11/12/2021 The Grainger College of Engineering 16 min read
Written by The Grainger College of Engineering
Every faculty member and researcher has been there. You’re going in to pitch a project or you’re meeting with a review panel considering your proposal, and nothing is landing. They nod politely. You keep plugging away. They ask the standard questions. You try to get them excited. “We’ve all gotten our share of blank stares,” said Professor Andrew Alleyne of the Department of Mechanical Science & Engineering. “They say ‘Thanks for coming. Do you want a pen on your way out?’” Everyone in the room knows things aren’t going anywhere.
Alleyne has had a very different experience with POETS, the Center for Power Optimization of Electro-Thermal Systems that he has led since it launched in 2015 with an $18.5 million award from the National Science Foundation. That initial support has been followed by $30 million in additional funding from NSF and others. POETS focuses on increasing power density in electrified mobility – that is, the amount of power that can be delivered in a given weight or amount of space. The POETS team looks at the technology behind electrification of everything involved in moving goods and people on land, air, and sea.
“In my time with POETS, talking to people about electrification, I’ve yet to sit down with anybody and have them say, ‘Nah, that’s not of interest to us,’ because they can see the market value of electrified mobility. There’s nobody in the automotive industry or the automotive supply chain that is turning a blind eye to this,” Alleyne said.
This interest is evident in the conversations that Alleyne describes, as well as industry’s public statements. In early 2021, GM announced that it plans to end production of gas and diesel vehicles by 2035. Jaguar and Volvo have both said they’ll do the same by 2030. Volkswagen is shooting for 2027. Companies like Tesla and Rivian have been electric-only vehicle companies from the start.
Those pledges from auto companies reflect growing demand. BloombergNEF’s 2020 Electric Vehicle Outlook reported that electric vehicle sales went from 450,000 in 2015 to 2.1 million in 2019. They predict that sales will reach 8.5 million by 2025 and 54 million by 2040. Those 54 million vehicles are expected to displace demand for 17.6 million barrels of oil per day.
And that’s just the automotive industry. The market for electrification solutions in aviation is expected to have a compound annual growth rate of more than 12 percent in the coming decades, according to analysts at Research and Markets. McKinsey predicts a $300 billion dollar market for micromobility – electric bicycles and scooters – by 2030.
It’s a promising landscape, and it has grand implications for climate change, transportation infrastructure, and the way we live. But it will require innovation like the transportation industry has never seen. POETS is driving that innovation. The center’s suite of research projects, technologies, and collaborations will be crucial to improving power density, such that companies can hit their targets, deliver their planned products, and create new products that we can only imagine today.
One of POETS most successful research efforts has been a multi-year project to develop a high-density traction inverter. Inverters are, effectively, the main electronics for propulsion in an electric vehicle. They provide power from batteries to drive an electric motor.
“Our traction inverter is about 10 times more power dense than the 2014 baseline that we started from. We had a goal of 10 times more power dense [over the course of the project]. We met that goal in five years with five more years still left on the clock. And we’ve done some even more power dense ones already,” said Professor Alan Mantooth, from the electrical engineering department at the University of Arkansas and deputy director of POETS. “We’re now applying it to road cars, off-road heavy machinery like bulldozers, and airplanes.”
The Most Important Question
POETS is composed of three technical thrust areas in system design and operation, packaging and integration, and component fabrication. Faculty members and research staff work across the University of Illinois Urbana-Champaign, Howard University, Stanford University, University of Arkansas, KTH Stockholm, and the University of São Paulo, as well as an ever-expanding roster of more than a dozen industry partners.
It’s a big, complicated system that looks at big complicated systems – so the team has thought consistently and intently about what it values, why it launched, and why it continues its work.
“The ‘why’ is the most important question to any endeavor. And this is one of the great things about Illinois,” Alleyne said. “We ask the ‘why’ questions. A lot of other people at other institutions only say ‘This is what a lot of people are doing, how do we do it too?’ Here at Illinois, we look at a situation and say ‘Why should we do this? Why is this a problem?’ That allows us to think big, because you get to the core of why you gather all this effort together to go after some challenge.”
Those “why” questions started in the early 2010s. Core members of the team homed in on the fact that mechanical systems were being replaced by electric drives. The trend spread across all forms of transportation – cars, busses, planes, trucks, and off-road equipment. The different modes were all at different stages of development and uptake, but electrified mobility wasn’t going to be a tiny segment of the overall industry or a fool’s errand anymore.
Electric vehicles were going to take over. Buying a Prius wasn’t just about being eco-friendly. It was about economics and performance. A modern electric vehicle will accelerate noticeably faster than most gasoline engines.
“You no longer had the giant fan belt on your engine running all your accessories. It was a motor generator putting electrical power out to provide power to systems all over your car, which improves efficiency. A lot of people will look at a problem today. What we did is that we went backwards in time and we were seeing these electrification trends across all mobility modes,” Alleyne said.
“It’s almost like studying the stock market before something hits and saying, ‘OK, this is going to be a real opportunity.’ And you see it across all modes of mobility. We started to ask why hasn’t this taken off faster? There are economic and political reasons. But some of the reasons were technical. And we knew we could have tremendous impact there at just the right time.”
A Co-Design Ecosystem
The potential impact of the POETS team wasn’t just a matter of timing. It was also a matter of thinking differently – moving out of silos that have traditionally dominated the field of power electronics and embracing a concept known as co-design.
“In the classical design cycle the electrical engineers do the initial work, developing the battery, the power electronics, the cabling, conditioning controls. And then they say, ‘Well, this is getting hot. Let’s throw this over to the mechanical engineers and tell them this has to be kept below 80 degrees C,’” said Professor Nenad Miljkovic, a mechanical engineering professor at UIUC who works on the inverter project.
“It really makes an inefficient design cycle because there’s no cross talk. Then you have these electronics and you have to couple them with a really bulky thermal system because you haven’t designed it holistically. It always results in an oversized system because you’re dealing with a layout or design that doesn’t consider all aspects of the design simultaneously from the beginning.”
Alleyne put a fine point on the problems with the classic design cycle: “One of the early generations of currently available electrical vehicles had four different cooling systems that they were running, because they were each developed for their individual electrical system – the motor, the inverter, the battery system. When the time came, each one had their own because they were under time pressure to launch this thing into the market. And the cargo space was horrible because they didn’t integrate anything.”
Co-design is crucial in this field in particular. People working in power electronics are constantly reconciling two competing, fundamental interests: Moving electricity and dissipating heat. That’s true whether they’re working on a microchip with millions of transistors packed onto it or a traction inverter that can move a 20-ton dump truck. By developing tightly integrated teams, with complementary experts in both areas and in packaging the final product, you can develop a much more power-dense product. And, if you’re doing it right, the process of designing them is less expensive and faster in the long term.
Reliable Density
Inside every electronic device, there are modules of electronics printed on circuit boards. Power modules – like those the inverter team works with – carry high current and voltage and are exposed to high electric and magnetic fields. That requires more cooling and additional protection than the electronics you would find in your television or portable speaker.
Working together, the inverter team designed those modules and cooling mechanisms, then packaged them into an inverter assembly capable of delivering 150 kilowatts of power. Demand for cooling is very inconsistent across the module. “Eighty percent of the module’s heat comes from, say, four devices, which make up maybe five percent of the board,” Miljkovic said.
In one of their inverter designs, each module in the inverter package is immersed in water. Heat is transferred from the module’s cooling plate to the water. The module is covered in a coating that serves as a stand-off between the electronics and the liquid. This setup prevents short circuiting, protects the electronics, and facilitates cooling very safely and efficiently. It also allows the team to use silicon carbide and gallium nitride in their electronics design. These materials enable them to rely on very high power levels that get very hot. The inventive, co-designed cooling system can handle the heat. Using a process called flow boiling, the water is allowed to evaporate and condense in order to regulate the device’s temperature.
“Phase-change heat transfer like flow boiling is ultra-efficient for removing heat. It allows you to absorb all that heat, very efficiently, at a constant temperature and really allows you to increase the power going to that device, and thus the power density,” Miljkovic said.
“Density is diametrically opposed to reliability, and cooling is really important to reliability. But we’ve really delivered reliable density here. A Toyota Prius is 55 kilowatts, and 150 kilowatts is a pretty fast car or a pickup. We’re just continuing to march forward. Building 150 and 250 kilowatt-sized devices that just get smaller and more compact,” Mantooth said.
“This is the way a co-design ecosystem is supposed to work.”
In this case, Mantooth and his team at Arkansas bring expertise in building the power modules. Professor Sonya Smith of Howard handles the thermal management, along with UIUC’s Miljkovic. Professor Paul Braun, of UIUC’s Department of Materials Science & Engineering, brings advancements in materials for cooling. Professor Debbie Senesky at Stanford knows extreme-environment electronics. Alleyne focuses on control algorithms to manage the inverter’s operation. Professor James Allison from UIUC’s Department of Industrial & Enterprise Systems Engineering works on packaging the overall system.
Most importantly, they all work together – on dozens of projects like the inverter project across more than 30 faculty members. “The beauty of all of this is there was enough overlap between Stanford and Illinois and Howard and Arkansas,” Mantooth said. “But it was complementary. It lets us communicate effectively. That’s always a good thing. Maybe there’s some redundancy, but that’s OK. Sometimes you need that. Materials to devices all the way to the integrated system. No matter what the needs, we were able to plug in somebody who contributed to the value chain.”
Blue Sky with Impact
These relationships create a strong foundation for new projects. “The inverter project thinks about heat dissipation, materials, new devices. That’s an area I would never work on on my own. I don’t have the background. My students don’t have the right expertise. But we do understand a lot about the fundamental and more engineering aspects of materials. And it’s clear that’s a critical part of the system,” said Paul Braun, who also leads the university’s Materials Research Laboratory (MRL).
“So if we have a new opportunity, we are able to go out and say, ‘Aha, we need someone that can do thermal modeling. We need someone who can build devices. We need someone that can understand how materials behave. Let’s bring them together.’ We can launch forward. The entire process of building the team [and executing on a given opportunity] can be done in months.”
The inverter project uses known materials while presenting complicated integration challenges. The team has to ask how the materials wear, how they interact, how to prevent oxidation, and other similar questions. But the POETS center also gives the researchers room for more novel work – work that, like the center itself, dovetails closely with the needs of industry and anticipates where the market for electrification will be in the future.
“It’s blue sky. But it’s blue sky that pays considerable attention to the impact of the work. High-risk, high-reward,” Braun said. “We work with the team on the problem of today. Then we use that knowledge to motivate the more fundamental projects that run in parallel. We know what the needs are, and we have a vision of a material that might meet that need. Then we go into the lab and try to do it.”
In Braun’s case, that work currently focuses on the nanostructuring of materials. If a material can conduct electricity, it’s always going to conduct some heat with the electricity. Electrons conduct heat on their own when they move, and there’s no getting away from that. But heat is also transferred by the movement of materials’ atoms in a phenomenon known as lattice vibrations. Heat causes the atoms to shake, and that shaking causes the atoms around them to shake as well, transferring the heat, even if they’re not moving electricity.
Developing materials that slow down the transport of lattice vibrations, therefore, is a promising way of improving the performance of electronic devices. Materials that conduct electricity well, while conducting as little heat as possible, can be used to build more power-dense and efficient devices.
As part of the POETS center, Braun’s team is exploring iron oxide. This crystalline material is a good electrical conductor. By growing the iron oxide to have tiny holes in it about 100 times smaller than a human hair, they can block the atoms vibrating one another without blocking electrons’ movement in the conduction of electricity.
“The way the electrons move is largely unchanged. The way the atoms move is changed a lot. The ratio of heat conduction to electron conduction improves by a factor of five or 10 when you put these little holes in,” Braun said. Using such materials, designers could, for example, place chips that generate the most heat closer together than they do currently, reducing the size of the cooling systems that they would require.
POETS’s culture of co-design and tightly integrated teams is crucial to the insights needed to do future-oriented work like that. It also encourages the sort of collaboration that makes securing funding for that work possible.
“Last year, funding for our associated projects – these are projects that we go after outside of our core funding from the National Science Foundation – that was actually larger than our core NSF funding. Close to four million bucks,” Alleyne said. That support came from places like the Department of Defense, the Department of Energy, and new industrial partners.
“We’ve pre-formed those teams who already know each other and know the strengths of each other. They’re from different disciplines, and they’re complementary. So now when they go present themselves as a team to some outside agency, they just rock. They’re able to finish each other’s sentences. And people can clearly say, ‘You know, if I’m going to invest several million dollars in a team, that’s the one I’m going to go after,’” he said.
ARPA-E, DOE’s advanced research arm, made exactly that sort of decision two years ago. The agency chose the POETS inverter team and Caterpillar to expand on their work. That project has already delivered an inverter system that is capable of 500 kilowatts – almost enough to power a bulldozer.
With that work complete, the team developed a hybrid electric aircraft inverter to drive the electric motor on a Cessna 337. These small aircraft have been around since the 1960s but are still widely used as island air taxis. “These things are gas guzzlers. They’re like 1971 Chrysler engines, and all they do is take off and land,” Mantooth said.
By replacing one of the 337’s two engines, they’ll be able to take off using both engines, while cruising and landing using only an electric motor. That change will massively improve their efficiency. A company based in Los Angeles, called Ampaire, is retrofitting several planes with an electric motor and the POETS team’s inverter. Testing and Federal Aviation Administration certification is under way.
These constantly growing collaborations and improvements are what Alleyne refers to as “reinforcement strategies.” They bring new people and ideas into POETS while maintaining the center’s clear mission, focus, and culture. “We’ve not quite doubled in size of faculty, but it’s getting close. The folks that have joined have seen, ‘Oh, wow. By being a part of this and keeping this momentum rolling, there’s this bigger sphere that I can also play in.’ You’ve got this feedback loop that naturally propels it,” Alleyne said.
The process reminded Mantooth of a conversation he had with the leader of an NSF Engineering Research Center like POETS more than a decade ago. “They were coming to the end of their ten years. I said ‘Would you do it again.’ And he said ‘Not for the money, but for the doors it opens up? You betcha.’”
Ideas and People
Most industrial and academic partners want two things, according to POETS Director Andrew Alleyne: Ideas and people. Projects like the traction inverter represent the research ideas that come from the center. The people, meanwhile, are well represented by the students that POETS prepares, nurtures, and graduates.
“I see them go off in their careers. They’ve walked into situations in the workplace and been like ‘OK, that’s not a big deal. I can handle that,’” Alleyne said.
“The students we’re producing, because we’re asking them to work across disciplines and asking them to be capable in one core discipline but have awareness of other disciplines and how their work goes back and forth, they tend to be in high demand from people who are building their workforce in this area.”