How the Solar Impulse team took alternative energy from dream to reality

While most of the country is still struggling with the concept of solar power on land, one team was busy using the panels to power something a little more ambitious: a plane.

The program, called Solar Impulse, is devoted to designing a plane powered entirely by solar panels, in hopes of demonstrating the potential of alternative energy.

This isn’t the first attempt at flying the plane, though it’s the longest journey they’ve had yet. In 2010, the Solar Impulse team managed a 26-hour day/night flight in a rough prototype after several shorter, daytime flights and years of research. In 2012, they began a flight across the country attempting to fly—via predetermined stops—from California to New York. In 2014? They take on the world.

The birth of an idea
In 1999, Betrand Piccard—who would go on to become the initiator behind the idea and one of two pilots—made the first ever nonstop flight around the world in a balloon and became inspired to find a way to travel around the world without using any fossil fuels. Piccard, a doctor, psychiatrists and aeronaut, found his dream in the Solar Impulse project.

Engineer Andre Borschberg, CEO of Solar Impulse and a former pilot in the Swiss Air Force, joins the 80-person team as the second pilot, though only one pilot is in the plane at a time. They began the project in 2003 with a budget of 140 million Swiss francs (about 147,741,720 US dollars).

The plane, called HB-SIA, has a 208 foot wingspan and weighs in around 1600 kgs (3527.4 lbs)—about the weight of a mid-sized passenger car—with an average speed of about 45 miles per hour. The entire flight plane included an initial takeoff in California with stops in Arizona, Dallas/Fort Worth, St. Louis and Washington D.C. before a final landing in New York City.

The challenges of a solar plane
Solar power—like any alternative energy—comes with a unique set of challenges, whether it’s on a house, in a field, or on a plane. The two main engineering challenges of a solar powered plan are energy and weight. Generally, the energy obstacles can be broken down into three groups: energy capture, energy storage and energy savings. For Solar Impulse, many of the solutions came in the form of a partnership with Solvay, a global chemical company based in Brussels.

Gathering the energy
The solar panels on the plane had to be lightweight and incredibly efficient. Designers needed the panels to be impervious to mechanical deformation, UV resistant and flexible, says Claude Michel, a chemical engineer and head of the Solvay/Solar Impulse partnership. The solution was a thin, lightweight film made from Solvay’s Halar ECTFE copolymer used to protect the photovoltaic cells from stressors and allow maximum sunlight. The cells themselves were produced by SunPower, a California company that specializes in high-efficiency solar cells.

To further improve the efficiency, the Solar Impulse team used a partially fluorinated polymer, which comes in tape form, to bind the cells together. In addition to being as stable as fluropolymers in harsh environment, the tape fills tiny holes in between the cells and improves the aerodynamics of the plane.

“The solar generator is made of 11628 unit mono crystalline Si cells displayed on the wings and on the rear stabilizer,” says Michel. “The total surface area of the solar generator is 200 square meters and the yield of these solar cells is just above 22 percent.”   

Storing and saving energy
The plane, equipped with a total of 400kg battery packs located in the engine gondolas, must store enough energy to fly through the night, according to Michel. Each of the gondolas (also called engine housing) contains one 100kg battery pack, an energy management system (EMS), an electric engine and propeller. The EMS controls whether the energy is sent directly to the engines or to the batteries.

“The electricity produced through the solar panels has to be stored in batteries so that Solar Impulse can fly through the night. The challenge was to increase the “energy density” of these Li batteries,” says Michel. “Solvay proposed new materials for the binder that makes the electrodes. We also proposed a chemical component for the electrolyte. With these materials, Solar Impulse was able to improve the energy density of the batteries by 50 percent.”

The pilots also use more creative means to utilize potential energy for the plane. At the end of the day, the batteries are full and the plane is at maximum altitude. The setup allows for three to four hours of gliding—and no energy usage—followed by eight hours of battery-powered flight.

Keeping it light
Obviously, keeping the plane light was a huge challenge that goes hand-in-hand with conserving energy.

“Weight was the obsession throughout the whole development phase. We defined technical objectives in terms of performance and the development focused on how to meet these objectives (mechanical, UV, ageing resistance, thermal insulation, energy yield, energy density, etc) at the lowest possible weight,” says Michel. “We also tried to find ‘passive solutions’ that did not require additional equipment. For instance, the temperature control in the cockpit (for the pilot) and the batteries housing is achieved using high performance insulation foams without using a single watt of energy.”

The wing spar, cockpit, engine housing and equipment parts were all designed with weight in mind. Materials for these structural elements are ultra-light foams or advanced polymers—mainly special polyamids, polysulfones, or polyethers—that replace metal parts wherever possible, says Michel.Specifically, the wing spar is made from a lightweight paper honeycomb between carbon fiber and coated with aqueous solution polyamide-imide for strength.

One downside to the solar plane is its construction reduces resistance to bad weather including cross winds, turbulence, rain or humidity, so the HB-SIA only flies in good weather.
“There is a low quantity of energy available for propulsion. It has a wingspan to weight ratio in which the wings are 63 meters for a weight of only 1600 kg,” says Michel. “It is a fully electrical plane, so it has to be protected against humidity.”

Lessons learned
“During this project we learned that the best source of renewable energy is just sparing energy,” says Michel. “When we have only limited amounts of energy available, we have to use it in a smart way. This makes two elements key, lightweight and low speed.”

After a successful landing of the solar plane at its final destination of JFK, the team will continue working on the HB-SIB, a more robust version of the HB-SIA, to take on a trip around the world in 2014.