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.
Water Systems
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 ft 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
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