In March this year, Swiss-based engineer Andre Borschberg and psychiatrist-aeronaut Bertrand Piccard kickstarted their aircraft at the Abu Dhabi airstrip. As the engine vroomed to life, the duo sat back to begin their first journey across the globe in an aircraft without a drop of conventional fuel. All it had was a powerhouse of solar cells.
Yes. I am talking about the aircraft that has set the stage for a future of solar-powered air travel – the Solar Impulse 2.
The Solar Impulse 2, which was expected to complete the circumnavigation in August, had to end the journey half way through the world in Hawaii following thermal damage. The duo is planning to restart its expedition in April 2016. Now while the aircraft is undergoing an overhaul, one concern that the pilots are hoping to tackle is its speed at night. News reports stated that the Solar Impulse 2 flew at a speed slower than that of a car during nights. This, not very surprisingly, has been a serious concern of several inventions in the past that relied only on solar energy to power up and thrive.
Blast from the past
In 1974, AstroFlight launched the first solar-powered aircraft named ‘Sunrise I’. Its bettered cousin, ‘Sunrise II’, was powered up with over 4,000 solar cells and could attain a theoretical height of 75,000 feet. However, due to an aero-elastic problem ‘Sunrise II’ broke up at around 22,000 feet. In 1980, AeroVironment launched the first human-carrying solar flight ‘Gossamer Penguin’. Later in 1995, NASA’s High Altitude Solar (HALSOL) project ‘Pathfinder’ reached 50,000 feet. Following this, ‘Pathfinder-Plus’ was launched, which was able to reach more than 80,000 feet.
Solar aircraft that track the sun
Here’s a possible solution for the makers of Solar Impulse to ensure the aircraft is as fast during day as at night. Solar powered aircraft that are aimed at flying continuously through day and night require efficient and lightweight solar cells. Most often than not, solar cells are fixed to the wings and the tail of an aircraft. Since these are fixed positions, their exposure to the sun is often not at optimal angles to recharge.
To handle this issue, the DLR Institute of Flight Systems built the ‘Solitair’ prototype in 1998 with solar panels that face the sun. Between 1999 and 2003, AeroVironment and NASA developed the ‘Helios’ prototype where the aircraft wing was segmented into plurality of solar cell covered sections connected by hinged joints that allowed them to tilt towards the sun.
Such a tilting of solar cells towards the sun has been described in a 2009 US patent application 20090292407. The patent talks of a technology that allows the tail of an aircraft to rotate in accordance with the angle and elevation of the sun, increasing optimum exposure of solar cells. Some other patents that describe such technology include US 5518205 and US 8746620.
But what does a solar-powered aircraft do in cloudy weather and during nights? The perfect technology available today is energy storage in regenerative fuel cells made of lithium polymer and lithium-ion.
A 1994 patent filed with the USPTO (US 5518205) describes a technology that helps convert onboard supply of water into hydrogen and oxygen during the day through electrolysis. At night, the same hydrogen and oxygen is re-fused by passing them through a fuel cell to produce electricity and water. While electricity can power the aircraft, the water can be reused during the next day.
In 2005, AC Propulsion developed the ‘SoLong’ aircraft using advanced Li-Ion batteries as energy storage means. ‘SoLong’ was able to stay airborne for two half nights.
Energy from static charges
When an aircraft collides with the particles existing in the earth’s atmosphere such as dust, fog, rain, snow, ice, etc., static charges are generated on its surface, whether the surface is made of metal or a dielectric material. This phenomenon is known as P-Static effect. This idea was explored in the patent US7898789, wherein static charges formed on the aircraft’s surface can be collected using electrodes. These static charges form an additional source of energy apart from the solar energy and can be used to power the solar aircraft at night times.
The Solar Impulse has several patented technologies to fall back on for support to optimize its fuel during the day and power up its resources at night. Whether the 2016 flight incorporate any of these technologies is something to wait and watch.
(Featured image source: https://pt.wikipedia.org/wiki/Energia_solar#/media/File:Helios_in_flight.jpg)