Illustration by NET Engineering
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Suburban Infrastructure for E-BusesDesign challenges and feasible answers
Which choice of charging system for e-buses should be made for long distance routes? What are the alternatives and the strong and weak points? But, above all, what assessment criteria should be followed to guide the planning choices? E-buses can now be considered a consolidated technology in urban contexts. Nevertheless, there are still some significant challenges in the design of public transport services with electric buses for suburban areas due to the distance covered by the service and the low number of stops.
Assessment of the planning criteria for a suburban electric infrastructure can be summarised in three macro-areas:
– Accessibility: i.e. the capacity of the vehicle, directly related to the volume taken up by the different types of batteries;
– Flexibility: temporary obstacles (constructions sites or other situations) that may impose significant re-routing and also infrastructure restrictions along the route that may make the inclusion of charging points difficult;
– Cost: there are clear cost differences between an urban electric infrastructure and a suburban one. It’s very difficult to make an objective assessment of the impact of planning choices on the overall cost of the infrastructure as it all depends on the specific features of each project.
In relation to the complex question of costs, recent research by McKinsey highlighted how the economic assessment for this type of technology is related to the type of service and how it operates in terms of number of stops, relative distances and the position of the bus depot and storage areas. However, it’s also linked to the type of traffic in terms of the level and frequency of congestion. McKinsey’s analysis, which assessed the cost of a short route and a medium-length one, showed how charging e-buses in the depot is more economical for short routes while an ‘Opportunity’ charging system is better for longer routes. Greater use of charging points can also reduce the cost – the consequence of a route where various e-buses are planned is a reduction in the cost of charging each vehicle.
As mentioned, there are various charging systems for e-buses – flash, depot, opportunity and wireless. The ‘Flash’ charge offers a high power charge through the pantograph. This type is power-hungry and could require demanding maintenance. The main advantage of the system lies in the contained costs and its easy integration. Charging in the depot, or overnight, is probably the best-known system and offers slow, very effective charging made overnight when the cost of electricity is lower. ‘Opportunity’ charging is high power and suitable for combination with depot charging. Its convenience lies in its use by more than one vehicle or bus route. Lastly, ‘Wireless’ charging is induction charging which, at present, is rarely used in Italy because of the high costs, especially for maintenance.
The actual charging of e-buses is via three different types of infrastructure – plug-in, pantograph and induction. The choice of the best option mainly depends on the charging strategy used. If we consider charging via pantograph, there are usually two options – with the pantograph raised or lowered. There are e-buses using just one method and others that can adopt both technologies, taking advantage of both systems, which can and will be able to co-exist.
So, trying to compare different charging systems, it could be said that charging in the depot has high battery costs against a lower infrastructure cost; on the contrary, the ‘mixed’ solution has cheaper, less voluminous batteries but higher construction and maintenance costs for the charging stations.
The size of the batteries presents challenges that are easy to imagine. The capacity of e-buses is one of the main factors contributing to their performance, which is clearly related to the duration of the battery charge and the frequency of the service. Therefore, it’s obvious that every choice potentially influences others. Starting from the assumption that the number of passengers on an e-bus is determined by the size of the battery pack, the smaller this is, the more passengers can be carried but with shorter duration of the charge. If, however, the battery pack is larger, the performance is better but there is less space available. In turn, the size of the battery pack determines the length of the route as the capacity of some batteries may not be sufficient for longer routes or those with steep hills causing greater energy consumption. Another case to consider is routes with stretches of motorway where all passengers must, by law, have a seat.
This is an example of the challenges faced and the effect generated by the answer adopted during transport planning of the Bus Rapid Transit (BRT) Bologna and that of the answer adopted. In the initial stages, the transport study was developed taking depot charging into consideration. By calculating the percentage of residual charge at the end of the route, the need to add another charging system in the stops was highlighted. In this case, the choice of ‘Opportunity’ charging, which could be combined with that in the depot, was found to be the most suitable for the context. In addition, the availability of each vehicle was increased with the resulting reduction in the need for vehicles.
The choice of electric vehicle must certainly be very carefully considered to give an adequate response to the demand for transport – assessing the technical features, the number of seats and the battery pack. A possible approach is the introduction of an alternative charging station at stops where the vehicle stays longest. As a result, the route must be careful planned, considering any obstacles from the very start but, in particular, designers must be aware of the challenges and opportunities that planning long distance public transport with electric vehicles currently impose and these must be visualised in advance, taken on board and interpreted.