|Design Handbook on Passive Solar Heating and Natural Cooling (HABITAT, 1990, 162 p.)|
|V. Basic design principles and strategies|
Because the sun's path across the northern sky (in the southern hemisphere) is low in winter, and high in summer, a house can be designed to allow the sun to enter and warm the house and warm outdoor living areas in winter and prevent the sun from striking walls and roof or from penetrating the house in summer.
Windows facing in a northerly direction receive useful sunshine for most of the day in winter and the desired period for access to the sun's rays in most of Australia is 9 a.m. to 3 p.m. In summer, unwanted sunshine can be very easily blocked by an overhang, pergola or other horizontal shading device.
Although the winter sun in the mornings and the evenings coming through windows facing east and west is pleasant visually, it provides very little useful heating. In summer those east-and west- facing windows receive a high proportion of the sun's energy and because the sun is low in the sky it is difficult to screen it out with conventional shading devices.
South-facing windows receive no direct radiation in winter and very little In summer. The low evening sun in summer may cause problems on an open site.
For the best passive solar design, the windows to living rooms and bedrooms should face in a northerly direction. Some flexibility of orientation is acceptable, however it has been found that the optimum orientation is within 20 degrees either side of north. A building oriented outside this range loses the benefits of winter sun. This is clearly demonstrated in figure 26 which illustrates graphs of mean daily solar radiation on vertical surfaces of various orientations in Sydney. Notice how the solar radiation received on a vertical surface facing north- east or northwest is almost the same year round. The importance of orientation is self-evident.
Figure 24. Optimum window orientation.
If there is a preference, then about 12 degrees east of north (approximately magnetic north in New South Wales) is best to let some sun in for an early warm-up in winter. Because north orientation is essential to passive solar design, it is important to choose a house block that allows north orientation to at least the major daytime living spaces. Trees or buildings could block access to sunlight, and this needs to be checked when siting dwellings.
The orientation of a building is determined usually by the position of the windows and the proportion of the plan. Excluding the internal spaces at this point the objective is to locate most glass on the north fae and to design the building so that its north and south fae are larger than the east and west faes.
Figure 26 also illustrates the relationship between Sydney's heating load profile and solar radiation incident on surfaces of different orientation. A diagram such as this is admittedly simplistic but it shows the effect of window orientation very clearly. The north orientation is the only one that is able to combat the heating load without creating enormous penalties in the summer.
It is often suggested that the optimum building plan proportions for a temperate climate is about 1: 1.5 with the longer fae to the north. This may be correct on the basis of simply the heat gains and losses but it will not necessarily be applicable to the requirements of a particular design and its site. The area of north-facing windows is perhaps more important and in Australia's temperate and cooler areas there is still a need for good cross-ventilation in the summer months. This requirement may well dictate a longer, thinner plan than the aforesaid optimum.
The way in which living units are planned effects the overall thermal efficiency. The greater the exposed external surface the greater the potential heat loss. Figure 25 shows that medium- density building is subject to smaller heat losses per living unit than detached cottages and likewise high-density building is subject to even smaller heat losses. In residential buildings this can be advantageous, provided the individual units have a reasonable access to sun from the north.
In high-density commercial buildings this compactness is often a disadvantage because it results in a year-round cooling load due to internally generated heat, and so a need for air conditioning. Figure 25 illustrates how density of planning relates to conductive heat loss. The heat loss rate will be proportional to the external surface area and its resistance (Aex × U). Solar access usually becomes more difficult as density increases.
Likewise, the solar gains in winter can be enhanced by the orientation and grouping of the various units. In developing these arrangements, it needs to be remembered that the sun is low in winter and high in summer. A roof's exposure is therefore high in summer. Light-coloured roofs will reflect much of that radiation.