Heat pumps

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Design: Solar Heating of Buildings and Domestic Hot Water

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.

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).

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