Natural Ventilation

Ventilation of buildings has been a major cooling technique throughout the world, as it provides cooling by using air to carry heat away from the building (convective cooling) and or from the human body itself (physiological cooling). It is based on the fundamental heat transfer mode of convection, when the air flowing next to a surface carries away heat, provided it is at a lower temperature than the surface. When it passes over the human body, it increases the evaporation rate from the skin and enhances heat extraction.

Besides, ventilation (passive, hybrid or mechanical) of indoor environment is also necessary to maintain the required levels of oxygen and air quality in a space.

Traditionally, ventilation requirements were achieved by natural means. In the majority of older buildings, infiltration levels were such as to provide considerable mounts of outdoor air, while additional requirements were satisfied simply by opening the windows. Modern architecture and energy conscious design of buildings have reduced air infiltration to a minimum in an attempt to reduce its impact on the cooling or heating load. Better construction has resulted in buildings sealed fro the outdoor environment. In particular, the construction of large glass office buildings, which do not allow the opening of windows has further eliminated the possibility of using natural ventilation for supplying fresh air to indoor spaces.

Provided that outdoor climatic conditions are favorable, the use of natural ventilation can reduce the cooling load, enhance thermal comfort conditions, and maintain indoor air quality. The effectiveness of natural ventilation techniques is determined by the prevailing outdoor conditions and microclimate, and the buildings characteristics (orientation, windows- number, size, location).

Outdoor temperature, humidity and wind velocity are determinant factors in the successful application of natural ventilation techniques. For cooling purposes, the incoming air should be at a lower temperature than the indoor air temperature. However, even at higher temperatures, the resulting air flow inside the space can cause a positive effect on the thermal comfort conditions of the occupants, since it increases heat dissipation from the human body, and enhances evaporative and convective heat losses. An increase in air velocity of 0.15m/s compensates for an increase of 1 degree Celsius at a relative humidity of 75%. Natural ventilation techniques for cooling purposes are also very effective during the night, when outdoor air temperatures are usually lower than indoors. As a result, the cooling load is reduced and peak indoor air temperature can be reduced by 1 to 3°C.

Air humidity is the most important limiting factor for the application of natural ventilation techniques. High levels of humidity have a negative influence on thermal comfort. As a result, in regions with high relative humidity levels during summer, the use of conventional air conditioning systems is necessary, to remove water vapor from the indoor air (dehumidification). Under such circumstances, natural ventilation should be avoided.