Dissertations / Theses on the topic 'Selective solar surfaces'

The optical properties of the absorber pipe in a parabolic trough collector isessential for the performance of the solar collector. The desirable propertiesare high absorptance (α) of solar radiation and low emittance (ε) of thermalradiation. A surface with these properties is known as a solar selective surface. There are several techniques used to produce selective surfaces, but one of the most common ones is electroplating. Research done by Vargas, indicates that optical properties of α = 0.98 and ε = 0.03 [1], which is superior to the best commercial alternatives (α = 0.95 and ε = 0.04 [2]), can be achieved by electroplating a Co-Cr coating on a stainless steel substrate. Additionally, Vargas used an electrolyte of trivalent chromium dissolved in a deep eutectic solvent, as opposed to the traditionally used aqueous electrolytes containing hexavalent chromium, which is toxic and carcinogenic. In this project, a coating of Co-Cr was electroplated on a stainless steel substratewith a method similar to that of Vargas in order to obtain a solar selective surface. The electrolyte was composed of ethylene glycol, choline chloride, CrCl3•6H2O and CoCl2•6H2O in a molar ratio of 16:1:0.4:0.2. The plating process was conducted using chronoamperometric electrodeposition with an applied potential of -1.2 V (against an Ag/AgCl reference electrode) for 15 min. The system was investigated using Cyclic Voltammetry (CV). The total absorptance was measured using UV-Vis spectroscopy, while the emittance was measured using an IR-thermometer. The microstructure and chemical composition was investigated using Scanning ElectronMicroscopy (SEM), Focused Ion Beam (FIB), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy. The thermal stability of the coating was investigated by exposingit to 400°C in air for 24 h. The electroplated coating is approximately 2.8 μm thick and exhibits a porousstructure with a surface of fine fiber-like flakes. The coating consists largely of Co hydroxides with low concentrations of Cr compounds, Co oxides and metallic Co. Hence, a satisfactory co-deposition was not accomplished, as the Cr concentration is low. The coating is not thermally stable up to 400°C, exhibiting signs of at least partially melting in the annealing process. The compounds in the coating were largely oxidized in the process. The electroplated surface does however exhibits strong selectivity, with a total solar absorptance of α = 0.952 and total emittance of ε = 0.32 at 160°C.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (page 32).
Current solar to thermal energy conversion technologies, including concentrated solar power (CSP) and solar water heaters (SWH) utilize absorber surfaces that collect incident solar radiation. However, these absorber surfaces emit thermal energy (at their temperature) in the infrared (IR) spectrum, resulting in decreased overall efficiency for solar-to-thermal conversion. Selective absorber surfaces are highly absorptive in the solar spectrum, yet highly reflective in the infrared spectrum and therefore have the potential to minimize thermal energy loss. Copper Oxide (CuO) nanostructures are a candidate selective absorber material due to high absorptivity in the solar spectrum (about 95%), relatively high reflectance in the IR spectrum, scalability, and ease of fabrication. The aim of this study was to analyze optical properties and thermal stability of CuO surfaces in order to assess its feasibility as a selective absorber material. CuO nanostructures were synthesized on copper via chemical wet processing. Samples were thermally cycled to simulate day/night cycles in a typical SWH application. A cycle consisted of 12 hours of heating at 200°C and 12 hours of cooling to ambient temperature. Samples were cycled 1, 2, 3, 8, and 10 times. Surface optical properties were characterized using Ultraviolet-Visible Spectroscopy (UV-Vis) and Fourier Transform Infrared Spectroscopy (FTIR) and compared to optical properties of Pyromark®, the industry standard. Reflectance in the IR spectrum of CuO samples was found to increase after initial heating, whereas the absorptivity decreased. This tradeoff in optical performance resulted in an overall efficiency that remained relatively stable between 0 and 10 cycles (69.5±1.6%, 70.2±1.6%, respectively). CuO samples were found to be roughly 10% more efficient (optical conversion) than Pyromark® (npyromark,3x = 59.5±0.7%), indicating that CuO samples have the potential to be an efficient selective absorber material.
by Julia G. Rubin.
S.B.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 101-107).
A thermoelectric generator is a solid-state device that converts a heat flux into electrical power via the Seebeck effect. When a thermoelectric generator is inserted between a solar-absorbing surface and a heat sink, a solar thermoelectric generator is created which converts sunlight into electrical power. This thesis describes the design and optimization of solar thermoelectric generators, with a focus on systems with high optical concentration which utilize multiple material systems to maximize efficiency over a large temperature difference. Both single-stage and cascaded (multi-stage) generators are considered, over an optical concentration range of 0.1 to 1000X. It is shown that for high-concentration Bi₂Te₃/skutterudite solar thermoelectric generators, conversion efficiencies of 13% are possible with current thermoelectric materials and selective surfaces. Better selective surfaces are needed to improve the efficiency of solar thermoelectric generators. In this thesis, ideal selective surfaces for solar thermoelectric generators are characterized. Non-ideal selective surfaces are also characterized, with emphasis on how the non-idealities affect the solar thernoelectric gencrator performance. Finally. the efficiency limit for solar thermoclectric generators with non-directional absorbers is presented.
by Kenneth McEnaney.
S.M.

One of the traditional approaches to reduce costs of solar energy is to use inexpensive reflectors to focus the light onto highly efficient solar cells. Several research projects have resulted in designs, where the excess heat is used as solar thermal energy.

Unlike a solar thermal system, which has a selective surface to reduce the radiant heat loss, a CPV/T (Concentrating PhotoVoltaic/Thermal) system uses a receiver covered with solar cells with high thermal emittance.

This project analyzes whether the heat loss from the receiver can be reduced by covering parts of the receiver surface, not already covered with solar cells, with an optically selective coating. Comparing different methods of applying such a coating and the long-term stability of low cost alternatives are also part of the objectives of this project.

To calculate the heat loss reductions of the optically selective surface coating a mathematical model was developed, which takes the thermal emittances and the solar absorptances of the different surfaces into account. Furthermore, a full-size experiment was constructed to verify the theoretical predictions.

The coating results in a heat loss reduction of approximately 20 % in such a CPV/T system and one of the companies involved in the study is already changing their design to make use of the results.

Functional coatings produced using scalable and cost-effective processes such as electrodeposition and etching lead to the creation of random roughness at multiple length scales on the surface. The first part of thesis work aims at developing a fundamental mathematical understanding of multiscale coatings by presenting a fractal model to describe wettability on such surfaces. These surfaces are described with a fractal asperity model based on the Weierstrass-Mandelbrot function. Using this description, a model is presented to evaluate the apparent contact angle in different wetting regimes. Experimental validation of the model predictions is presented on various hydrophobic and superhydrophobic surfaces generated on several materials under different processing conditions. Superhydrophobic surfaces have myriad industrial applications, yet their practical utilization has been severely limited by their poor mechanical durability and longevity. Toward addressing this gap, the second and third parts of this thesis work present low cost, facile processes to fabricate superhydrophobic copper and zinc-based coatings via electrodeposition. Additionally, systematic studies are presented on coatings fabricated under different processing conditions to demonstrate excellent durability, mechanical and underwater stability, and corrosion resistance. The presented processes can be scaled to larger, durable coatings with controllable wettability for diverse applications. Apart from their use as superhydrophobic surfaces, the application of multiscale coatings in photo-thermal conversion systems as solar selective coatings is explored in the final part of this thesis. The effects of scale-independent fractal parameters of the coating surfaces and heat treatment are systematically explored with respect to their optical properties of absorptance, emittance, and figure of merit (FOM).
Master of Science

Processos para a produção de revestimentos seletivos de baixo custo foram desenvolvidos utilizando-se técnicas de eletrode posição e imersão. A utilização de substratos facilmente encontra¬dos no comércio a custos relativamente baixos, como o alumínio e o ferro galvanizado e a simplicidade dos métodos utilizados, tornam esses revestimentos altamente competitivos com os melhores já de¬senvolvidos. Valores de absorbância solar em torno de 0,95 e emitância térmica (100Low cost production of selective surfaces were developed using immersion and electroplating techniques. Substrates easily found in the market, like aluminium and galvanized iron, and the simplicity of the methods make those coatings highly competitives. Properties like solar absorptance of 0,95, thermal emittance (100OC) of 0,10 and the good resistence against thermal and chemical degradation give indications that those coatings can be used for industrial and residential solar heating systems. Spec¬troscopy measurements in the visible and infrared range were used to characterize the surfaces. A detailed description of the methods are given, allowing a fast action to get the selective surfaces even by people who are not familiarized with electro¬ plating techniques.

La mission de la thèse est de réaliser et d'étudier par une approche 'bottom-up' des dépôts électrophorétiques (EPD) de nanoparticules (e.g. nanotubes de carbone, carbure de titane) pour la conception de nouveaux revêtements solaires photo-thermiques. Ces matériaux sont élaborés par EPD, procédé qui consiste à faire migrer, sous l'influence d'un champ électrique, des nanoparticules chargées en suspension vers une électrode. Il s'agira de réaliser des solutions colloïdales stables, à l'aide de stabilisant organique pour maîtriser les interactions entre nanoparticules et in fine contrôler l'organisation au sein des futurs revêtements (gradient de densité, épaisseur, rugosité). La stabilité sera étudiée par des DLS (diffusion de la lumière) pour déterminer le rayon hydrodynamique et/ou SAXS (diffusion des rayons X) en ce qui concerne la répartition en taille au cours du temps, ainsi que par vélocimétrie laser pour déterminer le potentiel zêta des nanoparticules stabilisées. Ensuite, les paramètres pertinents du champ électrique (intensité, pulse, durée) de l'EPD seront exploré, via une cellule électrophorétique couplé à un impédancemètre, pour obtenir des dépôts submicroniques caractérisés par MEB (microscopie électronique). La sélectivité optique des revêtements (UV-visible / Infra-rouge lointain) ainsi que les performances du matériau seront caractérisés par spectroscopie UV-Vis-FIR. Le lien entre la microstructure et les propriétés optiques obtenues sera particulièrement exploré pour pouvoir optimiser efficacement ces nouveaux matériaux. Outre l'application visée, ces travaux ont également une portée cognitive sur les mécanismes de déstabilisation d'une suspension colloïdale par un champ électrique ainsi que sur la coagulation et l'arrangement de colloïdes sur une surface
The main objective is to realize some photo-solar absorbers by electrophoretic deposition of nanoparticles, having some tuneable optical properties. This study takes place in the field of the development of a macroscopic object through a 'bottom-up' approach. The understanding of the mechanism of the deposition is crucial to design these new materials. The nanostructure of the coatings with density gradient will be elaborated by pulsed and variable electric field, and characterized by scanning microscopy and energy dispersive X-ray and alongside a modelling of the electrophoretic phenomenons (such as the electric field drop) will be investigated. The conversion efficiency of the tandem material, which has to display a high absoptance in the solar spectrum domain (0.5-2.5 µm) whereas a low emittance in the far infra-red (2.5-20 µm), will be calculated from the reflectance spectra of the UV-vis-NIR and the Fourier transform InfraRed spectroscopy in order to link the electrophoretic parameters to the spectral selectivity

Solar energy is increasingly being considered a promising solution to reduce the emissions of CO2 and green house gas. The performance of solar collectors largely depends on the ability to absorb incoming solar radiation with minimal thermal radiation losses. To weigh the potential absorbed energy to thermal losses, the performance criterion (PC) can be used, calculated as PC =α−xε, where α is absorptance, ε is emittance and x is a scaling factor < 1. It has been shown by G. Vargas et al. that Co-Cr alloys excibit great potential (α = 0.98 and ε = 0.03) for use in solar concentrators. The main goal of this project is to quantify the impact of key factors (controlled input variables) on an electroplating process of Co-Cr alloys, using the design of experiment (DOE) methodology. It is part of an ongoing collaboration between Absolicon and the physics department at Umeå university. Six factors were investigated using a fractional factorial (FrF) design. Data was collected through a series of experiments where stainless steel substrates were electroplated with Co-Cr alloys. The resulting samples were analyzed in terms of α and ε as well as the quality of deposition (QD). Using the experimental results, three models were made in a DOE-software called MODDE. Models are used to correlate the factors with each response, i.e. α, ε and QD. Ideally the predictive power of the models (Q2) should be as high as possible, and at least > 0.5. The analysis of variance (ANOVA) test was used to determine the significance of the models. Based on the models, the ’Optimizer’ tool in MODDE was used to predict two set of optimum factor settings, producing two samples, S1 and S2. S1 and S2 were evaluated in terms of α, ε and QD as well as chemical composition and structural properties of the coatings. The predictive power of the models was 0.49 for α, 0.38 for ε and 0.53 for QD. The predictive power of the models were therefore limited. ANOVA-test showed that the models for α and QD were statistically significant. For all three responses the significant effects were mostly two factor interactions. All three models showed significant lack of fit (model error) as a result of high reproducibility. S1 had the best PCAbsolicon (performance criterion for Absolicons solar collectors) of all samples with 0.858. S2 was not as good, even though it was predicted to have a higher value of PCAbsolicon by MODDE. EDS, XPS and SEM measurements of samples S1 and S2 showed that the two samples were very similar in terms of chemical composition. The main difference was that the coating of S1 was more porous, and also thicker than S2, 0.81 μm compared to 0.26 μm. Even though the models showed some predictive capabilities, the impact of the factors could not be fully determined. That is due to the nature of the FrF-design, which cannot accurately determine two-factor interactions.

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior
The using of alternatives materials to replace selective surfaces is a natural trend, because it usually looks for improvements on efficiency of surfaces at the same time that it tries to reduce costs. Composites are already used on obtainment of some selective surfaces, however, if the possibility to use residue that would be discarded was associated to these characteristics, providing an added-value, it would brings some benefits like a possible reduction of manufacturing costs. Thus, this thesis proposes the obtainment and study of selective surfaces for flat plate solar collectors for low cost by using residue from granite industry. Three different surfaces was studied, two of them of obtained on the laboratory, one is granite powder made and the other is a surface composed by a mixture of granite powder and CRFO (Cr0,75Fe1,25O3); the third surface is a commercial one, known as TiNOX. To perform the tests of the surfaces it was built an experimental stand, it allows simulating a solar collector conditions. The tests was performed in a stagnation condition, in other words, there wasnât any water flow inside tubes. Through this experimental apparatus it was possible to test the three surfaces simultaneously. The field tests showed that the highest temperatures were reached by granite powder surface, which reached an average temperature of 119 ÂC, while the granite powder and CRFO mixture surface reached an average of 96 ÂC. The TiNOX achieve an average temperature of 101 ÂC. The three surfaces was compared each other through an equation that gives a global heat loss coefficient. The granite powder surface was the one which achieved the lowest global heat loss coefficient.
O uso de materiais alternativos com objetivo de substituir superfÃcies seletivas à uma tendÃncia natural, pois geralmente se busca melhorias na eficiÃncia das superfÃcies ao mesmo tempo em que se tenta diminuir os custos. SubstÃncias compÃsitas jà sÃo utilizadas na obtenÃÃo de algumas superfÃcies seletivas, no entanto, se for associado a estas caracterÃsticas a possibilidade de utilizar resÃduos que iriam ser descartados, conferindo aos mesmos um valor agregado, isso traria alguns benefÃcios, como uma possÃvel reduÃÃo de custos de fabricaÃÃo. Desta forma, o presente trabalho propÃe a obtenÃÃo e o estudo de superfÃcies seletivas para aplicaÃÃes em coletores solares de placa plana de baixo custo originÃrio do resÃduo da indÃstria de granito. Foram estudadas trÃs diferentes superfÃcies, duas delas foram obtidas no laboratÃrio, a superfÃcie a base de pà de granito e a superfÃcie composta pela mistura de pà de granito e CRFO (Cr0,75Fe1,25O3); e a terceira superfÃcie foi uma superfÃcie comercial, conhecida como TiNOX. Para a realizaÃÃo dos testes foi construÃda uma bancada experimental de madeira, de forma que fosse possÃvel simular as condiÃÃes de um coletor solar de placa plana. Os testes foram feitos em condiÃÃo de estagnaÃÃo, ou seja, nÃo havia fluxo de Ãgua atravÃs de tubos no coletor. AtravÃs desse aparato experimental foi possÃvel testar as trÃs superfÃcies seletivas simultaneamente. Os testes de campo mostraram que a superfÃcie composta por pà de granito foi a que atingiu as maiores temperaturas, com uma mÃdia de atà 119 ÂC, enquanto a superfÃcie obtida com uma mistura de pà de granito e CRFO chegou a temperatura mÃdia de 96 ÂC, jà a superfÃcie comercial atingiu uma mÃdia de 101 ÂC. As superfÃcies foram comparadas atravÃs de uma equaÃÃo que fornece o coeficiente global de perda de energia tÃrmica. Os menores coeficientes foram obtidos pela superfÃcie de pà de granito

Domestic solar hot water units, if properly designed, are capable of providing all hot water needs in an environmentally friendly and cost-effective way. Despite 50 years of development, commercial technology has not yet achieved substantial market penetration compared to mainstream electric and gas options. Therefore, alternate designs are warranted if they can offer similar or greater performance for a comparable cost to conventional units. This study proved that such alternatives are possible by designing and testing two novel solar hot water systems (SHWS). The first system used compound parabolic collector (CPC) panels to concentrate solar energy and produce steam. The steam moved from a rooftop downward into a heat exchange pipe within a ground level water tank, heating the water, condensing and falling into a receptacle. The operation was entirely passive, since the condensate was pulled up due to the partial vacuum that occurred after system cooling. Efficiencies of up to 40% were obtained. The second system used an air heater panel. Air was circulated in open and closed loop configuration (air recycling) by means of a fan/blower motor and was forced across a compact heat exchanger coupled to a water tank. This produced a natural thermosiphon flow heating the water. Air recycling mode provided higher system efficiencies: 34% vs. 27%. The concurrent development of an analytical model that reasonably predicted heat transfer dynamics of these systems allowed 1) performance optimisation for specific input/starting operating conditions and 2) virtual design improvements. The merit of this model lay in its acceptable accuracy in spite of its simplicity. By optimising for operating conditions and parameter design, both systems are capable of providing over 30 MJ of useful domestic hot water on clear days, which equates roughly to an increase of 35°C in a 200 L water tank. This will satisfy, on average, daily hot water requirements for a 4-person household, particularly in low-latitude regions (eg. Queensland). Preliminary costing for these systems puts them on par with conventional units, with the passive, remotely coupled, low maintenance, CPC SHWS comparable to higher end models. The air heater SHWS, by contrast, was much more economical and easier to build and handle, but at the trade-off cost of 1) the need for an active system, 2) increased maintenance and running costs and 3) the requirement for a temperature control mechanism that would protect the panel body by dumping hot air trapped inside if stagnation were to occur.

Domestic solar hot water units, if properly designed, are capable of providing all hot water needs in an environmentally friendly and cost-effective way. Despite 50 years of development, commercial technology has not yet achieved substantial market penetration compared to mainstream electric and gas options. Therefore, alternate designs are warranted if they can offer similar or greater performance for a comparable cost to conventional units. This study proved that such alternatives are possible by designing and testing two novel solar hot water systems (SHWS). The first system used compound parabolic collector (CPC) panels to concentrate solar energy and produce steam. The steam moved from a rooftop downward into a heat exchange pipe within a ground level water tank, heating the water, condensing and falling into a receptacle. The operation was entirely passive, since the condensate was pulled up due to the partial vacuum that occurred after system cooling. Efficiencies of up to 40% were obtained. The second system used an air heater panel. Air was circulated in open and closed loop configuration (air recycling) by means of a fan/blower motor and was forced across a compact heat exchanger coupled to a water tank. This produced a natural thermosiphon flow heating the water. Air recycling mode provided higher system efficiencies: 34% vs. 27%. The concurrent development of an analytical model that reasonably predicted heat transfer dynamics of these systems allowed 1) performance optimisation for specific input/starting operating conditions and 2) virtual design improvements. The merit of this model lay in its acceptable accuracy in spite of its simplicity. By optimising for operating conditions and parameter design, both systems are capable of providing over 30 MJ of useful domestic hot water on clear days, which equates roughly to an increase of 35°C in a 200 L water tank. This will satisfy, on average, daily hot water requirements for a 4-person household, particularly in low-latitude regions (eg. Queensland). Preliminary costing for these systems puts them on par with conventional units, with the passive, remotely coupled, low maintenance, CPC SHWS comparable to higher end models. The air heater SHWS, by contrast, was much more economical and easier to build and handle, but at the trade-off cost of 1) the need for an active system, 2) increased maintenance and running costs and 3) the requirement for a temperature control mechanism that would protect the panel body by dumping hot air trapped inside if stagnation were to occur.

The thesis engage in structure of solar collectors in theoretical part. Secondly, theory of thermal spraying coating and anodizing is subscribed. In practical part of thesis is proposition of producing a spectral selective surface suitable for solar absorbers. Finally the properties of the samples are checked by thermographic camera shots.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq
The development of new materials and technologies have been shown to be very important when the goal is to make solar energy will be more economically attractive. One way to promote this attractiveness is increasing the efficiency of solar collectors, which can be done by use of so-called selective surfaces, or deposited coatings on absorbing plate solar radiation, which allow a maximum of absorbance in the solar spectrum while maintaining a minimum emittance in the infrared. Absorbers for producing films, it is necessary to use materials technology, energy and environmental efficiency, such as inorganic polymers (geopolymers). In the Earth's tropical regions, such as the state of Paraiba, there are materials with potential use for the synthesis of inorganic polymers that have not been evaluated, the Sugar Cane Bagasse Ash (SCBA) is one of them. So that this study sought to assess the SCBA's potential use as raw material for production of solar absorbing films. Through the use of two deposition techniques in three types of substrates, it was concluded that the SCBA has intrinsic selectivity, having potential use for the production of solar absorbing films. So that the best result set (film + substrate) exhibited peak and average absorption of respectively 88% and 65%. This surface was obtained from the SCBA.5.250 diluted in 10 g of H2O deposited by painting techniques on the glass substrate.
O desenvolvimento de novos materiais e tecnologias tem se mostrado de extrema importância quando o objetivo é fazer com que a energia solar venha a ser mais atrativa economicamente. Um dos modos de promover essa atratividade é aumentando a eficiência dos coletores solares, o que pode ser feito por meio da utilização das chamadas superfícies seletivas, ou seja, revestimentos depositados sobre placas absorvedoras de radiação solar, que permitem um máximo de absortância no espectro solar enquanto mantêm um mínimo de emitância no infravermelho. Para a produção de filmes absorvedores, faz-se necessário a utilização de materiais com eficiência tecnológica, energética e ambiental, tais como os polímeros inorgânicos (geopolímeros). Nas regiões tropicais da Terra, como é o caso do Estado da Paraíba, há materiais com potencial de utilização para a síntese de polímeros inorgânicos que ainda não foram avaliados, a Cinza do Bagaço da Cana-de-açúcar (CBC) é um deles. De forma que o presente trabalho procurou avaliar o potencial de uso da CBC como matéria-prima para produção de filmes absorvedores solares. Através da utilização de duas técnicas de deposição em três tipos de substratos, concluiu-se que a CBC apresenta seletividade intrínseca, tendo potencial de uso para produção de filmes absorvedores solares. De maneira que o melhor resultado do conjunto (filme+substrato) exibiu pico e média de absorção de, respectivamente, 88% e 65%. Essa superfície foi obtida a partir da CBC.5.250 diluída em 10 g de H2O depositada pela técnica de Pintura sobre o substrato de vidro.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico
O estudo de superfÃcies seletivas para coletores solares tÃrmicos, hà alguns anos, enfoca o uso de pastas e tintas para recobrir chapas de cobre ou alumÃnio, formando a placa absorvedora do coletor. Essas tintas ou pastas sÃo eletrodepositadas em placas metÃlicas, sendo este processo muitas vezes responsÃvel pelo levado custo de superfÃcies seletivas. O uso de compÃsitos obtidos a partir do Ãxido de cromo, Ãxido de ferro e Ãxido de titÃnio em um processo cerÃmico, utilizando a tÃcnica de deposiÃÃo por screen-printing pode ser uma alternativa para a obtenÃÃo de novas superfÃcies seletivas que tenham eficiÃncia prÃxima Ãs tintas e pastas jà comercializadas e com baixo custo de fabricaÃÃo. O objetivo deste trabalho à obter e caracterizar superfÃcies seletivas à base de cromo, ferro e titÃnio e comparÃ-las com outras superfÃcies jà comercializadas (Ãxidos metÃlicos), encontrar os valores da absortividade e caracterizar as mesmas por espectroscopÃa raman e infavermelho.
The study of selective coating for thermal solar collectors has been carried out with focus on the use of selective paints and coatings for copper and aluminum plates, which are used to manufacture the absorber plate. The processes, such as electrodeposition, used in these paints and coatings are expensive and they increasing the price of the final product. The use of composite obtained from chromium oxide, iron oxide, and titanium oxide in a ceramic process using the screen-printing deposition technique is an alternative to develop new selective coating with approximately the same thermal efficiency, but at a lower cost than the commercial ones. The results show that the absorptivity values measured with the new coatings are close to those presented by the manufacturers in the commercial sufaces. The objective is to obtain and characterize selective surfaces based on chromium, iron and titanium and compares them to other areas already marketed (metal oxides), find the values of absorptivity and characterize them by raman and infrared spectroscopy.

Dans les centrales solaires à concentration, le flux solaire est concentré sur des récepteurs pour chauffer un fluide de transfert jusqu’à 600°C. Dans le but d’améliorer leurs propriétés optiques, ces récepteurs peuvent être recouverts par des revêtements multicouches à sélectivité spectrale. Ces travaux portent sur le développement de structures optiquement sélectives innovantes, absorbantes dans le domaine du rayonnement visible et proche infrarouge (faible réflectivité) et peu émissives (forte réflectivité) pour le rayonnement infrarouge. Nous avons développé des empilements multicouches associant un métal réfractaire avec une céramique ce qui permet d’améliorer l’absorption dans le visible ainsi que la stabilité thermique. Les couches sont déposées par plasma combinant la PACVD et la PVD. L’étude de faisabilité du procédé transférable à l’industrie, son développement et son optimisation via la conception d’un réacteur innovant, le dépôt et la caractérisation des couches, leur association dans des empilements optiquement sélectifs performants et l’étude de leur vieillissement ont été réalisés
In concentrated solar power (CSP) plants, solar flux is concentrated on receivers to heat a transfer fluid up to 600°C. In order to improve their optical properties, these receivers can be covered by multilayered spectrally selective coatings. This work is devoted to designing and developing innovative spectrally selective structures showing strong absorption (low reflectivity) in the visible and near infrared range and low emissivity (high reflectivity) in the infrared range. We developed such stacks associating a refractory metal with high IR reflectivity and a ceramic which improves absorption in the visible range and thermal stability. The coatings were synthesized by plasma techniques, combining PACVD and PVD. Pre-industrial process feasibility, its development and optimization through the design of an innovative reactor, layer deposition and characterization, their combination in efficient optically selective stacks and the study of their aging have been achieved

Tese de Doutoramento - Programa Doutoral em Física (MAP- fis)
Esta tese tem como objetivo o desenvolvimento de revestimentos para absorção seletiva da radiação solar, que tenham um elevado coeficiente de absorção solar (α) (> 95%), baixa emissividade (ε) (<12% a 400 ºC), e com elevada estabilidade térmica, acima de 400 ° C ao ar e acima de 600 ºC em vácuo, de modo a que possa ser usado em sistemas de concentração da radiação solar (CSP) que utilizem temperaturas elevadas. Os revestimentos são multicamadas, que têm quatro a cinco camadas, sendo as duas primeiras, uma barreira de difusão e um refletor de radiação infravermelha, tungsténio. As restantes são constituídas por uma estrutura de dupla camada para absorção da radiação solar e uma camada antirefletora. Para a configuração das camadas de absorção, foram utilizadas três soluções diferentes. As duas primeiras soluções baseiam-se em camadas de nitreto / oxinitreto de metais de transição, nomeadamente a partir de crómio (CrAlSiNx/CrAlSiNxOy) e tungsténio (WsiAlNx/WSiAlNxOy). A terceira solução é baseada em compósitos cerâmico-metal de AlSiOx:W. Camadas individuais das diferentes soluções foram depositadas em substratos de vidro, aço inoxidável (SS) e silício (Si) para estudar a composição química, estrutura, propriedades ópticas e mecânicas. Os espectros de transmitância e reflectância das camadas individuais, depositados em substratos de vidro, foram simulados com o software SCOUT, de modo a calcular a respetiva função dielétrica, as constantes ópticas espectrais e espessuras. As multicamadas foram desenhadas utilizando as constantes ópticas das camadas individuais, utilizando também o software SCOUT. Utilizando os resultados das simulações, as multicamadas foram depositadas em substratos de aço inoxidável utilizando a técnica de pulverização catódica por magnetrão e o desempenho dos revestimentos foi verificado por meio da sua absorção solar, da emissividade e do seu comportamento quando sujeitos a tratamentos térmicos ao ar e em vácuo. Diversas técnicas de caracterização foram utilizadas para estudar os revestimentos, nomeadamente por microscopia eletrónica de varrimento (SEM), espectrometria de retrodispersão de Rutherford (RBS), difração de raios X (DRX), espectroscopia de infravermelho com transformada de Fourier (FTIR), espectroscopia de fotoeletrões de raios-X (XPS), espectroscopia de raios-X por dispersão em energia (EDS), análise de detecção de recuo elástico por tempo de voo (TOF-ERDA), espectroscopia Raman e espectroscopia UV-VIS-NIR. Os revestimentos multicamada apresentados nesta tese mostraram boa estabilidade térmica e resistência à oxidação após o tratamento térmico em vácuo a 600 ºC ou 580 ºC e tratamento térmico ao ar a 400 ºC ou 450 ºC. A solução baseada na estrutura WSiAlNx/WSiAlOyNx apresentou os melhores resultados, em termos de estabilidade térmica, resistência à oxidação e coeficiente de absorção solar, enquanto a baseada em CrAlSiNx/CrAlSiNxOy apresentou os valores mais baixos de emissividade à temperatura de 400 ºC. As alterações no coeficiente de absorção solar (αsol) e na emissividade (ε) são insignificantes na maioria dos casos. Em alguns casos, só foram reveladas pequenas mudanças nas curvas de refletância após o primeiro passo de tratamento térmico, não aparecendo alterações nos passos subsequentes. No caso dos revestimentos baseados em CrAlSiNx / CrAlSiNxOy, após o tratamento térmico em vácuo a 600 ºC, verificou-se que ocorreu a difusão de átomos de tungsténio da camada de W para o substrato de aço inoxidável. Assim, foi introduzida uma camada de barreira de CrAlSiNx entre a camada de tungsténio e o substrato de aço e fetuado o respetivo estudo, tendo-se verificado a não ocorrência da difusão do W.
This thesis has the objective to develop solar selective absorber coatings having simultaneously high solar absorptance (α) (>95%) and low emissivity (ε) (<12% at 400 ºC) together with high thermal stability above 400°C in air and above 600 ºC in vacuum, which could be used in the concentrated solar power (CSP) or in other high temperature applications. The coatings are multilayer stacks, that have four to five layers, being the first two, a barrier layer and a back-reflector tungsten layer. The remaining layers comprise a double film structure for phase interference finished by an antireflection layer. For the double absorption layer configuration, three different approaches have been used. The first two are based on transition metal nitride/oxynitride layers, namely from chromium as (CrAlSiNx/CrAlSiNxOy) and from tungsten as (WSiAlNx/WSiAlNxOy). Whereas, the third one is based on (AlSiOx:W) cermets. The single layers of each approach were deposited on glass, stainless-steel (SS) and silicon (Si) substrates to study their chemical composition, structure, optical and mechanical properties. The transmittance (T) and the reflectance (R) spectra of single thin layers, deposited on glass substrates, were modelled with the help of SCOUT software and the spectral optical constants and thicknesses were calculated. The multilayer designs were performed using the optical constants of the single layers and conducted with SCOUT software. The final multilayer stacks were deposited on stainless-steel substrates using DC magnetron sputtering technique and the functionality of the absorbers optical stacks was verified through solar absorptance, emissivity and accelerated thermal ageing treatments. All tandems and their individual layers were characterized by Scanning Electron Microscopy (SEM), Rutherford Backscattering Spectrometry (RBS), X-ray diffraction (XRD), Fourier-transform Infrared Spectroscopy (FTIR), X-Ray Photoelectron Spectroscopy (XPS), Energy Dispersive X-ray Spectroscopy (EDS), Time of flight Elastic Recoil Detection Analysis (TOF-ERDA), Raman spectroscopy and UV–VIS–IR spectroscopy. The absorber tandems presented in this thesis showed good thermal stability and oxidation resistance after vacuum annealing at 600 ºC or 580 ºC and air annealing at 400 ºC or 450 ºC. The approach based on WSiAlNx/WSiAlOyNx tandem showed the best thermal stability, oxidation resistance and it has higher solar absorbance, αsol, while the CrAlSiNx/ CrAlSiNxOy shows the lowest thermal emittance at the temperature (400 ºC). The variations in the solar absorptance (αsol) and the thermal emittance (ε) are negligible in most cases. In some cases, small variations in the reflectance curves after the first step of annealing were observed. After the annealing in vacuum at 600 ºC, tungsten diffusion from the back-reflection layer towards the stainless-steel substrate was found in the tandem based on CrAlSiNx/ CrAlSiNxOy. Thus, a CrAlSiNx barrier layer with higher nitrogen N content was included between the stainlesssteel substrate and tungsten. The influence of that barrier layer upon the W diffusion was also studied.
Financial support of FCT, POCI and PORL operational programs through the project POCI- 01-0145-FEDER-016907 (PTDC/CTM-ENE/2882/2014), co-financed by European community fund FEDER.

碩士
國立高雄師範大學
光電與通訊工程學系
101
In this thesis, the designs of circularly-polarized (CP) high-gain green antennas using solar module as frequency selective surface (FSS) for Fabry-Perot (F-P) cavity are presented. First, the analysis of the transmission and reflection coefficients of the square solar cell is adopted for predicting the resonant frequency of the circular polarization and high gain operation. Next, the green antenna is constructed, and consists of a circularly polarized patch antenna (radiating source) backed with a metal ground plate and a square solar module of 6  6 (or 8  8) solar cells. The effects of the resonant height, the rotating angle of the radiating source, and the dimension of the square solar module on the impedance bandwidth, the CP bandwidth, and the realized gain of the green antenna are investigated. For the case of the green antenna excited by a CP radiating source operated at the center frequency of 3.0 GHz, the achieved impedance bandwidth is 154 MHz (2.928 ~ 3.082 GHz), the CP bandwidth is 76 MHz (2.907 ~ 2.983 GHz), and the antenna gain within the CP band is 13.0 ~ 13.9 dBic. Furthermore, the green antenna excited by the radiating source operated at the center frequency of 3.5 GHz is also studied, the antenna gain within the CP band (88 MHz, 3.434 ~ 3.522 GHz) reaches to 16.0 ~ 18.6 dBic.

碩士
國立虎尾科技大學
光電工程系光電與材料科技碩士班
107
In this study, the applications of surface chemical treatments and molybdenum oxide carrier-selective contact layers on photovoltaic characteristics of screen-printed monocrystalline silicon solar cells were investigated. The silicon surface with highly interface trap density results in large recombination velocity. Thus, in this study, silicon surface passivation was demonstrated by various surface chemical treatments. Firstly, the chemical oxide layers on the silicon substrates were formed by HNO3 solution. The experimental parameters, including various concentrations, time and temperature, were presented. Furthermore, the surface roughness and chemical oxide were achieved by the mixed HNO3 and HF solutions and HNO3 solution, respectively. Then, the treated surfaces were immersed into the HNO3 solution. After two kinds of chemical surface treatments, MoOx carrier-selective contact layers and silver were evaporated by the thermal evaporation process, respectively. The experimental results show that photovoltaic conversion efficiency (CE) with 0.5% improvement can be addressed by the 30 wt% HNO3 solution for 45 s at the room temperature. The treated devices with a series resistance of 1.45 Ω-cm2, an open-circuit voltage of 665 mV, a short-circuit current density of 39.2 mA/cm2, a fill factor of 82.2 %, a interface trap density of 1.21×1014 cm-2eυ-1, and a CE of 21.48 % were demonstrated. The chemical oxide formed at the low concentration and long time was similar to the high concentration and short time by the HNO3 solution. Furthermore, the CE of device can’t be enhanced by the mixed HF and HNO3 solution. The mechanism can be attributed to the polished surface. The adhesion was decreased with increasing polished surface.