Natural Ventilation and Passive Design
When you design the building envelope and structure to control, or partly control the
internal environment, the quality of analysis needs to be more accurate and cover a wider
range of issues than simplified cooling load calcs.
A major step forward
A building with a reasonable amount of thermal mass can absorb and store energy for a number
of hot days while slowly rising in temperature. When the weather cools the stored heat will be
lost. These longer-term dynamics are the key to successful passive design. A building with good
solar protection and some thermal mass, with maybe night time ventilation with cool air, can
ride through a three to four-day heat wave keeping fairly stable internal conditions. This is
the major advantage simulation has over single design point calculation methods, it knows about
the transient performance over time! So, simulation moves on from single peak load analysis to
give an operational view of building performance over days, weeks and seasons. Tas uses the
response factor method for the transient analysis of the building structure, which is more
accurate than finite difference methods and can be up to ten times quicker in computational
speed.
What else is important?
The impact of solar energy on and in a building is critical. The 3D model is used for detailed
external shading calculations used in the simulation. Tas is unique in being able to shade
diffuse sunshine. This is very important for solar shading over windows and, if not considered,
can add ~2oC to predicted internal temperatures. Another unique function from Tas is internal
solar shading and tracking sunshine across spaces. It’s important to know how much solar
energy entering an atrium ends up in adjacent offices. Realistic analysis of glazed double
façade systems, with the possibility of natural or mechanical ventilation, requires good
modeling of the transition of solar energy across the various glazing elements.
Building simulation with integrated bulk air flow
By generating a natural ventilation air flow network between the apertures in the building
model, Tas is uniquely able to perform fast and stable simultaneous building simulation with
integrated bulk air flow for commercial-size buildings. The weather data used for simulation
has hourly values of wind speed and direction. This allows the calculation of variation in
wind pressure for all external apertures, which means that wind-driven ventilation rates and
direction are calculated for each hour. By simultaneously calculating internal air temperature,
the effects of buoyancy forces are also integrated into the analysis. It’s even possible to
have interaction between mechanical ventilation and natural ventilation forces. With all these
ingredients carefully modeled, Tas can produce hourly dynamic simulations that accurately
reflect reality, for any period of days up to a complete year. The resulting data can be
automatically processed to give frequency distributions of internal temperatures.