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

to 550 kWh/m 3 of distilled water and 4 to 12 l/m 2 .day of solar collector area, as
reported by Narayan et al [149].
GOR is directly proportional to heat and mass transfer effectiveness in the
evaporator and condenser or both the temperature drop of hot water in the
evaporator (ΔT hw ) and temperature rise of feed seawater in the condenser (ΔT cw )
and inversely to the temperature jump in the solar collector (ΔT coll ). When the HDH
system works efficiently, both the temperature drop in the evaporator and
temperature rise through the condenser become high, which reduces the required
temperature jump in the collector, i.e. the required energy input to produce same
amount of distillate.
The heat and mass transfer rates in the evaporator and condenser depend on the
following parameters, see the finite element illustration in figure (2.5):
1. heat and mass transfer
coefficients
2. gas-liquid contact area
3. driving forces for heat and mass
transfer (i.e. temperature
difference ΔT, and water vapor
concentration difference ΔC)
Liquid film
T l
C l
Gas film
Q
T g
The driving force for heat transfer is
the temperature difference between
the liquid film and the bulk of the
carrier gas. For mass transfer, it is due
to the difference in water vapor
pressure (or vapor concentration C) on
the liquid film surface and partial
pressure of water vapor in the bulk
gas. The gas liquid contact area is a
function of the geometric configuration
of the exchanger, packing shape and
size, and liquid and gas flow rates.
Packing
surface
Bulk liquid
m v
Liquid-gas
interface
C g
Bulk
gas
Figure 2.5: Finite element at the liquidair
interface in direct contact heat and
mass exchangers
2.8.3 Lumped model analysis
In this section a simplified lumped model of a HDH system as shown in Figure (2.4)
will be developed to explain how it works. Such a framework might help in
interpretation and making sense of literature results, which will be presented in the
following parts of this chapter.
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