Contents lists available at ScienceDirect Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr Optimal battery sizing procedure for hybrid trolley-bus: A real case study Luisa Alfieri a, ⁎ , Antonio Bracale b , Pierluigi Caramia b , Diego Iannuzzi a , Mario Pagano a a Department of Electrical Engineering and Information Technology, University of Naples Federico II, Via Claudio 21, 80125 Naples, Italy b Department of Engineering, University of Naples Parthenope, Centro Direzionale Is. C4, 80143 Naples, Italy ARTICLE INFO Keywords: Hybrid trolley-bus Repowering Design procedure Optimal sizing Experimental measurements ABSTRACT The paper investigates the renewal of a hybrid trolley-bus, powered by a 600 V DC overhead electrical grid. The analysis focuses on replacing the on-board internal combustion engine (ICE) with a battery-based power unit. A novel two-step optimization procedure is proposed for this purpose. The procedure compares the solutions in terms of the total cost sustained by the ownership. By means of an iterative method, the optimal size of the battery unit is designed as a function of power and energy requirements, taking into account cycle life, depth of discharge, working temperature, replacement, and the requirements of the transport service operator. Using the real measurements taken on a trolley-bus operating in the city center of Naples (Italy), several numerical si- mulations are performed. The simulations examine three alternative Lithium high specific power and energy batteries. The comparison of the results allows to select the best solution among the different technologies for the proposed application. 1. Introduction The major request to any public transport system in an urban area is the efficient movement of people and goods, within the general prin- ciples of reducing the emissions of greenhouse effect gases and im- proving the energy efficiency. Urban smart cities are thought as me- tropolitan centers, where fully digitalized communication and connectivity systems promote completely new and effective ways to coordinate the mobility based on the use of new power technologies [1]. The transition process towards a more sustainable public transport considers compulsory the renewal of low-efficiency and high-polluting internal-combustion engine (ICE) bus vehicles with bus vehicles equipped with electric powertrains [2–4]. Electric vehicles can suit, better than other solutions, the practical requirements and driving pattern considerations [5–8]. Moreover, studies on electric powertrains show multiple options and flexible designs, enabling a substantial re- duction of the vehicle cost, still ensuring an as reliable service as the one provided by conventional ICE buses [9]. The trolley-bus is a simple and relevant example of electric bus which is already operating in several European cities. Trolley-buses are electrical vehicles fed through an overhead contact line system [10,11], which restricts the buses to move along a fixed track. In order to extend the driving range beyond the overhead wires, trolley buses are usually equipped with on-board ICE units [2]. Aiming at nullifying local emissions, the literature examines the renewal of these buses with more sustainable on-board units. For instance, the authors of [11] design and optimize a battery unit for the required range extension. The potential energy savings of regenerative braking are studied in Ref. [10], pro- posing the use of a stationary supercapacitor energy storage device. Within the framework of electrically-powered hybrid buses, the litera- ture proposes various measures and different methods for sizing on- board units. For instance [12–14], propose an embedded system (i.e., battery and supercapacitor/fuel cell), which is designed under the purpose of optimizing the use of each technology, meeting both the load and transient power requirements. Dynamic programming and genetic algorithm are applied [14]. The design of a hybrid energy system is formulated in Ref. [15] as a minimization problem, taking into account both installation and charging costs. The authors of [16] propose a simulation approach to design the battery pack in terms of weight, volume, performance, and charging regimes. In Ref. [17] the optimal sizing of the hybrid energy storage system (including batteries and supercapacitors) on an electric bus is evaluated by means of Pon- tryagin's minimum principle algorithm. A similar hybrid energy storage system is sized in Ref. [18] by means of a multi-objective optimization methodology, which minimizes the operation cost of the storage sys- tems, its replacements and the fuel consumption, satisfying also the requirements of the bus daily journey. In Ref. [19], an energy storage system including battery and supercapacitor is sized in order to re- covery the braking energy of a trolley-bus; the sizing approach takes into account the aging-related degradation. In Ref. [20] a backward https://doi.org/10.1016/j.epsr.2019.105930 Received 13 November 2018; Received in revised form 14 May 2019; Accepted 1 July 2019 ⁎ Corresponding author. E-mail address: luisa.alfieri@unina.it (L. Alfieri). Electric Power Systems Research 175 (2019) 105930 Available online 09 July 2019 0378-7796/ © 2019 Published by Elsevier B.V. T