Good Day, Sunshine – The Amazing Solar Impulse 2

Every now and then something new comes along that causes you to want to dig down and find out a little more information. Tearing my attention away from Caitlin Jenner for a moment, I thought I'd take a closer look at the remarkable Solar Impulse 2 – an airplane powered solely by solar energy.

Hmm…. solely solar… alert an alliteration aficionado. But I digress.

At any rate, the SI 2 is engaged on a round-the-world trip, temporarily interrupted in June when the plane was stranded in Hawaii after suffering irreversible battery damage caused by overheating during the five-day flight from Japan. Assuming the money can be found to repair the damage and sustain the support team over the winter (€20M – a rounding error for The Donald, surely?) it will resume in April 2016.

While we wait, let's check out some of the technology that enables a human to traverse oceans aided by the sun, while avoiding the soggy fate of legendary aviatorIcarus.

Design : Including the Solar Impulse 1 prototype, the project has taken 50 engineers and technicians and 80 technology partners over 12 years from feasibility study to concept, design and construction. For the latter stages of the project. there are also 22 mission controllers. The principal partners include Solvay, Schindler, Omega and ABB.

Figure 1

Look Ma, it works! Solar Impulse 2 leaving Oman  (source: Solar Impulse)

Look Ma, it works! Solar Impulse 2 leaving Oman (source: Solar Impulse)

Construction : In order to maximize the aerodynamic performance, the plane has a wingspan of 236ft – greater than that of a Boeing 747 and only 3 feet less than the Airbus A380, the world’s largest passenger plane.

The airframe is made of composite materials: carbon fibre and honeycomb sandwich. The upper wing surface is covered by a skin consisting of encapsulated solar cells, and the lower surface by a high-strength, flexible skin. 140 carbon-fiber ribs spaced at 18-inch intervals give the wing its aerodynamic cross-section, and also maintain its rigidity. The whole plane weighs 5070 lbs., only about 100 lbs. more than a Chevy 1500 cargo van. Go (here to see an infographic about the structure.

Propulsion : Four sensorless BLDC motors, each generating 17.4 hp (13.5 kW), are mounted below the wings; they are fitted with a reduction gear limiting the rotation speed of the 13-ft diameter, two-bladed propeller to 525 rpm. The entire system is 94% efficient, setting a record for energy efficiency. Each engine's average power output over a 24-hour period is 15hp, about the same as a small outboard motor.

Such modest power gets the job done, albeit very slowly – Solar Impulse flies between 20 (23 mph) and 77 kts (88mph). This varies with altitude: at sea level, the minimum and maximum speeds are 20 kts and 49 kts (56 mph) respectively; at maximum altitude that increases to a minimum of 31.5 kts (36 mph) and a maximum of 77 kts.

Solar cells : The Solar Impulse 2 uses 17,248 monocrystalline silicon photovoltaic cells, each 135 microns thick, mounted on the wings, fuselage and horizontal tailplane, providing the best compromise between lightness, flexibility and conversion efficiency (23%). The cells take up an area of 2900 ft2 and can produce up to 340kWh of energy per day.

Figure 2

Installing the solar panels (source: Solar Impulse)

Installing the solar panels (source: Solar Impulse)

Batteries : The energy collected by the solar cells is stored in lithium polymer (soft-pack batteries), manufactured by Air Energy (Germany) with specific energy of 260 Wh/kg. The batteries are insulated by high-density foam and mounted in the four engine nacelles, with a system to control charging thresholds and temperature. Their total mass amounts to 1395 lbs., or just over a quarter of the aircraft’s gross weight.

In order to save energy, the aircraft climbs to 28,000 ft during the day to capture the maximum amount of solar energy, and descends to 5000 ft at night where it operates at reduced speed.

Cockpit : The cockpit is 134 cubic feet in size – just enough for the pilot plus essential equipment such as a parachute, life raft, and 6 oxygen bottles since it's unpressurized and unheated, with temperatures during flight ranging from -40o C to +40o C. There's also a fully-reclining seat for a cat nap or two; during a typical 24-hour flight cycle, the pilot rests 8 times, averaging between 5 and 20 minutes per session.

The cockpit must also contain 5 lbs. of food, 3 quarts of water, and a quart of sports drink per for each day of flight – up to six days total. I know you were wondering about bio-breaks on such a long flight (admit it); let’s just say that gravity plays a key role in the process.

Figure 3

Cockpit instrumentation (source:

Cockpit instrumentation (source:

There's a comprehensive suite of instruments, as shown in figure 2: from left to right the panel contains fuses, throttle, power management, autopilot, motor control, flight instruments and communications. Although the plane has only a single seat, piloting the plane for days at a time with almost no sleep is too much for a single pilot, so there are actually two – Bertrand Piccard and André Borschberg – who take turns.

You can find out more about the Solar Impulse project by visiting their web site. In the meantime, there's the little matter of that €20 million shortfall.

How about it, Donald? You might tie up the geek demographic.

2 comments on “Good Day, Sunshine – The Amazing Solar Impulse 2

  1. eafpres
    August 2, 2015

    Thanks for the great summary of the SIP2 mission.  I read on Fox News about the current status, and it seemed the project team were trying to downplay the battery failure.  They also quoted Bertrand Piccard, calling him an explorer and Chairman of Solar Impulse:

    “Piccard sees the high-tech plane as proving the potential of renewable energy and clean technology. “This was my vision when I created that project – it was to have an airplane that can fly with no fuel,” he told, during a phone interview last month. “This is fantastic, to prove that clean technology can achieve [the] impossible.””

    It is surprising as well, the article paraphrsed the team as saying “the temperature effect of quick ascents and descents in tropical climates was not properly anticipated”.  Using the figures you cited I roughly estimated 1000 man-years in the project.  Even if I'm off by 10, how can you spend 10s to 100s of man years on a project specifically designed to demonstrate the validity of a solar powered continuously operated vehicle, and overlook temperature issues that might adversely affect the batteries?

    I think this is comparable to the Challenger disaster.  However, even in that case the problem was likely anticipated and it was a series of poor decisions leading to launch that led to the catastrophic loss of the spaceplane and boosters.  

    Taking a less critical tone, I think this shows once again there is a lot to learn about the energy storage part of the equation for renewable energy.  I would rather have failures in a test demonstration than in commerially deployed systems.  Of course we have had a battery fire in a Boeing 787, another in a Tesla vehicle after hitting debris.  It can be hoped that this project proides invaluable infomation to make future designs more robust and commercial designs safer.

  2. michaelmaloney
    September 14, 2018

    It is indeed remarkable how far technology has leaped thus far. It is not cheap running an airplane but to know that there is now an alternative source of energy to run one, it does not get anymore exciting than this. We should still review the pros and cons of this new setup though, looking at the fact that the safety of the pilot and passengers still becomes the number one priority. It is not in any way similar to other much simpler tasks such as powering up a house on solar or driving a car because an airplane happens to be in the air. Thus, the level of risks involved is much higher and hence requiring much more work than others.

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