Powered by the Sun, Solar Impulse Takes Flight

Solar Impulse is a one-of-a-kind aircraft whose technology highlights the exclusive use of sun power for a transcontinental flight (Fig. 1).

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Fig. 1. The wing of the HB-SIA is filled with solar cells used to charge the batteries that power the motors and cockpit electronic systems.

“Our airplane is not designed to carry passengers, but to carry a message,” said pilot Bertrand Piccard. Pilot Andre Borschberg noted that “from the very start of the project we understood that our primary goal was to save energy.”

The Solar Impulse latest flight started on May 3, 2013 in San Francisco, and made stops in Phoenix, Dallas, St. Louis, Cincinnati, Washington DC before landing in New York City. The 3,511 journey took 105 hours and 41 minutes in the air. Average speed was 33.14 mph. Piccard and Borschberg alternated piloting the airplane.

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Version HB-SIA of the Solar Impulse was actually a prototype of what is envisioned as the forerunner of an airplane that will circumnavigate the world, the HB-SIB, whose construction began in 2011. The HB-SIB will have a larger cockpit that will allow the pilot to fully recline during flights lasting four to five days. It will have an increased payload, its electrical circuits will be isolated to enable flights in rain, and system redundancy will improve reliability. Its advanced avionics will allow trans-oceanic travel. Wingspan of the HB-SIB is 262.5 ft. compared to 208 ft. for the HB-SIA.

Solar Impulse HB-SIB requires development of new materials and new construction methods. For example, Solvay has invented electrolytes that increase the batteries energy density. Bayer Material Sciences is allowing the project to make use of its nanotechnologies. Decision is using carbon fibers that weigh less than any previously seen. The carbon fiber sheets are only 25 g/m², which is three times lighter than paper. By using carbon fiber construction the aircraft will weigh about the same as an average automobile.

The aircraft will undergo the same structural strength and vibration testing as the HB-SIA. Flight testing is scheduled for the Spring 2014, and the round-the-world flight is planned for 2015.

Solar Panels

Currently, there are about 12,000 solar cells on the HB-SIA’s wings and horizontal stabilizer. The HB-SIB will have 15,000 of them. This is more impressive than it appears because the panel building process is all hand made. Plus, the cells are 150 microns thick and rated at 45 kW, peak power. Cells were selected for their lightness, flexibility and efficiency, which is 22%. SunPower Corp. provides the cells, which are then meticulously put together, one by one. The process begins when a new batch of solar cells arrives, then are tested three times to verify their output voltage.

After 70 healthy cells are promoted they are strung together in series, providing 300 V. Following this is a layering process that places a plastic resin under a glass foil, and so forth, eventually laminating the strings. The “sandwich” is then cooked at 95 °C for seven hours before being placed on a mold that bends the cells into the desired shape, slightly rounded for the wings. Care is taken to ensure that nothing falls on the panels during the curing process. Any microscopic piece of hair, dust or insect could potentially cause a failure, rendering the panel unusable. It takes 10 to 15 hours to make a panel and 48 are needed for the HB-SIB.

A thin fluorine copolymer film protects the solar cells. These cells are brittle and have no mechanical resistance, but when covered with this film, they can be molded into the wing curvature without breaking. The resin is UV resistant, waterproof and only 17 microns thin.

Batteries

Solar panels charge the batteries many times during a typical long flight. Because they are part of the airframe their weight is critical. Plus, their efficiency and lifespan impact the success of the mission. A set of unique batteries will be employed in the HB-SIB. Made by the Korean producer, Kokam, they required extensive research to push their performance limits. The key lies in the complex chemical formula that has improved that battery’s oxidation issue, because they age faster and lose efficiency when oxidized. This technology is two years ahead of the industry, but it is the most that can be disclosed at this time. Ameliorating this usual ageing process allows Solar Impulse to have batteries able to guarantee 2,000 flight hours for the HB-SIB, compared with only 500 for the HB-SIA. Energy density of the HB-SIA batteries is 260 kW/kg (348 HP). Fig. 2 shows the battery compartment on the HB-SIA.

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Fig. 2. Battery compartment used for each motor in a gondola.

Each of the battery cells is a little different from the other. These cells do not like stress, extremely hot or cold temperatures impact their performance. Some days a cell is more efficient than others because of certain parameters that were applied the day before. Lithium-polymer can be charged up to 4.35 V, but that doesn’t necessarily means that on Friday it will exploit its full potential as it did on Thursday. Battery output voltage depends on the temperature conditions as well as the type of charge and discharge.

To ensure that lithium-polymer cells are fit for the aircraft, they must undergo numerous tests. These tests check the cell’s behavior in extreme temperatures, how much energy they can store, and for how long. Tests are also done for a better understanding of their reaction to different situations. The challenge is to find the optimum balance between usable lifespan and energy, which depends on temperature, cell voltage and current. For example, it was found that keeping a constant temperature or 25 °C inside the motor gondolas provides the ideal environment for optimized battery efficiency.

Moving Along

Solar Impulse uses Etel torque motors.Torque motor quality operating in extreme environmental conditions is an important characteristic. Performance must be maintained optimal and constant during non-stop flights for many hours. In addition, motor efficiency must be high to maximize use of precious solar/battery energy. These motors can reach efficiencies of 96%, and they weigh less than other types of motors.

The power plant for the HB-SIA consists of four torque motors, each of which is powered by 21 kWh (300 V) lithium-polymer batteries, providing 7.5 kW (10 HP) for twin-bladed propellers. Batteries and associated with each of the four motors are housed in gondolas under the wing. The gondola also includes a power management subsystem that controls charge/discharge and temperature (Fig. 3).

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Fig. 3. Each of the four gondolas house batteries and motors, along with a power management subsystem that controls the battery’s charge/discharge and temperature.

Thermal insulation conserves the heat radiated by the batteries to keep them functioning at very low temperatures encountered at high altitudes. Each motor is fitted with a reducer that limits propeller rotation to a 3.5 meter diameter within the range of 200-4,000 rpm.

Over an ideal 24-hour cycle, HB-SIA motors deliver a combined average of about 8 HP (6 kW). That’s roughly the power used by the Wright Brothers aircraft in 1903. The HB-SIB will employ more powerful motors and batteries.

Flight Instruments

Fig. 4. Instruments in the cockpit monitor aircraft power and enable the pilot to control the airplane, as well as send real time data to the Mission Team.

The cockpit’s electronic instruments (Fig. 4)have three main functions:

· Monitor the power supplied by solar panels to the motors and batteries.

· Communicate to the pilot the necessary information for controlling the airplane.

· Provide real time information to the Mission team that is monitoring the aircraft’s flight path and behavior from the ground.

A revolutionary new instrument was developed for the HB-SIA with collaboration from Omega and Claude Nicollier, who heads the Solar Impulse test flight team. It primary function is to inform the pilot within an accuracy of one degree, the bank angle of the aircraft (the turn or change of direction when it banks or inclines) must be below five degrees. For this reason the Omega instrument is connected to gyroscopes that are used to provide stability or maintain a fixed orientation.

Another key function of the Omega instrument is to inform the pilot of his real flight direction. This is important because the HB-SIA’s huge wingspan, combined with lightness, makes it very sensitive to air movements, especially crosswinds, which can cause it to drift. LEDs on the front panel indicate the flight direction to one degree. This also helps align the aircraft in the axis of the runway for landing.

On-board electronics have been optimized to combine lightness and maximum efficiency. In flight, the electronic systems undergo significant temperature variations between low and high altitudes; this must not affect their performance. Therefore, prototype circuits and devices have been destruction-tested with the test results used directly in the manufacturing process of the final systems.

Commendation

Based on what I have seen in the design of this aircraft, the team responsible for its success should be commended. They are innovative and paid great attention to even the smallest details. In addition, the test procedures employed in the design and manufacture of the aircraft were important in the ultimate success of the project. It is likely that much of this technology can be transferred to other “earthly” electronic systems.