Passive Cooling

Passive Cooling involves designing buildings for cooling load avoidance. Design strategies that minimize the need for mechanical cooling systems include proper window placement and daylighting design, selection of appropriate glazing for windows and skylights, proper shading of glass when heat gains are not desired, use of light-colored materials for the building envelope and roof, careful siting and orientation decisions, and good landscaping design.

Install fixed shading devices, using correctly sized overhangs or porches, or design the building to be "self-shading." Fixed shading devices, which are designed into a building, will shade windows throughout the solar cycle. They are most effective on the south-facing windows. The depth and position of fixed shading devices must be carefully engineered to allow the sun to penetrate only during predetermined times of the year. In the winter, overhangs allow the low winter sun to enter south-facing windows. In the summer, the overhangs block the higher sun.

Plant trees or bushes to shade the windows at the right time of day and season. Deciduous vegetation is often an attractive and inexpensive form of shading, because it follows the local seasons, not the solar calendar. In the warm south, where more shading is needed, trees leaf out earlier, while in the cold north, where solar heat is beneficial late into spring, trees wait until the weather warms up before they leaf out. Trees can be strategically planted on east and west sides to block the rising and setting sun. Bushes can be positioned to block undesirable low sun angles from the east or west, and deciduous vines trained to grow over trellises make easily controlled shading systems.

Evergreen trees trimmed so that their canopies allow low winter sun underneath but block the high summer sun can be very effective. Properly placed vegetation can also guide airflows toward buildings for natural ventilation and can block cold winter winds. Vegetation and groundcover also contribute to evaporative cooling around a building.

Vegetation used for shading should be properly located so as not to interfere with solar gain to buildings in winter. Deciduous trees can reduce winter solar gain by 20% or more and should not be placed in the solar access zone. Also note that trees require maintenance, pruning, watering and feeding. As they grow they change their shading pattern, and they can be damaged or killed, leaving the building exposed.

Consider awnings that can be extended or removed. Movable awnings are an old tradition and an excellent solution to the variation between seasons and the solar year. When rolled out in the summer, they not only provide deep shade but also lend a colorful touch to a building"s facade. When rolled up in winter, they allow more sun into the building and avoid snow loads and/or excessive weathering.

Consider exterior roll-down shades or shutters. An enormous variety of vertical shading devices are readily available. Wooden shutters are the most traditional. Also available are many exterior-grade fiberglass and plastic fabrics that cut out a significant amount of sunlight but still allow a clear view through the window. However, they do not prevent the glare problems caused by low-angle sun. Opaque steel or plastic roll-down shutters have proved reliable and long lasting. Although expensive, they can also provide additional storm and vandalism protection.

Limit east/west glass. Glass on these exposures is harder to shade from the eastern morning sun or western evening sun. Vertical or egg-crate fixed shading works well if the shading projections are fairly deep or close together; however, these may limit views. North-facing glass receives little direct solar gain, but does provide diffuse daylight. Other Cooling Strategies

Design the building to take advantage of natural ventilation. Natural ventilation uses the passive stack effect and pressure differentials to bring fresh, cooling air through a building without mechanical systems. This process cools the occupants and provides comfort even in humid climates. Buildings using this design will incorporate operable windows or other means of outdoor air intakes. Wingwalls are sometimes used to increase the convective airflow. Other features include fresh air inlets located near floor level, use of ceiling fans, and the use of atriums and stairwell towers to enhance the stack effect. Caution should be used not to increase the latent load (i.e., the increased cooling load resulting from condensation) by bringing in moist outside air.

Consider radiative cooling in appropriate climates. Radiative cooling, also known as nocturnal radiative cooling, uses design strategies that allow stored heat to be released to the outside. This strategy is particularly effective in climates and during seasons of the year when the daytime-nighttime temperature differences are meaningful. Night flushing of buildings uses radiative cooling principles. Building thermal storage serves as a heat sink during the day, but releases the heat at night, while being cooled with night air.

Consider ground coupled cooling. Ground coupling is achieved by conductive contact of the building with the earth. The most common strategy is to cool air by channeling it through an underground tunnel. Another strategy provides cool air by installing a tube in the ground and dripping water into the tube. This reduces the ground temperature through evaporation. Consider evaporative cooling strategies. This cooling method works when water, evaporating into the atmosphere, extracts heat from the air. Evaporative cooling is most appropriate in dry climates, such as the Southwest.

Use dehumidification in humid climates. Dehumidification is required in climates having high humidity levels, and therefore latent loads, during portions of the year. Common strategies include dilution of interior moisture by ventilating with less humid air, condensation on cooled surfaces connected to a heat sink, and desiccant systems.

References:
  1. Yazd-Iran, Passive Cooling and Heating Systems in Deseart Architecture