Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 193

by the increase of the heat transfer coefficients. This should usually result in rising
mass transfer coefficients within the absorber units. However, the mass transfer of
water vapour from the air to liquid desiccant is mainly influenced by the mass transfer
conditions within the solution film. Since the resistance to water vapour transport is
significantly higher within the solution film than in air, the mass transfer coefficient
depends mainly on these slower transport mechanisms. Furthermore, the decrease
of dehumidification with increasing air flow rates can be attributed to the time re-
duction for dehumidification within the absorber. As can be seen from Figure 5.77,
the calculated dehumidification and outlet return air temperatures fit the experimental
results well. This indicates that the heat andmass transfer conditions are well described
within the two developed models. However, for the HEAU the calculated decrease of
dehumidification is higher than the measured one, especially for LiCl. This can be
attributed to inaccuracies in the volume flow regulation (portable measuring instru-
ment) combined with imprecise control of the two humidifiers, used for the condition-
ing of return and ambient air, and further measurement inaccuracies (thermocouples,
humidity sensors, etc.).

Optimization Potential of the Heat Exchanger Absorber Unit (HEAU)

The experimental results demonstrate the good performance of the developed HEAU.
The high dehumidification together with the lowoutlet temperature of the return air can
be attributed to the integrated indirect evaporative cooling function. However, the spray
nozzles used for the distribution of the liquid desiccant over the return air channels
allow only high solution flow rates of at least 100 l h − 1 . In combination with the bad
surface wetting of 35-45%, just a small amount of the dehumidification potential of
the utilized salt solutions is used in the HEAU. If for example, the LiCl solution flows
once through the HEAU, under the examined conditions, the concentration is reduced
only from 43% to about 41.5% LiCl mass fraction. The flow rate of the solution
could be considerably reduced without decreasing the dehumidification rate, if the
same or higher surface wettings are reached. To illustrate this optimization potential,
further calculations have been carried out, where the surface wetting and the desiccant
flow rate were varied. The results are displayed in Figure 5.78, which illustrates the
reachable dehumidification of the return air versus the desiccant flow rate and the
reached surface wetting. The results were obtained for volume flows of 200m 3 h − 1 ,
return air conditions of 26 ◦ C, 55% RH and ambient air conditions of 32 ◦ C, 40% RH.
The reachable dehumidification increases with increasing solution flow rate and
increasing surface wetting. However, depending on the surface wetting percentage, a
dehumidification threshold value is reached with increasing solution flow rate. Con-
sidering the results shown in Figure 5.78, the main objective for further developments
of the HEAU should be the realization of an optimum solution flow rate in the range
of 20 to 40 l h − 1 combined with a surface wetting of

≥

60%. Under these conditions
the reachable dehumidification can be improved from 5.7 g water kg − 1

air to a maximum