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.
The challenges to electric flight
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.
Biofuels
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."
New designs
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."
