Passive Cooling Strategies - Low Energy Cooling for Sustainable Buildings - page 76

Figure 3.16 Cooling power and coefficient of performance of mechanical night ventilation system
and 155W for the exhaust fan, closely matching the design value. The specific total
consumption of 0 . 17Wm − 3 h is excellent. During night ventilation with a measured
supply air volume flow of 4021m 3 h − 1 at a pressure drop of 395 Pa, the supply air fan
consumed 1072W, while the exhaust air fan at a slightly lower flow and pressure drop
(3600 m 3 h − 1 and 356 Pa) required 850W. The specific total consumption related to
the supply air flow is 0 . 48Wm − 3 h, still a good value for ventilation systems, but 75%
higher than the design value.
Further measurements in several rooms were taken in 2006 during a very hot period
in July. Internal loads averaged over 24 hours were only 4Wm − 2 in the top floor and
5Wm − 2 in the bottom floor offices. External loads including sun shading systems
were 7 . 5Wm − 2 on average on the ground floor and 4 . 6Wm − 2 on the top floor. The
ground floor has a significant heat transfer to the ground due to the rather low insulation
thickness of the floor ( U -value of 0 . 35Wm − 2 K − 1 ). The measured heat flux via the
floor to the earth was between 2 and 3Wm − 2 on average. This is the same amount that
the measured heat flux removed from the ceiling during night ventilation (2 . 7Wm − 2
on average). The measured effective discharging time of the ceiling during mechanical
night ventilation was about 13 hours with a ceiling temperature decrease of 1 . 5 K. The
main limitation on a more effective night discharging of storage masses is the total air
exchange rate, which is limited here to 2 air exchanges per hour. The air outlet injection
to the ceilingworked effectively, as can be seen from the infraredmeasurements during
the day and after a period of night ventilation (Figure 3.17). Ambient air temperatures
during night ventilation were 4 . 2 K lower than room temperatures and a mean cooling
power of 7 . 6Wm − 2 was achieved.
There is a clear temperature stratification in the building of about 1 K for rooms with
the same orientation. This can be attributed to a heat flux of 2-3Wm − 2 via the rather
badly insulated ground floor, the radiative exchange with the cooler ground floor and
higher storage masses of the ground floor ceiling. Strong drops in room temperature