Hot on the Trail

More powerful EVs will produce more heat. Toyota Research Institute of North America is working on ways to manage it.

April 03, 2018

Heat Management -- Here's a sampling of some of the porous-coated heat spreader designs TRI-NA is testing in its search for more efficient electric and hybrid vehicle systems.

The heat is on. In more ways than one.
 
In 2015, Toyota Motor Corporation committed itself to the Toyota Environment Challenge 2050, a set of six objectives that go beyond zero environmental impact to achieve a net positive benefit to society. At the top of that very ambitious to-do list is “achieving zero carbon dioxide emissions from vehicles” within the next three-plus decades.
 
Making good on that pledge will likely mean transitioning away from the traditional internal combustion engine to some form of electrical propulsion, whether that be hybrid, battery electric or hydrogen fuel cell. But electric vehicles (EVs) that can replicate the power and range we’re accustomed to will require higher-density batteries, more potent electric motors, and next-generation power electronics.
 
Here’s the problem: Such electrified systems will produce more heat. A lot more.
 
Too Hot to Handle?
 
Think about how your smartphone turns into a hand warmer when you use it to play video games or some other processor-intensive activity. EVs, clearly, will need a lot more electrical power than that – in the range of 55 kilowatts for hybrids and 125 kW for full-on electrics. So that will mean coming up with a way to manage all of the heat the associated electronics dissipate.
 
Will this challenge prove to be too hot to handle? A team of scientists at Toyota Research Institute of North America (TRI-NA) in Ann Arbor, Michigan, says it’s getting a grip on it.
 
Since 2008, TRI-NA’s Electronics Research Department (ERD) has been looking at cooling technologies to manage that heat. ERD has participated in three core projects that show promise.


Joint Venture -- This smaller, lighter and more energy-dense power inverter was developed by TRI-NA in partnership with Wolfspeed and the U.S. Department of Energy.

Single-Phase Cooling
 
The first project involves single-phase liquid cooling for power electronics applications where multiphysics optimization methods are used in the design of customized fluid channel geometries. The super simple translation: It’s about passing coolant over a hot device (i.e. semiconductor) so that it extracts heat in the most efficient manner possible.
 
In current hybrid vehicles, this is accomplished by mounting the semiconductors on to cold plates through which a water-glycol coolant mixture is pumped. That’s going to be even more challenging in the not-so-distant future when it’s anticipated that power electronics will shrink to half their size in the never-ending push for lighter, smaller and more efficient vehicles.
 
You’d probably need a PhD in mechanical or electrical engineering to fully grasp TRI-NA’s solution to this problem. But suffice it to say that ERD has been making progress on ways to manage the dissipation of heat from power electronics in a more compact space.
 
Compact Power Inverter
 
As an example, in a second project, ERD partnered with a company, Wolfspeed, and the U.S. Department of Energy to develop a next-generation power inverter. The partnership’s state-of-the-art inverter is smaller, lighter, more power dense and more efficient – all of which will be critical to meet the needs of future EVs.


Bring to Boil -- This is a representation of "two-phase jet impingement" technology that aims to balance heat extraction with pumping power.

Two-Phase Cooling
 
The third project then ups the ante from single- to two-phase liquid cooling. This means the coolant, whether it be water or some other fluid, makes the transition from a liquid to a gas state (i.e. boils) to extract even more heat. The downside to this approach is that it tends to result in a big increase in the pressure of the cooling loop. To compensate, you need more pumping power. But that offsets the system’s overall efficiency.

ERD has been working on a technology called “two-phase jet impingement” that could balance heat extraction with pumping power.
 
Similar to when you put a pot of water on the stove, when the coolant starts to boil, bubbles form randomly. What ERD is trying to do is control and again optimize how this boiling process occurs. The choice of liquid coolant matters and so does the informed design of the cold plate structure.
 
It’s important to note that these projects aren’t just elaborate science experiments. TRI-NA’s efforts are definitely starting to bear fruit. Each fledgling technology has generated numerous patents, contributing to Toyota’s standing as the automotive industry’s lead innovator over the past eight years. And the projects have been honored (as two awardees and one finalist) by R&D Magazine’s R&D100 Award program, science’s equivalent to the film industry’s Academy Awards.
 
ERD believes that there is still much work to be done, but they are moving in the right direction. The Toyota Environment Challenge 2050 sets a very high bar. But with everyone around the world working together, Toyota believes it can meet this challenge.
 
By Dan Miller

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