Figure 15-8.Schematic of potable hot-water heating systems, using solar storage (tempering) tank ahead of the
conventional fueled or electric service water heater.
since heating and hot-water loads occur up to 24
hours a day, some type of energy storage system
is needed when using solar energy.
Practical experience in the industry, as well as
computer simulations and experiments, has
Figure 15-9.Typical DHW installation.
resulted in rules of thumb for storage sizing.
These guidelines provide storage sizes for which
the performance and cost of active solar systems
are optimized and relatively insensitive to changes
within the range indicated.
Since water has a specific heat of 1 Btu/1b-°F,
then 15 pounds of water storage is needed per
square foot of collector or 1.8 gallons of storage
is needed for each square foot of collector.
Since rock has a specific heat of 0.21 Btu/1b-
°F and rock densities typically contain 20 to 40
percent voids, then the optimum storage size is 0.8
ft3 per square foot of collector. Storage volumes in
this range store the equivalent overnight of 1 full
day of heating. A typical domestic hot-water
system is shown in figure 15-8. The use of two
tanks ensures that when hot water from the first
(tempering) tank is available, the auxiliary heat