15.07.2015   EuroSun2008-5

Evacuated Tube Reliability

After a year of operation several distinct patterns in the development of cracks in the evacuated tubes emerged. One of these involved the production sequence or, equivalently, the fin orientation and the other, the end of the tube where the crack occurred.

Подпись: the second half). Statistically, if one assumes that the entire production run is characterized by the overall fraction of cracked tubes of 0.05865 then the likelihood that the first half of the production run came from such a process is less than 0.3 percent. Moreover, after six years of operation only 3.6 percent of the vertically finned tubes had developed cracks, whereas the horizontally finned tubes continued to develop cracks at a much higher rate. Since the evacuated tubes were essentially hand built, this 3.6 percent failure rate is about what one would expect. The end caps of each end of the evacuated tubes were identical, each consisting of a dish shaped piece of glass and a metal cap bonded to the glass. At the top end a metal tubulation was brazed to the metal cap to provide flow of heated fluid. At the bottom end a metal tubulation was brazed to the metal cap to provide a means to evacuate the tube. Thus, only the top end was subject to both thermal stress (the 155C fluid) and mechanical stress (partial support of the fin and heat transport tube). One might expect the failure rates due to cracking to be higher at the top end of the tube than at the bottom. In fact the opposite occurred. Out of 19 cracked tubes after one year, 7 were cracked at their tops and 12 at their bottoms. Statistically, if one assumes that the true proportion of cracks at the top to be 60 percent, then there is only a 0.1 percent chance that one would observe seven or fewer cracks out of 19 at the top end. Optical Performance Modeling and Experimentation 2.1 Graphical Ray Tracing
Подпись: -80
Подпись: -100 1 1 1 1 1 1 1 1 1 1 -100 -80 -80 -40 -20 0 20 40 80 80 100
Подпись: Fig. 5: Rays Striking the Vertical fin ICPC at a Nominal Angle of 44 Degrees.
Подпись: Fig. 6: Optical Efficiency (Vertical Fin) from Nominal Angles of 15 to 165

Vertical and horizontal tube absorber orientations were produced in the first and second halves of the ICPC tube production run respectively. One year after installation 1.2 percent of the vertical fin orientation tubes and 9.8 percent of the horizontal tubes had developed cracks. This strongly suggests that there were distinct differences in the longevity of the vertically finned tubes versus that of the horizontally finned tubes (or, equivalently, of the first half of the production run versus

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Fig. 4 and 5depict the results of an animated Fig. 10: Rays Striking the Horizontal Fin

graphical ray tracing simulation that has been icpc at a Nominal Angle of 30 Degrees.

designed to investigate the optical perperformance of the ICPC. See Duff, et al [7]. Factors

incorporated are the transmittance of the glass tube, the reflectivity of the reflective surface, the gap between the tube surface and the fin and the absorptance of the fin. The sun ray is simulated as discrete uniform rays over a range of incident angles from 15 degrees to 165 degrees. The rays are followed through the glass envelope, to the reflector and to the absorber fin. The number of rays absorbed is recorded. The collector efficiency graph of Fig. 6 shows the amount of energy absorbed during a typical daytime period.