net electricity imports 0.06
solar
0.07
nuclear
8. 21
hydro
2.86
wind
0.26
geothermal
0.34
12. 45
2.83
electricity
generation
39. 52
6. 37
20. 46
27.06
rejected
energy
57. 43
0.26
0.30
4. 61
2. 17
0.06
8.70
residential
10.88
0.01
1. 29
0.41
4. 47
0.01
4. 43
1. 63
natural gas
22. 19
2.90
commercial
8. 15 6. 52 6. 52
0.62
0.02
energy
services
42. 32
0.06
0.10
4.97
3. 45
coal
22. 44
7.81
19.90
industrial
24.88
9.71
1.91
1.96
biomass
3. 37
0.41
21. 60
0.48
0.02
0.64
0.62
27.67
transportation
28.80
petroleum
39.95
7. 20
Where the energy goes Data from 2006 on how energy (in quads) is produced and used in the US. The numbers on the left indicate what percentage each type of energy
contributes to the total primary supply. Following each energy “stream” to the right shows how much of that energy is consumed for useful services and how much is
unused. (Note that all the numbers have been rounded.) Despite decades of research and development, sustainable-energy technologies (top left) account for only a tiny
fraction of the total energy flow. Altering this landscape will require breakthroughs in nanomaterials and chemical processes that convert energy efficiently between
photons, electrons and chemical bonds. Source: Lawrence Livermore National Laboratory and DOE
Without the
ability to store
electricity,
solar power
can never be
more than a
supplement to
fossil energy
generation
to the photocell, where it replaces the hole left by the
original electronic excitation. Once completed, the
electron round trip does no harm and leaves no chemical change. However, although solar electricity may
be fully sustainable in operation, it is not necessarily
fully sustainable in the construction or disposal of its
infrastructure – both steps require energy and emit carbon dioxide. These often-ignored full-life-cycle issues
must be considered when evaluating the sustainability
of energy technologies.
Despite the appeal of solar electricity, serious technical challenges block its widespread deployment.
Before the reach of solar electricity can expand, its costs
must fall below those of fossil electricity and must be
low enough to attract the majority of future demand
growth without artificial incentives. Achieving this will
require breakthroughs in understanding and controlling the fundamental nano-scale phenomena of photo-excitation, charge separation and charge transport in,
for example, high-efficiency mulitjunction solar cells
and in low-cost organic and thin-film solar cells (see
“Nanotechnology: does it have the energy?” on pp40–
45). An even greater and less-explored challenge is
utility-scale electricity storage to bridge the day–night
and cloudy–sunny cycles. Without the ability to store
electricity, solar power can never be more than a supplement to fossil energy generation.
As a derivative of solar energy, wind electricity shares
its sustainability profile, with the potential to satisfy
all three criteria. The barriers to wind electricity are
cost, utility-scale storage of electricity to bridge calm
days and long-distance transmission capacity to deliver
wind energy from its remote sources to urban population centres. The output of wind turbines is limited
by the weight of the generator that can be supported
on the tower. Superconducting generators can, however, lower the cost and land area required for wind
electricity by a factor of two because they produce
twice the output but are the same size and weight as
conventional generators.
As for fossil-fuel electricity plants, they can be made
more sustainable by capturing their carbon-dioxide
emissions and sequestering the gas in underground
geologic formations. Carbon sequestration prevents the
carbon dioxide from entering the atmosphere – a positive step toward “doing no harm”. This positive step is
balanced, however, by the challenges associated with
injecting the carbon dioxide underground. We know
little about how carbon dioxide reacts with the porous
rocks in which it would reside, and less still about how
far it might migrate during the thousands of years it
must remain there. Carbon dioxide is supercritical
under sequestration conditions, and the high temperature and pressure alter its reaction chemistry and
enable it to diffuse quicker through porous rocks.
These primary scientific challenges require a host of
studies of surface-reaction chemistry to identify reaction pathways, intermediate species, chemical kinetics and diffusion phenomena under simulated
sequestration conditions.
Beyond reaction chemistry, techniques for monitoring and modelling the migration of large quantities of
