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One of the most
cost-effective ways to include
renewable technologies into a
building is by incorporating
solar hot water.
A typical
residential solar water-heating
system reduces the need for
conventional water heating by
about two-thirds. It minimizes
the expense of electricity or
fossil fuel to heat the water
and reduces the associated
environmental impacts.
Solar Water
Heating for Buildings
Most solar
water-heating systems for
buildings have two main parts:
(1) a
solar collector
and (2) a storage tank. The most
common collector used in solar
hot water systems is the
flat-plate
collector.
Solar water
heaters use the sun to heat
either water or a heat-transfer
fluid in the collector. Heated
water is then held in the
storage tank ready for use, with
a conventional system providing
additional heating as necessary.
The tank can be a modified
standard water heater, but it is
usually larger and very well
insulated. Solar water heating
systems can be either active or
passive, but the most common are
active systems.
Active solar
water heaters
Active solar
water heaters rely on electric
pumps, and controllers to
circulate water, or other
heat-transfer fluids through the
collectors. These are the three
types of active solar
water-heating systems:
1.Direct-circulation systems use
pumps to circulate pressurized
potable water directly through
the collectors. These systems
are appropriate in areas that do
not freeze for long periods and
do not have hard or acidic
water. These systems are not
approved by the Solar Rating &
Certification Corporation (SRCC)
if they use recirculation freeze
protection (circulating warm
tank water during freeze
conditions) because that
requires electrical power for
the protection to be effective.
2.Indirect-circulation systems
pump heat-transfer fluids
through collectors. Heat
exchangers transfer the heat
from the fluid to the potable
water. Some indirect systems
have "overheat protection,"
which is a means to protect the
collector and the glycol fluid
from becoming super-heated when
the load is low and the
intensity of incoming solar
radiation is high. The two most
common indirect systems are:
a.
Antifreeze. The heat transfer
fluid is usually a glycol-water
mixture with the glycol
concentration depending on the
expected minimum temperature.
The glycol is usually food-grade
propylene glycol because it is
non-toxic.
b.Drainback
systems, a type of indirect
system, use pumps to circulate
water through the collectors.
The water in the collector loop
drains into a reservoir tank
when the pumps stop. This makes
drainback systems a good choice
in colder climates. Drainback
systems must be carefully
installed to assure that the
piping always slopes downward,
so that the water will
completely drain from the
piping. This can be difficult to
achieve in some circumstances.
Passive solar
water heaters
Passive solar
water heaters rely on gravity
and the tendency for water to
naturally circulate as it is
heated. Because they contain no
electrical components, passive
systems are generally more
reliable, easier to maintain,
and possibly have a longer work
life than active systems. The
two most popular types of
passive systems are:
1.Integral-collector storage
systems consist of one or more
storage tanks placed in an
insulated box with a glazed side
facing the sun. These solar
collectors are suited for areas
where temperatures rarely go
below freezing. They are also
good in households with
significant daytime and evening
hot-water needs; but they do not
work well in households with
predominantly morning draws
because they lose most of the
collected energy overnight.
2.Thermosyphon
systems are an economical and
reliable choice, especially in
new homes. These systems rely on
the natural convection of warm
water rising to circulate water
through the collectors and to
the tank (located above the
collector). As water in the
solar collector heats, it
becomes lighter and rises
naturally into the tank above.
Meanwhile, the cooler water
flows down the pipes to the
bottom of the collector,
enhancing the circulation. Some
manufacturers place the storage
tank in the house's attic,
concealing it from view.
Indirect thermosyphons (that use
a glycol fluid in the collector
loop) can be installed in
freeze-prone climates if the
piping in the unconditioned
space is adequately protected.
Learn more about
freeze-protected
piping and
research being conducted at the
National Renewable Energy
Laboratory.
As
you can see in the next chart
only a small amount of solar is
being used. It is estimated in
10 years solar will be a leader
in Renewable energy.
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