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

The optimal value of feed water temperature was recommended as 85°C which also
was confirmed in numerous preliminary tests for yielding optimal evaporation ratio at
reduced calcination and gypsum formation effects. This range of water temperatures
would enable using economical materials like polymers in construction of the units.
As secondary effect the costs stay low.
The following figures (2.15) and (2.16) show temperature and evaporation rate
dependency over the filling height of 300 mm for EnviPac at different brine
temperatures, different inlet air temperatures, and a mass flow of 360 l/h. The
dashed line delineates the evaporator outlet air temperature, hence temperature
levels decrease over the evaporator outlet. Local temperature minima at a filling
height of 200 mm depicts a local maximum in evaporation. Air develops a mass flow
of 0,05 kg/s with an entrance relative humidity of 25 % and an outlet relative humidity
of 95 %.
Table 2.1: Design parameters range of AQUASOL II [143]
Parameter
Range
Height of packing (H) 300 mm
Interfacial area
main factor to increase vapor production
Distribution system nozzles
Brine inlet temperature 85 °C
For the temperature range given in this application a height of 300 mm was found to
be ideal under natural draft operation. The irrigated area is variable and can be used
as parameter to increase the humidity ratio in the exit air and hence its velocity.
Indeed, the problem of pressure loss is evident and is leading to an optimum in filling
height. The diagram in figure (2.16) shows the evaporation rate [l/h] at different filling
heights with EnviPac filling material. It can be shown, that in this case, an optimum is
achieved at 300 mm. These results reveal that free convection air flow in HDH units
is very sensitive to the permeability of the bed. However, optimal filling height under
forced convection with the same packing material would be significantly higher than
with natural convection discussed above, as pressure loss may not be one of the
main governing factors in the overall heat and mass balance.
It was reported that the distribution system was the main problem of the evaporation
over the packed bed. The shower heads didn’t show good results, the nozzles had
good results in front of uniform irrigation. Against fouling and crystallization out of
sea water they won’t have a satisfying comportment in continuous use. The gutter
distribution would have better properties in this point, but its fabrication cost was
high. The realized distribution was irrigation by nozzles.
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