Comparing abrasion testing results on AR-coatings for solar receivers

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ScienceDirect Energy Procedia 00 (2015) 000–000 www.elsevier.com/locate/procedia

International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2014

Comparing abrasion testing results on AR-coatings for solar receivers T. Chiarappaa,*, F. Biancifioria, A. Moralesb, G. San Vicenteb and S. Santia Archimede Solar Energy, voc. Flaminia Vetus, 88 Fraz. Villa San Faustino 06056 - Massa Martana (PG) – Italy b Ciemat-PSA: Unidad de Concentración Solar, Avd. Complutense, 40 – 28040 Madrid - Spain

a

Abstract This paper aims to shed lights on specific ageing test methodologies, namely those related to investigate the durability of the AntiReflective (AR) layer coating the glass envelope of evacuated solar receivers, key component for linear focusing concentrating system. Due to the absence of dedicated norms, a bunch of different approaches have been proposed and developed, still not leading however, to a uniquely accepted laboratory test to be correlated to real plant operative conditions. As a consequence, the ageing test results are usually limited to material comparison only. This article is hence focused on demonstrating the dependencies of abrasion test on several variables, as coating process, samples geometry and investigation setup. Moreover, it will be argued the subtle involvements that might lead to a data misinterpretation. The final goal is hence to stimulate and possibly sketch a recommended best practice for ageing test investigation dedicated to solar glass coated with a suitable AntiReflective layer. © 2015 The Authors. Published by Elsevier Ltd. Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG. Keywords: Parabolic Trough; Linear Fresnel; Solar Receiver; Ageing Test

1. Introduction Solar radiation provides the Earth with a huge source of energy and therefore a feasible and “clean” alternative to fossil fuels. The exploitation of solar energy in widely extended plants with large dispatchability of energy leads to

* Corresponding author. Tel.: +39-075-8954-723; fax: +39-075-8954-823 E-mail address: [email protected] 1876-6102 © 2015 The Authors. Published by Elsevier Ltd. Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG.

T. Chiarappa et al/ Energy Procedia 00 (2015) 000–000

technological developments among which CSP (Concentrated Solar Power) represents one of the most known, widely distributed and mature technologies; Solar plant, power block and, most important, energy storage have been subject of intense investigations and large improvements over the last years. Within CSP applications, linear focusing systems are competing with tower technology in terms of costs, benefits and efficiency. An illustration of Parabolic Trough and Linear Fresnel system is provided in Fig. 1:

Fig. 1. Line focusing systems: Parabolic Trough (left) and Linear Fresnel (right).

The mirrors characterizing the linear focusing system concentrates the solar rays towards a focal line, where a row of solar receivers are placed. Fig. 2 provides a pictorial illustration of a solar receiver:

Fig. 2: solar receiver and its drawing with focus on the glass surface almost completely coated with an AntiReflective, AR, layer.

The solar receiver is designed to absorb the concentrate radiation and transfer it to a fluid flowing through the pipes, yielding hence a fluid at high temperature. Finally, thermal energy is transformed into electrical energy by mean of an auxiliary heat exchanger, a steam generator and a steam turbine. The desired properties of the solar receivers can be achieved by optimizing its optical efficiency: the designed suitable parameters, maximum absorptance and minimum emittance, are obtained by coating the stainless steel tube (the internal pipe, as sketched in the drawing of Fig. 2) with a spectrally selective thin film. A coaxial glass envelope (join to the steel pipe via glass-to-metal seals and bellows) is devoted to maintain a designed vacuum pressure in order to reduce thermal losses to radiative phenomena only. On the other hand, glass should not obstacle the concentrated solar rays pointing towards the inner pipe: to this aim, the glass envelop is coated itself with an AR, whose valuable observable is the transmittance, τ. In view of a durability estimation, this paper presents several subtleties that might affect the outcomes of abrasion test carried on the AR coated glass. The absence of norms and standards dedicated to solar receiver in CSP does not allow the matching between real field operative condition and laboratory experiments, promoting as mandatory a rigorous analysis of the effects involved by the experimental setup (imposed degradation phenomena) in order to estimate the degradation of the target performance of the solar receiver over its expected lifetime. In section 2 the coating is shortly described in terms of the transmittance increase thanks to the chosen AR solution and to the deposition technology adopted, dip-coating. Finally, that section discusses the measurement technique, focusing on the importance to achieve repeatability and full control over experimental errors. The abrasion test is described in section 3, quantitatively highlighting the involvements of the experimental setup. Section 4 presents the investigation outcomes, discussing the dependencies of the results on several setup parameters and proposing some best recommendations for this kind of similar ageing test.


2. Antireflective properties and dip-coating on glass The glass envelope of a solar receiver, see right drawing of Fig. 2, has to allow the concentrated sun rays to reach the inner stainless steel pipe; to this aim, a sol-gel polymeric solution has been developed and characterized by Ciemat (as patented in [1]) in order to act as an antireflective layer and hence increase the transmittance of the glass tube. Fig. 3 illustrates the optical benefits of this AR coating when compared to an uncoated, “bare”, glass sample:

Fig. 3: transmittance of bare glass and of AR-coated glass as a function of the radiation wavelength: for completeness, the solar spectrum curve has been superposed

The AR coating consists in a silica-based film deposited on top of each (interior and exterior) glass surface which leads to a total transmittance increase of about 5.0%. The deposition facility installed in Archimede Solar Energy is based on the well-known dip-coating technology which, among other advantages, allows a very strict control on the parameters temperature, viscosity and velocity. Dip-coating permits a highly repeatable process which is confirmed by the full agreement between the transmittance results obtained from samples deposited and measured in a laboratory and those processed in a modern industrial plant. Table 1 illustrates the solar weighted τ values for three samples, each measured three times. It should be noted that the measurement errors (statistical and systematic) are always below 1%; moreover, the conclusions drawn from the comparison presented in Table 1 are valid both for flat- as well as for curved- samples. Table 1. Solar weighted transmittance values, τ(sw), of AR coated glass samples deposited in the laboratory and in the industrial plant. Samples

Laboratory

Industrial Plant

τ(sw)

τ(sw)

1

0.971

0.973

2

0.970

0.972

3

0.972

0.973

Transmittance measurements have been carried on by mean of a spectrophotometer (Perkin-Elmer Lambda 950) equipped with an integrated sphere of 150mm in the wavelength range 300 – 2500nm (other measurements starts at 280, but for glass transmittance the impact is negligible) [2]. In accordance to ASME Standard 903-82 (92), the solar weighted transmittance τ(sw), is calculated by weighting the spectral transmittance, τ(λi), with the solar direct irradiance at the Earth surface, Edir, i, for each wavelength, λi, as given by the following Eq.1:


     E    sw   E   n

i 1

i

dir,i

i

n

i 1

dir,i

i

;

 i  

I s  ZLi  I a  ZLi 

(1)

with Is being the measured detector intensity, Ia the measured intensity in air and ZL(λ) the ZeroLine suitable for noise correction. Solar irradiance spectra have been chosen according to ASTM G173-03 for air mass 1.5. 3. Design of Experiment, DoE The polymeric solution realizes a porous structure on top of each glass surface, allowing the transmittance of solar glass to be increased by almost 5%. As a matter of fact, this physical layer could be subject to several degradation mechanisms when the receiver is exposed to outdoor atmospheric conditions [3], as for instance mechanical abrasion due to sand erosion, receiver cleaning [4] or the combination of dust and wind. Market available standard equipment for abrasion test, as that illustrated in Fig. 4 where the abrasive medium is inserted into the bottom side of the vertical abrading cylinder, are suitable for investigation of the resistance against mechanical stresses. For the scope of evaluate the robustness of the AR-coated solar glass, it should be stressed again that, for the time being, no correlation function to outdoor exposure mechanisms has ever been proved.

Fig. 4: Abrasion test facility used to perform the investigation discussed in the paper.

The mentioned absence of correlation to outdoor conditions does not fix boundaries for the values of the variables characterising the experimental setup, leading hence to results potentially not representative of the present CSP needs. Our Design of Experiment, DoE, has therefore been structured to investigate and evaluate the potentially damaging effects involved by different setting of the abrasion test equipment. Our collaboration has been focused on the following guidelines:  Reliability check of the transmittance, τ, measurements on the tested samples (flat and/or curved);  Disentangling the variable contributions, as for instance the geometrical properties of the abrasive medium;  Verification of the results for samples deposited in laboratory and in a modern industrial facility; In the framework of considering the abrasion test on AR-coated glass as an un-normed accelerated ageing, it is mandatory to clearly characterise and define the background of the whole experiment, what in the following is called Point Zero Configuration (PZC): an initial investigation of the characteristics of- and the effects induced bythe abrasive medium, in order to guarantee both a scientifically valid transmittance measurement as well as a reference result against which compare the mechanical resistance of the AR-coated glass. It should be self-evident that the transmittance, τ, has to be measured on a homogeneous surface, in order to obtain a comparable and scientifically reliable measured value. The left and right pictures of Fig. 5 illustrates respectively the comparison between tested surface conditions before and after cleaning (ethanol and a cotton pad),


needed to remove possible abradant remnants. The two samples in each picture results out of 10 abrasion cycles by using, left to right, two different abradants of the same diameter, 6mm: MIL E12397B and CS-10F from Taber, the latter clearly producing visible scratches.

Fig. 5: Comparison of samples undergone abrasion with different abradant before (left foto) and after (right foto) a dedicated cleaning procedure.

The right picture clearly demonstrates that the CS-10F is abrading the sample non-homogeneously. This conclusion is confirmed by the microscope analysis presented in Fig. 6, comparing the clean sample surface finishing after 10 abrasion cycles produced by the MIL E12397B (top pictures) and the CS-10F (bottom pictures). The pictures of each row refer to a 5X- magnification (central and edge abraded zone) and a 20X- magnification (on the central part of the sample), clearly demonstrating the different aggressiveness of the two abrasive medium.

Fig. 6: Left: two 5X microscope images spanning the abraded zone (edge and central region) for MIL E12397B, top, and CS-10F, bottom. Right: 20X magnification pictures of the central part of the samples.


It could be hence concluded that the abradant CS-10F leads to an inhomogeneous surface, which will therefore turn into a hardly representative measurement of solar weighted transmittance. Another available abrasive medium is the H-18, which however turn to be even more aggressive as demonstrated by Fig. 7, where the τ spectra of bare glass is compared to that of samples tested with H-18 and MIL E12397B:

Fig. 7: Transmittance of glass samples undergone: no-cycle, 150 cycles with MIL E12397B and 30 cycles with H-18 abradant.

The almost perfect overlapping between the transmittance of the bare glass and that of a sample undergone 150 cycles with abradant MIL E12397B proves that no degradation phenomena is induced by this abrasive medium. On the contrary, the abradant H-18 strongly scratches the bare glass sample already after 30 cycles, leading to drastically reduced transmittance and being hence classified as non-suitable to testing AR-coated glass. On top of the above consideration, abradant supplier [5] has been inquired for the feasibility of testing both on flat as well as on curved samples: within three different available abrasive medium diameters (¼”, ½” and ¾”) the manufacturer recommends only the use of ¼” for curved samples (solar receiver glass envelop of diameter 125mm) to get reproducible and reliable results. Finally, it is worth to stress again that only solar plant conditions could provide a criterion to choose the proper abradant to be used in laboratory experiments. Nevertheless, the characterisation of the PZC (Point Zero Configuration) is considered to be an undeniable request in order to scientifically investigate AR-coated glass. The reference transmittance values against which evaluate the τ losses along with the cycles is provided in Table 1. 4. Results The conclusions drawn from the previous section, led us to promote the following configuration as a comparative reference setting: 6mm diameter MIL E12397B, total weight of 350gr and cycling frequency of 7 cycles/min. While real plant operating condition could have not been yet reproduced, a comparison between some abrasion test configurations has been carried on, demonstrating the smooth transmittance decrease of the AR-coated samples and the independence from the deposition methodology.


4.1. Flat vs Curved Abrasion in parabolic trough solar plants affects a cylindrical glass envelope; on the other hand, laboratory test achieves the most precise results on flat samples. A correlation between the two geometries would be very welcome. Three flat- and three curved- samples had undergone an abrasion cycling test. Table 2 illustrates the solar weighted transmittance evaluated after 5, 10, 20, 40 and 60 cycles: the mean value and the standard deviation over three (flat or curved) different samples clearly indicate the highly reproducibility and accuracy of the experiment. Table 2. Solar weighted transmittance τ(sw) (%) decrease along the cycling Number of cycles 0

5

10

20

40

60

Flat

97.0 ± 0.1

96.5 ± 0.4

96.0 ± 0.3

95.4 ± 0.6

94.7 ± 0.3

94.6 ± 0.1

Curved

97.4 ± 0.1

96.6 ± 0.3

95.5 ± 0.2

94.9 ± 0.4

94.5 ± 0.6

94.0 ± 0.1

Fig. 8 illustrates on the left the variation of the transmittance spectra and on the right the solar weighted transmittance τ(sw) decrease both as a function of the number of cycles. In addition, the right plot demonstrates the almost identical behaviour for samples deposited in the laboratory and those deposited in a modern industrial plant. Observing the plots, it can be concluded that the complete removal of the AR coating happens after 60 cycles.

Fig. 8: Left: transmittance spectra variation after tens of cycles. Right: solar weighted transmittance degradation during the cycling.

The difference in the ultimate τ(sw) between flat and curved samples (right plot of Fig. 8) is traced back to the properties of their background bare glass. Nevertheless, the abrasion test on flat and curved samples show very similar degradation behaviour, allowing thus to extend the results related to flat samples also to curved ones, hence to infer conclusions for solar receivers to be installed on solar field. The error bars are within symbol dimension. 4.2. Effect of the weight The pressure of the abrasive medium on the sample has been increasing as it is expected to speed up the ARcoating degradation. Applying an extra load to get a total mass of 1000gr to the abrasion equipment and decreasing the frequency to 3.3 cycles/minute, a faster degradation has been observed as reported in the following Table 3: Table 3. Solar transmittance τ(sw) (%) decrease for heavier mass applied on the abradant Number of cycles

Flat

0

2.5

5

10

15

97.1 ± 0.1

96.0 ± 0.2

95.4 ± 0.2

95.0 ± 0.3

94.8 ± 0.2


As microscope pictures have not been taken, a clear evidence of unwanted scratches (those which are induced by a wrong setting, exaggerate mass of the abrasive medium in this case) could have not been explicitly demonstrate. This has been compensated by increasing up to 10 the number of sample on which the test has been carried on: the values of the standard deviation are perfectly comparable to those of Table 2, indicating the results being meaningful and not the signal of an uncontrolled contribution due to a wrong configuration setting. 4.3. Cleaning procedures and related effects The subtle dependencies of the transmittance measurements of tested samples on the cleaning technique have been already anticipated in section 3. The definition of a cleaning recipe is strongly dependent on the particular field of application and can easily lead to mistaken data thus to potentially wrong conclusions. In this subsection, some more evidence is provided in order to propose the recommended cleaning procedure to be applied to the preparation of samples soon before transmittance measurements. Fig. 9 proposes a comparison between the microscope pictures related to two different cleaning techniques when applied to samples undergone 1and 25- abrasive cycles, respectively left and right:

Fig. 9: comparison between different cleaning procedures (top vs bottom) for a single abrasive cycle (left) and a run over 25 cycles (right).

The so called “partial cleaning” is based only on water cleaning and drying with pressurized air; while the so called “full cleaning” consists out of the following steps: immersion in ethanol, wiping with a clear soft tissue and drying with pressurized air. As cleaning is not enhancing the quality of the AR-coating, it can be concluded that the top pictures of Fig. 9 are dominated by dust and abrasive medium residuals, agglomerations that can easily alter the τ measurement results. For the time being, the so called “full cleaning” will be referred to a best practice procedure. 4.4. Geometrical effects: abrader diameter Finally, an alternative concurrent test campaign has been carried on changing several parameters: diameter up to 19mm, cycling frequency to 25 cycles/min, total mass reduced to 295gr and abradant type (Weardisc CS-10 F). To get rid of larger diameter abrasive medium applied to curved samples, an extra adaptive tooling has been designed for the test to be possible [6]. The effects due to frequency increase are expected to be negligible. Three curved samples have undergone an abrasion test over a larger number of cycles until the AR-coating was completely removed. The resulting data are plotted in Fig. 10 where the almost constant plateau (τ(sw)=94.5% as the


AR is coating both sides of the samples) achieved above 150 cycles, clearly indicates that the configuration setting has led to a meaningful test, not excessively aggressive to scratch the underlying bare glass too.

Fig. 10: Behaviour of the progressive transmittance loss over up to 300 cycles.

Both plots of Fig. 10 demonstrate for the three curved samples an almost complete antireflective coating loss by 150 cycles, although the absolute value and the final still decreasing behaviour of Sample 3 remain to be fully understood. The following Table 4 provides the statistical analysis of the data illustrated in the plots above: Table 4. Solar weighted transmittance τ(sw) (%) decrease along the cycling Number of cycles

Curved

0

50

100

150

300

96.8 ± 0.0

95.8 ± 0.3

95.0 ± 0.5

94.8 ± 0.7

94.7 ± 0.5

Compared to the error bars of the reference samples, see Table 2, the standard deviations of the measured τ provided in Table 4 are slightly larger, due to the particular behaviour of Sample 3. The microscope analysis depicted in Fig. 11, confirms at first glance the conclusion drawn or the AR removal:

Fig. 11: microscope pictures illustrating the induced scratches on the central area of the AR-coating samples after 0-, 50-, 100- and 150- cycles.

Nevertheless, a successive dedicated investigation has revealed a significant difference between the pictures taken from the edge and from the centre of the abraded sample, as depicted in Fig. 12 (edge of the abraded area):

Fig. 12: microscope pictures taken from the edge of the abraded area of the AR-coating samples after 0-, 50-, 100- and 150- cycles.


The pictures composing Fig. 12 demonstrate that the AR-coating glass at the edge of the abraded area is not completely lost after 150 cycles. It can be concluded that the facility configuration setup and more precisely the large diameter of the abrasive medium (19mm), led to non-homogeneously abraded area, hence are not suitable for unambiguous and precise investigation campaign of a transmittance loss. It has been argued (not extensively demonstrated) that this conclusion could represent the explanation for the anomalous behaviour of Sample 3 in Fig. 10. This evidence would indicate a poor repeatability of the transmittance measurement technique since the samples are supposed to be identical. 5. Conclusions Abrasive test can be carried on in order to verify the mechanical resistance of an AntiReflective coated glass surface. Similar experiments are designed to investigate the durability of the optical properties characterising solar receivers in CSP. However the absence of suitable correlation function to outdoor conditions does not provide constraints for the test configuration setup, leading to results potentially not representative for present CSP needs. In this paper a rigorous method has been followed to estimate the validity of some setup configuration, disentangle the variable effects and indicating the condition under which the test can be considered valuable: the figure of merit to accept/reject the test has been chosen to be the reproducibility and homogeneousness of the test in terms of the measured solar transmittance, τ. The investigation of the so called Point Zero Configuration is therefore of primarily importance in order to define a reference initial state, analysing the outcomes of an experimental configuration setup on flat bare glass samples. It has been demonstrated the impact of several abrasive mediums (MIL E12397B , CS-10F and H-18) indicating the MIL E as the proper abradant to be used in the investigation. The extra load applied on the abrasive medium (up to 1000gr) has been proved to be applicable: in first approximation the τ behaviour along the cycling shows a linear dependence on the loaded mass, as can be argued by comparing the data reported in Table 2 and in Table 3. For the transmittance measurement to be reliable, cleaning procedures have been investigated and a suitable best practice has been indicated. In the final stage, the effects induced by wide diameter abrasive medium, up to 19mm, have been discussed: the drawn conclusion reflects the supplier recommendation, namely that when applied on cylindrical surface typical of solar receivers (diameter 125mm), ¼” abradant are more suitable than those with larger abrasive surface, the latter leading to an inhomogeneous scrubbed area on the sample. It is important to clearly stress again that the investigation is not intended to validate the absolute AR-coating resistance (cycle at which the AR-coating is completely removed), rather than to put emphasis on the methodology and possibly to the best practice to follow in order to get a safe and reliable test for material comparison.

References [1] Ciemat patent application number 02380215, Publication number EP 1329433, title: “Solgel process of porous coatings using precursor solution prepared by polymeric reactions” [2] Reflectance Measurements guidelines v2.5. TASK III. SOLARPACES. http://www.solarpaces.org/tasks/task-iii-solar-technology-andadvanced-applications/reflectance-measurement-guideline [3] Ye H., Zhang X., Zhang Y., Ye L., Xiao B., Lv H., Jiang B. Solar Energy Materials & Solar Cells. 2001;95. p. 2347-2351 [4] Arntzen M., Dreyer S., Specht J., Kuckelkorn T., Schimidt S., Sauerborn A. Proceedings of SolarPACES 2011 [5] http://www.taberindustries.com/linear-abradants [6] Pernpeintner J. et al, “Accelerated ageing of parabolic trough receivers – Overheating of the absorber coating, thermal cycling, bellow fatigue and AR-coating abrasion“, to be presented at the SolarPACES 2014