For those working on heavy rail, the way streets absorb tram infrastructure to show only running rails is impressive indeed. Even more impressive is that some streets are now absorbing the tram’s power supply as well, the result of a drive by the major tram manufacturers to make trams more attractive by eliminating the need for overhead wires.
This is actually a far from new idea as early trams collected current from ploughs in below-street conduits. These were connected to the tram through a slot in the centre of the tramway but were labour intensive, expensive to install, and vulnerable to items dropped in the slot. Blackpool has one of the world’s earliest electrically powered tramways. When it opened in 1885, it was powered by a conduit system that was converted to an overhead supply 14 years later. The conduit didn’t like Blackpool’s sea and sand.
For modern trams, there are now various second-generation catenary-less systems of which ground level supplies are just one solution. Others include the latest energy storage technology and overhead top-up at stations. All are the fruits of a commitment to innovation by manufacturers to challenge the conventional idea that trams must be powered from an overhead catenary.
As these companies compete for their share of the worldwide expansion of light rail systems, they strive to make their products attractive to their customers, the city authorities. They, in turn, need to convince the people of the city. Eliminating overhead wires removes visual intrusion which is a critical factor in historic cities. Moreover, utility work for its poles or fixings on people’s houses can also result in objections. Although public opinion is a significant issue, it will also be seen that energy efficiency and lower infrastructure costs also provide good reasons not to erect tram wires.
Ground Force – French-style
Alstom’s Alimoitere Par le Sol (APS) was the first second-generation catenary-less tram system. It began operation in Bordeaux, a UNESCO world heritage site, in 2003. Up until 1958, Bordeaux’s trams used the conduit system, and it had been assumed that the new trams would operate similarly. However it was decided that, as the old conduit system was not suitable, the new trams would have overhead wires. The resultant protest from both the public and the French Ministry of Culture resulted in the development of the APS system which is used for twelve kilometres of Bordeaux’s 44 kilometre tram network.
Alstom’s APS consists of a conduit, flush with the ground, on top of which are 8 metre long contact strips alternating with 3 metre long insulated segments. Inside this conduit are the supply cables and an antenna. There is a power supply box adjacent to every other insulated segment that feeds the adjacent contact strips. These are energised only when the antenna detects that the tram is wholly above the contact strip. To maximise power transfer time, contact shoes are in the centre of the tram.
The APS system had some initial teething problems. Like Blackpool in 1885, one of its initial problems was seepage and moisture. This required electrical housings and insulators to be redesigned. Now that these issues have been resolved, the Bordeaux tram system is 99.8% reliable and the city is now satisfied that it has a robust, reliable system.
Alstom claim that the installation cost of APS is comparable with that of tram catenary wire as the APS conduit requires minimal civil engineering work, being only marginally deeper than the slab track. In contrast, particularly in complex city areas, a catenary may require utility work for its masts and significant legal costs may arise if it has to be fixed to adjacent buildings.
A potential cost issue is maintenance of switchgear, with power supply switch boxes embedded in the street every 22 metres. Alstom advise that these are modular units which can quickly be replaced. Maintenance is by programmed replacement over a long period of time, enabling the units to be overhauled at service centres.
APS is also now in use as part of the Angers, Reims and Orléans tram systems. Outside France, work has started on APS tramways in Brasilia and Dubai which are planned to open in 2014. In Dubai the trams will have brushes to keep the contact strips clear of sand.
The Alstom APS system is not the only ground-contact system on the market. In Italy, Anasaldo have developed a modern version of the original conduit system. This has flush conductor and return rails in the centre of the tramway. In the troughing beneath is a flexible ferromagnetic belt which is lifted by magnets on the tram to energise the conductor rail. As the running rails do not carry return current, the system can be used for buses. The first use of this system will be on a section of Napoli’s tramway at the end of this year.
Also at ground level, but invisible, Bombardier’s PRIMOVE system is entirely hidden by the city’s streets. PRIMOVE uses buried inductive loops between the tracks to transmit power to trams. These loops need to be covered by a 40mm layer of non-conductive material such as resin, asphalt base or non-reinforced concrete which may need to be carefully installed or it might be vulnerable to heavy traffic.
Each looped cable segment is eight metres long and transmits 200kw. It is fed by an inverter which transforms 750 volt DC into 200 kHz AC. This system has transmission efficiencies of between 90% and 95%, which Bombardier advises is only 2% less than contact systems.
Like APS, PRIMOVE is only switched on when the tram is above it by a maintenance-free solid-state unit. Power transmission loops are generally located at stations and gradients as required by the tram network. The loops fit above sleepers and so involve no additional civil engineering costs.
Unlike APS, it does not continuously power the tram so energy storage is an essential aspect of the PRIMOVE system. Bombardier’s MITRAC Energy Saver uses super-capacitors and was originally designed to store energy from regenerative braking. Trials have shown savings of up to 30% of traction energy.
The PRIMOVE concept has been successfully demonstrated in Augsburg where Bombardier low-floor trams have been using it on an 800 metre spur line to the city’s exhibition centre since 2010. Further testing has been done at Bombardier’s e-mobility hub in Mannheim which opened in September 2011 and which has also tested PRIMOVE buses, minivans and cars from 3.6kW to 200 kW.
Trolley buses without wires
As PRIMOVE vehicles are not limited to rail or a fixed route, its equipment can easily be fitted to road vehicles. A battery powered bus running, say, 250 kilometres per day requires a six or seven ton battery. Bombardier have calculated that if PRIMOVE is used for charging, only a one ton battery is required and that a single charge point, located at a central part of the bus network could, typically, provide 20 charges a day with no effect on the bus service. Charging buses in this way also extends battery life. Thus for minimal infrastructure investment, it could enable a city to operate a fleet of electric buses with the same performance as diesel buses.
Although PRIMOVE buses would seem to offer huge environmental and cost benefits, these have yet to be demonstrated in practice. To demonstrate this concept, a pilot programme of PRIMOVE bus operation is planned for five cities in 2013 – Bruges and Lommel in Belgium and Augsburg, Braunschweig and Berlin in Germany. These pilots involve various types of buses of up to 200 kW and 18 metres long. In Braunschweig, the German Ministry of Transport has given a grant of €2.9 million for the PRIMOVE bus pilot on a 12 kilometre section of its bus network to become operational autumn 2013.
PRIMOVE’s greatest potential impact is its use in cars. To test this concept, the Lommel pilot bus scheme includes tests with a 22kW Volvo C30 car. Until now, battery size, range and time-to-charge have been insuperable constraints to the widespread introduction of electric cars. By removing these constraints, PRIMOVE has the potential to change motoring as we know it.
The storage solution
Siemens have been building trams since they provided the world with its first electric tram, powered from overhead wires in the Berlin suburb of Lichterfelde in May 1881. It’s therefore no surprise that they also offer trams without wires using their Sitras HES (Hybrid Energy Storage). This system has been in use in Lisbon, Portugal since November 2008 and has been selected by Qatar for its Doha tramway which will be operational in 2015 as part of the preparations for the 2022 World Cup.
HES is a modular system that can either be built into new vehicles or installed in existing trams, enabling them to run for distances up to 2.5 km without wires. It comprises two 820kg roof mounted units: a nickel-metal hydride cell (NiMH) battery and an MES (Mobile Energy Storage) unit using double-layer “super capacitors”. Batteries have a higher energy density than super-capacitors but take longer to charge. In the HES unit the respective stored energy of batteries and super-capacitors is 18 kWh and 0.85 kWh whilst the respective power output is 105 kW and 288 kW. For this reason, HES uses super-capacitors for acceleration and batteries for steady speed. Another advantage of such storage systems is that they eliminate power spikes when several trams accelerate at the same time.
Unlike APS or PRIMOVE, Siemens HES trams require an overhead supply. This may be conventional overhead wires on part of the network or a Sitras LCU (Local Charging Unit). The LCU is a short length of overhead conductor rail placed at stations or other stops that can deliver a 1,000 amp charging current during a typical 20-second station dwell time. HES is also charged from regenerative braking, which Siemens claim can reduce energy consumption by 30%.
A similar system has been developed by Spanish manufacturer CAF. This uses its ACR freeDRIVE which, when used in conjuction with the ACR evoDRIVE, developed to save energy from regenerative braking, offers up to 1.4 kilometres of catenary-free operation. It has been used on a 1.6 kilometre section of Seville’s tramway since 2010 and is also in use in the Spanish cities of Zaragoza and Granada .
In Nice, Alstom have fitted their Citadis trams with extra NiMH batteries for 500 metres catenary-free operation across two historic squares. On the Paris T3 tramline, a Citadis tram has been fitted with a bank of 48 supercapacitors to simulate catenary-free running.
Alstom is also trialling flywheel storage on a tram in Rotterdam. This is roof-mounted and runs in a vacuum at speeds around 20,000 rpm providing a net energy storage of 4kWh and 325 kW peak power. It will be interesting to see if a modern flywheel can compete with the increasing use of super-capacitors.
And the winner is….
With UNIFE predicting a 9.3% increase in the world’s urban rolling stock market over the next 5 years, tram manufacturers are doing all they can to increase their market share and catenary-less trams are one way to do this. However, the technology is quite novel. The oldest system, Alstom’s APS, is less than 10 years old and has now proved itself in service, although initially it had significant teething problems which have been resolved. Other systems offer great potential but are quite new and have yet to be subject to the intensive use needed to demonstrate their reliability.
All these systems have their pros and cons and what is best for one city will not be for another. For these reasons it would be wrong to name the best system. However, with its potential to increase the number of electric vehicles on the road, PRIMOVE offers an interesting potential environmental benefit.
The real winners are cities that now do not have to erect tram wires in their city centres. This not only preserves historic centres, but, in complex city centres, could reduce construction and utility costs. However, the case for catenary-less trams outside the city centre is less clear.
Another surprising winner, albeit in the long term, may be the motorist for whom fuel pumps could be a thing of the past as they drive PRIMOVE electric cars with the performance and range of today’s cars. Transferring technology from rail to road in this way shows just how innovative rolling stock manufacturers have become. There are, no doubt a lot more innovations to come.