INCREASING THE ENERGY EFFICIENCY OF TRAMWAY SYSTEMS
Gunther Dürrschmidt, TU Dresden, Prof. Dr. Michael Beitelschmidt, TU Dresden
This paper describes concepts for future trams to save between 20 and 40% of energy compared to current vehicles.
Due to shortage of natural resources and rising energy costs a reduction of the energy consumption of transport is not only ecologically worthwhile but also an economic need. Although urban rail transport in opposition to individual traffic is already local emission-free and featuring comparably low energy consumption there is still a huge potential for increasing the efficiency. This paper describes concepts for future trams to save between 20 and 40% of energy compared to current vehicles.
For the determination of possible savings, long-term measurement data from the Dresden Measurement Tram, a unique joint project of TU Dresden and 10 industrial partners has been used.The tram is equipped with devices to record electrical and mechanical values in every day passenger service producing approx. 10 TB of data on a measuring distance of almost 0.5 Mio. km since project start in 2009. For the evaluation of the energy consumption, the onboard systems of the tram can be divided into three main appliance groups: traction (40-45% share of net consumption), braking resistors (10-15%) and auxiliaries, including comfort systems for passengers (40-50%).
As most of the electric energy used for accelerating the tram is recovered by regenerative braking, the net consumption of the traction system is mainly formed by mechanical and electric losses in the traction chain.Coasting experiments to measure themechanical resistance forcesshow a variance up to 1 kNbetween different vehicle types1. This equals a reduction of approx. 40% of the mechanical running resistance on straight line that can be achieved by already existing smooth-running vehicles, which can be attributed to different construction of undercarriage and drive train. For quantification TU Dresden has developed a simulation tool using a parameterized vehicle model, which can replace formerly used equations published by Wende2.
Braking resistors, the second consumer group, are needed for the disposal of recovered braking energy if no other vehicles in the same substation sections are able to absorb the energy. Its13%-share is equivalent to an amount of 27,000 kWh of energy per year that is simply wasted to heat the environment. This can be avoided by the use of energy storage devices aboard the vehicle or in the substation. The measurement data evaluation leads to the conclusion, that currently available on-board storage devices (e.g. supercapacitors) are sufficient to store at least 95% of the wasted energy. However, they are unprofitable, ifmounted on every vehicle. As the use of the braking resistor is not equally distributed but most intensive at some suburban network branches, an early break-even can be realized if energy storage devices are onlyinstalled in these few substation sections.
The greatest potential to reduce the energy consumption of a tram can be found in the group of auxiliary and comfort systems, whose highly tempera-ture-related share is 40-50% equalling an annual mean power of 20-25 kW. It’s ranging from 7 kW daily averages in summer to 60 kW at very cold winter days. As passenger compartment heating makes up the difference, optimizing the heating system must have priority. Although an improvement of the heat transfer coefficient of the vehicle bodyhelps to avoid some percent of the thermal losses, the implementation of an intelligent control strategyis much more promising. If ventilation can be adapted to the passenger density, intake could be reduced by 70-80% saving a lot of power to heat thefresh air. Furthermorea remarkable share of the vehicle’s energy consumption could be saved by using preferentially regenerated braking energy for heating – a strategy which can decrease the heating consumption in winter, but especially in transitional periods like spring or fall conditions.
On the whole, all vehicle systems offer approaches for improvement. The evaluated data provides the possible optimization potential which can be added up to 100,000 kWh per year and vehicle, which is 40% of the total energy consumption of the vehicle. Although not all optimization measures can be realized profitable 20-30% savings should be attainable for future tramways.
1Dürrschmidt, G.; Beitelschmidt, M.: New Approaches for the Evaluation of the Driving Resistance of Tramway Systems Using Data of the Dresden Measurement Tram. Proceedings of 2nd International Conference on Railway Technology, Ajaccio, 2014
2Wende, D.: “Fahrdynamik des Schienenverkehrs”, Teubner, 2003.
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