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MASTERPLANNING LOW CARBON CITIES


Optimising shade and daylighting Site layouts can be designed to minimise the effects of the sun and reduce cooling loads. In hot climates, narrow street canyons provide shade to both streets and buildings. However, this traditional vernacular is inappropriate for vehicular access and can lead to trapping of pollutants and poor air circulation. Around tropical latitudes, streets


oriented along a north-south axis are more shaded than east-west orientated streets. The translation to lower radiant and air temperatures will lead to lower energy consumption. Depending on latitude, height:width ratios of more than two will reduce solar heat gains to north-south streets, whereas east-west streets will require ratios closer to four. In theory, sites can be laid out according


to their latitude and climate, in order to allow shading of direct solar radiation to be achieved while daylight levels are realised. This provides the potential to reduce energy consumption due to reduced lighting and heat gains. The use of surrounding buildings and other obstructions to provide shading of low-angle sunlight can


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significantly reduce the additional shading (and obstruction to light) required at glazing units. Analytical methods can help to optimise these effects at an early stage.


Improving air movement Air movement is most critical where pollutants are produced. Streets can be oriented along the prevailing wind directions to provide a constant low-speed aeration path. In a simple orthogonal plan this would mean streets that are perpendicular to the wind would have lower rates of ventilation. Alternatively, a higher overall average ventilation rate may be achieved in all the streets if they are aligned at 45 degrees to the prevailing wind direction. To enhance the ventilation in streets


perpendicular to the prevailing wind directions, wider streets or higher buildings on the downwind side of the street can be used to encourage downdraughts. Alternatively, wind-catching structures can draw wind into these streets. Coastal sites can often capitalise on sea breezes. Wide streets will tend to increase air movement (and disperse pollutants), but


August 2011 CIBSE Journal 25


We ought to think beyond the usual low-energy mantra of ‘passive building design first, then efficient systems, then building integrated renewables’


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