Passive cooling measures can play an important role in improving the energy efficiency of a building by reducing or even eliminating the need for auxiliary cooling. One passive cooling technique commonly used in private, public, and commercial applications is night purging. Night purging is the removal of heat from a building by bringing in cool nighttime air without the use of active HVAC cooling and ventilation.
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New buildings are becoming more and more insulated due to regulatory requirements, lowered U-values, and the continued focus on building heating usage. In a typical new office building with normal heat loads, the need for cooling can be required even at outdoor temperatures below 0 degrees Celsius. This is primarily due to high heat loads generated from people, lighting and computers, etc. Often this leads to buildings which have decreased the need for heating while increasing the need for cooling.
See where night purging is suitable
Night purging can be incorporated in the building’s natural ventilation strategy. Night purging in this case is a passive cooling method accomplished by the natural driving forces of the wind and/or thermally (stack) generated pressures. The heat absorbed by a building's exposed thermal mass during the day is released to the indoor air at night.
The indoor air is then purged by night ventilation. At the same time, external fresh air cools down the thermal mass which then acts as a heat sink during the following day. The night flushing involves automatically operable windows or louvres being opened for a pre-set period of time during the night, allowing a natural air flow through the building.
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Reduced building operating costs. Replacing hot and stale air with fresh nighttime air can help reduce building operating costs. This reduces the need for the HVAC system to be activated as soon as the building is occupied in the morning. The thermal mass of the building will be cooled, providing a fresher and cooler environment for occupants.
Occupant health and comfort. If hot and stale air is not removed, not only will the room feel stuffy, but airborne pollutants, such as carbon dioxide, may reach alarming levels. This can be potentially harmful for the occupants with symptoms such as headaches, dry and itchy eyes or a sore throat. In turn, this can have a negative effect on both productivity and satisfaction levels with the internal environment.
The efficiency of night cooling depends on the thermal properties of the building and on the local climate conditions, i.e. nighttime wind speed and the temperature swing of the ambient air.
It is particularly effective in climates that have cool nighttime temperatures as there will be a greater difference between internal and external temperatures. This is not to say that night purging cannot be effective in warmer climates. Even when internal and external temperatures are very similar, night flushing can still provide a means for airborne pollutants to be exhausted and allow fresh air to enter.
Security is a common concern when night purge ventilation is considered. This concern is alleviated by selecting high-level openings which minimize many risks such as break-ins. Insurance companies often prefer small openings at higher levels when night ventilation is present.
The risks from entrapment can be lowered by using intelligent actuators which are programmed with a pressure safety function. Furthermore, by using an intelligent actuator, in combination with an intelligent control system, the window openings can be controlled with precision. Normally, with a night purge ventilation strategy, the windows do not have to open fully in order to achieve effective cooling. Therefore, the system will help the building cool while maintaining the security of the building. The intelligent control system should also include wind and rain sensors which are able to detect when rain and wind speed limits are exceeded in order to close the windows and protect the building.
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