Brij Singh, Emily Cousineau, Paul Paret, Kevin Bennion, Sreekant Narumanchi
IMAGE LICENSED BY INGRAM PUBLISHING
Electrification of drivetrain systems is now seen as a major opportunity by the transportation industry across the globe to reduce the greenhouse gas emissions and revolutionize the travel patterns of millions of people. The cumulative number of plug-in hybrid and battery electric vehicles (EVs) sold in the United States has now surpassed 2 million in 2021, according to the International Energy Agency. The introduction of EVs in different classes of passenger vehicles and the continued drop in prices spurred by government incentives have attracted the attention of consumers despite certain barriers, such as higher initial cost and range anxiety. In the United States, the EV market share is now growing at an exponential pace, with the domestic automakers allocating a lion’s share of new and future car sales to EVs. Additionally, different market studies project decreasing cost and rising sales of medium- and heavy-duty electric trucks. Although electrification initiatives are strongly pursued in the on-road passenger vehicle market, off-highway sectors, such as construction, mining, and agriculture, are also gradually implementing electric drivetrain technologies in their machineries. As EVs grow in popularity on a global scale, innovative drivetrain technologies must meet the increasing energy demand by significantly increasing system efficiency.
Heavy-duty EVs present unique challenges for electrification: vehicle components must handle more power while staying within the safe operating temperature regime. A critical component in the electric drive systems of EVs is the power inverter, which converts the dc from the battery to ac and feeds it to the motors. In conventional inverters, silicon insulated gate bipolar transistors (Si IGBTs) were used as the switching devices; however, wide-bandgap (WBG) devices, such as Si carbide metal-oxide field-effect transistors (SiC MOSFETs) offer higher performance, efficiency, and increased power density. A collaboration between John Deere and the National Renewable Energy Laboratory (NREL) under the North Carolina State University-led PowerAmerica Institute—which was funded by the Department of Energy Advanced Materials and Manufacturing Technologies Office—led to the research and development of an SiC inverter for heavy-duty applications. This article describes in detail the thermal and thermomechanical design and analysis executed by John Deere and NREL as part of this research and development project.
The state-of-the-art thermal management technology implemented in the SiC inverter enables increased power density along with the ability to operate at high coolant temperatures. John Deere carried out an extensive demonstration of this inverter in the heavy-duty 644 K diesel-series-electric loader, shown in Figure 1. The SiC inverter has several notable achievements to facilitate energy-efficient power flow and to maximize the system efficiency of the power train applicable in the electrification of advanced machinery for on-highway and off-highway ground applications, as well as aviation with slight changes in control systems.
Figure 1. The in-field demonstration of the prototype SiC inverter in the 644 K diesel-series-electric loader resulted in increased vehicle fuel efficiency and has shown potentials for reductions in the daily operating costs.
Existing inverters from John Deere, such as the PD550 using Si IGBT devices, are heavy and material intensive. Si IGBT based powertrains struggle to compete with the combustion engine-based conventional powertrains Hence, these inverters act as critical barriers in the adoption of hybrid vehicles and EVs. The novel John Deere SiC inverter uses faster MOSFETs as switching devices to control power flow. Combined with the innovative thermal management system, this design increases power-density and efficiency while reducing material intensity. Also, this design encourages market adoption, where energy efficiency, fuel economy, performance, and system integration are important concerns.
A few unique features of the SiC inverter include:
As mentioned in the “Introduction” section, an inverter is a component in an EV that converts the dc power from any combination of the sources such as diesel-electric generator, battery, and fuel cell to the ac power used by the electric motor. By adjusting the current and frequency of the ac, the inverter can control the speed and torque of the EV motor. The amount of power through an inverter is limited by the maximum temperature of the semiconductor devices in the power module located inside the inverter; thus, thermal management is critical when scaling up inverter capabilities. In general, heavy-duty vehicles demand more power and torque from the inverter than the average light-duty sedan. As such, electric drive components in heavy-duty vehicles run an increased risk of overheating and shutting down, potentially causing significant damage to the vehicle. Figure 2 shows a prototype version of John Deere SiC inverter, and Figure 3 shows how the inverter is integrated into the 644 K diesel-series-electric loader.
Figure 2. The final prototypes inverter designed and test verified in conditions similar to the John Deere 644 K diesel-series-electric loader.
Figure 3. The John Deere 644 K diesel-series-electric loader drivetrain architecture used for the test verification of the prototype SiC inverter system. The thermal management system is located in the SiC inverter but uses the same cooling fluid as the diesel engine. PMAC: permanent magnet ac.
SiC devices are gaining popularity throughout the EV industry because of their improved efficiency and higher-frequency operation, allowing researchers and engineers in industries to use smaller and more efficient components with improved electrical performance compared to traditional methods. The collaborative research between John Deere and NREL led to an advanced thermal management system, which removes heat more efficiently and increases the power that can flow through the inverter. The design’s innovative thermal management enables an unmatched power density and keeps the system running safely and efficiently. Figure 4 illustrates how the thermal management system is integrated into the John Deere SiC inverter.
Figure 4. John Deere’s SiC technology has advanced power module-integrated thermal management for high-power density with high coolant temperatures to enable vehicle integration with the diesel engine cooling system.
The novel thermal management system incorporates perpendicular jet flow with advanced minichannel- and minimanifold-based cooling systems to extract heat from the power module and the inverter. This enables an impressive heat-transfer coefficient, as high as 93,000 watts per square meter per degree Kelvin (W/[m2-K]). This ensures that the modules can withstand high heat while keeping all areas of SiC power semiconductor chips at nearly identical temperatures, as shown in Figure 5, to eliminate the chances of developing damaging hot spots, which lead to thermal runaway. In addition, this design uses the existing diesel engine cooling system with temperatures up to 115 °C for a simplified engine-coolant–capable architecture. Conventional heavy-duty inverters, such as the PD550 power module or PD400 (both of which utilize Si IGBTs), require a separate coolant system with 70 °C WEG to operate successfully while ensuring the inverters’ durability.
Figure 5. Testing of the SiC thermal management design yielded uniform temperature results across the inverter and power module, reducing problematic hot spots and thermal runaway.
The thermal management operation of the John Deere SiC inverter is vital to the improved power density and the energy efficiency of the system design. The thermal management of mechanical systems relies on three primary factors: 1) the available area or volume, 2) the temperature difference between the hot semiconductor device and the coolant fluid, and 3) the heat-transfer coefficient. Addressing the first factor is not realistic for mobility applications, where space and weight constraints restrict the ability to modify the size of inverter systems. Many systems, including conventional hybrid EVs, focus on the second factor by introducing a separate dedicated cooling system; however, this design adds weight to the vehicle. The key to advanced thermal management design is through the third factor, by improving the heat-transfer coefficient and allowing the system to cool itself efficiently and continuously during operation.
The resulting SiC inverter developed by John Deere and NREL enables a high-power density design using the 115 °C WEG coolant already used for energy-efficient and high-performance vehicles. The use of the WEG engine coolant eliminates the need for a separate lower-temperature liquid cooling loop on the vehicle for a simplified vehicle architecture and improved power density. The team performed inverter evaluation experiments at 150-kW continuous and 200-kW peak power while cooling with the 115 °C WEG coolant. This innovative design achieved ∼99% efficiency during testing. In addition, the thermal design resulted in a 3.5 times footprint reduction in the SiC power module compared to the conventional Si IGBT modules used in a mass-produced PD550 inverter. Such a drastically reduced semiconductor module footprint is a significant advancement in this field and the first of its kind in a power level greater than 100 kW.
John Deere and NREL targeted three steps to improve the way the coolant fluid removes heat from the inverter to achieve a state-of-the-art heat-transfer coefficient as high as 93,000 W/(m2-K). A common strategy for the thermal management of EV inverters is to run a fluid—in this instance, the WEG coolant—parallel over the component’s surface to quickly transfer heat and cool the system.
The first step introduced jet impingement, using an array of jets to direct the fluid flow perpendicular—instead of parallel—over the surface of the power module. Figure 6 shows this perpendicular jet flow, with the blue lines illustrating how fluid hits the surface of the power module. This tactic increases heat transfer at the location of the impingement jets on the power module. Figure 7 provides another closeup look at the surface impingement within this innovative design, highlighting how the researchers computed the flow velocity to ensure a uniform velocity and mitigate potential erosion.
Figure 6. This illustration shows how impingement jets work. Fluid enters at numbered regions and impinges on the integrated module baseplate. The fluid then flows through a short channel region to remove heat from the power module. The highlighted blue regions show the coolant inlet and outlet. The system is designed to allow flow in either direction.
Figure 7. A closeup of the impingement shows the flow velocity as fluid hits the surface of the power module. The triangular shape of the minichannels ensures a uniform velocity throughout the system. If the velocity gets too high (i.e., >2 m/s), the fluid could erode the surface.
Next, after the coolant hits the surface of the integrated power module, the fluid is redirected into a series of minichannels and minimanifolds. These smaller channels offer an increased surface area to help cool the power module and to increase the effective heat-transfer coefficient of the system. The slope of these channels is explicitly designed to provide a uniform jet velocity through the system, alleviating the potential for heat buildup or hot spots. The consistent temperature throughout the system decreases thermal stresses, resulting in improvements in the reliability of the system. Figure 8 shows the manifolding channels across the power module.
Figure 8. Fluid can flow in either direction through this design. The WEG flows down to impinge on the surface of the power module and then is redirected up into another channel to repeat the impingement process.
Finally, the design ensures efficient fluid removal to reduce the pressure drop of fluid through the heat exchanger. To mitigate the high pressure drop that occurs with minichannels, the design uses a fingered manifold, as shown in Figure 8, to minimize the fluid path length through the minichannels. This advanced cooling system enables the modules to withstand the high heat flux while keeping the SiC power semiconductor chips at nearly identical temperatures. If an inverter system experiences inconsistent temperatures, it could shut down or cause permanent damage to the inverter components. As such, the thermal management system of the SiC inverter must be able to control the temperature across the module, regulating temperature and pressure through the consistent pumping power of the cooling system.
By eliminating the need for a separate cooling circuit, NREL’s novel thermal and thermomechanical research contributed to the inverter achieving 43 kW per liter power density. This is a significant improvement over baseline Si IGBT-based PD550 inverter system that offer a power density of 9 kW per liter. The redesigned inverter and power module creates a smaller and lighter SiC inverter system while maintaining the power levels needed to operate heavy-duty machinery. John Deere made additional updates to the packaging of the inverter, including a laminated ultra low inductance bus bar for improved electrical performance. The prototype SiC dual inverter test verified in the John Deere 644 K diesel-series-electric loader has combined two inverters into one design for further simplified packaging. The thermal and mechanical innovations in the SiC inverter design significantly reduced the inverter footprint, creating a smaller and lighter system. The lighter overall weight and improved performance have clear benefits to the fuel efficiency and operating costs and reduction in material intensity in the inverter manufacturing. Together, John Deere and NREL designed three generations of the SiC system, ultimately creating a market-ready prototype for heavy-duty vehicle early adopters. The developed SiC inverter is applicable to many vehicle forms and in all those, the SiC inverter will offer remarkably high fuel economy gains. For example, 1 million hours in-field data indicated that the John Deere current commercial 944 K diesel-series-electric loader leads to a 63% reduction in fuel use per hour—from 24 to 9 gallons per hour—over competing nonhybrid systems.
The SiC inverter technology will further aid in this reduction in fuel consumption by replacing lower-efficiency Si IGBT inverters. This fuel reduction equates to a decrease of 1,095 metric tons of carbon dioxide emissions per vehicle per year. Fuel efficiency measures in heavy-duty vehicle applications could save commercial vehicle operators in the United States approximately US$170 billion in annual fuel costs (https://nepis.epa.gov/Exe/ZyPDF.cgi/P100P7NL.PDF?Dockey=P100P7NL.PDF). The performance of the SiC inverter will provide additional fuel efficiency benefits, especially at lighter load operation. Figure 9 illustrates the efficiency improvements of the SiC inverter over the commercial John Deere Si inverter.
Figure 9. Low-speed and low-torque efficiency map of SiC MOSFET and Si IGBT traction inverters: (a) Si IGBT inverter’s efficiency map and (b) SiC MOSFET inverter’s efficiency map. The improved torque efficiency of the SiC inverter has demonstrated potentials for significantly improved fuel economy in heavy-duty vehicles.
Alongside the thermal management advances, the team introduced a novel gate-driver design to allow for high-temperature operation with advanced protection features, such as short-circuit protection, to keep the system operating efficiently and to minimize the risk of thermal runaway. This design features high-speed power switches—nearly five times faster than existing Si switches—and an added sensing circuit designed to detect abnormal conditions. Experimental data confirmed that the system operates efficiently in severe usage conditions, such as extended use, increased vibration, and environments with excessive dirt and dust.
The power modules are not only advanced in their performance; their reliability is industry leading. The researchers performed extensive thermal management and reliability research, both modeling and experiments, on the wide-bandgap power module. The team used steady-state and transient finite element thermal modeling along with computational fluid dynamics to evaluate and predict the performance of the thermal management design. Findings show that the heat exchanger designs kept the maximum device temperatures under operational limits in steady state as well as under transient real-world conditions.
Compared to other heavy-duty inverters, the novel SiC inverter developed by NREL and John Deere exhibits significantly improved power density while leveraging the use of existing WEG coolant up to 115 °C for advanced thermal management. In Figure 10, the features and capabilities of the SiC inverter are compared to two competitive IGBT-based inverters. The performance improvement of the SiC inverter was achieved through innovative packaging design, which prevented the heat flow to temperature-sensitive components, such as dc bus capacitor assembly. These innovations led to a compact packaging of the inverter power stage, as illustrated in Figure 11. Additionally, advanced cooling prevents heat flow from the electric motor to inverter and vice versa, as illustrated in Figure 12. Tables 1 and 2 illustrate the specific technology improvements and thermal management of the SiC inverter.
Figure 10. Comparisons to other IGBT inverters highlight the improved power density of the SiC system.
Figure 11. Illustrations of two SiC inverters tightly packaged on a common dc bus. (a) SiC power modules section and (b) complete power stage.
Figure 12. Active cooling of ac output bus bars prevents heat spread between inverter and electric motor.
Table 1. Technology improvements compared to the PD550 Si IGBT inverter using the dual prototype SiC inverter in the John Deere 644 K diesel-series-electric loader.
Table 2. Technology improvements specific to the thermal management of the prototype SiC inverter compared to the PD550 Si IGBT inverter.
Some additional transformational innovations of the SiC inverter technology are illustrated in Figure 13.
Figure 13. Cross-section of the power dense SiC inverter with illustration of key and enabling components.
The advanced thermal management of the NREL and John Deere SiC inverter shows major improvements over commercial inverters used for light-duty EV applications. Leading light-duty EVs—the 2012 Nissan LEAF, 2014 Honda Accord hybrid EV, and 2015 BMW i3—were compared to the SiC inverter. The SiC inverter outperformed across the board, including in thermal resistance, power density, heat-transfer coefficient, and coolant temperatures. Although light-duty vehicles can operate efficiently under the existing parameters, the comparisons in Figures 14–17 highlight how much innovation has gone into developing this heavy-duty–specific SiC inverter.
Figure 14. The lower resistance of the SiC inverter allows for more efficient heat removal compared to the device area. This translates into system benefits for the inverter.
Figure 15. The power density of the John Deere inverter is a vast improvement over existing systems. This includes the BMW i3 EV, which features an integrated heat sink with the power module.
Figure 16. The SiC inverter cooling system has a heat-transfer coefficient more than four times higher than the BMW i3 system. This higher heat-transfer coefficient enables a reduced design footprint, leading to its exceptional power density.
Figure 17. The exceptional thermal management system in the SiC inverter enables the use of a much higher temperature coolant compared to commercial systems.
Electrification of heavy-duty vehicles is slowly but steadily happening in the off-road industries, such as construction, mining, and agriculture. The high initial cost associated with these electrified vehicles can be offset by the long-term performance improvement of SiC inverters. Innovative thermal management designs play a big role in extracting the best performance out of SiC MOSFETs, as highlighted by the inverter discussed in this article, which was designed and developed under a collaborative project between John Deere and NREL. A potential implementation of SiC inverter in the John Deere commercial 944 K diesel-series-electric loader would result in a 63% reduction in fuel use per hour—from 24 to 9 gallons per hour over competing nonhybrid systems and will result in vehicle fleet reliability far greater than 1 million in-field operation hours. Future work will focus on extending the benefits of wide-bandgap technology aided by novel thermal and thermomechanical packaging to other vehicle architectures and platforms offered by the John Deere.
This work was coauthored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE). Funding (Award number DE-EE0006521) was provided by the U.S. DOE Advanced Materials and Manufacturing Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.
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Brij Singh (singhbrijn@johndeere.com) is with Deere & Company, Fargo, ND 58102 USA.
Emily Cousineau (emily.cousineau@nrel.gov) is with the National Renewable Energy Laboratory, Golden, CO 80401 USA.
Paul Paret (paul.paret@nrel.gov) is with the National Renewable Energy Laboratory, Golden, CO 80401 USA.
Kevin Bennion (kevin.bennion@gmail.com) is with Lockheed Martin, Littleton, CO 80120 USA.
Sreekant Narumanchi (Sreekant.narumanchi@nrel.gov) is with the National Renewable Energy Laboratory, Golden, CO 80401 USA.
Digital Object Identifier 10.1109/MELE.2023.3291255
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