Natural Ventilation and Mixed Mode
Dynamic Building Simulation With Fully Integrated Stack and Wind Pressure Driven
Air-Flow Analysis
This powerful combination, called Tas-Flows, provides a comprehensive design tool for
the development of successful passive design strategies. The effective use of thermal
mass, solar protection and natural, or mixed mode, ventilation regimes can all be
studied in a fully integrated simulation.
Opening of windows etc, and the use of solar controls, may be scheduled against a range
of performance parameters for the occupied spaces and external climate.
During the development cycle Tas-Flows has been used to evaluate the passive design
strategies for a number of projects. Notable amongst these is the new Powergen HQ, for
which monitored data is available on the behaviour of the occupied building.
Over the exceptionally hot summer of 1995 the building performed well, exhibiting many
of the behavioural characteristics predicted by the Tas-Flows simulations undertaken by
the design team.
The New Powergen HQ - Simulated and Monitored Performance
Simulation of air movement through the building required information on size and
location of both exterior and interior openings. It was also important to know the
relative position of internal spaces. This array of relational geometric data was
automatically derived from the 3D-Tas model of the building.
The PowerGen office areas are on three floors and open plan onto a central atrium.
In order to determine the performance at different locations, the open plan floors
were zoned into 4m wide strips across the width of the building. This was done for
the north and south sides and for each floor. The atrium was separately zoned at each
floor. Zoning of the open areas in this way allows free air exchange between adjacent
zones whilst generating data on comfort conditions for each.
In addition to the location of openings, data was input on their openable proportion,
with a schedule of when they would be allowed to be opened. The proportion of window
opening was controlled by changes in simulated zone temperature and external wind speed.
In the actual building motorised top windows are controlled, in groups, by a bems,
driven by air temperature in particular zones.
Schedules were also included in the model for operation of internal blinds for solar
control.
Prediction vs. Reality
When comparing the simulation predictions with the actual performance of PowerGen HQ
(see opposite page) it is clear that the characteristics of the building and its
ventilation control strategy are well represented in the computer model. The monitored
data, for the summer of 1995, does not include site solar data, so the simulation is
based on a similar hot period of detailed recorded weather data for 1994.
During very hot weather night ventilation is used to cool the building and during the
day, when external temperatures exceed those internally, the windows are closed. This
allows the building to remain below outside conditions even for prolonged periods of
extreme weather.This strategy is helped by the thermal mass built into the structure,
particularly the exposed concrete coffer which has an average thickness of 300mm. The
benefit of using thermal mass to create a long term dynamic in the structure can be seen
in both the simulated and monitored data, as internal temperatures rise only slowly over
the period of very high external temperatures.
Simulated Performance: Powergen HQ Days 179 to 196 BRE climate data 1994
Monitored Performance: Powergen HQ - 14/08/95 to 31/08/95
EDSL are grateful to the following organisations for permission to use the simulated
and monitored data for the Powergen building.
- Client: PowerGen PLC
- Architect (Shell & Core): Bennetts Associates
- Architects (Fit-out) Fletcher Priest
- Services Engineer: E Griffiths & Son
- Project Manager: Buro Four Project Services
- Structural Engineers: Curtins Consulting Engineers
- Controls: Kinross Control Systems Ltd