Making the building do the work: natural ventilation design
8 February 2012
By John Brodie
8 February 2012 –In the world of climate change and inefficient buildings everyone is clamoring for more efficient airconditioning systems and improved lighting to reduce energy loads and carbon.
Airconditioning uses a large proportion of energy in buildings.The rule of thumb in trying to drop air temperature down from something quite hot – say 38 degrees Celsius external temperature to a default comfort level of around 22 degrees – is that airconditioning uses around 40 per cent of the energy in a building.
If we stand back and take an objective look at building design and what inhabitant thermal comfort is really about we may be able to significantly reduce that airconditioning energy use by adjusting our notions of comfort.
We would need to consider natural ventilation as either the main option for ventilative cooling and providing fresh air or at least have it working in conjunction with airconditioning.
In an airconditioned building with comfort levels controlled through the commonly used temperature set points, there will always be 10-15 per cent of occupants dissatisfied.
The empirical research undertaken worldwide across a range of climates and buildings indicates a naturally ventilated facility will have around the same levels of user dissatisfaction.
We also need to remember that occupant comfort isn’t just based around temperature related conditions it is also based around air quality. Air quality in naturally ventilated buildings is higher than in airconditioned buildings.
If we utilise the hybrid technology of natural ventilation and airconditioning then we can significantly reduce the amount of energy used and carbon created to make a building comfortable.
Combine that initiative with the proven concept of adaptive comfort and how valuable that is in improving user comfort and reducing energy use in naturally ventilated buildings then natural ventilation should be considered as a major option across most of the Australian climate range.
The National Institute of Science and Technology, USA has some preliminary analysis software that indicates that with an adaptive comfort range of 20-27 natural ventilation cooling for around 78 per cent of the year in Sydney.
In Melbourne that figure becomes 93 per cent of the year because of climate conditions which are even more conducive to natural ventilation. This is using an internal heat load of 100 watts a square metre which is very high and would be considered the worst case scenario for internal heat loads in a commercial building. That is a lot of energy saving and reduced carbon. That is not to mention the major improvements in indoor climate that are often credited with increased user satisfaction, higher retail sales, productivity and improved learning outcomes.
Natural or passive ventilation has not been readily embraced in Australia for the past 50 years but in Europe and the hot dry humid climates of the Middle East and Asia it is traditionally very popular.
What these countries have learned is that natural ventilation must have the appropriate control strategies to work effectively in modern building types. Simply opening or closing a window manually has been shown to not be effective.
Users will open or close too early or too late. With a smart control system designed specifically for the nuances and broad range of criteria needed to control natural ventilation that still offer user control and override, natural ventilation will maximise comfort and energy efficiency.
There are many ways to assist the efficacy of natural ventilation through the building based on the use of pressure differences across the façade and inside the building.
If we examine the façade and footprint of the building and the surrounding buildings combined with the local climate data we are able to use that information to create areas of positive and negative pressure which will then push and pull air flow through the building.
Together with designs that utilise optimum positioning and shape of windows this can significantly increase the air flow and daylight penetration into a room.
These initiatives are generally either cost free or low cost and will have very little impact on the look of the building. Understanding air flow and the shapes as well as location of components that comprise the building façade are a key to assisting the designer make use of the “potential energy” just sitting there waiting to be captured.
Window shading is a prime example. Window shading is designed to keep the sun out of a facility in summer and if properly designed will encourage penetration of sunlight , in winter, if required.
But what about using the shades to encourage air flow into the building as required during the summer and, if required, deflect it during winter? This doesn’t mean the shades have to be movable. Working with pressure differences and air flow, again, can achieve this smart and simple design.
Sensible and small design changes to these façade items can have a major impact on the flow of air and day lighting into the buildings and the exhaust of air out of the building, hence improving inhabitant comfort.
If we couple these built form initiatives together and ensure we have the best typology for fabric including glazing then we can have a measurable improvement in air flow and daylight penetration over a typical building that hasn’t taken these ideas into account.
If we also look at external plantings and design of landscape areas to accelerate or decrease air flows and also temper the heat loads in those areas with suitable shading regimes we are adding again to the improvement of the external environment from a comfort point of view and also assisting the internal environment as well.
Natural ventilation offers up some interesting statistics. In schools, for example, natural ventilation and increasing outdoor air flow rates has been shown by the Environment Protection Authority the US to improve learning outcomes by 15 per cent over a typical school building, where natural ventilation has not been a key design driver . http://www.epa.gov/iaq/schools/pdfs/student_performance_findings.pdf
Concentration and recall is much higher in such environments. Energy usage is measurably reduced and maintenance costs are much lower. If we coordinate the smart natural ventilation control system and allow it to replace the building management system (at a much lesser cost) then we can use the natural ventilation system to run the lights, airconditioning, the shades and the blinds so all is coordinated and integrated.
In the current building set up if a window has to open automatically and a blind is in the way, a lot of design and coordination work is required to get the blind to talk to the window and the airconditioning to talk to the automated ventilation system and so on.
But if a specific natural ventilation control system is used then all those coordination headaches are removed.
Natural ventilation control is the hardest component for a building management system to control because BMS aren’t designed to control natural ventilation and the logarithms and expertise used in a true natural ventilation control system is not part of a typical BMS brief.
As we search for ever more efficient building types it is imperative that if we are serious about reducing the carbon emission from buildings (which is currently a key design driver), then we cannot afford to neglect natural ventilation as a serious option.
Research undertaken across thousands and thousands of users in an assortment of building types in different climatic zones around the world shows those users of naturally ventilated and hybrid buildings are more comfortable than or at least as comfortable as, users of an airconditioned buildings. (De Dear, Nicols, Roaf et al)
Imagine how much heat we will save from expelling into the atmosphere and how much energy we will save if we adopt even a hybrid system to cool our buildings.
Once internal temperatures get above, for example, 28 degrees and the outside temperature is even hotter users have the option to turn on their airconditioning. Sydney, however, has on average of only 15 days a year that over 30 degrees.
The cost of implementing natural ventilation into a facility is extremely low on a square metres basis and the options of automating windows or using other proprietary systems – for instance WINDATCHERSTM – can have pay back periods of between three to five years in a hybrid solution.
Energy savings for a mixed mode (hybrid) system are on average 59 per cent (Carnegie Mellon; 2004). An Australian study carried out in 2002 at Sydney University indicated the energy savings can be up to 79 per cent (D Rowe, 2002).
Through smart building design utilising simple initiatives that are low or zero cost, combined with knowledge and understanding of air flows linked to an adaptive comfort criteria, we can improve the inhabitants comfort while reducing energy and carbon emissions significantly. Surely a win–win situation.
John Brodie is the principal and founder of consultancy Vim Sustainability Pty Ltd and has an extensive background as construction project manager, design manager and builder and specialises in passive and low tech sustainable building design strategies.