Passive cooling options

As the climate warms, the demand for environmental cooling will inevitably increase. It would increase anyway: in developed countries the acceptable ' comfort zone' inside buildings is becoming narrower. Occupants are less and less willing to modify their clothing and habits in response to seasonal changes. Fifty years ago clerical staff thought nothing of wearing woollen underwear to the office, where men would work in three-piece suits, collar and tie for most of the year. In the height of summer, windows would be thrown open and jackets removed, but only the young and irresponsible would roll their shirtsleeves above the elbow. These days, winter and summer alike, office workers expect their environment to be maintained above 20°C and below 25°C - and also demand air conditioning in cars and public transport as a necessity rather than a luxury.

The elderly in higher latitudes were once seen as under threat from hyperthermia in winter, and thousands died every year in cold, damp, poorly insulated homes. Now summers offer no respite. Every prolonged heat wave sees deaths among the elderly hit new peaks. Unfortunately, while a number of well-established technologies exist for topping up internal temperatures with alternative energy in winter, alternative cooling technologies are less well known in temperate regions, where prolonged spells of hot weather have hitherto been a welcome novelty. Closer to the Equator, however, in areas like the southern United States and Australia, several cooling alternatives to conventional air conditioning have been developed which could be used at higher latitudes, particularly where ambient humidity levels in summer are relatively low. These techniques may not be capable of controlling internal temperatures and humidity levels as closely as traditional refrigerative air conditioning; but where the building occupants are prepared to accept greater variability they can offer very cost-effective solutions.

Passive cooling techniques have been around for millennia. Mediaeval builders in North Africa and the Middle East constructed houses and palaces with heavy floors, high ceilings, thick walls and roofs and minimal or non-existent glazing on the south side. Public buildings were often approached through a hierarchy of comfort zones: from full sunlight the visitor first entered an area shaded by vegetation, then progressed into a courtyard, shaded by more trees and cooled even further by fountains or pools. The final destination would be an internal space with low light levels and a tile or stone floor. Air moved through the building driven by differences in temperature and humidity, or by deliberately induced variations in air pressure between the interior and the outside world.

Passive Cooled Buildings
Passive cooling techniques can be incorporated into high technology buildings (Reproduced with permission from Mott Macdonald)

The simplest technique takes advantage of the stack effect. Warmer air is less dense than colder, and therefore rises above it. Air inside the building will be warmed by the bodies of the occupants, by lighting and any electrically powered equipment, and by solar gain in the building fabric. If the warmer air is allowed to escape from a vent at roof level, cooler air can be admitted at ground level to replace it. A more developed use of the stack effect is the solar chimney, basically an exterior vertical duct open at the top and with vents connecting it to the building interior. Located on the sunny side of the building, a solar chimney absorbs heat from the sun and transfers it to the air inside. The heated air rises, pulling in air from inside the building to replace it. This air will normally enter the interior of the building from the courtyard or a shaded area outside, although there are several other options. Internal barriers to air movement have to be minimised, either by an open plan design or by adequate internal venting between rooms and corridors. And there are practical limitations as to the size and geometry of the space that can be adequately vented by the stack effect or a solar chimney on its own.

Large public buildings right up until the nineteenth century not only had much greater thermal mass than most of their modern equivalents, but were often built with a network of ducts below their ground floor slabs which were filled with ice in summer, ice cut from lakes and ponds in the winter and stored in insulated ice houses until needed. Natural convection and/or a solar chimney were used to draw air over the ice and into the public spaces. This would hardly be an economic solution today, but other early techniques are more promising. One is to use the suction effect of the solar chimney to draw air down through a cool tower at the opposite extremity of the building.

Solar Venti Earth Cooling Kit
causing the hot air to rise, thus creating a vacuum.

Basic cool tower design

Cool towers have their roots in antiquity as well. They resembled solar chimneys but were filled with porous water jars which cooled the air as water evaporated from them. The cool, dense, humid air sank down into the building, pulling in warmer, dryer air from the top of the tower. Early modern equivalents used pressurised mains water to spray a fine mist down from the top of the tower, which had to be relatively tall to be effective. Variations on this system - also known as passive downdraught evaporative cooling -have been used on a number of more recent multi-storey buildings. More common are cool towers that use special fibre pads kept moist by a header tank arrangement, although most installations also use a small electric pump (discussed in Chapter 13). Solar chimneys have also been found to be less efficient at promoting the essential airflow than rooftop exhaust venturis coupled with an air scoop or windcatcher at the top of the cool tower. Naturally, cool towers and other evaporative cooling systems only work well where ambient humidity levels are low, and are dependent on an unrestricted water supply. Cool towers prefer rainwater, as a high dissolved mineral content in the water can soon clog the system.

Decorative fountains located close to the inlet of a ventilation system can also contribute to building comfort. Evaporative cooling ponds can be very effective if properly designed, either located adjacent to the building close to the intake point for the cooling air, or even on a flat roof. A strong loadbearing structure is needed for rooftop ponds, but if the structure is reinforced concrete it can act as an effective thermal mass heat sink. In its simplest form the pond - usually between 300 and 150 mm deep - is encouraged to lose heat by evaporation at night, cooling down the slab significantly. During the day the pond is protected against solar gain by floating insulated covers. Other versions use permanent shading to minimise

Venturi system

Venturi system

Spray Pond Cooling

Vent

Advanced cool tower design

Vent

Advanced cool tower design solar gain, allowing the pond to function during the day as well, although this obviously consumes more water. In all cases a good airflow across the surface of the water is needed to accelerate the evaporation process; so care must be taken in detailing the surroundings to take advantage of the prevailing winds. Trees close by can not only block wind flows but also radiate heat back to the pond, compromising its efficiency. Sometimes a fan or fans are the only answer, and are also useful during calm periods.

Air ducts or channels can be incorporated in the slab to encourage internal air circulation: water can circulate to floor slabs below by thermosyphonic action through cast in pipe networks. Some designs position a cooling pond on top of a water wall (see Chapter 3). Evaporative cooling can be very cost-effective - provided there is a suitable and reliable water supply - and more sophisticated active systems have been used successfully on large projects (see Chapter 13). Care must be taken to minimise the risks of legionella disease (legionellosis), see Chapter 2, and those of condensation within the building interior.

Windcatchers (badghirs) and wind towers have been a feature of Middle Eastern architecture for many centuries. Both operate on the same principle - that air will flow into an opening facing into the wind because it will be at a higher pressure than the air inside, but will be extracted from an opening facing away from it, due to the lower air pressure downwind of any obstruction. In its most basic form a windcatcher is no more than a unidirectional fabric or sheet metal scoop that can be rotated into alignment with the wind, to either suck air out of a building or conduct fresh air into it. A bi-directional windcatcher can perform both roles at the same time. Most developed versions are multi-directional and static: square shaped in plan with openings on each vertical face and adjustable vents or

Modern Windcatcher With Directional
Unidirectional windcatchers can be striking architectural features (Reproduced with permission from Arup - Mike Rainbow)

dampers. Cool night-time air is captured and exchanged for warm; the heavy structure of the traditional building is cooled down ready for the day. Extractive windcatchers were also used to draw in cool, humid air from underground water channels below the buildings - qanats.

Modern Windcatchers
Windcatchers, like this Iranian example, have been around for centuries (Reproduced with permission from Fabienkahn)
Windcatcher Qanat
The ultimate in mediaeval passive cooling - windcatchers and qanats
Monodraught Windcatchers

Modern multi-directional windcatchers are a well-developed option (Reproduced with permission from Monodraught)

Windcatchers have an advantage over solar towers because they can operate just as efficiently at night. Even in windless conditions ventilation will continue, driven by the stack effect. Modern variants use temperature- and time-sensitive electrically operated dampers to control flow in and out of the building (discussed further in Chapter 13). All such systems depend on consistent airflow across the building - trees or other buildings close by can seriously compromise their efficiency. The same limitation applies to wind-driven extractive fans, of which several types are on the market. Perhaps the most sophisticated of these is the award-winning VAWTEX design developed by Arup, Zimbabwe, and now in use on a number of major buildings in Europe as well as Africa. VAWTEX uses a twin blade Darrieus-type vertical axis wind turbine for power with a central spiral Savonius turbine to ensure self-starting (see Chapter 5), stands 3 m high, self starts when wind speed exceeds 1 m/sec and was first used as part of a passive heating/ cooling system based on underground thermal stores (see Chapter 15).

Skytherm System
VAWTEX wind-powered extraction units (Reproduced with permission from Arup - Mike Rainbow)

Night-time cooling of the building fabric works best where there is a large diurnal temperature range. Ideally, night-time temperatures should be below 20°C. There should also be some form of thermal store in the system: most commonly this is a heavy concrete floor slab or slabs, but basement level rock bins or gravel beds have also been utilised. Phase change materials (see Chapter 15) can also be used to increase the thermal storage capacity of traditional construction. Simply opening windows wide after ambient temperatures fall can be enough to make a significant contribution to occupant comfort the next day. Cool night air removes the heat stored up during the day, lowering the temperature of the thermal store ready for the next day. Again, in some circumstances there may be the risk of condensation on the surfaces of the thermal store.

Summer daytime operation

Prevailing wind

Summer daytime operation

Dampers open

Cross ventilation"

Prevailing wind

Dampers open

Cross ventilation"

Windows open

Temperature say 20 °C

Prevailing wind

Night-time operation (and mid-season)

Courtyard Cross Ventilation Analysis

Dampers open

Night-time operation (and mid-season)

Prevailing wind

Dampers open

Windows closed

Temperature say 5°C

Modern windcatchers in action (Reproduced with permission from Monodraught)

Opening windows may be an undesirable option for reasons of security or noise pollution, so windcatchers or other forms of venting may be preferred. It is important that the cool incoming air is maintained in close contact with the thermal store. Exposed soffits on concrete floor slabs are the norm, coffered or sinusoidal profiles are usually adopted to maximise heat transfer area. Some active cooling systems go further (see Chapter 13), with internal ducts in the slab or other measures to promote effective heat transfer.

An alternative approach is the totally passive US Skytherm system, which uses plastic water-filled bags resting on a corrugated structural steel roof deck, and removable insulated covers. This bears a superficial relationship to rooftop cooling ponds (see above) but no evaporation takes place. The water bags act only as a thermal store. With the insulated covers in place during the day, the water absorbs heat from the occupied space below and stores it until night falls. Then the covers are removed, and the bags radiate heat into the night sky. The cooling effect is most obvious in areas where clear night skies are the norm. Generally, most installations have been on the domestic scale and have relied on manual removal and replacement of the insulated covers.

Solar chimneys, windcatchers and wind-powered extractive fans can be used to draw outside air into the ground floor or basement of a building through earth tubes, also known as ground coupled heat exchangers - usually thin walled metal or plastic pipes buried at least 2 m deep around the building, although standard concrete drainage pipes have also been used successfully. In summer, at that depth soil temperatures are significantly lower than ambient air temperature, so the system yields cool air throughout the day and night as long as stale, warm air is leaving the building at higher levels. In areas with high ambient humidity levels condensation inside the tubes would be inevitable, so most designs set the tubes at a downward slope towards the building and direct any condensate into a basement level drain. Provision for regular cleaning and sterilisation of the tubes would have to be made. Intakes at ground level would have to be rugged and regularly maintained.

A commercially available earth cooling tube installation (Reproduced with permission from SolarVenti)

Two-stage passive cooling has also been tried. During the day, air flows first through a night-time cooled thermal store, usually either earth tubes, rock bins or a gravel bed, and then into a cool tower. When night falls the water is turned off. A windcatcher or solar chimney drives air movement. Most two-stage systems use fans to drive the circulation and powered evaporative coolers (see Chapter 13).

Whatever the overall level of technology adopted, passive cooling techniques will always be worthy of consideration. Architectural freedom need not be compromised; structural efficiency should not be affected. On large or particularly complex projects passive cooling may be inadequate on its own. But as part of an overall environmental management package, passive cooling can make an important contribution towards lower energy demand.

Coo ai allow drainage of water generated by condensation

Coo ai

Passive Cooling Chimney

allow drainage of water generated by condensation in the ground at a depth of approx. 300-600 mm as 20 lengths of 6.0 m

Pebbles to allow drainage of water generated by condensation in the ground at a depth of approx. 300-600 mm as 20 lengths of 6.0 m

Pebbles to allow drainage of water generated by condensation

Passive cooling solutions can be the most cost-effective (Reproduced with permission from Monodraught)

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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Responses

  • Elliot
    Are windcatchers effective in coastal areas?
    8 years ago
  • Doda Marino
    When are monodraught vents best used?
    7 years ago
  • Swen Pabst
    What is passive and low energy strategies?
    6 years ago

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