Environmental Engineering Reference
In-Depth Information
was up to 15 K above air inlet temperature. For tilted windows, measured air exchange
rates with the room are typically between 0.5 and 2 h 1 , if no cross-ventilation takes
place. Daily heat gains obtained were then between 36 and 86Wh m 2 d 1 for a
standard office geometry; that is, of the same order of magnitude as thermal gains
from people.
When ventilated photovoltaic fa¸ades are employed, it is possible to collect the
thermal energy produced in the air gap with thermal efficiencies between 15 and 30%.
Despite losses in thermal efficiency, it is recommended to work with gap sizes over
10 cm or to keep flow velocities very low in order to reduce pressure drops and obtain
high coefficients of performance between the heat produced and electricity employed.
In the best case, COPs of 50 could be achieved. In summer the thermal energy can be
used to prewarm air for desiccant cooling systems.
The magnitude of cooling loads in office rooms today is often not well known,
although it is a prerequisite for any cooling system design. The range and distribution
of internal loads were analysed in different offices in great detail over a 3-year period.
Internal loads were as high as 400-500Wh m 2 d 1 for a standard two-person office
situation, with about 20%due to the people, 5-10% to lighting and the rest to electrical
appliances such as computers and printers. Such internal loads are significantly higher
than external loads from a shaded fa¸ade. Comfort levels for typical German summer
conditions are good using purely passive night ventilation strategies. Measured air
exchange rates - mainly caused by cross-ventilation - were high in the case study
building with average summer values of 9 . 3h 1 . To remove daily loads of about
8-10 kWh at this air change rate, the temperature difference between room and outside
air must be 6 K for 9 hours of night ventilation. As this is not always the case in warm
summers, for example in the year 2003, here nearly 10% of all office hours were above
26 C.
Mechanical night ventilation leads to better control of night air flows, but the elec-
tricity consumption is significant. In a well-designed passive building in Tubingen,
measured COPs of the night ventilation were about 4.0 with maximum values of
6.0. Also, the rather low air exchange rates of the monitored mechanical ventilation
systems of about 2 h 1 were far from sufficient to decrease significantly the room air
temperatures. A much better performance was obtained from earth heat exchangers,
both air and water based. Here the average annual COPs were as high as 35 to 50. The
only disadvantage is the rather limited cooling contribution, as only fresh air is used
for the earth heat exchange and as water-based distribution systems such as activated
ceilings cannot deliver high cooling power.
At higher cooling energy demand, active cooling technologies are often required.
Different solar thermal cooling technologies were analysed and an overview of
existing projects was given. Design and monitoring results from two large desiccant
cooling systems were presented. It was shown that under German climatic conditions,
the coincidence of full regenerative operation and available solar power was rather
low. However, simulation studies demonstrated that auxiliary heating can be nearly
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