Why Solar-Powered Planes Are Still a Long Way From Carrying Passengers

Swiss pilot Bertrand Piccard made headlines on Saturday when he glided a solar-powered plane onto Moffett Airfield in California after a three-day journey across the Pacific Ocean. It's the most recent stop in an around-the-world trip that began in Abu Dhabi last spring and is intended to raise awareness about the importance of reducing carbon emissions through the use of clean energy.

The plane itself, "Solar Impulse 2," is a true zero-fuel aircraft, powered by more than 17,000 solar cells. It's designed to carry just one pilot - Piccard and his colleague André Borschberg have been tag-teaming the journey around the world - and has the wingspan of a jumbo jet, although it weighs only two tons.

The daring trans-Pacific flight has drawn global interest to the concept of electric planes, which have existed in various forms for several decades now. Some designs rely on solar cells, while others use various types of batteries, but the overall goal is the same: to achieve flight with minimal or no fuel burning.

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Electric aircraft are among the more ambitious technologies being researched around the world in an effort to reduce carbon emissions from aviation. It's a cause that's rapidly gaining international attention. Aviation is currently responsible for about 1 percent of all the world's carbon emissions - and as air traffic is expected to experience rapid growth in the coming decades, that proportion could quickly climb if no steps are taken to improve the fuel efficiency of aircraft. Some estimates have suggested that by 2020, emissions from aviation could be 70 percent higher than they were in 2005.

To that end, the UN's International Civil Aviation Organization (ICAO) proposed the world's first carbon dioxide emissions standards for aircraft back in February. And while some environmentalists have argued that the proposal did not go far enough, the action has placed aircraft emissions on the international radar - and scientists around the world are researching ways to reduce them.

Electric flight, however, may be among the technologies that are furthest from becoming practical. So far, most of the electric planes that have achieved flight have only been able to accommodate one or two people, and it will likely be at least a decade or two before the technology will progress to the point that it's commercially viable.

"The big challenge is the batteries," said David Zingg, director of the University of Toronto's Institute for Aerospace Studies. For electric planes to become competitive, their power sources need to be able to store more energy per unit mass - otherwise, their speed and weight capacities will remain impractically low.

"You can imagine in 20 years you can have an aircraft the size of a 737 that's electric - but you can't be sure," Zingg said. "That all depends on battery technology."

For smaller aircraft, the technology may even be able to work its way into the market within the next 10 years, said Sean Clarke, co-principal investigator on a Nasa project called Sceptor, which is working on an experimental electric propulsion-powered aircraft.

But in the meantime, there are plenty of other alternatives being explored that could start cutting emissions far sooner.

In March, United Airlines became the first American airline to use renewable fuel for commercial operations when it began using biofuel in flights between Los Angeles and San Francisco. However, others may be following suit soon. Both Southwest Airlines and FedEx, for example, also have contracts with biofuel producers that will allow them to start buying renewable jet fuel for future use.

The basic idea behind renewable fuels is to use biological sources - usually plant or sometimes animal matter - instead of oil. Many biofuel companies have developed "drop-in" fuels that are designed to work safely in existing jet engines - usually requiring mixing with traditional fuels - making them an easy way to cut down on carbon emissions without requiring costly mechanical alterations.

But there are some cons to consider. When biofuels first started to become competitive, there was concern that they were competing with food growers for agricultural land - an issue that's become more salient as concern heightens over the planet's rapidly growing population and the future of global food security. As a result, producers are increasingly focusing on fuel sources that can be grown on land that's unsuitable for food crops.

Additionally, Zingg pointed out, "there's a ton of work to be done to make the processing efficient enough that it's cost efficient compared to fossil fuels."

Making physical design changes to planes is another way of increasing fuel efficiency. Finding ways to reduce the drag on aircraft in flight is one important research area, Zingg said - for example, redesigning wings to improve the way air flows over the plane.

Nasa has focused a great deal of research on these types of design challenges in recent years. It's Environmentally Responsible Aviation project, which took place between 2009 and 2015, focused on solutions that would cut down on noise, pollution and carbon output and included research on more efficient engines, lighter-weight aircraft materials and new body designs. At least one of the resulting technologies - a new, more aerodynamic design for airplane wing flaps - is already on its way to becoming commercialized, according to the agency.

Other design research is ongoing. The Sceptor project, for example, which Clarke is helping to lead, is working to design a smaller, more aerodynamic and efficient wing than would normally be possible by equipping it with electric motors to help energize the flow that generates lift on the plane.

But it's not necessarily all about the technology, Zingg added. Even the way air traffic controllers guide planes can make a difference. Adopting procedures that allow for smooth, continuous descents rather than forcing planes to fly in inefficient landing patterns can help reduce carbon output. Similarly, an imaginative idea known as "formation flight" could be helpful as well, he noted - this is a concept in which planes fly in bird-like formations that take advantage of airflow and reduce drag.

In the end, integrating many carbon-cutting ideas together is likely to give the best shot at making a difference in aviation emissions, according to Zingg. There are plenty of concepts already commercially practical now - and one day, even solar-powered planes may join the mix.

"There's a huge range of different things that can be done - the most important thing is to try to do them all," he said. "Individually, they might have a modest benefit...but if you add them all up you can make a pretty big improvement."

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