Parabolic Trough Systems
Trough systems capture the sun's energy in a parabolically curved, trough-shaped reflector and then focus it onto a receiver pipe running along the focus line of the parabola. This focused solar energy heats oil flowing through the pipe, and the thermal energy is then used to generate electricity in a conventional steam generator.
A collector field comprises many troughs in parallel rows aligned on a north-south axis. This configuration enables the single-axis troughs to track the sun from east to west during the day to ensure that the sun is continuously focused on the receiver pipes.
Trough designs can store energy by accumulating the hot heat transfer fluid in insulated tanks, allowing for electricity generation several hours into the evening. Currently, all parabolic trough plants are "hybrids," meaning they use fossil fuel to supplement the solar output during periods of low solar radiation. Typically a natural gas-fired heater or a gas steam boiler/reheater is used, but troughs could also be integrated with existing coal-fired plants as pre-heaters.
Trough systems have proven their reliability in commercial systems. Thus far, 354 megawatts of solar trough capacity have been installed in California. These nine plants, called the Solar Electric Generating System (SEGS) plants, have provided power to the grid for a cumulative total of 100 plant-years.
Electricity costs from parabolic trough systems have demonstrated a downward trend. While the initial plants were producing power at a cost of roughly 36 cents per kilowatt-hour back in 1985, larger plants and improved operations and maintenance practices had reduced costs to about 15 cents per kilowatt-hour by the early 1990s. Repayment of initial capital investments and further reduction in O&M costs had cut power costs to about 10 cents per kWh in 2000. Future costs depend on many things such as technical progress, production rates and continuity, political, economic, and financial scenarios, and more importantly market needs and acceptance.
The SEGS plants have acted as an important proving ground for parabolic trough technology. Improved maintenance of the troughs over the years has boosted the availability of each collector to greater than 99.5 percent of the time.
Although solar trough development has been stagnant for more than 12 years, a new parabolic trough plant is now under development near Boulder City, Nevada. This plant could be the third-largest solar thermal power plant in the nation, with a capacity of 50 megawatts. The plant is expected to begin operating by March 2005.