Multilayer solar selective coatings for high temperature solar applications: from concept to design

Increasing the share of renewables in the energy mix has a key function for the security of energy supply and the reduction of greenhouse gas emissions from fossil fuels. The purpose of this thesis is to develop new solar selective coating (SSC) designs for high temperature applications in order to improve the performance of concentration solar power (CSP) plants. The main part of this thesis has been carried out in the company Abengoa, which is a world leader in the development of CSP plants, but also with the collaboration of other well-recognized academic organizations (Instituto de Ciencia de Materiales de Sevilla-CSIC, Center Tecnologic Manresa and the Helmholtz-Zentrum Dresden - Rossendorf) and the SME Metal Estalki. The improvement in efficiencies in solar thermal energy plants partially means the increase in the receivers’ temperature, reaching up to an average maximum temperature of 650ºC for superheated steam and molten salts receivers. There are several R&D approaches to substitute commercial absorber paints due to the degradation problems they show at high temperatures when exposed at air. The different routes include the development of new solar selective coatings fabricated using physical vapour deposition techniques. In this thesis, two potential candidates as solar selective coatings were selected: i) carbon–transition metal carbides nanocomposites (a-C:MeC) and ii) aluminium titanium oxynitride (AlTi(OxN1-x)) based coatings. The methodology followed in this thesis contains aspects of very high novelty including optical simulation, coating deposition using cathodic vacuum arc (CVA) and advanced characterization. The computer program CODE was used to simulate the reflectance spectra of different complete coating. Simulated reflectance spectra were compared with the measured reflectance of the deposited films to verify the agreement between simulations and experimental results. The simulations allowed predicting the optical properties of solar selective coatings with different thicknesses and with different materials avoiding a try and error approach. Special attention was paid to the simulation of optical constants. The knowledge of the coating microstructure revealed critical for a proper design of solar selective coatings. In this thesis, valuable insight into the most accurate way of simulating nanocomposite materials and oxynitrides is given. Several optical models were evaluated and their appropriateness described in detail. Different SSC stacks were deposited with pulsed filtered cathodic vacuum arc (PFCVA) and non-filtered CVA setup comparing different materials, compositions and thicknesses for each one of the layer that conforms the coating. After the deposition of single layers and complete SSC, the stacks were characterized employing a wide range of techniques. The optical properties were characterized by UV-Vis-NIR and FT-IR spectrophotometers; the elementary compositions were determined by Rutherford Backscattering Spectroscopy (RBS), Nuclear Reaction Analysis (NRA) and Elastic Recoil Detection (ERD) ion beam techniques; the crystal structure was studied with X-ray diffraction and Raman spectrometry was used to determine the chemical bonding of the carbon atoms; and finally, SEM and HR-TEM were employed to determine the morphology of the deposited thin films. The unique cluster tool (sited at HZDR) allowed the in-situ characterization of the films performance at high temperature. This novel technique provided a detailed study of the diffusion processes occurring at extreme temperatures in solar selective coatings of interest and the identification of their failure mechanism. The simulations, depositions and characterizations performed for the two selected candidate materials for SSC are thoroughly described in the following chapters. The introductory chapter 1 starts with a brief description of the advantages of thin films and coatings versus their bulk counterparts, followed by a summary of physical vapour deposition (PVD) techniques and the main growing mechanisms of thin films. The fundamental principles of the interaction of light with materials are also introduced, in order to get a better understanding of the thin film optical properties. The chapter ends with a summary of the current state of the art of SSC and the mechanisms employed to maximize the absorption of sunlight and to minimize the losses by thermal radiation. The methodology followed in this thesis for a complete design of solar selective coatings is fully explained in chapter 2. The design process starts with the selection of the material that composes each layer in the multilayer stack. Then, the different deposition systems employed are described followed by the complete characterization performed on the deposited thin films. Finally, the thermal treatment tests included in this work to analyse the durability in air at high temperatures and to predict their service lifetime are explained. Chapter 3 introduces the properties of transition-metal carbides and carbon as the individual components of the nanocomposite (a-C:MeC). Nanocomposite thin films consisting of interstitial metal carbide embedded in an amorphous carbon matrix exhibit a unique combination of properties which makes them very attractive candidates as absorber layer of SSC for thermo-solar applications. In a first step, optical simulation based on literature optical constants was employed for optimizing SSC. After this initial simulations, the selected a-C:MeC candidates were deposited and characterized. Following a thorough analysis of their composition and microstructure, the simulations were feedback with experimental data. The simulated complete coating based on these measured properties provided excellent selective optical selective properties (α>96% and ε600ºC<14%). A complete solar selective coating was deposited and analysed and afterwards a heating test was performed to study the stability of the coating at high temperature. Aluminium titanium oxynitrides were selected as candidate materials for SSC on basis of the state of the art described in the introduction of chapter 4. In this chapter, initial thermal treatment tests were performed to validate the stability of single oxynitride layers in air, showing no degradation at temperatures above 600ºC. A set of individual AlTi(OxN1-x) layers deposited by CVA were analysed in terms of composition, morphology and optical properties. A thoroughly study of the microstructure of the films, as a function of the oxygen content, was found to be key for a comprehensive analysis of the optical properties. A complete multilayer SSC was designed and deposited with optical simulations based on measured optical constants of each of the individual layers. Excellent agreement was found between simulated and experimental reflectance spectra. A solar selective coating with a simulated absorptance of 94,7% and an emittance of 5,6% is designed. To conclude, the thermal stability in air of the complete SSC was analysed by asymmetric and cyclic heating tests. Remarkable stability at temperatures as high as 650ºC after 750 hours of annealing was found for solar selective coatings based on oxynitrides. Chapter 5 described a novel technology for the in-situ characterization of coatings at high temperatures. This characterization is performed at the two materials candidates. In particular, an accurate knowledge of the variation of the dielectric function of thin films with the temperature and its relation to compositional and microstructural changes could help to prevent failures. The methodology employed combines a sequence of analytical techniques. An a-C:TiC thin film was studied first following the described methodology. Then, AlTi(OxN1-x) thin films with different oxygen concentration were investigated in order to understand the influence of the oxygen to nitrogen ratio on the optical properties and their failure mechanisms at high temperatures. No significant changes in optical properties and composition were found when heating oxynitride films in vacuum atmosphere at temperature above 800ºC, showing excellent high temperature stability. It is worth noting than a worldwide record of in-situ RBS measurement at 840ºC was performed in the framework of this thesis. The main conclusions of the thesis are discussed in chapter 6, including a comparative review of the different materials employed for SSC, along with the strengths and weaknesses observed for each one.

Author dc:creator

• Heras Pérez, Irene

Advisors dc:contributor.advisor

• Guillen Rodríguez, María Elena
• Krause, Matthias
• Escobar-Galindo, Ramón

Statement dc:rights

• Attribution-NonCommercial-NoDerivatives 4.0 Internacional

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc-nd/4.0/

Language dc:language.iso

spa

Handle dc:identifier.uri

http://hdl.handle.net/11441/47789

OAI identifier oai:identifier

oai:idus.us.es:11441/47789