Ventilative cooling

To reach thermal comfort conditions in the warm period, an effective ventilation strategy for temperature control (cooling) comprises measures that act to reduce the internal and solar gains as far as possible and to absorb heat in the fabric of the structure. The protection from intense heat gains may involve landscaping, building form, layout and external finishing, solar control and shading of building surfaces, thermal insulation, and the control of internal gains.

The aim of ventilation is to achieve high flow rates of cooler outside air through the building. Ventilation as a heat dissipation technique can deal with the potential for disposal of excess heat of the building to an environmental sink of lower temperature, which here is the external air. The dissipation of excess heat generally depends on the availability of an appropriate environmental heat sink, and of an appropriate thermal coupling between the building and the sink as well as sufficient temperature differences for the transfer of heat. The potential of heat dissipation techniques strongly depends on climatic conditions.

For the control of temperatures, passive daytime cooling or night cooling can then be very effective in moderate climates. This requires rather high flow rates, with a factor of 3 to 12 times the flow required for indoor air quality reasons in a medium-densely occupied office. This substantial difference leads to completely different systems for passive cooling, although the ventilation system for indoor air quality control may assist the control of temperatures during warm periods. The ventilation system for indoor air quality control will almost never be sufficient to take over the function of temperature control. With mechanical ventilation, the necessary system size and electricity demand is in opposition to the necessary flow rate.

Naturally ventilated buildings can offer good thermal comfort even in hot summer conditions. For example, in the summer of 2003 the office building Lamparter had less than 10% of the hours of use above 26 °C, but atriums can easily overheat [24].

Figure 2.19: Natural ventilation air change rates applied or measured [20,23,24].

2.2.4.1 Diurnal ventilation

Daytime ventilation is a simple strategy to enhance comfort by direct personal cooling. When internal temperature is felt too warm at still air, increased air velocities can compensate for higher room temperatures to achieve comfortable conditions by wind-chill. The increased airflow of outdoor air increases the limits of acceptable temperature and humidity as it affects evaporation and convection around the human body. High airflow rates are particularly useful when relative humidity is high as the higher air velocity increases the rate of sweat evaporation from skin, thus increasing heat losses in the thermal balance of human body [41].

Direct advective cooling with high flow rates replaces the warm internal air by cooler external air, and therefore the internal air temperature may closely follow the ambient air temperature.

Daytime ventilation can only be applied in an acceptable way if the indoor comfort may be achieved with outdoor air temperatures and with acceptable indoor air velocities. The distinct operation control regime should be considered with relatively high ventilation rates of the order of 5 to 10 ACH for direct ventilative cooling when appropriate.

0 5 10 15 20 25

Tokyo Liberty Tower (Aggerholm, 2002)

Lamparter Office (Eicker, 2008)

Singe Sided Ventilation (Breesch, 2006)

Cross Ventilation (Breesch, 2006)

Stack Ventilation (Breesch, 2006) Cross Ventilation Belgium (W

outers, 2002)

Air Change Rate per hour

Figure 2.20: Boundaries of outdoor air conditions within which indoor comfort can be provided by natural ventilation during the day, with indoor airspeed about 2 m/s (a very light breeze) [8].

2.2.4.2 Night-time ventilation

The idea of night-time ventilation in office buildings is to use the thermal mass of a building as a heat sink. The structure is cooled by convection during the night and is able to absorb heat in occupied hours. In principle, cool night air passes over a heavyweight building fabric and cools the thermal mass. The warmer daytime air will then be reduced in temperature when passing over the cooled slab. Therefore, night ventilation is particularly suited to offices, which are unoccupied during the night so that relatively high air changes can be used to provide maximum cooling effect without creating thermal discomfort. This strategy provides attenuation of peaks in cooling load and modulation of internal temperature with heat discharge at a later time (Figure 2.21). The larger the outdoor temperature swings, the bigger the influence of such storage capacity. The cycle of heat storage and discharge must be combined with means of heat dissipation, so that the discharge phase does not add to overheating.

Controlled night cooling must continue till the building is adequately cooled or occupied again. If the building structures are cooled to a too low level, the cooling process has to be interrupted before the end of the night in order to regain acceptable surface temperatures before the start of occupation. Night-time ventilation is an effective low energy cooling technique, especially in climates with relatively low

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peak summer temperatures during the day and medium to large diurnal temperature differences.

The distinct operation control regime should be considered with high ventilation rates when required. Typical ventilation rates for night cooling (typically of the order of 5 to 15 ACH) will exceed minimal rates needed for indoor air quality control, which during unoccupied hours may fall well below 1 ACH. Thus again, air quality control other than moisture control will usually not be needed to be considered during night-time ventilation.

Figure 2.21: Typical surface temperature cycle (black line) of an office concrete slab using the thermal mass as a heat sink with a constant flow rate of 10 ACH during night and day.

Night ventilation can be operated passively, e.g., window ventilation, or mechanically with fans. Natural night ventilation has the advantage of higher flow rates without fan electricity demand. Night ventilation can operate not only along with passive diurnal ventilative cooling strategies, but also with mechanical cooling.

Thus, night ventilation, if not a pure passive approach, is also of importance to reduce the energy consumption for mechanical cooling in summer.