Hybrid Energy Storage System To Increase Energy Efficiency During Regenerative Braking Of Urban Electrical Transport

A case study on the possibilities to increase energy efficiency during regenerative braking of public urban electrical transport (PUET) of Sofia by utilization of hybrid energy storage system (HESS), comprised primarily of supercapacitors, is presented in this paper. A HESS can be applied to increase energy savings and efficiency, and improve power quality. With the utilization of HESS during regenerative braking the energy can be stored and to be extracted afterwards back into the overhead catenary to supply travelling PUET vehicles. The utilization of supercapacitors as on-board storage devices has been proven on electrical buses. However, in the case of trolleybuses and trams it is not the case which presents the question of installing HESS in substations. In addition, the potential of utilizing photovoltaic (PV) installations to further reduce costs is explored. Based on obtained results regenerative braking is primarily occurring during a full stop and less whenever descending. Thus, the estimated energy capacity that is meaningful to be stored in HESS is rather small. Furthermore, estimated utilization of PV's can reduce energy costs by up to 13 %. Nevertheless, increasing efficiency of public transport is beneficial to society itself and cannot be compared to an economic profit.