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Next-generation traction batteries

Nice’s Citadis trams use battery power to cross the Place Masséna, as the city was keen to avoid the visual intrusion of overhead wires or the complexities of a third rail supply in historic squares. Image courtesy of N. Pulling

In the wake of COP26, key stakeholders in the rail industry are coming under increasing pressure to employ more sustainable ways of providing traction power. According to the UNIFE (Union des Industries Ferroviaires Européennes) association of European rolling stock builders and suppliers, rail transport is by far the most carbon-efficient mode of transport for both passengers and freight (Figure 1, right).

When it comes to reducing greenhouse gas emissions from rail networks, the greatest opportunity will be switching from diesel power to electrified lines. However, full route electrification can be both difficult and costly. A more affordable option is to adopt a hybrid method – a combination of partially-electrified sections, allied to onboard battery systems for any remaining non-electrified sections. It then becomes a balancing act of minimising costs and emissions.

Autonomous traction power has other advantages over just reducing emissions. In urban networks, a tram or metro train supplied by overhead or third rail sources may require an emergency traction supply in case of any electrical outages. An onboard battery solution here, for example, would give enough power for the vehicle to clear the line as well as offering enough auxiliary power for the doors, lights, and communications systems.

Figure 1. UNIFE study on the greenhouse gas emissions of various different modes of transport.

Figure 2. Jernbane-direktoraret’s cost-efficiency analysis of different rail traction technologies.

Traction battery solutions

Full electrification, by either overhead line or third rail, poses a number of challenges related to cost, delivery and safety. Bridges, tunnels, and low-traffic lines, for example, give rise to difficulties when installing electrified lines on certain sections of track. The most obvious alternative would be to partially electrify certain sections, applying onboard battery to power the vehicle for the rest of the time. This way, reduced and simplified electrification infrastructure can be installed more cost-effectively.

While there are a number of options to achieve autonomous traction, battery power has been found to be the most beneficial for the environment, economy, and culture. In its NULLFIB report (2019), Norwegian rail operator Jerbane-direktoratet investigated a variety of technologies (hydrogen, biogas, full battery, and partial electrification) to explore various potential avenues of achieving its goal of zero emissions.

Of these, battery power allied to partial electrification was identified as the most cost-efficient technology (see Figure 2). The Verband der Elektrotechnik (VDE), the German association for electrical, electronic and information technologies, found that the cost of BEMUs (Battery Electric Multiple Units) is comparable to Diesel Multiple Units (DMUs) over a 30-year lifetime, and will remain more competitive than hydrogen, even if the price of electricity were to triple.

Since trams already predominantly operate on electrified lines, installing onboard battery power is relatively straightforward and can be retrofitted in almost all cases. Not only does this provide traction power for non-electrified routes, it also increases vehicles’ energy efficiency on sections with fixed electrification infrastructure, especially during times of lower traffic.

Compared to older battery technology, today’s new-generation batteries have a high energy density and therefore a greater operating range away from fixed electrification systems. They are also smaller, lighter, and require less maintenance.

Saft – a company that has been specialising in advanced industrial battery technology solutions for over 100 years – has supplied and installed onboard traction batteries for urban transit systems across the world. The UK, France, and Russia are just a few of the countries using these technologies, including both lithium and nickel-based electrochemical solutions.

Evolving battery technology

Saft engineers installing and testing a battery system. Image courtesy of Saft

Nickel-based batteries have found their home in rail applications for many years. Typically, these are best-suited for onboard back-up supply, mainly for safety and communications systems, door controls, and passenger comfort systems such as lighting and heating, ventilation and air conditioning (HVAC). This is because they have a low life cycle cost, and provide high reliability, high availability, and are well-established in operation and maintenance practices. Nickel-based battery systems have even been implemented as an emergency traction solution in select cases, such as in underground metros in Moscow, which we will explore in more detail later.

New battery-powered tramway projects tend to focus on lithium-ion (Li-ion) batteries; this is a family of electrochemistries that has developed over the last 30 years.

Of the different forms of Li-ion, Lithium Titanate Oxide (LTO) is a relatively new type. It serves as the most suitable electrochemistry for rail applications as it balances operators’ safety, reliability, and lifecycle requirements. For example, LTO batteries’ cycle life is up to five times higher.

The trade-off is that, compared with other chemistries, LTO has a slightly lower energy density – the cells have to be larger to produce the same performance. However, when scaled up to the system level, this difference can be reduced by using the latest type of LTO cells that deliver a variety of benefits, including high power, a very fast charge, and a long cycle life.

With tight scheduling often a key area of concern for operators, charging time is, of course, an important consideration. Different batteries will charge at different speeds according to their internal electrochemistry and design. The latest LTO batteries can go from empty to fully-charged in six minutes – around a third of the time it would take for a typical Li-ion battery. However, both would be sufficient for most applications.

Another variable at play is the power supplied by the catenary or contact point; it is common for this to be the limiting factor in a battery system’s charging speed. It is also important to note that batteries are rarely charged from 0 to 100%, but instead receive regular top-up charges, either via the catenary system or through regenerative braking.

Battery system design and sizing

When designing the optimal battery for a specific rail application, multiple factors and variables must be considered.

Firstly, and arguably most importantly, is the operating profile of the tram. This includes the power profile of the lines, the planned service frequency, the maximum passenger load, and the route’s length and gradient. But rolling stock manufacturers and operators must also consider the environmental conditions, such as temperature. Since a battery is an electrochemical device, temperature affects the life and performance – higher temperatures, for example, will increase output but reduce battery life.

Regardless of the type of electrochemistry used, the capacity of every battery reduces gradually over time. So when sizing a battery, an engineer needs to include extra capacity to account for ageing. This ensures that the battery will still deliver sufficient performance towards the end of its life. Any extra capacity helps to cover rare events where a vehicle may be at maximum passenger load in particularly treacherous conditions.

Once these aspects are defined, the engineer can accurately model the volume and weight of the system.

Each OEM (original equipment manufacturer) needs to have its own clear priorities about safety, cost, lifetime, weight, volume, and maintenance. It isn’t as simple as choosing an ‘off-the-shelf’ solution.

Instead, it involves working side-by-side with a supplier to develop and fine-tune an approach to suit the needs of each tramway on an individual basis. This is a case of close collaboration with a manufacturer to not only select the optimum electrochemistry and size for the solution, but also to optimise the battery control system and system architecture.

At Saft, we have embraced a modular approach to delivering these solutions – one that combines the positive aspects of both off-the-shelf and tailor-made systems. This helps to reduce the time to market and, importantly, the cost.

By combining cells into modular bricks, they can be built up to deliver the right voltage and energy storage capacity for any given route. Allied to a Battery Management System, these can then monitor and control the performance and provide data to the vehicle’s management system. In this way, the solution remains bespoke, but keeps costs low.

The modular approach has brought about an innovative liquid cooling concept with various cold plates lying between the modules, drawing excess heat away using hydraulic connections.

Battery management and control are also essential. All Li-ion battery systems require electronics to manage their charge and discharge, but as batteries increase in size, this becomes more challenging. The latest thinking for larger batteries is to arrange them into ‘strings’, however different strings can fall out of synchronisation.

One solution to this is a sophisticated control system that can manage several of these strings at any one time, providing virtual battery operation. Through a singular control interface, an operator can manage all strings separately. For example, they can isolate one string for maintenance, and leave the others in operation.

Striding ahead in the West Midlands

The UK’s West Midlands Metro network has chosen an off-wire solution for sections of its expanded network. The CAF Urbos 3 trams feature Saft Li-ion battery modules mounted above the cab ends. Image courtesy of Neil Pulling

One tramway making use of this new Li-ion technology is in the UK’s West Midlands. When local mobility authority Transport for West Midlands was seeking to extend its tram network it sought alternatives to avoid the installation of expensive catenary infrastructure. In 2016, Saft was chosen to provide an onboard traction solution for CAF to retrofit to the West Midlands Metro tram fleet. After a visit by the manufacturer to the Saft factories, Li-on was identified as a suitable and safe solution for the application.

Battery systems were retrofitted onto the roofs of the system’s 21 Urbos trams. This solution allows the batteries to be charged on electrified sections of the network, letting the trams operate without the need for fixed overhead lines over several kilometres of new routes through the city centre.

Sizing the Li-ion batteries is relatively straightforward for tramway applications. As trams run the same route, day-in day-out, an engineer has an accurate estimate of how much power and energy the batteries will require. So, the maths are relatively easy to calculate once the maximum passenger capacity, the temperature range of the local climate, and variations in altitude along the route are known.

The lifetime requirement is another critical factor on battery sizing. A tram should be built for 30-40 years of reliable service with regular maintenance. In this case, Saft sized the batteries to provide a lifetime of at least seven years to match CAF’s maintenance intervals.

In case of emergency: Moscow

Saft is supplying MSX nickel technology battery systems for the 1300 new Moskva-2000
trains currently being delivered to the Moskva Metro. Up to 30% lighter and 40% smaller than conventional batteries, the batteries will enable the new vehicles to travel up to 6.5km (four miles) in the event of a power failure. Image courtesy of KSC Group

While batteries can be used to provide full traction power repeatedly throughout the day, they can also be used to provide just enough traction power in emergency situations. Moscow Metro, for example, wanted to boost passenger safety and comfort with an emergency traction solution for its new fleet of metro trains, the Moskva-2020. These would provide the trains with enough power to get to the next station in the event of a power outage.

Every metro train had to have the capability, at any time, to travel up to 6.5km (four miles), the maximum distance between stations on the line, and up a 40m incline within 1km (0.6 miles). The battery solution to be implemented was also heavily limited by the new train’s maximum operating weight and volume, and had to function reliably in temperatures that could easily drop to as low as -40°C during winter months.

These requirements offered an interesting challenge. Nickel-based MSX battery systems were chosen as the only solution on the market that could meet the demanding technical specifications for performance and size within the Moscow Metro’s approved technologies. MSX batteries are compact, light, and highly-resistant to extreme temperatures; additionally, the nickel technology offered reduced safety risks for the underground line.

From Birmingham to Belgium

The new tramway in Liège, Belgium, will feature trams equipped with onboard battery energy storage for off-wire operation; a mock-up of a CAF Urbos unit on display in the city’s transport museum. Image courtesy Mosbatho/CC BY 4.0

Following the success of the West Midlands project, CAF approached Saft to provide onboard batteries and accessories for 20 Urbos trams to be used on the future tram network in Liège, Belgium. A battery solution was required to allow for ‘wire-free’ operation over 3km (1.9 miles) of track.

In this application, the batteries feed into CAF’s Rapid Charge Accumulator (ACR) system. This allows the trams to be fast-charged at stops so they don’t require catenary charging over the rest of the route. The system also optimises overall energy consumption through regenerative braking, with the battery system capturing, storing and re-using recovered braking energy for later use in providing traction power. This improves energy efficiency, reduces overall costs, and lowers a tram’s overall environmental impact.

The batteries also need to provide reliable and safe operation for at least seven to eight years to match the service intervals of the tram, similar to the requirement in the West Midlands. Saft batteries were chosen for their battery control software, which optimises their lifetime by ensuring consistent ageing across all the cells in the system.

Nice: a wire-free solution

In 2005, the local authorities in Nice, France, wanted to extend the existing light rail service through two historic town squares. Aesthetic concerns were key as these areas needed to be kept free of overhead wires. Alstom, the rolling stock manufacturer, required the development and supply of a bespoke onboard traction power solution – the batteries had to be compact enough for installation on the tram’s roof, and needed to power the trams for roughly 500m through each town square.

A nickel-based traction battery system was specified for the city’s Citadis trams. Nickel technology was chosen for its excellent power storage in a compact, maintenance-free package. The unit was supplied as a ‘plug and play’ solution in a custom-built tray, with power, communication, and safety capabilities.

It is important to note that, even though this installation was completed roughly 15 years ago, the systems are still running, as the manufacturer endeavours to support any customer until the end of life of the tram. However, newer technologies, such as LTO, are available, giving newer customers more options to choose from.

Future innovations

The next step in technology is to support battery-powered traction for trams with less requirement for investment in catenary lines, building on the experiences of systems such as those in the UK’s West Midlands. These projects have used Li-ion batteries, but the next step will be the wider adoption of LTO as the latest-generation of rail batteries.

Looking even further ahead, the future of rail traction may lie in solid state battery technology, which eliminates the need for a liquid electrolyte. Hypothetically, this could have double the capacity of a Li-ion cell, whilst simultaneously improving safety further. Research is ongoing, and we could see solid state batteries established within the market by 2030.

Fixed electrification infrastructure not only comes at a financial cost, but also requires energy to build and maintain, which in turn produces greenhouse emissions. With regulations and budgets becoming more stringent, particularly in light of any new commitments resulting from COP26, we may very soon start seeing greater adoption of battery-powered traction as a more cost-effective and green solution.


Article appeared originally in TAUT 1009 (January 2022)