Comparison of on-board vs stationary energy storage systems for PV-powered multifunctional trolleybus grids
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Abstract
Trolleybuses are electric buses than run on electric power from overhead electric power lines (catenary) like trams do. Although is a service that dates all the way back to 1882, it has managed to regain interest in the recent years due to the constant electrification of various aspects of the world. This comes as a response towards environmental challenges as, even today, the majority of the energy necessary to cover our need comes from fossil fuels, making the transition to renewable energy sources more vital. One promising form of renewable energy source is solar energy and its use with photovoltaic (PV) systems. An interesting implementation of such systems is in catenary grids like trolleybus grids. Nevertheless, their integration has a low potential as the gaps in the trolleybus schedule and the intermittent behavior of the PV electric power leave a lot of excess PV electric energy unused. Possible solutions to this problem could be either the storage of this excess energy in on-board energy storage systems (OESS) or stationary energy storage systems (SESS). Besides that, in preparation for the future, more attention is given every day to the multifunctionality of trolleybus grids. This is related to parameters such as voltage drops, that have important role in rendering the trolleybus grid capable of being expanded with components other than trolleybuses, such as electric vehicle chargers.
The objective of this thesis work is to find out which energy storage systems (on-board or stationary) are most favorable for a PV-powered multifunctional trolleybus grid for increasing the PV system utilization and improving its multifunctionality. Parameters related to the multifunctionality of trolleybus grids can be the yearly electric energy consumption and the voltage drops.
For the conduction of this work is used an existing, verified trolleybus grid model of Arnhem in MATLAB that has realistic trolleybus electric power profiles as an input. By using the backward-forward sweep method are determined precise values such as the total electric power needed, voltage drops, ohmic losses and more. Also, is used as an existing, verified PV system model that provides the PV electric power as an output based on measured data. Then is developed a model for the on-board energy storage system which with the help of a constraint checking algorithm, that simulates the energy storage technologies, it provides new, adjusted trolleybus electric power profiles that are used as input to the trolleybus grid model. Finally, for the stationary energy storage system, the main trolleybus grid model is expanded with storage capabilities based on voltage control and a similar constraint checking algorithm for the storage technologies.
The results show that the impact of stationary energy storage systems on the PV system utilization and on parameters of the trolleybus grid in general is heavily correlated to the control strategy used. The technology, the PV system size, the trolleybus grid section, and the targeted outcome (improve the PV system utilization or reduce the voltage drops) have key role to the selection of the control strategy. The comparison of stationary energy storage systems to other energy storage systems cannot be straightforward. Besides that, on-board energy storage systems can perform better on parameters regarding the multifunctionality of the trolleybus grid such as the total yearly electric energy consumption and the mitigation of the severity of the voltage drops. On the other hand, stationary energy storage systems can perform well on a wide range of parameters according to their control strategy. For the one used in this work, they perform better on parameters regarding the PV system utilization, managing to have a positive impact.