From batteries to supercapacitors, from ground supply solutions to hydrogen fuel cells onboard vehicles , the technology exists to take down the overhead wires from our towns and cities and run trams – but what are the costs, and what are the benefits?
There has been discussion in recent years, much of it on these pages, of the need to reduce the capital costs of tramway construction and operation. One area of keen attention has been the reduction of the provision for overhead power supply, yet, fascinating though these ‘new’ systems are, it must be demonstrated that these technologies meet a real need and are not simply ‘solutions looking for a problem’. TAUT has brought together three experts with nearly a century’s combined experience on tramway development and engineering for an open discussion on the topic.
Scott McIntosh: ‘Aesthetic objections’ to tramway overhead are often raised on new schemes, however it must be asked how real are these objections. It is true that many modern installations are singularly ungracious, but the OLE on UK systems such as Sheffield and Edinburgh, for example, is very neat and unobtrusive.
Jim Snowdon: It is fair to say that, compared to first-generation overhead, the design on earlier second-generation systems was not exactly elegant, if anything because the designers had more background in railway electrification than tramways. A neat overhead network is achievable if the designer understands that its task is simply to get power to the tram and is not a means of interconnecting the substations.
Croydon’s overhead was, in many ways, a classic example of over-heavy construction, largely on account of the use of double contact wire, polyester span wiring and traction poles that were, by most standards, pretty industrial. To be fair to the designers, the double contact wire was required by the system specification, but a neater, and long-established approach, is to use single contact wire, reinforced electrically by a parallel cable run alongside the track.
Such an installation is slightly more complex and expensive, but it is far neater and the parallel feeder will have a long service life. It worked for the Victorians, who had far bigger problems with voltage drop, and it still works today in Europe and indeed on some of the more recent UK tramways. An added benefit is that with a single wire the weight and the tension in the overhead is halved. That in turn leads to less strain on the traction poles, particularly at curves, allowing thinner and less obtrusive poles to be used. Visual appearance is influenced a lot by the means by which the overhead is held up, with polyester cable being noticeably fatter than the steel wire traditionally used. The contractor’s response is often that while steel might be less obtrusive and longer lived, polyester costs less – a factor that is unfortunately significant when funding mechanisms force the emphasis on construction rather than whole-life cost.
Nico Dekker: It is quite unpredictable which cities consider aesthetics when making important design decisions. It would appear that often the overhead supply is, in aesthetic design terms, treated as the poor cousin to stop furniture. In reality the power supply equipment may be far more intrusive. German systems are particularly disappointing; French and Spanish tramways however show what can be done.
I have fought many a battle to convince authorities that spending an extra EUR100 000 and devoting a small amount of attention on the appearance of the overall infrastructure really pays off. Some arguments I’ve won, but others I’ve lost and every time I travel on such systems I cannot help feeling what could have been.
SM: There are claims that the installation of conventional OLE adds considerable cost to a tramway. There are the substations, poles and wires and the earth current leakage containment methods to be considered.
Research by the UK’s Passenger Transport Executive Group (now the Urban Transport Group) in the 1990s showed that this whole bill amounted to 20-30% of the total Capex.Is this still true and are the earth current protection methods and utility diversions adopted in UK tramways really necessary?
JS: So long as tramways use conventional dc traction supplies, whether via an overhead contact wire or any other system, with return via the running rails, the threat of stray current will be held against the tramway. This is unavoidable, but management of the risk can have a significant impact on the costs of a tramway.
The behaviour of electricity is the same in Europe as it is in the UK, although from the design of UK tramway track one could be forgiven for thinking that stray current was a major problem. The result is over-engineering. Traditional track elsewhere in Europe is founded on a mass concrete bed with the rails cast in, while the UK counterpart is a concrete slab filled with steel reinforcement, supporting rails set in various forms of polymer compound. This is more complex to build, and accordingly more expensive.
The same result is obtainable by providing the trackbed excavation with a plastic sheet liner before pouring the concrete slab; any reinforcement is minimal and intended to control cracking, not collect stray current.
ND: The capital percentage is probably a bit less than 20-30%, but a lot depends on the civil engineering cost of the whole scheme. It is a myth that stray current containment/mitigation adds much to the capital cost; careful use of the reinforcement in the trackbed in embedded track is all that is needed. Our company was involved in the Manchester Metrolink extensions, achieving cost-efficient designs.
There are few occasions where one would divert utility services for stray current protection; in most cases they are moved because they obstruct the tramway or access would be made impossible without major disruption to services in cases of emergencies. What is less clear though is how often such emergencies actually occur and if valid risk-cost analyses are made.
More work is needed to obtain a solid statistical picture of failure rates; if you can save five out of the 50 million spent on diversions costs and once every ten years you need to have a short section of two-way working for a few days then that may be worth it; at the moment it seems to be belt and braces.
Once the decision is taken to step back from heavily-reinforced structural track slab, opening up the track to get a utility crossing is much simpler. The girder rail used for street track will span quite a sizeable gap without additional support.
SM: There are three main approaches to ‘wireless’ operation:
1) Ground supply systems where surface conductors are switched on as the car passes over them, Ansaldo’s Tramwave and Alstom APS systems are examples of this.
2) ‘Short hop’, where the car’s onboard energy store is topped up at every stop and by recovery of braking energy (e.g. Siemens SITRAS HES (Hybrid Energy Storage), the current Chinese systems or the Brookville system used in the USA).
3) Systems where the car has a primary generator on the car, balanced with energy storage, such as the TiG hydrogen fuel cell system, the Parry LPG-flywheel and various forms of hybrid buses. Each has its own merits and each works very differently.
ND: At one stage, Bombardier promoted supercapacitors as a means of achieving modest runs ‘off wire’, and more recently advocated a system of inductive power supply from coils embedded in the road surface.
The latter seems to have gone a bit quiet lately however.
JS: Network Rail published a study into simpler ways of electrifying secondary lines about two years ago; I hope this stimulates firms to develop onboard storage solutions.
SM: Although the APS systems seem to work well in France there are still questions around their installation costs and reliability in harsher climates. As yet we seem to have little to no data on the maintenance cost of the infrastructure or the lifecycle of all that complex switchgear in the ground.
In France APS is only used on segregated sections of track, can one use the supply infrastructure in mixed running or is it only suitable for segregated tracks? I also wonder about skid problems for rubber-tyred
vehicles crossing the track. Will this lead to more Roe v South Yorkshire risks? 
We also need to know the difficulty of fitting the collection gear under the cars, particularly if they are 100% low-floor vehicles. Additionally, what is the cost of the kit, both the APS collectors and the batteries needed to cover for interruptions in supply?
JS: The Roe case highlighted the fact that having a steel tram rail standing proud of the road surface can be a hazard, to the extent that the actual path taken by road vehicles, especially cars, needs to be thought about quite carefully. Mainland Europe has a more relaxed attitude, possibly by virtue of the fact that street tramways are still commonplace and road users know what not to do. However, in the UK adding what is, in effect, a third rail in between the running rails could well give rise to significant issues for road traffic.
ND: In Bordeaux the issue of mixed running came up as it was causing deformation/excessive wear of the contacts at locations where the track crossed the carriageway. In general mixing with heavy highway traffic would not seem advisable. However, if the application of the ground feed is limited to historic centres, removal of heavy traffic from these centres could allow the use of APS.
SM: All the ground supply systems are proprietary, what happens if the operator wishes to buy additional cars from another car builder – will they be able to buy the kit and fit it to another manufacturer’s cars?
We have seen what happens when manufacturers cease to support a product (GEC Maglev, Bombardier GLT) is there a risk APS could become an orphan technology?
ND: There is an obvious issue with proprietary equipment with regards to single sourcing, but that probably applies to trams themselves as well. In other words, the design-life of the power supply is linked to the tram’s life.
JS: I would agree as far as the non-overhead collection systems are concerned; unlike the overhead contact wire – which has served everybody since Sprague came up with it – the other systems are proprietary. It is difficult to see Alstom, for example, letting another car-builder have use of its APS system.
SM: The technology for this seems to be moving apace and Chinese suppliers have developed an elegant system with simple overhead power supply integrated into the tramstop shelter. I recall proposing something similar in the 1990s, but the then promoter failed to take it up. This solution relies upon a reliable onboard energy store, probably both a battery for longer distances and capacitors to accept the rapid charging from energy recovery. These storage units must take up space in the passenger saloon, add weight and both have a finite lifespan. Do we have any accurate data on how long they last and how much they will cost to replace? I understand that the Roma trolleybuses that were fitted with batteries are out of use because the batteries cost too much to replace.
JS: I can’t comment on the Roma trolleybuses, but these components do have finite lives and replacement is not going to be cheap. There is also the matter of how rapidly the technology goes out of date to the point where re-equipment, rather than replacement, becomes necessary. This is not necessarily the first thing that the salesmen want to admit to…
SM: There is certainly a benefit in having a way of storing energy during braking and using it to ‘unstick’ the car from a stop. The energy demand profile of a tram shows a very high demand in the first few seconds and falling during its run to the next stop, so if some of the waste energy can be stored it can significantly reduce overall demand.
ND: There are major advantages in storing energy on the vehicle; we did some work on this for a project in Groningen in The Netherlands and even with a relatively short wireless low-speed section you do need quite a hefty battery/capacitor bank. However, despite the added weight (which could be as much as a tonne for all the equipment which you carry with you all the time), and the considerable replacement cost of tens of thousands of Euros per tram after perhaps seven or eight years, overall one can make energy savings benefitting the environment and of course reduce operating cost. Space can also be found on most trams without detriment to the passenger accommodation.
The main advantage comes from the ability to recuperate a large proportion of the braking energy as you are no longer dependent on the overhead line being receptive (I am afraid, the energy savings from traditional regeneration from the trams into the line are often overstated). The corollary is that there is effective “peak-lopping” of demand on the substations which means existing substations can power more trams in-section or new substations can have a lower peak rating.
SM: We are often told that ‘the perfect traction battery is only a few years away’, unfortunately we have been told that since 1880. Is it achievable in the near future?
ND: There has been a significant advance in battery technology using Li-Ion types which are inherently less sensitive to the age-old problem of rapidly decreasing life due to part-discharges. The use of supercapacitors combined with batteries is key to a successful longer-term solution; capacitors take the strain of rapid charge and discharge much better than the batteries. This does of course need a sophisticated power management package, but it then becomes feasible to make short hops off-line and to recover the energy efficiently. Where currently systems are equipped with longish wireless sections, it remains to be seen if the battery can last for a reasonably economic time as deep discharges limit their life severely. Therefore, technically it is possible to have long gaps between supply points, but it all comes down to economics. There is not really sufficient real-running experience to prove the applications yet.
Onboard power generation
SM: There seems to be a case for fuel cells in some road vehicles, particularly HGVs and PCVs that currently use diesel engines – is there a case for trams which have the benefit of an external, green, power source which can result in a relatively lightweight vehicle? The hydrogen fuel cell cycle from power generation to power at the wheels is also relatively inefficient, it can probably only be justified for tramway operations where there are large-scale supplies of renewable energy available at times of low demand.
The power can then be turned into hydrogen and ‘bottled’ for later use. This is certainly the argument deployed in Aberdeen (UK) for the use of hydrogen fuel cell vehicles and it is part of the rationale for the fuel cell tramway in Aruba. Is the equipment reliable in traction use and is it affordable in total lifecycle terms? And what will the Health and Safety people say about large tanks of hydrogen being carried through busy streets?
JS: We still have limited knowledge of fuel cell operations, although there are hydrogen-fuelled buses running in London, albeit only a small number. The small size of the fleet may be as much to do with the logistics of fuelling them as anything else, but there has certainly been no rush to increase their numbers.
ND: The first TIG/m streetcar has been operational in Oranjestad, Aruba, since December 2012. A second streetcar has been running since July 2013 and a third has joined the fleet. Operation seems to be satisfactory, but it would be valuable to have an operational report from Aruba in the near future.
I am afraid I do not know much about fuel cell technology other than to observe that the power density of the two elements and all the gear are not as good as diesel and petrol engines and even less than electric-motored vehicles where all power generation is effectively off-line, thus saving weight and space. The case for internal combustion-energy store systems seems to be very weak. Is this system worth considering for urban or interurban tramways? Are Parry People Movers making a mistake in moving from their short-hop concept to their ‘gas cruisers’?
SM: The use of combustion engine-powered urban transit vehicles using fossil fuels moves away from the concept of “zero emissions at point of delivery”. In a time where it is increasingly recognised that air pollution is a serious public health matter, using fossil fuels for public transport when better alternatives are available would seem a backward step.
ND: An often-overlooked option is to use energy storage mid-point between two substations or anywhere where significant regeneration is likely. It is a means to make the overhead line 100% receptive and payback time is relatively short. It also reduces the peak loads on the substations. But, this has the potential to make fault protection quite complex. The presence of large capacitors in the input stages of modern dc traction equipments made a significant difference to the characteristics of short circuits.
JS: Long live the electric tram! Other than using onboard hydrogen fuel cells, it is the only practical means of getting transport that is zero-emission at the point of use. Thereafter it is an argument between the relative efficiencies of bulk electricity generation and transmission versus that of using electricity to generate hydrogen and converting that back into electrical power on the vehicle.