EuroSun2008-2

Error in trough placement

Referring to the N-S positioning, the horizontal axis of parabolic trough (X in Fig. 1) should be oriented in the North-South direction, which corresponds to the axis of Earth rotation. The realisation of a precise alignment is quite complex; moreover the correct positioning should be maintained for long periods with the lower possible misalignment errors.

Errors in N-S axis positioning often occur during the installation of solar plants; while they rarely happen after some times of system working. The solar trough axis, instead of being perfectly aligned to the terrestrial axis, can form a few degrees angle with the N-S direction. The main consequences of this misalignment are collected energy losses, which are studied for a one-axis tracking collector with parabolic profile. In these studies, the reference parameter for the N-S misalignment is represented by the angle p between the horizontal axis of solar trough (X in Fig. 1) and the N-S direction. By a practical point of view it could be useful to visualise the N-S
misalignment angles p considering the associated displacements of trough extreme, presented in Table 1 for a solar trough of length 5 m.

The initial study examines the effect of N-S misalignment for three sun’s altitudes a, pertaining to the following months: a=23° in December, a=60° in August and a=69° in June. The effects of N-S axis misalignment are expressed using the collection efficiency, which is a crucial quantity to be considered in the application of sunlight exploitation. It is obtained as ratio between the light focused on the absorber and the light captured by the entrance aperture of collector.

The results are plotted in Fig. 9 as collection efficiency versus angle of N-S misalignment. The losses of collection efficiency are caused by the N-S misalignment combined with the effect of the different sun’s altitudes, corresponding to the different months. Obviously the maximum effect of collection losses is achieved in December; while the minimum losses are obtained in June, when a reaches its maximum value. For the data in Fig. 9 the absorber is a metal pipe surrounded by a glass tube and the layout parameters are: f=800mm, d=50mm, D=60mm,

T=3mm.

The successive study refer to different configurations for the solar trough, considering an angle of N-S axis misalignment P=15°.

The results in Figures 10-12 refer to a simplified absorber, consisting in a linear plate of width w and length 5 m. The examined layouts combine different focal lengths f of the parabolic mirror with two values of absorber width w, as follows:

1) f=500mm, w=50mm;

2) f=600mm, w=50mm;

3) f=700mm, w=50mm;

4) f=800mm, w=50mm;

5) f w 100mm. Fjg. 10. Absorbed light for N-S misalignment angle of 15°.

The results are summarised

in Fig. 10, plotting light received by the absorber versus suns altitude. the light received by the

absorber in Figures 10-12 is expressed in percentage with respect to the ideal case of sun’s altitude a=90°.

Finally, considering only two configurations of solar trough, the analysis focuses on the various errors in positioning the horizontal axis of collector. The considered North-South misalignment angles P are 5°, 10°, 15°. The layouts taken in to account are characterized by the following parameters: f=800mm, w=50mm and f=1000mm, w=100mm. Figures 11-12 show the results for the examined solar troughs. The curves show a significant dependence on sun’s altitude and the solar trough with f=1000mm and w=100mm experiences lower energetic losses.