Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 146

Absorber

t

y

H

W

Rib

z

B

Figure 5.36 Geometry of a finned air collector with flow channels of height H and width W and ribs
of thickness t to calculate the rib efficiency

collector size and component efficiency. A dynamic air collector model is part of the
model development.

Air Collector Model

The air collector model uses Nusselt correlations for laminar or turbulent flow condi-
tions and a rib efficiency for a channel of height H and width W derived by Altfeld
(1985) (see Figure 5.36). In addition, heat capacities were attributed to the three tem-
peratures nodes of the absorber, the air flow in the gap and the back insulation surface.
The energy balance for the three nodes was solved with the known air entry conditions
and then subsequent elements were calculated in the flow direction. The long-wave
radiative exchange between the absorber and back surface and the heat loss from the
absorber to the environment are calculated using the temperature values from the last
time step.
The accuracy of the dynamic model was studied on a sunny day with fast-moving
clouds. The collector volume flow through each of the 50m 2 collector fields is
3000m 3 h − 1 , which results in flow velocities in the 9.5 cm high channels of 9m s − 1 .
Temperature levels of 80 ◦ C or 47 K temperature increase are reached at a maximum
irradiance of 1000Wm − 2 . Whereas the steady-state model leads to very high tem-
perature gradients at changing irradiance conditions, the dynamic model is capable of
very precisely simulating the dynamic collector response (see Figure 5.37). The time
constant for the temperature change is approximately 15 minutes.

System Model

The desiccant unit in the full system model was simulated using static models for the
energy analysis, although models of rotating sorption wheels have been developed and
validated by laboratory results. Laboratory measurements at the University of Applied
Sciences in Stuffgart showed that the dehumidification is isenthalpic if regeneration
temperatures are below 75 ◦ C (see Figure 5.38). Only at higher temperature levels
does heat transfer from the regeneration side lead to an enthalpy increase during the
dehumidification.