Pre-Normative Charging Technology Roadmap for Heavy-Duty Electric Vehicles in Europe

Abstract

This paper presents a pre-normative roadmap that foresees the developments in the charging of heavy-duty electric vehicles (HD-EVs). It supports and facilitates the future standardization efforts of charging technologies by creating an overview of the popularity of charging technologies and the end users’ needs. The required input for the work was collected using a comprehensive investigation on the available charging technologies and their standardization, reviewing the existing roadmaps and research work, and conducting surveys and interviews of end users and technical stakeholders. According to the findings, a pantograph on the roof of a vehicle and plug-based charging are currently the most used charging interfaces. This trend is likely to continue in the future, since (1) pantographs on vehicle roofs, (2) pantographs on infrastructure, and (3) plugs were graded as charging interfaces with the highest potential by the participants of the technical survey. Static and conductive charging technologies show more potential than dynamic and wireless charging technologies. Nevertheless, inductive charging may be a future charging solution for HD-EVs if the current bottlenecks in the technology can be addressed. These bottlenecks include high prices, slightly lower efficiency, lack of standardization, the maximum achievable power, and safety concerns. Furthermore, interoperability was repeatedly mentioned as the main challenge for today’s charging technologies.

1. Introduction

In 2019, the European Commission (EC) established a set of policies through the European Green Deal [1] aiming to make Europe the world’s first climate neutral continent by 2050. To achieve this goal, the EC presented a “Fit for 55” package in the 2021 Commission work program to reduce emissions by at least 55% by 2030 [2]. Road transport is one of the sectors with the highest potential to reduce the net emissions of greenhouse gases. In 2020, the global transport sector generated 7.3 billion metric tons of CO₂, with passenger vehicles as the largest polluter, accounting for 41% of the emissions and followed by medium- and heavy-duty (HD) vehicles, being responsible for 29% of the emissions [3]. Therefore, decarbonizing medium and HD road transport and replacing them with electric vehicles would significantly support the EC’s zero emission goals.

One of the key factors affecting the success of new and expanding HD electric vehicle (HD-EV) fleet rollouts is the deployment of a purpose-serving and scalable charging infrastructure for HD-EV operations. Additionally, interoperability between various vendors (vehicles and chargers) is instrumental. Here, interoperability refers to the ability of vehicles and chargers of different brands to work together seamlessly and effectively, without restrictions. This requires the availability of robust charging standards. A guiding motivation of standardization is the cost reduction of infrastructure by assuring functionality, compatibility, and interoperability. Furthermore, standardization allows an optimized and scalable approach in the deployment of HD-EVs, as it does not bind the product choice to one solution or supplier.

On the other hand, as the number of HD-EVs increases in the market, their charging needs evolve. Therefore, the related charging standards and policies should be future-proofed and should meet the charging needs. This study supports and facilitates such standardization efforts for charging technologies by creating an overview of the popularity of charging technologies and examines the end users’ needs. It also introduces a technical roadmap showing the evolution of various aspects of charging technologies until the end of 2030.

After the introduction, in Section 2 of this paper an overview is presented of the currently available charging technologies for HD-EVs with the standardization status as well as a literature review of the available research, use cases, and roadmaps on charging technologies. Section 3 presents a description of the conducted surveys and interviews of this study. This is followed by their results analysis in Section 4 and Section 5. The surveys were carried out in the framework of the EU’s H2020 ASSURED project [4] and covered the perspectives of relevant stakeholders, including public transport authorities and operators, electric bus manufacturers, electric truck manufacturers, charging solution suppliers, national and international associations, cities, and research centers.

The new roadmap is presented in Section 6, and the parts of the roadmap are further explained in the following sub-sections. Section 7 concludes this study.

2. Literature Review

2.1. Commonly Available Charging Technologies

Generally, electric vehicles (EVs) can be charged when static (vehicle at standstill) or when dynamic (vehicle in motion). The charging solutions are further categorized by the power transfer method from the charger to the vehicle. The methods include conductive or contact charging, inductive charging, and battery swapping.

Table 1. Overview of available charging technologies for HD-EVs.

	Charging Technologies of HD-EVs			Pros	Cons
Conductive charging		Manual connector charging (power: 50 to 150 kW)		Good efficiency and less exposure to electromagnetic field compared to the inductive charging.	Maintenance costs because of the physical contact. For manual charging, need for dedicated staff to perform the charging. Visual impact, depending on the solution chosen and its location.
	Charging with automatic connection devices (ACD) (power: 150 to 600 kW)	Infrastructure mounted ACD	Roof-mounted ACD
		ACD connected to side or on roof of vehicle	Ground-based ACD
	Catenary			Small battery can be used in the vehicle, which decreases vehicle cost. Flexibility in terms of route and network design.	Installation of new overhead lines can be costlier than other conductive charging. Visual impact (no impact while running without wires).
Inductive charging	Static inductive			Better visualization compared to conductive charging.	Lower efficiency as compared to conductive charging.
	Dynamic inductive			Small battery can be used in the vehicle, decreases vehicle cost.	Lower efficiency compared to conductive charging. Heavy and costly construction works.
		Battery swap (not very common for HD-EVs in Europe)		Reduces the high upfront price of EVs by separating battery ownership and cost [5].	Complex and expensive infrastructure. A high level of standardization for the battery packs. High capital expenses of additional battery packs [6].

summarizes the commonly available charging technologies for HD-EVs.

2.2. Charging Standardization for HD-EVs

For the U.S. auto industry, the Society of Automotive Engineers (SAE) standard J1772 [7] is the governing document for EV charging. It defines the requirements for electric vehicle supply equipment (EVSE). SAE J3105 [8] standardizes the interface between the overhead infrastructure and the vehicle. The International Electrotechnical Commission (IEC) standard 61851 used in Europe [9] and Guobiao standards recommended (GB/T) 20234 [10] used in China were derived from the J1772 standard and have similar requirements, adapted for European and Asian AC line voltages. Standards for an overhead charging infrastructure are not fully established, and several committees have been formed in the U.S. and the European Union to form a set of standards.

Table 2. Main international charging standards for plug-based charging, inductive charging, and battery swapping for EVs (Table 6—ITF (2020) “Regulations and Standards for Clean Trucks and Buses: On the Right Track?”, International Transport Forum Policy Papers, No. 77, OECD Publishing, Paris [11]).

Charging Technology	Standard Number	Short Description of Standard
Plug-based charging	IEC 62196	Series of standards for conductive charge connectors (plugs, socket-outlets, vehicle connectors, and vehicle inlets) for electric vehicles
	IEC 61851	Series of standards covering safety-related specifications for charging stations, electromagnetic compatibility, and communication between the vehicle and charger (including vehicle-to-grid functionality)
	ISO 17409	Specifications for the connection of electric vehicles to an external electric power supply
	ISO 15118	Series of standards for vehicle-to-grid communication interfaces, protocols, and data requirements
	SAE J1772	Specifications for conductive charge connectors (plugs, socket-outlets, vehicle connectors, and vehicle inlets) for electric vehicles (most relevant for North America and Japan)
	SAE J2953	Requirements and specification by which a specific electric vehicle and charger can be considered interoperable
	SAE J3068	Electric vehicle power transfer system using an AC three-phase capable coupler
Inductive charging	IEC 61980	Series of standards and specifications for the equipment needed for the wireless transfer of electric power from the supply network to electric road vehicles
	ISO 19363	Safety and interoperability requirements for on-board equipment that enables magnetic field wireless power transfer for electric vehicle charging
	SAE J1773	Recommended practices for electric vehicle inductively coupled charging
	SAE J2954	Specifications on safety, interoperability, and electromagnetic compatibility of wireless power transfer for light plug-in electric vehicles
Battery swapping	IEC 62840	Series of standards for electric vehicle battery swap systems

presents the main international charging standards for plug-based charging, inductive charging, and battery swapping. While the technical standards for EV charging aim to cover a broad range of vehicle categories, the focus of the presented standards in Table 2 is on passenger cars, not HD-EVs.

In 2015, to tackle the lack of standardization for the charging infrastructure in Europe, the European Commission (EC) requested the European Committee for Standardization (CEN) and the European Committee for Electrotechnical Standardization (CENELEC) in a mandate 533 (M/533) to develop and adopt appropriate European standards or to amend existing European standards for an alternative fuel infrastructure [12]. According to this mandate, the developed European standards should include technical specifications with a single solution for electric bus (e-bus) supply connectors and socket outlets (derived from the standard developed for electric passenger cars and light duty vehicles, if possible) and a single solution for e-bus wireless charging.

In 2018, the CEN-CENELEC eMobility Ad Hoc e-Bus Steering Group, formed from the ZeEUS (the Zero Emission Urban Bus System) Project [13] Standardization Group, established the first version of their recommendation. It contained a manually applied Combined Charging System (CCS) combo Type 2 Mode 4 connector (up to 500 A and 1000 V) and three fast-charging solutions. The solutions were infrastructure-mounted, roof-mounted, and ground-level automatic connection devices (ACDs), hence differing from the EC’s request for a single solution for supply connectors [14].

Table 3. CEN-CENELEC recommendations for conductive charging of HD-EVs. Standards ISO 15118-20 Ed1 and IEC 61851-23-1 are still under development and not finalized yet.

Charging Subject		Connector (Plug-Based Charging)	Roof Mounted ACD	Infrastructure Mounted ACD	Ground-Based ACD
Communication	Application to network layer	ISO 15118-2 Ed1	ISO 15118-20 Ed1	ISO 15118-20 Ed1	ISO 15118-20 Ed1
	Physical to datalink layer	ISO 15118-3	ISO 15118-3	ISO 15118-8	ISO 15118-8
Electrical safety and EMC		IEC 61851-1, IEC 61851-21-2, IEC 61851-23
		ISO 17409 Ed1	IEC 61851-23-1 ISO 17409 Ed2	IEC 61851-23-1 ISO 17409 Ed2	IEC 61851-23-1 ISO 17409 Ed2
Mechanical interface		IEC 62196-3 Configuration FF	EN50696 Annex B	EN50696 Annex A	EN50696 Annex C

presents the list of CEN-CENELEC recommendations for the conductive charging of HD-EVs. Standards ISO 15118-20 Ed1 and IEC 61851-23-1 are currently being developed and are expected to be finalized in 2022. In 2020, SAE International issued the SAE J3105 standard for ACD charging.

In 2018, the Charging Interface Initiative (CharIN) initiated a task force to develop a “Megawatt Charging System” (MCS) to fulfill the market demand for charging electric trucks and buses with large battery capacities within a reasonable time. The MCS connector complies with CCS requirements and allows a bi-directional energy flow, with the ability to provide over 1 MW of charging. Its complete requirements and specification documents are expected to be finalized by 2022 [15].

2.3. Literature Review of Existing Roadmaps

Lucien et al. [16] provided a roadmap for electric truck charging infrastructure deployment. They categorized the changing needs of electric trucks into three charging use cases: depot charging (overnight charging), destination charging (typically at distribution centers), and public charging (dedicated truck locations along highways or at charging hubs in urban areas). According to Lucien et al., to cover half of the distance driven by trucks in Europe by electric trucks, policymakers should address these three charging use cases adequately. In the early phases of a transition to electric trucks, depot charging will serve about 80% of the truck charging needs, while destination charging will cover 15% of the total energy and public charging about 5%. However, in the long term, improving public charging will be necessary to fulfil the charging needs of urban and regional deliveries and long-haul operations, with an increasing dependency on public charging as longer trips are electrified.

Welch [17] presented a roadmap that forecasts the availability and cost range of charging use cases for e-trucks between 2018 and 2023. According to this roadmap, the charging technology would move towards high-power DC charging (150 kW–3 MW) at distribution centers and public charging locations, which would increase the infrastructure cost, while the demand for lower power charging (20 kW–50 kW) would be limited to night charging in depots. Sudhakar [18] presented a roadmap for commercial electric truck and bus charging infrastructure technology between 2010 and 2030 and concluded that the improvement of ultra-fast charging (with more than 1 MW capacity), interoperability between chargers, and network expansion would need to be the focal point for the next ten years.

Despite the anticipated increase of charging power, low AC and DC power will still be required for applications such as intercity fleets that have time to charge over long periods, either overnight or during off-operational hours at depots. On the other hand, DC charging with high power (over 400 kW) is suitable for fleets that must charge rapidly at dedicated facilities or along travel corridors. Therefore, the fleet charging solution and use case need to be defined according to the location and time needed to charge [17].

In general, a battery with high capacity requires a longer charging time compared to a lower capacity battery. For example, currently most e-trucks take more than two hours to fully (0.5 C or less) recharge on the fastest available charging systems, whereas other HD vehicles with a high battery capacity must plug in overnight to fully recharge a drained battery, as mentioned in the “Barriers to electrification in the heavy-duty sector” Geotab report [19]. The capability to fast charge (2 C or more) requires both the ability of the battery to receive the necessary current and the capacity from the EVSE to provide the necessary current and power.

E-trucks, with ranges of 150–300 km and with batteries with of 100–300 kWh capacity, are likely to play an increasing role in urban areas. These will primarily be for “last mile” delivery and for commercial vehicles that operate on a local route and return to a depot for re-charging on a regular basis. However, with the continuous increase in energy density, it is expected to have heavier vehicles with a battery capacity of about 500 kWh in the next five years. Nevertheless, the need for high battery capacities can be reduced by having a sufficient high-power infrastructure with easy access for vehicles with a short and medium mission length or fleets that have the possibility of fast and ultrafast opportunity charging, as argued by Nykvist and Olsson [20].

As the charging power and capacity of batteries increase the utilities also need to be prepared in advance to assure the grid readiness in response to the increasing demands. There are some studies on the impact analysis of highly powered HD-EVs on the distribution grid [21,22,23] which have shown the voltage constraints that the charging infrastructure may bring to the system. A solution to this problem could be to use smart and flexible charging to aid the grid in response to the HD-EV demand.

In the ZeEUS project [13], several market-based simulations for the optimal scheduling of bus charging were run to evaluate the revenue potential of different business models. According to this work, energy arbitrage combined with the reduction of demand charges resulted in modest savings for the charging supply organization. The savings from price-controlled charging (energy arbitrage business model) were the greatest when only overnight charging was allowed. However, in this case the battery cost was extremely high. Allowing opportunity charging lowered the total cost, which indicates that the high prices for power in the daytime could be compensated for by reduced battery and charger costs. This was even true if a service charge was added to the opportunity charging energy price to cover the infrastructure costs [24].

Dynamic charging solutions, such as overhead catenary or in-road charging systems, can extend vehicle ranges while reducing the need for heavy and costly batteries [17]. According to Lucien et al. [16], a pan-European dynamic charging system could be the most climate friendly and cost-efficient solution. On the other hand, different types of vehicles should also be able to utilize dynamic charging solutions, as discussed by Fyhr et al. [25].

To ensure the interoperability and the rollout of a dynamic charging system across Europe, however, European policy makers must provide a single standard for dynamic charging solutions [16]. Otherwise, companies will start pushing their individual charging solutions that may not be interoperable with others. This can already be seen with several companies and projects who have started piloting their own dynamic charging solutions in Europe, for example [26]:

• Elways in Sweden—a ground-based system [27,28];
• Elonroad in Sweden—a ground-based system, slightly different from Elways’s solution in being mounted on top of the road;
• eHighway in Sweden and Germany—overhead line-based system;
• Smartroad Gotland in Sweden and soon moving to Germany—a wireless electric road system.

Additionally, in the Highways England study [29], the feasibility of dynamic wireless power transfer was studied, also for HD-EVs.

3. Methodology

The required input for the technology roadmap in this study was collected through a comprehensive investigation of the available charging technologies and their standardization and by reviewing the existing roadmaps and research work. Furthermore, we conducted surveys and interviews of end users and technical stakeholders to obtain their perspectives on the future of charging technologies. These surveys and interviews are presented below.

Data Collection Using Surveys and Interviews

Two surveys, an end user survey and a technical survey, were carried out to collect the views of different stakeholders on the future needs and potential of charging technologies. The target vehicle groups in the surveys were battery-operated electric buses, trucks, and other HD vehicles. The charging technology types included in the surveys are presented in Table 1.

Table 4. Contents summary of surveys for the foreseen developments in the HD-EVs.

Survey Type	End User Survey	Technical Survey
Target Participants	Owners/users or future owners of electric buses, trucks, or other HD vehicles Examples: public transport companies, city governments, logistics companies, construction companies.	Organizations with in-depth knowledge of charging infrastructure for electric buses, trucks, or other HD vehicles Examples: charger manufacturers, charging service providers, research organizations, and electric bus/truck OEMs.
Goal	To maximize the adoption rate of future charging technologies standards by matching standards to the needs of the end user.	To smoothen and speed up future standardization processes for the charging infrastructure by creating an overview of the popularity of charging concepts and their perceived potential.
Expected results	An overview of the charging infrastructure user needs.	An overview of the popularity of charging concepts and their perceived potential.
Number of respondents	14	20
Respondents’ target vehicle groups	Electric buses: 14 Electric trucks: 0 Other HD vehicles: 0	Electric buses: 18 Electric trucks: 11 Other HD vehicles: 9

summarizes the target participants, goals, the expected main results, and participant statistics for the surveys.

A total of 25 participants responded to the end user survey. To ensure that the respondents matched the requirements of the target participants, the participants were asked at the beginning of the survey if their organization currently owns or uses the target vehicles (or aims to own or use the target vehicles in the future). The participants who answered “No” to these questions were not allowed to continue responding to the rest of the survey. Eleven (out of 25) of respondents answered “No”. Therefore, they were dropped, and the survey continued with 14 participants. The remaining 14 respondents were end users who were operating electric buses. A natural reason for the lack of respondents from trucks and other HD vehicle categories is probably that EVs are rare in these categories. This situation is expected to quickly change in the coming years.

The technical survey was answered by 20 participants, all of whom were experts in the field of charging technologies.

In both end user and technical surveys, the respondents were asked about their interest in participating in a follow-up interview after the survey, with the aim of discussing the participants’ answers to the survey in more detail. Eventually, 7 respondents were interviewed, 3 from the end user survey and 4 from the technical survey.

4. Results of End User Survey

4.1. Charging Technologies Owned or Used by End Users

A pantograph on the roof (roof-mounted ACD) and plug (connector) charging were the most used charging technologies among the end user survey participants (Figure 1). These technologies were almost three times more popular than infrastructure-mounted pantographs (infrastructure-mounted ACD) and overhead wire (catenary) solutions. The end user participants provided statements as reasons for their organizations to choose a specific charging technology (

Table 5. Reasons for organizations to choose a specific charging technology.

Pantograph on vehicle roof	Optimal use of time when the e-bus is at the terminal, during regular stops. Guarantees for the best system availability and the fastest opportunity charging. Charge with the same interface for opportunity charging and depot (not having both ACD and plug-in). Pantograph on the roof preferred to the infrastructure-mounted pantograph for maintenance reasons. For the pantograph on a vehicle roof, if a technical problem occurs in the pantograph, the infrastructure can be still used by other buses.
Plug	Plugs favored over infrastructure-mounted pantograph solutions, as the cost of the pantograph-based infrastructure was too high for the number of buses that were deployed with overnight charge solution.
Infrastructure-mounted pantograph	Public tendering for the complete package (bus and infrastructure). The infrastructure mounted pantograph system was the best offer.
Conductive charging	Conductive preferred to inductive charging, as the efficiency is higher.
Overhead wire	Able to use the currently existing contact network. The use of overhead wires and trolleybuses extended to hilly landscapes and areas with heavy passenger demand and full-day operation. Overhead wire technology chosen for fleet of older and historic trolleybuses. Technology suitable for places in a city that the infrastructure already exists for utilization.

). There were no single reasons above others when selecting the suitable charging technology, and the reasons varied from technological and economic ones to administrative and even historic ones.

4.2. Future Goals of End Users’ Organizations with Charging Technologies

The end users were asked to provide their organizations’ short- and long-term goals regarding the charging infrastructure. Some of their goals are presented in

Table 6. Future goals for charging technologies in end users’ organizations.

2020	Purchase f3 35 electric buses with dynamic charging, range up to 15 km
2021	Construction of 2 fast chargers, 360–380 kW 5 fast charging places 2-pole opportunity charging infrastructure for e-buses Overnight charging solution with pantograph-on-bus (large fleet of >30 buses) Overnight plug charging solution (city with few e-buses) Receiving 23 electric articulated buses
2022	3 fast-charging places Power connection for 8 MW
2024	BRT line with 20 buses with opportunity charging Wireless/inductive charging IMC infrastructure for standard articulated battery trolleybuses
2025	Purchase of 85 e-buses with dynamic charging, range up to 20 km Every new bus will be zero-emission bus with associated charging infrastructure
2030	The whole fleet if public transport buses will be zero-emission with associated infrastructure

. A clear tendency in the answers concerned moving from smaller to larger fleets, suggesting that electrification should increasingly be a system solution combining electric vehicles and their charging infrastructure.

4.3. Greatest Challenges with Charging Technology

The end users were asked about their organizations’ greatest challenge concerning charging technology. Some of the most mentioned concerns included a reliable communication line with easy diagnostics and finding locations for charger installation due to complications with land ownership and building the infrastructure as well as power availability.

4.4. Wishes for Future Charging Infrastructure Products and Services

The charging infrastructure products and services to be developed further in the future included cheap and reliable, high-power and ultra-fast charging (with power over 1 MW), more involvement from national energy suppliers, transformer stations, and wireless charging for HD vehicles.

4.5. Ranking the Priorities for the Charging Infrastructure

The respondents were asked to rank some charging infrastructure improvements and characteristics. According to their ranking, the reliability of the charging infrastructure is more important to end users than a lower cost infrastructure (Figure 2).

Furthermore, the ease of use for employees and ease of scaling up the infrastructure according to needs had a higher priority than the need for the infrastructure to match well with surrounding areas in terms of city planning (Figure 3).

4.6. Importance of Interoperability

Seventy-two percent of end users agreed that chargers and vehicles of different brands should be interoperable within one vehicle category (Figure 4). However, interoperability between different types of vehicles (e.g., buses and trucks) was considered much less important. Only 21% of the end user respondents believed that this type of interoperability was important (Figure 5).

This is an interesting result that could reflect the fact that the end user respondents do not operate fleets with different types of vehicles. This question is relevant for transport authorities and cities, especially when it comes to shared charging infrastructure. From the city perspective, if owned and deployed, a public charging infrastructure in public space should aim to serve most users. Up to now, this has been complex, as fleet operators such as bus, freight, and utility operators, have different needs and usage profiles.

4.7. Further Processing of the Results

The survey results were further processed in an ASSURED project workshop with freight and bus operators in November 2020. Participants elaborated on a scenario with a central charging hub. Such a hub could be run in three different ways: by the city (publicly owned), by a private energy operator (privately owned), or by an operator (operator-owned). Needs, requirements, barriers, and opportunities for the hub were preliminarily defined. The retro-planning conception of the scenario allowed the participants to share their views and suggestions as well as to identify go/no-go conditions. The questions addressed and the key takeaways are summarized in

Table 7. Results of the workshop.

Question	Answers
What are the possible advantages of a centralized charging hub?	Benefits of sharing the costs and risks of the infrastructure. Optimization of resources and higher efficiency, also in terms of usage of public space and the power grid. Accessibility and possibility of adding up capacity in terms of services and operation with increased multi-functionality. Improved market conditions, allowing more actors to take part and reducing the access to charging as a competitive advantage.
What are the challenges of a centralized charging hub?	Grid capacity and the logistics of sharing the charging points (pre-booking of chargers to avoid standing times). Adjusting the operation schedules of bus drivers’ breaks (for charging) and location close to the bus network.
What are the possible models for ownership and maintenance?	Independent third-party as owner, managing and developing charging options in cooperation with operators, suppliers, and authorities. Electricity suppliers in a similar structure to fuel stations. Privately-owned and operated fast charging infrastructure, with the price included in the electricity bill. City utilities as owner, since they are neutral actors, otherwise operator or user companies. In the case of publicly owned land, tender out the rental of space and operation of infrastructure to a neutral partner, maintenance included in a subscription fee from the operator.

.

The results of the workshop showed that the topic requires further discussion, ideally based on the identification of user needs and usage profiles.

5. Results of the Technical Survey

5.1. Charging Technologies Known to Technical Respondents

The respondents were asked to identify all the charging types they were familiar with. All the respondents were familiar with infrastructure-mounted and roof-mounted ACDs. However, only half of the respondents were familiar with battery swapping technology. Surprisingly, two (out of 20) respondents indicated that they were not familiar with plug-based charging (Figure 6).

5.2. Ranking the Potential of Different Charging Technologies

The results from the technical survey (Figure 7) clearly show that (1) a pantograph on a vehicle roof, (2) infrastructure pantograph solutions, and (3) charging plugs were graded as the highest potential charging technologies. Furthermore, static and conductive charging solutions have higher potential compared to dynamic and wireless charging, as shown in Figure 8 and Figure 9.

The respondents’ reasons for rating the charging technologies included lower costs of roof-mounted and infrastructure-mounted ACDs and plug charging compared to other charging technologies, as well as the higher efficiency and power of conductive and static charging compared to inductive and dynamic charging, and inadequate availability of standards for some of the solutions.

5.3. Challenges with Today’s Charging Technologies

The following aspects were the most frequently mentioned challenges with today’s charging technology, according to the technical survey respondents:

• Accessibility to sufficient power from the grid;
• Interoperability;
• Standardization;
• Batteries: capacity, cost, battery management system (BMS), aging;
• Installation: permissions, grid connection, space requirements;
• Fleet management, demand response and complexity.

These results are in line with the challenges identified in the ZeEUS Project [30] and in the ASSURED Project [31]. The ZeEUS project pointed out the need for standardization and interoperability for e-bus charging as the main challenge to ensuring e-bus deployment and further upscaling the fleet, for which the project held dedicated working sessions to address the topic. The ASSURED project took over this work and extended it to the development of high-power, fast-charging solutions for HD-EV charging.

The ZeEUS project also underlined the need for building strong cooperation with the energy sector and related stakeholders, which will be essential to ensuring the success of the deployment plans. Higher investment costs will result from the fundamental shift from vehicle to system procurement, which poses the challenge of higher upfront costs and investment needs, as operators will need to plan and deploy the charging infrastructure according to the deployment of the fleet.

In terms of management and operation, the main question considered was to develop new ways of operation, which included vehicle schedules and drivers’ timetables and staff training to acquire new skills, such as safety regulations, handling, and driving.

Operations planning requires IT tools for optimized fleet management, supported by data collection from the Fleet Management Solutions (FMS) and the CAN bus. Accessing these data is often an issue that operators and bus OEMs need to negotiate and approach together.

Additionally, the deployment of high-power fast-charging infrastructure and its integration with larger HD-EV fleets puts pressure on the energy grid in terms of capacity and accessibility. Issues including the selection of the location (depot or public space) need to be addressed jointly with city administration and energy operators. This ensures suitable, optimized solutions, as several aspects need to be considered. These include work permits, access to the grid, regulations to install a charging infrastructure in public spaces, timelines of the permits and work processes, and the possibility to share the infrastructure with other modes such as metros, trams, or trolleybuses.

Finally, the battery can represent almost 45% of costs for an electric bus. The development of battery technology with high energy densities is expected to increase vehicle range and reduce the costs. This will reduce the total cost of ownership (TCO) of HD-EVs even to near cost parity with diesel buses. Battery management and the second life of batteries are also key aspects for operators deploying large scale HD-EV fleet projects.

Challenges faced by bus operators concerning the deployment of charging infrastructure focus on finding suitable solutions for depot upgrades (in terms of location, increased energy needs, power capacity, and access to the grid), deploying interoperable systems, applying suitable smart charging strategies and energy storage systems to reduce the energy costs, and enabling optimized fleet operation through IT intelligence.

5.4. The Influence of Energy Storage Development on Charging Technologies

According to the technical survey results, development in energy storage (e.g., enhancing the energy density of the batteries) will increase the attractiveness of depot charging, smart charging, charging with renewable energy, and the driving range, while it will reduce the need for opportunity and en-route charging, as well as the weight of buses, charging times, and purchase and maintenance costs.

5.5. Different Aspects to Consider in the Long-Term (2030) Goals of Charging Technology

The following are the most frequently mentioned goals for charging technology in the long term:

• High-power opportunity charging (power over 1 MW, enabling super-fast charging of small batteries and fast charging times for large batteries).
• Minimizing grid instability through smart charging, vehicle-to-grid (V2G) or vehicle-to-X (V2X) charging, fleet and grid management, and local storage.
• Worldwide charging standards for all types of vehicles.
• Increased battery capacity with lower prices resulting in increasing depot charging, preferably via plug-in charging (as it is cheaper and requires low maintenance).
• Availability in all remote parts of Europe.

Table 8. Respondents’ opinions on including different aspects in the long-term goals of charging technology.

Aspects to Consider in Charging Technology Planning	Vehicle-Charger Interoperability	Interoperable Charging between Different Vehicle Types	Automated Charging without Human Involvement	Cyber Security
Proponent (%)	100%	90%	90%	80%
Opponent (%)	0%	10%	10%	20%
Reasons for opposing	NA	Not required, as trucks use mainly only CCS charging by plug. Opposing due to complexity.	Risk for safety. Unreliability of sophisticated systems.	Complexity Not an immediate threat, the necessity can be reviewed in 5 or 10 years.
Barriers to achievement	Lack of technical harmonization. Lack of standardization. Too many players. Complexity of standardization and misinterpretation.	Similar barriers to vehicle-charger interoperability. Different mechanical interfaces and charging technologies. Charging infrastructure adaptation.	Risk for safety (e.g., communication and cybersecurity issues) Reliability. Lack of suitable technology, including lack of automated authentication and payment methods, as well as functionality. Required space.	Hackers, human error. Lack of expertise. Lack of validation protocols. Ensuring protection of the communication. No definition of public key infrastructure (PKI) architecture at the EU level.
Solution to barriers	Robust and unified standards. Market maturity. Conformance and interoperability testing. Proper vehicle design and harmonization of solutions.	Similar solutions to the ones for vehicle-charger interoperability. Common mechanical interface for light and duty vehicles.	Well-defined standards. Further technology development of autonomous driving. Trusted and encrypted non-wired communication interface. Robust design.	Standardization and certification. Ensuring safe communication: encrypted communication. Definition of PKI architecture at the EU level. Collaboration of players to achieve cybersecurity test specifications.

summarizes the respondents’ opinions on including other aspects in the long-term goals of charging technology.

6. Pre-Normative Charging Technology Roadmap

By analyzing the data collected from the literature review, surveys, and interviews, a roadmap for charging technologies for HD-EV for the years 2021–2030 was developed. This roadmap is presented in Figure 10, and details of the roadmap, along with the challenges and opportunities for each technology, are presented in the following subsections. The thickness of the yellow bars in Figure 10 indicates the foreseen popularity of each topic in the roadmap.

6.1. Use Cases

The use cases are generic descriptions of different types of vehicles performing various operational transportation tasks. In Figure 10, the roadmap for depot and opportunity charging use cases is plotted for three different public transport bus services: feeder lines that bring people from a transit hub (or trunk routes) to a destination or vice versa; trunk routes, which are transport lines with very short headways and a distinguishable fleet; and Bus Rapid Transit (BRT), which includes public transport bus services with the operating characteristics and capacity of rapid transit systems.

For feeder lines, currently the fleets are charged at the depot overnight or at terminals during breaks. As the capacity of the batteries increases, it is expected that the fleet in feeder lines will be able to use the maximum available battery size and only recharge the vehicle at depots overnight. For trunk lines and BRT, both depot and opportunity charging will be utilized. The share of opportunity charging, however, would be larger than depot charging, since the fleets of these lines have long hours of operation and can share the charging infrastructure during operation.

Another technology that could facilitate the charging of the public transport fleets and make the overhead line technology attractive for bus traffic is the new e-Bus Rapid Transit (e-BRT) system. With e-BRT, battery-equipped buses recharge their batteries while running on sections with overhead lines, known as In-Motion Charging (IMC), and they can operate without connection to the overhead lines on battery power, providing more flexibility in their choice of route. As an example, the seaside resort of Rimini is currently running acceptance tests for its new IMC buses, and the vehicles are destined to serve the new Trasporto Rapido Costiero (TRC) express line from Rimini to Riccione. Furthermore, Trasporto Unico Abruzzese (TUA) is introducing the IMC buses to operate the eight-kilometer link between the two coastal cities of Pescara and Montesilvano.

For trucks, several types of trucks and truck–trailer combinations serve a multitude of different operations and requirements for both powertrain and infrastructure [32]. There are various ways to group the HD vehicles and their operations in use cases [33]. For the purposes of this paper, the following grouping of use cases is made:

• Urban, commercial, medium, and heavy-duty vehicles for various uses: deliveries, logistics, urban freight, refuse collection, utility vehicles, and earth moving and construction.
• Regional freight and logistics providing various transport for communities, agriculture, industry, trade, logistics chains, and hubs.
• Long-haul trucks in industrial freight and logistics with various trailer combinations up to high tonnages and single missions exceeding 400 km.

These use cases will have different charging requirements. The short-range urban and regional operations use private charging at depots, terminals, and hubs, where the available charging times could be a few hours per night. This depends on the scheduling of operations and possibly fast recharge opportunities during the operations or mission. The regional and especially long-haul use cases will likely need to resort to very fast charging in addition to origin-destination charging. The location, dimensioning, and operations of the corridor charging hubs for HD-EVs should be designed to support the electrification potential in the best possible way. Long-haul operations will be likely to utilize static charging, in-motion charging, and hydrogen and fuel cell solutions concurrently, with the most systemically viable combination.

6.2. Technology Development

6.2.1. Battery Development

Various vehicle categories and end use cases have different energy needs and charging specifications. The battery development in the presented roadmap is classified according to the vehicle categories, i.e., urban, regional, and long-haul trucks and buses. Generally, the longer the independent and continuous operation without charging that a use case requires, the larger the traction battery capacity needs to be from the design perspective and, secondly, the more the operations need to rely on fast opportunity charging en route (in addition to end stop charging).

Currently, various chemistries and designs are available and market-ready for EVs. Li-ion batteries with different chemistries are being continuously developed for buses and trucks. The battery technology should be selected according to the vehicle application, as it affects the choice of charging solution. Use cases requiring very fast charging need to have batteries and management systems suited for that purpose. For example, nickel–manganese–cobalt (NMC) batteries are suitable for opportunity charging use cases. NMC modular battery technology with a range of 640 kWh is being developed and tested for articulated electric buses and are expected to go into series production in the first half of 2021 [34]. Another technology in the market, especially for larger capacity battery packs, is lithium iron phosphate (LFP), which also has better safety than NMC. Alongside the capacity increase, the development is moving towards the improved life cycle of batteries, as well as scalable, modular, and lightweight designs. Recent years have brought rapid annual cost reductions to traction batteries combined with improved performance, largely explaining the much-improved market acceptance of these products and commodities.

Various next generation battery chemistries, such as semi or fully solid-state with advanced composite or Li-metal anodes or other ionic systems, are being developed. These technologies currently have a lower technology readiness level (TRL), but they could further make battery e-trucks and buses more competitive in the mid to long term [35].

As for articulated buses, there are OEMs that are manufacturing buses with a battery capacity of up to 400 kWh, which can be used for both depot charging and fast charging en route. This battery capacity may be sufficient for articulated buses for the next few years, as they drive at low speeds for short distances.

6.2.2. Charging Power

The terms “fast-charging” and “high-power charging” are not explicit when discussed in the HD-EV context with traction battery capacities and voltages, which are different from the light-duty (LD) EV context. Ultimately, the limiting parameter for a battery system in terms of charging is the charging current in relation to the capacity (referred to as the C-rate), and for the charging interface it is the current capability. The most relevant measures for charging are the time used for the charging event and the energy that can be on-boarded during that time.

In

Table 9. Terminology for HDV conductive charging and related C-rates. The values are typical, and they may vary.

Term	Charging Voltage	Charging Current	Charging Power	Battery Capacity	C-rate of Charging	Notes
Slow HD-EV charging	400 VDC, 800 VDC	60 A–400 A	50 kW–150 kW	50 kWh–250 kWh	0.2 C–1 C
Normal HD-EV charging	400 VDC, 800 VDC	200 A–800 A	150 kW–400 kW	50 kWh–250 kWh	0.5 C–2 C
Fast HD-EV charging	Up to 1.5 kVDC	300 A–1 kA	200 kW–1 MW	100 kWh–500 kWh	2 C–5 C	Over 800 V charging voltages not yet standardized
Ultrafast HD-EV charging	Up to 1.5 kVDC	800 A–3 kA	1 MW–4.5 MW	250 kWh–1 MWh	4 C–10 C	Over 800 V charging voltages not yet standardized

, our proposal for HDV charging classification, and the terminology of charging power vs. capacity values are presented. The battery C-rate describes the speed of charging, and it is related to the battery capacity C, expressed in As or more commonly in Ah. The unit for the C-rate is usually 1/h, and it denotes the rate at which a battery is charged. The C-rate can be also defined by the ratio of the charging power to the battery capacity, which is then known as the CP-rate [36,37]. Similar terminology for LDVs is used [38], but the power levels for HDVs are higher for each class than with LDVs, because their batteries are typically higher capacity. Other possible terms to be used include low-power, rapid, super, and high-power charging, but in our classification only the adjectives slow, normal, fast, and ultrafast are used, to avoid misinterpretations.

There are various enablers in the development of fast charging. Burnham et al. [39] investigated different infrastructural and economic aspects that require further consideration when deploying charging at 400 kW and above. The most important aspects were standardization, coordination, security, grid resources, power demand peaks, and costs.

Standardization is important to ensure safety and to increase interoperability and backward compatibility. Coordination between multiple utilities include EVSE network operators and authorities having jurisdiction over the permitting, siting, and regulation of charging stations. Cyber and cyber-physical security of the fast-charging infrastructure is rapidly becoming more important. Consideration of the existing grid resources and the planning of future fast charging installations and networks is also a key aspect, and it is related to the management of intermittent, high-power demand using fast charging stations. Finally, the costs of the charging infrastructure, installation, fast-charging capable battery electric vehicles (BEVs), and operation must be considered.

High-power DC charging systems and ultra-fast charging are under development, but their standardizations have not been finalized yet. According to the results of the surveys in this study, end users’ organizations are looking towards high-power and megawatt charging solutions for their short-term plans. According to the interviews, fast charging (typically several hundreds of kW and up to 1 MW) will be sufficient for commercial HD-EVs, and ultra-fast charging (above 1 MW) will be more suitable for long-haul vehicles and trucks. Nevertheless, the battery and vehicle components need to be developed further to withstand the high amount of current during ultra-fast charging (C-rate 4 or more). Considering the required technology development, the rollout of ultra-fast charging HD-EVs is not expected before 2025.

6.2.3. V2G Bidirectionality Potential

Bidirectional V2G technology is considered to act as a potential revenue source emerging from participation in the flexible energy market and will provide benefits to the grid, for example, through voltage and frequency regulations [40]. The bidirectional functionality could also enable fleet operators to optimize behind the grid connections concerning charging and energy use. Despite the advantages that V2G technology provides to both energy providers and consumers, the utilization of this technology depends on the use cases when it comes to HD-EVs. For example, V2G technology is cost effective for vehicles such as electric school buses if they have a short operating schedule and can be parked and connected to a charger for a long period of time during the day with few additional transportation tasks. Furthermore, commercial HD fleets could be another example in which V2G can be beneficial as long as the V2G technology does not interfere with the fleets’ operation scheduling [41,42,43,44,45], as they have predefined timetables and routes, and all of the fleets are usually parked for a long time at a centralized location (in depots or public parking places).

Communication standards (ISO/IEC 15118, IEC 61850) have been established for EVs to facilitate the V2G technology rollout. Nevertheless, the technology has not been widely used across Europe even for passenger EVs. In the following are some actions that will help to increase the use cases of V2G technology [46]:

• Testing of V2G protocols and standards;
• Developing advanced analytics and algorithms to predict charging patterns and to help shape grid optimization;
• Establishing a robust data collection infrastructure;
• Enabling the communication of system parameters and status, including market details in the electromobility value chain.

Considering these factors, we expect to see the utilization of bidirectional chargers with V2G technology in HD-EVs to start at a slow pace in mid-2024 and reach a mature market by the end of 2030.

6.3. Standardizarion

6.3.1. Charging Solution

Currently there are no European standards for inductive charging. However, international standardization work for wireless power transfer has already started, and it is scheduled to be carried out in relevant IEC project teams in close cooperation with the ISO. The standards for static and dynamic inductive charging are expected to be ready by the end of 2022 and the end of 2025, respectively. No standardization work for dynamic conductive charging and AC charging with on-board charger is scheduled for the time being.

6.3.2. Interoperability

The standards developed for HD-EVs facilitate the interoperability between charger and vehicle. These standards are either finalized or currently being finalized. However, to achieve fully interoperable solutions, the standards need to be simplified to be understandable for OEMs and end users. Furthermore, interoperability and conformance testing need to be widely available, and the communication challenges should be addressed effectively.

Standardizing the charging between different types of vehicles (e.g., trucks and buses) has been rarely discussed. According to the end user survey (Figure 6), this interoperability may not seem relevant, as currently operators do not operate buses and trucks at the same time. Nevertheless, 90% of technical respondents thought that the interoperability between different vehicle types should be part of future charging technology goals. As the number of HD-EVs increases, different stakeholders, fleet operators, and commercial businesses will be able to profit from this interoperability, for example at public charging hubs. The standardization work on such technology may start within the next two years.

6.3.3. Cyber Security

Secure communication between the vehicle and EVSE as well as between the charging station and grid are essential aspects for fast and smart charging. The secure communication protocols between the vehicle and the charging station are presented in the standard ISO 15118-2 [47]. In a broader sense, cyber security issues [48,49] are going to be included in the standardization in the near future, and proposals for charging infrastructure cyber security have already been made [50]. Having a detailed set of protocols that ensures cyber security will be essential soon, before significant cyber security challenges arise with the expansive usage of HD-EVs.

6.4. Charging Solutions

6.4.1. Conductive Charging

Due to simplicity and low costs, plug-based charging solutions remain one of the most common solutions for HD-EVs, especially for depot charging. Roof-mounted ACD will continue to be the most common solution with automatic connections for opportunity charging, primarily for urban buses. The application of infrastructure-mounted ACD solutions will continue as well but may not be used as widely as roof-mounted ACDs. One aspect that may attract the attention of the bus OEMs towards infrastructure-mounted ACDs could be the potential vehicle weight reduction and, thus, the increased vehicle capacity. For example, a roof-pantograph can weigh around 85 kg [51].

The popularity of ground-based and side-mounted ACDs will be less than other ACD solutions. These solutions may be used mostly in HD trucks or vehicles that do not have enough area on the roof to install an ACD. Furthermore, the standardization for these two solutions is not ready, which will slow down their journey to the market.

As for catenary charging solutions, currently a few demonstration catenary systems on highways have been developed and are being tested [35]. However, the expansion of this technology requires commitment from different countries and a well-defined set of standards. Until then, the share of catenary charging system in the expanding HD-EV market remains small.

6.4.2. Inductive Charging

Static and dynamic inductive charging solutions have been developed and are being tested for HD-EVs [52]. The market share of inductive technologies may increase slightly after the relevant standards are published. Nevertheless, these solutions will not be as popular as conductive charging solutions, unless their efficiency for HD-EVs in terms of cost and energy transfer is proven and their safety concerns are addressed.

6.4.3. Battery Swapping

The IEC has established IEC 62840 standard on EV battery swap systems [53]. A commission implementing decision C(2015)1330 [12] includes a standardization request for “a European standard containing technical specifications with a single solution for battery swapping for EVs”, to be completed by 2022. However, these standards do not target HD-EVs specifically, and no activity on battery swapping for HD-EVs exists in the market for the time being.

7. Conclusions

Charging standardization is one of the key elements in seamless transformation of HD fleets towards EV fleets, to ensure safety, interoperability, and compatibility of differ-ent charging solutions. Generally, developing standards is a time-consuming process that can last several years, during which new technology will be developed in the market. Therefore, for the developed standards to be future-proof, they must consider the upcoming charging needs and development. The aim of this paper is to present a pre-normative technology roadmap that foresees the developments in the charging of HD-EVs. This roadmap can support future standardization efforts and facilitate the standardization of HD-EV charging technologies by creating a clear overview of the popularity of charging technologies and the end users’ needs.

Furthermore, this paper provides an overview of different charging technologies for HD-EVs with comprehensive discussions on the state of the art, trends, enablers, and limitations. Therefore, this work can be of interest to academic audiences as well, not only to familiarize them with the charging technologies but also to help them to select research topics that are at the cutting edge of charging technology development.

The required input for the work was collected by reviewing the existing literature and conducting surveys and interviews on end users and technical stakeholders. According to the presented roadmap, roof-mounted and infrastructure-mounted ACDs in addition to manual connectors will continue to dominate the charging technology market for HD-EVs. Static and conductive charging have higher potential compared to dynamic and wireless charging. Nevertheless, inductive charging may be the future charging solution for HD-EVs if the current bottlenecks in the technology can be addressed. These bottlenecks include high prices, lower efficiency, lack of standardization, limitations in terms of maximum power, and safety concerns. Achieving interoperability was repeatedly mentioned as the main challenge concerning today’s charging technologies. To achieve fully interoperable solutions, the standards need to be simplified so that they are understandable for OEMs and end users. Furthermore, interoperability and conformance testing should be widely available, and the challenges in communication between the charger and vehicle should be addressed effectively.

Generally, as road transport electrification progresses and new use cases emerge, there is a growing need to build systems with higher battery capacity and charging power. Part of the higher charging power can be covered by moving towards higher voltage batteries and DC charging, but there will also be the need to increase the maximum currents in the cabling and interfaces between the charger and the battery.

In a future work, we attempt to make a techno-economic roadmap for charging of HD-EVs, which considers not only different aspects of technical development but also includes analysis of market maturity of solutions, policy-related topics, economic viability of various deployment scenarios, contractual and business models, system level TCO, etc.