Don't Book Your Solar-Powered Flight Yet

• An around-the-world flight by a solar-powered airplane is a remarkable achievement, but it does not signal that solar passenger planes are the next big thing.
• Compared to other options, solar's low energy density makes it an especially challenging pathway for pursuing large cuts in the emissions from aircraft.

Earlier this week the pioneering solar-powered airplane, Solar Impulse 2, completed its record-setting circumnavigation of the Earth, returning to Abu Dhabi. Just a few hours earlier, the US Environmental Protection Agency announced its intention to regulate greenhouse gas emissions from aircraft engines under the Clean Air Act. Over the last dozen years I have written numerous posts linking stories like these together, but this is one case in which I sincerely hope these events were entirely unrelated. That requires a bit of explanation.

Let's start by acknowledging the engineering talent and sheer courage involved in the flight of the Solar Impulse 2 (Si2). The aircrew and designers deserve all the kudos they will receive; they have earned a place in aviation history. However, notwithstanding the prediction of pilot Bertrand Piccard that, "within 10 years, electric aircraft could be carrying up to 50 passengers on short to medium-haul flights," I am skeptical that this project will be the forerunner of solar-powered commercial flight in the way that Charles Lindbergh's transatlantic flight in 1927 led to the first non-stop commercial flight across the Atlantic in 1938.

There's no anti-solar bias involved in that statement, just an appreciation of the constraints that physics and geometry (e.g., the "square-cube law") impose on the amount of solar energy an aircraft can harvest during flight with anything like current technology. Energy density is an essential factor in the economics of commercial air travel.

According to the website for the Si2, the aircraft is approximately "the size of a 747 with the weight of a car." That should be our first hint that scaling up to the performance and capacity of today's jets would be an even bigger challenge than the one these folks have just completed. During the course of its journey, which entailed over 500 hours of flight spread across 17 months, the Si2 collected and consumed electrical energy equivalent to a little over 300 gallons of kerosene-based jet fuel. By comparison, a Boeing 777, which is capable of carrying up to 400 people, burns an average of around 2,000 gallons of jet fuel per hour.

If you covered a 777's wings with the same 22%-efficient SunPower solar cells used by the Si2, they would generate the fuel-equivalent of less than 3 gallons per hour at noon on a cloudless day. Even allowing for the higher efficiency of electric motors compared to gas turbines, that is still orders of magnitude less than the energy necessary to push a fully-loaded jetliner through the sky at 550 miles per hour. (The Si2 averaged 47 mph.)

As the Financial Times reported, the near-term applications of solar-powered flight are likely limited to surveillance drones and other specialized platforms for which long-range fuel-free flight confers a big advantage. I could also envision lightweight, high-efficiency solar cells being used on next-generation commercial aircraft to provide auxiliary (non-motive) power, saving both fuel and emissions.

That brings me back to the EPA. The agency's stated rationale for targeting aircraft engines now is that they expect these emissions to increase in the future, and that reductions would lead to climate and health benefits. There's no mention of solar-powered aircraft, and I must trust that had nothing to do with their announcement.

The EPA's latest greenhouse gas inventory reported that in 2014 commercial and other aircraft accounted for 8% of US transportation-related emissions, and about 2% of all US emissions of CO2 and other greenhouse gases. It also showed that aviation emissions have fallen 22% since 2005.

Perhaps the growth they are worried about is proportional, rather than absolute, as emissions from electricity generation and other sources decline faster. However, compared to cars and light trucks that account for over 60% of emissions from transportation, and for which many emission-reduction options are available, aviation is a small and rather challenging focus for further reductions.  Those will likely rely on advanced biofuels, along with additional gains in turbine efficiency and airframe weight reduction. 

The website for Solar Impulse 2 acknowledges that its flight was intended to highlight the earth-bound applications of renewable energy: "Behind Solar Impulse’s achievements, there is always the same goal: show that if an airplane can fly several days and nights in a row with no fuel, then clean technologies can be used on the ground to reduce our energy consumption, and create profit and jobs." Solar-powered air travel for the masses seems pretty far off, and certainly not something we can count on for cutting our emissions.