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

extend the operation of the unit constructed in Jordan and Malaysia to be operated
in 24-h/d period by both Al-Hallaj et al. [132] and Nawayseh et al. [134].
Another approach for forcing the TTD to get narrow has been presented by Brendel
[154]. He used a forced convection in a multistage evaporator/condenser set-up
similar to Müller-Holst’s [5] concept but this system was run at different brine mass
flows to balance the heat capacity flows and yielding approximately 300 l/d.
However, this plant is far too costly and too complex for application in isolated arid
regions in developing countries.
2.8.4.3 Effect of air heating
The performance of the HDH system depends greatly on whether air or water is
heated [149]. It has been shown in the previous section that not only water heating is
essential but also hot water quality (i.e. inlet hot water temperature) at the entrance
of the evaporator plays the most important role in identifying the system
performance. Nevertheless, fresh water production efficiency can be enhanced with
air heating in addition to water heating. It has been experimentally observed by
Klausner and Mei [137] that the fresh water production rate increases when air is
heated prior to entering the diffusion tower. However, it is a well known fact that
solar air heaters are less efficient and less mature than solar water heating devices,
which is directly reflected on the specific cost of energy harnessed by each of them.
Also, from the various literature studies [149], it was observed that air-heated
systems have higher energy consumption than water heated systems. This is
because sensible heat content of water, which is supplied by air in the humidifier, is
not subsequently recovered from the water, unlike in the water-heated cycle in which
the energy transferred to air is recovered in the dehumidifier.
The air heated systems have been reported in the literature can be either single (e.g.
Abdel-Monem [145]) or multi-stage (e.g. Chafik [155]). Abdel-Monem [145] have
experimentally investigated the effect of heating water and air on the performance of
the HDH system illustrated in figure (2.11). The main parts of the laboratory system
are: air heater, water heater, cooling tower (humidifier), and air cooler (dehumidifier).
Both the feeding air and water are heated before they enter into the humidifier.
Within the operation range used in the experimental study, it was found that: the inlet
hot water temperature to the evaporator has the most important positive effect on the
productivity. Increasing both the inlet water and air temperature to the humidifier was
better than if each were increased individually; and increasing both water and air
temperatures is preferable than increasing their flow rates through the system to
increase the distillate rate. Increasing the cooling water to airflow rate ratio and
decreasing inlet cooling water temperature through the dehumidifier increase the
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