Passive Cooling Strategies - Low Energy Cooling for Sustainable Buildings - page 81

cross-ventilation dominated the air exchange. A significant disadvantage of manually
controlled passive cooling systems is the early evening heat gain through air changes,
which reduces the night cooling potential by 20-30%.
However, user-driven night ventilation concepts work very satisfactorily during
moderate Central European summers with no more than 150-200 h above 25 ◦ C.
Internal loads as high as 400Wh m − 2 d − 1 in a north-facing office can be removed,
so that less than 50 h are above 26 ◦ C. The same applies to a south-facing office with
lower internal loads of about 200Wh m − 2 d − 1 . During hotter summers with 300 h
above 25 ◦ C, nearly 10% of all office hours are above 26 ◦ C in both south- and north-
facing offices, which shows the limits of passive cooling concepts in warmer climatic
regions. An energy-efficient improvement is obtained if night ventilation is restricted
to the later hours of the night through the control of external or internal openings and
if a fan can increase the total air volume flow.
Mechanically driven night ventilation results were obtained from two further office
buildings in SouthernGermany. The ratio of cooling power to electrical power required
for the fans (i.e. the COP), was between 5 and 10, which is still a better performance
than conventional chillers provide. The night temperature drops were only modest,
about 3 K on average, which is due to the low air exchange rate of about 2 h − 1 . The
removed heat load was between 85 and 120Wh m − 2 per night, which is significantly
less than the usually generated internal heat loads. The limit to higher air exchange
rates is clearly the necessary electrical power, so that very low pressure drops at the
air inlet and within the tubing system are essential. Hybrid systems combining natural
and forced ventilation might also be an interesting alternative.