In warm sunny climates, excess solar gain may result in high cooling energy consumption; in cold temperate climates winter sun entering south facing windows can positively contribute to passive solar heating; and in nearly all climates controlling and diffusing natural illumination will improve day lighting.
Well designed sun control and shading devices can dramatically reduce building peak heat gain and cooling requirements and improve the natural lighting quality of building interiors. Depending on the amount and location of fenestration, there can be reductions in annual cooling energy consumption of 5% to15%.
Factors influencing the relative impact of shadow effects are site-specific and include differences in terrain elevation between involved properties, the height and bulk of structures, the time of year, the duration of shading in a day, and the sensitivity of adjacent land uses to loss of sunlight.
Shadows cast by structures vary in length and direction throughout the day and from season to season.
- Shadow lengths increase during the “low sun” or winter season and are longest on December 21-22, the Winter solstice.
- The winter solstice, therefore, represents the worst-case shadow condition and the Potential for loss of access to sunlight that a project could cause is greatest. Shadow lengths are shortest on June 21-22, the summer solstice.
- Shadow lengths on the spring and fall equinoxes, March 20-21 and September 22-23 respectively, would fall midway between the summer and winter extremes.
Useable sky-space, at the winter solstice, is that portion of the sky lying between the position of the sun (i.e., sun angle or azimuth) when it is 45 degrees to either side of true south – the portion of the sky covered or traversed by the sun between 9:00 a.m. and 3:00 p.m.
For either an active or passive solar energy system to work, it is not necessary for it to be exposed to sunlight from sunrise to sunset. In order to obtain the correct shading devices that will limit sun penetration on the external windows, a detailed sun analysis has been performed. This sun analysis considers the sun penetration for the following days of the year:
– December 22 (Summer solstice)
– March 22 (Autumn Equinox)
– June 22 (Winter solstice)
The solstices were used as they represent the extreme cases of the sun’s path, i.e. the summer solstice occurs when the altitude of the sun is at its highest and conversely the winter solstice occurs when the altitude of the sun in the middle of the day is at its lowest. This provides the full range of solar penetration, with all possibilities lying in between. The autumn equinox is used to represent an end point to summer and possibly less stringent shading requirements.
Shading analysis is one of the most essential steps in phase of solar energy system design or analysis. The effects of shading by one building upon another can be either positive or negative depending upon the site-specific circumstances of the properties involved. A potential benefit of shading for adjacent structures may be a cooling effect gained during warm weather. Negative consequences of shading include the loss of natural light for passive or active solar energy applications or the loss of warming influences during cool weather.