Impact/Purpose:

Sulfur dioxide (SO₂) and nitrogen oxides (NO_x) are the primary causes of acid rain. In the United States, approximately two-thirds of all SO₂ and a quarter of all NO_x result from the burning of fossil fuels to produce electrical power. The production of electricity using solar energy is an attractive (clean) alternative to burning fossil fuels. However, the cost of solar energy must be competitive with that produced by combustion processes for it to be widely used. The collector tubes used in current concentrating solar power (CSP) trough plants are prone to failure, which represents the single largest performance impact and operation and maintenance costs (Price and Kearney, 1999).

Description:

Seven coating systems, listed in Table 1, were evaluated. Pemco U-3101 and Neo 126 are commercial enamel coatings commonly referred to as ground and cover coats, respectively. Ferro PL214 is a commercially available black enamel coating. Ferro XG-210 is a clear enamel coating that was combined with four different black pigments, all of which are spinel-type powders.

Table 1. Summary of Coating Systems

Thick coatings (~ 20 μm) of the four enamels (Pemco Neo 126, Pemco U-3101, Ferro PL 214, and Ferro XG-201) were deposited on stainless steel substrates and evaluated. The white Pemco Neo 126 was excluded from further consideration because of its low absorption in the visible region. Pemco U-3101 and Ferro XG-201 exhibited similar behavior. Of the two, Ferro XG-201 was selected for further investigation. Ferro PL214, which showed high solar absorption throughout the spectrum, was also selected for additional studies.

Ferro PL214 coatings were deposited on stainless steel substrates using screen-printing inks with different solids loadings, resulting in coatings ranging in thickness from 1.25 μm to 24.6 μm. In general, both solar absorptivity and thermal emissivity increase with increasing solids loading (coating thickness). The solar absorptivity ranged from 0.8 to 0.92, which approaches the level of solar absorptivity desired of solar selective coatings, that is, unity for a perfect absorber. However, thermal emissivity was also high ranging from 0.63 to 0.94 at 450°C, compared to a desired value of zero for a perfect solar selective coating. The PL214 coatings were eliminated from further consideration because they did not show any significant selectivity.

Coatings containing Ferro XG-201 frit and four different spinel-type pigments were printed from inks containing 30 volume percent solids, which in turn contained a pigment volume concentration (PVC) of 30%. The coatings, which were approximately 20 μm thick, had solar absorptivities ranging from 0.891 to 0.946 compared to an ideal value of 1.0. The emissivity values (at 450°C) were substantially higher than desired, ranging from 0.828 to 0.922, compared to an ideal value of zero. The selectivity of the coatings prepared with 10456 and F-6331 pigments was slightly better than those prepared with the other pigments. Thus, these two pigments were selected for additional studies.

The effects of processing temperature, coating thickness, and pigment volume concentration on the optical properties of Ferro XG-201/Ferro 10456 coatings were investigated. Adherent coatings were obtained upon processing at 800–900°C. Solar absorptivity ranged from 0.889 to 0.922, and thermal emissivity (at 450°C) was high, ranging from 0.752 to 0.830. The solar selectivity did not change significantly with temperature or pigment volume concentration, ranging from 1.11 to 1.19.

The small average particle size of F-6331 pigments allowed the printing of thinner enamel coatings than possible with the 10456 frit. Experiments were conducted to determine the effect of solids loading and pigment volume concentration on the optical properties. In general, solar absorptivity increased and selectivity decreased with increasing thickness. Moderate solar selectivity was noted in thin coatings. As an example, coatings with 5% solids loading and 18% PVC were 3.2 μm thick, had a solar absorptivity of 0.867, a thermal emissivity of 0.549 (at 450°C), and a selectivity value of 1.58.

Coatings prepared with 5% solids loading and a F-6331 PVC of 25% were applied to stainless steel tubes in collaboration with a small business using a decal process. The optical properties of the samples prepared using the decal method were comparable to those obtained by screen printing. Additional coatings were also prepared with a “drop-in” substitute for the F-6331 pigment. The alternative pigment resulted in coatings with slightly lower absorptivity (0.892), but also lower emissivity (0.541 at 450°C) leading to a selectivity value of 1.65. The optical properties of coatings prepared with the alternative pigment were measured before and after heating in air at 450°C for 1,272 hours. The optical properties did not change, suggesting that the coatings are stable under the expected operating conditions.

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