Masonry Magazine April 1981 Page. 19
CONVECTIVE THERMAL
LOOP THROUGH
BUILDING ENVELOPE
SUMMER SUNLIGHT
CLOSED VENT
SOUTH
FACING
GLAZING
MASSIVE BRICK
THERMAL STORAGE
WALL
CONVECTIVE THERMAL
LOOP THROUGH BUILDING
ENVELOPE
INSULATED ROOF
INSULATED CEILING
Cavity Wall System, Heating Mode
FIG. 5
CLOSED
VENT
A schematic of the system in the heating mode is shown in Fig. 5. The vents are closed, which creates a convective loop around the entire shell of the building: through the floor, wall and roof/ceiling components. This thermal convective loop warms both the interior and exterior wythes of the building envelope. Since this operation warms the interior wythe, there is little or no heat loss through those portions of the building envelope. This system, properly designed and operated, may provide the most effective passive solar heating and cooling.
FACTORS AFFECTING PERFORMANCE
The effects of the environmental conditions and building use on passive solar heating systems are discussed in Technical Notes 43. These must also be considered for passive solar cooling systems, however, the necessary considerations of these factors vary for passive solar cooling systems. The major variations and additional effects which must be addressed specifically are: temperature, humidity and shading.
Exterior Design Temperature
The exterior design temperature may be such that the effective temperature range cannot be achieved or maintained within the structure. Since the effects of cooling are principally achieved by air movement, this may make the cooling system ineffective. One option is to take maximum advantage of the daily temperature swing. When the nighttime temperatures drop below the interior design temperature, the structure may be cooled during the night, delaying the time to heat up the next day. Caution must be used when considering the daily temperature swing to guard against overcooling in moderate climates.
Humidity
Humidity is an additional environmental factor not generally addressed in passive solar heating systems. In areas where the effects of high humidity cannot be eliminated by air movement, these simple versions of passive solar cooling systems may not be effective. Additional complex modifications to the basic passive solar cooling systems may be necessary to dehumidify the air.
Shading Devices
Operable shading devices are usually required in passive solar cooling systems. The shading devices are used to control the amount of solar radiation permitted to strike the system. This is necessary to prevent overheating, especially when the system is marginal because the effective temperature cannot be attained by natural air flow. In this case, the system should be completely shaded from the summer sunlight.
In instances where the system is providing cooling by night air intake, it may be advantageous to have the system shaded from the morning and possibly early afternoon sunlight. Exposure to only the late afternoon sunlight may result in sufficient performance to draw cool night air through the structure.
SPECIAL CONSIDERATIONS
The performance of the passive solar cooling system may be greatly increased by pre-cooling, or dehumidifying the air before introducing it into the structure. Pre-cooling and dehumidifying the air are both fairly straightforward concepts. Adapting the system for pre-cooling air is usually simple, but dehumidifying the air is much more complicated.
Pre-Cooling Air
Air may be cooled before it is introduced into the structure by providing underground ductwork or piping, and venting it to the surface as shown schemati-
HEATED AIR
EXHAUSTED
OPEN VENT OR LOUVERS
SUMMER SUNLIGHT
SOUTH
FACING
GLAZING
OPEN
VENTS
WARMED AIR RISES.
RADIANT HEAT
OPEN
VENT
EXTERIOR
AIR DRAWN
INTO THE
BUILDING
DUCT WORK FOR
PRE-COOLING AND
DEHUMIDIFYING
EXTERIOR AIR
Pre-Cooling and Dehumidification
of Exterior Air for Cooling
With Direct Gain System
FIG. 6