The transmittance-absorptance product describes the properties of the glass and the absorber. The products are for all four solar collectors varied in the interval from 0.77 to 0.90. As expected the best performance is seen with a high transmittance-absorptance product. Fig 5 shows the performance as a function of the transmittance-absorptance product for the solar collectors located in Nuussuaq. The

higher the product, the higher the thermal performance. The variations of thermal performance in the investigated interval are slightly higher in Nuussuaq than in Sisimiut and Copenhagen. An increase in the transmittance-absorptance product from 0.83 to 0.9 will result in a increased performance of about 12 % for all the collectors. A change in the transmittance — absorptance product from 0.83 to 0.9 results in an increased optimum tilt of the collectors of 6° for the collectors located

in Nuussuaq. In Sisimiut and Copenhagen the transmittance-absorptance product will only slightly influence the optimum collector tilt. The optimum orientation by changing the transmittance- absorptance product from 0.83 to 0.90 is changed in such a way that the collectors in Nuussuaq should be turned even more towards east to about 60° from south towards east. In Sisimiut the best azimuth for the collectors with curved absorbers is 2° from south towards west. The azimuth of the collectors with flat absorbers is 35 ° towards west from south. In Copenhagen a change in transmittance — absorptance product will result in the best azimuth to be 4° from south towards west for all four collectors. In improvement of the transmittance-absorptance product will result in great improvement of the thermal performance of all the collectors. With a mean collector fluid temperature of 60 °C, the improvement rages from 11 % to 14 %, with the highest improvement in Nuussuaq. The thermal performance increases with high mean collector fluid temperature for all collectors at all locations.