Angela Belcher - Shining brightly
Vast amounts of solar energy radiate to
the Earth constantly, but tapping that
energy cost-effectively remains a
David L. Chandler, MIT News Office
October 26, 2011
With the world's energy needs growing rapidly, can
zero-carbon energy options be scaled up enough
to make a significant difference? How much of a
dent can these alternatives make in the world's total
energy usage over the next half-century? As the
MIT Energy Initiative approaches its fifth
anniversary next month, this five-part series takes a
broad view of the likely scalable energy
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 MIT physics professor Washington Taylor, who co-teaches a course on the
physics of energy. A total of 173,000 terawatts (trillions of watts) 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 percent of the world's deserts — about 1.5 percent of the planet's total land
area — could generate about 15 terawatts of energy, given a total efficiency of 2 percent. 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.
Infographic: See the 'wedges' of alternative energy available
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
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.
Tomorrow: There are many sources that can make a contribution to our energy supply, but likely not at a major
scale in the near future.
Read part 1: "What can make a dent?"
Read part 2: "Where the wind blows"
Read part 4: "Harnessing the Earth, the atom and the leaf"
Read part 5: "Conserve, conserve, conserve"