Public Transport Decarbonization: An Exploratory Approach to Bus Electrification

The sales of zero-emission buses have been increasing in the European Union. In the last five years, the sales of this kind of vehicle have increased from 400 in 2016 to 2500 in 2021 [26]. By the end of 2021, the bus fleet of nearly 700,000 in the European Union comprised over 9000 electric and 20,000 natural gas buses [26]. The EU also requires national governments to encourage the achievement of the objectives targeted at the provision of sustainable modes of transport as well as the manufacturing of vehicles that release less pollutants.

One of the biggest problems faced when replacing diesel-powered buses with BEBs is the cost of the vehicles. The purchase of BEBs seems to not yet be cost-competitive with diesel-powered buses, although the purchase prices are falling, primarily due to falling battery prices, and this should make electric buses cost-competitive within the next few years [27]. Under some conditions, the use of hydrogen (initially in internal combustion engines and later predominantly in fuel cells) would be more cost-efficient than electric batteries in buses, but the presence of specific policies is relevant for the use of BEBs in urban public transport [28]. Moreover, the European Union is keen to develop strategies and directives for the electrification of public transport in cities, with a special focus on demonstrating how cost and energy can be saved by electrifying public transport and optimizing the use of existing infrastructure to develop new concepts and business cases [29,30].

The policies that have been implemented in the EU to reduce pollution originated in the transport sector and rely on two main standards: (i) carbon emission standards for fuels and (ii) the implementation of electric vehicles in the urban fleet. A study compared the outcomes of these two policies regarding the achievement of the targets for GHG emissions in the EU. The results show that the development of new technologies would aid the electric vehicle industry, mainly in the production of new and more efficient batteries, although carbon emission standards could be a better solution in terms of cost-efficiency as they contain an incentive to improve fuel efficiency. With endogenous technological progress, the cost of saving CO₂ emissions is reduced from some 200 EUR/ton of CO₂ in a static model to about 100 EUR/ton CO₂ [31].

Taking a specific look at the bus electrification scenario, Lu et al. [32] state that the life cycle evaluation of electric, hybrid, and diesel buses shows that the replacement of conventional buses with hybrid and electric ones can provide a good balance between financial and environmental needs. The same study shows that electric buses outperform hybrid buses in terms of GHG emissions in most European countries; the performance varies with the month of the year and the country. For example, the use of electric buses could reduce overall GHG emissions during summer compared to winter in Estonia and Poland, while in Malta and Cyprus, it would be the opposite due to weather conditions that make electric buses spend more energy during colder winters.

In another case study in Brazil, the electrification of the passenger transport sector is estimated to accomplish the most reduction in emissions if carbon pricing is increased and more efficient engines are used in buses, which means the electrification of the fleet [33]. On the other hand, even if the utilization of hydrogen fuel cell buses would reduce pollutant emissions, their cost (i.e., fuel economy, bus cost, and maintenance) would be 133% more expensive than conventional diesel-powered buses [34]. The purchase of electric buses, in this context, could either work on a full-lease model (i.e., a consortium formed between an energy utility and an electric bus manufacturer) or in a partial-lease model (i.e., energy utility solo performance), which would simplify the purchase and deployment of BEBs [35].

In Reykjavik, Iceland, where the electricity grid is fully decarbonized, the electrification of the passenger transport vehicle fleet is not sufficient to reduce the total indirect emissions; therefore, some more radical actions need to be taken, such as the use of MaaS options, behavioral change, densification of the urban fabric, and implementation of improved public transport [36]. Moreover, the provision of reliable public transport with proper infrastructure can lead to a modal shift, and, together with clean energy grids and BEBs, the targets for the reduction in GHG emissions can be achieved [37].

Results from a study performed in Greece [38] also corroborate the understanding that to reach European standards for GHG emissions, more action than the electrification of the bus fleet is needed, such as the application of techniques that contribute to efficient traffic management and the implementation of measures that upgrade public transport services. Another issue that needs to be addressed when replacing diesel-powered buses with less polluting and green ones is the infrastructure needed for fueling the new buses, which includes the infrastructure needed for CNG fueling stations, gas upgrade companies, and even charging stations for BEBs [39].

Despite the 1.10% annual growth in transport demand in British Columbia, Canada, when there is a shift to electric vehicles for urban journeys and when public transport is electrified, emissions in the long term can be decreased by 95% [40]. The benefits from the electrification of the bus fleet can be different depending on the contexts in which they are inserted, although the benefits of electrification are positive everywhere and can reach quite a large scale when compared to diesel buses [41].

Moreover, the energy that comes to the charging stations for BEBs needs to also be studied and analyzed because the power grid can affect the life cycle emissions of electric batteries. The environmental impact over the life cycle of a BEB is no doubt further reduced when compared to traditional buses; however, charging opportunities can be better specified to improve environmental gains [42,43]. As an example, the possibility of having a solar-powered charging station for BEBs could reduce operational costs by 8% during day-to-day operations [44]. Figure 2 illustrates how charging can be managed to improve the environmental sustainability of BEBs.

In addition to recharging the batteries and the power plans used to do so, the life cycle of the batteries needs to be investigated as well. Buses that are powered with heavier batteries seem to have higher battery-related emissions, and studies have shown that a battery capacity of 120 kWh has a better life cycle impact than a 60 or 300 kWh battery [46]. The adoption of traffic operations benefits the retrofit of energy and, in certain congestion scenarios, also affects the environmental performance of public transport, with positive impacts [47].

Despite the possibility of GHG emissions in some circumstances of electrification of the bus fleet in cities due to battery-related life cycle and energy production discharges, the replacement of diesel-powered buses with BEBs can represent a lowering in pollution when considering the trip itself [48,49]. A study performed in Portugal [16] showed that in a 14-year timeframe, it is possible to fully decarbonize the bus fleet of the country and consequently reduce GHG emissions to zero. This change would represent a reduction of more than 4 million tons of CO₂ emitted into the atmosphere from one country alone and only from urban buses [16].

In order to assess the life cycle of BEBs and how they affect the environmental burden of this mode of transport, it is important to consider either well-to-wheel or tank-to-wheel emissions [50]. The transition to BEBs is the most promising solution for an environmentally friendly urban bus fleet [51]; however, for cities to take full advantage of the decarbonization of the fleet, power grids need to comply with green energy levels and standards.

If electricity production becomes free of fossil fuels, electrified vehicles, as well as BEBs with external charging capabilities, could reach their full potential in mitigating global warming [52]. Also, the costs associated with recharging the vehicles can be lower than the prices of refilling diesel-powered public transport vehicles, which increases the costs for the overall management of buses.

A study by Jakub et al. [53] showed that the shift of electricity production to renewable and low-emission sources would significantly reduce well-to-wheel emissions, which is the main environmental burden of electric buses. In addition, it has been shown that the production of the bus itself has an insignificant burden when compared to the well-to-wheel phase when power grips are not decarbonized. And, as is expected, compared to diesel buses, the tank-to-wheel phase does not represent a significant environmental load.

Even if BEBs are much more expensive than other types of urban buses, their energy efficiency, zero-emission operation, reduced dependence on fossil fuels, and expected cost reductions make them a relevant choice for the replacement of diesel buses [54,55]. The deployment of BEBs in cities is better than conventional diesel buses over their life cycle regarding most of the indicators, such as carbon footprint and reduction in air pollution [56,57]. In some cases, like Vietnam, Sweden, and Norway, if BEB operations could have a supply of renewable energy, the carbon footprint could be reduced to 38.1 g CO_2eq/pkm [56,58,59].

Added to the fact that there is a reduction in emissions per passenger kilometer traveled, electric buses have an effective life cycle cost (EUR/KM) when compared to other types of buses. An assessment carried out by Lajunen and Lipman [54] stated that electric buses are more competitive than CNG and diesel hybrids when fossil fuel costs are higher and that BEBs with opportunity charging are more cost-effective than those that are charged overnight.

Thus, the life cycle costs and environmental externalities of BEBs can be affected in the future by some favorable trends. Tong et al. [60] exemplify that technological advancement in the future can corroborate the decrease in battery purchase costs as performance improves. In addition, energy policies from governments can create and improve on even more clean power plans, and BEBs are easier to integrate with intelligent control technologies. These opportunities can make the external costs of electric buses lower than the life-cycle external costs of conventional diesel buses by 2030 [60]. Figure 3 presents a diagram that represents the assessment of the life cycle of BEBs.

One of the main challenges of incorporating electric buses into the public transport fleet is to identify the most suitable solutions for each context, which comprises the most convenient tools that neither dramatically change the daily operation of the buses nor exceed the personnel, investment, and operational boundaries [30]. Thus, it is of utmost importance to better evaluate some aspects of the electrification of the bus fleet, such as the fleet size, charging models, the intended profitability, and the feasibility of the fleet.

The availability of BEBs in the market has been growing in the last few years, which can support the addition of this type of bus to public transport lines. BEBs are seen to have a high potential for urban public transport services because they can represent economic savings of up to 6.0% when compared to traditional buses. Even if the initial costs are higher than current diesel-fueled buses, BEBs are cheaper to operate, and they do not require a completely new infrastructure system but offer synergy with the existing electricity system [61,62,63]. They can also serve as a way to decrease the external costs of transport, which increases the convenience of having BEBs in the urban fleet [64].

A study performed in Canada shows that the addition of BEBs to the fleet and routes is capable of fulfilling the operational demands required in cities and that electric buses coupled with fast-charging technology would outperform regular BEBs [65]. However, fast-charging buses would need more electrical infrastructure and would not be a feasible solution in cities. On the other hand, regular charging stations for BEBs can be allocated on-route in highly dense service locations (e.g., downtown or central business districts) where several routes are operating to cover relatively smaller geographical areas, meaning the buses pass a main transit hub multiple times a day [66].

Studies show that the most efficient charging scheme is to install charging stations at the end stops of buses instead of along the route, although it is important to mention that a small portion of charging stations (10% to 25%) needs to be installed at stops along the route, depending on the optimization preference for the bus line [67,68,69,70]. Electric buses usually take 10 min to be charged, depending on the size of the battery and the power tension available, but are often charged overnight from 8 p.m. to 3 a.m. to secure capacity and availability of energy [71]. Also, the implementation of shared charging hubs for electric vehicles and electric buses can enable coordinated charging that reduces peak power demands and leads to savings in both initial capital investments and long-term peak demand charges [72].

The time of the day when BEBs are charged can also impact the overall cost of operation for this type of bus. In cities that have cheaper nighttime electricity prices, the BEB fleet can be charged at this time to benefit from reductions in refueling costs, as recharging during peak hours can lead to higher costs [42,73]. Moreover, the minimum battery capacity of each bus line must be identified so that the entire route can be completed during the day without the need for refueling, and the buses can be charged at nighttime at the end stop [74]. Figure 4 shows a schematic representation of the characteristics of the introduction of BEBs as a public transport option in cities.

In short, the success of the deployment of BEBs in cities when considering the cost-effectiveness of charging infrastructure and route assignment depends on the following main factors that need to be considered by the operator: (i) appointing routes according to the capacity of the batteries; (ii) placing charging stations at endpoints of the routes (end stops); (iii) charging BEBs at nighttime due to lower energy costs and availability of energy; and (iv) initial deployment of BEBs in more dense areas where they can cover more routes in a smaller area. The deployment of BEBs in cities in a way that regular charging can be controlled by the route chosen is the best way to reduce costs and increase the usability of this type of bus in cities [75,76,77,78].