Supercaps for a more efficient trolley system

Summary

The trolleybuses in Gdynia  run on a dedicated electricity supply network. The transport operator has increased  the energy efficiency of the system by making use of regenerative braking. Quick storage and release of the energy has been a limiting factor. Innovative supercapacitor technology, however, helped to overcome the barrier. The result of the measure was the installation of a supercapacitor on one of the substations along the trolleybus network and the testing of the system with expected savings of 20% energy consumption, provided that all the Gdynia trolleybuses are equipped with recuperative braking.

Implementing sustainable mobility

Modern trolleybuses that were operating in the City of Gdynia were equipped with regenerative braking which means that during braking the vehicles generated energy and fed it back into the supply system. This energy could be absorbed (used) by other vehicles but before this energy was lost when there was no demand by other trolleybuses which could use this energy.

The installation of a stationary supercapacitor bank at a substation reduced energy demand by saving energy generated by regenerative braking. Supercaps accumulated and stored the energy gained from regenerative braking and kept it until it was used by other trolleybus. The supercap bank was additionally equipped with measurement instrument, measuring the input and output energy. Energy savings could then easily be measured.

The main objectives of this measure were to:

• reduce energy power demand of the trolleybus system by 20% compared to current monthly demand
• improve the energy efficiency of trolleybuses and the existing infrastructure by recuperation of energy from braking  and storage
• set a national and European show case for innovative technology in public transport operation

Progress

Within the City of Gdynia’s trolleybus system there was previously no installation that would save energy generated by regenerative braking. Therefore, there was no possibility to make full use of recuperation energy in new trolleybuses. This energy was lost in braking resistors of trolleybuses. One possible solution was to install supercapacitor banks in vehicles, but it had a few disadvantages:

1. It would increase total weight of trolleybus and thus reduce passenger capacity (space for passengers).
2. It would more expensive than the installation of a supercap bank in a power substation because all vehicles would have to be equipped with supercap banks.

It has been possible to recover braking energy and other vehicles to use it. It has been also possible to recover 40% of energy used for acceleration. But as explained above, in the standard DC supply system this energy would be used only if one vehicle is braking and the second one is accelerating at the same time. Previous measurements have shown that in this way only half of this energy amount can be used.

Planning and preparation

A thorough analysis of options for installing the supercap bank was conducted at the beginning of the project. Analysis of options for installing the supercap bank installations was finalised in February 2013.

Implementation & demonstration

During the implementation phase the following tasks were executed:

• Preparing traction substation for supercap bank installation by interior rearrangement and rearrangement of existing electrical equipment in February 2014
• Installation of the supercaps device in March 2014
• Preparing calculation for cooperation of supercaps installation with overhead system, completed by computer simulation in April 2014
• Preparing technical project of substation rearrangement, with  Technical Project of Substation Rearrangement  report finalised in December 2013
• Activation of the equipment completed in March 2014
• Ongoing measurement of energy consumption since April 2014, and implementation Status Report on efficiency of installed supercaps finalised  in January 2015

Outcomes

Tangible outputs:

• 1 supercap bank with DC/DC inverter installed at the chosen trolleybus substation
• reduced energy power demand by 20% compared to the current monthly demand

Expected outcomes in the short-term to medium-term:

• experiences with new technologies for potential expansion or full deployment
• showcase for Poland and other countries

Expected outcomes in the mid-term to long-term:

• equipment of buses with supercap banks which work in combination with substations and enable higher autonomy of vehicles.