days in your area.) Winter clouds and cold temperatures mean that there isn't all that much solar energy to be had
anyway, and winter shutdowns of, say, three months reduce the heater's annual
Btu output by only 15 per cent or so. We feel winter shutdowns are the most
practical solution to freezeproofing:
Make sure your tank and all exterior plumbing can be drained completely.
Glazing
The best glazing material for batch heaters seems to be one of the specially designed solar plastics.
(Kalwall, Filon, 7410, etc.) They're less expensive than glass, they're much easier to work
with, they offer good resistance to breakage, and they have good optical and thermal
properties.
Collection Efficiency
In the simplest batch heater designs, the entire inner surface of the enclosure is painted flat black to absorb solar heat.
This technique works, but it's not ideal because using the box's inner surface as
a collector raises the temperature of the (next page)
Above There are only two ways in which this plumbing differs from the previous designs. First, a "vacuum
breaker" insures rapid and complete drainage of the tank. Second, the inner
tube, which carries hot water out of the tank, is insulated inside the tank with
a loose-fitting jacket of CPVC pipe. (Full plumbing details for this design
appear in the following article.) |
Illustration B Horizontal Two-Tank System
Above Here, a horizontal two-tank system forms a canopy for the home's patio
doors and is housed in a double-glazed, heavily insulated enclosure of its own. Wall-mounted systems such as this and the previous one-tank version must
be braced securely to carry the tank's considerable weight: Two 40-gallon tanks,
for example, weigh over 750 pounds when filled with water. If you live in earthquake-prone areas, check your building codes before attempting this type
of mount, or look into one of the ground mounts shown in the next two
illustrations.
Above Two-tank horizontal systems, if properly interconnected, can achieve higher output temperatures than single-tank designs because the incoming
cold water is kept away from the hot outlet. Even so, these tanks still cannot take
full advantage of warm water's tendency to rise. In this system, provision has been made for convenient draining of both tanks (for freezing-weather
shutdowns or for maintenance) by the additions of a drain valve and a one-way valve.
Illustration D Vertical One-Tank System
Below This design combines the good temperature stratification inside vertical tanks with the simplicity of a single-tank design. The enclosure is mounted
close to the house to minimize heat loss, and its sheathing matches the home's
original siding to provide a "built-in" appearance. Our batch heater shown on
the cover is based on this type of single-tank, vertical system. It uses a "selective
surface" absorber coating on the tank, triple glazing on the enclosure, and a
cusp reflector to provide superior thermal performance.
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