EuroSun2008-2

Recovering procedure

The effects of an incorrect placement of trough axis, in particular considering the North-South positioning, have been discussed in the previous sections. Referring to the solar trough with parabolic mirror and cylindrical absorber, this section proposes a correction procedure to recuperate the lost light.

The analysis presented in this paper has evidenced that the errors in trough axis positioning generate significant losses of collected energy. Moreover the quantity of missing energy always depends on the sun’s altitude over the horizon. The amount of lost light is graphically visualised in Figures 9-12, for N-S misalignment errors up to 15°.

The solution proposed to recover the lost light consists in using the rotation of the tracking system to compensate N-S misalignment errors. The methodology is completely empirical and its application starts simulating the situations under study, with the different N-S misalignments.

Then, for each case, a rotation angle is applied and the lost light is minimised.

This additional rotation angle is in the direction corresponding to the sun tracking for the N-S positioning. The rotation axis of this compensation is the X axis of Fig. 1, representing the horizontal axis of solar trough. The angular correction corresponds to the daily tracking of the sun position. The values of correction angle are experimentally determined maximising the light received by the absorber.

The results of the application of angular correction are reported in Table 2 considering an error of N-S axis misalignment P=10°. The configuration examined in this study includes a linear parabolic mirror of focal length f=800mm. The absorber is a metal pipe of diameter d=50mm, enclosed into a glass tube of diameter D=60mm and T=3mm.

The examined parameter is the light received by the absorber, expressed in percentage with respect to the impinging light. The reference parameter, in Column 2, is represented by the values obtained without N-S misalignment and without angular correction. These quantities should be reached by the values in Column 4, calculated for p=10° and introducing the angular correction. The empirically determined angle corresponding to each correction is reported in Column 3. It has been calculated for every value of sun’s altitude, indicated in Column 1. But probably the most important quantity is the percentage of recovered light in Column 5. It is obtained as ratio between the values in Column 4 and the reference values in Column 2.

A misalignment P=10° in N-S positioning introduces considerable losses that strongly depend on sun’s altitude. The application of a correction angle, using the rotation of the daily sun tracking, recuperates most of missed energy.

It has been verified that this recovering procedure can be successfully applied to the solar parabolic trough under study, considering N-S misalignment angles up to 15°. Over 95% of the lost energy can be recovered using this procedure, compensating the North-South misalignment errors.

Instead of considering the different months, all calculations are performed taking as reference the sun’s altitude over the horizon. Consequently the results can either be associated to the daily sun’s excursion, for North-South positioning, or to the different sun’s positions determined by Latitude and time, for East-West placement.

1. Conclusions

Ray tracing simulations have analysed a solar trough, whose main component is a linear parabolic mirror that concentrates the light over an absorber composed of a metal pipe surrounded by a glass tube. Since solar trough collectors perform sun concentration only in one direction, the sun tracking is realised only around this axis. The trough axis can either be placed parallel to the North - South direction or parallel to the East-West direction. In N-S positioning the tracking system follows the sun in its excursion during the day. In E-W placement the tracking system follows the sun’s altitude variations during the year.

Referring to the N-S positioning, we have preliminarily discussed the effect of sun’s inclination over the one-axis tracking collector. Considering the Latitude of Firenze (Italy) 43.75° N, the sun’s altitude is maximum (a=69°) in June and minimum (a=23°) in December. The simulations have evidences two main causes of energetic losses: on the light received by the collector aperture (IIN) and on the light received by the absorber (IABS). The amount of received light, normalised to the incoming light, results IIN=37% and IABS=16% for a=23° and it improves to IIN=40% and IABS=20% by introducing a mirror at a trough extreme; while for a=69° we obtain IIN=89% and IABS=72% (with lateral mirror IIN=90% and IABS=74%).

The successive study refers to the incorrect placement of trough axis with respect to N-S direction, taking into account misalignment angles up to 15°. The analyses investigate the interactions between collected light, N-S positioning error and sun’s altitude for different configurations of solar trough. For a parabolic mirror with f=800mm and absorber diameter d=50mm, the collection efficiency is strongly affected by errors in N-S axis positioning (parameterised by the misalignment angle P). If a=69° (June in Florence) the collection efficiency remains around 80% up to P=3°, then it decreases to 56% for p=4°. When a=60° (August) it maintains high values (>74%) for P<2°; then it drops to 23% for P=3°. Finally if a=23° (December) it immediately reduces from 43% (P=0°) to 2% (P=1°).

It can be concluded that the energy loss is considerable and it significantly depends on sun’s altitude a. In order to recover the missed light an empirical correction procedure is proposed and it is applied to the solar trough with parabolic profile and cylindrical absorber. For this configuration most of lost energy can be recovered for p <15°. With N-S axis misalignment P=10° the procedure recuperates 98% of lost light for a=20° and 99-100% for higher values of sun’s altitude. Therefore by using the daily tracking system it is possible to compensate the N-S misalignment errors to recover the missing light.

Acknowledgments

The research has been developed in the framework of the S. A.L. T.O. project. S. A.L. T.O. (Solar assisted cooling Toscana) is a research integrated project POR Ob. 3 Toscana 2000/2006 Misura D4, partially financed by the REGIONE TOSCANA-settore Promozione e sostegno della Ricerca (Tuscany Region). Thanks are due to the industrial partners FAIT group and CEVIT for their support to our activities in developing Solar Cooling Systems.

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