--by James Williams, FAA Safety Briefing
My relationship with electrification of transportation is, well, … complicated. I am simultaneously a strident advocate and a deep skeptic of electrification. People always want simple answers, but complex situations rarely offer them.
So here is where my dissonant world view on electrification comes from. I drive a range extender electric vehicle (EV). That means that my car is primarily electric but has a gas engine that can be used as a generator to recharge the battery for longer trips. The majority (about 75-percent) of my trips rely on just the battery. That number is skewed by a few longer trips where I didn’t have access to a charging station. In everyday life, I can go weeks or even months without burning a drop of gas. My experience with EVs has been overwhelmingly positive. So why do I doubt?
My skepticism is rooted in seeing the often-hyperbolic claims of those advocating for the technology. Like many new technologies, the benefits are trumpeted while the limitations are often ignored. In the long term, this consistent over-promising and under-delivering damages the public’s confidence in the technology. What follows is my hopeful skeptic’s view on the benefits of, and challenges for, aviation’s electric future. I’ll also take a look at a couple of projects that are making their way toward the general aviation (GA) community.
Electrical Elation
There are a number of great benefits that come from using electrical propulsion on an aircraft. Among the most recognizable is just how quiet this technology is. While the propeller still creates a noise signature, engine noise is all but eliminated. Along with that noise reduction is also a major reduction in vibration. This could be a major factor in reducing fatigue and creating a more pleasant environment for everyone.
Then you have the biggest potential advantage: “fuel” cost. According to the U.S. Energy Information Administration, Americans pay roughly 11 to 30 cents per kilowatt-hour (kWh) depending on where they live. The average for the country is about 13 cents per kWh. So to “fill up” the usable capacity on my EV (14 kWh) costs a whopping $1.82. That gives me somewhere between 40 and 70 miles of range depending on a host of factors from driving style to whether I run the windshield wipers. The straight operational economics (i.e., the direct operating cost per mile) work out to less than 40-percent of what a fairly efficient gas-powered car would run.
On the other hand, the total cost of ownership is a less rosy picture. There is a significant price premium on EVs compared to a traditional car. This is also true of airplanes. But the higher fuel costs in aviation may help offset that.
I have expressly ignored the larger sustainability argument for electrification: No matter how worthy a cause sustainability is, we won’t see large scale adoption until a strong economic case can be made. I think we’ll get there, but there are a few challenges on the way.
Energy Density is a Harsh Taskmaster
There’s no way around this one. Energy density is probably the greatest challenge in the electrification of transportation. This applies to EVs but is even more critical to airplanes, as they are more sensitive to added mass. The real problem is that as a fuel, petrochemicals are actually really, really, good in terms of energy density. For reference, jet fuel has a specific energy of about 11.9 kWh per kilogram (kWh/kg). Gasoline is about 12.9 kWh/kg and diesel is 13.3 kWh/kg. Lithium Ion (Li-ion) batteries come in between 100 and 243 Wh/kg. To put it another way, the worst petrochemical (jet fuel) has 48 times more energy per unit of mass than the best Li-ion battery. In practical terms, that means that for every kilogram of fuel you are looking to replace, you need between 48 and 54 kilograms of battery. Keep in mind that this is specific energy measured at the cell level, without any of the additional packaging, wiring, and cooling capacity that has to be built into the battery pack. When these factors are considered, EV batteries wind up having between 100 and 168 Wh/kg.
There is a weight savings from electric motors, which are significantly lighter than an internal combustion engine (ICE). An installed Rotax 912, a common Light Sport ICE, weighs in at about 64 kilograms, while an equivalent electric motor only weighs 11 kilograms. But this weight savings isn’t nearly enough to offset the massive battery weight relative to standard fuel tanks.
Will energy density improve? Yes, but the consensus on that rate of improvement seems to be five- to eight-percent per year and research suggests that we may be approaching the limits of the current technology. There is promise in solid-state battery technology. Solid-state batteries replace the liquid or gel electrolyte in the battery with a solid electrolyte. This change offers much better packaging, cooling, and energy density capability than traditional Li-ion batteries. Solid-state batteries do exist and will provide a major step forward. The catch is that to build them in large scale is astronomically expensive. Therefore, the widespread application of solid-state batteries will likely take many years, even with massive private and public research efforts.
Chemistry, Cobalt, and Capitalism
If you’re in the battery business, chemistry is the name of the game. Batteries of all kinds use chemistry to store electricity and the exact nature of that chemistry can have significant effects on the performance of the batteries. That’s why there’s so much focus on battery research. Any potential gains could have huge economic benefit to those who discover and commercialize them. But that search to find even better chemistry can lead to interesting materials. And those materials have concerns of their own.
One key material is cobalt. Cobalt is a metal that is used in a number of applications, notably Li-ion batteries. Battery manufacturers have been working to reduce the amount of cobalt in their cells because it is expensive and prices have been climbing. Cobalt also requires significant processing as it is very rare in its pure form. Cobalt’s supply chain is also a concern. The majority of cobalt is mined in the Democratic Republic of Congo as a byproduct of copper mining. About half of the cobalt supply is refined in China. An expensive metal with massively increasing demand and a geopolitically sensitive supply chain is another challenge for electrification.
Then there’s the industry’s dirty little secret. None of the headline-grabbing car makers actually make their own batteries — not even Tesla and its Gigafactory. Battery cells for Tesla’s Model S and Model X vehicles are made by Panasonic in Japan, and its Model 3 battery cells are made by Panasonic in the Gigafactory in Nevada. This approach is not unique to Tesla; it’s actually standard practice in the industry. GM, Hyundai, Daimler, Ford, and Volkswagen buy cells from LG Chem and BMW buys cells from Samsung.
This industry practice has an interesting aspect for us in GA. It is a potentially tremendous benefit in that all of the research and development in battery technology can be easily transferred. This is a massive game-changer. In the electrical space, any advancement made by any battery manufacturer can be directly dropped in. That’s huge. But on the flip side, it also means that you are competing for cells with all of these other users.
So where does that leave us? About five years ago, I would have said that we were very much in the experimental/proof of concept phase. Today, we are beginning to see the transition to the potential to field truly functional GA airplanes with electric propulsion. Let’s take a look at two projects and where they stand right now.
Pipistrel Alpha Electro
Pipistrel is a Slovenian manufacturer known in the United States for its Light Sport Aircraft. In 2017, the company introduced an electric version of its Alpha airplane. The Electro swaps the standard Alpha’s Rotax engine for a 60 kW (80 hp) electric motor. The Electro sports a 21 kWh battery capacity split between two packs located ahead of and behind the cockpit. This capacity allows for one hour of flight time with a 20-minute reserve.
During development, Pipistrel thought that battery swapping would be the way to go for fast “refueling.” But the company soon found that it was possible to charge the batteries in less than the flying time of the aircraft. The key was to manage battery temperatures to keep them as cool as possible. Reducing temperature increases charging speed. The current battery packs are air-cooled, but Pipistrel is actively researching liquid cooling to bring the charging time down even further. Initial testing has yielded charge times less than 30 minutes after one hour of flight time. This would allow operational usage on par with gas-powered trainers.
This change of philosophy is actually the kind of smart thinking you want to see from engineering companies, and it shows the value of these pioneering efforts. What looks good on paper can often be less effective in reality. Today, the focus of almost all Li-ion battery users is in improving charging efficiency and speed. Modern large batteries are sophisticated and expensive pieces of equipment. They are also heavy. The Electro’s battery packs are still removable, but this feature is now used for maintenance and storage rather than “refueling.” With regard to energy innovation, the Electro has another trick up its sleeve. Much like an EV, the Electro is able to regenerate power through the wind-milling of the propeller. This means that when you’re descending, you can actually add power to the battery.
The Electro meets the LSA criteria with one significant caveat. At the time of this writing, the FAA’s LSA regulations do not allow for powerplants other than a reciprocating engine. Earlier this year, Pipistrel delivered four Electros to a pilot training program in California and expects to start full-scale deliveries next year. The Electro is aimed squarely at the training market with focus on initial training to solo, which should be easily accommodated by the current battery capacity. But at the moment that training agenda is on hold until the Electro’s LSA status can be resolved.
Sun Flyer
The next project is Bye Aerospace’s Sun Flyer. While the Alpha Electro is targeted primarily at the LSA/Cessna 152 market, the Sun Flyer is more of a Cessna 172 competitor. The company has stated it is planning to sell two- and four-passenger versions of the airplane. The Sun Flyer 2 is the two-passenger version slated to have a 3.5 hour endurance from a 92 kWh battery pack with a 90 kW (120 hp) motor. That size is in line with some of the largest packs in the EV space. The planned four-seat Sun Flyer 4 is listed to have a 4.2 hour flight endurance with a 105 kW (140 hp) motor. The Sun Flyers are being certificated under part 23. The Sun Flyer project is still in development as the Sun Flyer 2 prototype made its first flight in April 2018. Bye Aerospace anticipates certification in 2020.
These projects are just examples of what exists now and what’s coming. The challenges are real and daunting, but the benefits can be very meaningful. The vanguard of this electrical revolution is arriving now in the form of airplanes like the Alpha Electro. The Electro has to make certain sacrifices, namely in range, in order to meet weight and cost metrics. Airplanes like the Sun Flyer 2 will arrive in the next few years to alleviate some of those range concerns, albeit at the cost of a much heavier and more expensive battery pack. Manufacturers are building in modularity in battery solutions to allow for the potential upgrades that could make current aircraft much more usable.
But all of these projects are incredibly important in helping us build out the potential that exists. There are many assumptions that need to be tested. Some of them will prove correct and others not so much. How should we design our charging networks? What’s the ideal battery size? Do our current procedures work for electric aircraft? How do I schedule an electric fleet? How fast will it pay back the price premium of an electric aircraft? These questions are just the tip of the iceberg.
We might think we know, but until we’ve tested it in the real world we don’t. The experience gained from these pioneers will tell us just how close our electric future is. In the meantime, I’ll continue as a skeptic — but a hopeful one.
James Williams is FAA Safety Briefing’s associate editor and photo editor. He is also a pilot and ground instructor.