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

packing are its heat and mass transfer performance, the quality of water, pressure
drop, cost and durability [149]. Al-Hallaj and Selman [2] summarized the main
technical results reported by various research groups. Almost all the investigators
state that the effect of water flow rate on the performance of the unit is important.
The effect of air flow rate on productivity is termed insignificant by all authors except
Younis et al [135]. Also, all researchers express preference for natural convection
since air flow rate has no significant effect on unit productivity. However, forced
circulation could be feasible with another cost-effective source of energy such as
wind energy. The effect of air flow ate is only noticeable at temperatures around
50 o C, as reported by Al-Hallaj et al [132].
It is concluded that HDH technology has great promise for decentralized small-scale
water production applications, although additional research and development is
needed for improving system efficiency and reducing capital cost [149]. Al-Hallaj and
Selman [2] attributed the higher production cost of HDH systems compared with
reverse osmosis (RO) and multi-stage flash evaporation (MSF) units to three main
reasons:
1. HDH units rely upon natural draft which results in low heat and mass transfer
coefficients and a large surface area for the humidifier
2. The HDH systems are applied for small desalination capacities and scale
economies can not be realized in capital investment
3. Film condensation over tubes is typically used, which is extremely inefficient
when non condensable gases present. Thus, a much larger condenser area
is required, and the condenser accounts for the majority of the capital cost.
2.8.9 Conclusion and spot light
The extensive literature on HDH systems reveals that in spite of the large and
valuable amount of research and development effort invested, there is still room for
development and optimization of such systems. To improve the performance of HDH
systems, previous studies have investigated various configurations of the HDH
cycle. These efforts focused on improving heat and mass exchanger designs or
using multi effect humidification of air. A multitude of alternative water-heated HDH
systems has been presented. However, few interesting approaches focused on
direct air heating compared to direct water heating, which proved to be much more
energy efficient than air heated systems. The multi effects of heating/humidification
of air which was investigated by Chafik [155], the combination of air heating and
water heating which was presented by Abdel-Monem [145], and the hybrid latent
heat storage and spray flash evaporation which has been proposed by Miyatake et
al. [141] are approaching the main interest of the present work. A novel approach for
the multi effects of heating/humidification of air and improvement of the HDH cycle
will be presented in the next chapter.
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