MIL-HDBK-1003/19
4.1.2 Daylighting. The daylight delivered to the interior of direct
gain buildings is an additional resource that is available year-round.
Pleasing uniform illumination can be achieved by using blinds that reflect
sunlight toward white diffusive ceilings. The artificial lighting system in
many buildings imposes a significant load on the cooling system that may be
reduced by daylighting because the fraction of visible light in the solar
spectrum is greater than the visible fraction of incandescent or fluorescent
lighting.
4.1.3 Radiant panels. Radiant panels are simple passive solar systems
that are inexpensive and well suited as retrofits to metal buildings. A
sketch of a radiant panel system is presented in figure 2. Note that the
solar aperture consists of one or more layers of glazing material placed
over an uninsulated metal panel. The metal panel would ordinarily be a part
of the building shell so that a retrofit is constructed by simply glazing an
appropriate area on the south side of the structure. Any insulation or
other poorly conducting material should be removed from the inner surface of
the glazed portion of the metal panel to facilitate heat transfer to the
interior.
Solar radiation is absorbed on the outer surface of the metal panel
after passing through the glazings. The panel becomes hot and gives up heat
to the interior by radiation and convection. Thermal mass must be included
inside the building shell as with direct gain systems. Usually, only a
concrete slab will be available before retrofitting a metal building and it
may sometimes be necessary to add water containers to achieve the desired
thermal capacitance. Radiant panels perform on a par with direct gain
buildings and are likely to be less expensive when used as retrofits to
metal buildings.
4.1.4 Thermosiphoning air panels. Thermosiphoning air panels (TAPs)
are also appropriate for use on metal buildings either as retrofits or in
new construction. Two configurations occur in practice and the first,
which is referred to as a frontflow system, is illustrated in figure 3.
Again there are one or more glazing layers over an absorbing metal surface
but, in this case, the metal panel is insulated on the back side. Heat
transfer to the interior occurs via circulation vents cut through the metal
panel and its insulation at the upper and lower extremes. Solar radiation
absorbed on the the outer surface of the panel is converted to heat and
convected to the adjacent air which then rises due to buoyancy forces and
passes through the upper vent into the living space. The warm air leaving
the gap between the inner glazings and the absorber is replaced by cooler
air from the building interior that enters through the lower vents. In
this manner, a buoyancy driven loop is established and sustained as long as
the temperature in the air gap exceeds that in the living space. Passive
backdraft dampers or manually operated vent closures must be employed to
prevent reverse circulation at night. Backdraft dampers are usually made
of a lightweight plastic material suspended above a metal grid such that
air flows freely in one direction but is blocked should the flow attempt to
reverse.
The second type of TAP configuration, illustrated in figure 4, is called
a backflow system. In a backflow system, the flow channel is behind the
absorber plate rather than in front of it. An insulated stud wall is
constructed a few inches behind the metal panel and vents are then cut at
the top and bottom of the wall. Air in the flow channel thus formed is
heated by convection from the back of the absorber panel and a circulation
loop is established in the same manner as in a frontflow system.
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