From Berlin to Buenos Aires, Moscow to Melbourne, overhead catenary is a familiar necessity along tramway and light rail networks around the world.This has been the case for more than a century, having its genesis when Werner von Siemens presented the world’s first tram using the system in 1881. Steady evolution has brought improvements in reliability, safety and efficiency, such as the transition from trolley pole to pantograph, and the advancement of regenerative braking.
While still generally accepted as achieving the best balance of performance, safety and cost, the installation of overhead wire brings several challenges and compromises. Electricity supplied at several hundred volts through unshielded wire in public spaces can bring safety hazards which must be controlled. Infrastructural necessities such as movable drawbridges can create challenges in providing continuous electrical supply and ground paths along alignments that cross waterways. Visible aerial wires can spoil important sightlines, such as of historic buildings or parks, reducing the appeal and public acceptance of a system.
For new systems, relocation and modification of other overhead infrastructure, such as traffic signals and bridges, can make catenary installation difficult and expensive. The continuing popularity in electrically-powered urban rail over the last few decades has inspired planners, engineers and the public to look for alternative arrangements so that these concerns can be addressed. These alternatives can fundamentally be grouped as relocated continuous electrical supply, and onboard electrical energy storage.
Relocated continuous electrical supply
Subways have for many decades commonly used a third-rail supply, allowing underground tunnels to achieve a much lower profile than would otherwise be required. When running above ground, this kind of system removes the visual obstruction from the skies, and would theoretically also solve the interface challenges with overhead infrastructure. However this classic arrangement is of course incompatible with in-street running, or even occasional road and pedestrian crossings. Recent derivatives of this philosophy for tramway applications feature flush-mounted conductor rails running between the tracks, with the conductor rail segmented by electrical or mechanical means, activated when a train is detected above each section.
At this stage of development, however, this has proven to be significantly more expensive to install than traditional overhead catenary, due to the amount of equipment and controls required to ensure public safety. In addition, the system has a high degree of dependency between vehicle and wayside, with proprietary interfaces limiting an operator’s flexibility in any system expansion, and vehicle retrofits. Furthermore, the continuous circuit does not solve the difficulty in connecting tramways over moving bridges.
“While at this stage such distances are only possible in favourable and controlled circumstances, this achievement gives a glimpse of what the future of tramways may hold.”
Onboard electrical energy storage
On lines with heavy traffic, regenerative braking has added significantly to the efficiency of modern tramways. However, for the regenerated braking energy to be of use, other vehicles must be both within the same non-insulated catenary network section, and in need of higher power, such as during an acceleration phase.
Furthermore, even within the same non-insulated network, the further the current must travel, the higher the catenary losses. Without these circumstances playing out ideally, the regenerated electrodynamic braking energy is primarily fed to the braking resistor to be radiated to the atmosphere. The advancement of double-layer capacitors and batteries over the past 15 years has allowed engineers to explore ways that this energy can be captured and reused more reliably.
Batteries and double-layer capacitors each offer a path towards the ultimate solution to the problems raised earlier: eliminating the requirement for a continuous power supply throughout the alignment. However, in detail, their respective benefits and detractions are somewhat opposite to one another, because of their contrasting energy and power density. With unlimited weight and space, each could fulfil the power and energy properties of the other, but within the strict weight and spatial restrictions of modern rail vehicles these differences are brought into sharp focus.
Double-layer capacitors feature an impressive power density, but a low energy density. Therefore, while relatively little energy can be stored within a given spatial and weight envelope, it is rapidly exchanged between storage in the capacitor bank and usage in the propulsion system. This characteristic lends itself towards short bursts of high power, but cannot endure sustained power consumption over a longer duration.
Batteries, on the other hand, offer high energy density but low power density. Sizing of battery systems is driven by multiple factors, including instantaneous power intake and outtake to match acceleration and deceleration requirements, redundancy and rescue scenarios, while reaching acceptable depths of discharge so the life of the batteries is not compromised. Cell-based monitoring and management of charge level and thermal properties is a key factor in the successful implementation of such systems. In limited-space applications, the lower instantaneous power available from a battery system can limit acceleration, as well as the power which can be recuperated back into the storage system during regenerative electrodynamic braking.
Ultimately, the optimal solution for ‘off-wire’ running would be for a system with the energy density of batteries, and the power density of capacitors. Researchers are developing new technologies to overcome the disadvantages of each method, and find solutions that bring a happier medium.
In the meantime, combined systems such as that being implemented on Siemens’ Avenio for Qatar Education City, use batteries and capacitors alongside one another to exploit the benefits of both approaches. However, a two-pronged approach is not practical for every system.
The making of a World Record
At this point in time, the most promising best-of-both-worlds solutions have been found through new battery chemistries, providing higher power density, with improved charge and discharge rates. In particular, Lithium-Ion batteries have reached a window whereby the energy required for a tram to traverse distances of over one kilometre, with a discharge rate sufficient to provide acceptable acceleration of more than one metre per square second, can all be installed within the available roof space of an otherwise-standard light rail vehicle. It was using this approach that Siemens, with technical partners Corvus Battery and Medcom Power Solutions, developed a system to be installed on a newly-manufactured S70 LRV.
In 2013, work started on the conceptualisation and design of a prototype system, with the aim of advancing each company’s expertise in energy storage for rail vehicles while endorsing and promoting the technology in the market. The system featured two packs of Corvus Lithium-Ion batteries, each containing 96 four-volt cells, arranged in parallel, each managed by a robust battery management system. Buck-boost converters from Medcom would then handle the voltage step-up and step-down controls for discharge and charge operations between the battery packs and the conventional propulsion system.
Siemens approached its longest-standing US light rail customer, Metropolitan Transit System (MTS) of San Diego whose latest Series 8 vehicles were in production, to provide a test vehicle – MTS enthusiastically obliged. Optimisation and tuning was performed on the test track at Siemens’ engineering and manufacturing hub in Sacramento, California, during which time the system was shown to have excellent efficiency and durability.
With functionality confirmed, MTS agreed for the vehicle to be shipped downstate to San Diego, where more advanced testing would continue on a real-world alignment with gradients and curves, not to mention a variable climate between the seaside and inland hills. Through this process, the team was able to identify areas to optimise the system, such as improving the effectiveness of the water-cooling of the batteries and the interface between the propulsion system and the energy storage control. The team calculated that with these advances, and given the efficiency of the system that had been shown in field testing, the vehicle would be able to challenge the existing Guinness World Record for the longest distance travelled by a battery-powered tram on one charge.
On 15 July 2014, a team from Siemens, Medcom and Corvus assembled with MTS operational staff in San Diego to make an official attempt at the World Record. Before leaving the MTS depot at around midnight, the battery packs were sealed with tape and signed by one of two independent witnesses. The tram was driven to the designated test location on the MTS Green line with the energy storage system disengaged, utilising power from the overhead catenary. At the test location, the energy storage system was engaged, and the batteries were charged via the pantograph connection to the overhead line. Once the battery system was fully charged to 390V, the pantograph was lowered for the duration of the record attempt, and therefore the batteries could not be charged further.
The tram was driven 10km (6.2 miles) at a restricted top speed of 40km/h (25mph) – representative of streetcar running in mixed traffic operation – back and forth, up and down hills, through curves, until the batteries were depleted to the recommended minimum depth of discharge. By that point, about 50 minutes later, the vehicle had travelled a distance of 24.596km (15.283 miles), eclipsing the previous record by more than 30%.
While at this stage such distances are only possible in favourable and controlled circumstances, this achievement gives a glimpse of what the future of tramways may hold. Driven by the high volumes required to support the nascent hybrid automobile market, battery size, weight and cost are simultaneously and rapidly being reduced. Performance, in terms of both energy and power density, is increasing.
The implementation of rapid charging while stationary at passenger stops, as is planned for Qatar Education City, can allow a catenary-free sky along whole networks, but even being able to traverse small sections without continuous power supply brings increased flexibility in planning alignments, and reduced installation expense.
This feature originally appeared in Tramways & Urban Transit – February 2015 issue (925).