Beamed power for electric aviation

(A different version of this was published in Spectrum magazine)

Imagine it’s the year 2050, and you’ve booked a cross-country flight on an electric airliner. Your plane rolls down the runway and leaps into the air, engines buzzing with the power of an incredible battery. But as you rise above the airport, the battery starts to give out. It’s nearly out of charge. Your ascent slows, the engines quiet. Some passengers gasp.

You sit back in your seat, swirl your drink. They must be new to this.

Then, just as it seems like the plane can’t stay in the air any longer, a large antenna comes into view on the horizon. Energy flows invisibly from the antenna to the plane. The engines power back up, and the plane keeps climbing…

Beamed power for electric aviation

A crazy, futuristic idea. Something physics almost forbids, and the FAA certainly would. But compared to other options for decarbonizing aviation, is it that crazy?

Electric planes are a tough proposition. The best rechargeable batteries have effective energy densities ~20x lower than jet fuel. Since an airliner carries 25-50% of its mass in fuel already, an all-electric airliner would have to fill its entire cargo with batteries—no room for passengers, sorry—and it’d still barely make it a tenth as far as an ordinary jet. Given that the best batteries have improved only 3x in the past 3 decades, it’s safe to say that batteries won’t power commercial air travel as we know it any time soon ^[1].

So can we forget about storing energy altogether, and instead beam power to airplanes electromagnetically through the air?

Your thoughts might first run to shining high-powered lasers at planes, which is a federal offense for a good reason. Or you might think of airliners losing power and falling out of the sky the moment they climb above clouds or lose connection.

But microwave radiation can generally pass through clouds, and could be completely absorbed by receivers on the underside of planes, with essentially zero risk to passengers.

Microwaves can also be effectively collimated – formed into tight beams—so that they can run laser-like from place to place, using technology known as phased arrays. Phased arrays work on the principle of constructive and destructive interference. Throw two pebbles into a calm lake, and you see rays of taller and shorter ripples radiating into the lake where crests and troughs from each pebble overlap. Throw five pebbles in an even line, and you’d begin to see a directionality to the waves, perpendicular to the row of pebbles. Throw 50,000 pebbles in a perfect line, perfectly spaced, and keep throwing them in perfect rhythm… you get the idea.

The physics of phased array power transmission are governed by the diffraction limit—how well those electromagnetic “ripples” can be brought back together at a receiving antenna. Practically, that means power is transmissible when D1 D2 > λ R, where D1 and D2 are the sending and receiving antennae diameters, λ is the radiation wavelength and R is the distance over which two antenna apertures communicate.

For a first approximation, let’s consider a big airliner with a wing and body area of about 1000m² (D1 » 30m). Let’s guess that we need to send energy about 100km (the line of sight for a plane at 10km cruising altitude would be about 360km in flat terrain). At a 5cm microwave wavelength, that suggests that we could power the plane with base stations with diameter of D2 > 150m, spread out every 200km or so. That’s gigantic but perhaps not unreasonable. Imagine 3 or 4 football stadium sized arrays spread along the route from LAX to SFO, or between BOS and DCA.

But that’s the theoretical limit. How close have engineers gotten to this in practice? People have dreamed of beamed power—on earth, or even to earth from space—for more than a century. So there have been plenty of attempts. Microwave demonstrations have passed power from station to station and even energized drones in the air. In one of the more impressive historical efforts, NASA in 1975 sent 30kW of power over a distance of about 1.5km using a 26m dish.

But electronic technologies have improved since the 1970s. Older systems would have used cavity magnetrons, like in a microwave oven. The magnetron is a “whistle” for electrons. Breathe electricity in, and radiation comes out. That simplicity means a magnetron is efficient, but it can be challenging to orchestrate many magnetrons in the precise concert needed to collimate a beam of energy (50,000 pebbles, perfectly spaced, in perfect time…). In more recent years, advances in semiconductor technology have allowed a single oscillator to drive a large number of solid-state amplifiers in near-perfect phase coordination. Spread those amplifiers over an area, and space them at intervals of half the desired wavelength, and you can collimate a microwave beam much more tightly than was possible before.[ii]

In a recent demonstration hosted by Airbus, a startup called Emrod showed just how powerful this approach could be. Inside a cavernous hangar in Germany, Emrod used their phased-array technology to beam 550W of power 36m between two 1.8m arrays. Impressively, Emrod was able to keep over 95% of the microwave energy in a tight beam between the arrays. While the end-to-end power transmission efficiency was only 30% (solid-state amplifiers can be less efficient than magnetrons), Emrod claims that they will be able to achieve 80% efficient transmission over much larger distances. Extrapolating from Emrod’s projection, that could translate to efficiencies of about 40% over distances near the diffraction limit for a given emitter-receiver pair. In a nod to potential applications in aviation, Emrod has patented the integration of a lightweight, wide-angle antenna into an aircraft fuselage, with an eye on beachhead markets in EVTOL (electric vertical takeoff and landing) aircraft.

When asked specifically about power beaming airliners, Emrod suggested a configuration with a higher number of smaller ground stations which match the aspect ratio of a plane’s wings. In this setup, they said they’d expect achieve >70% efficient microwave power beaming to larger planes at cruising altitudes.

So beaming energy from phased arrays to airliners might not be entirely crazy. But hold on to your butts. There’s still some turbulence ahead for this idea:

Receiving the energy will be hard

The receiving antenna must intercept the power beam and rectify the signal back to useable electricity—hence it’s called a “rectenna”. The ideal rectenna would be approximately circular, and oriented perpendicular to the power beam—a flying saucer when it’s directly over the emitter, but more like an errant frisbee the rest of the time. Since the plane will ordinarily be seen front- or end- on by the array, the engineering constraints of aerodynamics (favoring small frontal area) and of power beaming (favoring large frontal area) are at cross purposes. Satisfying both will involve major tradeoffs.

That leaves us with a more realistic rectenna area of perhaps 125m2—the wing area on a Boeing 737. Like the transmitter, the rectenna will be covered with a large array of solid-state elements. Since the power we need in the array is on the order of 30MW (takeoff power for the 737), we have a power density of ~25W/cm2 on the wing. With elements spaced again at half-wavelength intervals, that means ~150W per each cm-scale element. This is perilously close to the maximum power density of any solid state power conversion equipment—the top mark in the IEEE/Google challenge was near 150W per cubic inch.

And don’t forget that this equipment will have to work in an electric field of ~7000V/m—about three times as strong as in an ordinary microwave oven (if a microwave oven creates a corona discharge between the tines of a fork imagine what will happen inside these chips). It will also have to weigh very little, and add very little aerodynamic drag. In short, while the rectenna may not involve impossible physics, designing a good one could be a fiendish engineering challenge.

Spectrum issues

So you’ve probably already deduced that the windows on our beamed-power airplane will have those same little grids as on the window of your microwave oven to keep you from cooking in your seat. But what do all those microwaves do to things outside the plane?

Surprisingly little, physically. A bird flying over our emitter station might encounter a heating of ~400W/m2—less than the heat from the sun on a sunny day. Emrod personnel said it’d take “more than 10 minutes to cook a bird”. True, there will be a focal point somewhere between the emitter dish and the plane with a much higher local flux, but it’s moving at a good fraction of the speed of a plane. Possibly this is not a showstopper.

Legal challenges might be worse—and not just from the Audubon society. If we still have to turn off our cell phones during takeoff to quiet radio noise, do you think the FAA will let us put the entire plane in a microwave oven? And the ITU – the global body that manages radio frequencies—might have even more to say. 30MW beamed to a plane would be a ~10¹⁰ times stronger signal than ordinary at 5cm wavelengths (a band currently reserved for amateur use). Even with 99% collimation, that leaves our leaked signals a factor of 10⁸ greater than approved transmit strengths. Surmountable, perhaps, but only with very good lawyers.

Infrastructure cost

So far, compared to what we need to invent for the planes, the system we’d need to build on the ground seems quite modest. Stadium-sized arrays (plus or minus the parking lots). We build bigger grid-linked ground arrays of high tech semiconductor gizmos on the regular: solar farms. But when you start to consider the massive electrical transmission lines that need to link these emitter facilities, the fact that many flights occur over mountainous terrain with less than 100km line of sight (necessitating either higher power, or more emitters), and that 1/3 of all airline miles occur over oceans, the infrastructure cost starts to swell. Clearly, financing this project would be an undertaking on the scale of the Eisenhower interstate system, or national high-speed rail.

But all great ideas sound crazy at first, right? 😊

There might be workarounds for many of these issues. Direct microwave-to-thermal power might obviate many of the rectenna challenges. A sawtooth altitude profile—with the plane climbing towards each emitter station, and gliding down afterwards—could help with the power density and field-of-view issues, as could larger, slower planes or flying-wing designs. Perhaps using existing municipal airports or siting emitters near solar farms could reduce some of the infrastructure cost. Perhaps companies like Emrod will find beachhead markets and radically streamline phased array technology as they scale. Perhaps.

Overall, concatenate some rosy assumptions, and beamed power looks undervalued on the market for decarbonized aviation ideas. At very least, it’s not the craziest idea.