhead catenary, and the capital and maintenance savings that may be achieved due to this. It is therefore important that detailed lifecycle analyses are included as part of any package looking at the feasibility of implementing OESS or HFC technology.
Smaller, lighter, faster
Whilst OESS and HFC technologies are still in their relative infancy compared to conventional overhead electrification, there is no doubt that future developments are worth exploring and will see these technologies being used more widely. As industry confidence grows, standards, best practice and precedents will grow too, which will likely result in these solutions becoming more commonplace within vehicle procurement specifications.
It is also likely, alongside this growing industry confidence and expertise, that OESS and HFC technology will develop rapidly, resulting in smaller, lighter, faster charging and more energy-dense solutions which can solve many of the issues explored within this article.