accelerated aging of a solar absorber material subjected to highly concentrated solar flux

Available online at www.sciencedirect.com ScienceDirect Energy Procedia 49 (2014) 1673 – 1681 SolarPACES 2013 Accelerated aging of a solar absorber material subjected to highly concentrated solar flux A Boubaulta*, B Claudetb, O Faugerouxb, G Olaldea a Processes, Materials, and Solar Energy laboratory (PROMES-CNRS), rue du Four Solaire, Odeillo, 66120 Font-Romeu, France Processes, Materials, and Solar Energy laboratory (PROMES-CNRS), Université de Perpignan-Via Domitia, 52 av Paul Alduy 66860 Perpignan, France b Abstract To support the fast growth of Concentrating Solar Power (CSP) technologies, R&D efforts need to be continued to minimize costs Reducing manufacturing, processing and maintenance costs is key to reach a competitive low production price for electricity From this perspective, one issue that becomes increasingly significant is the service lifetime of solar absorber materials In solar tower receivers, absorber materials are subjected to very intense solar flux and cyclic thermal stresses The chemical and physical aging mechanisms such as corrosion and oxidation, or the possible apparition of cracks or delamination are responsible for the change of the material’s surface and bulk properties over time This causes the thermal performance of the material to decline more or less rapidly Taking a two-layer solar absorber material (metal + coating) that is commonly used in power tower receivers (SOLAR TWO, GEMASOLAR, SOLHYCO), we have designed several accelerated aging tests by analyzing the thermal behavior of the material in permanent and variable regimes The experimental tests consist in subjecting the material to cyclic irradiance of variable intensity, amplitude and period to determine the fastest and most realistic tests A Solar Accelerated Aging Facility (SAAF) was built for this purpose It is made of a 2-meter diameter parabola concentrating the solar radiation (up to 16,000 times) onto a material sample that is cooled by air or water in direct contact with its rear face A flux sensor coupled to a shutter allows us to apply the desired solar irradiance while an infrared pyrometer monitors surface temperature of the sample Following different constant-irradiance solar aging treatments, the properties that are characteristic of the aging are estimated The normal solar absorptance of each sample is evaluated by a solar optical fiber reflectometer The thermophysical properties of the paint such as the thermal diffusivity, effusivity, conductivity and the thermal contact resistance between the paint layer and the metal substrate are estimated by inverse method using the measurements of an impulse photothermal experiment In addition, untreated samples are characterized in order to provide rare data on the 'not aged' material *Corresponding author Tel.: +33-468-307-734 E-mail address: antoine.boubault@promes.cnrs.fr 1876-6102 © 2013 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Selection and peer review by the scientific conference committee of SolarPACES 2013 under responsibility of PSE AG Final manuscript published as received without editorial corrections doi:10.1016/j.egypro.2014.03.176 1674 A Boubault et al / Energy Procedia 49 (2014) 1673 – 1681 Careful attention should be paid to the preliminary vitrification heat treatment carried out to give the black paint its optimal absorption properties before being used The results of the aging tests show a slight deterioration of the properties corresponding to the first aging signs of the material The solar absorptance and the thermal effusivity of the paint coating are the most affected properties during the tests © 2013 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license © 2013 The Authors Published by Elsevier Ltd (http://creativecommons.org/licenses/by-nc-nd/3.0/) Selectionand andpeer peerreview review scientific conference committee of SolarPACES 2013responsibility under responsibility of PSE AG Selection byby thethe scientific conference committee of SolarPACES 2013 under of PSE AG Final manuscript published as received without editorial corrections Keywords: CSP, solar, durability, aging, receiver, absorber, coating, paint, absorptance, diffusivity, effusivity, thermal contact resistance Introduction Solar power tower receivers are exposed to highly-concentrated solar flux Solar radiation is absorbed at their surface and transferred as heat to the heat transfer fluid Their efficiency is a key parameter of the global power plant efficiency The materials used in solar receivers have to withstand the strong degradation mechanisms that occur in extreme conditions of radiation and temperature Their thermal performance generally decreases until they need to be restored or replaced The aging behavior of materials that are exposed to high solar flux is not well known It is necessary to study the durability of these materials by performing accelerated aging tests Absorber tubes are the most common receiver design for commercial solar power tower (SOLAR TWO, GEMASOLAR, IVANPAH) They are generally composed of a metallic alloy coated with an absorbing paint Exposed to the concentrated solar flux, the fluid temperature usually reaches 700 K The solar receiver prototype SOLHYCO based in the Plataforma Solar de Almeria (PSA), Spain, aims to reach higher working fluid temperatures of up to 1073 K This receiver is made of Inconel 625 tubes coated with Pyromark 2500 black paint The aging of this two-layer material is investigated We first built a numerical thermal model in order to find specific thermal and radiative conditions in which material aging is enhanced A Solar Accelerated Aging Facility (SAAF) is developed and used to perform accelerated aging tests at high constant solar irradiance for several exposure times Aging is defined by the evolution of characteristic properties that have influence on the thermal performance of the material The model allowed us to identify these properties as the solar absorptance, the thermal conductivity, and the thermal contact resistance between the coating and the metallic substrate These properties need to be monitored during aging tests as they characterize the aging state of the material A solar optical fiber reflectometer is used to estimate the solar absorptance, and an impulse photothermal method is used to estimate the thermal conductivity and the thermal contact resistance of each solar-treated sample Nomenclature a thermal diffusivity, m2.s−1 b thermal effusivity, W.m−2.K−1.s−1/2 Rtc thermal contact resistance, m2.K.W1 te exposure time, s Greeks D absorptance M solar irradiance, W.m2 A Boubault et al / Energy Procedia 49 (2014) 1673 – 1681 λ 1675 thermal conductivity, W.m−1.K−1 Indices i incident p relative to paint S solar Exponents ˆ hemispherical A normal Material SOLHYCO is a high temperature solar receiver prototype developed by the Deutschen Zentrums für Luft- und Raumfahrt (DLR) at the Plataforma Solar de Almeria (PSA) [1] It is depicted in Figure The solar radiation is absorbed by Inconel 625 tubes coated with a high-temperature absorbing black paint called Pyromark 2500 Pressurized air at 4.5 bar is heated up from 873 K to 1073 K in order to power a 100 kW turbine concentrated solar flux Figure SOLHYCO receiver with metallic absorber tubes coated with black paint (credit : P Heller [2]) The absorber tubes are 1mm-thick, and the paint coating is 15 microns-thick This two-layer material can withstand temperatures of up to 1300 K and is relatively stable at high temperatures [3] To investigate the aging dynamic of this material, experimental tests are carried out on flat rather than tubular samples It enables a better control of the radiative and thermal conditions of the material, as well as easier and more reliable material characterizations Thus, 50 mm u 50 mm square pieces of Inconel 625 are painted with Pyromark 2500 The thickness of the coating is controlled by Scanning Electron Microscopy (SEM) 1676 A Boubault et al / Energy Procedia 49 (2014) 1673 – 1681 Before being exposed to accelerated aging conditions, the samples need to undergo a vitrification heat treatment The treatment consists in putting them in an electric oven where they are cured hours at 522 K then heated up at 811 K then held 15 minutes at this temperature Finally, the heating is stopped and the samples are allowed to cool down slowly According to the paint manufacturer (LA-CO Industries, ex TEMPIL), the vitrification treatment enables the paint coating to reach a nominal normal solar absorptance of 0.964 This value is experimentally verified by measuring the normal solar absorptance before accelerated aging solar treatments (paragraph 5) Aging solar treatments A Solar Accelerated Aging Facility (SAAF) was developed at PROMES laboratory (Odeillo, France) [4] The facility is depicted in Figure parabola radiative flux sensor kaleidoscope sample test-bed shutter heliostat Figure Solar Accelerated Aging Facility (SAAF) Solar energy is concentrated by a parabola A rotating blade shutter controls the amount of incident solar flux A radiative flux sensor coupled with the shutter enables us to apply either constant or cyclic solar irradiation thanks to a Proportional-Integral-Derivative (PID) regulator A 20 mm u 20 mm u 60 mm kaleidoscope stands to homogenize the concentrated solar flux that hits the surface of the sample The sample is held in a ceramic test-bed in which flows the heat transfer fluid In this case, pressurized air at bar is used The samples are treated with the SAAF Each one is exposed to one constant irradiance level and one exposure time Tests are performed for five uniform incident irradiances Mi (52 kW.m2, 104 kW.m2, 173 kW.m2, 346 kW.m2, and 692 kW.m2) and two exposure times te (1000 s and 3000 s) There is a linear relation between the surface temperature and the incident solar irradiance [5]: T with T in K and Mi in W.m−2 1.176 u10 3 M i  293 (eq 1) A Boubault et al / Energy Procedia 49 (2014) 1673 – 1681 1677 Material characterization 4.1 Solar optical fiber reflectometer A solar optical fiber reflectometer called DISCO is used to estimate the normal solar absorptance of the samples [6] An optical fiber sends a solar radiation beam perpendicularly to the surface of the sample The reflected flux is collected by a series of seven receiving optical fibers that are distributed every ten degrees (see Figure 3) Measurement area Receiving fibers Emitting fiber Figure Estimation of the directional normal solar reflectance distribution By integrating the directional normal solar reflectance distribution over the whole hemisphere, it is possible to estimate the hemispherical normal reflectance in the solar spectrum, termed USAˆ Thus, the normal solar absorptance αSAcan be calculated with the formula: DS A 1 US Aˆ (eq 2) 4.2 Impulse photothermal method ('Flash method') The thermal conductivity of the coating as well as the thermal contact resistance at the interface play an important role in the thermal performance of the absorber material [4] One way to estimate their value is to use a photothermal method coupled with an inverse method [7] Here the front face flash method is used A picture of the optical bench is shown in Figure 1678 A Boubault et al / Energy Procedia 49 (2014) 1673 – 1681 Figure Optical bench of the impulse photothermal experiment A laser impulse hits front face of the material (coated surface) The temperature increases instantaneously before decreasing An infrared detector measures the thermal response of the surface This response is compared with an analytical model, which enables the identification of the thermophysical properties of the coating [5] Results Unexposed samples are first processed with the two characterization devices The average values for the normal solar absorptance, thermal diffusivity, effusivity, and conductivity of the paint coating, as well as the thermal contact resistance between the paint coating and the metal substrate are given below These values are taken as reference or 'initial' values to be compared with those of aged samples αSA = 0.968 ap = 5.08 u 106 m2.s−1 bp = 2.78u 103 J.m−2.K−1.s−1/2 λp = 6.26 W.m−1.K−1 Rtc = 2.57 u 106 m2.K.W−1 Subsequently, the aged samples are characterized Figure 5, Figure 6, Figure 7, Figure 8, and Figure 9, show the values of the normal solar absorptance, thermal diffusivity, thermal effusivity, thermal conductivity and the thermal contact resistance for each solar treatment at different irradiances and exposure times The error bars show the estimation uncertainty for each measurement 1679 A Boubault et al / Energy Procedia 49 (2014) 1673 – 1681 0.980 0.975 0.970 αS┴ (-) te = 1000 s 0.965 te = 3000 s 0.960 0.955 0.950 200 400 600 800 Mi (kW.m−2) Figure Normal solar absorptance of the paint coating according to the irradiance and the exposure time of each solar aging treatment 1E-04 3500 1E-05 te = 1000 s te = 3000 s bp (J.K–1.m–2.s–1/2) ap (m2.s–1) 3250 3000 2750 te = 1000 s 2500 te = 3000 s 2250 2000 1E-06 1750 200 400 Mi (kW.m−2) 600 800 200 400 600 800 Mi (kW.m−2) Figure Thermal diffusivity of the paint coating according to the irradiance and the exposure time of each solar aging treatment Figure Thermal effusivity of the paint coating according to the irradiance and the exposure time of each solar aging treatment 20 1E-04 18 14 12 10 te = 1000 s te = 3000 s Rtc (m2.K.W–1) λp (W.K–1.m–1) 16 1E-05 te = 1000 s te = 3000 s 1E-06 0 200 400 Mi (kW.m−2) 600 800 Figure Thermal conductivity of the paint coating according to the irradiance and the exposure time of each solar aging treatment 1E-07 200 400 600 800 Mi (kW.m−2) Figure Thermal contact resistance between the paint coating and the metal substrate according to the irradiance and the exposure time of each solar aging treatment 1680 A Boubault et al / Energy Procedia 49 (2014) 1673 – 1681 For each property, different evolutions are observed The properties that are most affected by an increase in the incident irradiance are the normal solar absorptance and the thermal effusivity The respective estimation uncertainties on these two properties are 0.1 % and % The increase in the absorptance seems to be inconsistent with an expected faster aging at higher irradiance At highest irradiances (692 kW.m2), the absorptance decreases while the thermal effusivity increases slightly This phenomenon suggests that the initial vitrification heat treatment is still occurring at the beginning of the aging treatments This may also be accentuated by a change of the surface condition which has an impact on the absorptance value Electron Dispersive Spetroscopy (SEM-EDS) and X-Ray Diffraction (XRD) analyses would be useful to understand the aging mechanisms at microscopic scale Similar observations are made for the thermal diffusivity and conductivity which have a strong dependency on the irradiance for an exposure time of 1000 s and a low irradiance, while for te = 3000 s, no evolution is observed with irradiance The influence of the exposure time is significant on the solar absorptance and the effusivity for which the values are lower for a longer exposure time of 3000 s The evolution of the thermal contact resistance is not clearly determined The results suggest that the coating remains stuck to the metal substrate with regard to the exposure times and irradiances studied Conclusion The aging of materials exposed to highly concentrated solar flux can be evaluated by monitoring specific thermoradiative and thermophysical properties A solar accelerated aging facility is used to subject a particular absorber material to different solar treatments at several irradiances and exposure times Two devices  a solar optical fiber reflectometer and an impulse photothermal method  allow monitoring the most aging-characteristic properties with accuracy However, the uncertainties generated in the preparation and solar treatment of the samples have not been assessed yet For example, it would be possible that the thermal contact resistance varies nonnegligibly along the interface between the coating and the metal in a sample, although the estimation of its value is done with a relative good accuracy (10 % uncertainty) The characterization of solar treated samples shows that the normal solar absorptance and the thermal effusivity are the most affected properties The vitrification heat treatment that was performed on the samples before the aging treatments seems to have been incomplete In support of this assumption, new recommendations on the vitrification process that have recently been provided by the paint manufacturer (LA-CO) and by the literature [8] advise to extend the duration of the heating process Further experimentations with longer exposure times are needed In particular, the influence of cyclic variations of the solar irradiance will be assessed Materials aged in real working conditions need to be characterized to find an accelerated aging method that is relevant and correlates with normal aging Acknowledgment The authors would like to thank Jean-Jacques Serra of the DGA-TA laboratory References [1] Amsbeck L., Buck R., Heller P., Jedamski J., Uhlig R., Development of a tube receiver for a solar-hybrid microturbine system, SolarPACES 2008, March 4-7, Las Vegas, USA, 2008 [2] Heller P., Solar-Hybrid Power and Cogeneration Plants (SOLHYCO), Final public report, SES6-CT-2005-019830, 2010 [3] Pacheco J E., Final test and evaluation results from the Solar Two project, Sandia National Laboratories, Albuquerque, Nouveau-Mexique, Etats-Unis, rapport SAND2002-0120, 2002 [4] Boubault A., Claudet B., Faugeroux O., Olalde G., Serra J-J., A numerical thermal approach to study the accelerated aging of a solar absorber material, Solar Energy, 86, 11, pp 3153−3167, 2012 [5] Boubault A., Claudet B., Faugeroux O., Guerin N., Olalde G.,Study of the aging of a solar absorber material following the evolution of its thermoradiative and thermophysical properties, High Temperatures  High Pressures, in-press, 2013 [6] Hernandez D., Antoine D., Olalde G., Gineste J M., Optical fiber reflectometer coupled with a solar concentrator to determine solar reflectivity and absorptivity at high temperature, J Sol Energy Eng., 121, pp 3135, 1999 A Boubault et al / Energy Procedia 49 (2014) 1673 – 1681 [7] Beck J.V., Arnold K.J., Parameters estimation in engineering and sciences, Wiley, New York, pp 234-247, 1977 [8] Ho, C.K., A.R Mahoney, A Ambrosini, M Bencomo, A Hall, and T.N Lambert, Characterization of Pyromark 2500 for High-Temperature Solar Receivers, ESFuelCell2012-91374, in proceedings of the ASME 2012 Energy Sustainability and Fuel Cell Conference, San Diego, CA, July 23-26, 2012 1681