IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS, VOL. 15, NO. 3, JUNE 2014 1193 Comparison of Three Electrochemical Energy Buffers Applied to a Hybrid Bus Powertrain With Simultaneous Optimal Sizing and Energy Management Xiaosong Hu, Member, IEEE, Nikolce Murgovski, Lars Mårdh Johannesson, Member, IEEE, and Bo Egardt, Fellow, IEEE Abstract—This paper comparatively examines three different electrochemical energy storage systems (ESSs), i.e., a Li-ion bat- tery pack, a supercapacitor pack, and a dual buffer, for a hy- brid bus powertrain operated in Gothenburg, Sweden. Existing studies focus on comparing these ESSs, in terms of either general attributes (e.g., energy density and power density) or their impli- cations to the fuel economy of hybrid vehicles with a heuristic/ nonoptimal ESS size and power management strategy. This paper adds four original contributions to the related literature. First, the three ESSs are compared in a framework of simultaneous optimal ESS sizing and energy management, where the ESSs can serve the powertrain in the most cost-effective manner. Second, convex optimization is used to implement the framework, which allows the hybrid powertrain designers/integrators to rapidly and optimally perform integrated ESS selection, sizing, and power manage- ment. Third, both hybrid electric vehicle (HEV) and plug-in HEV (PHEV) scenarios for the powertrain are considered, in order to systematically examine how different the ESS requirements are for HEV and PHEV applications. Finally, a sensitivity analysis is carried out to evaluate how price variations of the onboard energy carriers affect the results and conclusions. Index Terms—Convex optimization, electrified vehicle, energy management strategy, energy storage, optimal sizing. I. I NTRODUCTION T HE current transportation system is heavily dependent on fossil fuels, resulting in serious concerns on energy and economic sustainability, as well as environmental impact in densely populated areas. Endeavors are being actively un- dertaken to alleviate this dependence, in which transportation electrification has been recognized as one of the most promising Manuscript received July 22, 2013; revised October 16, 2013; accepted December 4, 2013. Date of publication January 9, 2014; date of current version May 30, 2014. This work was supported in part by the Swedish Energy Agency and in part by the Swedish Hybrid Vehicle Center. The Associate Editor for this paper was L. Li. X. Hu, N. Murgovski, and B. Egardt are with the Department of Sig- nals and Systems, Chalmers University of Technology, 412 96 Gothenburg, Sweden (e-mail: xiaosong@chalmers.se; nikolce.murgovski@chalmers.se; bo.egardt@chalmers.se). L. M. Johannesson is with the Department of Signals and Systems, Chalmers University of Technology, 412 96 Gothenburg, Sweden and also with the Vik- toria Swedish ICT, 417 56 Gothenburg, Sweden (e-mail: larsjo@chalmers.se; lars.johannesson@viktoria.se). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TITS.2013.2294675 solutions [1]–[3]. It has been projected that electrifying the whole transportation system could reduce energy dissipation by 80% of the current level [4]. The use of electricity is able to diversify the power sources of vehicles, thus downsizing or even getting rid of low-efficiency internal combustion engines (ICEs), while facilitating the growth of renewable energy in the power sector, e.g., wind and solar energy sources and hydro- electric power [5]. A good interaction between the transporta- tion and power sectors is therefore anticipated to accomplish a sustainable energy future [6]. Two key technologies accelerating an evolution toward elec- trified transportation are hybrid electric vehicles (HEVs) and plug-in HEVs (PHEVs). HEVs/PHEVs are attracting increas- ing attention from automotive industry and academia, owing to better fuel economy and lower exhaust emissions in comparison to conventional ICE vehicles [7]–[12]. The performance of HEVs/PHEVs is affected by a multitude of factors, in which the characteristics, sizing, and control of the energy storage system (ESS, onboard electricity carrier) are instrumental. For exam- ple, the fuel consumption, recuperation efficiency, and drivabil- ity of HEVs/PHEVs highly depend on the specific power and energy of their ESSs [13]–[15]. Additional characteristics of ESS are lifetime, safety, cost, etc. [16], [17]. Several survey papers have compared the characteristics of prevalent vehicular ESSs, including batteries, supercapacitors, flywheels, and fuel cells [18]–[21]. It has been demonstrated that the electrochemical power sources, i.e., battery and su- percapacitor technologies, possess better overall performance than flywheels, thus being the most common options for current electrified vehicles. Although the fuel cell, another electro- chemical system, is a good choice of vehicle prime mover, the long time constant restricts its usage as an ESS [21]. More specifically, Li-ion batteries have been perceived as one of the most promising options in the battery group because of its significant advantage in energy density. Compared with Li-ion batteries, supercapacitors typically have higher power density and longer lifespan, enabling a faster response to vehicle power demand, whereas their energy density is much lower [22], [23]. A dual energy buffer combining two electrochemical power sources with complementary characteristics, such as a Li-ion battery pack and a supercapacitor pack, has been also proposed and analyzed [19], [21], [24]–[26]. Superior performance could 1524-9050 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.