Solar Impulse 2 (Si2) is not the first solar airplane, but it is the first to fly day and night, without any fuel, only using energy stored in its batteries.
The brainchild of Bertrand Piccard and André Borschberg, Si2 is the first solar airplane to have achieved an oceanic crossing: 5 days and nights from Nagoya, Japan, to Kalaeloa, Hawaii. Piccard, a medical doctor specialized in psychiatry, explorer and aeronaut, who made the first non-stop round-the-world balloon flight, is the initiator and chairman. Borschberg, an engineer and graduate in management science, a fighter pilot, and a professional airplane and helicopter pilot, is the co-founder and CEO.
Piccard and Borschberg, with aspirations to make Si2 the First Round-The-World Solar Flight, powered only by the sun, with no fuel or polluting emissions, set out to send one, clear message: if an airplane can fly several days and nights in a row with no fuel, then clean technologies can be used on the ground to reduce our energy consumption, and create profit and jobs.
If we utilize the plane’s technologies on the ground, we could potentially halve our world’s energy consumption, save natural resources, and improve our quality of life. This message is being spread by the pilots to the general public, students, key decision-makers, and entrepreneurs all over the world.
The plane looks a bit unusual, which the team thinks will help further spread the message of the project worldwide. The wingspan of a Boeing 747 Jumbo Jet, the weight of a family car, the power of a small motorcycle, Si2 is the largest aircraft ever built with such a low weight. Let’s break it down a bit:
Energy + Batteries
Si2 has more than 17,000 solar cells, collecting up to 340kWh of solar energy per day and representing 269.5 m2! The energy collected by the solar cells is stored in lithium polymer batteries, whose energy density is optimized to 260 Wh / kg. The batteries are insulated by highdensity foam and mounted in the four engine nacelles, with a system to control charging thresholds and temperature. Their total mass amounts to 633 kg, or just over a quarter of the aircraft’s allup weight. In order to save energy, the aircraft climbs to 8,500m during the day and descents to 1,500m at night.
Motors + Speed
The solar airplane has an average power over 24 hours of a small motorbike (15 hp) with a maximum power of 70 hp (four 17.5 hp engines). Four brushless, sensorless motors, each generating 17.4 hp (13.5 k), mounted below the wings, and fitted with a reduction gear limiting the rotation speed of a 4 m diameter, twobladed propeller to 525 rev / min. The entire system is 94 percent efficient, setting a record for energy efficiency. Si2 can fly at the same speed than a car, between 36 km/h and 140 km/h. At sea level: minimum speed of 45 km/h and maximum speed of 90 km/h. At maximum altitude: from 57 km/h to 140 km/h.
Energy Efficiency
The systems on the airplane for collecting and storing energy have also been designed to minimize energy loss. The solar cells, batteries, and motors have record beating energy efficiency: 23 percent for the solar cells, and 97 percent for the motors, which only lose 3 percent of energy against 70 percent for standard thermal motors.
So where is Si2 today? Well, they’re a bit behind due to weather conditions, “Today, we have decided to postpone the flight from Lehigh Valley International Airport to JFK, New York City due to worsening weather conditions. The weather analysis prior to the takeoff indicated a change in our predictions, with showers marking the flight path to New York. Prior to our decision to give the green light to fly to JFK, we were aware of potential showers closing in behind the Si2 flight path, however they came faster and have intensified beyond what we could have anticipated.”
Let it be known, however, that this globetrotter is not new to the travel game. Si2 has completed 13 flights in total thus far, with just a couple flights left until victory.