Green Buildings: Solar Chimneys
As we strive for more energy efficient and cost effective buildings with improved indoor air quality, engineers and architects are looking to natural ventilation as a means to achieve greater performance and comfort. Along with natural...
As we strive for more energy efficient and cost effective buildings with improved indoor air quality, engineers and architects are looking to natural ventilation as a means to achieve greater performance and comfort. Along with natural ventilation, designers of sustainable buildings are also using solar chimneys. The technology has been around for centuries and was developed both by the Persians and the Romans as a method for keeping people in buildings comfortable when the environment outside was warm. Solar chimneys are sometimes referred to as thermal chimneys as they rely on temperature differences to induce air to move upwards through buildings. Wind pressure and direction also play a key role in their operation.
Solar chimneys can be part of natural ventilation or mixed-mode ventilation systems. They are typically configured as an air shaft to remove air from the building and are terminated at least one half-storey above the highest floor. Their size and shape depends on the amount of natural ventilation needed on each floor and the number of floors served.
The “solar” reference comes from the fact that typically these devices are located on the south side of a building to gain the maximum sun exposure during the day. This strategy creates a zone of high heat at the top of the chimney, which in turn induces air flow upwards due to convection air currents and buoyancy effects.
Maintaining air paths
The flow of outside (natural) air into the building can be from operable windows, doors, louvered openings, earth tubes, trickle vents and any other suitable opening. The use of operable windows has been gaining in popularity and is supported by sustainable rating systems such as LEED (which gives a credit under “LEED NC 2009 Credit 6.2-Controllability of Systems: Thermal Comfort”). Post occupancy evaluations have shown that occupants prefer to have operable windows and studies have shown that employee sick days have been reduced in buildings with this feature.
In a building designed for natural ventilation, designers using solar chimneys must take care to ensure that air paths to the chimney are not restricted by walls and doors. When partitioning is required for tenant suites, transfer air openings will be required to allow the air to flow to the solar chimney intakes and must be sized generously so as not to create an excessive pressure drop and inhibit air flow. The transfer air openings should be at high level (minimum 2.2 metres above floor level) to create a cross flow ventilation regime. They can be constructed of transfer air ducts or transoms above doors.
Additional measures should be taken to maximize the use of natural ventilation on days when there is little wind or stack effect. Fans with variable speed controls and sound attenuation should be used to assist with drawing air through the building. This technique is known as fan-assist natural ventilation.
In our Canadian climate most locations will require a building to have a mechanical system to provide fresh air during extreme conditions when the operable windows have to be closed, as noted in ASHRAE Standard 62.1-2010: Ventilation for Acceptable Indoor Air Quality. Since in these circumstances the mechanical systems are being operated during extreme weather conditions, it is important to provide them with heat recovery equipment.
Integrated design and
The design of the natural ventilation system should be part of an integrated design process (IDP) as it involves all design team members. They include the architect (operable windows, air paths, building envelope), structural engineer (slab openings, thermal mass), electrical engineer (electrical motors, dampers), and mechanical engineer (thermal load and fresh air calculations, fan sizing for fan assist mode, building controls).
Additional technical support is recommended from the micro-climate specialist (consultants who study wind patterns and emissions around buildings) and the energy modeller. The micro-climate specialist should consult on the locations for fresh air openings and solar chimneys with respect to prevailing winds and possible re-entrainment of emissions from on-site or neighbouring sources. Such sources might be boilers, fume hoods, exhaust from emergency generators, or loading docks.
The energy modeller should provide feedback to the design team on the size, quantity and location of the solar chimneys by using advanced building software such as IES-Virtual Environment, eQUEST, or Thermal Analysis Software (TAS). Such programs can predict internal temperatures based on the envelope design and the internal heat gains (from people, lights and equipment), which can be very useful information for the design team and owner.
The design of the solar chimney should also consider components such as the velocity of air (typical design air velocity of 2.0 m/s maximum to match ideal wind speed), the selection of dark coloured material on the upper part of the chimney (to heat up the top of the chimney and induce thermal draft), and the discharge arrangement at the louvres. At the Evergreen Brickworks (see p. 35 and above), discharge cones were added to guide the air through the louvres because in the fan-assist mode the discharge air from the fan within the chimney was rebounding off the inside top of the chimney. The air was then bouncing back down the shaft, creating a positive pressure at the top floor corridor. The louvres were also installed with low leakage insulated dampers to keep cold air from entering the building during heating season.
Some solar chimney/natural ventilation schemes make use of motorized clerestory windows in atriums, and the fan-assist mode is accomplished through the smoke exhaust fans. The dual duty use of the smoke exhaust fans is cost-effective and ensures the fans are regularly exercized.
Occupants must play
One of the key items with this type of design feature is the training of the occupants and building operators. The occupants must be made aware that they have a responsibility in opening and closing their windows. Measures can be incorporated into the building automation system (BAS) such as high and low temperature alarms triggered by space thermostats. These can send a signal to the building operator if windows are left open when outdoor conditions are unsuitable. Or after hours the BAS can send a signal to the security desk staff to close windows if needed. Occupants who leave their windows open should be notified that they need to follow building protocols in the future.
The BAS system should also be used to determine when it is appropriate to naturally ventilate the building. Sensors for outdoor air temperature, humidity and carbon dioxide levels, wind speed and a rain sensor should be provided as part of the control system, and the designer should work with the building operator and owner to determine the appropriate low and high levels of operation within all these variables. More energy savings are possible if the natural ventilation operating hours are increased as much as possible and if interior temperatures are allowed to rise beyond normal North American standards.
In any scheme considering the use of solar chimneys and natural ventilation the cost of the solar chimney and associated equipment such as fans, variable speed drives and silencers — not to mention the loss of usable floor space — must be weighed against the goals of the project. Using natural ventilation and expanding the typical range of thermal comfort parameters such as inside temperature and humidity will reduce the cost of the mechanical plant compared to a fully air-conditioned building. In the case of the Evergreen Brickworks building, the mechanical cooling plant size was reduced by over 50% and the natural ventilation system has been used right into the hot summer days of July.
In conclusion, solar chimneys provide a functional and unique design feature to buildings. They allow th
e building to be energy efficient and allow user control. Careful design using the integrated design team approach is necessary and energy modelling should be used to ensure proper air movement through the building.cce
Centre for Green Cities, Evergreen Brickworks, Toronto
At the Evergreen Brickworks, Centre for Green Cities building (a LEED Platinum candidate) in Toronto, the use of natural
ventilation software allowed the design team to determine that the originally proposed number of chimneys had to be
increased from two to three to ensure good air flow through the building.
The building is five storeys high comprising 5,000 m², with a typical floor plate of 50 m x 17 m. It has solar chimneys from Level 2
to Level 5. The chimneys have acoustically-insulated sheet metal elbows at each of the grilles that incorporate fire dampers
and motorized dampers. The elbows serve as sound traps to reduce noise transfer between floors. Fans in the chimneys
have silencers to minimize noise transfer down the shaft. Multi-speed ceiling fans were also used throughout the naturally ventilated spaces to improve the occupants’ comfort conditions.
The Evergreen Brickworks project was designed by Diamond Schmitt Architects, dTAH Architects, Stantec Consulting (mechanical/electrical), Halsall (structural/LEED) and AECOM (civil).
Mike Godawa, P.Eng. is a principal and partner in the consulting firm The Integral Group of Toronto. He was the mechanical engineer-of-record with Stantec for the Evergreen Brickworks project in Toronto.