Passive cooling is more difficult than passive heating. It is easier to insulate a house to hold on to internal heat when it is cold outside and then let in a controlled amount of solar gain to be stored in thermal mass. Passive cooling requires a method to take heat out of a house when it is warmer outside than it is inside.
Passive cooling opportunities can be explored through the use of the bioclimatic chart (Figure 15.1). An overlapping array of cooling methods is plotted out above the comfort zone. The area of the chart that is above the comfort zone labeled natural ventilation provides the easiest and most straightforward method of cooling when the air temperature is above the comfort range. Air motion across a person’s skin speeds evaporation from the surface of the skin. The evaporation draws energy off the skin causing a cooling sensation. Air motion can be natural from breezes coming in through open windows or artificial from ceiling fans.
The area of the chart labeled high thermal mass overlaps part of the natural ventilation area and also extends above the natural ventilation area. The area where the high thermal mass overlaps the natural ventilation area calls for ventilating a house at night to cool off the thermal mass of the house and keeping the house open to natural ventilation during the day. The cooled down thermal mass will draw heat by radiation away from occupants, while the air motion cools them by speeding the evaporation of sweat.
As a climate gets very hot and dry the temperature swing between day and night becomes large. This large diurnal temperature swing can be used to cool thermal mass off during the night. During the day the outside temperature is too hot for air motion cooling to be effective, so the house is closed up during the day. The cool thermal mass cools people by radiation exchange and cools the inside air by convection.
Also overlaid on top of high thermal mass and natural ventilation is an area labeled evaporative cooling. When the air is hot and dry, moisture can be added to the air, which causes the air temperature to drop at the expense of raising the relative humidity. The water vapor evaporating in the air takes energy out of the air molecules causing a decrease in molecular velocity and thus a decrease in air temperature. Evaporative cooling requires air flow through a building. Air flowing in receives the evaporative cooling. Air has to flow out somewhere else for the process to work.
FIGURE 15.1 Cooling strategies are mapped out in the area above the comfort zone on the bioclimatic chart.
Source: Brown and DeKay 2001.
Air flows from high pressure to low pressure and in a relatively open environment, minor obstructions will not substantially alter the general direction of the air flow. However, where there is an urban area with houses and trees close together, or a forested area, the wind speed at the ground will be significantly lower than the reported air speed. Air speed for climate data is usually measured in an open area on a high pole. The air speed near the ground in a built up area will be at best one-third of the reported air speed (Olgyay 1963, 39).
When air flows around an object like a building, it creates a high pressure zone on the upstream side of the building and low pressure zones along the sides, over the top, and especially on the downwind side of the building (Moore 1993, 180).
FIGURE 15.2 Ventilation cooling requires inlets and outlets for air to flow.
Air flows through a space from the high pressure side toward the low pressure side as long as there are openings to let the air in on the high pressure side and let it out on the low pressure side (Figure 15.2). Maximum air flow volume happens when the inlets and outlets are of equal size. Maximum air speed inside the space happens when the inlets are smaller than the outlets (Olgyay 1963, 104–105). The faster the air speed across a person’s skin, the larger the evaporation of sweat, and thus the larger the cooling sensation. Since ventilation cooling functions by increasing evaporation off skin surfaces, as the humidity gets high, ventilation cooling becomes less effective.
Inlets and outlets that are on adjacent walls should be spread as far apart as possible to create air flow throughout the entire space. Inlet and outlet windows located close to one corner of a room will localize the ventilation effects near that corner, thus not providing air motion cooling relief in the rest of the room.