Power Generation - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 196

car that used monocrystalline silicon cells of the advanced, laser-grooved
buried-grid type. The 1993 winner was powered by 20% efficient mono-
crystalline silicon PV cells, which provided an average speed of 85-km per
hour over the 300-km course.
Solar cells do not use most of the energy of sunlight. Light from the
sun has an average temperature of about 6300°K. The Helmholtz ratio of
sunlight is about 95%. This means that theoretically it should be possible
to convert 95% of the radiant-energy to electricity.
In practice, solar cells may only convert about 10% of the radiant en-
ergy into electricity. The other 90% of the sunlight is converted into ther-
mal energy.
When the sunlight strikes the semiconductor material, an electrical
potential is created by dislodging electrons due to the impact of the pho-
tons. Sunlight is made of photons that contain different amounts of pho-
ton-energy at different frequencies. The semiconductor material cannot
readily be matched to convert all types efficiently. This means that some
photons will not be converted at all because they have too little photon
energy and some photons will only have a part of their energy converted
to electricity because they have too much photon energy.
Solar cells that have several layers of different semiconductors can
be much more efficient. Each layer can be matched to a specific photon en-
ergy range.
Another type of solar cell separates the light into different colors.
Then each color is converted using a different type of semiconductor for
higher efficiency.
Solar cells may also use lenses or parabolic concentrators to convert
more of the radiant energy into thermal-energy. The lens increases the
brightness of the light and converts some of the photon energy into ther-
mal energy. A vapor cycle engine could be used to convert this thermal en-
ergy into electricity. This could provide an efficiency of nearly 30% in large
installations. An advantage with this system is that during the night or on
cloudy days fuel can be burned in a separate boiler to operate the system.
With the right solar concentrator and engine, efficiencies of over 50% are
possible. A modern gas turbine combined cycle can have up to 60% effi-
ciency and uses a much lower temperature of combustion than the sun.
The difficulty with solar energy involves the path that is used to get the
sunlight into the process. Most collection schemes allow radiant-energy to
escape. One way coatings generally only work to keep some of the lower
energy rays from escaping but not the higher energy rays.