Experimental and Numerical Analysis of a PCM-Supported ...

Klausner and Mei [136] described a diffusion driven desalination (DDD) process for
mineralized water distillation. The investigations focused on optimization of the DDD
system both experimentally and numerically. A practical difficulty that arises in film
condensation is that heat transfer is tremendously degraded in the presence of noncondensable
gases. In order to overcome this problem, they used a direct contact
condenser. For this application the warm fresh water discharging the direct contact
condenser is chilled in a conventional shell- and -tube heat exchanger using cooling
seawater water. A portion of the chilled fresh water is directed back to the direct
contact condenser. The distillate is discarded as the fresh water production.
Haddad et al. [140] presented another alternative condensing system for basin type
solar stills as depicted in Figure (2.18). An external condenser, constructed as a
packed bed storage tank filled with sensible heat storage material (rocks), was
integrated with the still. The packed bed condenser was cooled during the night,
using a radiative cooling panel by circulating water into the packed bed condenser
and the radiative cooling panel.
The cooling panel utilized the cold effective sky temperature, which normally is 10-
25ºC lower than the ambient temperature, in order to cool the rock domain in the
packed bed storage during the night. The packed bed tank is installed at a higher
level than the solar still level and is connected to the solar still by a vertical duct. At
the beginning of the day light, the tank temperature was lowered to nearly effective
sky temperature and water was evacuated from the storage tank. By buoyancy
forces and reduced back pressure created in the condenser, the vapor was sucked
through the duct between the still and the condenser.
Several advantages were reported for this system. The heat loss was reduced since
the temperature inside the still was lowered, the low temperature of the condenser
enhanced the condensation rate, and consequently this lowered the vapor partial
pressure in the still resulting in higher evaporation rate.
When humid air is fed into a dry porous medium which is at a lower temperature, the
humidity condenses and the condensate trickles down by gravity. As elapsed time
increases and a condensate is formed within the porous medium, the temperature
difference between the humid air and porous medium decreases and the
condensate flow rate diminishes.
Vafi and Sozen [13] developed a model for analyzing the behavior of a packed bed
of encapsulated phase change material (PCM) and a condensing flow through it.
Thermal charging of the packed bed was analyzed and compared for a sensible heat
storage material as well as for different PCM storages. It was found that storage of
thermal energy of condensation would be most efficient when a PCM with melting
temperature close to the lower limit of the operation temperature range chosen. It
was also found that for a given particle size and nominal particle Reynolds number,
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