Solar water heaters are made up of collectors, storage tanks, and,
depending on the system, electric pumps.
Most commercially available solar water heaters require a well-insulated
storage tank. Many systems use converted electric water heater tanks
or plumb the solar storage tank in series with the conventional water
heater. In this arrangement, the solar water heater preheats water
before it enters the conventional water heater.
Some solar water heaters use pumps to recirculate warm water from
storage tanks through collectors and exposed piping. This is generally
to protect the pipes from freezing when outside temperatures drop
to freezing or below.
Types of Solar Water Heaters
Solar water heaters can be either active or passive. An active system
uses an electric pump to circulate the heat-transfer fluid; a passive
system has no 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.
Solar water heaters are also characterized as open loop (also called "direct")
or closed loop (also called "indirect"). An open-loop system circulates
household (potable) water through the collector. A closed-loop system
uses a heat-transfer fluid (water or diluted antifreeze, for example)
to collect heat and a heat exchanger to transfer the heat to household
water.
Active Systems
Active systems use electric pumps, valves, and controllers to circulate
water or other heat-transfer fluids through the collectors. They
are usually more expensive than passive systems but are also more
efficient. Active systems are usually easier to retrofit than passive
systems because their storage tanks do not need to be installed above
or close to the collectors. But because they use electricity, they
will not function in a power outage.
Open-Loop Active Systems
Open-loop active systems use pumps to circulate household water
through the collectors. This design is efficient and lowers operating
costs but is not appropriate if your water is hard or acidic because
scale and corrosion quickly disable the system.
These open-loop systems are popular in nonfreezing climates such
as Hawaii. They should never be installed in climates that experience
freezing temperatures for sustained periods. You can install them
in mild but occasionally freezing climates, but you must consider
freeze protection.
Recirculation systems are a specific type of open-loop system that
provide freeze protection. They use the system pump to circulate
warm water from storage tanks through collectors and exposed piping
when temperatures approach freezing. Consider recirculation systems
only where mild freezes occur once or twice a year at most. Activating
the freeze protection more frequently wastes electricity and stored
heat.
Of course, when the power is out, the pump will not work and the
system will freeze. To guard against this, a freeze valve can be
installed to provide additional protection in the event the pump
doesn't operate. In freezing weather, the valve dribbles warmer water
through the collector to prevent freezing. Consider recirculation
systems only where mild freezes occur once or twice a year
at most. Activating the freeze protection more frequently wastes
electricity and stored heat.
Closed-Loop Active Systems
These systems pump heat-transfer fluids (usually a glycol-water
antifreeze mixture) through collectors. Heat exchangers transfer
the heat from the fluid to the household water stored in the tanks.
Double-walled heat exchangers prevent contamination of household
water. Some codes require double walls when the heat-transfer fluid
is anything other than household water.
Closed-loop glycol systems are popular in areas subject to extended
freezing temperatures because they offer good freeze protection.
However, glycol antifreeze systems are a bit more expensive to buy
and install, and the glycol must be checked each year and changed
every 3 to 10 years, depending on glycol quality and system temperatures.
Drainback systems use water as the heat-transfer fluid in the collector
loop. A pump circulates the water through the collectors. The water
drains by gravity to the storage tank and heat exchanger; there are
no valves to fail. When the pumps are off,the collectors are empty,
which assures freeze protection and also allows the system to turn
off if the water in the storage tank becomes too hot.
Pumps in Active Systems
The pumps in solar water heaters have low power requirements, and
some companies now include direct current (DC) pumps powered by small
solar-electric (photovoltaic, or PV) panels. PV panels convert sunlight
into DC electricity. Such systems cost nothing to operate and continue
to function during power outages.
Passive Systems
Passive systems move household water or a heat-transfer fluid through
the system without pumps. Passive systems have no electric components
to break. This makes them generally more reliable, easier to maintain,
and possibly longer lasting than active systems.
Passive systems can be more expensive than active systems, but they
can also be more efficient. Installed costs for passive systems range
from about $1,000 to $3,000 more, depending on whether it is a simple
batch heater or a sophisticated thermosiphon system.
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