Solar Impulse: First in Solar Flight

Solar Impulse - Solar Airplane
Solar Impulse 2 over San Francisco. Corbis via Getty Images / Getty Images

On 26 July 2016, pilot Bertrand Piccard landed a very unusual plane in Abu Dhabi, in the United Arab Emirates. The Solar Impulse Two was the first solar powered airplane to fly around the globe without using a single drop of fuel. This record is a great milestone in the search for transportation technology that does not rely on fossil fuels for propulsion.

The Planes: Solar Impulse 1

The project was initiated in 2003 by the Swiss adventurer Bertrand Piccard, who previously had been a copilot in the first navigation around the entire globe in a hot-air balloon. He was later joined by André Borschberg, an engineer and entrepreneur, in the building of a solar powered aircraft. Their work led up to a prototype named Solar Impulse 1. This first effort showed that long flights were possible with an airplane powered by solar energy captured by photovoltaic cells on the wings and stored in on-board batteries. Solar Impulse 1 completed flights from Spain to Morocco, and across the United States, breaking many distance records for solar-powered flight.

The Planes: Solar Impulse 2

Construction of the second prototype, Solar Impulse 2, began in 2011 and was funded by private corporations and the Swiss government. The plane is built as a single honeycombed carbon-fiber wing with a one-person cabin hanging below it. The total wingspan is 208 feet (16 feet longer than a Boeing 747), and the entire upside of the plane is covered by 2,200 square feet of photovoltaic solar panels.

The energy collected by the panels is stored in lithium polymer batteries. These cells power four electric motors, each generating 10 hp transferred to a propeller. The entire plane weights about as much as a Toyota Camry. 

The plane is flown with a suite of state-of-the-art electronics, including control instruments, navigation tools like GPS, and communication equipment, both satellite and VHF. Besides the electronics, the cabin is very basic. Surprisingly, it is not pressurized, even though the aircraft routinely reached altitudes above 25,000 feet. Insulation keeps the inside air warm enough. The single seat reclines, allowing the pilot 20 minute naps when he needs it. A series of alarms wakes him up if the flight controls need immediate input, but otherwise a simplified autopilot system can maintain flight altitude and direction on its own.

The Itinerary

The solar aircraft started its now famous circumnavigation in Abu Dhabi on May 9 2015, heading east.  The entire trip took 17 separate legs, with pilots Piccard and Borschberg alternating at the commands. Frog-hoping through Asia, the plane stopped in Oman, India, Myanmar, China, and then Japan. After a month-long wait for favorable weather, Borschberg flew for almost 118 hours straight to reach Hawaii, at the same time establishing a new endurance flight record.

Damaged batteries grounded the adventurers for 6 months, time needed for repairs and to wait the return of favorable conditions in term of weather and amount of daylight. On April 21, 2016 the Solar Impulse 2 made the crossing from Hawaii to Mountain View (California) in 62 hours, and eventually reached New York City. Traversing the Atlantic Ocean took 71 hours, with a landing in Spain. The rest of the trip consisted in one long flight from Spain to Cairo, in Egypt, followed by the triumphal arrival in Abu Dhabi, 16 and a half months after their departure. Total flight time was 23 days, at an average speed of 47 miles per hour.

The Challenges

Besides the obvious technical challenges involved in building the plane, the Solar Impulse project had to deal with some interesting issues. For example:

  • To accommodate the absence in solar energy during nighttime, the aircraft reached its maximum flight elevation during the daytime, at the same time filling its storage batteries. At night the plane slowly lost altitude, essentially experiencing a powered glide with the motors on but minimizing the draw of energy from the batteries.
  • During the 5-day long Japan-to-Hawaii leg of the trip, the aircraft batteries overheated as there was too much insulation around them to allow for proper cooling during heavy power demand. Replacements had to be ordered, delaying the flight a full 6 months. The new equipment included a fail-safe system to protect the batteries in case of over-heating and some pilot-operated venting ports to increase airflow when needed.
  • As long-distance fliers know, remaining seated for long periods of time puts people at risk from developing potentially dangerous blood vessel clots. Without even the option of walking the aisle to the bathroom, the Solar Impulse pilots relied on a series of seated yoga and Pilates movements for exercise and stretching. 
  • About that bathroom, where did the pilots go? Well, they didn’t have to go anywhere, as a trap door was judiciously positioned in their seat.

Environmental Significance of the Solar Impulse 2 Flight

The Solar Impulse airplanes are not only record-chasing vehicles, but more importantly technological development and innovation platforms. Many of the corporate sponsors of the project developed technologies and tested them on the planes. For example, engineers formulated protective chemicals to keep the solar panels as effective as possible under harsh conditions. These types of innovations are already being re-purposed for other sustainable energy projects.

Similar engineering breakthroughs have been made with regards to the lithium-polymer batteries used on Solar Impulse 2. There are many commercial applications for these energy-dense batteries, from consumer electronics to electric vehicles.

Solar powered flight is not going to transport people commercially anytime soon, but is likely to be achieved by small, lightweight, automated aircraft capable of being airborne months or years at a time. These solar drones will be able to offer similar services as satellites but for a fraction of the cost.

Perhaps the most important contribution of the Solar Impulse project, however, was the circumnavigation record as a stunning demonstration of the immense potential of solar energy. It provided powerful inspiration to the engineers (and future engineers) developing creative solutions for our carbon-free energy future.