There are basically
three types of solar
collectors for heating
water: flatplate,
evacuatedtube,
and concentrating.
The most
common type, a
flat-plate collector, is
an insulated,
weatherproof box
containing a dark
absorber plate
under a transparent
cover. Evacuated-
tube collectors
are made up of
rows of parallel, transparent glass tubes.
Each tube consists of a glass outer tube
and an inner tube, or absorber, covered
with a coating that absorbs solar energy
but inhibits heat loss. Concentrating collectors
for residential applications are usually
parabolic-shaped mirrors (like a trough)
that concentrate the sun’s energy on an
absorber tube called a receiver that runs
along the axis of the mirrored trough and
contains a heat-transfer fluid.
All three types of collectors heat water by
circulating household water or a heat-trans-
fer fluid such as a nontoxic antifreeze from
the collector to the water storage tanks. Collectors
do this either passively or actively.
Passive solar water-heating systems use
natural convection or household water
pressure to circulate water through a solar
collector to a storage tank. They have no
electric components that could break, a
feature that generally makes them more
reliable, easier to maintain, and possibly
longer lasting than active systems.
An active system uses an electric pump to
circulate water or nontoxic antifreeze
through the system. Active systems are
usually more expensive than passive systems, but they are also more efficient.
Active systems also can be easier to retrofit
than passive systems because their storage
tanks do not need to be installed above or
close to the collectors. Also, the moving
water in the system will not freeze in cold
climates. But because these systems use
electricity, they will not function in a
power outage. That’s why many active systems are now combined with a small
solar-electric panel to power the pump.
The amount of hot water a solar water
heater produces depends on the type and
size of the system, the amount of sun
available at the site, proper installation,
and the tilt angle and orientation of the
collectors. But if you’re currently using an
electric water heater, solar water heating is
a cost-effective alternative. If you own a
swimming pool, heating the water with
solar collectors can also save you money.