Dr. Martin Schulz, Global Principal, Application Engineering, Littelfuse
With the electrification of heavy-duty or commercial vehicles, charging larger batteries than the ones in electric passenger cars becomes necessary. As time is money especially in logistics, either assigning idle times for charging or increasing charging power are preferred options. This leads to three different charging scenarios.
Use Case I: Fleet Operation and Depot-Charging
Modern technology in batteries and cutting-edge power semiconductor solutions allow the design of highly efficient infrastructure. Figure 1 depicts the modern version of depot charging for bus fleets.
Depot charging is the preferred option for local fleet operations, particularly for buses and any kind of delivery vehicle. These are operated on rather fixed routes and are idle for a period of hours during night-time.
This comes with a reduced demand for charging power as well as with further options in energy management. Including stationary batteries, decoupling the time of charging buses from times of having excess energy becomes an option.
Common battery-electric buses today feature battery capacities in the range of 250 to 500 kWh, enabling them to operate one shift without charging. A charger in a depot only needs to recharge one vehicle overnight. Even in the case of recharging 80 percent of 500 kWh in 6 hours, 70 kW of power is sufficient. Of course, for the whole depot, this is multiplied by the number of vehicles to be charged at the same time.
A typical schematic of a charger includes an input stage that can adapt the DC-link-voltage, a stage for galvanic isolation, and an output rectifier as seen in Figure 2.
Chargers are typically built in a modular approach from subsystems that can be stacked to increase the output power. Common designs feature 15-60 kW per subsystem and the choice of components varies with the output power requirement and cooling preferences. While units with forced-air-cooling in a range of 10 to 15kW are widely built with discrete devices, units with higher power levels use liquid cooling and are mostly constructed from power modules.
Paralleling units is an option to increase output power. The technique can also be used to form redundancy. This enables system operation at lower power in case of failure in a single module instead of losing the complete system.
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Figure 2. Bidirectional charger schematic and recommended components
Depot-charging also is a door-opener for secondary grid services. Stationary energy storage helps to reduce the load to the grid and during high energy demand even supports the grid. Load balancing and scheduled charging also become an option. Charging times can be aligned with periods of having excess energy with correlating low, or even negative energy prices at night.
In a fleet of vehicles with a fixed schedule, not all of them must be fully charged at the same instant. Even sharing the energy between the vehicles is possible and those vehicles that are not scheduled to be in service can contribute their stored energy as well. Holistically, depots as larger industrial areas could become solar power stations as well.
Use Case II: Opportunity Charging
Operating a fleet of vehicles along predefined routes opens the option to extend the driving range by adding smaller amounts of energy more frequently. This is the so-called opportunity charging which works best if it takes place in a fully automated manner.
Two solutions are recommended for this way of charging.
Pantographs are mechanical systems that allow large electric contacts to move over larger distances and safely contact their counterparts. Pantographs are a proven reliable technology and are widely used in tramways and railway applications. Depending on the mounting position, pantographs can be separated into top-down and bottom-up systems. While bottom-up systems are mounted on the vehicle and contact the station, top-down mechanics are part of the station, lowered down to the vehicle. Figure 3 gives an example, how charging by pantograph can be set up.
Construction of the infrastructure remains restricted to the roadside. Thus, such an installation can be built as an upgrade to existing stations in case a suitable power supply is available locally. As this is rarely the case, buffering the station by battery storage is a widely appreciated solution to decouple high-power charging of the vehicle from recharging the stationary batteries. It is common to apply power levels of 125-250 kW.
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Figure 3. Top-down pantograph for opportunity charging
Charging voltage and current are aligned between the station and the vehicle’s battery management systems before the charging process is started. Because of the high power involved, charging via pantograph is always DC-charging with direct access to the vehicle’s battery.
For future installations, pantographs are foreseen as the recommended solution especially for autonomous vehicles as no plug or wire is involved that needs to be handled precisely. The systems can easily handle vehicles with different heights and can be constructed to tolerate misplacements between station and vehicle.
Wireless Power Transfer (WPT), also popular for mobile devices like smartphones, can be upgraded in power to suit the needs of large-scale energy transfer. Wireless power transfer for vehicle-scale systems is described in detail in SAE J2594. Wireless charging systems inherently consist of two independent parts that exchange energy via magnetic flux. To avoid sacrificing too much of the transfer efficiency, SAE J2594 sets a target for the transfer efficiency to reach 80 percent at least. To fulfill this requirement, series-compensated resonant circuits as drawn in Figure 4 can be used, operated in a frequency range of 80-140 kHz.
There is a multitude of input rectifier topologies to be considered, including static diode rectifiers as a cost-optimized solution or thyristor-based versions. Due to superior EMI behavior, the reduced effort necessary for filtering, and adjustable DC-link-voltage, the Vienna Rectifier is an often-seen solution. With the high switching frequency of 80 to 140 kHz to drive the sending coil, as required by the standard, IGBTs with low switching losses or SiC-MOSFETs can be considered for the DC-DC-conversion stage.
Inductive chargers need to be installed in a place where the vehicle can run over it. In contrast to pantographs, this has a more severe impact on the infrastructure, especially in public traffic. Therefore, inductive charging is a suitable solution mostly for semi-public areas. Baggage trolleys on airports for example can benefit from wireless power transfer as the power levels, energies involved, and topographic conditions suit the use case.
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Figure 4. Series-compensated, resonant WPT setup
Use Case III: Individual Long-Haul Operation
Traveling on random routes as in long-haul logistics demands individual high-power charging similar to today’s gas stations. This high-power charging needs to become part of the existing infrastructure to allow for seamless integration of electric trucks into the mobility sector.
With a DC voltage up to 1500 V and a maximum charging current up to 3000 A, charging at rates exceeding 2 MW becomes possible.
At 2 MW charging, 500 kWh to go another 300km can be delivered in about 15 minutes which is well covered by a break a driver has to do to comply to legal requirements. However, urban low-voltage 3-phase grids up to 400V would not support this level of power.
In this scenario, local supply powered from the medium-voltage regime needs to be considered as a prerequisite. Though buffering by stationary batteries is a potential option, the capacity for the storage would become comparatively large.
Having to work from a medium voltage transformer leads to a promising option for chargers in the megawatt regime. Instead of scaling up the structure used to charge passenger cars, the well-established scheme used in electrolysis can be followed. Figure 5 depicts the correlating high-power setup.
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Figure 5. High-power charging topology with B12C, also noted as B6C-2P
This approach only features a single stage of energy conversion and displacing the stage of galvanic isolation from smaller individual converters to the medium-voltage transformer enhances the efficiency of the power conversion stage to exceed 99 percent. At the same time, it minimizes the number of resources per kW installed, and an assembly built from presspack components reduces space demands.
When tapping into the megawatt regime, solutions based on thyristors combine outstanding efficiency with the unprecedented lifetime and reliability of capsule-type devices.
Such infrastructure systems demand a high number of operating cycles and pose extraordinary expectations towards time-in-service. Both need to be considered during the design phase’s early stages. Though topology and technology may appear outdated, the higher efficiency along with the lower cost and reduced space requirements make it the obvious choice. This will be especially important when the future autonomous commercial vehicles demand even higher power ratings to reduce charging times further as no recreational break for a driver is required.