4.1 The passive solar still

Mb, Mf, Mw, and Mg: heat mass are unit area of the basin, the air, the water in the

Tb, Tf, Tw, and Tg: respectively, the basin, the airflow, the still water, and the

wout and win: the air-vapor mixture's moisture contents exit and enter the still

� � � � � 103 (30)

c

μkΔT !<sup>1</sup>=<sup>4</sup>

; ΔT is the dew point temperature difference between the

ΔTLc � �<sup>0</sup>:<sup>25</sup> (31)

(32)

(33)

hout and hin: respectively, the enthalpies of air exiting and entering the still (J/kg). The air enthalpy exiting the still hout can be computed as the temperature

hout ¼ Tf þ wout� 2500 þ 1:81Tf

The yield of the distillate in the solar still depends on the air and the glass temperatures. Water will condense on the glass surface only when the airflow dew point temperature Tfd is higher than the glass temperature Tg. In this case, the amount of the distillate produced from the glass mew-g can be computed from

<sup>m</sup>\_ ew�<sup>g</sup> <sup>¼</sup> qcon�<sup>g</sup>

qcon-g = hcon-g(Tf – Tg): heat transfer by condensation from the airflow to the

<sup>k</sup> <sup>¼</sup> <sup>0</sup>:<sup>943</sup> <sup>g</sup><sup>2</sup>sinβhfgL<sup>3</sup>

where Lc is the length of the glass, in m; Lc = Ls; k is the thermal conductivity, in W/m K; g = 9.81 m/s<sup>2</sup> gravity constant; β is the slope of the glass, in degree; ρ is the

hfg

still, and the glass (J/m<sup>2</sup>

glass temperatures (°C).

Tf function as follows:

(kg/kg).

(kg/s m<sup>2</sup>

):

air density, in kg/m<sup>3</sup>

and Tb, can be found.

38

glass. Using the Nusselt to calculate

.°C).

Distillation - Modelling, Simulation and Optimization

hfg: latent heat of vaporization of water at Tf (J/kg).

hfg: latent heat of vaporization of water at Tf, (J/kg).

Nu <sup>¼</sup> hcon�gLc

airflow and the glass, in °K; μ is absolute viscosity, in Pa s.

3.2 The dehumidifying coil and preheating coil calculation

active distillation system is presented in [9].

Using the properties of the air at Tf = 40°C, one can achieve

qcon�<sup>g</sup> <sup>¼</sup> <sup>70</sup>:<sup>93</sup> sin<sup>β</sup>

Hence, with five formulae from (18) to (19), five parameters Tg, Tw, Tf, wout,

The calculation of dehumidifying and preheating coils has been studied and is shown in [11, 12]. However, a clear and detailed procedure for simulating the performance of dehumidifying coils was not available in these references. Hence, a numerical model of the performance of the dehumidifying and the preheater coils in this research was developed from the handbook and the standard. The calculation procedures for the psychometric properties of humid air were given in [11]. A detailed procedure for simulating the coils of preheating and dehumidifying of an

Figures 4 and 5 show the computed distillate yields and still water temperatures from the mathematical model compared to those from the experiments. As shown in these figures, the simulation model developed in this study gave very accurate calculated results. Hence, one can confidently use this program to simulate solar passive stills.

Figure 4.

The water calculated and measured temperatures in a passive solar still.
