Solar cooling systems

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Design: Solar Heating of Buildings and Domestic Hot Water

MIL-HDBK-1003/13A

For greenhouses: To determine solar gain: S = 1200 Btu/ft2 of

glazing per clear day, S = 700 Btu/ft2 per average day. Double

glaze only south wall. Insulate all opaque surfaces to R20,

outside foundation to frost line to R10, minimize infiltration with

caulking. Thermal mass = 5 gal of water or 1-2/3 ft3 of gravel

per square foot of glazing. If storage is thermally isolated from

greenhouse, air should be moved at 10 ft3/min per square foot of

glazing through the storage (McCullagh, 1978).

Rules d. or e. have to be followed by rule a. to estimate the storage

required.

2.7 Solar cooling systems. The state-of-the-art of solar cooling has

concentrated primarily on the developmental stages of systems in the last few

years. Various methods have been researched, and some demonstrated, but only

a few systems have been installed for other than research purposes. Solar

cooling systems are attractive because cooling is most needed when solar

energy is most available. If solar cooling, can be combined with solar

heating, the solar system can be more fully utilized and the economic

benefits should increase. Solar cooling systems by themselves, however, are

usually not economical at present fuel costs. Combining solar heating and

cooling systems is not easy because of the different system requirements.

This can best be understood by summarizing the different solar cooling

techniques.

As with solar heating, the techniques for solar cooling consist of passive

systems and active systems. The passive systems use some of the techniques

discussed in Section 2.6 and further sources of information are Mazria

(1979), Anderson (1976), NCEL (1983), and Bainbridge (1978). For active

solar cooling systems the three most promising approaches are the heat

actuated absorption machines, the Rankine cycle heat engine, and the

desiccant dehumidification systems. A brief summary of these systems is

given here and a more detailed explanation can be found in Merriam (1977) or

other sources in the literature.

2.7.1 Absorption cooling. Absorption cooling is the most commonly used

method of solar cooling. An absorption refrigeration machine is basically a

vapor-compression machine that accomplishes cooling by expansion of a liquid

refrigerant under reduced pressure and temperature, similar in principle to

an ordinary electrically operated vapor-compression air conditioner. Two

refrigerant combinations have been used: lithium bromide and water, and

ammonia and water. There have been a number of proposed solid material

absorption systems also. Figure 2-24 shows a typical lithium bromide (LiBr)

absorption cooler. In the absorption cooler, heat is supplied to the

generator in which a refrigerant is driven from a strong solution. The

refrigerant is cooled in the condenser and allowed to expand through the

throttling valve. The cooled, expanded refrigerant receives heat in the

evaporator to provide the desired cooling, after which the refrigerant is

reabsorbed into the cool, weak solution in the absorber. The pressure of the

resulting strong solution is increased by pumping and the solution is

available to repeat the process.