One of the hard targets for people concerned about greenhouse gas emissions are commercial aircraft. Commercial aviation is a driver of economic activity and growth; it creates mobility and access, promoting commercial and cultural exchange and understanding, both of which contribute to peace. But those big beautiful jets rely on a very high energy density fuel, namely fossil fuels, and there is substantial concern that nitrous oxide emissions, water vapor, and sulfides have a different and possible more harmful impact when released at altitude, than if they were just released on the ground. This concern provides incentive to pursue research in electric and hybrid-electric aircraft. In the same manner that electrification of automobiles puts the choice of fuel at the electric generation plant, electrification or hybrid electric propulsion of aircraft moves the propulsion of aircraft away from dedicated fossil fuels to a variety of options on the ground. True, currently most power plants in the United States burn too much coal to make a net reduction – see this AIAA paper by Dr. Terry Thompson and me showing how our national electricity generation plant constrains how much electric aircraft can reduce GHG emissions (http://goo.gl/cA9qXk). The bright spots are 1) California, whose power plants make electric propulsion a GHG winner in the near future; and 2) growing independent action by individual states and generation facilities to voluntarily move away from coal to renewables and to natural-gas fuels.
A number of companies are currently working to launch accessible small electric aircraft. They include ESAero, Joby Aviation, Zee Aero of California, Evolo from Germany, and Ehang of China. (See images.) The X-57 pictured is currently under construction, as a prototype, with NASA funding. The Evolo and Ehang prototypes are currently doing flight testing. All three are completely electrically propelled aircraft, with electric motors instead of the piston engines commonly found in small personal aircraft.
Aircraft piston engines are large and expensive and subject to wear and overhaul; most of the nearly 200,000 small privately owned “general aviation” (not commercial) aircraft in the US have only one engine, due to the expense. Generally a small single-engine fixed wing aircraft has 150-180 hp. Two engines are safer and provide more power than one, but are much more expensive. When an engine fails in a single engine aircraft and won’t restart, the pilot has only one option: land quickly. Depending on how high you are, generally your landing spot is within view, because gliding range is a few miles at best. Two engines buy the pilot another option: to continue flying. The new electrically powered aircraft tend to have more than eight motors and propellers, because going to electric motors opens up whole new design options, due to the scaleability of electric motors. There is a lot of weight and mass in the piston engine that produces the combustion and propulsion. Electric motors have no pistons, so they weigh less and take up less mass, which allowed aircraft designers to experiment with putting them anywhere on the aircraft, without taking a weight penalty in the airframe structure. Laws of aerodynamics dictate that the part of flight requiring the most power is the takeoff. For gas piston engines, size is driven by take-off power, meaning a big engine is not needed in cruise. With multiple electric propellers, some of the propellers can be turned off after takeoff. Being able to size different motors differently, with different pitch or rotation and location has meant that electric aircraft designers have more freedom to shape flows and vortices and reduce aircraft noise. Joby Aviation’s S4 has 18 propellers producing about 300 hp. As a benchmark, most automobiles are also in the 150-200 hp range.
Aircraft innovation generally starts with small aircraft and works its way up the size scale. A few years ago (2011-2), prize competitions for student design teams such as the Green Flight Challenge were being posed to find out if electric flight was achievable in prototype aircraft. The ungainly 2-cabined Pipistrel Taurus G4 achieved Green Flight goals (https://www.youtube.com/watch?v=O0JhS_HuMvg), and aerodynamic engineers went to work on streamlining an electric aircraft built from scratch. The achievements in small electric aircraft are prompting research at large civil aircraft manufacturers. Airbus is building the E-Fan, a small electric aircraft with two 60 kW motors and Lithium-ion battery packs sufficient for 1-hour flights.
But there are technical challenges on the way to scale. Currently (July 2016) Siemens offers one of the largest motors available for flight, at 261 kW, or about 350 hp. The Siemens motor turns at 2500 rpm, making it fast enough to use without gearing. The first challenge is motors of sufficent size and scale to fly a 144,000 lb (weight of a B737) aircraft instead of the 4000 lb aircrafts being prototyped. Scaling up the motor depends on materials and on electric field research. Current aircraft electric motors are built around the same central-shaft architecture as a turbofan; the electromagnetic field creates a rotating flux around the shaft. Some research is looking at expanding the flux field to a wider diameter, such as the blade diameter. Small business jet aircraft like the Lear 35 weigh around 9000 lbs. empty, which is only twice as large as the Siemens motor supports; so business jets may be achievable platforms for electric motors in the next five years.
Near term for the large transport aircraft (for example, B737 scale), research in this area is looking at hybrid electric drive train propulsion to power the fan blades, where there are two power sources driving the propeller, or a hybrid engines and motors. There is a weight penalty for two propulsive sources, which has to be counterbalanced by a desirable payoff. Those payoffs are the cheap cost of electricity, fuel cost savings, and carbon tax avoidance. Electricity is about a fourth to a tenth of the cost of fossil fuels when converting by useful energy output. The fuel trade-off is better for small aviation; small aircraft often pay two to two and a half times as much as jets for a gallon of fuel.
Assuming that the motor can go bigger and several can be put on the plane, or that electric motors are used in combination with smaller gas turbine engines, the next challenge is the battery pack. Energy density of lithium-ion batteries, the current state of the art for batteries is about 4 kW of power per kg of weight. A B737 burns about 2000-3000 lbs of fuel in an hour of flight, but would require about 40,000 lbs of batteries to power the same flight. Research in this area is accelerated by portable electronics devices. It’s estimated that a doubling of energy delivered per kg using magnesium-based batteries is at least 15 years out. A quadrupling of energy density is needed to power purely battery-powered large civil transport; but research to get to a doubling may accelerate the progress to the next breakthrough. Battery research has advanced greatly through the widespread availability of electron microscopes, to understand the aging processes of new compounds, and the ability to manipulate very small compounds. The research being done today on battery chemistry was literally not possible in the 1970s.