Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 194

7

6

5

4

3

4 8 12 16 20 24 28 32 36 40

Solution flow rate / l h -1

surface wetting 30% surface wetting 40%

surface wetting 60% surface wetting 80%

surface wetting 100%

Figure 5.78 Calculated dehumidification of the return air in the HEAU by varying the desiccant flow
rate and surface wetting

of 6.5 g water kg − 1

air . Also, for the construction of further absorber units the utilization
of non-corrosive materials, like polyethylene or polypropylene, is imperative. Since
these materials are extremely hydrophobic, the main objective of a high surface wet-
ting is harder to reach than for metal surfaces. Special surface structures or mineral
surface coatings may offer a practicable solution under these circumstances. Another
challenge, for high surface wetting and the distribution of the liquid desiccants, is the
intended low desiccant flow rate. This problem may be solved using special spray
nozzles or eventually thin membrane tubes on top of the heat exchanger. Considering
the results of Saman and Alizadeh (2001), further improvements can be expected if a
counter-flow arrangement between the liquid desiccant and the return air is realized,
instead of the current parallel flow arrangement. On the other hand, an acceptable
distribution of liquid desiccants with spray nozzles is then only practicable for very
low air velocities. This requires low air flow rates, leading to large absorber units.

Conclusions on Liquid Desiccant Cooling

A new desiccant cooling system is proposed, which shifts the air drying and humid-
ification process completely to the return air side. The supply air is cooled using an
efficient heat exchanger, so that hygienic problems are avoided. The system solution
requires a highly efficient desiccant process to compensate the transfer losses through
the additional heat exchanger. Different technological solutions were experimentally
and theoretically analysed for the return air drying. A solid material rotating desic-
cant wheel can dry best in an isenthalpic process with a corresponding temperature
increase, leading to dehumidification rates around 4 g water kg − 1

air . Liquid desiccants