Calculations with different glass tube radius are carried out. Other parameters are here changed as well: For the collectors with the curved absorbers, the glass tube centre distance and the radius of the absorber fins are changed as well. For the collectors with the flat fins the glass tube centre distance and the width of the fins are changed as well. For Seido 5-8 and 1-8 the radius of the glass tubes is varied from 0.035 m to 0.070 m. For Seido 10-20 with curved and flat absorbers the glass radius is varied from 0.02 m to 0.05 m. The thermal performance is shown as a function of the glass tube radius for
Copenhagen the glass tube radius almost does not influence the optimum collector tilt. The change in the optimum orientation follows the same tendency as seen with a change in the other parameters. The collectors in Nuussuaq should be turned more towards east. In Sisimiut the collectors with the curved absorbers should be turned slightly from south towards west, and the collectors with flat absorbers should be turned 35° from south towards west. In Copenhagen the change in the orientation is only about 2° more from south towards west. In Fig 7 the thermal performance of the collectors in
Nuussuaq is shown with an increased tube radius for all the collectors.
Mean collector fluid temperature [ Cl
Fig 7. Improvement of the thermal performance as a result of an increase
in tube radius in Nuussuaq.
The last simulations were done with the models using the improved centre distance, larger strip angle and width and larger transmittance-absorptance product. The thermal performance of the improved
collectors in Nuussuaq in seen in Fig 8. The overall improvement of the collectors results in an increase in thermal performance of up to 9 % with a mean collector fluid temperature of 60 ° for the collectors in Nuussuaq. For Sisimiut the improvement of the collectors is about 5 % for a mean collector fluid temperature of 60 °C. In Copenhagen the least overall improvement is detected. The highest improvement is in Copenhagen seen for Seido 10-20 with flat absorbers.
 L. J. Shah, S. Furbo, (2005). Theoretical investigations of differently designed heatpie evacuated tubular collectors, Denmark.
 L. J. Shah, S. Furbo (2005). Utilization of solar radiation at high latitudes with evacuated tubular collectors, Denmark.
 J. Fan, J. Dragsted, Rikke Jorgensen, S. Furbo (2006). B^redygtigt arktisk byggeri i det 21. arhundrede, Denmark.
 J. Fan, J. Dragsted, S. Furbo (2007). Validation of simulation models for differently designed heat-pipe evacuated tubular collectors, Denmark.