Powering trams (part two)

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, <0.7t/m should be attainable, with even lower energy use as a consequence, therefore enabling a 40% reduction compared to conventional designs.

What should the future be?

We need to get back to basics. Whatever the primary power solution, we should be doing all we can to minimise energy consumption and get the maximum benefit from the low rolling resistance of steel-wheel-on-steel-rail (as little as 10% of that as a rubber-tyred bus).

Using onboard energy storage we can considerably increase energy recovery through regeneration, and also reduce line current, enabling much lighter overhead line equipment and feeders with fewer and smaller substations to be possible. This will also enable some degree of off-wire running, which could be of benefit in depots or for short stretches through historic areas where objections exist to the erection of overhead lines. Used in conjunction with wheel-motors, traction current demand could be reduced about ten-fold.

Use of local energy storage is now the norm within electronic equipment – called ‘decoupling’, the modern world of computers, radio, television and many more everyday devices would be impossible without it.
A tramway system is just a much larger version.

One must exercise caution concerning the appropriate application of onboard energy storage however, as many of these systems seem to be a complicated way of avoiding a simple copper wire. OLE is wrongly perceived to be a major cost of tramways (it is actually under 10% for most standard installations), and being visually intrusive; and has been tainted by the problems on mainline railways, which in the UK at least have suffered from gross over-specification in many cases.

After all, we must remember that one of the key advantages that tramways and light rail systems have always enjoyed compared to internal combustion-engined road vehicles is that they don’t carry large, heavy and cumbersome power sources with them!

A tram on street routes only travels at moderate speed and certainly doesn’t need to be designed for any more than 80km/h (50mph) operation. With such relatively low speed, direct wire suspension is possible – there are many examples around the world of where this has been achieved in an unobtrusive and sympathetic manner – and where this is not possible the use of slender poles (perhaps of composite construction) can be employed. Wherever possible these should also double up for street lighting to further reduce clutter.

Our Victorian and Edwardian predecessors used elegant ornamental poles and scrollworked bracket arms, often embellished with the city coat-of-arms, to enhance the streetscape. They were proud of their
tramway systems, heralding the signs of a progressive city – so why don’t we do this now? There are many examples of ‘heritage’ street lighting installations in prime locations, so there is precedent for also using such fixtures for OLE.

Hybridised systems predominantly led by well-proven and well-designed overhead line systems with the addition of onboard energy storage would likely be the most energy-efficient combination – also offering the lowest overall lifecycle cost. A lower current demand will also help negate the perceived issue of earth current leakage due to voltage drop in the rails.

In conclusion, we need to use OLE infrastructure that is thoughtfully-designed, not over-specified, and learn from our forebears in making it a visual asset. It does, after all, advertise a city that has an efficient transport system – where a wire is, there will soon be a tram! 

Article appeared originally in TAUT 1000 (April 2021)