Passive Cooling 

Natural ventilation depends solely on air movement to cool occupants. Window openings on opposite sides of the building enhance cross ventilation driven by breezes. Since natural breezes can't be scheduled, designers often choose to enhance natural ventilation using tall spaces within buildings called stacks. With openings near the top of the stack, warm air can escape, while cooler air enters the building from openings near the ground. Ventilation requires the building to be open during the day to allow air flow.

   

High thermal mass depends on the ability of materials in the building to absorb heat during the day. Each night the mass releases heat, making it ready to absorb heat again the next day. To be effective, thermal mass must be exposed to the living spaces. Residential buildings are considered to have average mass when the exposed mass area is equal to the floor area. So, for every square foot of floor area there is one square foot of exposed thermal mass. A slab floor would be an easy way to accomplish this in a design. High mass buildings would have up to three square feet of exposed mass for each square foot of floor area. Large masonry fireplaces and interior brick walls are two ways to incorporate high mass.

   

High thermal mass with night ventilation relies on the daily heat storage of thermal mass combined with night ventilation that cools the mass. The building must be closed during the day and opened at night to flush the heat away.

   

Evaporative cooling lowers the indoor air temperature by evaporating water. In dry climates, this is commonly done directly in the space. But indirect methods, such as roof ponds, allow evaporative cooling to be used in more temperate climates too.

Ventilation and evaporative cooling are often supplemented with mechanical means, such as fans. Even so, they use substantially less energy to maintain comfort compared to refrigeration systems. It is also possible to use these strategies in completely passive systems that require no additional machinery or energy to operate.

To cool a building utilizing cool underground air, a closed circuit of pipes is laid 4 to 6 feet (1.2 to approx. 2 metres) beneath the ground outside the building. A small fan is then used to force air through the system and blow cold air into the building.

One variation on this is the have one end open to the surface and draw the air out through a vent pipe.

Another variation is the wind chimney, which is sometimes employed in deserts. In the direction of the prevailing cool winds, a scoop-like vent is placed and a pipe is laid under the ground from it to the house. Where it reaches the house, the air passes over a container filled with dampened charcoal. This causes evaporative cooling of the air before it enters the home. Charcoal is used both as a sponge to hold the water and as a means of keeping the water fresh.

Another strategy put into effect in tropical and sub-tropical areas is the shade house. A shade house is simply an area on the shaded side of a home with a vine-covered trellis to create a shaded sanctuary. Often these shade houses will contain an outside kitchen to prevent the heat of cooking from entering the home. Subtropical areas would have two kitchens, one inside for the winter, and one outside for the summer. Urban temperate areas suffering from the thermal island effect would also benefit greatly by the shade house strategy.

If you are living in a tropical or sub-tropical region and building a new home, you can increase airflow through the home by substituting the simple “box” design for one with more corners in it (see diagram). This will allow greater airflow through the home.

In tropical regions, it is beneficial to design a traditional tropical home with wall vents and a vaulted roof of permeable material to draw cool air in and push hot air out. (See diagram.)

The Encyclopedia of Alternative Energy and Sustainable Living:  http://www.daviddarling.info/encyclopedia/C/AE_cooling_refrigeration_and_air_conditioning.html