Still considering the solar heating plant, the thermal performance of the evacuated tubular collector is compared to the thermal performance of the newest (Vejen N. K., Furbo S., Shah L. J. (2004). ) Arcon HT collector. The collectors are facing south and tilted 45° in Copenhagen. In Ummannaq the collectors are facing south and tilted 60°.

It can be difficult to compare the thermal performances of flat-plate collectors and tubular collectors as the effective area of a flat plate collector typically is defined as the transparent area of the glass cover and the effective area of a tubular collector can be defined in many ways. In the present comparison, the tubes are placed close together so that there is no air-gap between the tubes and the outer tube cross­section area (=L-2-rc-N) directly corresponds to the transparent area of a flat-plate collector.

Fig. 11 shows the thermal performance per m2 collector as a function of the solar fraction of the solar heating plant for the two collector types. Here, the solar fraction is defined as:

Qauxiliar

Qto

First of all, the figure shows that the tubular collector has the highest thermal performance for both locations. Further, it can be seen that the Uummannaq curves decreases more rapidly with increasing solar fractions. This is due to the lower air temperature in Uummannaq.

The figure also shows that the ARCON HT collector has a better thermal performance in Copenhagen than in Uummannaq, whereas the tubular collector performs best in Uummannaq. The main reason for the result is that there is much more solar radiation ‘‘from all directions’’ in Uummannaq and this radiation can better be utilized with the tubular collector.

Conclusions

A new TRNSYS collector model for evacuated tubular collectors with tubular absorbers is developed. The model is based on traditional flat plate collector theory, where the performance equations have been integrated over the whole absorber circumference. On each tube the model determines the size and position of the shadows caused by the neighbour tube as a function of the solar azimuth and zenith. This makes it possible to calculate the energy from the beam radiation.

The thermal performance of an all glass tubular collector with 14 tubes connected in parallel is investigated theoretically with the model and experimentally in an outdoor collector test facility. Calculations with the new model of the tubular collector vertically placed and tilted 45° is compared with measured results and a good degree of similarity between the measured and calculated results is found.

Further, the collector model is used in a model of a solar heating plant and a sensitivity analysis of the tube centre distance, collector tilt and orientation with respect the thermal performance per tube is investigated for the two locations Copenhagen (Denmark) and Uummannaq (Greenland). The results show that the optimum tilt and orientation is about 45° south for Copenhagen and about 60° south for Uummannaq.

Finally, the thermal performance of the evacuated tubular collector is compared to the thermal performance of the newest Arcon HT collector. Here, the results show that the tubular collector has the highest thermal performance for both Uummannaq and Copenhagen. This analysis also illustrates the differences in the thermal behaviour of the two collector types: The ARCON HT collector has a higher thermal performance in Copenhagen than in Uummannaq, whereas the tubular collector performs best in Uummannaq compared to Copenhagen. The main reason for the result is that there is much more solar radiation ‘‘from all directions’’ in Uummannaq and this radiation can better be utilized with the tubular collector than with the flat plate collector.