Investigating electric vehicles as energy storage systems for an urban tram network to promote the energy efficiency

Electrification of the transport sector is a necessary measure required to lower greenhouse gas emissions. However, electrified light rail systems suffer energy efficiency problems due to the ineffective recovery of the braking energy regenerated during train or tramcar braking events. Utilizing the growing number of electric vehicles (EVs), and therefore the increasing number of EV batteries, as energy storage for the rail could provide a novel solution to address the challenge.

Using tramcar operational data collected with a GPS data acquisition system, and the Matlab and Simulink modelling and simulation environment, this research initially gains insight into the energy balance of a local tram system, highlighting that the unrecovered and thereby wasted braking energy could be equivalent to approx. 50% of the overall energy consumption. This research later simulates the addition of a stationary energy storage system (SESS) to the tram network, and demonstrates the energy-saving achieved. Additionally, the simulation also suggests that a small but optimal capacity, and an installation on the mid-stop of an energy supply section, are beneficial to achieving better energy-saving and economic returns. Subsequently, this study designs two energy storage systems (ESSs), the EV energy storage system (EVESS), which solely exploits EV batteries for energy storage, and the combined ESS (CESS), which integrates the EVs with a sub-system of a stationary battery. Both ESS arrangements were found to successfully deliver energy-saving to the tram system. Moreover, a control approach was designed for the CESS to prioritise the charging of the EVs. The CESS could therefore refresh the capacity of the stationary battery more frequently and achieve a better overall energy-saving. Finally, this work compares the energetic and economic performance of the SESS and CESS addition to the network, and concludes that the CESS is more advantageous than the SESS alone. A network-wide CESS installation was therefore examined to assess its economic feasibility, and it was found economically viable if the electricity delivered to the EVs was charged at a reasonable price.