Plenty Road track maintenance
Read 17-minute stories and join #onboardbookclub
E-Class trams on Route 11 & new passenger info displays - all part of improving Melbourne’s tram network
Infrastructure Tasmania boss Allan Garcia considers new bridge and light rail projects
Nalder finds light rail ‘unviable’
New East Brunswick tram terminus being built in second phase of Route 96 upgrade
Prime Minister Tony Abbott uses ACT light rail project as example of how to fund public transport
Man injured while working on light rail network in Sydney's CBD
Fuel cell tram framework agreement
Adelaide tram drivers to stop work
In our last issue, we explored the basics of tramway and light rail electrification, as well as a few suggestions for how we could improve the efficiency of transmission and regeneration. This time we will consider the potential benefits – and drawbacks – to some of the alternatives to the well-proven overhead contact lines for providing traction power, as well as a number of recommendations for how trams and LRVs can be made more energy efficient.
Alternatives to the overhead wire
Various other solutions exist for transmission of electrical power to the tramcar. Ground supply contact systems, for example, also originated in the 1880s, but fell out of favour in the early to middle years of the 20th Century.
The ‘conduit’ system, used in London and other cities, utilised conductors placed in a trough between the rails, accessed through a slot, with a plough pick-up mounted underneath the tram to transfer the electricity.
Another variation was the stud contact system, in which contacts were installed between the rails which only became ‘live’ under the tram. Modern evolutions of this approach use similar segments of conductor rail that are only energised underneath the vehicle, separated by ‘neutral’ sections.
Ground supply technology has recently seen something of a limited renaissance, with modern applications in France, China and the UAE, amongst others. However, these solutions are generally more expensive to install than traditional overhead lines, and can suffer in wet or icy conditions, or from obstructions from litter or other detritus found in the urban environment in the same way as their 20th Century predecessors.
Another alternative is onboard energy storage. Batteries have been used (with varying levels of success) for over a century, although significant improvements in design, battery chemistry and control electronics have seen their popularity increase over the past decade. For example, the UK’s West Midlands is now on its second generation of battery propulsion, having pioneered it in 1890!
Supercapacitors store energy physically rather than in chemical form, so are able to be discharged and recharged many more times over. However, their energy density is generally less than batteries on a kilo-for-kilo basis so they are therefore more suitable for intermittent operation with high power peaks. Developments in technology, however, mean that ultimately they will likely overtake batteries; they store electrons… and there is not much that is smaller.
The supercapacitors and their charge control do have losses; like all capacitors, they have internal resistance, but this is very low, and the charge control system also has some loss (the total will be around 10%). The weight of the system to handle just acceleration energy would be around 300kg.
A good solution for a system with many steep gradients may therefore be a hybrid, using both supercapacitors and batteries. The latter would be protected from very high peak currents by the former, so could be smaller and have a longer life.
Both battery and supercapacitor solutions require additional charging infrastructure and run through a number of cycles during a normal service day:
• Prior to use, the storage system must be fully charged, either during off-service times at the depot, or in-service through catenary charging or induction systems
• Once the tram leaves the charging point and accelerates to operational speed, the system begins to discharge as it bears the power demands of traction systems and any other auxiliary equipment (lighting, passenger information systems, heating and air-conditioning systems)
• Once up to speed, power demand reduces considerably as only the small rolling resistance and no-load losses need supporting, plus auxiliaries
• Upon deceleration (for example at a tramstop, junction or curve), kinetic energy from the unit’s traction motors is recuperated and transferred back into the battery or supercapacitor, increasing charge.
A full service day with minimal recharging necessitates a greater energy store onboard the vehicle, but this comes with a considerable weight penalty that has the knock-on effects of increasing both track and wheel wear.
‘Opportunity recharging’ is the common method, as seen on next-generation Chinese tramways, as it only needs smaller and lighter batteries. But this requires the delivery of a very high current in a short space of time, something that is less than desirable for the electricity grid, and will come with a large maximum demand charge. To overcome this, the charging station sometimes also incorporates an energy store – although this will have its own losses. The charging process itself comes with associated losses, typically around 20%.
Another popular option is ‘discontinuous electrification’ as practised, for example, by West Midlands Metro, where only short sections of the route are operated away from the overhead line.
What about hydrogen?
The current vogue appears to be for hydrogen as an alternative fuel – arguably the modern equivalent of gas or compressed air-driven trams of bygone days, although fuel cells are a much better way of utilising gas for tractive power (see TAUT 998 for more on fuel cells).
Hydrogen, however, is not a primary fuel – it has to be produced by chemical reduction, usually by the electrolysis of water which is then split into its constituent elements of hydrogen and oxygen. The resultant hydrogen gas is then compressed to around 350bar (5000psi) and requires either piping, or bottling and transport to the refuelling infrastructure in the depot.
In terms of overall efficiency as an energy carrier, it comes in at about 30%. Proponents of fuel cells say this doesn’t matter as with ‘green hydrogen’ the electricity used for electrolysis comes from renewable sources such as windfarms which operate overnight with little other demand, and therefore the production process is virtually free. However, with the projected future demands on power grids of millions of electric cars also charging overnight, this assumption needs to be challenged.
The next step is the fuel cell. This combines hydrogen and oxygen from the air to produce electricity. The efficiency of this is around 65%, so heat is also produced; this could be used for heating in winter, but is a loss in summer. The reaction produces water vapour, which is expelled. It should be noted however, that the nitrogen component of the air is also expelled, so the claim that only water is exhausted is not strictly true.
A fuel cell will produce electricity only as long as fuel (hydrogen) is supplied, and has a limited power capacity, so that usually an auxiliary energy store, battery or supercapacitor, is also required for operation. It must be further borne in mind that refuelling requires dedicated infrastructure, with consequent cost, and appropriate safety measures are required for the storage of hydrogen at high pressure.
In Germany, recent studies indicate that electrolysis current has to be a quarter the cost of traction current for hydrogen to be economical and that with current technologies battery-electric rail vehicles are up to 35% cheaper than their hydrogen-electric alternatives.
A weight-loss plan
As well as reducing the electrical losses and improving energy recovery, another key way of improving trams’ overall efficiency is by reducing the vehicles’ weight. Not only will this reduce the input energy required, but it will also lessen the infrastructure loading and wear – increasing the lifespan of a system’s fixed components and minimising the expense and disruption of premature renewals.
A number of projects are underway to reduce overall vehicle weights through the use of composite materials – such as Coventry’s Very Light Rail R&D programme – with an aim of creating a vehicle of less than one tonne per linear metre. But aside from using alternative materials and processes, there is another way…
A high proportion of the weight of any rail vehicle is in the running gear, located underneath the passenger carrying area. Using permanent magnet wheel-motors will give an intrinsic weight saving, but these can also be steered by varying their relative speeds, removing the need for bogies and therefore reducing the wheel count.
This is achieved by using independent wheels which are pivoted on their vertical axis, without a conventional axle, and are steered around curves so that the wheel is always tangential to the rail, greatly reducing wear, energy and noise. It also reduces the requirement for space as bogies or short trucks are no longer required, so longer body sections (about six metres) are possible, reducing the number of articulations; and they contribute to a similar weight of less than 1t/m.
An energy consumption of less than 1KWh/km for a 30m vehicle with 220 passengers, with stops at 500m intervals, and speed up to 50km/h (30mph), should be achievable. This represents a line current of only 42A for traction when energy storage is used, against about 500A rms conventionally. When combined with the use of composite materials for the bodyshell,
This article first appeared on www.tautonline.com
About this website
Railpage version 3.10.0.0037
All logos and trademarks in this site are property of their respective owner. The comments are property of their posters, all the rest is © 2003-2021 Interactive Omnimedia Pty Ltd.
You can syndicate our news using one of the RSS feeds.