The effect of vitrification temperature upon the solar average absorptance properties of Pyromark Series 2500 black paint

ABSTRACT:
A significant drop in production efficiency has occurred over time at the Solar One facility at Barstow, California, primarily as a result of the degradation of the Pyromark Series 2500 black paint used as the absorptive coating on the receiver panels. As part of the investigation of the

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problem, the solar-averaged absorptance properties of the paint were determined as a function of vitrification temperature, since it is known that a significant amount of the panel surface area at Solar One was vitrified at temperatures below those recommended by the paint manufacturer (54OoC, 1000'F). Painted samples initially vitrified at 230'C (450°F), 315'C (600°F), 371OC (700°F), and 480'C (900'F) exhibited significantly lower solar-averaged absorptance values (0.02 absorptance units) compared to samples vitrified at 540T (1000'F). Thus, Solar One began its service life below optimal levels. After 140 h of thermal aging at 37OOC (700'F) and 540'C ( 1 0 0 0 O F ) , all samples, regardless of their initial vitrification temperatures, attained the same solar-averaged absorptance value (as = 0.973). Therefore, both the long-term low-temperature vitrification and the short-term high-temperature vitrification can be used to obtain optimal or near-optimal absorptance of solar flux. Further thermal aging of vitrified samples did not result in paint degradation, clearly indicating that high solar flux is required to produce this phenomenon. The panels at Solar One never achieved optimal absorptance because their exposure to high solar flux negated the effect of long-term low-temperature vitrification during operation. On future central receiver projects, every effort should be made to properly vitrify the Pyromark coating before its exposure to high flux conditions.

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