Electric buses: A review of alternative powertrains

Renewable and Sustainable Energy Reviews 62 (2016) 673–684

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Electric buses: A review of alternative powertrains Moataz Mahmoud a,n, Ryan Garnett b, Mark Ferguson a, Pavlos Kanaroglou c a b c

McMaster Institute for Transportation and Logistics (MITL), McMaster University, Hamilton, ON, Canada L8S 4K1 Centre for Spatial Analysis (CSpA), McMaster University, Hamilton, ON, Canada L8S 4K1 School of Geography & Earth Sciences, McMaster University, Hamilton, ON, Canada L8S 4K1

art ic l e i nf o

a b s t r a c t

Article history: Received 19 February 2015 Received in revised form 8 January 2016 Accepted 3 May 2016

Evidence suggests that the role of electric buses in public transit is important if we are to take steps to reduce climate change and the environmental impacts of fossil fuels. Several electric alternatives are currently operationalized, and the debate about which is most suitable is attracting considerable attention. This article provides a detailed review of various performance features for three categories of electric buses: hybrid, fuel cell, and battery. Economic, operational, energy, and environmental characteristics of each technology are reviewed in detail based on simulation models and operational data presented by various scholars in different contexts. The study develops a holistic assessment of electric buses based on side-by-side comparison of 16 features that best inform the decision making process. The review indicates that the selection process of electric technology is highly sensitive to operational context and energy profile. In addition, it highlights that hybrid buses will not provide a significant reduction in GHG and would be suitable only for short-term objectives as a stepping-stone towards full electrification of transit. Battery and fuel cell buses are arguably capable of satisfying the current operational requirements, yet initial investment remains a major barrier. Overnight Battery Electric Bus is advocated as the most suitable alternative for bus transit contexts given the expected improvements in battery technology and the trend to utilize sustainable sources in electricity generation. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Electric bus Fuel cell Hybrid Well-to-Wheel review

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674 Overview of electric buses technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674 Market trend for electric buses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 675 Performance features of electric buses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 675 4.1. Economic performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676 4.2. Operational features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676 4.2.1. Range & charging time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676 4.2.2. Infrastructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677 4.3. Environmental performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677 4.3.1. Well-to-Tank (WTT) GHG emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 4.3.2. Tank-to-Wheel (TTW) GHG emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 4.3.3. Well-to-Wheel (WTW) GHG emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 4.4. Energy efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 4.4.1. WTW energy efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679

Abbreviations: ATR, Auto Thermal Reforming; CAGR, Compound Annual Growth Rate; CNGHEB, Compressed Natural Gas Hybrid Electric Bus; DHEB, diesel hybrid electric bus; FCEB, Fuel Cell Electric Bus; GHEB, gasoline hybrid electric bus; GSR, gas steam reforming; GREET, greenhouse gases, regulated emissions, and energy use in transportation; HEB, Hybrid Electric Bus; H2-WE, hydrogen - electrolysis of water; MJ, mega-joules; NGSR, natural gas steam reforming; RED, renewable energy directive; UCs, Ultra Capacitors; TCO, Total cost of ownership; WTT, Well-to-Tank; BEB, Battery Electric Bus; CNGB, Compressed Natural Gas Bus; DB, diesel bus; EM, electric motor; GB, gasoline bus; GHG, greenhouse gas; H2-NGSR, hydrogen - natural gas steam reforming; ICE, internal combustion engine; NGCC, Natural Gas Combined Cycle; PHEV, plugin hybrid electric vehicle; SD, single-deck bus; USC, Supercritical Steam Cycle; TTW, Tank-to-Wheel; WTW, Well-to-Wheel. n Corresponding author. E-mail addresses: [email protected] (M. Mahmoud), [email protected] (R. Garnett), [email protected] (M. Ferguson), [email protected] (P. Kanaroglou). http://dx.doi.org/10.1016/j.rser.2016.05.019 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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4.4.2. Energy Storage Systems (ESS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 5. A holistic review of electric buses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683

1. 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– 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 technoeconomic 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 [7,10–16], 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 nontechnical 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.

2. 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 power source for the electric engine. Hybrid electric technology uses both an internal combustion engine (ICE) and an electric motor (EM), in various configurations, to provide wheels with traction power [27]. Hybrid buses are configured in two distinct forms: series and parallel. In parallel configuration, Fig. 1-a), both engines (ICE and EM) are connected to propel the vehicle. Traction power could be derived independently from the ICE and the EM, or through a combination of both. In series configuration, the on-board ICE, often referred to as generator, is used to generate electricity that is either transferred to the EM or stored in an on-board battery package as highlighted in Fig. 1-b) [27,29,30]. Several other configurations are available for hybrid buses that are based on the fuel source for the ICE, such as gasoline, diesel, natural gas, and biofuel [20]. Hybrid buses are often configured based on the required degree of hybridization [8,28,30]. High or low hybridization ratio refer to the energy output ratio from the EM and the ICE respectively [31]. The demands for a high hybridization ratio have led to the development of a plug-in hybrid technology [30]. Plug-in hybrid configuration follows series hybrid settings with an additional feature that allows the on-board battery to be recharged with an external electric source. This provides an electric only drive option without using the ice/generator for a limited range [30]. Fuel cell technology is an alternative method for the electrification of buses [32]. Fuel cell technology is based on powering the electric motor with electricity generated from fossil fuel.

FC–HEV for lower life cycle impact BEB suffer from various limitations that should be addressed before full operation CNG for long express operation, BEB for local short service Xu [12]

Modeling/case study operational data

Review of operational and technological demands Diesel, Biofuel (B20), Hybrid (Series & parallel), Compressed Natural Gas Life-cycle emission (CNG), and Battery Electric (BEB) Review Li [23]

Ribau [7]

Simulation (ADVISOR)

FC–PHEV for operation efficiency,

Energy consumption and cost benefit analysis (CBA) Operation optimization

Hybrid (Series & parallel), Plug-in Hybrid, and Battery Electric (BEB) Hybrid Electric (HEV), Fuel Cell Hybrid (FC–HEV), And Fuel Cell Plug-in Hybrid (FC–PHEV) Battery Electric (BEB) Lajunen [9]

Simulation

Overnight Battery Electric is preferable in terms of cost effectiveness Plug-in Hybrid & Battery Electric Total cost of ownership (TCO) Opportunity Battery Electric, and Overnight Battery Electric Nurhadi [18]

Life cycle impact assessment Diesel, Compressed Natural Gas (CNG), Biogas, and electric trolley

Modeling/case study operational data Simulation Kliucininkas [26]

Life cycle assessment Diesel, Hybrid, and Battery Electric Modeling Cooney [25]

CNG and Hybrid buses have lower lifecycle GHG emissions than diesel buses, but at an increase in costs. Battery Electric is preferable in specific U.S states due to variation in electricity generation profile Biogas and electric trolley buses Life-cycle assessment of costs and greenhouse gas emissions Modeling McKenzie and DurangoCohen [10]

Energy characteristics Trolley electric Life cycle (LC), energy savings (ES) and Each technology would fit specific context based on GHG reduction (GR) (LCESGR) energy profile and operational conditions

Trolley battery electric (BEB) Diesel, Gasoline, Coal-derived DME bus (CDMEB), Coal-derived methanol bus (CM100B), Compressed Natural Gas (CNG), Hybrid Electric, Fuel-Cell, and Battery Electric Ultra-low sulfur diesel, hybrid diesel-electric, Compressed Natural Gas (CNG), and Hydrogen Fuel-Cell Review Modeling Kühne [22] Ou [11]

Type of study Source

Table 1 Recent studies of electric buses in transit.

Technology recommended Scope of analysis Alternative powertrains

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Unlike the conventional ICE whereby fuel is burned to generate dynamic movement, the fuel cell technology generates electricity from fuel through an electrochemical process [29,33]. This process converts the chemical energy stored in the fuel cells into electrical energy [20]. Fuel cell technology could be configured to assist electric battery in a hybrid mode, or as the main power source of the electric engine as illustrated in Fig. 2. The Battery Electric Bus, often described as pure electric, is powered by electricity that is stored in an on-board battery package (Fig. 3). The engine configuration of this technology does not include any mechanical parts [8,20,33,34]. The Battery Electric Bus is operationalized in two forms: opportunity, and overnight [35]. The differences between the two types are based on range and charging time [1,35]. The opportunity electric bus has a smaller battery package that offers a limited range (20–30 miles) [35] and full charge (80–100%) can be achieved within 5–10 min. In contrast, the overnight electric bus contains a relatively larger battery package with a range of up to 200 miles and a much longer charging time (2– 4 h).

3. 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 Compound Annual Growth Rate (CAGR) of 26.4% as illustrated in Fig. 4. However, the market distribution of electric buses shows that the Asia Pacific regions, mainly China and India, will dominate the electric bus market with estimated CAGR of 6.3%. This is compared to 3.9% in Europe, 3.6% in North America, and 6.3% in Latin America as detailed in Fig. 5. Taking into account the Asia Pacific bus market share (40.9%), the estimates show that 75% of all electric buses will be introduced in Asia Pacific [5]. . In addition, the 2020 estimate shows that although electric buses will dominate the North America market, this market penetration will be driven mainly by hybrid technology. Based on Pike's estimations [6], the market profile (Fig. 6) shows that, within the electric bus segment, hybrid buses will represent 73% of the electric bus market. The market share of BEB and FCEB will be relatively small at shares of 8% and 19% respectively [6]. In Europe, both FCEB and BEB are steadily increasing market penetration so as to overtake HEB market share by 2030. However, all market estimations are mainly premised on the maturity of current technologies that are subject to significant changes when all electric technologies are equally mature [36].

4. 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: ● Economic aspects such as: capital cost, infrastructure investments, maintenance, and operational costs, ● Operational aspects such as: range, acceleration, charging time, availability, and infrastructure, ● Environmental aspects such as: GHG emissions, noise, and air quality, and

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Fig. 1. Hybrid Electric Bus (HEB) configuration.

Fig. 2. Fuel Cell Electric Bus (FCEB) configuration.

Fig. 3. Battery Electric Bus (BEB) configuration.

● Energy aspects such as: energy source, energy consumption, and fuel efficiency. This section provides a detailed review of these four domains and develops the foundation for a holistic assessment of electric buses. 4.1. Economic performance Total cost of ownership (TCO) has been identified as one of the main barriers for the implementation of electric buses [3,9,22]. TCO includes manufactured price and the costs for maintenance, operation, energy distribution, infrastructure, emission, insurance, and end-of-life [35]. TCO has been calculated in the literature based on operational data [11,35,37], or on life cycle analysis models [7,10,15]. Although various efforts have been made to calculate the TCO of electric buses, it is evident in the literature that there is a lot of uncertainty in the estimation of TCO [9]. TCO calculation is highly dependent on operational and logistics aspects, for instance, fuel price (diesel, electricity, and hydrogen) plays a significant role in the TCO estimations [3]. The differences in emission cost/penalty and taxation policies influence the TCO as well [9,13]. Therefore, the TCO review presented in this study is based on averages obtained from relevant literature. For manufacturing price, all electric buses are more expensive than their diesel counterpart, as is detailed in Table 2. The added

cost is attributable to the cost of electric components (i.e. battery package, electric motor, and auxiliary system) [38]. It is apparent that there is no significant variation in the price between parallel and series hybrid buses. For BEB, the opportunity bus is relatively cheaper than the overnight bus due to the smaller on-board battery package [9,38]. FCEB is the most expensive electric bus available in the market with an average manufactured price of 2 million USD. In terms of operational cost, electric buses perform better than the diesel bus. BEB provides an average reduction of 80% in running cost. While HEB, series and parallel, reduce operation cost by an average of 8–15%. However, in terms of infrastructure cost, the opportunity BEB is considered the most expensive option. Although FCEB and Overnight BEB require major infrastructure modification for filling/charging stations at depot, the Opportunity BEB requires a higher density of charging points along routes. Estimates suggest one charging point for each 10– 20 km [20]. The Fuel Cells and Hydrogen Joint Undertaking [35] developed TCO estimations based on operational data of electric buses across Europe. The TCO estimations considered a wide array of indicators that include the costs of purchase, financing, operation, infrastructure, and emission penalties. Their findings, based on 60,000 km annual mileage and a 12-year bus lifetime, show that the Overnight BEB is the most expensive electric alternative for urban buses based on total cost of ownership, followed by FCEB, and Opportunity BEB. Both series and parallel hybrid are reported to have almost similar TCO compared to diesel buses as highlighted in Fig. 7. However, other studies have argued that the TCO of electric buses is very sensitive, not only to expected cost reductions of electric components (i.e. battery price, auxiliary system), but also to the utilization level of the service. Several studies have echoed that under high, and even moderate utilization scenarios, electric buses can be an economically competitive choice compared to diesel and CNG buses [9,38,42]. The FCH-JU [35] reported that the TCO for some electric buses will drop significantly by 2030 with an average of 30–50% for FCEB and BEB (Overnight & Opportunity). The TCO for HEB (series and parallel) and diesel buses is also expected to drop by an average of 1–5% in 2030 as highlighted in Fig. 7. 4.2. Operational features 4.2.1. Range & charging time In general, range is the key barrier that reduces the attractiveness of electric mobility in many choice contexts [43]. The term “Range Anxiety” has been always echoed as a limitation of the operational features of electric buses [38]. The term refers to the unsuitability of the electric range for daily travel activities. Although the issue has been frequently highlighted in the literature, several electric buses have already addressed the range concerns. The HEB provides an overall range similar to diesel buses and parallel hybrid provides an additional all electric range of 10 km. Also, FCEB provides a full electric range similar to diesel

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Fig. 4. Global bus market share: new purchases [5].

buses with an average of 4 300 km [35]. However, the range issue is apparent in the BEB category: overnight BEB has an extended range of 250 km in a single charge (i.e. BYD–BE 40 ft), while opportunity BEB has a limited range of 30–40 km (i.e. Proterra EcoRide BE35). To overcome range limitations, recent studies have combined both refueling time and range in a single indicator due to their cumulative direct impact on service scheduling [20,44]. This indicator is referred to as operational flexibility [35]. Miles and Potter have argued that Battery Electric Buses are not flexible in operation due to the influence of charging time on schedule. They have further argued that, due to charging time and range limitations, two Battery Electric Buses would be required to replace a single diesel bus in maintaining the same schedule [1]. However, this review argues that when considering the added range for 5 min refueling/recharging (using diesel bus as a threshold), all electric buses provide similar performance to the diesel counterpart except Overnight BEB. Opportunity BEB provides relatively good performance with 20/30 km of range for each 5 min, compared to only 4 km for the Overnight BEB as detailed in Fig. 8. Currently, there are several schemes in place that are implemented to overcome the long charging time for Overnight BEB. Several operators have introduced battery exchange schemes/stations to overcome the long charging time for batteries [19]. In contrast, the opportunity BEB benefits from opportunistic charging at stations/stops during the boarding and/or alighting of passengers. It is argued that opportunity BEB can operate seamlessly for 24 hours [35].

minimize the cost and GHGs of the hydrogen pathway [40]. For the BEB category, several infrastructure installations are required. BEB Overnight requires super/fast charging station(s) at the depot and additional supply of batteries if a battery-swapping scheme is in place. BEB opportunity can operate with various alternatives for charging infrastructure such as: charging spots, overhead charging poles, and inductive charging [45]. While the installation of charging spots/poles does not require major modification to the current infrastructure, the required large number and the distribution of these spots acts as a barrier to the implementation of the Opportunity BEB [45]. In addition, the impact of charging stations on the utility grid is also considered a roadblock for the implementation of Opportunity BEB especially in big metropolitan areas. Other operational features including availability, acceleration, vibration, and noise all show that electric buses provide similar performance to the diesel bus. The electric bus operates with lower noise and vibration due to the absence of mechanical parts, and provides a relatively acceptable acceleration (10 s 0–30 km/h compared to 7.5 s for diesel) [35]. Recent performance data also shows that FCEB and BEB bus provide an average of 85% and 90% availability (operating as for the planned schedule) compared to diesel and hybrid buses [24,35].

4.2.2. Infrastructure Another major operational aspect for electric buses is infrastructure. Hybrid buses do not require any special infrastructure and can operate in the current infrastructure settings [37]. The FCEB requires a hydrogen filling station. The ideal locations for such a station is recommended to be on-site at the depot to

The environmental performance of different technologies has received considerable attention in recent years from both academics and service providers. The environmental benefits of electric powertrains are promoted as the main motivation for the electrification of mobility choices [8,20]. Environmental performance is introduced in the literature in the form of Well-to-Wheel

Fig. 6. North America electric bus market profile, modified from [6].

4.3. Environmental performance

Fig. 5. Distribution of estimated bus market share in 2020 [5]. * Between brackets percentage refers to global market share.

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Table 2 Manufacturing, maintenance, running, and infrastructure costs of electric buses (SD-12 m).a Powertrain ICE HEB HEB FCEB BEB BEB

Diesel Series Parallel Overnight Opportunity

Unit price $

Maintenance cost $/km

Running cost $/km

Infrastructure cost $/km

TCO $/km

280,000 410,000 445,000 2,000,000 590,000 530,000

0.38 0.24 0.26 1.2 0.2 0.2

0.8 0.68 0.76 0.53 0.15 0.15

0.04 0.04 0.04 0.16 0.15 0.26

2.61 2.98 2.85 5.71 6.83 3.97

a Data collected in Euro are converted to USD using an exchange rate of 1 Euro¼ 1.241 $, and Kilometers are converted to miles using 1.00 km ¼0.62137119 miles. The cost estimations represent an average of available data in the literature [7, 9, 10, 13, 35, 37-41]. Running cost is explicitly identified in some studies as fuel cost, while other studies incorporated fuel and maintenance cost as running/operation cost, hence data obtained from these are excluded.

Table 3 WTT GHG emissions for diesel, hydrogen, and electricity (gCo2eq/Mj). Context

Gasoline Hydrogen

Electricity

Source

223 (USmix) 130 (CAmix) 150 (EUmix) 60 (CANmix) 289.6 104.47 116.1 110.22

GREET model [17]

H2 – NGSR H2 – WE

Fig. 7. Total cost of ownership (TCO) of electric buses 2012/2030 [35].

US

19

265

256

EU

13.8

306

–

Canada

21.7

–

–

China Spain Italy Portugal

12.4 14.62 14.2 14.2

– 150.72 98.2 69.45

– 136.83 110.9 112.1

RED model GHGenius [11] [14] [15] [7]

contexts as well. It is evident from these models that gasoline fuel provides the lower WTT GHG emissions for each mega-joule (MJ) compared to electricity (EU-mix) and hydrogen (NGSR) fuel as illustrated in Table 3. It is also apparent that the fuel production method has a significant impact on the resulting GHG emission (i.e. the electricity profile in China, EU, and US). It is argued that renewable energy-based production methods are the ultimate means for reducing WTT emissions to zero [46]. However, the WTT assessment provides only a partial illustration of the overall environmental performance. Fig. 8. Driving range of electric buses.

(WTW) assessment of Green House Gas emission (GHG). WTW assessment integrates the generated emissions in two stages: Well-to-Tank (WTT) and Tank-to-Wheel (TTW). WTT measures the GHG emissions of fuel (i.e. diesel, hydrogen, and electricity) at both production and distribution stages, while TTW measures the GHG emissions of the fuel during the usage stage. In this section each powertrain is assessed based on both WTT and TTW aspects using different scenarios for energy supply. 4.3.1. Well-to-Tank (WTT) GHG emissions Well-to-Tank assessment provides quantified measures of GHG emissions during energy production and distribution. The assessment is simply carried out through the identification of energy production methods, feedstock, and the distribution pathways [17]. Due to the significant variation in energy production methods (i.e. fossil fuel, renewable, and biofuel), and distribution pathways (i.e. road, rail, pipelines, and on site), several models have been developed to calculate the WTT GHG emissions [17]. These include the GREET model in U.S, the GHGenius in Canada, and the RED model in Europe. Other studies have been carried out in other

4.3.2. Tank-to-Wheel (TTW) GHG emissions On the other hand, Tank-to-Wheel assessment of GHG emissions estimates the local emissions produced during bus operation [15]. Since TTW results are highly sensitive to context (i.e. driving conditions, congestion, average speed, and number of stops), the TTW assessment is typically carried out in one of two forms; operational data or vehicle simulation models (i.e. ADVISOR) [47]. The TTW results, for a standard SD 12-m bus, show that both FCEB and BEB operate with zero local GHG emissions, while the GHG output of HEB is based on the propulsion degree of hybridization [5,17]. The Diesel–HEB produces an average GHG of 790–970 g Co2eq/km, while CNG–HEB has an average of 700–800 g Co2eq/km [14,15,37]. Table 4 illustrates the TTW GHG emissions for different powertrains. 4.3.3. Well-to-Wheel (WTW) GHG emissions The Well-to-Wheel GHG evaluation of electric buses provides an overview of the environmental performance for each technology. It shows that both FCEB and BEB have great potential to reduce GHG emissions. DHEB serial and parallel are contributing to 20% and 13% of the GHG reduction respectively. FCEB with both NGSR and WE hydrogen production methods contribute to an average 74% reduction of GHG emission compared to diesel bus.

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Table 4 Tank-to-Wheel GHG emissions (gCo2eq/km) – SD 12 m bus. Context/Powertrains

DB

GB

CNGB

GHEB

CNGHEB

DHEB

FCEB

BEB

Source

Italy China Spain Developed countries Europe

1311 1171.32 1326 1290 1005

1396 1171 – 1075 –

1079 893 – 946 1014

1201 – – – –

807 – – 769 –

1084 – 796 794 796

0 0 0 0 0

0 0 0 0 0

[15] [11] [14] [37] [35]

Table 5 Well-to-Wheel GHG emissions. Powertrains

Energy source

DB Diesel CNGB H2 – mix DHEB – serial Diesel DHEB – parallel Diesel FCEB H2 – Central NGSR FCEB H2 – WE BEB electricity – EU mix BEB Electricity – renewable

WTT GHG (gCo2eq/ km)

TTW GHG (gCo2eq/ km)

WTW GHG (gCo2eq/ km)

Average % reduction of GHG compared to DB

218 157 172 188 320

1004 1014 796 870 0

1222 1171 968 1058 320

NA 4.17% 20.79% 13.42% 73.81%

305 720

0 0

305 720

74.96% 41.08%

20

0

20

98.36%

While in the BEB case, the electricity pathway (WTT GHG emissions) has a significant influence on the overall environmental performance. The BEB based on EU-electricity mix provides a notable 41% reduction in GHG. BEB with renewable-based electricity source is considered the ultimate solution for a ZERO GHG transit as detailed in Table 5. In this respect, Kennedy [48] identified a WTT emission threshold for electricity, and argued that electricity with carbon density lower than 600 T Co2e/GW h contributes to overall WTW emission reduction compared to diesel. 4.4. Energy efficiency Electric buses operate with different sources of energy. These include electricity for BEB, hydrogen for FCEB, and fossil/bio fuel for HEB [3,49]. Each source of energy possesses unique characteristics that influence the performance of electric buses. These characteristics include energy generation, energy storage, and energy consumption. These are identified as the main criteria for optimizing the performance of electric buses as they provide a clear indication on the overall energy efficiency [3,47]. Energy efficiency is often determined as the net volume of energy required for one km travel. This section reviews the energy efficiency for each powertrain based on Well-to-Wheel (WTW) assessment. WTW energy efficiency integrates two stages; Wellto-Tank (WTT) that include energy generation, delivery pathway, and energy storage, and Tank-to-Wheel (TTW) that include energy utilization for traction power [7,14,15]. 4.4.1. WTW energy efficiency The efficiency of electricity varies significantly depending on the production method (Fig. 9). Renewable energy based production is considered the ultimate method with 100% efficiency. The Natural Gas Combined Cycle (NGCC), and Coal Supercritical Steam Cycle (USC) production methods provide an average of 50% efficiency [46,49,50]. Mixed method production, which is the most common in Europe, contributes to an average of 40% efficiency [46].

Hydrogen is produced using different methods that include: renewable energy (electrolysis), natural gas steam reforming (NGSR), gasoline steam reforming (GSR), and coal (gasification). Hydrogen is also produced on-board through an Auto Thermal Reforming (ATR) method. Unlike electricity, the efficiency of hydrogen depends on both the production method and delivery pathway. On-board ATR is identified as the most efficient hydrogen production method as detailed in Fig. 9, while NGSR is the most efficient fossil fuel-based hydrogen production method, and the most commonly used today accounting for 75% of world hydrogen production [14,46]. WTT energy efficiency is calculated based on the ratio between the net volume of energy generated to the energy consumed during the process. The functional unit of the WTT stage is often expressed for one mega-joules (1 MJ) of fuel or energy in the forms of liquid, gas, and electricity [14]. In this respect, oil-based fuel remains the most efficient source of energy on WTT bases, based on current energy production methods, with an average of 3.82 MJ/km followed by hydrogen (7 MJ/km) and electricity (11.90 MJ/km) as detailed in Table 6 [14,15,37,51]. However, renewable energy based electricity is arguably the most efficient, and indeed desired, source of energy, especially when considering natural resources and energy security aspects [52]. TTW energy consumption varies significantly due to driving conditions (i.e. congestion, geography, and number of stops) and propulsion configurations (i.e. degree of hybridization, battery type, and fuel cell type) [7,37,53]. It is evident in the literature that different operational contexts lead to different rates of energy consumption for the same technology [7]. TTW energy consumption is often expressed in the form of diesel equivalent mile per gallon (mpg), or mega-joules for each km (MJ/km). In this respect, BEB provides the highest energy efficiency on TTW bases, with fuel consumption of 6.76 MJ/km, followed by FCEB (10.48 MJ/km) and series HEB (10.81 MJ/km) [37,40]. WTW energy efficiency, often described as energy economy, is calculated based on the combination of WTT and TTW stages. Several attempts show varied results for WTW energy consumption/efficiency due to energy pathway and energy generation method [7,11,14,15,37]. Table 6 provides the averages from the available literature, and highlights that renewable-based electricity for BEB provides the best energy efficiency alternative for electric buses with energy consumption of 10.33 MJ/km. However, with the EU-mix electricity profile, the BEB consumes an average of 18.66 MJ/km. Both FCEB and Series HEB provide an average of 26% reduction of energy consumption compared to diesel bus (20.66 MJ/km). 4.4.2. Energy Storage Systems (ESS) The energy efficiency of electric buses is highly dependent on Energy Storage Systems (ESS) [20,53,54]. The efficiency of ESS is evaluated based on three main criteria including energy density, power density, and cost. Energy density refers to the amount of energy stored in a unit volume, while power density refers to the amount of power for a unit volume and the time rate for energy transfer [53–55]. ESS could be classified into three categories that include Ultra Capacitors (UCs), fuel cells, and batteries. Ultra

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Fig. 9. Production efficiency of electricity and hydrogen.

5. A holistic review of electric buses

Table 6 Well-to-Wheel energy consumption (MJ/km).a Powertrains

DB DHEB – series DHEB – parallel FCEB

Energy source

WTT (MJ/ km)

Diesel 3.82 Diesel 3.45 Diesel 3.31 H2 – central 7.00 NGSR FCEB H2 – WE 4.45 BEB – overnight Electricity – 11.90 EU mix Electricity – 3.57 renewable a

TTW (MJ/ km)

WTW (MJ/ km)

Average % reduction of energy consumption relative to DB

16.84 10.81 12.81 10.48

20.66 15.26 16.12 17.48

NA 26.14% 21.97% 15.39%

6.76

14.93 18.66

27.73% 9.68%

10.33

50.00%

1 litter of Diesel ¼33.6 kW h ¼35.9 MJ.

Capacitor ESS stores the energy by separating negative and positive charges, which means that there are no chemical variations. This contributes to a long life cycle [20,53]. UCs are characterized by high power density and low energy density that allows for faster charging/discharging rates. Therefore, it is very appropriate for recovering electricity from regenerative breaking. Ultra Capacitors are suitable for short-range Battery Electric Buses, and it is commonly used for Opportunity BEB, and for HEB in a hybrid mode [53]. The Fuel Cell (FC) energy storage system generates electricity through an electrochemical process using fuel as anode and oxidant as cathode. Although various combinations could be used in fuel cell [56], hydrogen is considered the ideal fuel due to its high energy density compared to other types of fuels. However, due to the relative low energy density of fuel cells in general, a large storage of hydrogen is required on board [53,54,56]. Batteries, as an energy storage system, provide a variety of options in terms of energy and/or power density as illustrated in Fig. 10. Lithium-Ion (Li-ion) batteries are the most common type for BEB buses and provide suitable balance for both energy and power densities [20,55], while Nickel–Metal Hybrid (NiMH) are more suitable for HEB [20]. Recent studies show significant advancements in battery technology that are at the prototype stage [54,55]. The new technologies, such as Lithium-air (Li–air), and Lithium–sulfur (Li–sulfur), promise to achieve as much as double the energy density of Li-ion batteries as illustrated in Fig. 10 [54]. Furthermore, Nano technology applications for fuel cells are under development to enhance response time and energy density [20].

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 side-byside comparison for three different electric technologies and six powertrains. The comparison draws upon the previous review and details the performance of electric buses based on 16 indicators as illustrated in Table 7. In addition, the study develops a detailed visual comparison (Fig. 11) where each electric bus is portrayed against the diesel counterpart. This allows a simple comparison between different powertrains and provides a vivid portrait for each powertrain. Accordingly, and for visual purposes only, the study assigns numerical values for all performance features associated with each powertrain. The normalization is carried out by transforming the numerical and nominal values for all performance features into a standard 5-point Likert scale. The index is developed as follows: Y ¼ a þ ðX  AÞðb  aÞ=ðB  AÞ

ð1Þ

Where, Y¼new value, X ¼the original value, b¼5 (highest value of the 5-point scale), a ¼1 (lowest value of the 5-point scale), A¼lowest value of the original data, B ¼highest value of the original data. Several insights emerge from the side-by-side comparisons. Hybrid buses show similar performance features as diesel bus in all aspects, except for GHG emissions and energy consumption. It is evident that HEB provides an average of 20.8% reduction in GHG emissions and 26.1% savings in energy consumption. Fuel cell buses are characterized by high initial cost and the total cost of ownership is 188% higher than the diesel bus. With moderate infrastructure installations, fuel cell buses provide similar operational features as diesel with zero local emissions. Renewablebased hydrogen FCEB contributes to a 75% reduction in GHG emissions, and 27.7% savings in energy consumption, which is slightly better that the NGSR-based hydrogen FCEB which contributes to 73.8%, and 15.4% reduction in emissions and fuel consumptions respectively. Opportunity Battery Electric Buses could be seen as the most appealing choice with zero local emissions and a reasonable price tag. Total cost of ownership is 52.1% higher than diesel bus. However, in order to satisfy operational requirements, especially range limitations, Opportunity BEB requires major infrastructure

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Fig. 10. Battery technologies for electric vehicle applications, modified from [54].

Table 7 Holistic review of electric buses. Engine technology Fuel type

Unit

ICE Diesel

HEB – series HEB – parallel FCEB Diesel Diesel Hydrogen – NGSR, WE

BEB – overnight Electricity – EU mix, Renewable

BEB – opportunity Electricity – EU mix, Renewable

Unit price $ Maintenance cost Running cost Infrastructure cost Total cost of ownership WTT GHG emission TTW GHG emission WTW GHG emission WTT energy consumption TTW energy consumption WTW energy consumption Range Availability Acceleration time (0–30 km/ h) Infrastructure modification Refueling/Recharging time

US $ US $/km US $/km US $/km US $/km gCo2eq/km gCo2eq/km gCo2eq/km MJ/km MJ/km MJ/km km % Seconds

280,000 0.38 0.8 0.04 2.61 218 1004 1222 3.82 16.84 20.66 450 100 7.5

410,000 0.24 0.68 0.04 2.98 172 796 968 3.45 10.81 15.26 450 100 8.1

445,000 0.26 0.76 0.04 2.895 188 870 1058 3.31 12.81 16.12 450 100 7.9

2,000,000 1.20 0.53 0.16 5.71 320, 305 0 320, 305 7, 4.45 10.48 17.48, 14.93 450 85 9.2

590,000 0.20 0.15 0.15 6.83 720, 20 0 720, 20 11.9, 3.57 6.76 18.66, 10.33 250 90 10

530,000 0.20 0.15 0.26 3.97 720, 20 0 720, 20 11.9, 3.57 6.76 18.66, 10.33 40 90 10

Nominal Minutes

As is 5

As is 5

As is 5

Moderate 10

Moderate 240

Major 10

installations. Renewable-based electricity for opportunity BEB shows significant reductions in GHG emissions (98.4%), and energy consumption (50%), while the EU-mix electricity profile contributes to 41.1% reduction in GHG emissions and 9.7% savings in energy consumption. Overnight BEB provides satisfactory range and operational features with a total cost of ownership 161.7% higher than diesel bus. Battery exchange schemes are currently recommended to overcome the long charging time. With no local emissions, Overnight BEB provides similar GHG emission and energy consumption as opportunity BEB. Both are sensitive to the electricity profile. This holistic assessment shows that successful implementation of electric buses is highly sensitive to context. Energy logistics, network structure, and energy profile are among the factors that have significant impacts on the performance of electric buses. It is evident in the review that the energy profile has a significant impact on the selection of suitable powertrain. For instance, BEB will perform better in Canada and Sweden due to the renewablebased electricity profile. In the US, since electricity production generates high emissions rate, FCEB is more suitable. This sensitivity is clearly reflected in the literature where varying powertrains are claimed to be more suitable. Tzeng and colleagues (2005) advocate for Hybrid Electric, while the Fuel Cells and Hydrogen Joint Undertaking [35] conclude that “Opportunity e-buses and hydrogen fuel cell buses are the most promising zero local-emission powertrains”, which contradicts the recommendations of Nurhadi [18] that argues overnight “would be more preferable in term of cost

effectiveness”. Other scholars argue for the integration between different technology such as; “battery electric vehicle with fuel cell range extender” [33], and “FC–PHEV for achieving higher operation efficiencies, and FC–HEV for lower life cycle impact, lower cost in general, and highest financial savings potential” [7]. Based on the review, this study argues that Overnight Battery Electric Bus is potentially the most desirable electric powertrain for bus transit. This argument is grounded on three pillars: a) The change is inevitable; a shift in energy profile towards renewable energy is inevitable due to the current consumption rates of fossil fuel resources. It is estimated that oil production will decline after 2030 [57]. Oil reserves are expected to decline to 20% of current levels in 2040, and the same for gas and coal, which are expected to last for 20 additional years [52]. Accordingly, renewable based energy, especially electricity, will be the most desired avenue for energy production in the post oil area [57]. b) Lapses, not consistent steps; hybrid and internal combustion technologies are reaching their potential. Although consistent small progress is expected in the future towards emission reduction, it will not be sufficient to achieve the global emission targets [1]. Hybrid technology is often seen as a bridge to full electrification [57], while fuel cell technology is also considered a short-term solution because it is highly dependent on fossil reserves. That is being said, battery technology is expected to gain significant share in the mobility market. Huge advancements in battery technologies are anticipated that will triple or quadruple vehicle range with very short charging time (i.e. Zn-air batteries). c) Context sensitivity will matter no more;

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Fig. 11. Side-by-side comparison of electric and diesel buses. a) All Electric buses – Numerical scale indicates transformed values not the actual values. b) HEB – Series diesel. c) HEB – Parallel diesel. d) BEB – Opportunity (EU - mix & Renewable). e) BEB – Overnight (EU - mix & Renewable). f) FCEB (Central NGSR & Renewable - WE).

given the two previous points, the future energy profile will be similar in different contexts, and thus context will have lower impact on the operational features of electric buses specially from an environmental perspective. Therefore, this study argues that implementing Overnight BEB buses in the bus transit context will benefit both short-term emission targets with zero local emission,

and long-term integration with the expected energy profile. This will provide incentives for R&D to enhance the technology in terms of affordability and reliability.

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6. 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 process as well as future research directions. The study reviewed four performance features of electric buses including economic, environmental, operational, and energy efficiency. Several messages have emerged; most notable is that the performance of all electric buses is highly sensitive to energy profile and operational demands. Therefore, a single technological choice will not satisfy the varied operational demands of transit services. That is being said, Battery Electric Bus (BEB) coupled with renewable/clean source of electricity is arguably the ultimate solution that provides zero net emissions as well as various configuration of range and charging time (opportunity & overnight). However, given the mixed-source electricity generation profile across countries, the environmental merits of BEB are dependent on the volume of GHG emitted during electricity generation. Hybrid buses could be seen as a stepping-stone toward full electrification. The review shows that hybrid powertrains are not providing significant reductions for both energy consumption and GHG emissions. However, due to the strict operational requirements in transit service, especially as it relates to operating hours and charging/refueling time, hybrid buses are considered a viable choice that satisfies these operational constraints. Fuel cell technology could be seen as a competitive choice in contexts with longer driving range, yet the review shows slow market penetration of fuel cell technology. The slow pace could be attributable to the current R&D focus of major manufacturers on battery electric technology. It is evident that the electric powertrain provides different features compared to its diesel counterpart, yet the demands of transit operation are well-aligned with diesel powertrains. Accordingly, there is an imperative need to rethink the design parameters of transit operation if we are to promote electric powertrain technologies in transit context. Future research that integrates the features of electric buses with network/schedule optimization is very important in delineating a path towards full transit electrification.

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-inChief Dr. Lawrence Kazmerski and the two anonymous reviewers for the insightful comments and feedback.

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