The investigation of the thermal performance of collectors with coloured absorbers has been performed by AEE INTEC in Gleisdorf, Austria using the dynamic collector test method according to EN 12975-2. Three collectors connected in series have been tested simultaneously. Hence, an equal mass flow through all collectors has been maintained. To assure the same input conditions for all collectors, the heat transfer fluid (water) has been cooled after each of the collectors to the same temperature. The coloured collectors were measured in comparison to a collector with black solar varnish coating on a aluminium absorber and a collector with black selective coating on a copper absorber respectively. Two different absorber materials have been used, because the involved project partners use these absorbers respectively. The test series with black solar varnish and the colours blue and grey has been performed using aluminium absorbers. For the test series with black selective coating and the colours green and auburn copper absorbers have been used. The influence of different glass covers (structured and antireflective coated glass) has been investigated for two of the collectors. Figure 1 shows the test site with three at aEe INTEC.

The tests have not been performed at a tilt angle of 90°, although this would be the standard orientation of a fagade collector, because the incidence angle of the direct radiation (testing time: summer) would have been too small for an accurate determination of the incident angle modifier. This would also have resulted in an inaccurate calculation of the conversion factor.

For all test series, a broad range of different conditions for solar irradiation, temperature differences between collector and ambient, amount of diffuse solar irradiation and wind conditions has been monitored at the outdoor test facility. Diagrams of the different parameters, showing the distribution of the measured values have been used to decide, whether enough data had been collected or measurements had to be continued. The data were analysed with a program developed by AEE INTEC. The parameters of the dynamic model were derived from the measured data using multi-linear regression. Making assumptions according to EN12975-2 the static efficiency curve of the collectors was calculated from the dynamic model.

Collector efficiency aluminium absorbers

Hemispherical irradiance in collector plane (45°): 800 W/m2

The result for the blue coating on an aluminium absorber shows that the thermal performance is comparable to the one of the black solar varnish coated absorber. At low temperatures the efficiency of the blue coating is slightly lower, but at higher temperatures the selectivity of the blue coating has an advantage and shows higher efficiency than the black one.

The grey absorber shows a 16% lower conversion factor than the black absorber due to the lower absorption. The slope of the efficiency curve is comparable to the one of the blue solar varnish coated absorber.

Efficiency curves copper absorbers

Hemispherical irradiance in collector plane (45°): 800 W/m2

coll. 4-selective- antireflective coated glass

coll. 4-selective-structured glass coll. 5-green-antireflective coated glass

coll. 6-red-antireflective coated glass

coll. 6-red-structured glass

The conversion factor of the green coating on a copper absorber is about 6 % lower than the one of the selective coated absorber. At a temperature difference of 45 K between collector mean temperature and the ambient, the efficiency of the green collector is about 14 % lower than the one of the selective absorber.

The conversion factor of the red coated absorber with the structured glass cover is about 9 % lower than the one of the selective coated absorber. At a temperature difference of 45 K the efficiency is about 14 % lower than the one of the selective absorber. The red absorber shows lower emission than the green one.

The measurements showed that the influence of the anti-reflective coating of the glass cover is lower than expected but statistical significant. Two independent test series have shown almost no difference in the efficiency with different glass covers.