What are the Challenges of Electric Heavy-Duty Vehicles?September 25, 2020 by Steve Arar
With bigger vehicles come bigger problems with electrification.
Economic and climate concerns are pushing the automotive industry toward electrification. In general, vehicle electrification poses several challenges such as high price, low driving range, and a dearth of chargers. In this article, we’ll focus on the technology challenges and trends pertaining to electric medium- and heavy-duty vehicles (MHDVs).
High-Current SiC Increase Efficiency and Power Density
An EV converts about 60% of the electrical energy stored in the battery to power at the wheels. With this low level of efficiency, the cooling requirements of the system can be demanding, particularly when dealing with high power systems such as electric heavy-duty vehicles.
eCascadia is a company manufacturing electric big rigs, which are currently undergoing pilot testing in Southern California. Image used courtesy of The New York Times
To address thermal management issues, MHDV manufacturers cannot apply conventional IGBT based solutions and need to employ more efficient semiconductor switches such as silicon carbide (SiC) devices in the drivetrain.
Typical SiC devices can operate at high temperatures of about 200°C with peak temperatures of over 600°C. This, along with lower power losses, make thermal management of SiC-based solutions easier and enables a smaller and cheaper heatsink.
Another reason that SiC devices can increase the system power density is that these devices can be operated at a higher switching frequency. This reduces the size of passive components such as capacitors and inductors.
The following figure compares Toyota’s SiC-based power control unit with a silicon-only design.
A PCU with silicon power semiconductors (left) and a PCU with SiC power semiconductors (right). Image courtesy of Toyota
In this case, the SiC technology reduces the size of the power control unit by about 80%. From the efficiency point of view, SiC technology can improve the performance of certain blocks, such as the inverter, by about 77% compared to an IGBT-based design.
SiC seems to be one of the dominating power switch technologies in future EVs. However, high current SiC devices need to be developed for electric MHDV applications. Currently, SiC switches are available at relatively low currents while 200 A devices rated at a junction temperature of 175°C are required for MHDV electrification.
PM-Based vs. Non-PM-Based Motors
Electric motors for EV applications should be affordable and provide high torque at zero speed, high efficiency, and high power density.
The traction motor selection heavily depends on the application requirements. For example, induction motors might be suited for applications where reliability and affordability are the key factors, while permanent magnet (PM) synchronous motors might be a useful fit for mining truck applications that commonly require huge surges of power.
Although the requirements of an application determine the motor type, some general trends are observable: electric motors for MHDV applications are either PM-based or non-PM-based. PM-based motors typically offer the highest efficiency and power density, but they are costly and there are reliability concerns due to the brittleness of the magnets—especially in the harsh environments of MHDV applications.
Non-PM-based motors cannot provide the efficiency and power density of PM-based motors but they can be more affordable and reliable than PM-based solutions. The table below compares PM-based motors with other common technologies, including DC motors, induction motors (IMs), and switched reluctance motors (SRMs).
An evaluation of electric-propulsion systems. Image (adapted) courtesy of Mounir Zeraoulia et. al
Many electric MHDVs are currently using PM-based motors because of their efficiency and power density. However, considering the availability issues of the PM materials, researchers are attempting to develop non-PM-based motors that can offer the efficiency and power density of PM-based motors at a more affordable price.
The Challenge of Battery Cost
A major challenge with electric trucks is hiding the cost of the required lithium-ion batteries. The battery pack is currently estimated to cost about $200 per kilowatt-hour. Even with $100 per kilowatt-hour, a 500 kWh battery pack would cost $50,000. This is about 30% the cost of a conventional truck!
It is worthwhile to mention that although researchers attempt to gradually reduce the battery cost through innovative techniques, the availability issue of certain critical elements, such as cobalt, nickel, and lithium, can be a problem in the future and affect the battery price.
That’s why researchers are looking for cobalt-free lithium-ion batteries that provide high performance at a more affordable price.