SUSTAINABLE LIGHT RAIL
By Prof. Lewis Lesley: Claverton Conference 24/26th October 2008
Light rail is a passenger transport system using steel rails to support and guide electrically power vehicles, running on street with other traffic and on separate dedicated lines. Normally light rail is driven “on sight” without railway signaling, so it can share road space or road alignments, and mix safely with road vehicles.
Ideally light rail should enjoy 100% priority over other traffic, through dedicated lanes and the pre-emption of traffic lights. Sustainable light rail emits no CO2 in the operating cycle, using renewable generation. When attracted car trips are included, light rail reduces total CO2 emissions. It is also financially viable so not vulnerable to public spending squeezes. Consistent market research and experience over the last 50 years in Europe and North America shows that car commuters are willing to transfer some trips to rail-based public transport but not to buses. Typically light rail systems attract between 30 and 40% of their patronage from former car trips. Rapid transit bus systems attract less than 5% of trips from cars, less than the variability of traffic.
2.0 Modal shift to light rail
It follows that light rail lines should be built parallel to the busiest roads, with the highest volumes of car traffic. Only by diverting car trips will light rail reduce urban greenhouse gas and pollution. Building light rail to serve areas of low car ownership or high unemployment will rarely make significant ecological contributions.
People drive cars for among other reasons their convenience, speed and dependability. Despite the rising cost of fuel, cost is rarely the most important factor in car use. Using taxis is usually cheaper than having a car. For light rail to achieve significant market penetration, journeys must be fast, waiting times short and operations 100% reliable. As a rule of thumb people will not wait for longer than the ride time, since it would usually be faster to walk. For short urban trips of under 10mins, the service interval needs to be less than 10 minutes, ideally about 5 minutes, giving an average wait of less than 3 minutes. A “no timetable”, convenient, turn up and go service is the first requirement to attract committed car users.
Fast journeys need direct routes and high operating speeds. Light rail service speeds should be above 25km/hr. Here there is an interaction with stop spacing. Closer stations mean shorter walks but a slower operating speed. Widely spaced stations mean longer walks but a faster speed. The local population density will determine the optimum stop spacing, to minimise overall journey times, and give a competitive alternative to car use. This optimum will also maximise catchments and hence patronage, and depending on fares, revenue.
Park and ride is vital both for maximising the attraction of external car trips, reducing traffic and raising patronage. The factors on the siting of park and ride stations are well determined and proven in practice. The terminals of tram lines make good park and ride stations, since there will usually be a tram waiting to depart, which is important in maintaining passenger confidence. Such terminals will be sited either on the edge or slightly out of town. This would make the installation of wind generators acceptable, and provide renewable power.
3.0 R & D for sustainable tramways
Tramways have developed incrementally over the last 150 years. Recently the trend has been to use modified heavy rail technology, which has increased costs and the weight of trams. This paper concentrates on vehicle technology, although other aspects of tramways have also been addressed to reduce costs or improve performance, especially track and power supply systems. The increase in weight per passenger space has been noticeable in the last 20 years. Contemporary trams have masses of about 200kg per passenger, compared to 125kg for buses.
Table 1 Comparison of vehicle weight per passenger
With frequent start and stops of urban operations, most of the energy used is due to mass and maximum speed.
KE = 0.5mv2
The two critical variables in urban transit energy use are vehicle mass and the maximum speed. Many designers have tried to capture braking energy to reduce overall consumption. Electrical regeneration into batteries or back to the power grid, or mechanically, e.g.into flywheels, have produced at best 25% power reductions, ignoring any weight penalties from this equipment. Halving the tram weight will automatically halve power consumption.
Reducing the maximum speed can be achieved by optimising stop spacing and using the maximum rate of acceleration that is ergonomically safe for standing passengers. Most trams accelerate at about 1.0m/s2. Trams that can accelerate at 1.8m/s2, on a 400m stop spacing, can reduce the maximum speed needed by 3%, to achieve the same operating speed. A 3% lower maximum speed means 6% less energy used per start stop cycle.
Historically trams had steel under frames and wooden bodies for a low mass. Learning from the automotive and air industries, all metal integral bodies can have the same mass per passenger space as wood but be much stronger and safer.
Although low voltage dc power systems still supply tramways, the operating voltages have increased from 500 to 750. This means that the current used has not increased in line with tram weights, since;
W = IV
Increasing the voltage by 50% means the current has only increased by 33% for trams twice as heavy. Nevertheless a contemporary 40tonne tram will draw about 1400amps when accelerating. This creates a large voltage drop between the tram and substation, reducingperformance, and to increase the number of trams operating means strengthening the substations and power distribution. Originally trams were powered by dc motors using variable resistors (rheostats) for acceleration, another source of energy wastage in heating resistances. More recently power transistors were used to chop the dc voltage, to give variable voltages and so vary motor speed. Today most trams use ac motors powered by variable frequency and variable current inverters, fed from the dc Overhead Line
The market for tramcars is limited, with a world production of less than 1000 annually. In comparison over 100,000 buses are built annually, so can afford to invest in product development, and in comparison to trams be more technically innovative. This is compounded by the replacement cycle for buses being typically 10 years, and 30 years for trams (70 in Blackpool). In comparison to private cars, trams appear obsolete long before they are worn out.
Still running in Blackpool after 100 years
Even at the most optimistic levels of investment in new tramways, the vehicle volumes will not be large enough to support mass production, the usual way to reduce unit costs. The TRAM research project begun in 1988 has turned this logic around by identifying mass produced components off the shelf (COTS) from other industries, that can be used unmodified on trams. The challenging part has been to manage the interface between different standard components, and the rail environment, which in some aspects is kind and in others harsh.
The R&D process began with simulations and calculations, progressed to bench and laboratory testing, then the construction of a slave vehicle using a redundant 1930!s tram in Blackpool. Finally a full size prototype vehicle was built and tested in Blackpool. From that experience, the tram was rebuilt, re-equipped with the next generation COTS and tested in Birkenhead and Blackpool.
Slave tram being fitted at Carnforth Depot
One of the advantages of COTS is that the original equipment manufacturers (OEM) are engaged in constant product improvement. This means the tram builder can piggyback on that to achieve more advanced vehicles with a minimum of investment. As an example the power train COTS used on the City Class tram has a high level of energy efficiency, and continuous power optimisation to reflect varying vehicle loads and track conditions. In overall terms, the power train is better than 90% efficient.
Prototype tram at Fleetwood on clearance tests
A team from the Electrical Engineering Dept. of the University of Manchester measured power consumption and compared it with other contemporary trams. (TABLE 1). This found the city class uses significantly less than similar capacity and performance trams, and less even that smaller, lighter but older trams in Blackpool. Part of the reason for the higher energy efficiency is of course lower vehicle mass, and part through the advanced power electronic COTS.
Table 1 Power consumption of various tramcars
Prototype at Blackpool Pleasure Beach 2006
How do radical new capital technologies get introduced when the market is dominated by the public sector, where a minimum of three previous satisfied customers are needed ? We might reflect how quickly mobile phones would have been introduced in Britain, if telecoms was still a monopoly of the GPO ? This “Catch 22” can only be broken by private sector initiatives, where the risk is taken by the investors who will get the rewards from commercial projects.
Coping with steep hills and tight curves in Birkenhead Changing the fundamental economics of tramway projects, away from a subsidised public sector controlled environment is however the subject of another paper ? Suffice it to say that privately funded projects are being progressed, which will enable the rate of new tramway projects in the UK to increase from the average of one every 4 years achieved over the last 20 years of public funding, to several a year. It might thus be possible to catch up with German levels of tramway provision in 20 years, rather than 150. Several of these privately funded projects are linked to complementary renewable power generation, making the tramways energy self sufficient, zero CO2 and when attracted car trips are factored in, a net reduction of urban CO2.
To reduce CO2 in urban travel, significant numbers of car trips must be attracted to public transport (or cycling ?). Buses have not been able to achieve that, and also need imported fuels that add to CO2 and other health threatening emissions. Tramways are proven worldwide to attract up to 50% of their patronage from previous drivers. Technical R&D to improve the efficency of trams, with the adoption of COTS to reduce costs, makes new tramways CO2 reducing and give value for money. With the reduction of the cost of PVs, and better power density batteries or super capacitors, the roof of a tram is big enough to be the renewable power generator, not needing any other power supply. That however is the subject of another presentation.
New City Class nearing completion on Blackburn production line 2008
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