Given comprehensive hourly weather data, Szokolay showed in The Thermal Design of Buildings (RAIA Press, 1987) that it is possible to apply objective tests to determine the potential benefit of passive and low energy building design strategies on occupant comfort. This technique clearly shows the most effective strategy or strategies suitable for application to any given climate and should basically be the starting of every design.
The methodology for determining the effect of various passive design strategies involves plotting hourly conditions on the Psychrometric Chart, a graph used to represent the state of air under any set of conditions. Overlaid on the graph is a comfort band derived from the monthly thermal neutrality temperature for that climate (shown in yellow below). The percentage of plotted points inside this band is the percentage of time the climate provides conditions that would be considered comfortable.
Select Technique: :: All :: Passive Solar Heating :: Thermal Mass :: Night Purge Ventilation :: Natural Ventilation :: Direct Evaporative :: Indirect Evaporative
It is also possible to overlay on this graph the potential effects of passive design strategies applied to a building (shown above in red). These effects are based on a fundamental set of assumptions laid out by Olgyay and Szokolay. These basically extend the area of the comfort zone based on their ability to moderate the effects of climate. For example, the effect of thermal mass is to provide radiant temperatures that counteract to some extent the effects of both low and high air temperatures. This extends the comfort zone both up and down the dry bulb temperature (DBT) scale depending on the monthly average temperature and the extent of diurnal variation.
It is therefore possible to determine the effectiveness of particular passive design strategies by simply counting up the number of points inside both the base comfort area and the extended area and dividing them by the total number of points. If this is done for each month of the year and expressed as a percentage, a set of bar graphs can be generated that clearly show the relative effectiveness of each technique. The yellow bar represents the base percentage in the yellow area alone. Thus, an effective strategy would significantly increase the percentage over and above the base.
Once the individual effect of each technique has been determined, it is possible to look at the effect of combinations of techniques. For example, the following graph shows the monthly effect of the natural ventilation, thermal mass and passive solar heating, all working together to achieve comfort more than 90% of the year. Such graphs can be generated based on different occupancy hours and basically calculate the percentage of time that environmental conditions are within the comfort band for each of the 12 months of the year, and for the year overall (the far right column).
All of these calculations make some assumptions as to the moderating effects of passive systems within a building. By default, it is assumed that the techniques used are of average efficiency with adequate thermal insulation used when required. The passive solar heating values refer to the use of an indirect solar gain system with a glazed area of 20% of the equator-facing facade.
The Weather Tool program displays this data in the Psychrometrics panel, from which you can also interactively adjust parameters such as the glazing ratio, insulation and design efficiency of the different passive design systems. The passive design graphs are available as the first item in the Settings menu.
Related Topics
- Climate Analysis - Example
- An example of the use of such an analysis to determine the viability of a night-purge ventilation system in Perth, Western Australia.
The Climates of Western Australia: A comparison of a range of different climates and the most appropriate passive strategies.
- Passive Systems
- An overview of passive systems.
Related Links
- Bio-climatic Design Central
- http://www.suryakund.com/BCDC/