Solar design Passive solar design

Since the sun drives every aspect of the climate it is logical to describe the techniques adopted in buildings to take advantage of this fact as 'solar design'. The most basic response is referred to as 'passive solar design'. In this case buildings are designed to take full advantage of solar gain without any intermediate operations.

Access to solar radiation is determined by a number of conditions:

• the sun's position relative to the principal facades of the building (solar altitude and azimuth);

• site orientation and slope;

• existing obstructions on the site;

• potential for overshadowing from obstructions outside the site boundary.

One of the methods by which solar access can be evaluated is the use of some form of sun chart. Most often used is the stereographic sun chart (Figure 5.1) in which a series of radiating lines and concentric circles allow the position of nearby obstructions to insolation, such as other buildings, to be plotted. On the same chart a series of sun path trajectories are also drawn (usually one arc for the 21st day of each month); also marked are the times of the day. The intersection of the obstructions' outlines and the solar trajectories indicate times of transition between sunlight and shade. Normally a different chart is constructed for use at different latitudes (at about two degree intervals).

Sunlight and shade patterns cast by the proposed building itself should also be considered. Graphical and computer prediction techniques may be employed as well as techniques such as the testing of physical models with a heliodon.

Computer modelling of shadows cast by the sun from any position is offered by Integrated Environmental Solutions (IES) with its 'Suncast'

Figure 5.1

Stereographic sun chart for 21 March program. This is a user-friendly program which should be well within normal undergraduate competence (www.ies4d.com).

The spacing between buildings is important if overshading is to be avoided during winter months when the benefit of solar heat gain reaches its peak. On sloping sites there is a critical relationship between the angle of slope and the level of overshading. For example, if over-shading is to be avoided at a latitude of 50oN, rows of houses on a 10° north-facing slope must be more than twice as far apart than on 10° south-facing slope.

Trees can obviously obstruct sunlight. However, if they are deciduous, they perform the dual function of permitting solar penetration during the winter whilst providing a degree of shading in the summer. Again spacing between trees and buildings is critical.

Passive solar design can be divided into three broad categories:

• attached sunspace or conservatory.

Each of the three categories relies in a different way on the 'greenhouse effect' as a means of absorbing and retaining heat. The greenhouse effect in buildings is that process which is mimicked by global environmental warming. In buildings, the incident solar radiation is transmitted by facade glazing to the interior where it is absorbed by the internal surfaces causing warming. However, re-emission of heat back through s

the glazing is blocked by the fact that the radiation is of a much longer wavelength than the incoming radiation. This is because the re-emission is from surfaces at a much lower temperature and the glazing reflects back such radiation to the interior.

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