All rise

The application of ventilated facades to tall buildings requires both broad and analytical approaches to assess the benefits to the constructor, owner and operator.

The Green facade can be multifunctional – active or inactive, and has numerous benefits. This includes offering solar gain reduction, fresh air ventilation, providing an acoustic barrier, allowing access for maintenance, as well as the ability to use clear glass without excessive solar gains. It can also contribute to occupant comfort levels, overall thermal performance, and energy to the building.

Design of the ventilated facade and the outcomes and benefits are characterised by the physical location of the building and the climate.

The Australian Building Codes Board is set to introduce mandatory minimum performance standards in terms of energy consumption and hence new approaches are being sought on the design of facades. This reflects the situation in the UK, where legislation is driving clients to think more about energy use in their buildings.

There are a number of sophisticated computer simulation and modelling software packages available that allow engineers and architects to accurately predict performance. Perhaps more importantly, they can also show the economic benefits to the building operator from the day to day operation of the facade.

For example, Brisbane magistrates court building in Australia has been designed to provide multiple benefits to both the users and operator. It is an 11-storey building purposely constructed for the Department of Justice in Queensland.

Brisbane magistrates court
The brief for the project was issued as a design competition and involved meeting certain energy performance criteria. The winning design came from ABM/Cox Rayner Architects and utilises a north facing thermal facade.

The orientation of the building towards parklands provides the Justice Department with a unique opportunity to enhance the quality of the common areas of the courts to allow uninterrupted vistas of the gardens. It offers a naturally calming effect on the users and takes away some of the often unfriendly nature of typical courthouse interiors.

The wall has been designed primarily as a ventilated window with extensive internal and external shading and provides a substantial thermal buffer in terms of transmission gains or losses. The primary aim is to reduce energy costs while providing a substantial amenity in terms of the internal appeal of the space.

Orientation of shading
The shading has been evaluated in terms of the exposure to the sun during periods of peak solar load. During summer months there is virtually no solar impingement on the fenestration. See figure 1 for an illustration of the solar array during peak load periods.

Heat recovery is possible through the north facing wall. Low winter, spring and autumn sun angles introduce sufficient solar load gain within the window cavity to provide continuous heat recovery.

This effectively is free heating and provides sufficient energy to service the needs of the common areas of the entire building. Temperature rises of up to 30°C can be achieved with up to 800 W/m² direct solar radiation¹.

The control strategy for use of this free heating involves use of the existing 'all air' air conditioning system to recycle the warm air to the space.

When space temperatures reach comfort levels and equilibrium is maintained, the excess warm air is expelled via the building's relief air system.

The need for an all air system to provide this function is established by the extreme depth of the building and the fact that an internal atrium is located in the space directly between the facade and the courtroom floors.

Thermal performance
The thermal performance of any double skin facade is dependent on a number of factors, including:

• width of skin;
  
• shading – internal and external;
  
• mechanics of air flow;
  
• overall heat transfer coefficient;
  
• absorptive and reflective qualities of the glazing and structural members;
  
• orientation;
  
• direction of prevailing winds if the facade is open to the elements;
  
• mode of operation.

There is an overall energy balance for the facade which varies considerably in the building as the difference between solar gain and outdoor air temperature for both winter and summer conditions is substantial.

The respective components of the energy balance for the magistrates court building can be seen in figure 4 on page 51, and are as follows:

• Rp – primary solar load on interstitial space
  
• Rra – primary reflected and thermal load from external and internal facade element
  
• Rf – secondary solar load on occupied space
  
• Ta – temperature of introduced air
  
• P – primary thermal load to inside and outside via direct transmission
  
• Rr1 – primary reflect solar load
  
• Rs – secondary direct load resulting from radiation
  
• Rint – radiated internal load from floor
  
• S – secondary direct load resulting from direct transmission.

The facade is shaded to prevent any direct solar loading during the summer months, as well as minimising the effect in autumn and spring. During winter the facade is in direct sunlight from 0800h to 1600h providing a renewable resource in heating mode. The direct comparison between the single glazed high performance facade and the double skin is shown in table 1.

This example of cooling offers a significant reduction in peak loads on the northern facade in the absence of any direct solar radiation during the summer months. Peak cooling loads occur in January.

Energy reduction for lighting and air con
At the magistrates courts, there are substantial benefits in terms of meeting the client's brief for a 'four-star' energy rating for the building. Some Australian states have promulgated a ratings scheme which provides a rudimentary assessment of the buildings performance when compared to benchmark assessment criteria.

These basic reductions coupled with the extensive lateral shading from the staircase structure, overhead shading and free heating capabilities provide significant improvements in the performance of the building.

Cfd analysis of thermal wall
A computational fluid dynamics (cfd) analysis has been conducted as part of the schematic design process to prove the tangible benefits in terms of the reduced cooling and heating loads on the building. The model was based on the following boundary conditions and the results are as follows:

• Exhaust of 190 l/s total from 400 mm x 250 mm openings at each end of high-level exhaust plenum.
  
• Make-up air via continuous 100 mm high slots 100 mm above the floor of inner pane (summer 23°C, winter 20°C).
  
• Adjacent air temperatures were 32°C outside and 23°C inside in summer, 9°C outside and 20°C inside in winter.
  
• Internal loads due to absorbed solar heat from glass floor and ceiling.

The results are summarised in tables 2 and 3, opposite, and also as a cross-sectional diagram showing temperature changes and air flow in figure 3.

Circulation is down the cool inner pane and up the warmer outer pane in summer. Velocities are low. In winter the direction is reversed and the airflow is much stronger due to the higher heat gains.

Solar chimney
A naturally ventilated facade that is openable to allow the introduction of outdoor air at the base and relief air at the top will, through stack effect, act as a solar chimney.

This arrangement relies on cool air being drawn in at the base of the facade to replace the rising warm air and thus the cavity is ventilated to reduce the solar gains. Wind effects can supplement the rate of flow through adopted capture or induced airflow devices at the top or bottom of the facade.

Research² has indicated that a wall cavity in excess of 300 mm will provide effective ventilation with an opening equivalent at least to half the cavity width.

The solar chimney principle has been adopted to an integrated solution whereby the sensible heat gains are actually returned to the occupied space for use in building warm-up and heating functions.

The solar chimney solution could also be adopted through the use of heat recovery equipment at the exit point to the facade.

The heat recovery could be stored as warm water in the insulated tanks and utilised as pre-heated domestic water for the building.

In the latter two examples the rate of airflow would be controlled via thermally-operated dampers or solar/wind powered fans to regulate the airflow and provide the optimum airflow rate.

Photovoltaics and economics
The inclusion of photovoltaics (pv) within a ventilated facade is an option – albeit an expensive one. Locating photovoltaics in the non-vision panels, such as spandrels, could provide sufficient electricity recovery to allow storage and thus provision of power to operate the active elements of an interactive facade.

As new technology pv cells become more affordable and available their support technology will improve to the point where their use will be more commonplace. The titanium technology developed in Australia has shown substantial promise in this area.

Double leaf facades cost anywhere from US$1000-2000/m², more than a comparable single leaf facade. When compared to the energy savings in this example there appears to be little benefit in short-term economics and payback for the building owner. Further, annual standing costs in terms of maintenance and cleaning will add to the case against the use of these facades.

Other non-economic benefits include occupant comfort. At the Brisbane magistrates court one of the benefits to the occupants is a view of the recently completed Roma Street Parklands from almost any location in the public areas of the floors (see impression, left). The courthouse complex requires areas that relax, and the view supplies the perfect tonic.

This feature is based on a paper delivered at a conference held by the Council on Tall Buildings & Urban Habitat in November 2001.