Buyer's Guide to Solar Heating
(Page 2 of 5)
December 2006/January 2007
By Dan Chiras
Two Kinds of Heaters
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Solar hot-air systems have been around since the 1950s. Today’s systems fall into two categories: open-loop and closed-loop. The difference lies in the source of air entering the system.
In open-loop systems, collectors draw in cold outdoor air, heat it and transfer the heated air into buildings. Collectors used in open-loop systems, known as transpired air collectors, incorporate a dark-colored, perforated metal facing, called the absorber plate. There’s no glazing over the plate; the sides and back are insulated to reduce heat loss. Sunlight striking the absorber plate of a transpired collector heats it. Air is drawn through the plate by a blower and is piped to the interior of the building.
Closed-loop systems, the most popular option today, draw cool air from the house, heat it and return the heated air to the interior. These collectors include glass or other clear glazing over an absorber plate, typically made of dark-colored metal. Its surface is roughened to increase air turbulence so that air absorbs heat from it more effectively. The collector’s back and sides are insulated.
Air from the home enters the bottom of the panel and moves up, either behind the absorber plate (back-pass collectors) or in front of it (front-pass collectors). Air moves in and out of the collector through ductwork running through the wall or roof—the shorter the better. Small registers are mounted inside on the air intake and outlet openings.
Of the two types of glazed closed-loop collectors, back-pass collectors are more common. Back-pass designs use single panes of glass; front-pass designs are double-glazed to reduce heat loss. That makes back-pass collectors somewhat less expensive to manufacture and about 50 pounds lighter than front-pass collectors and, therefore, a little easier to install.
Most systems are thermostatically controlled and include backdraft dampers on the outlets to prevent reverse convective airflow at night, which would suck heat out of a building.
Heat Output
Closed-loop solar hot-air panels can substantially boost the temperature of air flowing through them. According to the U.S. Department of Energy, air entering a glazed collector at 70 degrees is typically warmed an additional 70 to 90 degrees. Open-loop transpired air collectors may provide considerably less heat than glazed collectors.
Chuck Marken, president of AAA Solar and solar thermal editor of Home Power magazine, says transpired air collectors increase the temperature of the air flowing through them only about 20 degrees, which is probably of little value to residential structures.
Closed-loop residential solar hot-air systems can produce impressive results. Steve Andrews, a residential energy expert based in Denver, Colo., for example, installed a collector to heat the bottom 500 square feet of his tri-level home, which was usually 5 to 6 degrees colder than the rest of his house. He found the collector made a big difference during sunny winter days and the following evenings. “Overall, the comfort improvement was dramatic,” he says.
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