NCER Assistance Agreement Final Project Summary

 

Date of Final Report:  March 31, 2007

EPA Agreement Number: X-83254101-0

Center: Center for Environmental and Energy Research (CEER)

Project Title: Robust, Spectrally Selective Ceramic Coatings for Recycled Solar Power Tubes

Investigator(s): William M. Carty and Doreen Edwards

Institution(s) of PI(s): Alfred University

Research Category: Congressionally Mandated Center

Project Period: October 1, 2005 – September 30, 2006

 

Description and Objective of Project: 

Sulfur dioxide (SO2) and nitrogen oxides (NOx) are the primary causes of acid rain.  In the United States, approximately two-thirds of all SO2 and a quarter of all NOx 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.1 

Parabolic trough solar power plants concentrate sunlight onto stainless-steel tubes that contain a heat-transfer oil, which is pumped through a boiler to create steam that drives a turbine and generates electricity.  Each collector tube is coated with a solar-selective coating and is encased in glass tube.  The glass tube is evacuated to minimize conductive and convective heat loss and to prevent oxidation of the solar-selective coating.  A flexible metal bellows and glass-to-metal seal are used to seal the steel tube within the glass tube and to maintain vacuum.  In some tubes, a breach in the vacuum causes the solar-selective coating to oxidize, vaporize, and deposit on the inner surface of the glass tube, which dramatically decreases tube efficiency. 

The main objective of this project was to develop a robust, ceramic coating that can be used to refurbish (recycle) failed collector tubes and to possibly manufacture new tubes so as to circumvent many of the problems found in the current tubes.  In this project, seven coating systems were evaluated as potential candidates for refurbishing failed collector tubes.  These included two commercial black enamel frits, a commercial white frit, and a commercial clear frit combined with four different commercial black pigments.  Coatings ranging in thickness from submicron to tens of microns were applied to flat stainless steel substrates using screen printing and heated to form robust, glass-ceramic coatings.  The coatings were characterized using scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), UV-Vis and IR spectroscopy, and x-ray diffraction (XRD).  Coating properties and performance, in terms of solar absorptivity and selectivity, were correlated to processing variables to define conditions that produced the best coatings.  With additional industrial support, the technology developed in this EPA-supported project was transferred to a small business that applied coatings to 12-foot-long tubes, which will be tested in commercial solar energy generation systems. 

 

Summary of Findings: 

Seven coating systems, listed in Table I, 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 I. Summary of Coating Systems

Frit

Pigment

Mean
Particle Size (mm)

Solids Loading
(volume %)

Pigment volume concentration (%)

Pemco Neo 126

None

NA

30

NA

Pemco U-3101

None

NA

30

NA

Ferro PL214

None

NA

10- 50

NA

Ferro XG-201

Pemco G636

3.8

30

30

Ferro XG-201

Pemco G-595

1.2

30

30

Ferro XG-201

Ferro 10456

2.2

30

5- 30

Ferro XG-201

Ferro F-6331

<0.10

3-30

5 - 60

 

Thick coatings (~20 mm) 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 mm to 24.6 mm.  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, i.e. unity for a perfect absorber. However, thermal emissivity was also high ranging from 0.63 to 0.94 at 450 oC, 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 ~ 20 mm thick, had solar absorptivities ranging from 0.891 to 0.946 compared to an ideal value of 1.0.  The emissivity values (at 450 oC) 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 oC.  Solar absorptivity ranged from 0.889 to 0.922, and thermal emissivity (at 450 oC) 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 mm thick, had a solar absorptivity of 0.867, a thermal emissivity of 0.549 (at 450 oC), and a selectivity value of 1.58.

Coatings prepared with 5% solids loading and a F-6331 pigment volume concentration (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 Òdrop-inÓ 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 oC) leading the 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 oC for 1272 hours.  The optical properties did not change, suggesting that the coatings are stable under the expected operating conditions.

 

Conclusions: 

Moderately selective solar absorbing enamel coatings were developed in this work.  The best coatings were achieved with Ferro XG-201 enamel loaded with F-6331 spinel-type frit (or its equivalent).  As an example, one of the best coatings (prepared with 5 volume % solids and 18 % PVC) exhibited a solar absorptivity of 0.867 and thermal emissivity of 0.549 at 450 oC, leading to a selectivity of 1.58.  The optical properties of coatings developed in this project were not as good as those of coatings on commercial solar-collector tubes, which are typically reported to exhibit >0.95 solar absorptivity and <0.15 thermal emissivity.  Nevertheless, the coatings are expected to perform better than bare stainless steel. The ultimate test of the developed coatings will be their performance in field testing, which is ongoing.

 

References:

  1. Price, H. and Kearney, D., ÒParabolic-Trough Technology Roadmap: A Pathway for Sustained Commercial Development and Deployment of Parabolic-Trough Technology,Ó US Department of Energy, National Renewable Energy Laboratory document, 40 pp., 1999

 

Publications/Presentations:

  1. Harlan J. Brown-Shaklee, ÒSpectral Selectivity of Enamel Coatings for Solar-thermal Conversion,Ó M.S. Thesis, Alfred University, 2006

2.   A publication will be prepared and submitted for publication in Solar Energy Materials in Summer 2007

 

Supplemental Key Words:  spectrally-selective coatings, solar coatings, solar absorptivity, glass-ceramic composite coatings

 

Relevant Web Sites:  http://ceer.alfred.edu/