Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement
562
absorber temperature to about 80°C, which is the required
maximum operating temperature for domestic hot water
and space heating applications.
2. MODELING PARAMETERS
2.1 Modeling Software
Theoretical modeling was done applying a software
generated by AEE-INTEC GmbH (Gleisdorf, AUT), which
provides a comprehensive theoretical mathematical
description of flat-plate solar collector performance[3]. It
allows for the evaluation of collector conversion factors
and for the determination of efficiency graphs. For the
present investigation specific functional elements, such as
thermotropic layers have been implemented additionally.
Basically this software considers direct, diffuse and
scattered solar irradiation reaching the absorber,
transmission, absorption and reflection on multi-layer
collector glazing including the thermotropic layer, solar
absorption and reflection as well as thermal emission of
absorber coatings, heat transport from absorber to fluid and
heat losses by convection, thermal conduction and radiation
due to collector glazing and casing[1].
2.2 Collector Configuration
For the model collector a specially designed 10mm thick
triple-wall sheet with channels for transport of water
(Solarnor, NOR) was used as the polymeric absorber[4].
Black absorbers with solar absorptivity α of 0.95 and
infrared emissivity ε of 0.9 as well as absorbers with
spectrally selective coating exhibiting a solar absorptivity α
of 0.95 and a infrared emissivity ε of 0.05 were considered.
Heat transfer medium was water with a flow rate of
50kg/(m²h).
Two types of collector covers were implemented. On the
one hand a polycarbonate single-wall sheet exhibiting a
thickness of 4mm was used as collector glazing. On the
other hand the collector was covered by a 10mm
polycarbonate twin-wall sheet with intrinsic air filled
channels (width 9.5mm; spacing layer thickness 0.5mm)
acting as an additional insulation layer.
A theoretical thermotropic layer was attached to the back
side of the different glazing types. For the considered
thermotropic layer constant solar transmittance and
reflectance values in the clear and opaque state were
assumed. In the switching temperature range a steep
switching with a linear change from the light transmission
to reflection was chosen. Solar transmittance of the film in
clear state was set to 85 and 90%. In opaque state, the
transmittance was assumed to decrease to values between
10 and 60%. Layer absorption was neglected.
The air gap between collector glazing and absorber was
10mm. The solar collector is confined by a 50mm and a
30mm thick insulation on the rear side and the lateral
surface, respectively.
As to solar collector position an angle of 45° to the
horizontal was selected. Ambient air temperatures of 0, 20
and 30°C were considered. Solar irradiations of 1000, 1100
and 1200 W/m² were modeled.
3. RESULTS AND DISCUSSION
3.1 Effects of Thermotropic Glazing on Collector
Efficiency
In Fig. 1 the efficiency graphs for a collector with
single-wall sheet glazing and black absorber (α=0.95,
ε=0.90) at solar irradiation of 1200 W/m² are compared at
ambient air temperatures of 0 and 30°C. It is observable
that a thermotropic layer with residual hemispherical solar
transmittance of 10% in the opaque state can provide
adequate overheating protection for this solar collector. To
achieve maximum absorber temperatures of 90°C,
switching temperatures of the thermotropic film between
45 and 50°C are required. At elevated ambient temperatures
the efficient working range of the collector is reduced to
temperatures up to 55 and 60°C. As shown in Fig. 2 the
effect of ambient air temperature on collector efficiency is
less pronounced for a collector with twin-wall sheet glazing
operating under the same environmental conditions. Even
at elevated temperatures, the efficient collector working
temperatures exceed 60°C. This indicates that for domestic
hot water applications collectors with twin-wall sheet
glazing and thermotropic layers are more appropriate.
Thermotropic materials with excellent switching properties