Solar-Powered Plane Faces the Human Factor

The solution, they say, is more technology.

Solar Impulse stands out among airplanes partly because its range is limited not by fuel but the stamina of the pilot.

The single-seat airplane’s cabin is unpressurized, so the pilot must wear an oxygen mask. It has no bathroom. With a grand total of 40 horsepower, the average cruising speed is 38 knots, or about 44 miles an hour, so progress is slow. Even if a pilot can tolerate 26 hours in the tiny cockpit — the record so far — an oceanic flight is hard to imagine.

“The seat is like a really bad economy seat on an airliner,” said Gregory Blatt, a managing director of the company, which is based in Lausanne, Switzerland. “The next version will be like a good business-class seat.”

That version, which is supposed to be able to fly five or six days at a time, will have a bigger seat, a cabin large enough to move around in, some plumbing and various other upgrades, like an autopilot. But that means a heavier plane that will require more efficient solar cells, electric motors and other improvements.

A Solar Impulse plane, an eye-catching proof-of-concept vehicle, arrived at the Smithsonian Air & Space Museum’s branch at Dulles Airport over the weekend from St. Louis, part of a photovoltaic barnstorming tour. The plane’s designers say the successor airplane, already under development, needs crucial but incremental improvements.

The four motors on the existing model, for example, are 90 percent efficient in turning current into torque, far better than most motors on the ground. But the new ones are 94 percent efficient, said Bertrand Piccard, chairman of the company and one of the two pilots. (Some inefficiency is desirable, because the energy that does not go into turning the propellers instead becomes heat, which is needed to keep the motors and the batteries warm at cruising altitude, around 29,000 feet. The batteries are tucked into the pods under the wings that hold the motors.)

The solar cells on the wings and the horizontal tail, from the SunPower Corporation of San Jose, Calif., are about 22.7 percent efficient, Mr. Blatt said. New cells will be better than that, he added. The existing plane carries 11,628 solar cells.

The existing cells are of a type already in use, but they are used in Solar Impulse in a new way. They do not cover the skin of the wing, but rather are the skin of the wing: thin monocrystalline cells, given a slight bend so the wing creates lift. The wingspan is huge, about the same as a Boeing 747. The whole plane weighs only about 3,500 pounds, so what aeronautical engineers call “wing loading,” or force of the air on the wing, is low.

The lithium-polymer batteries, from Kokam, a Korean company, are also a commercial product.

Ernest Moniz, the Energy secretary, who visited the hangar on Monday to look at the plane, praised it not only for using new technology but for integrating it into a useful package.

“Energy efficiency and system integration are enormously important areas,” he said. Efficiency is crucial because the propulsion system is not quite as big as one on a large motorcycle.

One innovation is the foam used in the structure of the plane, which has unusually small pores. Dan Reicher, a former assistant secretary of energy now at Stanford Law School, said that the foam had already found its way into refrigerators, because it is a better insulator.

The plane is scheduled to end its tour at Kennedy International Airport in New York early next month, depending on weather. Because it is so light and has such a large wingspan, it is vulnerable to gusts at low altitude, said André Borschberg, the company chief executive and the other pilot.

As a result, it takes off and lands at night, when there is no solar warming of the earth, which produces wind. And landing is an elaborate problem. Its main landing gear is a single wheel, and to keep its wing tips from scraping on the ground, the team deploys two people on electric bicycles to sweep in and hold the wings up as the plane touches down.

With a touchdown speed of 26 knots, about 30 m.p.h., this is not difficult. But it requires a runway without much other traffic, or it becomes like riding a bicycle on an interstate highway. At Kennedy, the plan is to circle at low altitude near the runway until there is a big gap in the traffic, Mr. Piccard said.