Before refrigeration technology first appeared, people kept cool using natural methods: breezes flowing through windows, water evaporating from trees and fountains as well as large amounts of stone and earth absorbing daytime heat. These ideas were developed over thousands of years as an integral part of all building designs. Today this is called "passive cooling" and, ironically, is considered an 'alternative technology', as if untried and untested compared with reliable and robust mechanical cooling that requires complicated refrigeration systems. By employing passive cooling techniques in modern buildings however, you can often eliminate the need for mechanical cooling or at least significantly reduce the size and cost of the equipment.
Main Objectives
The two main objectives in any passive cooling design are to exclude unwanted heat gains as much as possible and to generate cooling potential wherever possible.
Excluding Heat Gains
There are three main sources of heat gain in summer that have to be dealt with. These are direct solar radiation, high outside air temperatures and internal gains from occupancy, lighting and equipment.
Dealing with Solar Radiation
Unwanted heat from solar radiation can enter a space either directly through a window or indirectly through opaque elements of the building fabric, heating up the outer surface and increasing heat flow by conduction (see the topic in sol-air temperature for more details). The best way of dealing with either is to prevent it from reaching building surfaces in the first place, however a range of techniques can be used:
- Shading Devices
Devices such as wide roof overhangs, shading fins, thick vegetation or external shutters can be used to protect windows and wall surfaces. It is also often possible to shape the building such that some parts of it are self shading. - Double Roof Systems
A double roof system uses a ventilated air gap between an upper exposed roof and a lower protected roof. Much of the solar gain from the upper leaf is carried away by the air before it can pass to the lower leaf. - Surface Colouring
If opaque building elements exposed to solar radiation are painted white or very light colours, much of the incident radiation can be reflected away from the surface. - Insulation
Any surface that is exposed to high levels of solar radiation in summer should be well insulated to reduce the transfer of heat. The best location for this insulation is on the outside surface, however this may not always be practical. In climates with a high diurnal range in summer (hot days and cold nights), it may be preferable to store daytime heat for release later at night when the temperature falls. In this case, exposed surfaces should comprise a thick layer of heavyweight material with a high thermal capacitance and a thermal lag of around 8-10 hours. - Thermal Mass
This too applies only to climates with a high diurnal range. In this case, lots of thermal mass is used in the interior of the building to even out temperature fluctuations. To be effective when used in this way, the mass must be exposed internally and not covered over with carpets, cupboards or panelling. There is some design flexibility here as, depending on the exact conditions at night, some delayed conduction gain may be desirable or not. If so, the form of the building can be designed such that exposure is limited to certain surfaces at specific times to control the collection and release of conducted heat. If not, then shading, surface colour and insulation should be used on the outside surfaces to reject solar gains. It may also be desirable to use a night-purge ventilation system to cool the mass down at the end of each day (see the night-purge ventilation section). - Solar Control Glass
If external window shading is not practical (though I cannot think why), highly reflective glass can be used. This won't be as effective and may even be against local planning regulations in some places where bright solar reflections are a hazard. Heat absorbing glasses and internal blinds/curtains are the least effective option as they allow the radiation to enter the space (either directly or as long wave radiation from the heated glass) before shading the occupant (see the topics on solar control and solar control glasses for more information).
The equator-facing facade of a building is the easiest to control solar radiation on as the position of the Sun in the sky throughout the year is favourable to the use of simple horizontal shades. The east and west facades are much more difficult to shade due to high-intensity low level Sun in the early mornings and late afternoon. There is some scope to take advantage of the changing sunrise/sunset positions between summer and winter using angled vertical shades, however such systems must be designed with great care. The east and west facades can be protected using large deciduous trees or a trellis system with creeping vines, however vertical shades are the usual option.
Dealing with High Outside Temperatures
The main pathway for ambient heat energy from the outside air to enter a building is through infiltration: air moving through cracks, apertures and even porous elements of the building fabric. Even well-built, relatively airtight buildings can expect at least 1/2 and air change per hour, with most buildings being more in the order to 1 or 2 air changes per hour.
If the outside air temperature is 36°C, this can be a significant heat gain. This can be minimised by:
- Airtight Construction
This requires using windows and doors with good quality airtight seals as well as caulking and sealing cracks/gaps around them. - Non-Porous Materials
Using less porous materials in the building fabric can prevent some infiltration, however careful consideration must be given to the potential for interstitial condensation when using materials with high vapour resistances.
The second major pathway for ambient heat energy to enter a building is via conduction through the building fabric. The temperature difference between the inside and outside surface of a material is the main driving force behind conductive heat flow. Whilst it is not really possible to affect the magnitude of this temperature difference without unduly affecting internal comfort, there are ways of minimising the flow:
- Insulation
The same issues relating to the use of insulation and solar radiation are relevant here. The interplay between insulation and thermal mass will depend entirely upon the diurnal range and night-time conditions. - Minimised Fenestration
Glass is a very good thermal conductor. The more glass in a facade, the more heat will flow in from the outside regardless of shading. Furthermore, large areas of hot glass can generate convective loops within spaces, quickly and effectively heating the air inside. The convective effect can be prevented by the use of curtains with pelmets, however these will only slightly reduce the conduction flow.
Dealing with Internal Gains
The simplest way to deal with internal loads is to remove them from the space. However, most of the time it is not possible to reduce them this way without impacting on the amenity of the space. A dealer's room will generally be full of computers, a pottery workshop will have a kiln, etc. Even in relatively cold climates, most offices will not need auxiliary heating as high internal gains are usually a problem year round. There are however ways to minimise these loads:
- Maximise Daylighting
In terms of the number of lighting lumens per watt of heat energy, daylight is the most efficient way of lighting a space. Careful design of fenestration that maximises daylight (whilst excluding direct sunlight) can not only save electrical energy but also reduce internal gains. Combined with an intelligent daylight-linked dimming system, lighting amenity can be ensured whilst still achieving savings. - Energy Efficient Lights
Lighting installations can be a significant load in many buildings, especially open-plan offices where a uniform light level has been designed for over the entire floor area. The use of more efficient lamps and luminaires, occupancy sensors, daylight dimming and task lighting can significantly reduce the overall lighting load. - Zoning
Careful planning and spatial compartmentalisation can be used to separate high-gain spaces from low gain, so that ventilation and other strategies can be applied to a different degree to remove heat generated by internal gains.
Generating Cooling Potential
Once all the unwanted gains have been dealt with, it is often necessary provide additional cooling in the hotter months of some climates. There are only really three sources of passive energy that can be used for cooling: solar radiation, air movement and geothermal mass. Even then, the climatic conditions must be right for any of them to be effective. For example, in hot humid equatorial climates the skies are often overcast during the day (reducing direct solar radiation), the air is often very humid (nullifying the effect of air movement) and the high year-round average air temperature mean that ground temperatures can be above comfort comfort levels. This situation can make generating cooling potential somewhat of a problem for the passive designer. If, however, your particular climate is not so extreme, then the following systems can be very effective.
Related Links
- Passive Solar Cooling - Sustainable Building Sourcebook
- http://www.greenbuilder.com/sourcebook/PassSolGuide3.html