Characterization of Pyromark 2500 for High-Temperature Solar Receivers

Abstract

Pyromark 2500 is a silicone-based high-temperature paint that has been used on central receivers to increase solar absorptance. The cost, application, curing methods, radiative properties, and absorber efficiency of Pyromark 2500 are presented in this paper for use as a baseline for comparison to high-temperature solar selective absorber coatings currently being developed. The directional solar absorptance was calculated from directional spectral absorptance data, and values for pristine samples of Pyromark 2500 were as high as 0.96–0.97 at near normal incidence angles. At higher irradiance angles (>40°–60°), the solar absorptance decreased. The total hemispherical emittance of Pyromark 2500 was calculated from spectral directional emittance data measured at room temperature and 600°C. The total hemispherical emittance values ranged from ∼0.80–0.89 at surface temperatures ranging from 100°C – 1,000°C. The aging and degradation of Pyromark 2500 with exposure at elevated temperatures were also examined. Previous tests showed that solar receiver panels had to be repainted after three years due to a decrease in solar absorptance to 0.88 at the Solar One central receiver pilot plant. Laboratory studies also showed that exposure of Pyromark 2500 at high temperatures (750°C and higher) resulted in significant decreases in solar absorptance within a few days. However, at 650°C and below, the solar absorptance did not decrease appreciably after several thousand hours of testing. Finally, the absorber efficiency of Pyromark 2500 was determined as a function of temperature and irradiance using the calculated solar absorptance and emittance values presented in this paper.

Levelized Cost of Coating (LCOC) for selective absorber materials

“…The annual thermal energy absorbed was assumed to be affected by the collector efficiency, receiver efficiency (which includes the selective absorber efficiency), and parasitic losses. The degradation rates and costs associated with materials, application, reapplication, downtime, and degradation are based on available data from Solar One [1], Ho et al [6], and consultation with engineers from Babcock & Wilcox (Table 2). Table 2) assuming Q=600 kW/m 2 and T=700°C…”

“…Table 3 shows the input parameter uncertainty distributions used in the analysis. The nominal values were based on Pyromark 2500 and available data from Solar One [1], Ho et al [6], and information from other power tower applications. The uncertainty distributions about the nominal values were assumed to be uniform with minimum and maximum values based on available data (e.g., radiative properties of candidate materials being developed for this project) and professional judgment.…”

“…A significant amount of effort and studies have focused on the development of high-temperature solar selective coatings with high solar absorptance and low thermal emittance [3][4][5][6][7]. Typically, the metric used for selective absorber coatings is based on the net absorptance of energy relative to that of an ideal absorber.…”

“…Pyromark 2500 is relatively inexpensive, easy to apply, and has a measured solar absorptance of 0.96 (new) [6]. However, with a thermal emittance of 0.87 it suffers from large thermal losses during high temperature operation.…”

“…Pyromark® Series 2500 high temperature paint has been used on previous CSP central receivers ( Figure 2) and is considered a standard [1,2,6]. Pyromark 2500 is relatively inexpensive, easy to apply, and has a measured solar absorptance of 0.96 (new) [6].…”