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

The performance of the condenser is mainly dependent, among other factors, on the
temperature difference between the humid air mixture and the condenser surface,
and the condensation surface area. The desired condenser design should enhance
both above factors in addition to simplicity and economic considerations.
As a direct consequence, it is important to note that effective heat recovery in such
type of condensers require a larger heat transfer area for improving the overall HDH
system performance. For example, Bourouni et al. [128] used 3000 m length of
polypropylene tubes in the condenser. In another design Orfi et al. [156] used a
seawater cooled condenser that contains two rows of long copper cylinders where
longitudinal fins were soldered to their outer surfaces. The feed seawater flows
inside the cylinders while the water vapor condenses on the finned surfaces. The
condenser has a heat-transfer surface area of 1.5 m 2 and a total length of 28 m of
the coil. The multiple paths of the cooling medium inside tubes of small diameters
increase both the pressure loss and scale formation, which makes the fouling
tendency inherently high.
Furthermore, this configuration also has a rather restrictive coupling between the
performance of the condenser and the performance of the whole system, since the
mass flow rate of feed seawater (i.e. the condenser coolant) should be same as in
other system components (i.e. evaporator, and solar collector). For a given solar
collector area, increasing the mass flow rate of feed seawater increases the energy
efficiency of both the condenser and solar collector, while decreases the evaporator
effectiveness due to lowering its inlet hot water temperature. Due to existence of
non-condensable gases, the condenser performs better at higher mass flow rates of
cooling seawater than the optimum designed flow rate in the evaporator. The overall
thermal performance of HDH system is therefore extremely restricted by such a kind
of complex trade off.
Direct-contact condensers have several advantages that tend to alleviate these
problems. This type of condensers makes use of spray columns or packed beds in
countercurrent/concurrent flow regime to bring together the cooling medium (e.g.
freshwater in HDH cycle) and humid air in direct contact along the bed height. On
contrary to non-contact heat exchangers, direct contact condensers exhibit higher
intensity of heat transfer without heat resistance through the tube walls and provide
a large specific interfacial area with a minimum pressure loss. As a result, direct
contact condensation technique is widely used in industrial processes, and in
chemical and nuclear applications due to the required low driving potential and high
efficiency.
Several research studies (136, 137, 147, 148) report that direct contact
condensation of humid air is an attractive alternative for small-scale HDH
desalination units. Dawoud et al. [148] argued that it is simple, inexpensive, not
susceptible to fouling, and has relatively high heat transfer rate and low pressure
47