Technology INTERNATIONAL Internationales Verkehrswesen (74) 1 | 2022 49 Charging infrastructure and-charging methods for electric buses Focus on the city of Shenzhen in China which has the world’s largest fully electric fleet of buses Electric bus fleet, Charging infrastructure, Shenzhen The movement towards a sustainable and greener future aims to transform the transport sector. Therefore, the public transportation sector is currently undergoing a major transition. In comparison to diesel buses, electric buses have a limited driving range and the charging process takes longer. However, they offer an improved environmental balance. Cities like Shenzhen provide an example on how to transform public transport with a fully electric bus fleet in operation. Elisabeth Gütl T he European Union’s Clean Vehicles Directive (CVD) sets ambitious goals and obliges member states of the EU to focus on the procurement of alternative initiatives in the public sector. Until 2025 many member states have committed to procuring at least 45 % of ‘clean’ buses in the vehicle category ‘M3’ (used for the carriage of passengers, having a maximum mass exceeding 5 tonnes). That means they are powered by sources such as electricity, hydrogen biofuels or natural gas. Half of them must be run with zero local emissions, thus they must be either powered by electricity or hydrogen. By the end of 2030 this percentage will increase to 65 % [1]. Hence many countries from the EU have focused their efforts on eliminating diesel buses and replace them with electric buses. But there are still obstacles to tackle. The Image: Volvo Buses INTERNATIONAL Technology Internationales Verkehrswesen (74) 1 | 2022 50 driving range of electric buses is lower than that of diesel busses and the recharging time is longer. Therefore, a suitable charging infrastructure is necessary and the right charging method needs to be taken into account [2]. Taking a closer look at possible charging methods it can be differentiated between AC or DC charging, inductive charging and battery swap stations [3]. AC or DC charging Looking at AC (alternating current) or DC (direct current) charging of electric buses the main distinction is between a normal power recharging point and a high power recharging point. A normal power recharging point allows a transfer power of less than or equal to 22 kW. A high power recharging point allows a transfer of electricity with a power of more than 22 kW [4]. For electric buses the charging power varies depending on the type of use and the charging strategy. Lower charging power is typically performed with overnight charging of electric buses. This means that, for a common battery size of electric buses, between 200 to 300 kWh [5], with a charging power of e.g. 50 kW, a 300 kWh battery can be charged in around 6 hours in a depot overnight. Higher charging power with up to 500 kW is usually performed by charging with a pantograph and can recharge the battery rapidly. A pantograph is an apparatus that collects power through contact with an overhead line either mounted on the roof of an electric bus or for example mounted on the roof of a bus station (see figure 1) [6]. Inductive charging Inductive charging is a contactless wireless technique that recharges electric vehicles and transmits power by electromagnetic induction [8]. Simply spoken it consists of two parts: the primary coil and the secondary coil. The primary coil (transmitter coil) can be, for example, imbedded in the street and the secondary coil (receiver coil) is on top of the electric bus. The working principle is like a transformer: power is transferred across an air gap without any mechanical contact [9]. Inductive charging can be carried out both stationary and dynamic, thus an electric vehicle can be also charged while it is in motion. Inductive charging offers many advantages. No human intervention is required for the charging process and there are no conductive losses because no conductive wires are used for charging. On the other hand inductive charging systems are still under development and setting up the infrastructure can be cost-prohibitive [10]. Battery swap stations Battery swap stations offer the advantage of replacing the whole battery with a recharged one. The battery change happens fully automated at battery swap stations and the battery can thus be replaced quickly en route [11]. One design solution to achieve an efficient battery swapping process is to produce electric buses with an integrated roof-top mounted battery exchange system. If the battery needs to be recharged the electric bus can utilise a battery swap station along its route and swap the fully automated battery in less than one minute [12]. Building up battery swap stations is expensive, however they offer the advantage to recharge empty batteries more flexibly, for example when electricity demand is low [13]. Shenzhen model Shenzhen is the first major city in the world that has managed to fully electrify buses in the public transport system. By the end of 2017 the entire bus fleet of about 17,000 buses in Shenzhen was fully electrified. Since the initial investment costs for a suitable charging infrastructure as well as the procurement cost of electric busses are rather high, Shenzhen adopted a new business model to tackle the challenging transformation (see figure 2) [14]. The parties involved in this newly adopted business model are the electric bus production company, a financial leasing company, the bus company, the charging facility operator and the government. A key point of the model is how the vehicle and battery are purchased separately. The electric bus produced by the electric bus production company is purchased by the financial leasing company, while the battery is purchased by the charging facility operator. After purchasing the electric bus, the financial leasing company leases the bus to the bus company for eight years. The charging facility operator is responsible for establishing the charging infrastructure, maintaining it and for the charging cost. To use this service, the bus company pays a maintenance service fee to the charging facility operator. The government subsidises the electric vehicles. [15]. Regarding the charging mode, several charging modes have been evaluated, in the end Shenzhen opted to go for large-scale DC fast charging stations. Most of the buses in Shenzhen’s public transport system have an operation distance of 190 kilometres per day. This means that they mainly need to recharge at night. [16]. Shenzhen opted to replace diesel buses for several reasons. Busses in public transport do frequently stop which means that diesel busses are not operating in their optimum operating point. Furthermore, diesel buses do emit more particles as for example gasoline-powered vehicles do. Therefore, electric busses contributed to a more liveable and environmentally-friendly city by improving the air quality in Shenzhen [17]. Figure 1: Charging with a pantograph [7] Technology INTERNATIONAL Internationales Verkehrswesen (74) 1 | 2022 51 Future prospect: Solid-state battery Up to now different types of lithium batteries are still the dominating type of batteries used in electric vehicles. Among them lithium-ion batteries are most commonly used since they are low in weight and offer a high energy storage potential. But the driving range is limited and the operating temperature has an impact on their performance and longevity of lithium-ion batteries [18]. Furthermore, lithium-ion batteries use liquid electrolyte, to allow the movement of ions between anode and cathode, which is flammable and therefore a safety hazard. Solid-state batteries replace liquid electrolyte by a solid electrolyte which is not flammable and hence leads to a safer vehicle operation [19]. Another notable benefit is the higher energy density and therefore a longer driving range since they can use a lithium metal anode. Currently solid-state batteries are still under research and challenges like cost effective manufacturing or finding the right type of electrolyte are still to be overcome to make them ready for mass market use [20]. Conclusion Transforming the public transport sector and replacing diesel buses by more environmentally-friendly electric buses is one measure to help prevent climate change. Building up the infrastructure and choosing the right charging method can be challenging. Shenzhen in China already underwent this transformation and shows an example on how to successfully reshape the public transport sector. Shenzhen adopted a new business model and opted for DC fast charging stations to recharge their electric buses. Although there are other approaches that can be considered, Shenzhen shows us one possible way forward on how to support public transportation electrification and this could have positive consequences for other cities in the future. ■ REFERENCES [1] Directive (EU) 2019/ 1161 of the European Parliament and of the Council of 20 June 2019 amending Directive 2009/ 33/ EC on the promotion of clean and energy-efficient road transport vehicles. [2] Rogge, M.; Wollny, S.; Sauer, D. U. (2015): Fast Charging Battery Buses for the Electrification of Urban Public Transport—A Feasibility Study Focusing on Charging Infrastructure and Energy Storage Requirements. In: Energies 8(5), pp. 4587-4606 [3] Arif, S. M.; Lie, T. T.; Seet, B. 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In: MRS Energy & Sustainability 8(1), pp. 33-39 Elisabeth Gütl, Dipl.-Ing. Project Manager, Great Wall Motor Austria R&D GmbH, Kottingbrunn (AT) elisabeth.guetl@gwm.at Figure 2: New business model of Shenzhen to electrify the public transport system [15]