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 MIL-HDBK-1003/13A
As with active solar systems and heat pumps, there are endless variations of
the passive technique, limited only by one's imagination. There are systems
that use water on the roof (SKYTHERM of Harold Hay) to absorb heat directly,
and there are clever ways to insulate glass at night by blowing styrofoam
beads between two glass panes (BEADWALL of Steve Baer). Also natural objects
such as earth berms to protect from winds and trees which shade in summer and
let light pass in winter should be considered. Figures 2-20 through 2-23
show various representations of some of these passive techniques used either
by itself or in conjunction with air collectors and thermosyphon systems.
Although passive systems are rather simple in construction and design, their
performance analysis is often complicated by a vast interplay of many
components. Mazria (1979) is a good source of design information for passive
systems. Here are some "rules of thumb" that should be useful for passive
designs:
a.
South-facing passive storage walls in direct sunlight should have a
minimum of 30-lb water storage or 150-lb masonry (concrete) storage
per square foot of south vertical glazing. If the storage media is
not located in direct sunlight, four times this amount will be
needed (Balcomb, undated). Mazria (1979) recommends at least 5-6
gallons water storage (about 45 lb) per square foot of south glass.
b.
Shading of south windows should be used to reduce summer and fall
overheating. One effective geometry is a roof overhang which will
just shade the top of the window at noon (solar time) sun elevation
of 45 deg. and will fully shade the window at noon sun elevation of
78 deg. F (Balcomb, undated).
c.
The best thickness of a Trombe wall is from 12 to 16 inches. The
masonry should have a high density - at least 100 lb/ft3.
Thermocirculation vents can be used to increase daytime heating but
will not increase nighttime minimums. Vents should have
lightweight passive backdraft dampers or other means of preventing
reverse flow at night (Balcomb, undated).
d.
Two to three square feet of south-facing double glazing should be
used for each Btu/deg. F-hr of additional thermal load (i.e.,
exclusive of the glazing). This will give 70% to 80% solar heating
in northern New Mexico (Los Alamos) for a building kept within the
range of 65 deg. F to 75 deg. F. See Balcomb (undated) for example
of how to use this method.
e.
An easier to use rule in place of (d) is that given by Mazria
(Mazria, 1979). For a well-insulated space in 40 deg. N latitude
in cold climates (outdoor temperature = 20 deg. F to 30 deg. F) the
ratio of south glazing to floor area is in range 0.20 to 0.25 to
maintain an average space temperature of 68 deg. F over 24 hours
(e.g., a 200 ft2 floor space needs 40-50 ft2 of south glazing).
In temperate climates (35 deg. F to 45 deg. F outdoor temperature)
use ratios in the range 0.11-0.17.*
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