Natural Ventilation: A Review for Buildings in Rio de Janeiro
As living standards improve and people expect a greater degree of comfort, the demand for air conditioning has grown. This has caused an increase in electricity use – in Rio de Janeiro, air conditioning accounts for 20 % of electricity usage . Passive design measures that reduce the need for air conditioning are likely to:
- Save money (due to reduced running costs)
- Reduce peak electricity load and improve the resilience of electric supply
- Reduce greenhouse gas emissions
Natural ventilation relies on pressure differences to move fresh air through buildings – and is one design element that should certainly be considered for Rio de Janeiro. Passive airflow serves to cool both building and occupants, and can significantly improve the energy efficiency of buildings by reducing reliance on air-conditioning. Savings of up to 30% of total energy may be possible in some circumstances , .
Sea breezes in Rio de Janeiro provide good opportunities for promoting airflow, though these breezes may not penetrate inland.
Is Natural Ventilation Suitable for Rio de Janeiro?
The purpose of natural ventilation is to passively help human beings maintain thermal comfort without any change in air temperature. Natural ventilation is effective because exposure to a light breeze removes heat from the human body by convection and evaporation. Convective heat loss is significant if air temperature is less than skin temperature. This usually means that convective heat loss is useful when air temperatures do not exceed 35 deg C, which is the skin temperature of a clothed individual in a hot environment. So long as relative humidity does not exceed 80%, airflow helps increase evaporative heat loss through insensible sweating, where sweat evaporates as it is produced and is not noticeable [4–6].
In Rio, ambient temperatures are usually below 35 deg C. Even in the hottest months (January & February) daytime relative humidity is frequently below 80%. Under these conditions, natural ventilation can help people tolerate high temperature and humidity without discomfort. As such, incorporating natural ventilation into buildings in Rio is likely to improve energy efficiency by reducing the need for air-conditioning.
Natural ventilation should be strongly considered for Rio de Janeiro – the climatic conditions are such that properly designed and implemented natural ventilation has the capacity to improve thermal comfort, and significantly reduce energy consumption, running costs and greenhouse gas emissions.
Temperature and humidity levels will occasionally exceed the capacity of natural ventilation to provide a comfortable environment, and backup air-conditioning systems should be considered at the design stage. It should also be noted that making alterations to partitions, moving plant & equipment around the building and users who do not understand the system can render natural ventilation less effective.
Types of Natural Ventilation
The specific design of natural ventilation varies according to building type and site conditions. It should be noted that wind rose data for a given location is usually only an approximation – usually taken from the local airport.
Careful design is needed for natural ventilation to be effective. To remove heat from the body, air needs to flow across the skin – so design needs to facilitate breezes at the right level. High windows might remove rising hot air from a room, but they are unlikely to promote the physiological cooling effect . For Rio de Janeiro, the main methods of natural ventilation are:
- Wind – air blows in one opening and out another
- Buoyancy –cool air enters the building at low level, warms, rises and exits at a higher level
Wind causes positive air pressure on the windward side, and negative air pressure on the leeward side of a building. With careful design of building orientation and openings, wind can be used to maximum advantage. Orientation is covered elsewhere in this toolkit.
When the prevailing wind occurs parallel to a wall, natural ventilation can be maximised by careful design of openings. For example, windows can be hinged appropriately so that air is scooped in by the first window and allowed to flow out by the second window.
When the prevailing wind flows perpendicular to a wall, windows on either side of the building should be staggered in order to maximise mixing of air . In general, creating high and low pressure areas around buildings connecting these appropriately through the building will create beneficial air flows.
Louvre style windows allow building occupants to control how much air enters the building, and they can help to direct air downwards to maximise the cooling effect of natural ventilation. Windows or louvres should be arranged so that air is taken in at low level and vented at the highest point of the room. This helps to optimise natural ventilation and provides maximal cooling for occupants .
Where possible, buildings should be designed to facilitate cross ventilation, whereby prevailing winds flow in one side of a building and out the other. In the case of Rio, this will mean windows on the north and south aspects of buildings. Fortunately, placing windows on these facades will also serve to minimise solar gain – though such windows should be appropriately shaded – for more information on this, see the section on reducing solar gain in this toolkit.
Studies suggest that in hot humid environments (such as that found in Rio) effective ventilation is best achieved by large operable windows on either side of a building, with one of them facing the prevailing wind. If necessary, prevailing winds can be “funnelled” into the building by proper positioning of trees, shrubs and hard-landscape elements. Appropriately placed internal partitions can also help channel air through the occupied zone of a building .
Carefully sited wing walls can help to optimise internal airflow rates, as they can increase or decrease the pressure differential between inlets and outlets. Such devices can act as vertical wind catchers – effectively “catching” skewed winds when used appropriately . Wing walls can also act as solar shades.
Buoyancy ventilation works when air enters and is warmed by the building occupants and internal heat sources (computers, appliances etc.). This causes the air to become less dense, whereupon it rises and is vented at a higher level – a phenomenon known as the stack effect. This method of ventilation works due to a temperature differential, with warmer air on the inside of a building. It is unlikely to work successfully in Rio – if the inside air was warm enough to drive the stack effect, it would not be a comfortable living environment. Using solar chimneys to drive the stack effect may be an option.
Buoyancy ventilation can be achieved by using chimneys heated by solar energy to drive the stack effect. Hot air at the top of the building rises and exits through the heated chimney, which creates a pressure differential and causes air to be sucked in from lower levels. The solar chimney design typically consists of a solar collector angled to receive solar irradiation with a chimney to ensure adequate stack height [10–12]. There are variations on the solar chimney theme, but the basic operating principle is the use of solar energy to drive a stack effect . Solar chimneys can be usefully combined with light wells or atria to maximise natural lighting and further improve energy efficiency.
With abundant solar energy in Rio de Janeiro, the use of solar chimneys or solar towers to drive natural ventilation should be strongly considered.
Underground Heat Exchange
The cooling effect of natural ventilation can sometimes be augmented if incoming air is led through underground ducts buried beneath or next to the building. This allows heat in the air to be dissipated before entering, improving the effectiveness of natural ventilation. The use of buried ventilation pipes has been gaining ground in Europe, where demonstration projects and simulation tools have been developed . Under certain circumstances the daily temperature oscillation can be dampened with only 15-20 cm earth around the pipes. However, this technique is most successful for buildings with daytime occupancy. Furthermore, earth cooling tubes are less effective in hot humid environments, and the cooling potential of in Rio de Janeiro is likely to be low .
Mixed Mode Ventilation
Ventilation can be a mixture of active and passive, with ceiling fans being used when natural ventilation is inadequate. Under some circumstances, distributed fan controlled cooling systems may be easier to manage than a pure natural ventilation system. Mixed mode ventilation allows natural ventilation to be used most of the time, or to serve particular zones. For example, transitional zones may not require active cooling measures if properly designed for passive cooling.
Night Time Ventilation
Night time ventilation, or night flushing, can successfully prevent heat build up under some circumstances . Night ventilation can cool the ambient air as well as removing stored heat from walls, floor and ceiling slab. This helps to cool the building fabric and reduces cooling load during the daytime – saving on costs and improving energy efficiency . In some cases, windows can be designed to automatically open at night allowing air to be purged. This approach can be combined with thermal mass ceilings which absorb heat during the day and release it during the night. A good example of this in practice is the CH2 Building in Melbourne .
Night time ventilation is generally seen as less effective in hot humid environments (i.e. Rio), since the air temperatures at night are generally high. However, some studies suggest that this technique does have the potential for cooling when combined with a thermal mass that absorbs heat during daytime occupancy. This would be most appropriate for commercial buildings that are used during the day. It has been suggested that a reduction of indoor temperature of roughly 3-6 deg C below exterior air may be achievable .
Strategies for night-ventilation techniques in combination with thermal mass in hot humid environments should be examined for use in commercial buildings in Rio de Janeiro.
Natural Ventilation: Summary
Natural ventilation should be considered for buildings in Rio de Janeiro. Properly designed and implemented natural ventilation has the potential to significantly reduce energy consumption, running costs and greenhouse gas emissions whilst maintaining thermal comfort. It should be noted that natural ventilation systems can be rendered ineffective by moving equipment around the building or by users who do not understand how the system works. These limitations should be considered at the design stage.
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