SOLAR ENERGY
Our primary source of clean, abundant energy is the sun. The sun deposits 120,000 TW of
radiation on the surface of the Earth, far exceeding human needs even in the most aggressive
energy demand scenarios. The sun is Earth’s natural power source, driving the circulation of
global wind and ocean currents, the cycle of water evaporation and condensation that creates
rivers and lakes, and the biological cycles of photosynthesis and life. Covering 0.16% of the land
on Earth with 10% efficient solar conversion systems would provide 20 TW of power, nearly
twice the world’s consumption rate of fossil energy and the equivalent 20,000 1-GWe nuclear
fission plants. These comparisons illustrate the impressive magnitude of the solar resource,
providing an energy stream far more potent than present-day human technology can achieve.
All routes for utilizing solar energy exploit the functional steps of capture, conversion, and
storage. The sun’s energy arrives on Earth as radiation distributed across the color spectrum
from infrared to ultraviolet. The energy of this radiation must be captured as excited electronhole pairs in a semiconductor, a dye, or a chromophore, or as heat in a thermal storage medium.
Excited electrons and holes can be tapped off for immediate conversion to electrical power, or
transferred to biological or chemical molecules for conversion to fuel. Natural photosynthesis
produces fuel in the form of sugars and other carbohydrates derived from the reduction of CO2 in
the atmosphere and used to power the growth of plants. The plants themselves become available as biomass for combustion as primary fuels or for conversion in reactors to secondary fuels like
liquid ethanol or gaseous carbon monoxide, methane, and hydrogen. We are now learning to
mimic the natural photosynthetic process in the laboratory using artificial molecular assemblies,
where the excited electrons and holes can drive chemical reactions to produce fuels that link to
our existing energy networks. Atmospheric CO2 can be reduced to ethanol or methane, or water
can be split to create hydrogen. These fuels are the storage media for solar energy, bridging the
natural day-night, winter-summer, and cloudy-sunny cycles of solar radiation.
In addition to electric and chemical conversion routes, solar radiation can be converted to heat
energy. Solar concentrators focus sunlight collected over a large area to a line or spot where heat
is collected in an absorber. Temperatures as high as 3,000°C can be generated to drive chemical
reactions, or heat can be collected at lower temperatures and transferred to a thermal storage
medium like water for distributed space heating or steam to drive an engine. Effective storage of
solar energy as heat requires developing thermal storage media that accumulate heat efficiently
during sunny periods and release heat slowly during dark or cloudy periods. Heat is one of the
most versatile forms of energy, the common link in nearly all our energy networks. Solar thermal
conversion can replace much of the heat now supplied by fossil fuel.
Although many routes use solar energy to produce electricity, fuel, and heat, none are currently
competitive with fossil fuels for a combination of cost, reliability, and performance. Solar
electricity from photovoltaics is too costly, by factors of 5–10, to compete with fossil-derived
electricity, and is too costly by factors of 25–50 to compete with fossil fuel as a primary energy
source. Solar fuels in the form of biomass produce electricity and heat at costs that are within
range of fossil fuels, but their production capacity is limited. The low efficiency with which they
convert sunlight to stored energy means large land areas are required. To produce the full 13 TW
of power used by the planet, nearly all the arable land on Earth would need to be planted with
switchgrass, the fastest-growing energy crop. Artificial photosynthetic systems are promising
routes for converting solar energy to fuels, but they are still in the laboratory stage where the
principles of their assembly and functionality are being explored. Solar thermal systems provide
the lowest-cost solar electricity at the present time, but require large areas in the Sun Belt and
breakthroughs in materials to become economically competitive with fossil energy as a primary
energy source. While solar energy has enormous promise as a clean, abundant, economical
energy source, it presents formidable basic research challenges in designing materials and in
understanding the electronic and molecular basis of capture, conversion, and storage before its
promise can be realized.
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