Bioclimatic (Natural Cooling)

Strictly defined, the term ‘passive cooling’ applies only to those processes of heat dissipation that will occur naturally, that is without the mediation of mechanical components or energy inputs. The definition encompasses situations where the coupling of spaces and building elements to ambient heat sinks (air, sky, earth and water) by means of natural modes of heat transfer leads to an appreciable cooling effect indoors. However, before taking measures to dissipate unwanted heat, it is prudent to consider how the build-up of unwanted heat can be minimised in the first place. In this context, natural cooling may be considered in a somewhat wider sense than the strict definition above suggests, to include preventive measures for controlling cooling loads as well as the possibility of mechanically assisted (hybrid) heat transfer to enhance the natural processes of passive cooling.

A useful design strategy for the overheating season is to first control the amount of heat from solar radiation and heated air reaching the building, then to minimise the effect of unwanted solar heat within the building skin or at openings, next to reduce internal or casual heat gains from appliances and occupants and finally, where necessary, to use environmental heat sinks to absorb any remaining unwanted heat. In practice a combination of these cooling techniques is almost invariably in operation.

Fixed or adjustable shading devices, or shading provided by vegetation and special glazing may be used to reduce the amount of solar radiation reaching the building.

Passive cooling strategy.
External heat gains due to solar radiation can be minimised by insulation, reduced window sizes, thermal inertia in the building envelope, reflective materials and compact building layout.

Infiltration gains can be reduced by cooling the incoming air and by reducing its infiltration to a minimum necessary for comfort and health.
Internal gains can be reduced by the use of more efficient lighting and appliances and appropriate control strategies for their operation and by the use of natural daylight wherever possible to replace artificial lighting.

Ventilation using cooled fresh air driven through the building by naturally occurring differences in wind or air pressure can help to reduce internal temperatures.

Several methods of natural cooling, including increased air speeds to maximise perceived levels of cooling, ground and evaporative cooling to reduce the temperature of ventilation air and night-time cooling of the building by radiative heat loss to the sky and enhanced ventilation, can help to maintain comfortable indoor conditions.

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