Novel ICPC

After a year and a half of operation there were two distinct glass crack development patterns. One was related to the production sequence and the other to the end of the tube where the crack occurred.

Tube End: The end caps shown in figure 4 were used on both ends of the evacuated tubes. They consisted of a dish shaped piece of glass and a metal cup bonded to the glass using a housekeeper seal.

12mm metal pipes were brazed to the metal cups to provide flow of heated fluid at the top end and as a means to evacuate and tip off the tube at the bottom end. 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). Therefore, one might expect failure rates due to cracking of the glass 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 located at the top out of a total of 19. Thus, the evidence supports that there is no difference in quality due to absorber orientation and that the higher operating temperatures do not cause higher rates glass cracking.

Production Sequence: One and a half years after installation, 1.2 percent of evacuated tubes in the first half of the production run versus 9.8 percent of the tubes in the second half had developed cracks. This strongly suggests distinct differences in quality between the first half of the production run and the second half. Statistically, assuming that the entire production run is characterized by the overall fraction of cracked tubes of 0.059, the likelihood that the first half of the production run coming from such a process is less than 0.3 percent. Moreover, after six years of operation only 3.6 percent of the original vertically finned tubes had developed cracks, whereas the horizontally finned tubes continued to develop cracks at a much higher rate. Since fabrication methods typically trend toward higher quality (lower failures during operation), the first part of the production run seems to be a better baseline for expected manufactured tube characteristics.