Solar Impulse voyage abandoned after irreversible battery damage

The Solar Impulse 2 “adventure” is over. The lithium-ion batteries have overheated and have been irreversibly damaged. It may fly again in April 2016. Even if it does, that cannot – by any stretch of creative hype – be considered a part of this journey which has effectively been abandoned. By April 2016 a new plane could be built. The planned duration of the whole flight was to have been 5 months. But 4 months after starting in Abu Dhabi the plane has been grounded in Hawaii after completing 17,800 km of its planned 35,000 km journey.

The much hyped journey of Solar Impulse 2 as a solo flight circumnavigating the world was never really about technology or science. It was all about perceptions and PR. The project has been made into a symbol for solar energy  with claims that

Solar Impulse is the only airplane of perpetual endurance, able to fly day and night on solar power, without a drop of fuel. 

This revolutionary single-seater aircraft made of carbon fiber has a 72 meter wingspan (larger than that of the Boeing 747-8I) for a weight of just 2,300 Kg, equivalent to that of a car. The 17,000 solar cells built into the wing supply four electric motors (17.5 CV each) with renewable energy.

During the day, the solar cells recharge lithium batteries weighing 633 Kg (2077 lbs.) which allow the aircraft to fly at night and therefore to have virtually unlimited autonomy.

This flight has done little to demonstrate “perpetual endurance” or “unlimited autonomy”. Every flight starts with fully charged batteries (presumably charged from the grid and that would be fossil energy) and the solar cells need to top up the charge lost during night flying during the day. What has actually been demonstrated with the maximum flight length of five days is that – at best – the stored charge declined by 20% each day. Moreover the batteries suffered irreversible damage after a 5 day flight. That suggests fatal-flaws in the design of the plane, both in the sizing of the solar panels and in the design of the batteries. Firstly, either the power absorbed during flight has been under-estimated or the recharging capacity has. Secondly the immaturity of the battery technology and the inherent risk of over-heating during recharging has popped up again (as with Tesla, Volt and the Dreamliner).

Certainly it has demonstrated the endurance of the pilot flying solo and all credit to him for that. But it has not provided much in the way of new science or demonstration of engineering or technology. That solar cells work and solar energy can be converted into electricity is not new. That battery technology is still struggling to provide efficient, reliable and sustainable charging and recharging has been demonstrated but it did not need a plane to do that. That the cost and weight of extra cells needed to compensate for the lack of solar energy at night is still a major challenge, remains the state of the art.

Following the longest and most difficult leg of the round-the-world journey which lasted 5 days and 5 nights (117 hours and 52 minutes), Solar Impulse will undergo maintenance repairs on the batteriesdue to damages brought about by overheating.

During the first ascent on day one of the flight from Nagoya to Hawaii, the battery temperature increased due to a high climb rate and an over insulation of the gondolas. And while the Mission Team was monitoring this very closely during the flight, there was no way to decrease the temperature for the remaining duration as each daily cycle requires an ascent to 28’000 feet and descent for optimal energy management.

solar impulse route . based on BBC graphic

solar impulse route . based on BBC graphic

Log (BBC):

LEG 1: 9 March. Abu Dhabi (UAE) to Muscat (Oman) – 772km; in 13 hours and 1 minute

LEG 2: 10 March. Muscat (Oman) to Ahmedabad (India) – 1,593km; in 15 hours and 20 minutes

LEG 3: 18 March. Ahmedabad (India) to Varanasi (India) – 1,170km; in 13 hours and 15 minutes

LEG 4: 18 March. Varanasi (India) to Mandalay (Myanmar) – 1,536km; in 13 hours and 29 minutes

LEG 5: 29 March. Mandalay (Myanmar) to Chongqing (China) – 1,450km; in 20 hours and 29 minutes

LEG 6: 21 April. Chongqing (China) to Nanjing (China) – 1,241km; in 17 hours and 22 minutes

LEG 7: 30 May. Nanjing (China) to Nagoya (Japan) – 2,852km; in 44 hours and 9 minutes

Leg 8: 28 June. Nagoya (Japan) to Kalaeloa, Hawaii (USA) – 7,212km; 117 hours and 52 minutes.

The story is now being spun madly to get some PR benefit, but if the objective was to demonstrate unlimited autonomy then it has been a fiasco. Five days and nights is a long way from being unlimited.

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