|Design Handbook on Passive Solar Heating and Natural Cooling (HABITAT, 1990, 162 p.)|
|I. Principles of passive solar architecture|
If use is to be made of the sun's heat, then it has to reach buildings when it is useful. Generally, the sun should be able to reach the collection area between 9 a.m. and 3 p.m. in winter with as little and interference as possible. Trees on the site or the neighbors' site or perhaps those the householder plans to put in, might shade the vital areas of the building. This needs to be checked and the building located to minimize any such interference. During the winter, solar energy comes from approximately northeast at 9 a.m. and north-west at 3 p.m., about 90 degrees.
Figure 3. Sketch showing how the sun should have good access to windows that act as solar collectors.
In summer, there are often cool breezes which if directed through a building will help cool it. Weather Bureau information gives details of the direction for a particular area, but this should be checked on-site because other buildings, hills or trees might deflect those breezes to another direction. Likewise in winter, the cold winds should be deflected away from a building. They tend to come from the south, the south-west and the west. With simple local information about a site, it is possible to plan to optimize winter sun and summer breezes and to block winter winds by careful placement of obstructions such as trees, fences, hedges or a garage. If a client has not yet bought a site, then one with the right aspect should be sought. It is most Important to remember to let the sun in from the north.
The most straightforward way to use the sun's energy to warm a house is called the "direct-gain system". Here the sun is simply let into the building through correctly positioned and carefully sized windows. When describing the way buildings must face reference is being made to the direction that the main windows face. The windows of living rooms and, if possible, bedrooms should face towards the north. There is some latitude and so anywhere between 30 degrees east or west of north is acceptable. If there is a preference. then about 10 degrees east of north is best, to let some sun in for an early warming.
Figure 4. Use of landscaping and the building bulk deflect the cold winter winds to accept cool summer breezes.
It is important to try to avoid windows on the east or west sides, because these sides of a building receive more of the sun's heat in summer than they do in winter. The north side is quite different. It receives more heat in winter than summer, because the sun is high in the sky In summer and low in winter. Therefore, east- and west-facing windows can cause a building to overheat in summer much more so than north- or south-facing windows.
Figure 5. Example of a building having good performance and acceptable internal conditions.
Shade for north-facing windows is relatively easy with a simple overhang: this will still retain the view. East- and west-facing windows would need a full cover in summer which will restrict the view to the outside throughout most of the summer months.
Glass lets heat out in winter and in in summer by conduction approximately 10 times faster than an insulated wall, and three times faster than a brick-veneer wall. This is another reason for not having windows too large. The rate at which heat passes out through glass is fastest at night when it is very cold, so good curtains with insulating linings should be drawn across after sunset. These curtains should be close-fitting at the floor and the sides and have pelmets at the top. In an office, it may not be practical to do this, so It is necessary to accept that the rooms will be a little colder at night, when normally everyone will have left.
Sunlight through the roof using a clerestory may be practical where normal-level windows cannot be located to face north. Such a solution should be designed so that all the criteria of size and shading apply. Clerestory windows in the roof should be vertical in preference to sky domes and the like. The sun is overhead in summer and so much stronger than in winter on any surfaces near horizontal, such as roofs.
When renovating a building, consideration should be given to moving windows from the east and west sides to the north. Where this is not possible, clerestory windows should be considered. Unfortunately, terrace houses that face east or west are extremely difficult to improve in order to optimize solar energy, if not impossible.
"The sun porch" is another passive solar system which has tremendous potential for many houses, flats and units. Many buildings already have these but they lack a few details. In the 1930s they were called sun rooms and many houses were built with them. It Is important, agaun, that the glass faces north. Such a space with a large area of glass will trap a lot of heat. The space should, therefore, be connected to the rooms needing heath, with a simple exhaust fan at the top and a return ventilator at the bottom, as indicated in figure 6. The easiest place to install the fan is probably at the top of a window, but the exact position is not too important and will vary for each example. The sun porch then becomes a solar collector. There are many of these in Sydney already, and all that is needed is the fan and ventilators. At night, the temperature drops quite low in sun porches because of the large quantity of glass. Therefore, both the fan and the ventilator at the bottom should have a shutter or flap to be closed at night. Besides being a solar collector, the sun porch also serves as a useful room. This system is ideal for older-style houses where the fabrics and materials may be a delicate colour and would otherwise be damaged by lots of bright sunshine coming in all day during the winter.
Figure 6. The sun porch concept.
In summer, it is important that the sun porch be well vented to the outside, so that the summer heat does not build up. One way to do this is to have sliding glass windows that allow 50 per cent of the glass to be opened. Another way of controlling the summer heat would be external shutters or blinds.