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Direct radiation is intercepted by only a portion of the mirror at a time,
thus this collector does not collect as much solar energy as a focusing
collector which tracks the sun.  It is, however, less expensive to install
and maintain.  The absorber tube is encased within an evacuated tube to
reduce heat losses.
Many other types of concentrating collectors have been developed which
produce high temperatures at good efficiencies.  However, the potentially
higher cost of installing and maintaining tracking collectors may limit their
use in some applications.  These points should be addressed early in project
development when tracking collectors are considered.  In addition,
concentrating collectors must be used only in those locations where clear-sky
direct radiation is abundant.  Testing requirements for concentrating
collectors are referenced in Section 1.2.
2.2 Energy storage and auxiliary heat.  Since effective sunshine occurs
only about 5 to 6 hours per day (in temperate latitudes), and 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.  The design of the storage tank is
an integral part of the total system design.  Although numerous storage
materials have been proposed, the most common are water for liquid collectors
and rock for air.  These have the advantages of low cost, ready availability
and well known thermal properties.
Precise heat storage sizing is not necessary, but economics and system design
to determine the optimum range of sizes.  The temperature range wherein
useful heat is stored is important in determining optimum system size.  If
the volume of storage is too large, the temperature of the storage medium
will not be high enough to provide useful heat to the building.  Also,
overdesigned storage requires excess floor space.  If the storage is too
small, the storage medium temperature will be too high, resulting in low
collector efficiency.
Practical experience in the industry as well as computer simulations and
experiments have resulted in general rules of thumb for storage sizing.
These guidelines give storage sizes for which the performance and cost of
active solar systems are optimized and relatively insensitive to changes
within the range indicated.
The optimum size of storage for active solar systems is 15 Btu/deg. F/ft2
of collector area (Kohler, 1978).  The range is 10-20 Btu/deg. F/ft2
(200-400 KJ/deg. C/m2).  For water or air systems application of the rule
gives the following.
Water systems*.  Since water has a specific heat of 1 Btu/lb-deg.
F, then 15 lb of water storage are needed per square foot of
collector or considering the density of water, 8.33 lb/gal or 62.4
lb/ft3, then 1.8 gal of storage are needed for each square foot
of collector (range 1.2 to 2.0 gal/ft2).  The range in SI units
is 50-100 liters/m2.


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