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 commercial operation.
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 engineâ€”usually a Stirling cycle engineâ€”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.