PERFORMANCE CHARACTERIZATION OF A MULTI-STAGE SOLAR STILL - Energy Research Developments: Tidal Energy, Energy Efficiency and Solar Energy - page 141

The number of cells intervening in such an apparatus is not fixed. Some prototypes
having been used in North Africa (Senegal, Algeria, Libya, Tunisia, …) include up to six
stages. The number of cells depends on the operating conditions. Some works have shown
that the production of the two last cells of a distiller that includes six cells is very low. In a
previous work [28], we have shown that in the Tunisian climatic conditions, it is not
necessary to implement more than three stages. In the present work, it will be considered that
the solar distiller includes three stages.
The three cells run under the same conditions. It is assumed that they have the same
thermal yield

q

l , i

R = , i = 1 2

(5)

q

f , i

where

q . q .

is the heat flux density provided at the level of the the cell. Thus is the heat

f , i f , i

q for i=1 . For the other values of i ,

flux density imposed to the face of the first cell, thus

q , represents the density of the latent heat flux released by the condensation at the level of
the previous cell. Knowing the value of the yield R previously determined, the different
values of the latent heat flux density

i

q , m ,

, thus of the distillation mass flow rate , can be

i i

deduced as

q

l , i

m

, =

(6)

d i

Lv

The total production of the apparatus is obtained by summing

m = m + m + m

(7)

d , t d , d , 2 d , 3

In this calculation, it is assumed that the latent heat flux released by condensation at the
level of a cell is totally transmitted to the liquid film of the following cell.

2.5

2

1.5

1

0.5

0

200 400 600 800 1000 1200 1400

q f (W/m²)

Figure 7. Distillate flow rate of a three-stage solar distiller: Influence of the heat flux density.