IEEE Talks Transportation Electrification: Prof. Pat Wheeler

Prof. Patrick Wheeler is a member of the IEEE Transportation Electrification Community who specializes in the electrification of aircraft and also serves as Head of the Department of Electrical & Electronic Engineering and as the Professor of Power Electronic Systems, Faculty of Engineering, University of Nottingham, UK. In this interview, Prof. Wheeler explains the drivers and benefits of increased electrification of aircraft. 


Question: Would you briefly define power electronics and describe how it applies to aviation?

Pat Wheeler: Power electronics is the enabling technology used in power converters, which are used where a compact, efficient way of converting electrical energy from one frequency and voltage to another is needed. Power electronics are used in everything from the power supplies in your mobile devices to very high-power converters used in the electric grid. In aviation applications many of the power converters come in the mid-range between these two extremes. 


The driver for power electronics in aviation is a desire to use more electrical systems for functions formerly performed by either hydraulic, pneumatic or mechanical system. This shift in technology has the potential to save weight, increase functionality and provide a range of other benefits. 


Traditional aircrafts today have four energy systems. Reducing those four to just one produces the weight savings. Electrical energy is the most flexible and controllable. Having intelligent control over an aircraft’s power converters provides a lot of information that can be used for diagnostics and prognostics, which are, respectively, understanding a current problem and anticipating a future one. 


Question: What are some of the challenges in this seemingly beneficial shift to the increasing electrification of aircraft? 


Wheeler: When we increase the amount of electrical energy used on an aircraft and there’s a power converter between the supply and every single load on that aircraft, it comes with built-in challenges in terms of reliability, failure modes and the size, weight and heat of those power converters. Making power electronics as reliable as possible produces a packaging problem as well as a material problem. For example, how do we dissipate the resulting heat? That’s a thermal problem. How we design and build power converters to provide the functionality we want is critical, but we must do so in a manner that increases reliability, for obvious reasons. These are the salient challenges. 


Question: Where would you place your work in the context of the overall topic of the electrification of transportation? 


Wheeler: You may recall that transportation electrification is not a new idea. If you look at diesel electric rail-related motors we’ve been electrifying transportation since the 1950s. I also like to remind people that the first electric cars were designed about 180 years ago. What's changing is people's perception of the environmental impact of fossil fuel use and their desire to make transportation more environmentally friendly and sustainable and, frankly, more affordable. 


The electrification of transportation is a good way of doing that. So, for instance, cars and motorbikes today are being designed as hybrid electric or all-electric vehicles. The problem with all-electric vehicles today is the efficiency and cost of energy storage, exactly as it was 180 years ago. There’s a limited range for the vehicle defined by the amount of electrical energy we can store in current systems. That’s improving slowly, but it’s not completely solved yet. We need the next generation of battery technology before we get to a totally viable solution.


These hybrid and all-electric cars need the same technologies as aerospace. They need power electronics, which did not exist when the first electric cars were designed nearly two centuries ago. The development of power electronics is one of those big, enabling technologies that provide control of electrical energy. 


To place aircraft into this context, we are now using power electronics as I described earlier to replace other energy systems in terms of auxiliary systems, actuation systems, fuel pumping and so on. But over the next 10 or 15 years we will see more work directed towards the actual electric propulsion of aircraft, to actually make the aircraft fly electrically. We’ve seen a few, small prototypes already, but it will take years to achieve the safe, reliable electric propulsion of aircraft based on batteries or other energy sources. 


Question: You’ve compared cars’ and aircraft’s similarities. How would you contrast them to underscore their unique differences? 


Wheeler: In both cases, we want to understand the fundamentals of why things fail, how they fail and when they're going to fail and power electronics will aid in gaining those understandings. A slight difference between airplanes and cars is that most people don't drive their cars for 18 hours a day, 365 days of the year, but that's exactly what we expect from a commercially deployed aircraft. So the hours of use of an aircraft are far higher than they are for the automotive industry. The automotive industry is looking for cost reduction whilst getting a reasonable, cost-effective product lifecycle or lifetime that no longer exceeds a decade.. 

In aerospace and aircraft design, each plane is so expensive to build and to buy that the manufacturers design it to last as long as possible, but with a reasonably well-defined lifecycle or lifetime when equipment will need replacing. An auto manufacturer wants to understand failure modes and such so they don’t pay too much for components. Why design a component for 30 years when the vehicle it serves isn’t expected to last more than 10 years? Whereas in aerospace and aircraft design, manufacturers want to understand component failure modes and lifecycles so they can work out rational maintenance and replacement cycles for reliable service and, of course, safety. 


Question: We’ve discussed power electronics in aviation in somewhat general terms. Where are we on the curve of commercial implementation? 


Wheeler: Power electronics have been on aircrafts for very many years, so there's nothing new about putting power converters on aircraft. What has changed and is still changing is the number and the size of those power converters. There are far more power converters on Boeing 777s than there are on Boeing 787s, for example. And the size of those power converters and the amount of electrical energy they're processing is going up, so the electrical system on the 787 is about 10 times the size of a conventional aircraft. We’re seeing a step change in the number of power converters, in their size and in the power level of those power converters. 

When you do that in any industry – particularly an industry that flies hundreds of people in one craft across continents and oceans at seven miles above the ground – that puts a lot of pressure on everyone working on those power converters. They must work, we must understand their reliability and their failure modes. We have to understand their cost basis and so on. The result of that commercial pressure has been quite a revolution in power electronics because as an industry we've had to respond to those new demands, and aviation is quite a demanding application area. 


From my perspective, that’s why the present is an exciting time to work in the power electronics field, because there are a lot of problems to consider, a lot of people wanting solutions and a lot of potential profit to be gained for the companies that produce the solutions.