Electric buses: A review of alternative powertrains
Introduction
Initiatives to reduce transit emissions, the commitments associated with the Kyoto protocol and instability in oil prices are compelling policy makers to implement alternative technologies that will replace oil-dependent mobility. Despite significant efforts to enforce standards in order to reduce emissions generated from the traditional internal combustion engine, projected reductions are unlikely to meet the emission targets of the Kyoto protocol [1], [2], [3], [4]. It is evident in the literature that alternative technologies are essential if we are to reduce the emission footprint of the road transport sector. Although, different technological solutions have been operationalized in recent years, oil-based mobility still holds the lion׳s share in the transport market and the market penetration of alternative technologies is still very small [3], [5], [6].
The implementation of new alternatives for road transport depends on various factors that are well addressed by the conventional petrol/diesel counterpart [7]. These factors involve, but are not limited to, energy logistics, cost-benefit assessment, infrastructure, and public acceptance. In this respect, public transit offers superior potential for considerable market penetration of alternative technologies, especially in the context of city buses [1]. Bus transit provides fixed routes, centralized depot locations, and shared infrastructure, among other factors, that are suitable for the implementation of alternative technologies. In such a context, the technology could be operationalized, tested, and optimized all the while reducing emissions [8]. Currently, several powertrains for urban buses have been introduced in the market. Each offers specific advantages that could be utilized to maximize emission reduction. However, selecting a suitable powertrain for each context depends on various factors such as cost, network structure, energy source, and driving conditions [1], [9]. A trade-off between different features is required for optimal utilization of each technology.
There are several studies that model and quantify the techno-economic and environmental impacts of electric buses. These studies are mainly developed across three domains environmental, economic, and energy, which are thoroughly reviewed in the following sections. In a nutshell, environmental models investigate potential GHG emission reductions from electric buses [7], [10], [11], [12], [13], [14], [15], [16], energy consumption models investigate energy efficiency of electric buses [9], [11], [17], and economic studies focus on the cost-benefit analysis of implementing electric buses in transit [9], [18]. Other studies focus on the operational constraints of electric buses [1], [16], [19], [20], and the perspective of stakeholders towards the implementation of electric buses in transit [21].
However, literature on electric buses is developed across many technical and non-technical disciplines as highlighted in Table 1. Several models and methods have been developed in different parts of the world, which are not necessarily linked in the literature [1]. It could be argued that the electric bus literature is fragmented; consequently there is a growing need for a comprehensive review of the literature, as well as, for developing a unified volume that combines reviews on both technical and non-technical aspects of electric buses. Some reviews of electric bus technology in transit have been developed to overcome this issue; Kühne [22] was among the early scholars to review the potential of electric buses in transit. His effort is followed by attempts to investigate the applications of electric buses in transit [23], and the market shares of electric buses across the world [24]. However, there is a lack of reviews that accommodate different powertrain configurations across a wide variety of technical and non-technical aspects.
This study builds on previous attempts and aims at providing a comprehensive review of electric bus features and their potential as a replacement for diesel buses in transit operation. Namely, the study focuses on Hybrid Electric Bus (HEB), Fuel Cell Electric Bus (FCEB), and Battery Electric Bus (BEB). Initially, an overview of the configurations of electric powertrains is provided. Market forecasts are illustrated in section three. A review of economic, environmental, operational, and energy features of electric buses is detailed in section four. Results are in turn utilized to generate a holistic comparison of electric buses, along with diesel, on 16 performance features of electric buses in section five. Lastly, a concluding section highlights the future on electric buses in transit operation, and presents avenues for future research.
Section snippets
Overview of electric buses technology
Electric buses operate by different degrees of electrification that depend on the configuration of the propulsion system [1], [27]. These include, but are not limited to, Hybrid Electric (series and parallel), Fuel Cell Electric, and Battery Electric (overnight and opportunity) [7], [28]. With the exception of parallel hybrid, all systems share a central concept that the propulsion energy is derived from an electric traction drive system. The main difference between these technologies is the
Market trend for electric buses
The market share of electric buses has featured steady growth in recent years. In 2012, electric buses had 6% of new purchases globally [5]. This share is distributed among key players around the world such as Asia Pacific, Europe, and America (South and North). Several attempts have been made to forecast the potential market share for electric buses; most notable are the efforts of Frost and Sullivan. According to their estimates, electric buses will hold 15% of global market in 2020 with a
Performance features of electric buses
Electric buses offer various operational features that differ from diesel buses [1]. These operational features should be considered and weighted for successful implementation of the technology [21]. Features include:
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Economic aspects such as: capital cost, infrastructure investments, maintenance, and operational costs,
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Operational aspects such as: range, acceleration, charging time, availability, and infrastructure,
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Environmental aspects such as: GHG emissions, noise, and air quality, and
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Energy
A holistic review of electric buses
Multiple criteria govern the decision-making process for selecting a feasible alternative technology in the bus transit context. A given technology may be suitable for a certain context but not for others. Given the multi-criteria nature of the problem and the trade-off during the decision-making process, a comparison between different technologies is essential in order to better inform service providers and decision-makers on the implementation of feasible alternatives. This section develops a
Conclusion
This study presents a review of electric bus technologies in the transit context and is aimed at three in particular: hybrid, fuel cell, and battery. The review is informed by both simulation models and operational data disseminated in the literature, and develops a comparative analysis of 16 performance features for electric powertrains. Overall, this review provides a unified appraisal of the capabilities and features of electric bus technologies that better informs the decision-making
Acknowledgment
This article emerges from a research project funded by Social Sciences and Humanities Research Council of Canada (SSHRC) Grant no: 886-2013-0001, with additional support from Automotive Partnership Canada. The views expressed by this article are those of the author and do not necessarily reflect those of the funding authority. The authors would like to thank the Editor-in-Chief Dr. Lawrence Kazmerski and the two anonymous reviewers for the insightful comments and feedback.
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