Beyond wind and
solar power, a variety of carbon-free sources of energy—notably biofuels,
geothermal energy, and advanced nuclear power—are seen as possible ways of
meeting rising global demand.
But many of these may
be difficult to scale up enough to make a major contribution, at least within
the next couple of decades. And a full accounting of costs may show that some
of these technologies are not realistic contributors toward reducing emissions—at
least, not without new technological breakthroughs.
Biofuels have been an
especially controversial and complex subject for analysts. Different studies
have come to radically different conclusions, ranging from some suggesting the
potential for significant reductions in greenhouse gas emissions to others
showing a possible net increase in emissions through increased use of biofuels.
For example, a 2009
study from Massachusetts Institute of Technology’s (MIT’s) Joint Program on the
Science and Policy of Global Change found that a major global push to replace
fossil fuels with biofuels, advocated by many as a way to counter greenhouse
gas emissions and climate change, could actually have the opposite effect.
Without strict regulations, that study found, the push to grow plants for
biofuels could lead to the clearing of forestland. But forests effectively
absorb carbon from the air, so the net effect of such clearing would be an
increase in greenhouse gases entering the atmosphere, instead of a decrease.
Another recent MIT
study, by researcher James Hileman of MIT’s Department of Aeronautics and Astronautics,
found that replacing fossil fuels with biofuels for aviation could have either
positive or negative effects—depending on which crops were used as feedstock,
where these were located, and how the fuels were processed and transported.
Key to biofuel’s
success is the development of some sort of agriculture that wouldn’t take away
land otherwise used to grow food crops. There are at least two broad areas
being studied: using microbes, perhaps biologically engineered ones, to break
down plant material so biofuels can be produced from agricultural waste; or
using microscopic organisms such as algae to convert sunlight directly into
molecules that can be made into fuel. Both are active areas of research.
For the former, one
problem is that traditional processes to break down cellulose use high
temperatures. “You really want these conversions to go on at low temperature,
otherwise you lose a lot of energy to heat up” the material, says Ron Prinn,
the TEPCO Professor of Atmospheric Science and co-director of the MIT Joint
Program on the Science and Policy of Global Change. But, he adds: “Given the
ingenuity of bioengineers, these conversion problems will be solved.”
Tapping the Earth
Geothermal energy has huge theoretical potential: The Earth continuously puts
out some 44 TW of heat, which is three times humanity’s current energy use.
The most promising
technology for tapping geothermal energy for large-scale energy production is
so-called hot dry rock technology (also called engineered geothermal), in which
deep rock is fractured, and water is pumped down into a deep well, through the
fractured rock, then back up an adjacent well after heating up. This heated
water can then be used to generate steam to drive a turbine. A 2006 MIT study
led by professor emeritus Jefferson Tester, now at Cornell University, found
potential to generate 0.5 TW of electricity this way in the United States by
2050. And a new study by researchers at Southern Methodist University found
that just using presently available technology, there is a potential for 3 TW
of geothermal electricity in the United States.
In principle, this
power source could be tapped anywhere on Earth. As you drill deeper, the
temperature rises steadily; by going deep enough it’s possible to reach
temperatures sufficient to drive generating turbines. Some places have high
temperatures much closer to the surface than others, meaning this energy could
be harnessed more easily.
Using this method, “there are thousands of years’ worth of energy available,” says Professor of
Physics Washington Taylor. “But you have to drill deeply,” which can be
expensive using present-day drilling methods, he says.
“There’s a lot of
energy there, but we don’t quite have the technology” to harness it cost-effectively,
he says. Less expensive ways of drilling deep into the Earth could help to make
geothermal energy cost effective.
Most analysts agree nuclear power provides substantial long-term potential for
low-carbon power. But a broad interdisciplinary study published in 2011 by the
MIT Energy Initiative concluded that its near-term potential—that is, in the
first half of this century—is limited. For the second half of the century, the
study concluded, nuclear power’s role could be significant, as new designs
prove themselves both technically and economically.
The biggest factors
limiting the growth of nuclear power in the near term are financial and
regulatory uncertainties, which result in high interest rates for the upfront
capital needed for construction. Concerns also abound about nuclear
proliferation and the risks of radioactive materials—some of which could be
made into nuclear weapons—falling into the hands of terrorists or rogue
And, while nuclear
power is often thought of as zero-emissions, Prinn points out that “it has an
energy cost—there’s a huge amount of construction with a huge amount of
concrete,” which is a significant source of greenhouse gases.
A bewildering variety
of other sources of energy have been discussed. Some, such as fusion power—harnessing
the process that powers the sun itself—require significant technological
breakthroughs, but could conceivably pay dividends in the very long term.
Others have inherent
limits that will, for the foreseeable future, make them much smaller
contributors to energy production. For example, the power of waves and tides is
a potential energy source, with the world’s oceans producing a total of 3.75 TW
of tidal power. But, practically speaking, the most that could ever be captured
for human use is far less than one terawatt, Taylor says.
With any energy
source, it’s crucial to examine, in great detail, the total process required to
harness their power. “Every one of these has an energy or environmental cost,”
Prinn says. “Nevertheless, this should not deter their consideration. It should
instead spur the research needed to minimize these costs.”