The answer is blowing in the wind
Well designed and carefully controlled natural ventilation helps reduce the carbon footprint of buildings and achieve good environmental control. Richard Arnott explains.
Not only does natural ventilation bring in fresh outdoor air to ensure safe healthy and comfortable conditions for building occupants but it also provides cooling without the need for mechanical systems. As an alternative to mechanical air conditioning, natural ventilation can reduce a building’s total energy consumption by 20 to 30%.
A carefully designed natural-ventilation system can reduce building operation costs by lowering the energy consumption for air-conditioning and circulating fans. The design for natural ventilation should maximise both the wind- and stack-driven ventilation design concepts.
The goal is to increase air flow or reduce heat gain so that the natural ventilation can effectively cool the spaces in a building. In certain cases however, mechanical ventilation and cooling systems may be needed to supplement the natural ventilation. Reducing heat gains means that less air flow will be required to remove the heat, so there will be a reduced need for mechanical cooling systems, resulting in reduced energy consumption.
Research in real buildings during operation has highlighted the positive correlation between natural ventilation and reduced energy consumption. Completed in August 2007 and designed by architects 3XNielsen, the 12 500 m2 Oerestad High School is the first new school to be built in Copenhagen in 35 years. The school features an innovative open-plan design intended to provide optimal learning conditions.
The design called for a natural-ventilation strategy with cooling air drawn in through automatically controlled windows in the facades. As the air becomes heated, it is drawn into a central atrium, rises and is expelled through vents in the roof. The building is spread over four floors and is divided into 27 different zones.
To establish the performance of natural ventilation in the school, one zone on the ground floor, two zones on the second floor and one zone on each of the remaining floors were analysed.
In the UK it is recommended that indoor temperature should not exceed 28°C for more than 120 h a year. In Denmark the recommendations are for fewer hours — with a maximum number of 100 h at 26°C and no more than 25 h at 27°C being the standard.
Temperatures were measured in a hot summer week between 20 and 27 August 2007, which is during the Danish term time. In five zones the temperature exceeded 26°C for 2 to 21 h. Only in two zones did the temperature exceed 27°C. Only in two zones did the temperature exceed 27°C, and only for 7 h.
The use of natural ventilation in the Oerestad High School reduced energy consumption considerably. Energy consumption between August 2007 and April 2008 peaked in January when the total consumption was 16.25 kWh/m2, of which 12.25 kWh/m2 was heating and hot water. Total energy consumption for the whole period was 102 kWh/m2, equivalent to 30 kg/m2 of carbon dioxide. According to the Carbon Trust, typical energy consumption in UK schools is 174 kWh/m2.
Lessons learned at Oerestad are being applied in the UK in the £43 million Southampton City College. The thermal mass of exposed concrete ceilings will help control heat fluctuations caused by external weather conditions in summer and winter to reduce heating and cooling loads. These in turn will reduce the energy consumption of the building and aid natural ventilation.
Extensive environmental modelling ensures that a natural-ventilation strategy achieves comfortable teaching conditions within the building and meets the strict design criteria set out by the college.
Exploiting the thermal mass of the building combined with natural ventilation in the perimeter rooms through openable windows, cross ventilation and the stack system reduces energy consumption of the building and CO2 emissions.
WindowMaster, which supplied the natural ventilation system at Oerestad, was also selected for Southampton City College for its ability to provide a complete installation — in particular the central computer control system. In addition to the window actuators and sensors, the system includes a weather station.
The weather station monitors external conditions around the building, including wind speed and direction and changes in wind pressure on the facades. These parameters are fed into the computer control system and analysed along with readings taken from sensors fitted internally. Windows around the building are then precisely controlled and positioned to keep air quality within rooms fresh and the temperature constant.
To provide a comfortable and healthy indoor climate it is necessary to provide precise millimetre-by-millimetre control of window openings and intelligent modes of operation suitable for the specific building. This enables the windows to be opened at the optimum distance to control noise while keeping energy consumption to a minimum.
Reducing energy consumption and carbon emissions associated with buildings can be achieved using natural ventilation to reduce size and/or the need for cooling equipment and energy use. Natural ventilation is a renewable strategy, and educating building owners, operators, and occupants on its functional and energy-saving advantages is important.
Richard Arnott is WindowMaster’s market manager for the UK and Ireland.
