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How important is solar shading to passive solar building design?

Posted by Alex Barrett on 24 April 2017 at 12:10 pm

Trying to get indoor spaces to be the right temperature is a constant struggle. During cold parts of the year we use large amounts of energy for heating, but when warm weather arrives we suddenly want to cool down our buildings, either with fans or air conditioning. There seem to be relatively few occasions when the indoor temperature is just right.

One way around this problem is passive solar design. This is a building principle that seeks to maximise heat gain at some times of the day, and the year, while reducing it at others. A lot of this can be done using solar shading techniques, which employ shutters, blinds and other forms of shade to control internal temperature and ensure that living spaces remain comfortable all year around.  

Warming up

Sunlight warms up the surfaces which it illuminates, and this energy can be harnessed to provide space heating, a process called “solar gain”. Greenhouses and conservatories both work on this principle. The glass walls of the structure allow light to enter, warming up the interior. They prevent heat from escaping both by stopping the warm air from leaving the structure and by trapping infrared radiation, which is blocked by the glass.

A passively heated room will generally have a large bank of windows. These are pointed in the direction where they will get the most sun, generally the equator facing side of the building. The room also needs a large thermal mass in which to store the incoming heat. This could be a concrete wall, or a tile floor. If a room has a large enough area of glass to let light in, and good enough insulation to retain this heat throughout the day, then it massively reduces the need for space heating from other sources.

Another option is the Trombe wall, this is a solid wall, with a sheet of glass mounted on the sunward side of it. This glass traps heat, which warms up the wall. Heat is stored within this solid material, and reemitted into the interior of the building as infrared radiation.

Keeping cool

Solar gain is great in cold weather. But at warmer times of year it can cause rooms to overheat. Consequently solar shading is a vital component of passive solar design.

Blinds can be deployed to reduce the amount of light that enters the structure and so cut down on the heating that occurs. However these reduce the amount of light the room gets as well as the heat. Fortunately there are several quite elegant solutions to reduce solar gain, while keeping rooms well lit.

The height of the sun in the sky varies over the year. In the winter the sun is fairly low, while in the summer it is much higher. Consequently the angle at which sunlight hits a building changes with the seasons. This means that putting a ledge above a window blocks light from the high summer sun. However in the winter, when more heat is needed, the sun will be low enough to shine through the window without being blocked by the ledge.

Because the path the sun takes across the sky is very well known it is possible to position solar shades extremely accurately. Brise Soleil, or sun breakers can be seen above windows on many buildings. These consist of a series of metal slats, which block sunlight at specific times, providing lightweight shading for the interior.

Deciduous trees can also be used to provide shading at certain times of year. In the summer their leaves block direct sunlight, reducing solar gain. In the winter the trees don’t have leaves, so more sun reaches the property.

Solar Access

The main limitation of passive solar design is that a building needs to have good solar access in order to benefit from it. If a tall building or stand of trees casts too much shadow on the building then it could disrupt the house’s energy balance.

Providing access to sunlight is now an important part of urban planning. One solution is to work out a “solar envelope” within which buildings can be constructed without blocking solar access to their neighbours. Taller buildings can be constructed in some parts of the city block, but smaller buildings will have to be placed in other areas, so that their shadows don’t disrupt adjacent plots.

References

Passive solar design:

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Image Credit: Brise Soleil at the YouGen Offices, Alex Barrett 2016

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