Solar Impulse Airplane Lands in Arizona, Shows Strengths and Limitations of Solar-Powered Flight

The solar-powered airplane Solar Impulse 2 landed at a Phoenix-area airport on Monday night, completing the most recent leg of its around-the-world voyage to cheers from an enthusiastic crowd of well-wishers.

Slowly but surely, the electric airplane is making history: It is powered solely by the solar panels that cover its wings, and by the batteries those panels recharge during the day. Its main mission is to promote renewable energy, not to set speed records.

About 250 people, including local residents, aviation buffs, and student pilots turned out for the 9 p.m. landing at Goodyear Phoenix Airport, and the giant airplane didn't disappoint. Visible at first only by its bowed array of navigation lights, the giant craft floated down like a feather from a near-flawless night sky, the low whine of its engines audible just before touchdown. The plane's wingspan is 208 feet — nearly that of a Boeing 747.

It had departed Moffett Federal Airfield outside San Francisco 17 hours earlier, with pilot André Borschberg at the controls. A previously scheduled flight last week was scrubbed because of high winds.

It's the second time the Solar Impulse project has come to Phoenix — the first was three years ago in a previous, slightly smaller version of the plane that landed Monday. The project dates back to 2003, when the first model was built. Its organizers, including pilots Bertrand Piccard and Borschberg, hope to complete the mission to circumnavigate the globe this year following problems that kept Solar Impulse 2 in Hawaii for much of last year. The airplane will head eastward later this week, en route to a planned Atlantic Ocean crossing and, by July, eventual touchdown in Abu Dhabi, the United Arab Emirates capital where it kicked off its latest global attempt in March 2015.

"It was a beautiful flight," Borschberg said after emerging from the one-person cockpit to address the assembled fans and news media. "There is a lot of meaning in this landscape."

Not only is the Arizona desert a historical site as a longtime training ground for military and civilian pilots, but Arizona is beginning to use its solar resources for energy, he said.

Piccard, who joined Borschberg on the tarmac for a speech, said solar power has become a "mature" technology that people can use immediately in their lives.

"It's not science fiction," Piccard said. "It's the present."

With 14,000 photovoltaic cells on its wings, the airplane has gone as long as five days and nights without stopping or refueling. During the calm days that it's able to fly safely, Solar Impulse 2 typically climbs to about 22,000 feet — above the clouds that block the sun's rays — then uses minimal power during a long, slow descent with its excellent glide ratio of 20:1. A "virtual cockpit" web users can access during flights showed on Monday that the solar panels were generating 10 to 14 kilowatt-hours of power for the daylight portion of the trip to metro Phoenix.

The remarkable airplane shows off the success of solar power — but it also demonstrates the technology's weaknesses. Solar Impulse 2 had to be designed from the most lightweight materials possible and can be manned only by one person, because the sun doesn't provide enough energy to lift heavy things off the ground.

"I'm not saying solar airplanes will be transporting 200 passengers anytime soon," Piccard said in a separate interview.

In fact, it's doubtful that a solar-powered airplane will ever transport 200 passengers — not by directly using the sun's radiation, anyway.

Although Piccard, chair of the Solar Impulse organization, compares the plane's journey to Wilbur and Orville Wright's famed first flight in December 1903, a key difference is that solar power has an inherent limitation: Under perfect conditions, for a few hours a day, the sun lays down about 1,000 watts of power per square meter. Most photovoltaic panels convert that energy to electricity with only 15 percent efficiency, greatly reducing the potential.

But even if efficiency were 100 percent, Solar Impulse 2 could only carry about seven times its current weight of about 3,500 pounds. In other words, something as a big as a jumbo jet can never be solar-powered, says Peter Rez, a physics professor at Arizona State University who is working on a book about renewable energy, The Simple Physics of Energy Use.

Solar technology is "anything but 'mature,'" Rez told New Times, noting that the Solar Impulse 2 can't fly into a headwind, and must take full advantage of prevailing tailwinds to increase its ground speed. Piccard used west-to-east global wind patterns to his advantage in 1999 when he floated around the world nonstop in 19 days in a helium balloon. It would be much tougher to go the other way, Rez said.

"The average speed is about the same or less than the speed of the prevailing winds at the altitude it needs to fly at to be above cloud," said Rez, a general-aviation pilot. "That means even if they wanted to go east-west, they'd end up going west-east," but slower than they're going now in that direction.

Another problem: the heavy batteries that must be brought aboard. Solar Impulse staff say the airplane spends about two-thirds of its time powered directly by the sun and the other third on battery power. By weight, gasoline provides about 60 times as much energy than the best lithium-ion batteries now available, Rez said.

As Elke Neumann, a Solar Impulse spokesperson, conceded, the carbon footprint of the Solar Impulse project has been enormous. The program has been going on for more than a decade, employs more than 100 people and has cost more than $170 million, paid for by sponsors including Google and Moët Hennessy.

People could consider using less energy in the future instead of more, Neumann said when reminded that demand for air travel keeps increasing and aircraft manufacturers are building jets that can hold 500-plus passengers.

"Maybe we don't need so many people in one plane," she said.

Piccard echoes that idea, saying that "energy efficiency" needs to be as important to the future as renewable energy.

While using less energy seems like common sense, the reality isn't always so simple. In 2012, Arizona Public Service was allowed to tack on an extra fee because the investor-owned utility had lost money because customers were using less electricity. APS and other utilities in Arizona and around the nation have levied similar fees against solar customers, who — according to the utilities — don't pay their fair share of infrastructure costs. The fees on solar customers by APS and Salt River Project have led to declines in the number of rooftop installations over the past year or more in solar-resource-heavy Arizona.

The fight over those fees and the "net metering" payments made to solar customers for the electricity their rooftop panels generate grew to near-crisis mode last month, with solar companies on one side preparing to spend $3 million of their customers' money on a citizens' initiative that would lock in lucrative power-purchase prices, and utilities and conservative lawmakers on the other side preparing to spoil the initiative with their own ballot measure. Last week, Governor Doug Ducey stepped into the fray and, at his request, both sides dropped their plans for ballot measures and agreed to sit down at the negotiating table for the next two weeks.

Addressing the crowd in front of the solar-power-assisted airplane, Bertrand Piccard expressed his hope that Arizonans are willing to continue subsidizing solar power.

"We didn't plan to come here during the debate [over net metering], but if it helps, we'll be happy," the pilot said.