Powering the planet: Chemical challenges in solar
Nathan S. Lewis*†and Daniel G. Nocera†‡
*Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125; and
‡Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139-4307
Edited by Edward I. Solomon, Stanford University, Stanford, CA, and approved August 11, 2006 (received for review May 25, 2006)
Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by
midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil
energy resources, particularly coal. However, the cumulative nature of CO2emissions in the atmosphere demands that holding atmo-
spheric CO2levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment
of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply
from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing
more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of
insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An
especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a
year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific chal-
lenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of
chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon
manity in the 21st century. Energy secu-
rity, national security, environmental
security, and economic security can
likely be met only through addressing
the energy problem within the next
10–20 yr. Meeting global energy de-
mand in a sustainable fashion will
require not only increased energy
efficiency and new methods of using
existing carbon-based fuels but also a
daunting amount of new carbon-neutral
energy. The various factors that con-
spire to support the above far-reaching
conclusions and the basic science
needed for the development of a large-
scale cost-effective carbon-neutral en-
ergy system are the focus of this paper.
the globally averaged energy intensity (i.e.,
the energy consumed per unit of GDP).
The world population was ?6.1 billion in
2001, and in the scenario represented in
Table 1, the global population is projected
to increase by 0.9% yr?1to ?9.4 billion
by 2050. World per capita GDP was
?$7,500 per capita in 2001. In the Table 1
scenario, GDP?N is projected to increase
at the historical average rate of 1.4% yr?1
to ?$15,000 per capita by 2050. No coun-
try has a policy against economic growth,
so this increase in GDP?N seems quite
reasonable and in fact may well be modest
given the rapid economic growth being
experienced by China and India at
present. With no changes in the globally
averaged energy intensity, the world en-
ergy consumption rate would grow, due to
population growth and economic growth,
by 2.3% yr?1, from 13.5 TW in 2001 to
he supply of secure, clean, sus-
tainable energy is arguably the
most important scientific and
technical challenge facing hu-
The Global Energy Perspective
In 2001, worldwide primary energy con-
sumption was 425 ? 1018J, which is an
average energy consumption rate of 13.5
terawatt (TW) (1). Eight-six percent of
this energy was obtained from fossil
fuels, with roughly equal parts from oil,
coal, and natural gas. Nuclear power
accounted for ?0.8 TW of primary
(thermal) energy, and the remainder of
the energy supply came mostly from un-
sustainable biomass, with a relatively
small contribution from renewable
Future energy demand is projected to
increase considerably relative to that in
2001. The most widely used scenarios for
future world energy consumption have
been those developed by the Intergovern-
mental Panel on Climate Change, an or-
ganization jointly established by the World
Meteorological Organization and the
United Nations Environment Program
(after Scenario B2 in ref. 2; E˙? (869 EJ?
yr)?(106TJ?EJ)?(60?60?24?365 s?yr) ?
27.54 TW (TJ, terajoule; and EJ, exa-
joule). The scenario outlined in the last
two columns of Table 1 is based on ‘‘mod-
erate’’ assumptions and hence is reason-
ably viewed as neither overly conservative
nor overly aggressive.
To better understand this scenario,
the top half of Table 1 breaks down the
rate of energy consumption, E˙, into
three fundamental factors (3):
where N is the global population, GDP?N
is the globally averaged gross domestic
product (GDP) per capita, and E˙?GDP is
?40.8 TW in 2050. However, the global
average energy intensity has declined con-
tinuously over the past 100 yr, due to im-
provements in technology throughout the
energy production, distribution, and end-
use chain. In anticipation of continued
improvements in technology, the global
average energy intensity in the Table 1
scenario is projected to decrease at ap-
proximately the historical average rate of
0.8% yr?1, from 0.29 W?($ yr?1) in 2001
to 0.20 W?($ yr?1) by 2050. This decrease
offsets somewhat the projected increases
in population and per capita GDP, so that
the world energy consumption rate is in-
stead projected to grow by 2.3% yr?1?
0.8% yr?1? 1.5% yr?1, from 13.5 TW in
2001 to ?27 TW by 2050. Hence, even
factoring in a decrease in energy intensity,
the world energy consumption rate is pro-
jected to double from 13.5 TW in 2001 to
27 TW by 2050 and to triple to 43 TW by
The Global Energy Challenge Presented
by Consumption of Fossil Fuels
Many sources indicate there are ample
fossil energy reserves, in one form or
another, to supply this energy at some
reasonable cost. The World Energy As-
sessment Report estimates of the total
reserves (i.e., 90% confidence that the
Author contributions: N.S.L. and D.G.N wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS direct submission.
Abbreviations: TW, terawatt; GDP, gross domestic product;
PV, photovoltaics; GtC, metric tons of carbon.
†To whom correspondence may be addressed. E-mail:
email@example.com or firstname.lastname@example.org.
© 2006 by The National Academy of Sciences of the USA
October 24, 2006 ?
vol. 103 ?
no. 43 ?
BIOPHYSICS. For the article ‘‘Drift and breakup of spiral waves in
reaction–diffusion–mechanics systems,’’ by A. V. Panfilov, R. H.
Keldermann, and M. P. Nash, which appeared in issue 19, May
8, 2007, of Proc Natl Acad Sci USA (104:7922–7926; first pub-
lished April 27, 2007; 10.1073?pnas.0701895104), the authors
note that on page 7922, right column, the first sentence in
Mathematical Model, ‘‘Our RDM model is based on a three-
variable Fenton–Karma RD model for cardiac excitation (15),
coupled with the soft-tissue mechanics equations described in
refs. 12 and 16 . . . , where ?(x) is the standard Heaviside step
function: ?(x) ? 1 for x ? 0 and ?(x) ? 0 for x ? 0,’’ should
instead read: ‘‘Our RDM model consists of RD equations
developed by F. H. Fenton (personal communication) and is
based on a three-variable Fenton–Karma RD model for cardiac
excitation (15), coupled with the soft-tissue mechanics equations
described in refs. 12 and 16 . . . , where ?(x) is the standard
Heaviside step function: ?(x) ? 1 for x ? 0 and ?(x) ? 0 for x
? 0.’’ Additionally, on page 7923, left column, beginning on line
10 of the text, the formula for Isiis incorrect in part. The portion
of the formula appearing as ‘‘(0.46 ? 0.085 ? tanh[k(u ? 0.5)])’’
should instead appear as: ‘‘(0.23 ? 0.085tanh[10(u ? 0.65)]).’’
Thus, the corrected formula should read Isi(u, w) ? ?(u ?
0.2)uw(0.23 ? 0.085tanh[10(u ? 0.65)]). Finally, on page 7926,
in the first sentence of the Acknowledgments, the authors would
like to more specifically acknowledge the assistance of Dr.
Fenton. Therefore, ‘‘We thank Dr. F. Fenton, Prof. P. J. Hunter,
and Dr. P. Kohl for valuable discussions’’ should instead read:
‘‘We are grateful to Dr. F. H. Fenton, who kindly provided
equations used in the construction of our RDM model, and to
Prof. P. J. Hunter and Dr. P. Kohl for valuable discussions.’’
These errors do not affect the conclusions of the article.
IN THIS ISSUE, MEDICAL SCIENCES. For the ‘‘In This Issue’’ summary
entitled ‘‘Carvedilol sidesteps G proteins,’’ appearing in issue 42,
October 16, 2007, of Proc Natl Acad Sci USA (104:16392), the
figure caption appeared incorrectly. The online version has been
corrected. The figure and its corrected caption appear below.
Carvedilol recruits ?-arrestin to the ?2-adrenergic receptor. The ?-arrestin2-
GFP is shown in green.
PERSPECTIVE. For the article ‘‘Powering the planet: Chemical
challenges in solar energy utilization,’’ by Nathan S. Lewis and
Daniel G. Nocera, which appeared in issue 43, October 24, 2006,
of Proc Natl Acad Sci USA (103:15729–15735; first published
October 16, 2006; 10.1073?pnas.0603395103), the authors note
that in Fig. 1, the charges shown in the solar fuel cell are on the
wrong sides of the cell. The holes should be at the anode, and
the electrons should be at the cathode. This error does not affect
the conclusions of the article. The corrected figure and its legend
uses light to run the electron and proton flow in reverse. Coupling the
electrons and protons to catalysts breaks the bonds of water and makes the
bonds H2and O2to effect solar fuel production.
H2and O2are combined in a fuel cell to generate a flow of electrons
December 11, 2007 ?
vol. 104 ?
no. 50 www.pnas.org