IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009
From Figure 1 the steady-state internal heat flows of the vessels are as follows.
Evaporator: Q = m&
⋅ ( h − )
& (1)
E, int v 10
h8
Condenser: Q = m&
⋅ ( h − )
& (2)
C, int v 7
h8
Q& = & & & &
(3)
,
m ⋅ h − m ⋅ h + m − m ⋅ h
Absorber: A int v 10 sol,
w 1
(
sol,
w v
)
5
Q& = & & & &
(4)
,
m ⋅ h + m − m ⋅ h − m ⋅ h
Generator: G int v 7
(
sol,
w v
)
4 sol,
w 3
In equations (1) to (4),
& is the mass flow of the diluted solution and m&
v
is the mass flow
m
sol , w
of the refrigerant water vapour which is of equal value at state points 7 and 10 in Figure 1.
Equations (1) to (4) are based on the calculation of internal enthalpies for LiBr/water solution
and refrigerant vapour which requires sound knowledge of these properties. For
computational purposes this means that a property database of both LiBr/water and water
vapour has to be implemented into the simulation model and usually requires timeconsuming
iterations.
The main aim of the model presented is to account for the dynamics. Therefore, no detailed
steady-state simulation with all state points was performed but a simpler approach was taken
which considers the most important physical properties only. This may be refined later on but
is not intended to be the focus of this work. This kind of refinement will not change the basic
findings of this report. So, to keep the model fast, equations (1) to (4) can be simplified using
the latent heat of evaporation and sorption, r and l, as shown in equations (6) to (9). This
approach has been proposed by Ziegler [4] to avoid a detailed enthalpy calculation for each
state point. Please note that the latent heat is taken at condenser pressure. The dependency
of latent heat on temperature is treated according to Plank’s rule [4].
Moreover, not all internal temperatures in Figure 1 are being calculated in the model.
Instead, each vessel is being modelled using external and internal mean temperatures of the
respective fluids (heat carrier and working fluid) as well as a mean vessel temperature.
External mean heat carrier temperatures ϑX
are being calculated using the arithmetic mean
temperature of inlet and outlet temperature. Internal mean temperatures
page 64
T X are assumed to
be the mean of the equilibrium temperatures of strong and weak solution and of the