MIL-HDBK-1003/13A
f.
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.
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