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Microclimate and Site design

Siting and Site layout followed by landscaping can improve the microclimate around a building, taking advantage of existing topographical features, adjacent buildings and vegetation for solar protection. Good site layout can also take greater advantage of local breezes by the formation of air funnels and also aid natural ventilation by staggering of the building layout. The presence of water and vegetation on the site can also be used for natural cooling. Good site layout can reduce cooling loads appreciably by optimising natural solar protection and local breezes. For example, streets with an east-west orientation can help reduce summer solar gain on south facades. Staggered layouts can enhance natural ventilation. Proper siting of the building can provide good comfort conditions within the building as well as reducing energy costs to a large extent.

Elements of site design that can be used for efficient natural cooling include landscaping, orientation to sun and wind, building shape and planning and natural ventilation.

Landscaping

Landscaping can improve the microclimate in both summer and winter, providing shading, evaporative cooling and wind channelling in summer, or shelter in winter. Vegetation absorbs large amounts of solar radiation in summer helping to keep the air and ground beneath cool while evapotranspiration can further reduce temperatures.

Use of shade trees, shrubs and vines can provide protection from solar radiation in summer. Careful planting of trees for wind channelling can provide efficient ventilation and circulation of summer breezes in addition to creating attractive spaces for outdoor activity. However some care should be taken in the choice and placement of vegetation on or near the building to avoid structural damage.

Grass and other ground cover planting can also influence the microclimate, keeping the ground temperature lower than most hard surfaces as a result of evapotranspiration and their ability to reduce the effect of solar radiation. This happens due to the shading provided by the grass which prevents radiation from reaching the ground resulting in a difference between asphalt and lawn being as much as 25 degrees F (figures taken from Climatic design, Energy efficient building principles & practises/ Donald Watson and Kenneth Labs).

Windbreaks can enhance air pressure difference around buildings and improve cross ventilation. Hedging, for example, can allow a gentle breeze to filter through the foliage, while a masonry windbreak can create a calm, sheltered zone behind it. Gaps in windbreaks, openings between buildings or openings between the ground and canopy of trees can create wind channels, increasing wind speeds by about 20%.

Water can also be used effectively for cooling of internal as well as surrounding environment. Ponds, streams, fountains, sprays and cascades can be used where water is available in summer. These are particularly effective in dry conditions where relative humidity levels are low.

Orientation to sun and wind

The orientation of the building on site is very important to achieve reduced heat gain and improved wind circulation and ventilation. The major openings in the building envelope should be placed on the North while the south face should be adequately protected from heat gain by using shading devices or vegetation. Prevailing wind direction should be taken into consideration while deciding the position and size of the openings to ensure proper cross ventilation. This can go a long way in improving comfort conditions within the building.

Building shape and Planning

The configuration of the building and the arrangement of internal spaces according to function can help to influence the exposure to incident solar radiation, the availability of natural daylight and airflow in and around the building.

In general, a compact building will have a relatively small exposed surface, or in other words a low surface to volume ratio (SVR). This can offer advantages for the control of heat gains through the building skin without conflict between design priorities for winter and summer months. There are a range of other options to improve thermal performance including courtyards, construction on pilotis, use of wing walls, etc. however the relationship between form and thermal transmission are not very critical as a number of strategies are available to counteract its negative effects. More important are the effect of building form on wind channelling and airflow patterns and the opportunities for enhancing the use of daylight.

Natural Ventilation

Ventilation provides cooling by using air to carry heat away from the building and from the human body. Air movement may be induced either by natural forces (wind and stack effect) or mechanical power. Airflow patterns are a result of differences in pressure patterns around and within the building. Neighbouring landforms such as slopes and valleys can be used to increase the exposure to summer breezes along with proper orientation to wind. Openings should be oriented to catch the prevailing summer breeze.

Air moves from high-pressure regions to low pressure ones. When the outside air temperature is lower than the inside air temperature, building ventilation can exhaust internal heat gains or solar heat gain during the day and cool air during the night if required. Indoor air movement enhances the convective exchange at skin surface and increases the rate of evaporation of moisture from the skin. Evaporation is a very powerful mechanism for cooling which may bring a feeling of comfort to the occupants under hot conditions. However, to be effective the surrounding air should not be too humid (relative humidity less than 85%). Turbulent air movement will hinder both of these mechanisms of heat removal.

Both the design of the building itself as well its surrounding spaces can have a major impact on the effectiveness of natural cooling through ventilation. The rate of air flow through the building will be affected by location, sizing and air flow characteristics of the openings (wing walls, louvers, overhangs can be used to direct summer wind flow into the interior), the effect of indoor obstacles to air movement (open plan spaces promote air flow), the effects of the external shape of the building in relation to wind direction, etc. The total airflow normally depends on a combination of buoyancy and wind pressure differences, and is affected by the size and location of openings.

Proper placement of openings along with the use of wing walls can greatly enhance the effect of ventilation by increasing wind pressure differences and consequently air velocity within the space.

Other strategies for improving ventilation are wing walls, wind towers and solar chimneys.

Wing walls

Many buildings having rooms with lust one external wall are difficult to ventilate naturally. Even rooms with two windows placed as far as possible will offer limited ventilation. However, research has found that airflow in rooms with two windows on one wall can be further augmented by the use of wing walls (small walls perpendicular to the main wall). These projections create positive pressure over one window and negative over the other, achieving cross ventilation of the room by drawing air in on one side and forcing it out on the other side.

Wind towers

Wind towers draw upon the force of wind to generate air movement within the building. There are various systems based on this principle. The wind-scoop inlets of the tower oriented toward the windward side capture the wind and drive the air down the chimney. The air exits through leeward openings in the building. Alternatively, the chimney cap is designed to create low pressure at the top of the tower, and the resultant drop in air pressure causes the air to rise through the chimney. A windward opening should be incorporated in the system as an inlet. The buoyancy of the warm air inside the building aids this process. Both these systems can be combined in a single tower providing both admittance and exhaust of air thus creating a self sustained system.

Solar chimney

Solar chimneys use the sun to warm the internal surface of the chimney. The buoyancy generated due to the temperature difference help induce an upward flow along the plate. The chimney width should be close to the boundary layer to prevent backward flow.

Limitations of wind induced ventilation

Wind induced ventilation would be an ideal strategy if winds were in a steady direction and intensity (greater than 3m/s). In reality, however winds are extremely variable and detailed weather data is not readily available for most sites. Also the rate of air changes in a naturally ventilated system will vary and therefore cause some inconvenience. Also a detailed study of the effect of measures taken to enhance ventilation has not been made due to which reliable information on the subject is not available.