So, having had a look at various aspects of the Solar Impulse 2 (SI2) journey, we are left with the ‘elephant in the room’ question: will solar energy ever be used to power commercial aircraft? If SI2 can be run on solar energy, then would it just be a matter of scaling up the technology to commercial levels? To answer these questions, we sat down with Dr. Alan Epstein, Vice President of Technology and Environment at Pratt & Whitney.

Q: So, Dr. Epstein, let’s get straight to the main question. Does the success of Solar Impulse 2 mean that we could see fully solar-powered commercial aircraft in the future?

Dr. Epstein: First of all, the Solar Impulse journey is a remarkable feat and an inspirational one, as well. It is an historic technological and organizational achievement. However, using solar energy alone to power a full-size commercial aircraft is, unfortunately, neither technologically feasible nor physically possible. To put the question in context, all the energy produced by the SI2 aircraft wouldn’t be enough to power the onboard systems of a commercial aircraft, let alone the enormous fans needed to take off, climb and cruise close to the speed of sound an aircraft carrying hundreds, or even dozens, of people.

Q: So this is not simply a question of scale? Could a technology company not simply design an aircraft with enough solar panels on it to absorb enough power to power an aircraft of the appropriate size?

Dr. Epstein:  It isn’t a matter of technology. Even if perfect solar cells that weighed very little were available, there simply isn’t enough sunlight falling on the earth at any one place to supply sufficient power for an aircraft to fly at the speed and with the payload needed for commercially viable transport. And, making larger wings to provide more area for solar cells won’t help, since they would be heavier and produce more drag, thus needing even more power.

Q: So should the flight of solar impulse be seen purely as an impressive achievement in its own right, or does it have any implications for commercial aviation in the long term?

Dr. Epstein: I think the Solar Impulse team has been very clear that their project should be seen as inspirational, and I agree that it certainly is. That said, solar energy has great potential for reducing aviation’s impact on climate change. We must break our dependence on fossil energy for powering our airplanes. The obvious role of solar energy is through the growth of plants on the ground. This plant bio-mass can then be converted to jet fuel in a sustainable manner. At least five such fuels have been certified for use to date and proven on hundreds of commercial airline flights. The CO2 savings is about 80% now and can grow in the future with additional technology. The use of waste material, such as corn stalks left over after a harvest, looks especially attractive as it can reduce fuel cost, increase farmer income and not impact food prices. I think such bio-fuels will be an important contributor to aviation’s quest for environmental improvements in the years and decades ahead.