Solar Hot Water
Nearly 300 homes in San Diego have
solar water heating systems, and some have solar electric systems. The
solar water heating collectors on the roof look like skylights.
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.
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 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:
-
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.
-
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:
-
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.
-
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.