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The collector has an airtight and insulated metal or wood frame and a black metal plate for absorbing heat with glazing in front of it. Solar radiation heats the plate that, in turn, heats the air in the collector. An electrically powered fan or blower pulls air from the room through the collector, and blows it into the room(s). Roof mounted collectors require ducts for supplying air from the room(s) to the collector and for distribution of the warm air into the room(s). Wall mounted collectors are placed directly on a south-facing wall. Holes are cut through the wall for the collector air inlet and outlets.
Simple "window box collectors" fit in an existing window opening. They can be active (using a fan) or passive. In passive types, air enters the bottom of the collector, rises as it is heated, and enters the room. A baffle or damper keeps the room air from flowing back into the panel (reverse thermosiphoning) when the sun is not shining. These systems only provide a small amount of heat, since the collector area is relatively small.
Central Heating Systems - It is possible to integrate solar air collectors into a central heating system, which generally requires a storage component. These types of systems are rarely installed now, but the concept received a great deal of attention in the late 1970s and early 1980's, when federal tax credits spurred innovations in an industry that was in its infancy.
Many of these systems stored solar heat from the collectors in a large bin of rock. In a typical design, a blower circulates warm air from the collectors through a large duct into a plenum (a mixing space at the top or side of the bin), then through the rocks, which absorb most of the heat, and into a second plenum at the bottom or other side of the bin. The air then returns to the collectors for reheating. When the house requires heat, air is pulled from return ducts in the house in a reverse direction through the bin and is pushed into supply ducts for heating individual rooms. Thus, the rock bin serves as storage and as a heat exchanger. Bin temperatures can reach 140°F (60°C). If the air in the bin is too cool, a back-up system heats the air leaving the bin to the desired temperature before distributing it.
A rock bin requires 1/2 to 1 cubic foot (0.014 to 0.028 cubic meters) of volume for every square foot of collector area. (This is two to three times more space for storage materials than liquid system tanks.) Rocks of uniform size (3/4 inches to 1 1/2 inches [19 to 38 millimeters] in diameter, such as river rock, are ideal. The rock must be clean, dry, and without any dirt or gravel. The rocks must be kept dry inside the bin to prevent problems with mold, mildew, and insects.
An ideal location for a bin is a crawl space or basement, because warm air naturally rises to the living space, though they can be located outdoors, above or below ground. The bins can be made ocinder blockck, concrete, or wood. They must be tightly constructed and sealed to prevent air leaks and moisture intrusion, and well insulated. Treated wood needs to be sealed with a plastic liner to keep gases released by the plywood from entering the air.
Because of the difficulty in keeping the bin dry, and thereby avoiding the growth of mold and mildew on the rocks, rock bin storage is only appropriate in very dry climates. Even then, it may be difficult to control mold and mildew, posing potentially severe indoor air quality problems. Also, the electrical energy necessary to power the fans to move the air through the system reduces its overall efficiency and cost effectiveness. These issues and the large volume of storage area needed are the reasons why few if any of these systems are now installed.
One option is to store heat from the collectors in a water tank. This requires an air-to-water heat exchanger similar to a car radiator located exterior to the tank. A blower circulates warm air from the air collectors across/through the heat exchanger and back to the collectors when the air in the collectors is at a specified temperature. A thermostat controls the blower fan. The heat is then distributed from the storage in the same way as in a liquid system (see below).
Liquid Systems
Liquid solar collectors are most appropriate for central heating. They are the same as those used in solar domestic water heating systems. Flat-plate collectors are the most common, but evacuated tube and concentrating collectors are also available. In the collector, a heat transfer or "working" fluid such as water, antifreeze (usually non-toxic propylene glycol), or other type of liquid absorbs the solar heat. At the appropriate time, a controller operates a circulating pump to move the fluid through the collector. The liquid flows rapidly through the collectors, so its temperature only increases 10-to-20° F (5.6-to-11°C ) as it moves through the collector. Heating a smaller volume of liquid to a higher temperature increases heat loss from the collector and decreases the efficiency of the system. The liquid flows to either a storage tank or a heat exchanger for immediate use. Other system components include piping, pumps, valves, an expansion tank, a heat exchanger, a storage tank, and controls.
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