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MIL-HDBK-1003/13A
temperature of the water will rise.  When the water temperature is high
enough (about 104 deg. F), heat can be extracted directly from the water by
means of water-to-air heat exchanger.  In this mode of operation, the heat
pump is shut off.  Auxiliary electrical resistance heaters are provided to
make up the balance of the heat load if the heat from the heat pump or water
air heat exchanger is not sufficient to meet the demand.  Normally this could
be "off-peak" power for the auxiliary heater.
When used for cooling, the heat pump transports heat from the building to the
water in the storage tank thereby causing the temperature of the water in the
tank to rise.  During spring and fall, when it is not unusual to have a light
cooling load during the day and a light heating load at night, the heat in
the storage system is simply shuttled from the building to storage during the
day and from storage to the building at night, and the solar collectors are
used only to make up for lost heat.  During periods of prolonged cooling
demand, the heat pumped into the storage tank might be sufficient to cause
the temperature of the water to rise to where the heat pump will no longer
operate.  Thus, provision must be made for rejecting excess heat.  One method
is to add a cooling tower to the system to cool the water.  Another method is
to circulate water through the solar collectors at night and reject heat by
radiation to the night sky.  During periods of high cooling load it is not
desirable to also add heat to the storage tank by circulating water through
the solar collectors.  Therefore, when the system is in the cooling mode the
solar collector circuit can be used to heat DHW.
The "parallel" SAHP system is shown in Figure 2-18.  The solar heating system
and the heat pump operate in parallel.  Solar heat is used directly rather
than being transferred to a storage medium and then transported into the
building with a heat pump.  This system is essentially a direct solar heating
system with an air-to-air heat pump as a backup heating system.
The choice of a "best" system is difficult to make due to the many variables
involved.  For example, in addition to the two configurations shown in
Figures 2-17 and 2-18, one could examine a series system with low cost
(unglazed) collectors, or a series system with air-collectors and rock
storage, or a parallel system with low cost collectors, etc.  Each system
would be highly dependent on geographical location, type of construction,
etc.  One such analysis done at NCEL comparing several systems to a stand-
alone air source heat pump, showed the "parallel" system to have the best
comparative performance (Kirts, 1978).  More information about heat pump
systems can be found in Kirts (1978).
Each heat pump configuration should be considered on a case-by-case basis.
The analysis of these systems is beyond the scope of the worksheets given in
this report, and the reader is directed to more sophisticated computer
programs such as those in Durlak (1979b).
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