Passive Cooling Strategies - Low Energy Cooling for Sustainable Buildings - page 61

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Passive Cooling Strategies
Cooling of buildings can be achieved at very different energy consumption, ranging
from zero energy for purely passive over low-energy consumption for earth heat ex-
change up to high electrical energy requirements for active compressor chillers. The
application of different systems depends strongly on the cooling load, which has to
be removed.
Ambient air can be directly used to cool office buildings through night ventilation or
daytime ventilation using earth heat exchangers. Night ventilation can rely solely on
buoyancy or wind-driven natural forces, whereas fans are required to drive the volume
flow through the earth heat exchanger or to support the night ventilation volume flow
control. The energy which can be removed at night depends on the air exchange
rates, the convective heat transfer from all surface areas and the temperature swing of
ambient air.
Zimmermann states that daily cooling loads should not exceed 150Wh m − 2 for
night ventilation to be efficient, and that night-time ambient temperature should be
at least 5 K below room temperature for more than 6 h at air exchange rates of 5 h − 1
(Zimmermann 2003). If summer nights are very cool with ambient temperatures below
16 ◦ C, loads up to 250Wh m − 2 d − 1 can be removed. Shaviv and colleagues claim that
achieving 20 air changes per hour is important for locations in Israel and recommend
forced night ventilation strategies, if natural ventilation does not reach this air exchange
rate (Shaviv et al ., 2001). Measured night air changes in low-energy office buildings
such as the Fraunhofer Institute of Solar Energy in Freiburg, Germany, and the railway
building DB Netz AG Hamm on the other hand are only 2-5 h − 1 (Pfafferott, 2003