It’s quite an achievement (in fact, it’s a first) to fly an airplane across the United States using only sun as fuel. It’s even more remarkable considering that Solar Impulse covers a portion of the multistage cross-country journey after the sun goes down.
While the solar-powered airplane, dubbed HB-SIA, is flown in five stages from San Francisco to New York, Solar Impulse engineers and technicians are busy preparing the way for the next-generation version (HB-SIB). HB-SIA will be retired at the end of its journey across the U.S.
Capturing the sun’s energy during the day and storing it for later use at night are obviously critical engineering imperatives. For HB-SIB, Solar Impulse plans to use more solar cells to capture more energy and also use more efficient battery cells to store more energy.
HB-SIA, which was preparing for the final D.C.-to-New York leg of its cross-country journey as this article was being written, uses 11,628 cells from California solar cell producer SunPower. They are applied to the surfaces of the wings (10,748 cells) and horizontal stabilizer (880 cells) and serve not just an energy-collection function but also an aerodynamic one.
On HB-SIB, 15,000 SunPower silicon solar cells from the company’s Maxeon line will be used, each 135 u thick (about the width of a human hair) and rated at 22.7% efficiency.
At midday, Solar Impulse notes, each square meter of land surface receives from the sun the equivalent of 1000 W or 1.3 hp of light power. Over 24 h, the hourly figure averages 250 W/m². With 200 m² of photovoltaic cells and a 12% propulsion-chain efficiency, the HB-SIA plane’s motors achieve an average power of 8 hp. The company has not said what the figure will be for SIB.
The solar cells are strung together into 300-cell panels. The panels are encapsulated in films supplied by Solvay that allows them to bend in a slightly rounded mold so they can be fitted to the rounded wings. Solvay says the films (see image for list of Solvay products used in the airplane) present the best compromise between mechanical performance, radiation resistance, transparency, and mass. For HB-SIB, it was decided that both the top and bottom of the panels should be protected by film, not just the top as with HB-SIA, because the second-generation aircraft is expected to encounter worse weather conditions in its around-the-world flight.
Forty-eight solar panels are used on HB-SIB.
The electrical energy produced by the solar cells is stored in lithium-polymer batteries from Kokam, and Solvay supplies materials for the battery-cell electrodes and electrolytes, as well as battery housings. Solvay and Solar Impulse say the batteries for HB-SIB are improved over HB-SIA’s in terms of energy density, but they have not said by how much. The Swiss chemicals company was the first to join the Solar Impulse project, in 2004, and supplies 11 products that are used in about 20 applications and about 6000 individual parts that make up the structure of or equipment in the airplane.
The four batteries on HB-SIB weigh a combined 400 kg, a quarter of the aircraft’s total mass of 1600 kg. Other aircraft specifications are a wingspan of 63.4 m and a length of 21.85 m.
Another important partner for Solar Impulse is Schindler Group, although the Swiss company does not supply any parts for the aircraft. The elevator and escalator supplier provides support in the form of financial and technical assistance. “The insights Schindler and Solar Impulse have gained from working together with the top researchers and technical experts in their fields bring us closer than ever to achieving this common goal of developing trailblazing technology that safely moves more people with less energy,” Schindler’s U.S. unit said in a statement to Aerospace Engineering.
Testing of HB-SIB is coming to a close, and completion of full-airplane assembly is foreseen by the end of this year. Fight testing is scheduled for 2014. The around-the-world flight will take place in 2015.