While the components of an active system are more compact than are passive components, they are also less likely to be integral parts of the structure it self; so the collectors, in particular, are by no means hidden. An active system will require a collector area equal to about 1/4 to 1/3 of the square footage of the space to be heated. In other words, a 2000 square foot house will need approximately 500 to 700 square feet of collector to generate 75 percent of its heat, which is about the maximum heat most solar systems produce cost-effectively.
• Flat-plate collectors: There are several kinds of solar heat collectors, of which the flat-plate collector is the simplest, least expensive, and most frequently used. Although flat-plate collectors come in several forms, they are all made up of an insulated box with a heat absorbing plate at the bottom, a heat transfer component (pipe or duct tubing) in the middle, and a clear cover at the top.
The absorber, which may be made of copper, aluminum, or some other material, is ordinarily coated black to enhance its heat collecting abilities. Or it may be coated instead with a chemical “selective coating” designed to absorb a high proportion of the available heat while losing little of it through emission.
The primary purpose or the clear cover is to prevent heat loss from the collector by convection. Its secondary purpose is to keep the absorber plate clean. Most collector covers are made of tempered glass, although plastic and fiberglass are also used.
The cover should be removable, so it can be replaced if that becomes necessary; and it should be sealed. The seals keep the collector from leaking heat, as well as from gathering dust.
Thermostatically-controlled heaters will not turn on until they are warmer than the space they are designed to heat. Therefore inadequate insulation, which causes heat loss through the back and sides of the collector, is likely to require increased collection area. Since collectors cost about $30 to $45 per square foot plus installation, you have a vested interest in minimizing your collector’s square footage requirements and seeing that its insulation is adequate. Usually it should equal at least the R-value of your wall insulation; but if your collector is set on the ground instead of on the roof (see below), it may require an A-value as high as your ceiling’s.
At present, flat-plate collectors, in all their variations, are the only real option in a completely active solar heating sys tem. Compared with the other choices they are easy to use and readily avail able. Still, you might want to know of two other possibilities for the future, concentrating collectors and evacuated-tube collectors.
• Concentrating collectors: These are made with mirrors and lenses, and focus the heat they collect in the same way you would use a magnifying glass to start a fire. They can reach extremely hot temperatures—up to 1000 F. But be cause you are not likely to use 1000°F of heat in your home, such temperatures are very inefficient since in those ranges a high proportion of the heat generated is lost. Concentrating collectors also need direct sunlight—they can't use reflected or otherwise diffused light— and must therefore be capable of tracking the sun. This ability, of course, can create a whole new set of mechanical problems for the owner.
• Evacuated-tube collectors: These collectors are most effective at temperatures that far exceed anything needed to heat an ordinary house. If they ever become economically feasible for purposes other than industrial ones, they will be good house-sized solar heating components because their vacuum construction prevents them from losing heat in the cold or wind. This makes them reliably efficient in a way that flat plate collectors (which do lose heat to the ambient atmosphere) are not.
Placing the Collector
Banks of flat-plate collectors are most commonly mounted on the roof or on a wall with a generally southern orientation. Such a setting keeps them away from the dangers of foot traffic, and off ground space that may be used in other ways. Some solar researchers suggest that collectors be mounted instead on racks on the ground for easy access, and to minimize the expense of improving the structure of your house to hold the added weight.
Where you place your collectors will be determined by a wide variety of factors. For example, if you need collectors only for your swimming pool, and it is not very close to your house, you may decide to mount them on racks. (rack-mounted collectors).
• Orientation: A collector must be positioned to receive as much direct sun light as it can. However, since the sun moves in an annual cycle as well as a daily one (rising farther north in summer than in winter, and riding lower in the sky in winter than in summer), and since flat-plate collectors are usually mounted in fixed positions, they can not track the sun. Under such circum stances, facing due south is the most desirable position—in the US, the sun is always south of the ecliptic. However, morning haze and the relatively warmer afternoon temperatures make a slight southwesterly orientation even better. In fact, any orientation within 20 degrees either side of due south will give the collector enough sun to work efficiently, as long as it gets at least six hours of unobstructed access to whatever sun there is, every day, including December 21—the shortest day of the year.
• Tilt: This is the angle at which the collector is aimed at the sky. The ideal tilt is your house’s latitude plus 10 to 15 degrees for winter heating. Most commercially installed collectors tilt at 45 degrees, which about equals the sun’s position in the North American sky.
If you are building your own system and want to be precise about orientation and tilt, remember that these numbers are generalizations, for use as guidelines. Specific situations demand specific solutions and you will need to explore the peculiarities of your own environment before you place your collectors. Speaking with an experienced solar architect or engineer, and with satisfied solar users, will give you in formation you’ll never find in books or web sites.
Once your collector has gathered in all that heat, the storage unit is where it stays until you need it. For all practical purposes, three materials are currently regarded as successful for solar heat storage. Research continues into others.
Solar collectors can be placed virtually anywhere, as long as
their exposure to sunlight is not impeded. When placed on the roof (top),
they are out of the way of foot traffic, and least likely to be shaded
by nearby trees or buildings, but the roof’s structural supports may
have to be strengthened to accommodate their weight. If shadows from
nearby objects do not create problems, you can simply place collectors
out of the way where their orientation and tilt are optimum (middle).
Or they can be located at ground level (bottom) where winds that ordinarily
carry away some of the collected heat are minimal, and where no added
structural support for the house is necessary.
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