Natural Ventilation

Natural ventilation is a passive cooling strategy that consists of using natural forces, such as wind and buoyancy to drive cool outdoor air through a space. If well implemented, it can considerably contribute to reducing the cooling energy consumption of a building. Moreover, natural ventilation is not only beneficial to reducing energy consumption in a building; its high flowrates also lead to higher levels of indoor air quality than mechanically cooled buildings. It has been found that lower contaminant levels in a space can lead to increased occupant productivity. Given that nowadays humans spend about 90% of their time indoors, increasing productivity even by a small percent can result on increased revenues for companies, as well as potential reductions on health care costs.

Natural ventilation is by no means a new technology: before the use of electricity were widespread, it was one of the only methods available to keep spaces at comfortable temperatures, even in extremely hot climates. It is still widely used in residential settings, particularly in the developing world. For example, wind catchers are still commonly used in the Middle East to take advantage of the wind blowing in any direction to ventilate multi-story houses.

The two main goals of natural ventilation are to improve the indoor air quality (IAQ) and to reduce the cooling/ventilation energy consumption of a building. Both of these goals must be fulfilled by guaranteeing that the indoor thermal comfort conditions are acceptable. If this condition is not met, the natural ventilation system will most likely be replaced by an air conditioning system by the building occupants or owner.

Challenges of designing for natural ventilation

Despite all of the advantages of using natural ventilation, the strategy is rarely used to cool office or commercial buildings in the developed world. This is, in part, because the performance of naturally ventilated systems is highly dependent on the building geometry and the weather conditions, forcing the designer to account for several additional factors very early in the building design to guarantee occupant comfort. And while modeling the effects of natural ventilation in a single room is fairly simple, understanding –let alone modeling– the coupled effect of its airflow dynamics and heat transfer processes inside a larger building can be very complex. This has led to a lack of simple yet robust tools to guide the architect through the implementation of a natural ventilation system in the early stages of building design, forcing the architect to chose between the risk of designing a natural ventilation which may not work (and may even lead to higher energy consumption) and the security of a mechanical ventilation and cooling system which he/she knows will keep the occupants comfortable, rain or shine.

Natural ventilation to control IAQ

The use of natural ventilation to control indoor air quality is of particular interest during the winter time. (The minimum flowrate required to ensure an appropriate IAQ is considerably lower than that  needed to ensure occupant thermal comfort in the summer time.) At this time of year, windows are opened such that the minimum required airflow rate is met, and not more. The physical framework to model natural ventilation to control IAQ in the wintertime is slightly different than that to provide cooling in the summertime for two reasons: the indoor temperature –if controlled by a heater with a set point thermostat– does not depend on the airflow through the space, and physics of the airflow through cracks (or very small openings) varies slightly from that of flow through large openings (windows).

Natural ventilation for summertime cooling

Using natural ventilation to prevent overheating within a building presents a very different challenge to maintaining acceptable IAQ standards. For summertime cooling, important considerations are internal heat loads and external solar gains, as well as building characteristics, such as thermal mass and insulation level, and the overall building floor and site layout. The higher the airflow availability, the greater the cooling effect.

In regions where the ambient temperature is too high during the day but low enough at night, the use of thermal mass and a night cooling/flushing strategy can help achieve acceptable indoor thermal comfort conditions with the use of  natural ventilation.

Natural ventilation and thermal comfort

It has been found, that the comfort levels occupants of naturally ventilated buildings do vary with outdoor temperature. This is because people naturally adapt their clothing levels from season to season, and will increasingly wear warmer clothes when the air is colder, and wear lighter garments when temperatures are higher. They will even adapt to hourly changes in weather conditions: they will open and close the windows depending on the amount of draft desired.

This human adaptation to outdoor conditions widens the traditional thermal comfort ranges, with occupants feeling comfortable at temperatures lower than the minimum and higher than the maximum acceptable conditions in a mechanically ventilated space. This provides an even greater advantage to using natural ventilation, and can lead to larger energy savings if the adaptive comfort range is considered in the building controls, rather than the traditional range.

The adaptive thermal model has one caveat: it only applies in spaces where the windows are operable. This means that if a building is being naturally ventilated by bringing outdoor air indirectly (i.e., not directly from a window), then the standard thermal comfort model for mechanically ventilated spaces must be used.

Tools to model natural ventilation

Despite of all of its advantages, NV is rarely considered as a cooling strategy in new buildings. This is due to a lack of tools that can help the architect implementing this strategy properly during the early stages of building design, when aspects like building dimensions and orientation –critical to the performance of NV– are still flexible.

The ideal tool to modeling the effects of a building’s geometry and orientation of natural ventilation throughout the year during early design stages would need to have the capacity to:

• Couple the dynamics of airflow and heat transfer to estimate the dynamics in the entire building.
• Rapidly perform transient simulations using weather data.
• Model the transient effects of thermal mass, to predict the potential of heavily massed buildings as well as night cooling strategies.
• Require only the input parameters that are relevant to the physics of natural ventilation.
• Allow the user to easily modify inputs, to rapidly compare different scenarios.
• Perform fast simulations.
• Be accessible and informative to architects and designers.
• Provide accurate predictions on terms of thermal comfort conditions, which would allow to estimate the natural ventilation potential of a building.

Currently, NV can be modeled through two types of simulation tools: multi-zone models and computational fluid dynamics (CFD). Multi-zone models provide a highly simplified view of the building’s airflow dynamics, relying mostly on a network of zones linked by airflow resistances, and provide very little –often times too little– detail of the airflow and thermal dynamics in each zone. On the other hand, CFD is a tool that can provide highly detailed calculations of the specific airflow and heat transfer dynamics in each region of a building, often providing much more information than is needed to make an informed design decision.