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

Air circulates in a closed loop, hence is assumed to be fully saturated with
water vapor at local temperatures
Feed seawater is equal to ambient temperature and it is constant at 25°C
Fresh water production rate is prescribed at a constant value of 1000 l/ day,
while the HDH unit size, air flow rate, and solar collector area have to be
adopted to obtain same distillate rate under different boundary conditions
Solar irradiation intensity is constant at 800 W/m 2
The solar collector area will be varied to maintain constant specific heat input
per unit of feed seawater flow under different mass flow rates, i.e. constant
temperature jump of feed seawater through the collector
2.8.3.2 Results of lumped model
All calculations of this simplified model were done using Excel. Figure (2.6) shows
the influence of hot feed seawater mass flow rate on the inlet hot water temperature
and GOR. As the seawater flow rate increases the inlet hot water temperature
entering the evaporator (T 2 ) decreases (i.e. , as the outlet water temperature from
the condenser decreases with increasing mass flow rate due to the assumption of
constant distillation/condensation rate. To maintain constant distillation rate with
decreasing the inlet hot water temperature, this is compensated by increasing the air
mass flow rate, while keeping the water temperature jump through the collector
constant by increasing the collector area linearly with the water mass flow rate, as
shown in figure (2.7). Since the specific heat input per liter of water mass flow is
constant, GOR decreases with decreasing the water mass flow rate, as the energy
input to the system increases at a constant distillation rate.
However, since the outlet air temperature from the evaporator and its humidity
content depend on the inlet hot water temperature, for obtaining a constant distillate
rate the air mass flow rate should be increased with increasing seawater mass flow
rate to substitute the temperature fall of inlet hot water. This fact is described by
equation (2.7) and shown in figure (2.7).
Following equation (2.12), the effect of feed seawater mass flow rate on the NTU is
shown in figure (2.8) as a function of air to water mass flow ratio. Results show that
the performance of the unit (in terms of low NTU) increases with inlet hot water mass
flow rate below 850 l/m 2 /h under the prescribed boundary conditions. Beyond this
limit, it is observed that the performance of the unit decreases when the hot liquid
flow rate increases. A critical air to water mass flow rate corresponding to the
minimum NTU can be determined from the plot, as the humidity content is varying
exponentially with air temperature (see figure 3.4). At this value, a maximum amount
of evaporated water can be obtained at a water recovery ratio of 5% (the ratio
between distillate rate and feed seawater mass flow rate).
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