Developments in Power Systems

Developments in alternative power systems will mean that tomorrow’s tramways can be sympathetic to both aesthetics and the environment.

It was the mayor of Bordeaux’s ambition to see his city designated a UNESCO World Heritage Site that pushed wire-free trams from aspiration to reality. The beauty of the port city’s historic architecture was being blighted by cars and car parks.

Bordeaux wanted an at-grade transport system, as an underground metro project had failed. The solution to the problem was a light rail network but there was one huge caveat: mayor Alain Juppé did not want catenary wires spoiling the views of Bordeaux’s streets, squares and buildings.

Thanks to the commitment of Bordeaux’s politicians and the determination of many engineers, the city did get a new tramway, with its wire-free section, in 2004 – as well as its UNESCO status in 2007. The selected off-wire solution sees the vehicles powered at ground level, and is working well in Bordeaux and in several other cities which have subsequently used the same system.

The next generation of wire-free trams are likely to go a step further, combining ground level power supply with on-board storage to take advantage of regenerative braking and provide a better capability for running over short de-energised sections. We are now seeing the development of tramways with sections that are both wire-free and use less electricity.

Wire-free trams are not a new concept. Since electricity took over from horses at the beginning of the 20th century, engineers have been looking for ways to power trams without the catenary wires.

At the time that Bordeaux was looking for wire-free solutions, there were at least three systems under development, none of them proven. Alstom chose one of those three possible technologies and purchased the patent for its design; this was to become the Alstom APS (Aesthetic Power Supply) system.

The Bordeaux politicians made a bold move in agreeing to fund the development of APS. However they were shrewd too: in return for their investment, they agreed a deal with Alstom where they received royalties for subsequent schemes that used the power system.

The APS system, powered by 750 V DC which is supplied via cables and switching boxes buried in the ground, works by activating the current in a conductor rail only when a tram is directly over it. The conductor rail is divided into alternate 8m-long conductive sections and 3m-long neutral sections, with the traction current collected by two collector shoes under the tram

It was a long, hard, struggle to produce a reliable system. The biggest challenge was ensuring that the buried power boxes, which contain sensitive electronic and electro-mechanical components, were robust enough to cope with the operating conditions.

There was a lot at stake during this period, not only the time and money that had been invested. The reputations of the organisations involved – including SYSTRA which was project manager for the tramway scheme – were also at risk.

Since Bordeaux’s first tram ran in 2004, other French cities followed suit with Alstom APS on sections of their lines: Reims, Orleans, Angers and Tours. But the most extensive use of ground-level power has been in Al Sufouh Dubai, the first ever tramway to be entirely powered without a catenary wire in sight.

Al Sufouh is unusual, in that the choice of a wire-free solution has not been driven by the need to preserve the aesthetics of a historic cityscape. Here the choice of ground-level power was made in order to meet the aspirations of client Dubai Road Transit Authority, to whom SYSTRA was consultant, to create a state-of-the-art system which would enhance Dubai’s status in the region.

The APS system has to be adapted for Dubai’s hot and dusty environment with brushes fitted to the trams to sweep sand from the third rail and air-conditioning for the buried switch boxes that power the rail.

Cuenca in Ecuador will also make use of APS over a 2km stretch of tramway which runs through the city’s historical centre, also a UNESCO World Heritage Site. Due to start operation in 2016, the 10km tramway will be the city’s first.

Constraints and challenges

The biggest drawback of the first generation of ground-level power supplied trams is that they cannot make use of regenerative braking. This means that they are 15 to 20 percent less energy efficient than a traditional catenary system with regenerative braking, with the resulting financial and environmental impacts.

On-board energy storage solutions have had their own challenges too. Perhaps the most significant is the amount of energy they are able to store; if one station’s recharging facilities are out-of-use, there are question marks over how far a tram could continue without having to conserve energy by turning off air conditioning for instance.

There are also concerns over the performance of the current generation of supercapacitors. Their life expectancy has not been proven and could be as low as seven to 10 years.

However, supercapacitor and battery technology is developing every day. Hyundai Rotem has developed a lithium-ion polymer battery which it claims can run for 50km on a single charge. The firm is supplying a hybrid catenary and battery-powered tram system for two new tram lines in Izmir, Turkey, the manufacturer’s first ever tram contract.

Issues around lack of interoperability and competition for ground-level powered systems have been a deterrent to some would-be clients. Alstom patented the design of its vehicles which are powered by APS, but was asked by the French government to share the patent of the current collection interface only with others during the procurement of phase 3 of Bordeaux’s tram network.

The future for wire-free trams will see a combination of both on-board storage and ground-level power or local (at passenger stops) overhead charging facilities. This allows the power from regenerative braking to be stored, improving energy efficiency even above a traditional system where the electricity is returned to the catenary wire with a loss of around 7 percent of energy. Manufacturers are targeting 30 percent energy savings.

This combined solution will be put to work on Rio’s tramway in Brazil. Here Alstom is supplying its Citadis tram system which will run 80 percent on APS and 20 percent on supercapacitor.

Despite the ongoing improvements to wire-free solutions, it seems unlikely that they will take over completely from catenary supplied trams. Catenary systems remain the most reliable and also require less capital outlay and less spend on maintenance during operation.

Additional equipment for collecting and storing energy can add up to 10% of the price of a vehicle. Ground level equipment, such as cables, junction boxes, inverters, a third rail and switches can add up to €1.8m per km of double track. The layout of the track is also more complex and some solutions are not compatible with a grass track bed. Maintenance of ground level equipment is also more costly than looking after catenary wires.

However, in situations such as Bordeaux, where a tram network won’t be an accepted solution without the ability to be catenary free, this technology is extremely important. We can also expect the development work carried out and underway on supercapacitors and new generation batteries will find its way onto trams running on conventional overhead wires, with resulting improvements in energy efficiency and operating costs.