Investments in renewable power and fuel climbed from $45 billion in 2004 to $270 billion a decade later. Almost every industry is moving towards green energy but the aviation industry is facing major challenges in it. We do not have enough technology till date to commercially use any kind of renewable energy in aerospace industry. The aerospace giant Boeing had tested it high-altitude pseudo-satellite called Odysseus. the company claims it to be the world’s most capable solar-powered autonomous aircraft. similarly, Airbus has launched its solar aircraft ‘Zephyr’ which is of UAV category and will be used for surveillance of military and other environmental purposes.

odysseus boeing solar plane
Odysseus Boeing solar plane

On the other hand, the newly born company Solar Impulse is working for long-range solar aircraft for commercial uses. it has also got good response and, solar Impulse 2 had completed a world trip too. But, is these solar aircrafts are environment-friendly? and can this nightmare be a practical thing? what are the various challenges in making a solar-powered aircraft?

zephyr, airbus Soalr UAV
zephyr, airbus Solar UAV

Are solar planes environment-friendly?

Just consider the most popular Solar Impulse 2 features 17,000 solar cells crammed onto its wings size equal to that of Airbus A340 (63.40 meters), along with four lithium-polymer batteries to store electricity for nighttime. Yet that’s still only enough power to a single passenger, at a maximum speed of just 43 miles per hour.

On the same side, a Boeing 747-400 running on jet fuel can transport some 400 people at a time, at top speeds of 570 miles per hour. Unless we see some truly shocking advances in module efficiency, it’ll be impossible to cram enough solar panels onto a 747’s wings to lift that much weight around some 370 tons.

It is also not possible to batteries charged by solar on the ground, since that would add even more weight to the plane, vastly increasing the energy needed for takeoff. The main problem about the batteries is energy density as compared to fossil fuel. The energy density of the most advanced lithium-ion batteries is 1MG while that of currently used jet fuel is 43 MJ. I had already published in my other blog in details about energy density and Electric planes.

Hurdles in the success of solar aircraft

One of the major challenge is, both the sun and the plane are constantly moving in the sky, so the angle of capture for the sun to hit the panels is highly variable. Because of this, the solar panels do not capture as much energy as they could if they were, say, on a roof.

Harnessing_Solar_PowerAnother problem with solar-powered flight is harnessing enough energy for speed. “There is a cubic relationship between speed and how much power is needed to move an object through the air,” We know photons strikes on the solar cells and by p-n junction mechanism are converted into electrical potential that powers electric motors in the plane, but the currently available solar aircrafts are only capturing about 10-20 % of the energy from the sun. That equates to a speed of only 50 miles per hour.

If we could harness 100% of the solar energy, the top speed would be 100 miles an hour which is far from 600 miles per hour of a Commercial passenger jet. The power-to-speed relation means that solar power ends up being a not very good solution if you want to develop it from commercial point of view.

Considering the weather conditions, solar-powered planes face even more bumps—both literally and figuratively. Because these types of planes are built with enormous wingspans and delicate, lightweight solar cells (some are as thin as a piece of hair), they are more vulnerable to adverse weather conditions.

The problems we find here have valid point, but how far till now we are able to cure it. As you know the solar impulse 2 is a successful model, it has completed the world trip in just 7 stoppages. therefore let’s see the construction of solar impulse 2.

 

soalr impulse 2
pic credit: BBC news

 

Working and construction of Solar impulse 2

The Solar Impulse has a wingspan the size of an Airbus A340 (63.40 meters) layered with state-of-the-art solar monocrystalline silicon cells, each 150 microns thick and chosen for their lightness, flexibility and efficiency. This massive wingspan is partnered by a super lightweight fuselage (1,600kg) built around a carbon fiber-honeycomb composite using a sandwich structure of a series of 120 carbon fiber ribs.

Four gondolas are attached to the wings each containing a 10hp motor, a lithium polymer battery set and a management system controlling charge/discharge and temperature. Surrounding these is a raft of thermal insulation used to conserve the heat radiated from the batteries and keep them functioning at a height of 8,500 meters, where the temperature can drop to -40°C. Its cockpit, where every piece of instrumentation has been specially developed and redesigned to save energy.

In order to be energy efficient each of the Solar Impulse’s twin-bladed propellers, which are capable of operating in a 200-4,000rpm range, are limited by a reducer to ensure energy is not wasted when solar energy is in abundance. At midday each meter-square area of Earth receives the equivalent of 1,000 watts of solar power, an average of just 250W/m2in 24 hour period, much energy is enough for each of its 8hp engines. This means as little energy as possible needs to be wasted during the more prolific day, as it needs to be stored for the night time where the battery reserves are all that stands in the way of the Impulse losing total power.

There is an interesting fact, the 8hp produced by the Solar Impulse’s engines is the same amount of power the Wright brothers had available to them in their historic 1903 flight.

Also as discussed above the Boeing and Airbus is making high altitude never landing solar UAV’s (plane). though these are aimed to work as satellites, but how different these two are from low orbit satellites in terms of power as the solar energy availability and climatic conditions are quite different.

 

juno space craftThen, how satellites get enough energy in space?

Getting enough power in space is a big problem for the satellites powered by solar panels. Solar panels used in satellites need to be big to supply usable energy to a spacecraft. The International Space Station’s eight solar arrays contain thousands of solar cells and take up an area half the size of a football field. Its arrays can produce up to 120,000 watts of energy, enough to power 40 homes.

Solar panels designed to fly to space are much more expensive than ground-based panels. Because weight and volume on the spacecraft are at a premium, they use high-efficiency cells. Space agencies have pioneered many of the technologies used in solar panels. The European Space Agency recently announced that they enabled a very thin solar cell, just 0.1 millimeters thick, that provides 30 percent efficiency by sandwiching together 4 different layers of materials and can absorb wider wavelengths of sunlight.

The most efficient panels on the commercial market only convert 22.5 percent and most are in the 15-17 percent range NASA’s Opportunity Rover, set down on the surface of Mars back in January, 2004. It’s now spent more than 5,000 days exploring the Red Planet, searching for evidence of past water. In the best conditions, its solar panels generate about 140 watts during the Martian day. It needs about 100 watts if it needs to drive anywhere.

While the solar energy at Earth is about 1300 watts per square meter, the intensity drops to 1/25th, or 50 watts per square meter by the time you get to Jupiter. This is all thanks to the inverse square law, where the intensity at any distance is equal to the inverse square of that distance.

This is why NASA’s Juno spacecraft, is such a feat of power engineering. The spacecraft is equipped with three solar panel arrays 9-meters long, covered with 18,698 separate solar cells. If Juno was at Earth, it would be able to generate 14000 watts of electricity, but out at Jupiter, it can only generate 500 watts.

On the basis of this, we can say it is quite difficult and costly in earth’s environment to have a permanent flying UAV. As engaging such a large space by aircraft wings is not possible. But any kind of hybridization could lead to its possibility.

carbon foot printThe idea behind the solar plane is basically correct: We do need to rethink the way we fly. After all, burning jet fuel carries huge environmental costs. Aviation now accounts for 3 percent of humanity’s global warming footprint, and it’s one of the fastest-growing sources. The aviation industry is currently facing heavy pressure to reduce its CO2 emissions after 2020 in order to mitigate climate change.

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