Leaves are the ultimate solar panel. If we're going to power more of the world with the sun, we're going to need to imitate plants, one way or another.
Enough solar energy strikes the earth in one hour to power our civilization for a year, and futurists like Ray Kurzweil see us moving to an all-solar civilization in the span of a single human lifetime.
But getting to a civilization powered entirely by renewables isn't going to happen with current technology. Traditional solar panels might become cheaper, but the laws of physics say they can't become that much more efficient. Not to mention that the resources required to build the kind of desert-spanning solar farms that would be required to replace even a percentage point or two of our current energy mix boggle the mind.
But there is another way, and it borrows directly from nature: "solar leaves." They've become a shorthand for the idea that if we can only make solar panels cheap enough, no one will bother to use anything else ever again.
There are a number of competing visions for how a solar leaf will work and what it might be made from. Daniel Nocera at MIT is working on a combination of catalysts that use sunlight to--like a leaf--directly convert water into hydrogen . (Which can then be burned or used in a fuel cell.) Another group of MIT researchers have successfully printed solar cells onto paper, conjuring visions of a future in which solar cells are as cheap as the morning paper.
But making these kinds of solar cells into a true replacement for our current energy system will require technology that isn't merely cheap; in some sense, it needs to be disposable. (Obviously, a closed production cycle where 100 percent of its recyclable components are reclaimed would be optimal.)
In addition to price, disposability respects the fact that all energy infrastructure, and especially parts of it that are continually exposed to the sun's powerful radiation, has to be periodically replaced. And while Nocera's vision of a water-bathed, hydrogen-spewing lozenge is tempting, this may be a case where it's more important to imitate the form of nature rather than its exact mechanism.
This is the vision of researchers at Germany's University of Technology Chemnitz, who have just published work on durable solar cells that could be printed with traditional processes. Imagine a copy shop in Nairobi spooling out a continuous ribbon of "solar leaves" at up to 15 meters per second on a single machine and you've got the idea. These solar leaves could be cut to whatever length the user prefers, then connected to a battery or home with simple alligator clips.
Printed solar cells still require some kind of substrate (usually metallic) whose widespread adoption might deplete world reserves. But what if we could make solar leaves out of nothing but carbon?
Jiaxing Huang and colleagues at Northwestern University are working on a solar cell that is made out of nothing but three different forms of carbon: balls (fullerenes), sheets (graphene), and tubes (carbon nanotubes). Best of all, all three can be mixed up in a test tube full of water. It's what chemists call aqueous chemistry. This makes the production of these solar cells biomimetic from start to finish, and implies that some day we'll be making solar panels on paper with little more than graphite as a feedstock.
Of course, we don't have to wait decades to see what effect solar leaves will have on the success of solar power. Already, solar panels known as thin film solar, which are produced by something like a traditional printing process, are a $3 billion business. One research firm projects that this market could explode to $44 billion by 2017.
Thin film isn't as efficient as silicion solar cells, but the world's abundance of sunshine makes that irrelevant. These days, it's all about finding an patch of ground and throwing up inexpensive panels. Plants have been covering every inch of earth's surface with what are essentially solar panels for billions of years--now it's our turn.
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