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MIL-HDBK-1003/13A
2.5.3 Heat pumps.  Heat pumps have been mentioned in previous sections as a
possible choice for auxiliary heaters.  Some manufacturers are combining
solar systems with heat pumps for the purpose of reducing auxiliary energy
costs.  When a heat pump and a solar system are combined in this manner, the
system is usually called solar assisted or solar augmented heat pump (SAHP)
system.
Solar assisted heat pump systems can be configured in many different ways.
For example, the solar collectors can be either water or air types, the heat
storage medium can be water or a solid material such as rock or brick, and
the heat pump can be of either the air-to-air design or the water-to-air
design.  But heat pumps have a characteristic which can limit their
effectiveness: the efficiency and capacity of a heat pump decreases as the
temperature of the heat source (usually outdoor air) decreases.  This
deficiency can be overcome, however, by using solar collectors to gather the
sun's energy for the purpose of keeping the heat source in the temperature
range required for efficient heat pump operation.
a.
Air-to-air heat pumps.  Some air-to-air heat pumps function very
well as an auxiliary heater at temperatures down to 20 deg. F.
Below these temperatures, they suffer in efficiency and
performance.  When solar assisted by heat from a rock-pebble
storage bed and air collectors, the heat pump adds much to the
performance of the solar energy system.  Without such a solar
assist, air-to-air heat pumps have limited utility in cold
climates.  Their use should be carefully checked with the local
utility and pump manufacturer.  The heat pump also provides cooling
during the summer.  It thus has year-round utility.  Heat pumps
should be comparison-shopped.  The purchaser should look at the
cost, performance, service, and expected life.  Units differ
considerably from manufacturer to manufacturer (Montgomery, 1978).
b.
Liquid-to-air heat pumps.  The liquid-to-air heat pump is an ideal
auxiliary heater when coupled with liquid solar storage.  It
operates at very low cost.  And it greatly enhances solar energy
collection by drawing down the temperature of the solar storage
water to as low as 45 deg. F.  It should be considered for all
installations, except those with existing fossil fuel furnaces and
no need for summer cooling, (Montgomery, 1978).
Out of the many SAHP configurations which could be used, the two most in use
are called the "series" and "parallel" configurations.  Figure 2-17 is a
series SAHP system.  When the system is used for heating, water from the
storage tank is circulated through water-cooled collectors where it is heated
before returning to the storage tank.  Warm water from the storage tank is
also circulated through a water-to-air heat pump.  Heat is removed from the
water and transported to the indoor air by the heat pump and the water
returns to the storage tank at a lower temperature.  If heat is added to the
water in the tank faster than it is removed by the heat pump, the
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