Passive cooling is a method that incorporates design and technology to cool a building without the use of power. Traditional cooling techniques, such as air conditioning or evaporative cooling, rely on power to operate compressors and fans that forcibly cool a space. Such power is most typically purchased from a local utility and is generated by burning fossil fuels that can have degrading effects on the environment. Passive cooling techniques require no power and range from simple, such as shading the building, to more complex, such as strategic building design. Use of such techniques may reduce or eliminate the need for powered cooling.
In warmer climates, particularly arid regions, cooling is the primary environmental problem for buildings. There are two primary sources of unwanted heat that need to be addressed to keep the interior of the building comfortable for occupants. Direct sunlight on a building, especially shining on windows, is a major source of heat gain for most buildings. Infiltration of warm air through cracks and crevices and heat transfer through building materials also affect the interior temperature.
Generally, the simplest method of cooling a building is to prevent it from being heated in the first place. Strategic orientation of a building to minimize direct solar exposure, especially during the hottest part of the day, can significantly reduce its heat load. Once a structure has been built, the use of sunscreens or awnings as well as strategic landscaping may help avoid direct exposure to the sun. Where direct exposure cannot be avoided, such as on a roof, reflective techniques may be employed to prevent unwanted heating. For example, a roof may be coated with a reflective, white coating to reduce heating over a large, unprotected area.
Passive cooling techniques can also prevent heating by reducing the infiltration of heat into the structure. Heat naturally flows from higher temperature areas, such as outdoors in the summer, to lower temperature areas, such as a comfortable home interior. Caulking and adding weatherstripping around windows and doors, for example, reduces the area through which unwanted hot air can enter a structure. Insulation is material with a high resistance to heat flow. Adding insulation to a building impedes unwanted transfer of heat from the hotter outdoor environment to the interior space that is being kept cool.
Although much heating can be prevented, often it is not possible to keep a structure at a comfortable temperature through avoidance alone. Another passive cooling strategy involves cooling the structure through convection. Convection is a process wherein heat is transferred from one location to another by the movement of molecules of a gas or liquid.
Vents and windows strategically incorporated into a building’s design, for example, can route hot air out of the house while drawing in cool air to replace it. Hotter air is more buoyant than cooler air and rises naturally on its own. Vents placed near the ceiling on the side of the house away from prevailing winds can help this hotter air escape the structure. To replace it with cool air, vents and windows may be placed low on the side of the house in the path of the prevailing wind. It is then a simple matter of opening vents and windows to encourage natural convection to cool the structure.
More complex passive cooling methods may also be incorporated into a building’s design, such as earth cooling tubes. Earth cooling tubes take advantage of the cooler temperatures below the surface of the earth to cool a structure. Buried underground as they lead into a structure, earth cooling tubes draw outdoor air into the building through an underground route. As it passes through these tubes, air looses heat to the much cooler ground through which it travels on its way into the structure. Other passive cooling techniques are typically used in conjunction with earth tubes to set up a natural convection through the structure and vent hotter air.
While not suitable for use in all locations, passive cooling is employed in many areas for energy efficiency resulting in cost savings and minimal environmental impact. It is generally most suitable for warmer climates and arid regions. Passive cooling can avoid, in part or in whole, the expense of cooling techniques that are driven by powered devices such as fans and compressors. This energy conservation strategy can also benefit the environment by reducing the need to burn non-renewable fuels that emit waste byproducts harmful to the environment.