|box as a whole and increases heat loss. The resulting higher temperatures also
may cause glues, plastics, or other heat-sensitive components to deteriorate.
Better batch heaters use some sort of shiny surface to reflect the incoming sunlight onto the tank. For example, it's a simple matter to line the enclosure with aluminum foil or other shiny metal. And if the heater is equipped with movable insulating doors, the backs of the doors should also be covered with reflective materials so they can be angled during the day to reflect additional sunlight onto the tank.
Flat reflective surfaces like these help raise the collection efficiency of batch heaters, but have some built-in limitations. Hold a pencil up to a mirror (an excellent flat reflector) and you'll see why: The image of the pencil occupies only a small fraction of the mirror's surface, and you can see a reflected view of you and the room to either side of the pencil. A flat reflector in a batch heater works the same way. A large amount of the reflected solar energy simply misses the cylindrical water tank altogether and is bounced right back out through the glazing.
The best batch heaters use a curved reflector to collect and accurately focus sunlight on the tank. In our design, the reflector is made from two intersecting curves, the point of which is called a "cusp." (See Illustration E.) A cusp reflector keeps the incoming solar energy
focused on the tank all day long as the sun moves across the sky, and can even gather diffuse energy that's been scattered by clouds or haze. In fact, this type of reflector offers many of the same advantages provided by a high-tech sun-tracking collector, but without the added costs and complexity.
A batch heater should be constructed carefully from high-quality materials to insure a long life and minimal maintenance. Cheap caulking, for instance, may save a dollar or two now, but later
compromise the enclosure's weather-tightness -- hardly a bargain. Likewise, all plumbing components should be
selected for resistance to corrosion. We recommend glass-lined, stone-lined,
or heavily galvanized tanks, coupled with
low-cost plastic plumbing.
In order to work properly, a batch heater needs a good location. First of all, it should be located as close as possible to your existing water heater to minimize heat losses from the connecting pipes. Second (and more obviously), it needs plenty of sunlight.
It's easy to find a good solar site. You start by determining where true or "solar" south is. (It's usually different from magnetic south as shown by a compass.) The fastest way to find true south is to drive a stake into the ground and observe its shadow at solar noon, when the shadow forms a precise north-south line. (South is toward the sun.) Solar noon is the time exactly halfway between sunrise and sunset, and may or may not coincide with 12 p.m. on the clock. You can find the times of sunrise and sunset in any almanac or daily newspaper.
Ideally, a solar heater should face due south. But if your home is off the mark, don't give up. Any location that lets you mount your batch heater so that it faces within about 20 degrees east or west of due south will provide upwards of 90 percent of the energy available at a due-south orientation, and that's still pretty
Of course, shadows will ruin the performance of even a perfectly oriented solar system, so you need to be sure your south-facing location will remain essentially shade-free during the prime solar collection hours of 9 a.m. to 3 p.m. You can estimate sun and shadow at your site with an ingenious method developed by New York's Energy Task Force:
Stand where you want to place your collectors and face true south. Hold your left arm out straight, level with your
Illustration E Cusp Reflector Cross Section
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26 RODALE'S NEW SHELTER JULY/AUGUST 1981