In a physical sense, a passive system is one that uses only locally available energy sources and utilises the natural flow paths of that energy to produce work. In other words, no auxiliary equipment (such as fans or pumps) are required to make the system to function - the transmission media is directed by induced convection currents or reflected/refracted to where it is needed to do the work.
In an architectural design sense, the work that needs doing is usually the heating, cooling and lighting of enclosed spaces. The kinds of low-grade ambient energy systems available within most building sites are actually well suited to these tasks, as is evident by our ability to survive prior to the discovery of electricity.
On-Site Energy Sources
There are many sources of energy available locally within a building site. These include direct and diffuse radiation from the Sun, air movement from winds and temperature differences, biomass from vegetation, as well as geothermal and hydro-kinetic sources. This does not mean that the site has to be covered in solar panels and huge wind turbines, with a massive concrete dam underneath and the occasional geyser spewing steam Yellowstone Park-style out of the ground. Instead, windows can be designed to allow in natural light and heat from the Sun, and opened to cooling afternoon breezes. Rainwater can be collected from the roof and stored in the ceiling, using gravity to feed the taps and faucets below. Earth can be bermed high against rear walls to protect them from large changes in outdoor air temperatures. Alternatively inlet air can be drawn through underground cavities providing some level of geothermal cooling in summer and heating in winter.
The effective use of these low-grade energy sources in a building requires only some careful thought and a little innovative design. Many projects have shown that such buildings do not have to cost any more than less carefully designed buildings, and can be significantly cheaper to run. For more information, see the topic on alternative energy systems.
In more recent times, the term passive design has moved beyond meaning simply 'using passive systems', to encompass the general design of energy-efficient and low-energy buildings. In this modern world of compromise, the concept of passive design does not even preclude the use of low-energy active systems.
The basic idea of passive design is to allow in daylight, heat and airflow only when they are most beneficial, and to exclude them when they are not. This includes the storage of ambient energies where possible, for distribution later when there may be greater need. The full range of passive techniques are considered, such as the correct orientation of the building, appropriate amounts of fenestration and shading, an efficient envelope, maximum use of daylighting and the appropriate level of thermal mass, as well as the use of renewable resources in preference to non-renewables.
More conventional systems using fans and pumps can be used where a small initial energy input can be used to yield a relatively high output. This includes technologies such as evaporative cooling units and heat pumps.
The list of passive design techniques given here is by no means meant to be exhaustive. However, for the purposes of categorisation, the following two topics cover passive heating and passive cooling systems.
Good passive design for thermal comfort is based on the following six major principles:
- Orientation of frequently used areas towards the equator (north in the southern hemisphere, south in the the northern hemisphere), to allow maximum sunshine when it is needed for warmth, and to more easily exclude the sun's heat when it is not.
- Glazing used to trap the sun's warmth inside a space when it is needed, with adequate shading and protection of the building from unwanted heat gain or heat loss.
- Thermal mass to store the heat from the sun when required, and provide a heat sink when the need is for cooling.
- Insulation to reduce unwanted heat losses or heat gains through the roof, walls, doors, windows and floors.
- Ventilation to provide fresh air and capture cooling breezes.
- Zoning of internal spaces to allow different thermal requirements to be compartmentalised when required.
Buildings should be planned in such a way that benefit is obtained from shaded indoor and outdoor living areas when the weather is hot and sunny indoor and outdoor areas with wind protection when the weather is cold.
Well designed buildings should be oriented, and the spaces arranged in such a way, that the majority of rooms face towards the equator. In this way the eastern and western sides are exposed to the low-angle summer sun in the morning and afternoon. The high angle of the sun in the sky in summer makes it easy to shade windows using only a generous roof overhang or horizontal shade. The longer north/south sides of the building benefits from the low angle sun in winter. The roof overhang or shading on the equator side should allow the Sun to shine into the building when its warmth is required in winter, and provide adequate protection from high-angle Sun in summer.
If the majority of windows are designed into the equator-facing wall, sun penetration into the building will be maximised. Living areas should be sited to gain maximum benefit from cooling breezes in hot weather and shelter from undesirable winds in winter. This does not mean that the orientation of the building should be varied from north towards prevailing breezes as it does not have to face directly into the breeze to achieve good cross-ventilation.
Within the internal planning, rooms such as dining and recreation area that require more heat during the winter months should be placed on the equator side. Rooms that are used for short periods of time during the day can be placed towards the rear, or more effectively, as buffer zones on the west side to protect living areas from the hot afternoon Sun (for example bathrooms, laundry, ensuite, entry corridors, stairs, bedrooms, bars).
For more detailed information, see the orientation topic.
Windows, glass doors, panels and skylights play a crucial role in admitting heat and light, and can have a significant impact on energy consumption. They are also the most difficult parts of the building envelope to adequately insulate. Care needs to be taken to ensure that windows are positioned, sized and protected so as to get the most benefit from winter sun while avoiding overheating in summer and heat loss in winter. For more information, see the overshadowing and shading design topics.
Thermal mass is basically the ability of a material to store heat. It can be easily incorporated into a building as part of the walls and floor. Thermal mass affects the temperature within a building by:
- Stabilising internal temperatures by providing heat source and heat sink surfaces for radiative, conductive and convective heat exchange processes.
- Providing a time-lag in the equalisation of external and internal temperatures.
- Providing a reduction in extreme temperature swings between outside and inside.
Material selection to capitalise on thermal mass is an important design consideration. For instance, heavyweight internal construction (high thermal mass) such as brick, solid concrete, stone, or earth can store the Sun's heat during winter days, releasing the warmth to the rooms in the night after it conducts through. Lightweight materials such as plasterboard and wood panelling are relatively low mass materials and will act as insulators to the thermal mass, reducing its effectiveness. Lightweight construction responds to temperature changes more rapidly. It is therefore suitable for rooms that need to heat or cool very quickly.
For maximum energy efficiency, thermal mass should be maximised in the equator-facing sides of a building. Any heat gained through the day can be lost through ventilation at night. In using this technique, the thermal mass is often referred to as a 'heat bank' and acts as a heat distributor, delaying the flow of heat out of the building by as much as 10-12 hours.
Thermal mass design considerations include:
- Where mass is used for warmth, it should be exposed to incident solar radiation.
- Where mass is required for cooling, it is better placed in a shaded zone.
- Buildings may be preheated using electric or hot water tubing embedded in the mass (mostly concrete floors).
- Buildings may be pre-cooled using night-purge ventilation (opening the building up to cool breezes throughout the night), although this requires significant amounts of exposed mass, and may be necessary only at certain times of the year.
- Thermal mass is particularly beneficial where there is a big difference between day and night outdoor temperature.
For more detailed information on how it works, see the thermal mass topic.
Insulation specifications are another important design feature. The building envelope provides a barrier against the extremes of the outdoor environment, allowing the thermal comfort levels indoors to be adjusted to suit the occupants. This might require heating or cooling depending on the season and location of the building. The energy required for heating or cooling will be greatly reduced if the building envelope is well insulated to reduce incidental losses. This means insulating the ceiling, walls and floor of the building, an easy task during construction, but often more difficult for existing buildings.
Insulation reduces the rate at which heat flows through the building fabric, either outwards in winter or inwards in summer. In temperature controlled buildings, this will result in significant energy savings and increased thermal comfort. In passive buildings, it means that any low-grade energy available will be more effective at its job of heating or cooling.
Insulation has an additional benefit it that it also reduces noise transfer through the fabric, however its resistance to both fire and insects should also be major considerations. Proper installation is also essential to maximise performance, and there often local and international standards to cover the fire safety and health aspects of installation.
For more detailed information, see the thermal insulation topic.
Ventilation of a building is critical during summer as the building must provide sufficient ventilation and breeze paths to assist with cooling. For warmer climates doors and windows should be positioned to facilitate prevailing cooling breezes. An analysis of local wind directions at different times of the year may be necessary in order to best locate windows and design systems to 'catch' or funnel the breezes through them.
To maintain indoor air quality, the opportunity to provide clear breeze paths through a building should be maximised to encourage air flow for night time cooling in summer and 'flushing out' the accommodation, by removing stale air that contains CO2, water vapour, and mould.
For more detailed information, see the natural ventilation topic.
Substantial savings can be made through proper zoning. Rooms requiring heating such as dining areas can be heated without having to include less frequently used rooms such as hallways, bathrooms or bedrooms. The following strategies could be incorporated into the design to allow for zoning in winter:
- Air locks to the main entries to the building (for example, entry, laundry).
- Similar activity rooms grouped together (for example, bedroom zone, living zone, wet or bathroom zone).
- Grouped areas need to be sealed with tight fitting seals to all four sides of the door.
- Bio-Climatic Design
- Shows how given comprehensive hourly weather data that it is possible to apply objective tests to determine the potential benefit of passive and low energy building design strategies on occupant comfort.
- Passive Solar Heating - Sustainable Building Sourcebook
- Solar Radiation Data Manual for Buildings