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Sustainable Building: Passive Solar Design
Print 2008-08-13 22:52  

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Passive solar design involves the use of solar energy – that is, energy from the Sun – to provide a significant proportion of the heating, cooling and ventilation required by domestic dwellings, and other buildings. It is, in fact, one of the simplest applications of solar energy, but, unlike some other forms of renewable energy, it directly replaces the conventional energy consumed in your home.
Passive solar design is a proven technology, but not a technology that generates power, per se, and can therefore be implemented at little, or no cost. Passive solar design can, nevertheless, provide a cost-effective method of reducing energy costs, minimising your reliance on fossil fuels for heating, or cooling, as well as improving the overall comfort of your home. Passive solar design is, of course, fundamentally a design process, and the location, and orientation, of your home are key factors in optimising that process. Building elements, such as windows, and awnings, for example, affect the flow of passive energy in your home, and should also be taken into consideration, along with the local climate. Passive solar design is, perhaps, best applied in new homes, where the orientation, window technology and other building materials can be chosen to provide maximum benefit, without adding unnecessarily to the cost of construction. Almost any type of architecture, however, can be modified to accommodate passive solar design.          

Types of Passive Solar Design

The simplest form of passive solar design, in domestic situations, is known as “direct gain”. Direct gain relies on the use of windows, shutters, etc. to allow sunlight to penetrate a living space, where heat is absorbed in the so-called “thermal mass” – the walls, and floors – of that space, itself. Sunlight is usually admitted through a series of south-facing windows, and its heat energy – known as “short cycle” solar energy – is transmitted to masonry walls and floors, darkly coloured to increase their absorption properties. When the sun sets, and the air in the living space cools, the heat energy stored in the thermal mass is transferred to the air, by processes of conduction, radiation and convection. The efficiency of a direct gain passive solar design depends on the size of the glazed area, which determines the amount of sunlight admitted, and the size, and characteristics, of the thermal mass, which determines how much heat energy can be stored. The ideal ratio between the two depends, largely, on your geographical location, but most direct gain passive solar designs feature a glazing area that corresponds to 7%, or more, of the floor area. Bear in mind, too, that if the thermal mass of your home is too small, relative to the size of the glazed area, this may result in your living space becoming uncomfortably hot. The thermal mass must also be insulated, from the ground, and from outside temperatures, to prevent the accumulated heat energy from draining away.

Another form of passive solar design, known as “indirect gain”, also collects and stores energy from solar radiation in some form of thermal storage material. The thermal storage material is often positioned between south-facing windows and the living space – in what is known as a “Trombé” wall, after the French engineer, Felix Trombé – but may also involve water tanks, or earthed roofs, for the slow release of heat energy. The Trombé wall, however, is the most common approach, and typically features a masonry wall, 8 to 16 inches in thickness, with a single, or double, thickness of glass in close proximity to its outer surface. Sunlight enters through the glass, is absorbed by the outer surface of the wall, and stored in its thermal mass. Once again, when the temperature of the air in the interior falls below that of the wall, heat radiates from the wall into the living space. The rate of heat transfer through masonry, however, is slow – typically 1 inch per hour – so that a Trombé wall radiates heat for several hours.

The third form of passive solar design is known as “isolated gain”, and typically features a “sunspace”, or “solarium”, which can be opened, or closed, to the remainder of the home, at will. Sunspaces, by their very nature, involve an abundance of glazing, and as a result may experience high heat gain, and high heat loss. Variations in temperature can be regulated to some degree by thermal mass, and window technology – low emission, selective coatings, for example – but most isolated gain passive solar designs include doors, windows, or vents, at ceiling, or floor, level, to prevent these variations from disturbing the temperature balance in the home, as a whole. The thermal mass of a sunspace may, again, be composed of masonry wall, masonry flooring, or water containers.

Orientation & Other Considerations

The orientation of your home is a major determining factor in the efficiency of a passive solar design, of any kind. Ideally, the longest wall of your home should face due south, or “solar” south, or, at least, within 30° east, or west, of solar south. The further away the alignment is away from north-south, the greater the reduction in heat gains during summer, and winter, but between these limits efficiency should only decrease by between 5% and 15% of the optimum. Remember, however, that due south is not the same as “magnetic” south, and that, if you wish to determine due south with a compass needle, you need to take into account the so-called “magnetic declination” – the number of degrees difference between magnetic south, or north, and due south, or north – that is applicable at your latitude and longitude.

Environmental, and climatic, factors, too, can influence the efficiency of passive solar design, in some cases just as much as the orientation of your home. Knowledge of your local weather patterns, and topography, for example, may allow you to exploit natural features, such as trees, or hills, to improve the heating, and cooling, effects on your home. A hill, or evergreen trees, to the north of your home, may reduce the impact of cold, winter winds, while deciduous trees – trees that shed their leaves, in autumn – to the south, can provide shade in the summer, but still allow the maximum amount of sunlight to penetrate your home when it is most needed, during the winter months.

Categories: Green Building, Design and Planning


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