Air travel is a common butt of jokes, but the truth is that air travel has shaped the modern world in ways we can’t even imagine. You can now be almost anywhere in the world in less than a day. It’s relatively cheap and very safe.
The one thing aircraft are not is eco-friendly. They are supreme gas guzzlers and significant contributors to climate change. Despite the fact that modern planes are much more fuel efficient, it turns out that the absolute increase in air travel means that total pollution is increasing.
Despite things like Skype and FaceTime, people still seem to want air travel for some reason, so we need to figure out a way to make aircraft emissions-free. The logical thing to do is make them electric, just as cars are now rapidly transitioning to electrical power. Sounds easy right?
Fly the Lightning
In recent years it has become the norm for radio controlled aircraft to be electric. They used to be smelly fuel-powered contraptions, but then efficient electric motors and lithium batteries came on the scene. Now we have incredibly powerful model aircraft with no emissions. These technologies have also led to the creation of drones. Combined with powerful mobile computer chips, drones have revolutionized several industries.
So why not scale this technology up to full-sized aircraft? Believe it or not, that’s an idea that’s been around for more than a century. In the late 1800s there were a few airships that were powered by electricity, but the devices used to store the electricity were so heavy that not even an airship attached to a hydrogen balloon was practical.
It wouldn’t be until the coming of nickel cadmium batteries that energy densities were high enough to make an electric plane possible. A glider called the Mauro Solar Riser used these batteries and 350 watts of solar cells to reach a height suitable for gliding. Hardly a passenger plane, but a strong proof of concept.
In the 90s we started to see all-electric planes, with actual flight certification coming in 2003 to a plane known as the Lange Antares. This plane used lithium ion batteries. While it was also a glider, it was self-launching craft capable of reaching 3000 meters with a full charge of its battery.
Today we have some electric craft that look almost ready for prime time. Thanks to the popularity of drones, many of these are large electric multirotors. The Volocopter is one example, but I really like the Ehang 184, which is basically a drone taxi. It’s the closest to a flying car we have at the moment. Many VTOL electric craft are coming out of the woodwork, but pure electric battery craft are still very limited in terms of total flight time and range.
Alternate Power Sources
Because even lithium batteries don’t have enough capacity to give an electric craft a practical range, alternative sources of electric power have to be considered – something that will generate electricity, with the batteries acting as temporary storage. Fuel cells are a good candidate. Liquid fuel still needs to be carried on the craft, but it gets converted to electrical energy in the fuel cell, with no emissions other than water vapor. Fuel cells are coming along nicely, but they’re not yet ready for primetime.
Some crazy engineers are even suggesting that we beam power from the ground using microwaves. This can actually work and has been shown with model craft, but it will probably be a long time before this can be scaled up, if ever.
Solar power comes in alongside these wonderful clean energy sources as a potential solution in both the short and the long term. Solar panel efficiency is now good enough to create relatively high amounts of power. This means solar-powered flight could become more than just a curiosity. Since electric motors can spin a propeller with plenty of torque for flight, this raises the idea of a solar-powered electric aircraft. Indeed, these craft already exist, but it turns out there are many ways to approach this problem.
How Solar Aircraft Work
In my article on solar cars I note that it’s not really possible to have an electric car that runs only on solar power, since there isn’t enough surface area to generate the needed power. This means that you have to top up from another source. Right now the best example can only regenerate 30 kilometers of range per day.
When it comes to solar-powered aircraft, there’s a lot more surface area to work with. The length of the wings and body can be covered in panels. If you fly at a high enough altitude there’s also a lot more solar energy coming in as well, without all that cloud cover in the way. Even with a plane’s wingspan, it turns out that a purely solar aircraft needs more surface area than the typical plane allows, which is why the solar planes that have flown the furthest and the longest are gliders with huge wingspans covered in panels.
The electricity generated by the panels usually drives the electric propellers. Traditional solar gliders have many of these. Newer generations of solar electric planes may look more like the planes of today, but we’ll get to that later.
While solar electric aircraft are not as strong as the planes that carry hundreds of passengers and not as fast as jet fighters, solar gliders have set endurance records that no plane that needs fuel will ever match. Thanks to having an onboard renewable source of energy, solar planes can stay aloft for days and weeks, and some day will even go for months. If the technology develops well enough, an unmanned solar aircraft may never have to come down except for maintenance. That means it will be an entirely new class of aircraft, more like a satellite or flying station that can work on long-term functions normally reserved for spacecraft.
The Hybrid Solution
If solar aircraft are going to be the future of flight, the planes will have to use hybrid power sources. At the very least, powerful batteries will be needed to get through the night or to get the total range needed. Battery development is speeding along, and we might see lithium batteries with much more energy density. So in the future, an aircraft may be charged up on the ground from an emissions-free power source, and then when in the air it will stay charged through solar power or at least have its range extended.
Alternatively, one of the other power sources, such as fuel cells, could keep the batteries charged in conjunction with solar cells. There will always be an energy storage system such as batteries, but solar panels will have to improve their efficiency significantly before you could have something like a solar 747!
Where We’re At
A craft called the Solar Impulse 2, which is purely solar-powered, holds the record for a single fixed wing plane going all around the world. The only catch is that it took sixteen months. That’s because the plane’s batteries were severely damaged in the attempt and needed lots and lots of repairs. Still, it’s an amazing feat.
More recently, Airbus set an insane record with a craft that flew for 26 days at a height of 70,000 feet. It’s called the Zephyr and is intended as a “pseudo satellite”. In other words, it will fly in the stratosphere (which is above just about everything else but below vehicles in low-earth orbit). With an enormous wingspan of 25 meters and powered by sunlight, it will do some of the jobs we use satellites for now. Such planes can be used as internet providers, spy systems, radar systems, and plain old photographic drones. Unlike satellites, they can easily change course as needed.
These machines are much cheaper and more flexible than satellites and are set to have a serious impact on the sorts of services that can now be provided. It’s a great example of how solar craft are poised to change how we live in the modern world. It may be a long time before we see a fully solar passenger craft, but solar power along with another green energy source is going to change the way we fly forever!