| National design handbook prototype on passive solar heating and natural cooling of buildings |
|V. Basic design principles and strategies|
This section considers strategies that can be adopted to regulate overheating. Whilst some strategies can be quantified using reasonably accurate design methods such as the admittance method to predict peak Indoor temperatures or computer-simulation techniques, others are more elusive although well known in use. One of the main difficulties is that most cooling strategies rely on user action and accurate knowledge of their behaviour and such climatic information as detailed wind patterns. If these were really known to the designer then the quantifying process would be simpler.
In many cases of poor design, the internal thermal climate Is more severe than the external climate due to uncontrolled external environmental gains (windows not adequately shaded and external walls and roof not adequately Insulated) or unnecessarily large internal energy gains. A good design solution will provide an indoor thermal climate that starts from the position of being better than the outdoor environment.
In much of the year overheating the inside of buildings is the result of excess solar heating and internally-generated heat reaching the interior spaces. This is certainly the case where the ambient air temperatures are no greater than about 27 °C. In such cases, it is usually practicable to maintain comfort conditions with appropriate control of those heat gains (such as shading of windows and exhausting internal heat) and stood ventilation and air movement patterns.
Where air temperatures are above reasonable comfort levels it is necessary to apply other strategies which will collect or soak up the excess heat for disposal into the cooler earth or to the cooler night air. In these cases the design approach should be as follows:
(a) Reduce solar gains to the interior by correctly designed shading;
(b) Minimize conductive gains by shading walls and other surfaces as appropriate and Insulating the external fabric of the building:
(c) Minimize the effects of internal gains (lights and other appliances) by exhausting the heat;
(d) Design night ventilation openings to optimize the cooling of thermal sinks (thermal mass):
(e) Allow for appropriate air movement (ceiling fans and the like) to raise the occupants' comfort threshold;
(f) Design for minimum air infiltration during the day when external air is 3C greater than the upper comfort limit.
1. Control of heat gain through external fabric
Discomfort from overheating is the result of the body not being able to dispose of the heat generated at an appropriate rate. This state in turn may be the result of external heating influences or a person's activity rate. In the design of buildings the aim Is to cater for people whose activity rate is appropriate to the situation, i.e., when designing a gymnasium allowance is made for the increased activity rate of the occupants by providing more ventilation than might be the case in another situation. In this section the primary concern is with excess heat from the sun or from appliances being used within the building enclosure.
The unwanted heat generated by appliances (cooking stoves in houses or computers and the like in an office environment) can usually be dealt with by ventilation in temperate or cool climates. This technique will at least bring the surrounding conditions to that of the incoming ventilating air. In excessive cases other techniques are applied using heat pumps and the like to remove the excess generated heat.
In most cases for the building designer, the major source of excess heat is the sun in two forms; incident solar radiation and ambient air temperature above an acceptable comfort level. In both cases, the first strategy should be to provide some form of barrier or filter to reduce this overheating effect. The second is to provide positive means of effective natural cooling. The strategies chosen for a project should suit the specific characteristics of the climate and also enhance the architectural design solution.
The reduction of overheating is best achieved by exclusion of the unwanted heat rather than its later removal (often by air conditioning). Therefore the external fabric should be insulated as discussed earlier (including the appropriate use of RFL insulation in summer driven climates). Appropriate shading of opaque external elements is desirable especially where the sol- air temperature effect is significant. The solutions available to the designer range from suitable landscaping to shelter the walls and roof from solar gains (unfortunately these solutions are slow to establish unless already existing) to more sophisticated screens and vented wall linings built as an integral part of the building structure. Transparent elements of the external fabric such as windows and skylights are usually the main source of concern and so will usually require shading from direct sun in the non-heating seasons. The design of shading is discussed separately in this book as it is both a key factor in the reduction of overheating and in visual design of the building. Unwanted solar energy can be excluded by various techniques that can all be grouped into the following general categories.
(a) External devices or systems that deflect the sun's rays (either fixed or adjustable, natural or man-made, to suit the particular application). This is the most positive and effective approach with the least chance of failure;
(b) Specific treatment of the transparent element to change its transmission properties. Generally such solutions are fixed and so will also minimize sun penetration at times when it may be desirable, i.e., winter. A number of variable transmission glasses are being developed such as electrochromic glass, where an electrical force is used to alter the transmission properties of the glass. These should be available in the next few years;
(c) Internal devices that reflect some energy back out and convert other energy from radiant to sensible heat by heating up the air around the shading device. These devices, such as reflective blinds and curtains. are less effective than external barriers but are easier to operate and maintain;
(d) Each of these has its place in the designers "palette" and the successful designer will choose the best one for the job.
2. Passive cooling
In many locations it is necessary to apply some positive cooling strategies to overcome overheating resulting from climatic factors, even after reducing direct solar heat gain to a minimum. This situation usually occurs where the ambient air temperature is either close to the upper limit of the comfort zone or above it and is, sometimes, exacerbated by high humidity levels.
Where the ambient air temperature is within the comfort zone and humidity levels are low it may be possible to satisfy comfort needs with the heat control strategies discussed above. If these limits are exceeded in one way or another the following approaches may be appropriate. either individually or in combination. Some are more climate-dependent than others, such as evaporative cooling which is only applicable where the wet bulb temperature is in the range of 12°C to 24.5°C and relative humidity is less than 80 per cent.
Varying degrees of cooling in a building can be achieved using natural sources such as:
(a) The night air: night-time convective cooling of a thermal mass or heat sink within the building fabric;
(b) The upper atmosphere: radiant nocturnal cooling of the building fabric to the night sky, causing a chilling of the roof structure which can then take up heat the next day;
(c) Water evaporation: evaporative cooling of the air and/or the building structure;
(d) Earth cooling: storage and recovery of energy from the sub- surface soil.
Cooling systems using these sources have been in existence for a long time but in some cases little is known about how to quantify their effectiveness. Water evaporation for cooling is well understood when used in a defined environment of cool air within a mechanical systems. Little is known about how to define the evaporative cooling effect of water sprayed into the surrounding landscape or the free use of water in semi-enclosed spaces such as an atrium or centrally-located courtyard. Some work has been done on this by authors such as Givoni and Lesuek but there is still a long way to go. Many designers have their own knowledge of specific cases from personal experience: they have tried ideas built on the available knowledge and their own developed sense of logic. The young designer can learn from studying the natural and the built environment, noting the cases where a space is comfortable in an otherwise unpleasant environment and observing what makes it better; the use of sprayed water. a large shaded mass that soaks up the heat of the day etc.
Night-time convective cooling used to cool the structural mass of a building can be most effective where the vapour pressure is generally below 2.27 kPa and the diurnal temperature range is large (above about 10 deg.C.) and night time temperatures fall below about 20°C. In areas where the vapour pressure is between 2.27. kPa and 2.67 kPa, nighttime convective cooling is still workable but it may be necessary to use air circulation techniques whilst the building is closed to maintain comfortable conditions. These limits are discussed in detail in chapter VI - Bioclimatic analysis. Almost all places in Australia that require cooling. except for the warm-humid areas, can achieve a useful cooling contribution from this technique. Usually natural ventilation during the day is undesirable under such conditions and so indoor air movement must be created using fans until the day cools sufficiently to open the building to the outside air. The night-time cooling of the structural mass of a building by convection will lower the daytime temperatures by about 2-4C compared with the same building not ventilated at night. The external fabric of such a building should be well insulated to minimize daytime conductive and solar heat gains to the interior and the structural mass in particular.
Radiant nocturnal cooling can be achieved in areas where the night sky tends to be clear during hot weather, generally in all Australian climates except the warm-humid areas. The power of such systems is limited to about 70 W/m², which is minimal for most purposes. In most areas of Australia where there are clear night skies sufficient for such a system there is also a large diurnal temperature swing which will provide cool night air as discussed above, which can be used more simply and at less capital expense.
Evaporative cooling has been used extensively throughout the hot-arid areas of Australia for many years. Mechanical systems that force evaporatively cooled air through the building in much the same way as an air-conditioning system are most effective and economical to run. The main energy requirement is to drive the ventilating fan. In many of the places in Australia where evaporative cooling is most needed there tends to be a shortage of clean water suitable for such equipment. As a result the dirty or brackish water that is often used tends to clog the evaporator pads and increase the maintenance costs. In the more temperate areas of Australia, when on certain occasions the wet bulb temperature is below about 17°C, it is possible to use evaporative cooling but more often the humidity levels are too high resulting in unpleasant warm humid conditions inside.
Passive uses of evaporative cooling such as open water in roof ponds and courtyards have been effectively used throughout history, but as stated before it is difficult to quantify their benefits. Even today most of the design details are based on anecdotal experiences. The water in a pond will follow about 1C above the wet bulb temperature with diurnal swings dependent on the depth of the pond. If water is used in a concrete roof pond then the ceiling below can be expected to reduce the indoor temperature by up to 3C below the average outdoor dry bulb air temperature.
Earth cooling can be utilized in many climatic areas and its effectiveness has been demonstrated by those who have constructed their whole building underground. It is possible to draw air through tubes or passages beneath ground that has been cooled by water evaporation and then into a building for cooling. This technique was used in Alice Springs in a dwelling built for the Flying Doctor Service earlier this century. There have been other examples tried but generally the capital cost is quite high and the passages difficult to maintain free of harmful bacteria.
3. Ventilation and air movement
Natural ventilation of a building will initiate passive cooling during the summer. The exhaustion of excess warm air and the intake of cool air help to lower interior temperatures. To facilitate the exit of warm air, dampered vents should be located at high points in a building.
Natural thermal pressure differentials provide airflow in domestic dwellings. In still conditions ventilation occurs due to this effect. In windy conditions thermal pressure differentials are insignificant compared with the effects of the wind. Ventilation due to temperature differences can be increased by extra storey heights or by providing heat to the upper end of a chimney which is the principle of the solar chimney. If natural thermal pressure differentials do not produce sufficient flow velocities, fans, turbines, or plenums can be used to accelerate them. Attic fans will, if needed, more quickly remove hot air that accumulates during the day. Operable vents, connecting the building space with the attic, provide a pathway for the upward flow of hot air. The motion of outside air can often be used to induce interior air movement without the aid of fans. Wind can be used to power a turbine or can be directed in such a way that pressure changes result which move inside air.
Buildings themselves act as large-scale ducts and should be designed with options for unobstructed movement of air.
Ventilation will generally occur if a pressure differential exists across a building. This differential may be enhanced by the shape and orientation of the building with regard to the prevailing wind direction and openings placed to utilize those pressure differences.
Openings in opposite walls produce a different interior air movement than openings on adjacent walls. Similarly the position of the opening in each wall will influence the path of the interior air stream. The area of openings may be varied for different effects. However, maximum ventilation occurs by having equal size of inlet and outlet openings.
Internal air flow may be further affected by roof overhangs, awnings and fenestration. The main air stream can be directed towards the ceiling for winter ventilation and towards the occupied zone for summer comfort.
Internal walls, partitions and floor-to-ceiling furniture will affect the air flow pattern. Whether the air flow is perpendicular to or parallel to the direction of the main stream flow will modify the air flow. The number of internal openings that the air must flow through to reach the outlet opening will further modify the ventilation produced within the building by reducing the volume of flow.
4. Circulating fans for air movement
Where natural wind patterns are not available or buildings have to be closed to keep out an overheated environment. it may be desirable to use mechanical aids to induce air movement throughout the space or building. The advantage of using a fan for air movement compared with natural ventilation is that its effect is more controllable and so can be directed as needed away from work areas where papers or other materials can be disturbed.
Personal low level fans can be used close to the occupant and directed in such a manner that others are not affected by H. Ceiling fans are economical to operate and provide a broad area of air movement. Generally the blades move slowly creating a gentle movement which need not be disturbing. This type of fan is ideal in cases where air movement is needed for comfort in an otherwise closed space such as a massive building in a hot-arid climate. This fan type can effectively mix the air in the space bringing it in contact with the structural mass cooled the night before. The values in figure 59 illustrate how air movement at comfortable rates can be provided over a reasonable area from one ceilingmounted fan. The distribution covers a much wider area than does the vertical type fan. The only disadvantage being that for safety the units should be mounted at about 2.5-m necessitating ceiling heights of 3-m (regulations in Australia require that the blades be mounted at not less than 2.2 m above floor level.
Daylight is important for more than just vision. Studies have shown that daylight is important to satisfy many human physiological reactions. It is most important that designers consider both daylight quality as well as daylight quantity. In this section it is intended only to highlight the need to incorporate good daylighting design with passive solar and natural cooling technologies. A brief summary of sources of daylight are given for general guidance. Designers should refer to other more detailed texts for design guidelines.
Sources of daylight include direct sunlight, (high intensity), diffuse skylight and diffuse reflected light from buildings. ground. objects
Direct daylight is the result of allowing direct solar radiation into a space, and so in hot times of the year it will be more often excluded or at least filtered to reduce the heating effect of the sun. Direct sunlight often be a serious source of glare unless internal surface colours and textures are chosen to avoid the problem. Direct daylight/sunilght is a valuable design modelling tool as it is always on the move and changes in intensity as the day passes. The designer should not lose sight of this magnificent component of a designers vocabulary.
Diffuse light comes from the sky hemisphere (that component reflected and refracted through the Earth's atmosphere) or is reflected from the surfaces of other objects and if too bright it can cause glare. Diffuse
Good daylighting design is to do with the quality of light rather than just quantity. Glare from daylight can be a serious source of discomfort especially when direct sunlight is reflected off another object either inside or outside the building. In a residential situation, occupants are more able to adjust their position to avoid glare, whereas in other spaces where people are working in a fixed pattern it is more difficult to move and so potential glare sources must be considered in the basic design.
Direct sunlight may be necessary for heating and as a source of light and so it may be better to direct it away from work surfaces and other potential glare-producing surfaces. The use of reflective surfaces outside the viewing range such as light shelves and louvres are an excellent way to overcome this problem. Many designers have utilized these techniques, as Is shown below, to produce glare-free interiors with good natural daylighting levels and a dynamic visual quality that exploits the changing nature of natural light.