tide 0.0004%
other 0.5%
wind 0.064%
nuclear 7%
renewables 13%
solar 0.039%
gas 21%
hydro 2.2%
coal 25%
combustible
renewables and
waste 10.6%
geothermal
0.414%
oil 34%
A long way to go A breakdown of the world’s total primary energy supply in 2004 shows that less
than one-sixth of our energy comes from renewable sources. Within this category, the lion’s
share consists of combustible renewables, which usually fall short of the “do no harm” criterion
for sustainability.
A new leaf
Biology incorporates
many exemplary
sustainable-energy
strategies.
Pick and mix
So which of these more sustainable energy alternatives
should we develop? The expected doubling of global
energy demand by 2050 is too daunting a challenge to
be met by any single technology – we are likely to need
many or even all of them. Some of the most sustainable options require significant scientific breakthroughs before they can be implemented. We can,
however, follow a dual course of phasing in portions
of sustainable technologies as breakthroughs make
them cost-competitive, while aggressively pursuing the
research needed to meet the remaining challenges and
achieve even greater sustainability. In this framework,
carbon sequestration, high-efficiency nuclear electricity and plug-in hybrid vehicles are near-term solutions that will precede the deployment of large-scale
solar and wind power generation, utility-scale electricity storage and all-electric vehicles.
The scientific advances needed for these more sustainable energy technologies reflect a fundamental sea
change in the role of materials in energy technologies.
With traditional fossil energy, the important materials
are the fuels, which are valued for their high energy
content and low cost. Burning fuels to produce heat is
the first step in the traditional energy-use chain. This
heat is then converted by an internal combustion engine to mechanical motion for transportation or by a
turbine and generator to electricity for a diversity of
energy services.
Sustainable energy technologies, in contrast, tap into
underused energy flows like sunlight or wind and use
these to produce electricity or fuel directly, without a
combustion step. The important components are the hi-tech materials and chemical processes that initiate and
control the conversion of energy between photons, electrons and chemical bonds via nano-scale phenomena.
Unlike the fuels of fossil-energy technology, which are
valued as a raw commodity, the hi-tech materials of sustainable energy are valued for their ability to coordinate
sophisticated nano-scale energy-conversion processes.
Many decades of advances in observing and modelling phenomena at ever smaller length scales and
shorter timescales mean that science is poised to enter
a new era where researchers will not only observe, but
also control these nano-scale conversion processes.
This new capability promises powerful breakthroughs
in raising the efficiency and lowering the cost of sustainable energy technologies.
Breaking the scientific bottleneck
Despite 30 years of research and development, the deployment of sustainable energy technologies has hardly
affected the global energy mix. The world is still over
80% dependent on fossil fuels, much as it was in 1970.
The reason is remarkably simple: the cost of alternative energies is significantly higher than fossil fuels, and
the energy enterprise, driven by economics, will always
choose the lowest available cost. This simple fact clearly
identifies the basic deployment challenge for sustainable energy: we must make fundamental scientific
breakthroughs in materials and chemical processes,
and exploit them to make sustainable energies cheaper
than fossil fuels.
That we have not succeeded in doing so in three
decades indicates the magnitude of the challenge. Existing sustainable technologies do not control materials and chemistry at the sophisticated level needed to
cost-effectively convert sustainable sources into useful
energy. But unlike fossil energy, which after a century
of development operates near its theoretical maximum
efficiency, sustainable energy technologies are still in
their infancy. There is generous room for improvement
in raising efficiency and lowering cost before intrinsic
limits are reached; we are, in effect, where we were with
the steam engine in James Watt’s day. Multijunction
solar cells, for example, where two or more semiconductors tuned to different band-gap energies are connected in series, have a theoretical efficiency of over
50%, well above the 22% of the best single-junction
silicon solar cells now in commercial production.
Indeed, silicon solar cells themselves symbolize the
impact of materials breakthroughs on sustainable energy technologies. Their efficiency has risen from 6%
in the 1950s to 22% today because of dramatic improvements in control of the purity, perfection and precision
doping of silicon. Similar big improvements in efficiency
and cost await other sustainable-energy technologies –
provided we aggressively pursue scientific research
to control the materials and chemical processes that
govern nano-scale energy conversion. Discovering and
designing these hi-tech materials and processes is the
grand challenge of sustainable energy. ■
More about: Sustainability
V S Arunachalam et al. 2008 Harnessing materials for energy
(special issue) MRS Bulletin 33 261–477
Basic Energy Sciences Advisory Committee 2007 Directing
Matter and Energy: Five Challenges for Science and the
Imagination; 2008 New Science for a Secure and Sustainable
Energy Future www.sc.doe.gov/bes/reports/list.html
J Baxter et al. 2009 Nanoscale design to enable the revolution
in renewable energy Energy and Environmental Sci. 2 559
M Eikerling et al. 2007 Driving the hydrogen economy
Physics World July pp32–36
D Hafemeister et al. (ed) 2008 Physics of Sustainable Energy
(Berkeley, 2008) (AIP Conf. Proc. 1044) (Melville, AIP)
Science 2007 Energy and sustainability (special Issue) Science
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