Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 186

c P,a T R,in − T R,out c PD T R,in x R , in − T R,out x R,out + + x R h V

h c,R A
m R

T R −

T S

c P,a x R T R,in +

h d ) = + x R ( h V +

Mass balance:

m S
m R

x R = (5.17)

Boundary Conditions The boundary conditions for temperatures T , humidity ratios
x (kg water kg − 1

air ), mass flow rates m and concentration of the salt solution ξ S (e.g. kg LiCl

kg − 1

H 2 O ) are:

at y = 0 ,T A,in = T A, 0 ; x A,in = x A, 0 ; m W,in = m W, 0

at z = 0 ,T R,in = T R, 0 ; x R,in = x R, 0 ; m S,in = m S, 0 ; ξ S,in = ξ S, 0

To reduce the complexity of the given problem, the following assumptions and sim-
plifications have been made in the developed model:

There is no heat transfer to the surroundings from the absorber unit.
The temperature gradient between the water and the desiccant film across the thin
separation plate is negligibly small ( T W,out = T S,out ).
The transferred specific enthalpy, during the absorption process h ab or during the
evaporation process h ev is calculated using the element inlet temperature of the
return air T R,in or of the water T W,in .
The heat and mass transfer coefficients on the return and ambient air sides are
calculated using the element inlet temperatures of the return air T R,in , liquid desic-
cant T S,in , ambient air T A,in and water T W,in .

Good wetting of the heat exchanger plates with liquid desiccant on the return air
side and with water on the exterior air side is critical for achieving a good performance
of the HEAU. Since the wetting of the surfaces depends on surface properties and heat
andmass transfer conditions, a correct model prediction of the wetting factors is nearly
impossible. For the absorber unit a wetting factor for each surface has been defined
and iteratively calculated during the model validation using the experimental results.
The study of the flow pattern of the desiccant solution and water film within the
heat exchanger unit requires the film thickness and velocity of the liquid film to be
known. Under the assumption of laminar continuous film flow, the film thickness can
be calculated from the Nusselt falling film theory by neglecting the contribution of
the free surface. The equation for computing the solution film thickness δ S (m) is
a function of the solution mass flow, the viscosity µ S (kgm − 1 s − 1 ), the density ρ S