Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 112

1.0

0.9

Adsorption and absorption
single effect

Absorption
double effect

0.8

0.7

0.6

0.5

0.4

0.3

Flate plate Vacuum tube

0.2

0.1

0.0

0.00 0.05 0.10 0.15 0.20

( T m - T o )/ G / m 2 K W -1

Figure 5.1 Collector efficiency as a function of reduced temperature

Commercial adsorption systems, either open air-based systems or closed adsorption,
are designed for heating temperature ranges around 70 ◦ C. Single effect absorption
chillers start at operating temperatures of about 70 ◦ C. Commercial machines are often
designed for average heating temperatures of 85-90 ◦ C. Double effect systems require
150 ◦ C and more.
An actual database of solar thermal collectors with manufacturer efficiency infor-
mation was implemented in the software environment INSEL and used to calculate
efficiency curves as a function of the difference between mean fluid temperature T m
and ambient air temperature T o , normalized by the incident irradiance G (the so-
called reduced temperature difference). At an irradiance of 800Wm − 2 and a summer
ambient air temperature of 30 ◦ C, the reduced temperature difference for heating tem-
peratures of 70 ◦ C is 0.05 and gets higher at lower irradiance levels. For a mean
fluid temperature of 90 ◦ C the reduced temperature is 0.075 and for 150 ◦ C, 0.15. In
Figure 5.1 it can be seen that current flat plate collectors available on the market can
be well used for driving temperature levels around 70 ◦ C, but are less efficient than
vacuum tubes at heating temperature levels of 90 ◦ C. Adsorption or single effect ab-
sorption machines can both operate at about 70 ◦ C driving temperatures, if evaporation
temperatures are high and cooling water temperatures low. For double effect chillers,
only vacuum tubes or parabolic concentrators can provide the required temperature
levels.
Themain features of the dominating absorption cooling technology and an overview
of the historical development will be presented in the following together with some
project results from desiccant cooling systems and a study of innovative cooling sys-
tems for low-power applications.