Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 188

Table 5.9 Diffusion coefficient parameters for water transport in salt solutions

LiCl-H 2 O CaCl 2 -H 2 O

τ 1 0.52 0.55

τ 2 − 4 . 92 − 5 . 52

τ 3 − 0 . 56 − 0 . 56

For the salt solutions used, the parameters τ 1 , τ 2 and τ 3 are as given in Table 5.9.
The distribution of the molar water concentration C ( X ) across the liquid desiccant
film thickness can be approximated by a linear function (see Figure 5.74).
With the boundary conditions, C ( X

= 0) = C 2 and C ( X = δ S ) = C 1 , the concen-

tration C ( X ) (mol/l) is

C 1 − C 2

C ( X ) = X + C 2 (5.22)

δ S

0 is assumed to be equal to the average concentration
in the previous differential volume element of the HEAU. Furthermore, it is assumed
that the concentration at X

The concentration C 2 at X =

δ S is constant within each differential volume element.
The diffusion coefficient for water in salt solutions (m 2 s − 1 ) can be described as a
function of the molar water concentration (Klopfer et al ., 1997):

=

= D ( C 2 )exp C ( X )

− C 2

ln D ( C 1 )
D ( C 2 )

D ( C ) (5.23)

C 1 − C 2

Figure 5.74 Concentration gradient within the liquid desiccant film