Wind-Driven Natural Ventilation Systems. PRESENTED BY NIRAJ MISTRY

Transcription

1 . Wind-Driven Natural Ventilation Systems PRESENTED BY NIRAJ MISTRY

2 Natural Ventilation Natural ventilation makes use of the forces of wind and differences in air to move air through a building. Main Types: Cross Ventilation Single-sided ventilation Passive stack ventilation

3 Cross Ventilation In its simplest form, cross ventilation drives air through open windows on the windward side of the building. Wind-induced pressure differences drive the air across the building. Open windows on the opposite side allow the stale air to escape.

4 Cross ventilation Benefits: A high rate of ventilation is possible under favourable weather conditions. Cross ventilation can be used in relatively deep-plan spaces with two or more perimeter wall containing windows that can be opened. Good occupant control. Low cost Cross ventilation can be designed to link with thermal mass.

5 Cross ventilation Limitations: Effective cross-ventilation requires a relatively clear path for air to flow across the space. Partitions should be kept under 1.2m, and tall cupboards should be placed between windows on the perimeter wall. Natural ventilation is dependant on the presence of wind for good ventilation. Inappropriate window design and positioning may result in disruptive draughts and papers being blown off desks. May not be appropriate for noisy polluted environment such as town centres.

6 Single sided ventilation Cross-ventilation of some spaces may not be possible due to fixed or structural partitioning. Where this is the case, singlesided ventilation may be appropriate. If windows are designed with this in mind, a room depth of up to 6-7m can be satisfactorily ventilated in this way

7 Single-sided ventilation Benefits: The simplest form of natural ventilation. Good occupant control for cellular spaces with workgroups of between 6-10 people. Windows should be tall, ideally with top and bottom openings. A space which can be daylit by perimeter windows on one side is generally suitable for single-sided ventilation up to a depth of around six to seven metres. Low cost.

8 Single-sided ventilation Limitations: Dependents on the presence of wind for good ventilation. Inappropriate window design or positioning and poor control of opposite elements can lead to occupier discomfort. Inadequate control over ventilation, either due to poor automation or unused manual controls, can lead to windows being opened while heating is on, wasting energy. Not suitable for deep-plan spaces. May not be appropriate for noisy, polluted environments. Motorised windows can be noise and distracting and prone to illogical operation

9 Passive stack ventilation Stack ventilation is driven by temperature (and therefore pressure) differences between the hot air in the occupied space and the cooler external air. Warm air rises upwards in the building and exists through one or more high level openings. The air displaced from the building causes cooler fresh air to be drawn into the building through low-level openings, such as windows and vents.

10 Passive stack ventilation Benefits: Temperature difference is the driving force, therefore stack ventilation is not reliant on wind to work effectively. Stack ventilation is particularly effective for nighttime cooling when there is the greatest differential between internal and external temperature. Stack ventilation systems are often combined with wind-driven ventilation for improved daytime performance. Fans can be used to drive ventilation on hot still days.

11 Passive stack ventilation Limitations: The air outside must be cooler than the internal air for stack ventilation to work. A relatively clear path is required for air to travel easily from low level to the high-level ventilation opening. A greater height stack is needed to increase effectiveness of the stack. Upper floors can overheat if there is insufficient height difference between those levels and the stack ventilation shaft. Short-Circuiting can occur between lower and upper floors if the ventilation paths are not well designed and tested, either on a computer or by physical modelling

12 Windows The most basic natural ventilation system is the provision of openable windows.

13 Windows

14 Windows

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18 Ventilators, dampers and louvres Natural ventilation can be achieved using the building fabric. Here simple vents have been created in the brickwork, while inside the ventilation flow rate is controlled by simple vanes called dampers.

19 Trickle ventilation Trickle vents can be simple grilles in a building s window sill or wall. Trickle ventilators are designed to provide the required minimum fresh air rate, particularly in winter, but without the increase in heating energy loss caused by opening windows.

20 Ventilation

21 Key design and application checks Natural ventilation is unlikely to cope with heat gains exceeding 40 W/m2 A naturally ventilated open-plan office cannot be controlled in summer to stable temperatures typical of a mechanically, air-conditioned space. Natural ventilation is intrinsically variable check performance.

22 Thermal mass How is thermal mass usefull? The use of exposed thermal mass, to reduce peak temperatures and improve thermal stability. Exposed concrete, masonry or other thermal mass present, such as furniture, has the ability to store and release heat (depending on it temperature relative to the surrounding air.) Can be used with a night cooling regime to reduce daytime peaks. Cooler thermal mass will cool the air in the space by natural convection as it moves across the surface. The lower surface temperature also introduces a radiant cooling effect. It has been shown that as little as 100mm of concrete can be sufficient to provide good damping of the diurbal temperature swing when used with night cooling.

23 Thermal mass A rammed-earth wall, constructed from waste soil excavated from the building site, has been used to provide some thermal mass in this naturallyventilated office.

24 Night cooling What is night cooling? Night cooling involves using the ventilation system to remove heat gains that have built up during the preceding day. Night cooling techniques are most effective when a thermally heavyweight structure is exposed to enable heat transfer. Heat gains of approximately 20 W/m2 can be offset. Night cooling offers the potential to minimise or even avoid the use of mechanical cooling and improve the internal conditions in naturally ventilated buildings.