PERFORMANCE CHARACTERIZATION OF A MULTI-STAGE SOLAR STILL - Energy Research Developments: Tidal Energy, Energy Efficiency and Solar Energy - page 132

h depth of the cavity (m)
l length of the water film (m)
L v latent heat of vaporization (J/kg)
m feed flow rate feed (l/h)
m v flow rate evaporated water (l/h)
m d flow rate distillated water (l/h)
m out flow rate of the rejected brine (l/h).
q f heat flux density provided to the water film (W/m²)
R yield
T temperature of the fluid (°C).
T c temperature of reference, equal to that of the cooled plate (°C).
T feed temperature of the feed water (°C).
x, y Cartesian coordinates (m).

Greek Letters

δ thickness of the water film (m).

Subscripts

c reference (relative to the cold plate)
v water vapor
feed feed

Introduction

In distillation, two antagonist phenomena with respect to energy intervene: evaporation
which consumes energy and condensation which returns it. In some situations, in order to
conceive robust and reasonably low-cost processes, these phenomena are gathered in a single
cell: this is the case of the capillary film solar distiller [1, 2]. To be efficient, an optimization
study is then necessary. Several works dealt with the modeling of these exchanges so as to
understand the mechanisms of heat and mass transfer that accompany evaporation. However,
because of the physical complexity, simplifying hypotheses are adopted which prevent from
taking into account all the phenomena which occur in the process. To make the study more
reliable, additional experiments are necessary to quantify the influence of the operating
parameters on the yield of distillation. Evaporation comes from a thin film which falls along a
wall heated with a constant heat flux density. The vapor thus formed condenses on the
opposite wall maintained at a uniform and constant temperature. Both walls are vertical and
form the active walls of a rectangular cavity of large form factor, equal to 10. The evaporation
of a film falling along a heated wall presents great practical interests: it involves large heat
exchange coefficients for low temperature gradients. According to the expected application,
the interest concerns the mass transfer (drying a film), or the heat transfer (cooling the wall
which supports the film). Several authors have studied the heat and mass transfer which