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Efficient heating and ventilation key to energy savings for trams in Brussels, ELIPTIC study shows

Posted on: Wednesday, April 12, 2017 - 20:40

STIB has completed a study to find out whether significant energy savings could be made to Brussels’ tram system by recovering more energy during braking. The study showed that actually, the highest potential for energy savings is not in harnessing power lost through braking, but in making auxiliary features such as heating and ventilation more energy-efficient. The feasibility study focussed on tram lines 7, 19 and 94 and involved a measurement campaign and the development of models and simulations of the three tram lines.

One of the possible ways to reduce energy consumption in a tram network is to capture the energy that trams generate during braking. Normally, some of this energy is reused by the vehicle auxiliaries (e.g. heating and cooling) while some is sent back to the overhead line to power other vehicles. The remaining energy, i.e., the part that could not be re-used, is let out through the tram’s brake resistor. A variety of technologies such as energy storage and reversible substations can allow the tram to recover and reuse this braking resistor’s energy, and the study aimed to estimate the amount of energy that is lost and to propose means to capture and reuse it.

The results of the measurement campaign show that most of the braking energy generated by the tram is already reused and that only a very small share is dissipated by the braking resistors. In line 94 and line 7 around 97% of the braking energy generated by the vehicles is reused and only around 3% is wasted in the braking resistors. Therefore, even if all of the wasted energy could be reused, potential to reduce energy consumption is very low.

The feasibility study shows in which part of the line there is a high rate of reuse of the energy and where the energy is wasted in the resistors. Where there is a strong traffic density and line interconnections, the line receptivity is high, and almost no energy is wasted. On parts that are more isolated, a higher share of energy is wasted.

Auxiliaries such as heating and air conditioning were found to consume a lot of energy. The average auxiliaries’ consumption during the measurements represented around 40 to 49% of the trams’ net consumption during the summer months and up to 60% when the heating is on. This energy share depends on the weather conditions, but even on days when heating and ventilation systems are not used, this value represents around 30% of the trams’ total consumption.

Several measurements to reduce the energy consumption have been proposed, but it seems clear that to achieve any significant energy savings, the focus should be given to reducing the auxiliaries’ energy consumption.

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