does not come on; also less total fuel is used to
bring the smaller second tank up to temperature.
Single-tank arrangements are not recommended
because they frequently activate the heating
element every time there is a draw of water, rather
than wait for the solar collectors to provide
additional heated water. The two-tank
arrangement (fig. 15-9) avoids this control
problem. Two-tank arrangements are suited to
retrofits since the second tank (the water heater) is
already there. A variation would be to use a heat
exchanger (copper coil) (fig. 15-10) in the
tempering tank collector loop for freeze
protection. The tempering tank then becomes an
inexpensive unpressurized tank.
Another method of heat storage in air systems
is latent heat storage. Latent heat is stored in a
material as it changes state from a solid to a liquid.
Materials that have melting points near the
temperatures supplied by solar collectors store
heat as they melt and release it as they resolidify.
The two materials that have received the most
attention are salt hydrates and paraffins.
Water may be stored in a variety of containers
usually made of steel, concrete, plastic, fiber glass,
or other suitable materials.
Steel tanks are commercially available and
have been used for water storage. They are
available in many sizes and are easy to install.
However, steel tanks are susceptible to corrosion
and should be lined or galvanized. Dissimilar
metal at pipe connections should be separated by
high-temperature rubber connections or galvanic
corrosion will occur. Steel tanks must be well
insulated to reduce heat losses.
corrosion-resistant and installed easily. They are
available in many shapes and sizes. Although
many commonly fabricated tanks begin to soften at
temperatures above 140°F, there are more
expensive, specially fabricated tanks available that
can withstand temperatures up to 150°F. The
types of plastics needed to store large quantities of
water at high temperatures can be more expensive
than steel. Buried tanks must be protected from
groundwater and resist buoyant forces. The tank
must be reasonably accessible for repairs. In mild
or warm climates, an outdoor location may be
Domestic Hot-Water Systems (DHW)
Domestic hot-water systems (without space
heating) may use lined, insulated, or pressurized
tanks similar to the conventional water heater.
Appropriate temperature-and pressure-relief valves
must be used. Since it is possible for solar
collectors to reach hot temperatures, a tempering
or mixing valve should be used. A typical
two-tank installation with proper valves and
connections is shown in figure 15-9.
To size the collectors and storage tank, you
must estimate the hot-water consumption of the
facility or building. The hot water consumption
rate for a typical family home is 20 gal/day/person.
When the hot-water consumption rate is more than
average, use 30 gal/day/person. So, 80 to 120
gal/day should serve a typical four-person family.
A variation of the DHW system is the
thermosiphon system. It uses the principle of
natural convection of fluid between a collector and
an elevated storage tank. The advantage is no
pump or controller is needed. The bottom of the
tank should be mounted about 2 feet higher than
the highest point of the collector. This is the main
disadvantage because structural requirements often
prohibit the weight of a water tank on a high point
of the structure. Also, since the thermosiphon
system is connected directly to the potable water
supply, it cannot be protected from freezing. A