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2.7.4 Other cooling methods.  Other methods, using solar heating equipment
but not direct solar energy, should also be considered.  These methods chill
the thermal storage unit of the system during the night and use the chilled
medium to provide the daily cooling load.  Methods of chilling the storage
include radiation of the heat to the night sky and heat exchange with the
night air cooled or uncooled by auxiliary means.  The chilled storage is used
directly, via heat exchange with the building air.  Both rockbed and water
storage are suitable since the only additional hardware required is that to
route the fluid.  A heat pump can be used during the day to cool the building
and reject heat to the thermal storage unit.  The thermal storage is then
cooled by using the solar collectors for night sky radiation.  From
experimentation in Arizona, Bliss obtained a nightly heat rejection quantity
of 360 Btu/night/ft2 for a black cloth radiator.  Analytical estimates can
be obtained using an effective clear sky temperature of 25 deg. F (14 deg. C)
lower than the ambient air temperature.  The advantage of this system is that
the same equipment (collectors and heat pump) can also be used for heating
(Section 2.5.3).  In systems with dual storage units, the heat pump transfers
heat from one to the other - cooling the first and warming the second.  The
cool fluid in the first unit is circulated to the house while the
concentrated heat in the second is discharged to the outdoors.
An evaporative cooler can be used coupled with a rockbed storage unit.  Night
air is evaporatively cooled and circulated through the rockbed to cool down
the pebbles in the storage unit.  During the day, warm air from the building
can be cooled by passing it through the cool pebble bed.  This method is not
very effective in humid geographical areas.
The storage volume can also be cooled using a small refrigeration compressor.
Most through-the-wall air conditioners use such compressors to cool the
indoor air.  This unit acts as the backup or auxiliary cooling system -
analogous to the backup heating system.  If operated only at night, its
capacity can be as small as half that of an independently functioning unit
and still meet peak cooling demands.  Nighttime operation will be
particularly wise if electric companies charge more for electricity during
times of peak loads on hot summer afternoons.  An even smaller compressor can
be used if it operates continuously night and day - cooling the storage when
not needed by the house (Anderson, 1976).
2.7 5 Estimating system size.  The sizing of cooling system components is
dependent on hardware, climate, and economic constraints.  The cooling unit
must be sized so as to provide the maximum cooling load under conceivable
adverse conditions of high humidity and low or erratic solar insolation.
The collection area required is dependent on the fraction of the cooling load
to be provided by solar.  Very large collector areas may be required for 100%
solar cooling under adverse conditions of high humidity and low insolation.
Although a detailed calculation method, as provided in the worksheets in the
following sections for heating systems, is not available for solar cooling,
an estimate of the required collector area can be made by the equation:

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