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MIL-HDBK-1003/13A
Another major drawback of latent heat storage is that heat is stored at an
average temperature with essentially no thermal stratification occurring in
the storage unit.  A high level of thermal stratification maximizes thermal
performance because low temperature fluid can be delivered to the collectors
and high temperature fluid can be delivered to the heat load.  For example,
the high degree of thermal stratification in rock-beds results in the
delivery of 79 deg. F air to the collector and 120 deg. F to 150 deg. F air
to the heat load.  In comparison, latent heat storage in Glauber's salt
occurs near an average temperature of 90 deg. F; thus air at 90 deg. F is
delivered to both the collectors and the heat load.  Due to the problems
discussed, latent heat storage has not received widespread use.
Since it is not economically justifiable to store huge quantities of heat,
most solar systems cannot be depended on to provide 100% of the building's
needs.  Depending on the geographical area and size of the system, about 40%
to 80% of the heat requirement is the average to design for.  Therefore
auxiliary heaters are necessary.  They should be sized to provide all the
energy requirements, although in some cases, again depending on location, it
may be possible to increase storage volume and provide less than 100% backup
auxiliary heat.  This is especially true if the use of passive solar designs
can be incorporated with active systems.  Passive designs are discussed
briefly in Section 2.6.
The auxiliary heater should operate automatically as needed, use the most
economical fuel, and share a common heat delivery system with the solar
system.  Often a heat pump is a good choice in that it can serve both as an
auxiliary heater and work together with the solar system.  In retrofit
situations, the existing heater would be the choice.
2.2.1 Storage tanks.  Water may be stored in a variety of containers
usually made of steel, concrete, plastics, fiberglass, or other suitable
materials.
Steel tanks are commercially available and have been used for water storage.
They are available in many sizes and are relatively 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 minimize heat losses.
Concrete tanks are durable, but may be difficult to install.  Concrete tanks
cast in place, prefabricated septic tanks, or large diameter pipes may be
used for water storage.  A high temperature sealant or lining should be
applied to the interior of the tank to prevent seepage of water through the
tank.  Although concrete is less conductive than steel, concrete tanks should
also be insulated to reduce thermal losses.  Leaks are difficult to repair.
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