Concentrating Solar Power
Concentrating solar power technologies use reflective materials such as
mirrors to concentrate the sun's energy. This concentrated heat energy is
then converted into electricity.
Parabolic trough systems use curved mirrors to focus sunlight on an
absorber tube filled with oil or other fluid. The hot oil boils water to
produce steam, which is used to generate electricity. Since 1985, nine
power plants in the Mojave Desert called the Solar Electric Generating
Systems (SEGS) that use parabolic trough technology have been in full
Power tower systems use a large field of sun-tracking mirrors, called
heliostats, to concentrate sunlight onto a receiver on the top of a tower.
The sun heats a fluid inside the receiver. An early U.S. demonstration
plant, Solar One, used water as the fluid, generating steam in the tower
to drive a turbine to generate electricity. The plant was later converted
to Solar Two, which used molten salt as the fluid. The hot salt could be
stored, then used when needed to boil water into steam to drive a turbine.
A dish/engine system uses mirrors in the shape of a dish to collect and
concentrate the sun's heat onto a receiver. The receiver transfers the
solar energy to a heat engine—usually a Stirling cycle engine—that
converts the heat into mechanical energy, which drives a generator to
produce electricity. The receiver, heat engine, and generator are
integrated into one assembly that is mounted at the focus of the mirrored
dish. An alternative approach, called the open Brayton cycle, passes air
through a porous medium in the receiver, causing the air to heat and
expand rapidly. The hot air is then fed into a separate gas turbine that
drives a generator to produce electricity.
Concentrators use reflective surfaces of aluminum or silver on the front
or back surface of thin glass or plastic. Researchers are developing new
reflective materials, such as advanced polymer films, that are less
expensive to produce than glass. Stretched membranes are thin reflective
membranes stretched across a rim or hoop. Another membrane stretched on
the back creates a partial vacuum. This forces the membranes into a
spherical shape, which is the ideal concentrator shape.
Researchers are working with utilities on experimental hybrid power towers
that run on solar energy and natural gas. A similar solar/fossil fuel
hybrid is being developed for dish/engine systems. The advantage of hybrid
systems is that they could run continuously.
Concentrating solar power is the least expensive solar electricity for
large-scale power generation, and has the potential to make solar power
available at a very competitive rate. As a result, government, industry,
and utilities have formed partnerships with the goal of reducing the
manufacturing cost of concentrating solar power technologies.
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