Solar water heating panels in their simplest form are made from a sheet
of metal painted black which absorbs the suns energy. Water is fed through
the panel in pipes attached to the metal sheet and picks up the heat in
the metal. . The system is usually installed on the roof.
flat-plate collector is a metal box with a glass or plastic cover (called
glazing) on top and a dark-colored absorber plate on the bottom. The sides
and bottom of the collector are usually insulated to minimize heat loss.
Sunlight passes through the glazing and strikes the absorber plate, which
heats up, changing solar energy into heat energy. The heat is transferred
to liquid passing through pipes attached to the absorber plate. Absorber
plates are commonly painted with "selective coatings," which
absorb and retain heat better than ordinary black paint. Absorber plates
are usually made of metal—typically copper or aluminum—because
the metal is a good heat conductor. Copper is more expensive, but is a
better conductor and less prone to corrosion than aluminum. In locations
with average available solar energy, flat plate collectors are sized approximately
one-half- to one-square foot per gallon of one-day's hot water use.
Low Temperature Collectors
Low temperature solar collectors are typically unglazed flat plate collectors,
intended to operate at temperatures only 5 to 30 degrees above ambient
temperature. Low temperature liquid collectors are used for swimming pool
heating. With light glazing and enclosure, they are used as air collectors
for agricultural low-temperature applications such as crop drying.
Plastic low-temperature collectors have been used widely for swimming
pool heating. However, because of the deteriorating effects of ultra violet
radiation and stagnation temperatures on some plastic solar collectors,
metal collectors are being more widely utilized. Unglazed collectors with
aluminum absorber plates and copper water passages appear to be most cost
effective over the typical metal collector lifetime of 20 years or more.
All-copper collectors for swimming pool heating also work well, but are
generally more expensive for the same performance characteristics. Copper
is preferred over any other metal for water passages because of its high
conductivity and compatibility with water. Almost all other metals must
be separated from direct contact with the water being heated by a heat
exchanger, which seriously reduces the collector efficiency.
Medium Temperature Collectors
Medium temperature collectors typically are flat-plate collectors, enclosed
in an insulated case, with one or two glazings. The intended temperature
range of operation is from about 15 to 200 degrees F above ambient temperature.
For the lower end of this range, single-glazed collectors with non-selective
absorber plates are most cost effective. In the middle and high end of
the range, selective collectors with one or two glazings become more cost
effective. Typical applications include water heating, space heating and
some medium temperature industrial heating uses.
High Temperature Collectors
High temperature collectors include some overlap from flat plate collectors
in the medium temperature range, with selective absorber plates and heavy
insulation, and may have temperature capabilities enhanced in installation
by being mounted in a sun-tracking system