Solar water heating systems, which use liquids as heat-transfer fluids, need protection from freezing in climates where temperatures fall below 42ºF (6ºC).
Don’t rely on a collector’s and the piping’s (collector loop’s) insulation to keep them from freezing. The main purpose of the insulation is to reduce heat loss and increase performance. For protecting the collector and piping from damage due to freezing temperatures, you basically have two options:
- Use an antifreeze solution as the heat-transfer fluid.
- Drain the collector(s) and piping (collector loop), either manually or automatically, when there’s a chance the temperature might drop below the liquid’s freezing point.
Source: EERE, US Department of Energy
Concentrating Solar Power’s (CSP) relatively low cost and ability to deliver power during periods of peak demand—when and where we need it—means it can be a major contributor to the nation’s future needs for distributed sources of energy. Large-scale concentrating solar power technologies include parabolic troughs and power towers.
There are more than 350 megawatts of parabolic trough systems connected to the electricity grid in southern California. Photo credit: Warren Gretz. Illustration: open domain.
Parabolic troughs have a proven track record as a technology that can function effectively for large-scale power needs. They currently provide the least expensive way to produce solar electricity. Parabolic-trough systems concentrate the sun’s energy through long rectangular, curved (U-shaped) reflectors. The reflectors are tilted toward the sun, focusing the sun’s energy on a pipe that runs down the center (focal point) of the trough. The sun’s energy heats oil flowing through the pipe, and the hot oil then is used to boil water in a conventional steam generator to produce electricity.
Power towers have also shown their efficiency in demonstration projects and one is in commercial operation in Spain, but they are not yet in use commercially in the United States. A power tower system uses a large field of reflectors to concentrate the sun’s energy onto the top of a tower, where it heats a receiver filled with a heat-exchange fluid, such as oil. The heated fluid is then used to generate electricity in a conventional steam generator. Systems using molten salt as the heat-exchange fluid retain heat very efficiently, enabling them to produce electricity up to three hours after the sun has set.
Source: EERE, U.S. Department of Energy