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OCR for page 33
R&D Challenges in the Chemical Sciences
to Enable Widespread Utilization of
Renewable Energy
Nathan S. Lewis,
California Institute of Technology
When looking at the present utilization of primary power and options for
tuture sources of energy, a number of questions can be envisioned. Where do we
presently get our power from and what are the costs? What is the role for the
chemical sciences in renewable energy technology? To answer these questions, it
is useful to review the present primary power mix how much energy is con-
sumed, from what sources, future constraints imposed by sustainability, and the
theoretical and practical energy potential of various renewables. Once these issues
have been addressed, it is possible to identify the challenges for the chemical
sciences to economically exploit renewables on a scale commensurate with our
energy needs.
The mean global energy consumption rate in 1998 amounted to 12.8 TW
(383 quad/year, of which only 10 percent was used for electricity (Figure 5.1~.
Oil, gas, and coal constitute almost 80 percent of this total energy consumption.
The mean U.S. energy consumption rate in 1998 was 3.3 TW (99 quad/year), 15
percent of which was used to generate electricity.
At present, the use of renewables for energy accounts for only a small per-
centage of total energy requirements. Of the 12.8 TW of global power consumed
in 1998, power from biomass accounted for only 1.21 TW, while the largest
renewable source consumed hydroelectric still accounted for only 0.3 TW.
One possible explanation for the limited use of renewables may be cost. With
one exception, the use of fossil fuels for electricity generation in the United States
is presently by far the least expensive option. The cost per kilowatt-hour for
il quad = 1015 Btu = 1.055 x 10~8 I; ~ quad/year = 0.0334 TW.
33
OCR for page 34
34
TW
Os~0
hi' ^
an'
FIGURE S.1 Mean global energy consumption, 1998.
ENERGY AND TRANSPORTATION
2.96
2.7 4~
'''1
5- ,
4 -
3 -
2 -
O 0> ~ ~ )~
0.286
0.286
0.828
a;\ c\
electricity derived from coal is 2.1 cents; for natural gas is 3.6 cents; and for oil,
3.9 cents. This compares with a cost per kilowatt-hour for wind of 3 to 5 cents,
and for solar approximately 22 cents. However, it must be noted that electricity
derived from nuclear power is quite competitive with fossil fuels, with a cost of
2.3 cents per kilowatt-hour.2 3
Proven reserves of oil, natural gas, and coal tend to underscore that fossil
fuels will remain an abundant inexpensive resource base for the foreseeable future.
Proven oil reserves are expected to last at least 40 years, natural gas should last at
least 70 years, and coal reserves are adequate for 200 years. By adding reserves
that are likely to be found, the oil supply would then last between 50 and 100
years, natural gas 90 to 275 years, and coal at least 2,000 years.
In light of this supply, it is reasonable to conclude that renewables will not
play a large role in primary power generation unless one of two things happens.
Either technology breakthroughs on these renewable sources of energy reduce the
cost of these sources significantly below what they are today, or there are unpriced
externalities introduced that significantly increase the cost of fossil fuels. For
example, environmental concerns may result in the introduction of carbon taxes
or subsidies for carbon-neutral technology.
Looking to the future, it may be possible to make some predictions regarding
primary power demands and the expected environmental impact over the next 50
2These costs represent the costs of capital equipment and fuel but not waste disposal or environ-
mental remediation.
3Electricity, however, is high-value energy, and the cost per Joule of energy produced by consum-
ing fossil fuels to make heat is approximately a factor of 5 to 10 times less expensive.
OCR for page 35
R&D CHALLENGES IN THE CHEMICAL SCIENCES
35
to 100 years based on demographics. The global population is expected to rise to
10 billion-11 billion people by 2050, and GDP growth is expected to rise an
average of 1.6 percent a year, which is its historical average. Balanced against
these factors, energy consumption per unit of gross domestic product (GDP) is
expected to decline 1 percent a year, mainly as a result of increases in energy
efficiency in the industrialized world.
Based on these projections, total primary power consumption is expected to
rise from 12 TW in 1990 up to 28 TW by 2050. The overriding question is from
where this 28 TW of power will be derived.
The above projection, as previously stated, depends on increased efficiency
in energy usage. Sustaining the historical trend of carbon intensity in the energy
mix implies that by 2050 the energy economy will actually be more efficient than
one run entirely on natural gas. This can only be accomplished if there are signifi-
cant contributions from carbon-neutral power. Based on projected carbon dioxide
emissions from fossil fuel usage, it is estimated that even with increased use of
fossil fuels over the coming century, these fuels will not be enough to meet the
expected 28 TW of power demanded by 2050. There will be an estimated 10 TW
shortfall that must be derived from carbon-neutral sources in order to meet the
primary power needs of the planet.
Obviously, this is not an insignificant amount of power. Ten terawatts was
the entire global power production from all sources in 1990. In addition, if atmo-
spheric carbon dioxide emissions are to be stabilized, there will be an even greater
need for renewable energy. For example, if man-made releases of carbon dioxide
are to be stabilized at 550 ppm twice the preindustrial level of carbon dioxide in
the atmosphere 20 to 30 TW of carbon-neutral primary power would be needed.4
This is a daunting amount of power. Without economic policy incentives, the
needed technology to meet these demands will probably not be in place soon
enough to meet this demand by 2050. In fact, meeting the goal of commercial
carbon-neutral power capable of producing 10 to 20 TW by the mid-21st century
could require efforts comparable to the Manhattan Project or the Apollo space
program.
Where such large amounts of power can be derived from must be examined.
If this power is to be carbon neutral, the technologies needed must also be exam-
ined. To develop these technologies, the challenges for the chemical sciences
must be identified.
The five most common renewable sources of primary power are hydro-
electric, geothermal, wind, biomass, and solar. Hydroelectric power is considered
by many to be a model energy source. It is clean, relatively benign environmen-
tally, nonpolluting, and relatively inexpensive. However, the global theoretical
potential of the hydrology of all the world's precipitation and all of the energy
4Hoffert, M. I. et al., Nature, 395:881. 1998.
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36
ENERGY AND TRANSPORTATION
flows of the waters on Earth provide only 4.6 TW, far below the goal of 20 TW of
power. The technically feasible amount of hydroelectric power is far less, only
about 0.7 TW. The installed capacity of hydroelectric power is already 0.5 TW.
Therefore, there is not much additional power available for exploitation from this
source.
Power derived from wind presents significant onshore potential. Two large
geographical areas the Great Plains of the United States and the region from
Inner Mongolia to northwest China present significant expanses of land suit-
able to utilize wind. If 6 percent of the dedicated land in the Great Plains were
used for wind farms, approximately 0.5 TW of power could potentially be
obtained. As with hydroelectric, this is a significant amount of power but far
short of the projected 20 TW of additional power that will be required.
Globally, it is theoretically possible to obtain 50 TW of power from wind
stations on land, but with practical land usage about 4 percent of all the land
that has enough wind to make power generation economically feasible the
potential amount of power that can be derived is approximately 2 TW. The off-
shore potential for wind power generation is larger, but there is the significant
requirement of being close to an electrical grid to make it practical.
Distribution is a key concern with wind generation of power. If, for example,
the Great Plains were used to generate large amounts of power for the United
States, this power would not be consumed locally, and there are constraints to the
electrical grid. An efficient method of power storage would have to be found.
Presently the methods available for power storage carry too high a penalty in
terms of energy loss to make them economically feasible for wind as a significant
source of energy.
Biomass as a source of a large percentage of the world's power has signifi-
cant obstacles. Biomass requires large areas because the process is very ineffi-
cient. Only 3 percent of the total sunlight that is incident per unit area on a plant
is actually stored in free energy by photosynthesis. This is sufficient for biological
needs but is difficult to exploit as a source of primary power. To meet the goal of
20 TW of additional power, biomass would require 4 x 10~3 m2 of land, and the
total landmass of the earth is 1.3 x 10~4. Clearly this is not a viable option.
It is possible to look at the situation with biomass from a different perspec-
tive. The amount of land with crop production potential in 1990 was 2.45 x 10~3
m2. In order to support 9 billion people in 2050, 0.416 x 10~3 m2 of additional
land will be required for crop production. The remaining land available for
biomass energy would then be 1.28 x 10~3 m2. This would result in a projected
total of 7 to 10 TW of power.
This would be a massive undertaking, requiring that almost all of the crop
production potential on the planet be utilized. In addition, there are significant
obstacles, not the least of which is the issue of water resources. Also, cellulose
derived from biomass must be readily converted to a liquid fuel preferably
ethanol presenting a challenge for the chemical sciences.
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R&D CHALLENGES IN THE CHEMICAL SCIENCES
37
Nuclear power, which will be discussed in greater detail in other presenta-
tions in this workshop, would require 10,000 new 1 gigawatt nuclear power plants
in order to provide 10 terawatts of additional energy. Building one of these power
plants every 2 miles along the California coast would provide only 300 of the
needed 10,000 power plants. Clearly, it will not be practical for nuclear power to
be the sole source of this additional power.
Solar power is still another option for noncarbon primary power. Theoreti-
cally there is 1.2 x 105 TW of solar energy potential. However, if solar cells are
assumed to be 10 percent efficient, realistic land estimates lead to a practical
value of 600 TW of available incident solar power, leading to 60 TW of generated
power with 10 percent conversion efficiency.
To generate 20 TW of power using solar cells with 10 percent efficiency
requires approximately 0.16 percent of the world's landmass, including 8.8 per-
cent of the landmass of the United States. To generate 12 TW of primary power
by this method would require 0.1 percent of the Earth's landmass, including 5.5
percent of the United States.
These numbers are still quite large, but compared to other methods for gener-
ating noncarbon primary power, solar power appears to be the most compelling
method. To achieve anything approaching these numbers using solar energy
requires one of three approaches. A low-efficiency low-cost method is through
photosynthesis. Alternatively, a highly efficient process yet high-cost method uses
photovoltaics. A third method utilizes semiconductor liquid junctions and photo-
catalysts with the ultimate goal of using sunlight to split water into hydrogen and
oxygen or to make electricity. Both the cost and efficiency of this process are
moderate.
Production capacity for solar electricity is currently limited to about 100 MW
per year, even though it is a subsidized industry. This industry is growing
rapidly on the order of 30 percent per year although it must be noted that this
is from a small base. Solar electricity currently makes up 0.1 percent of total
electricity production.
The rate of progress for a variety of different technologies shows increasing
photovoltaic efficiency, yet most of these technologies have to contend with
physical limitations. Silicon or crystalline semiconductors have very high con-
version efficiencies, yet these crystals are very costly to make because the grain
sizes must be large. When smaller grain sizes are used, these semiconductors are
cheaper to make, but they have much shorter lifetimes. Single-crystal silicon can
be replaced with a less expensive organic material, but these organic films cur-
rently also have short lifetimes and therefore produce devices with low efficiency.
Based on these physical limitations it appears unlikely that, with normal
market forces and normal research advances, these technologies will provide the
amount of power needed economically by 2050. What is likely to be needed is the
implementation of a new solar technology that initially offers less performance
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38
ENERGY AND TRANSPORTATION
but at a far lower cost, and one that eventually overtakes present technologies as
it improves.
In terms of technologies for photovoltaics, a grand challenge would be to
devise alternatives to massive single crystals, which clearly cannot be produced
economically. One way to achieve this goal would be to passivate grain bound-
aries in order to get polycrystalline samples to act as a large single crystal.
Electron transfer agents could be used to link together grain boundaries, thereby
allowing electrons to move from grain to grain without inducing recombination at
the grain boundary edges.
One particular method for achieving this goal that is currently being investi-
gated utilizes titanium dioxide, an inexpensive pigment found in white paint.5
The TiO2 is coated on a glass slide and sensitized with a dye to absorb sunlight.
Electricity can be generated with 5 to lo percent efficiency. While there are still
questions about the long-term stability of this material, it does represent a new
and inexpensive approach to photovoltaics different than technologies presently
used. Other approaches with different light absorbers, such as interpenetrating
polymer networks, nanocrystals, and other inexpensive approaches to solar energy
conversion, should be explored as well.
Photoelectrolysis is a technology that converts light into both electrical and
chemical energy. Solid SrTiO3 is used in a photochemical cell where it absorbs
sunlight and effectively splits water with high quantum yield. Electrolysis of water
using this process can be sustained almost indefinitely. However, the band gap
for SrTiO3 (3.4 eV), is in the ultraviolet range, and materials with a lower band
gap either are not stable in water, cannot sustain the electrolysis of water, and/or
cannot absorb sunlight efficiently. Catalysts are also needed to effectively con-
vert the photogenerated charge into chemical fuels.
SUMMARY
To meet the increased demands for primary power in the 21st century, normal
economic driving forces appear to indicate that the demand for 28 TW of power
without unacceptable environmental consequences could result from a combina-
tion of wind, solar, biomass, and nuclear power. However, sources such as wind
and solar require new technologies to effectively store and transport power with
little loss.
Another important consideration with solar power as a source of energy is
that it inherently provides electrical power. Only about 10 percent of energy
consumption is presently in the form of electricity, whereas the other 90 percent
is used for heating, transportation, and industry. Even if electricity were used to
meet part of these needs, it would not be used to meet all of the remaining 90 per-
5B. O'Regan and M. Gratzel, Nature, 353:737. 1991.
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R&D CHALLENGES IN THE CHEMICAL SCIENCES
39
cent of power consumption unless direct photochemical or efficient electro-
chemical methods for energy storage or fuel generation were developed.
To make fuels that are storable and transportable, there are two primary
chemical transformations to consider. One is the conversion of carbon dioxide to
methanol, and the other is splitting water. Methanol could be used in a fuel cell
where it is converted to carbon dioxide. This must be converted back to methanol
to close the carbon loop. Alternatively, if hydrogen is used as a replacement fuel
for carbon-based fuel, a hydrogen fuel cell that utilizes the product of solar or
electrical water splitting would be available for transportation.
Whichever of these alternatives is ultimately adopted, the need for additional
primary energy sources is apparent. In addition, the case can be made for sig-
nificant carbon-neutral energy systems in the future. These technologies present
significant challenges for the chemical sciences. For solar power, inexpensive
conversion systems must be developed that include effective energy storage.
Advances in the chemical sciences will also be needed to provide the new
chemistry required to support an evolving mix of fuels for primary and secondary
energy.
Representative terms from entire chapter:
chemical sciences
