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It pays to act naturally on the ventilation front

Natural ventilation systems can reduce energy wastage in commercial applications, but only if they are specified correctly and installed properly, says Laura Callaghan
The fundamental difference between natural and mechanical ventilation is that natural ventilation uses encapsulated wind and solar power to fully ventilate the building. Natural ventilation is driven by wind and stack effects based on outdoor wind speed and indoor and outdoor temperature and pressure differences. In order to maintain a comfortable environment, with the correct air temperature and velocity, a combination of window vents, extract grilles and ventilation turrets or stacks are required.

Both temperature and CO2 sensors are also necessary to control ventilation rates. Natural ventilation is a popular method of ventilating offices, restaurants and educational facilities, and is particularly effective in open spaces that have a high occupancy, such as warehouses, distribution centres, gymnasiums, sports halls, assembly areas and supermarkets. Natural cooling eliminates the need for mechanical air conditioning, which leads to low energy consumption, low operating costs and low maintenance costs.

Benefits of natural ventilation over mechanical ventilation
Cost-effective: Natural cooling eliminates the need for mechanical air conditioning, which leads to low energy consumption, low operating costs and low maintenance costs. Night purging reduces day-time cooling needs by refreshing the room with cooler night-time air, lowering the temperature of the building structure, to provide a comfortable environment for occupants the following morning.

Sustainable: Harnessing natural wind power and temperature buoyancy to ventilate a building means that no fossil fuels are needed to run mechanical fans, which contributes significantly to energy conservation.

Healthy: A constant supply of cool fresh air eliminates 'sick building syndrome', commonly in mechanically ventilated buildings, re-circulated air can cause occupants to become ill. A continuous supply of fresh air replacing stale air increases concentration levels. By replacing open windows, security risks are also reduced.

Carbon dioxide is often measured to ascertain indoor air quality. It indicates the number of occupants and if levels of CO2 are high, adequate ventilation is not being provided. Exposure to high levels of CO2 is not dangerous, but may affect performance - some studies have shown it can lead to a lack of concentration and symptoms of sick building syndrome. A build-up of CO2 may indicate a build-up of other contaminants. CIBSE recommends a CO2 concentration of no more than 900 parts per million (ppm) in occupied spaces to control odours and create a comfortable environment. By incorporating CO2 monitoring within a ventilation system, ventilation rates in each area can be designed to be dependent on occupancy.

Ventilation demand will change depending on the season and occupancy levels. In a school, for example, occupancy in each room varies throughout the day, so a controlled ventilation rate using CO2 sensors would be both cost effective and energy efficient.

Preventing energy wastage
Incorporating CO2 sensors in a natural ventilation system will prevent wastage of energy in buildings where occupancy varies during the day. By incorporating CO2 monitoring throughout a school the ventilation rate can be designed in each area to be dependent on the occupancy in the room.

In summer, ventilation rates will be controlled by temperature sensors, but lower winter ventilation rates will be controlled by CO2 sensors. This limits the fresh air entering the building in order to maintain the internal temperature.

There are numerous options to allow for the most effective airflow throughout the building. Natural ventilation systems are not suitable in areas with a high level of pollutants, fumes and odours - eg kitchens, laboratories, toilets and areas with high functional heat gains. Operating theatres also need to be mechanically ventilated so that outside air cannot bring potential pollutants into this type of environment.

Mixed mode systems can be used to minimise the cost of a mechanical arrangement by using natural ventilation where possible, solar-powered systems can reduce energy demand where power-assisted fans are required

Solar powered natural ventilation
Harnessing natural wind power and temperature buoyancy to ventilate buildings significantly reduces energy consumption that would have been used to operate a more traditional mechanical arrangement. In order to maintain a comfortable environment, with the correct air temperature and velocity, temperature and CO2 sensors maintain a controlled ventilation rate again significantly contributing to energy conservation.

Solar power is commonly used to assist in the effective operation of a natural ventilation system, particularly during the summer at times of high occupancy when an increased ventilation rate is required. A solar photovoltaic cell is often installed on the roof of the Penthouse Turret itself, powering the fans situated within the Turret allowing 24-hour supply or extract ventilation.

During the day in the summer months in particular there is a build up of solar gain and human heat gain within all building types, natural ventilation Penthouse Turrets allow a fresh supply of air throughout the night to lower the internal temperature of the building structure for the next morning providing a fresh feel for occupants with 100 per cent security as there are no open windows. This process will incur virtually no energy costs. Night-time purge can be further enhanced via the battery assisted fans within the solar-powered Penthouse Turret.

// The author is marketing manager of GDL Air Systems //
10 September 2012

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