How to Save Millions on Air Conditioning by Designing Passively Cooled Buildings
Air conditioning is by far the greatest consumer of electricity in buildings in hot countries, but it needn't be so.
Back in the warm days of August (at least in my part of the world) I wrote about how buildings in sunbelt countries could save millions by using active solar cooling. But architects designing buildings for regions that otherwise would require air conditioning can also use passive solar techniques to keep them cool, and in many cases successfully eliminate the need for expensive air conditioning.
Right: A design for a mosque using passive solar and evaporative cooling.
Passive solar cooling operates in two stages:
- Do your best to prevent the sun from reaching the building or gaining the interior of the building during the periods when it is in danger of overheating.
- Then employ passive techniques to remove unwanted hot air.
Different techniques are available depending upon the climate, i.e. whether it is dry or humid.
Passive cooling for warm/hot, dry climates
Here is a summary of the techniques available for hot, dry climates with a large temperature variation from day to night use:
- high interior thermal mass;
- exterior superinsulation;
- highly-reflective OR green roofs, with insulation and a radiant barrier beneath;
- night ventilation;
- phase change materials;
- air vents;
- diode roofs;
- roof ponds;
- wind towers;
- ensure correct exterior shading on windows;
- closing windows and deploying shutters at sunrise to keep out the hot daytime air;
- direct evaporative cooling.
Above: How a wind tower on a building is used for cooling cooling.
Cooling is provided by radiant exchange with the massive walls and floor plus optional techniques explored in more detail below. Open staircases, etc. may provide stack effect ventilation, but observe all fire and smoke precautions for enclosed stairways.
Curved roofs and air vents are used in combination where dusty winds make wind towers impracticable: a hole in the apex of a domed or cylindrical roof with a protective cap over the vent directs the wind across it and provides an escape path for hot air collected at the top.
Arrangements may be made to draw air from the coolest part of the structure as replacement, to set up a continuous circulation and cool the spaces.
Passive cooling for warm/hot and humid climates
This is a summary of the techniques available for warm and humid climates, with little temperature variation from day to night:
- airtight and superinsulated construction;
- a radiant barrier beneath the roof deck;
- highly-reflective or green roofs, with insulation beneath;
- phase change materials;
- daytime cross-ventilation to maintain indoor temperatures close to outdoor temperatures.
- fresh air brought in through a dehumidifier through the crawlspace or basement by using underground pipes or use solar-powered absorption chillers;
- avoid drawing in unconditioned replacement air that is hotter or more humid than interior air;
- avoid open areas of water such as pools;
- the Passivhaus standard permits 15 kWh/m2.yr of cooling energy to be used, which has proven to be sufficient in almost all cases because the Passivhaus system is highly effective in reducing unwanted heat gains;
- As above, curved roofs and air vents are used in combination where dusty winds make wind towers impracticable.
Large buildings will require detailed modelling. Power may be required for anti-stratification fans and ducts.
Some tactics for providing shading to prevent the sun from reaching the building are:
- Covering of rooves and courtyards with deciduous vegetation (allowing winter access) such as creepers or grapevines permits evaporation from the leaf surfaces to reduce the temperature. At night, this temperature is even lower than the sky temperature.
- Green rooves, earthenware pots laid out on the roof, and highly-reflective surfaces (e.g. painted with titanium oxide white paint/whitewash) are all techniques practiced widely.
- Coverings that in the daytime insulate the roof but automatically withdraw at night exposing the roof to the night sky, allowing heat to leave by radiation and convection.
- Horizontal overhangs or vertical fins prevent overheating while preserving natural daylighting.
- For east and west walls and windows in summer: vertical shading and/or deciduous trees and shrubs.
- For south-facing windows: horizontal shading.
- Shutters, closed in the day.
- Highly textured walls leave a portion of their surface in shade.
Right: A summary of different shading types.
The design of natural ventilation systems varies on building type and local climate. The amount of ventilation depends on the careful design of internal zones, and the size and placing of openings.
Wind-induced ventilation is helped by siting the ridge of a building perpendicular to summer wind direction, with minimal obstruction to the wind.
In a multi-storey building, the rooms or zones on the outside faces may be separately controlled, depending upon the degree of sophistication present in the building, its size and location. If hot air is present above a set temperature, it is allowed to escape at whatever rate is necessary to preserve the comfort of this zone's occupants, via controlled venting into a vertical space – atrium or stairwells/lift shafts – positioned centrally or on corners (with glass sides to aid the process).
At the top of this space, louvres, perhaps in clerestories or skylights, allow a controlled amount of hot air to escape, again, at whatever rate is necessary for comfort of the whole building's occupants. Basement windows allow cool air in.
- Offset inlet and outlet windows across the room or building from each other.
- Make window openings operable by the occupants but controlled by the building management system.
- Provide ridge vents at the highest point in the roof that offers a good outlet for both buoyancy and wind-induced ventilation.
- Allow for adequate internal airflow.
- In buildings with attics, ventilate the attic space to reduce heat transfer to conditioned rooms below
It's not always possible for a building to be completely natural ventilated. In such cases fan assistance is required. Thermostats, dampers and fans would be connected to a building energy management system.
Ventilation chimneys include caps to prevent backdrafts caused by wind. These adjust according to the wind intensity and direction and increase the Venturi effect.
Turbines may be deployed to increase ventilation. Self-regulating turbine models are available. There are several styles of passive roof vents: e.g., open stack, turbine, gable, and ridge vents, which utilise wind blowing over the roof to create a Venturi effect that intensifies natural ventilation.
This uses wind speed differences to move air, based on the idea that the faster air moves, the lower its pressure. Outdoor air farther from the ground is less obstructed, with a higher speed, and thus lower pressure. This can help suck fresh air through the building.
Bernoulli's principle multiplies the effectiveness of wind ventilation and is an improvement upon simple stack ventilation. However, it needs wind, whereas stack ventilation does not. In many cases, designing for one effectively designs for both.
The BedZED development in south London (above) utilises specially-designed wind cowls which have both intakes and (larger) outlets; fast rooftop winds get scooped into the buildings. The larger outlets create lower pressures to naturally suck air out.
Solar chimneys for cooling
Solar chimneys are employed where the wind cannot be relied upon to power a wind tower. The chimney's outer surface (painted black and glazed) acts as a solar collector, to heat the air within it. (It must therefore be isolated by a layer of insulation from occupied spaces.)
Pre-cooling air with ground source intakes
Right: Earth-air tunnel.
Also known as an earth-air tunnel this is a traditional feature of Islamic and Persian architecture.
It utilises pipes buried a few meters down, or underground tunnels, to cool (in summer) and to heat (in winter) the air passing through them.
They can lower or raise the outside replacement air temperature for rooms which are buffer zones between the interior and exterior temperatures.
Trombe wall effect for cooling and heating
A double skin façade is employed, the outer skin of which can be glass or PV panels. The cavity between the array and the wall possesses openings to indoors and outdoors at both high and low levels. operate more efficiently anyway).
Evaporative cooling is used in times of low or medium humidity. As water is evaporated (undergoing a phase change to water vapour), heat is absorbed from the air, reducing its temperature. When it condenses (another phase change), energy is released, warming the air. This is the same for all phase change materials.
Right: passive solar cooling with a courtyard.
Evaporative cooling can be direct or indirect; passive or hybrid.
- Direct: the humidity of the cooled air increases because air is in contact with the evaporated water – can be applied only in places where relative humidity is very low.
- Indirect: evaporation occurs inside a heat exchanger and the humidity of the cooled air remains unchanged – used where humidity is already high.
- Passive: where evaporation occurs naturally; incoming air is allowed to pass over surfaces of still or flowing water, such as basins or fountains;
- Hybrid: where mechanical means are deployed to control evaporation.
Phase change materials (PCMs)
PCMs utilise the air temperature difference between night and day. In the daytime, incoming external air is cooled by the PCM-storage module, which absorbs and stores its heat by changing its phase state (e.g. solid to liquid).
At night-time the substance reverts to solid form, releasing its heat by being cooled by the now cooler external air. Commercially available PCMs are chosen based on the temperature of their phase change relative to that required in the space to be moderated.
Night ventilation, or night flushing relies upon keeping windows and other openings closed during the day but open at night to flush warm air out of the building and cool thermal mass which has heated up during the day.
It relies upon significant temperature differences between day and night time (which must be below 22°C / 71°F) and some wind movement.
The above combines edited extracts from one of my published books, Solar Technology, and a forthcoming title: Passive Solar Architecture Reference Pocketbook.
David Thorpe is the author of
- Solar Technology: The Earthscan Expert Guide to Using Solar Energy for Heating, Cooling and Electricity
- Energy Management in Buildings: The Earthscan Expert Guide
- The 'One Planet' Life: A Blueprint for Low Impact Development
- Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency, and
- Energy Management in Industry: The Earthscan Expert Guide