Russ Hewett, NREL/PIX05642 Figure 3. Parabolic-Trough Collector 5 About the Technology Glazing frame Single or The basic principle of solar water heating double glazing Temperature is intuitive and straightforward. A dark sur- tolerant face is positioned to absorb sunlight and gasket convert it to heat. Water or another heat Outlet transfer fluid passes along that hot surface manifold to pick up the heat—either for direct use or for transfer through a heat exchanger to the end use. Simple solar water heating has Box been around since ancient times. More than Inlet Flow passages manifold a million solar water heaters have been Temperature installed in the United States.
Most of these Absorber Backing tolerant were installed during the 1980s, when tax plate insulation 02279809m credits were available, and nearly all use Figure 1. Flat-plate collector. "flat-plate" collectors, which essentially place the absorber surface inside an insu- lated box. (See Figures 1 and 2.) Parabolic-trough solar water-heating sys- tems carry solar water-heating technology a step further by concentrating the sunlight before it strikes the absorber. Mirrored sur- faces curved in a parabolic shape linearly extended into a trough shape focus sunlight on an absorber tube running the length of the trough (Figure 3).
A heat transfer fluid—usually a solution of water and antifreeze—is pumped around a loop through the absorber tube of the collector. There it picks up heat and then goes to a heat exchanger where it either directly heats potable water or heats a thermal storage tank. (See Figure 4.) As concentrating systems, parabolic troughs use only direct radiation. Unlike flat-plate collectors, they cannot use diffused sunlight. Using tracking systems makes up Figure 2. Flat-plate collector array providing domestic hot water for U.S.
for this inability to collect more of the direct Environmental Protection Agency headquarters in leased space in the Waterside Mall light, but cloudy skies become a more criti- in Washington, D.C. cal factor. Parabolic-trough collectors are therefore much more likely to be effective in areas such as the Southwestern United States that have good solar resources domi- nated by direct-beam sunlight. (See Figures 5 and 6.) To make efficient use of the tracking sys- tems and of the much higher temperatures that can be generated by a concentrating system, it is more cost effective to build a Glass encased absorbent-coated larger system that will be used continuously.
receiver tube Parabolic troughs are generally impractical Flex-hose to carry for small systems, but can be much less transfer fluid to/from receiver tube Parabolic shape expensive than flat-plate collectors if the aluminum trough system is large enough. Parabolic-trough with reflective coating Glazing frame Single or double glazing Temperature tolerant gasket Outlet manifold Box Inlet Flow passages manifold Temperature Absorber Backing tolerant plate insulation 02279809m Glass encased absorbent-coated receiver tube Flex-hose to carry transfer fluid to/from receiver tube Parabolic shape aluminum trough with reflective coating 02279803m