Advanced Thermal Management for Power-Dense Power Electronics and Electric Motors Improves EVs

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Ever since the first gasoline-fueled car rolled off the assembly line 130 years ago, engineers have continually worked to improve vehicle design and function. Electric vehicles (EVs) have been gaining ground in the marketplace over the last decade, so the race is on to improve their cost, range, reliability, and efficiency to ensure they can perform on par with their gasoline-powered counterparts and consumers can purchase them with confidence.

As EVs are further developed, new materials and enhanced configurations to improve the thermal management systems of power electronics and electric motors is necessary for peak EV performance. Thermal management continues to be a growing field as components used in various applications are required to be smaller, more affordable, and more powerful.

Finding new ways to disperse or remove heat from motors remains a challenge because high-density motors emit more heat in a smaller volume. NREL's Advanced Power Electronics and Electric Machines (APEEM) team focuses on developing advanced cooling methods for next-generation applications. This includes ongoing work on R&D Award-winning Wide Bandgap (WBG) technology to enhance the performance of EVs.

Additional ongoing research goals include:

• Improving heat transfer designs
• Evaluating, identifying, and overcoming heat removal challenges through modeling and analysis
• Partnering with other collaborators to advance the most promising motor thermal management technologies.

There are several ways to address these challenges. One is by improving cooling systems. A recent journal article co-authored by NREL and UQM Technologies Inc., titled "Experimental Characterization and Modeling of Thermal Contact Resistance of Electric Machine Stator-to-Cooling Jacket Interface Under Interference Fit Loading" featured in the ASME Journal of Thermal Science and Engineering Applications, outlines an electric motor cooling design and the groundbreaking characterization studies that followed.

The article outlines a motor design with a high-performance cooling jacket wrapped around the stator (the stationary part found in electric motors). By incorporating the cooling jacket into the motor case, the motor can be effectively cooled as components heat up during motor operation, increasing power density and improving electric motor reliability. As motors are required to be more compact while providing high-power (heat-producing) output, understanding the impact of stator-to-case thermal contact resistance (TCR) is critical to ensuring these motors are cooled properly.

The importance of this study is that it provides accurate values for stator-to-case TCR, improving knowledge of cooling designs, specifically by providing clean, objective data on the thermal behavior of the interface between the electric motor stator and cooling jacket. According to research engineer and lead author Emily Cousineau, "Prior to this paper, there wasn't any reliable data in the open literature for this critical property, and since this work we've been able to validate other thermal electric machine models."

This valuable information allows researchers to design cooling systems that can be optimized correctly to meet the need for more compact designs. This work also helps electric machine designers accurately estimate the stator-to-case TCR for a range of electric machines without having to resort to expensive experimental tests or overly conservative estimates.

"As computer simulations become more sophisticated, engineers tend to rely on them more during the design phase. If you can start with more accurate data for material and interface properties, you may be able to save some expense of testing and prototyping. This helps to drive down the cost of new designs," Cousineau said.

Another way to provide improved thermal management of components is with the development of new thermal interface materials (TIMs). TIMs facilitate the removal of heat generated from the operation of electronic, electro-chemical, and mechanical devices. They work by filling in interstitial gaps, reducing thermal contact resistance between a heat source and heat sink. As electronics get smaller and more powerful, and as the moving components of machinery operate at higher speeds, additional heat is created. Researchers are continually improving TIM chemistries to enable them to further relieve thermal stresses in next-generation devices and applications.

NREL collaborated with Texas A&M University (TAMU) on a Defense Advanced Research Projects Agency (DARPA)-funded project to characterize the thermal conductivity and resistance of new, high-performance TIMs. This partnership resulted in a journal article, "Chemically Linked Metal-Matrix Nanocomposites of Boron Nitride Nanosheets and Silver as Thermal Interface Materials," featured in Nanotechnology on a novel, high-performance TIM that utilizes the chemical integration of boron nitride nanosheets (BNNs), soft organic linkers, and a copper matrix. This new, high-thermal conductivity amalgamation is capable of thermal resistance one-third lower than any other state-of-the-art TIM and has the potential to satisfy thermal management needs as components are further miniaturized.

According to research engineer Xuhui Feng, "The novel structure gives superior thermal and mechanical properties that satisfy the rigorous thermal/reliability requirements of high-power-density components. This thermal interface material will significantly advance the development of smaller, lighter but more powerful electronic components in various fields, such as solar, automotive, consumer electronics and aerospace."

Researchers at NREL and TAMU studied conventional TIMs, which include thermal greases, polymer composites, and solders, assessed their shortcomings, and then identified a way to improve them. Two ways to resolve the issues with current TIMs are to further improve the thermal properties of a compliant matrix, or further improve the mechanical properties of a high-thermal-conductivity matrix. By involving metal nanocrystals coordinated with organic ligands grafted on BNNs in the form of mesoscale metal-organic framework, this new class of TIMs was created.

These advanced chemically integrated metal/organic/inorganic hybrid nanocomposites are a promising start to providing thermal management solutions that can enable the development of more advanced thermal management systems in high-power-density components for EVs and other electrical devices.

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