The sunlight
that reaches Earth every day dwarfs all the planet’s other energy sources. This
solar energy is clearly sufficient in scale to meet all of mankind;s energy
needs—if it can be harnessed and stored in a cost-effective way.
Unfortunately, that’s
where the technology lags: Except in certain specific cases, solar energy is
still too expensive to compete. But that could change if new technologies can
tip the balance of solar economics.
The potential is
enormous, says Massachusetts Institute of Technology (MIT) physics professor
Washington Taylor, who co-teaches a course on the physics of energy. A total of
173,000 TW of solar energy strikes the Earth continuously. That’s more than
10,000 times the world’s total energy use. And that energy is completely renewable—at
least, for the lifetime of the sun. “It’s finite, but we’re talking billions of
years,” Taylor
says.
Since solar energy is,
at least in theory, sufficient to meet all of humanity’s energy needs, the
question becomes: “How big is the engineering challenge to get all our energy
from solar?” Taylor
says.
Solar thermal systems
covering 10% of the world’s deserts—about 1.5% of the planet’s total land area—could
generate about 15 TW of energy, given a total efficiency of 2%. This amount is
roughly equal to the projected growth in worldwide energy demand over the next
half-century.
Such grand-scale
installations have been seriously proposed. For example, there are suggestions
for solar installations in the Sahara, connected to Europe via cables under the
Mediterranean, that could meet all of that
continent’s electricity needs.
Because solar
installations of all types are modular, the experience gained from working with
smaller arrays translates directly into what can be expected for much larger
applications. “I’m a big fan of large-scale solar thermal,” says Robert Jaffe,
the Otto (1939) and Jane Morningstar Professor of Physics. “It may be the only
renewable technology that can be deployed at very large scale.”
And we do know how to
harness solar energy, even at a colossal scale. “There’s no showstopper, it’s
just a matter of price,” says Daniel Nocera, the Henry Dreyfus Professor of
Energy at MIT.
Nocera foresees a time
when every home could have its own self-contained system: For instance,
photovoltaic panels on the roof could run an electrolyzer in the basement, producing
hydrogen to feed a fuel cell that generates power. All the necessary
ingredients already exist, he says: “I can go on Google right now, and I can
put that system together.” Nocera’s own invention, a low-cost system for
producing hydrogen from water, could help over the next few years to make such
systems cost competitive.
In principle, we know
multiple ways of generating electricity from the sun (direct photovoltaic, or
solar thermal energy used to drive a turbine); of storing that energy (in batteries,
by pumping water uphill, or by separating water into hydrogen and oxygen using
an electrolyzer); and of converting that stored energy into electricity when
it’s needed (using fuel cells powered by hydrogen, for example). Some kinds of
solar power are already cost competitive, at least in some settings, and prices
have been moving steadily downward.
“Costs have come down
very dramatically” for solar power, says Ernest J. Moniz, the Cecil and Ida
Green Distinguished Professor of Physics and Engineering Systems and director
of the MIT Energy Initiative, “but it’s still not that cheap.” And even as the
price of solar panels themselves has dropped, there has been little reduction
in the costs associated with installing them.
Like nuclear power,
Moniz says, solar is characterized by high initial costs, but very low
operating costs. But one significant advantage solar has over nuclear is “you
can do it in smaller bites,” rather than needing to build multibillion-dollar
plants.
Solar energy is a
vibrant research topic, attracting scientists interested in many different
approaches. For example, MIT researchers Angela Belcher and Paula Hammond are
exploring approaches to solar power that would harness the power of biological
organisms to create solar devices; Penny Chisholm and Shuguang Zhen are looking
into the possibility of directly harnessing the photosynthesis done by plants
or single-celled organisms; and various researchers including Vladimir Bulovic,
Michael Strano, Tonio Buonassisi, Jeffrey Grossman, and Yang Shao-Horn, among
others, are working on ways of improving the efficiency or lowering the costs
of solar photovoltaic cells.
Still others are
pursuing a variety of approaches to solar thermal energy: using the sun’s heat
to power turbines or to heat homes or water. A significant breakthrough in any
of these areas could make solar power an economically viable option for the
world’s energy needs. This year, for example, Alexander Slocum and others
published a proposal for a solar thermal system that could provide steady, 24/7
baseload power for utility companies, helping to make it cost-competitive with
other sources.
Other researchers are
studying ways to make effective solar-power systems using common, inexpensive
materials. For example, cadmium telluride is a very promising material for
solar cells. But it turns out that tellurium, one of its ingredients, is “rarer
than gold,” Jaffe says. “We need to be able to make solar cells out of common
materials, or at least things that are not exquisitely rare,” he adds.