M. Tech. Electrical Engineering. / Investigates the thermal performance of solar water heating systems on low cost housing environment as a component of the measurement and verification process of the sustainable energy management.

Condition monitoring of machines and structures is commonly utilized in order to prevent failures before they can occur. For these reasons, data such as temperature, vibrations or displacements are collected and analysed. Sensors collect this information, which is sent to a base station to be examined. Wired sensors have been used since the appearance of condition monitoring maintenance; however, wireless sensors are becoming more popular in this area. The use of wired sensors can be very expensive, due to the cost related to the installation and maintenance of the wiring between the sensors and the base station. In wind turbines, wired sensor networks are starting to be substituted by wireless sensor networks. However, for tidal turbines, such as those developed by Delta Stream, this is still a challenge. The use of batteries to supply energy to sensors is not an optimal solution for turbines that are located in remote areas. Batteries have a limited life and their replacement is costly and complicated. Thus, alternative sources of energy have to be found. The environment found in a tidal turbine provides several sources of profitable energy, such as vibration and temperature differences which can be used to supply energy by means of energy harvesters. The aim of this project is to demonstrate the operation of self-powered short-range wireless sensor nodes for a potential use in a Delta Stream nacelle of tidal turbine. This project focuses on the wireless communication inside the nacelle (where most of the sensors are located) using a land protocol (Zigbee), and the energy harvesting using waste heat by means of thermoelectric devices. In order to prove the operation of the whole system (thermoelectric generator and sensor node), a power management circuit was also constructed and tested.

Increasing efforts have been put into research about thermoelectric materials for the last few decades, especially recently, faced with the crucial demand for new energy and energy savings. Among the potential candidates for new generation thermoelectric materials are the intermetallic clathrates. Clathrates are cage-structured materials with guest atoms enclosed. Previous studies have shown lower thermal conductivities compared with many other bulk compounds, and it is believed that guest atom vibration modes are the reason for such thermal behaviors. Several models, including the Einstein oscillator and soft potential models, have been used to explain the guest motion. However the characterization of the anharmonic oscillating motion can be a challenge. In this work, Nuclear Magnetic Resonance (NMR), heat capacity and transport measurements have been used to study several clathrate systems, especially the well- known type-I Ba8Ga16Sn30, which has been reported to have one of the lowest thermal conductivities for bulk compounds. In this material the strong anharmonic rattling behavior was investigated and analyzed according to a double well potential model, yielding good agreement with the experimental results. Furthermore, the resistivity and heat capacity results were studied and analyzed according to the influence of the anharmonic contribution. This offered a way to connect the NMR, transport and heat capacity properties, providing an advantageous way to study strongly anharmonic systems. In further work, several related intermetallic materials were examined for their structure, motion and NMR properties. Dynamical and electrical behaviors were investigated by studying the magnetic and quadrupole NMR spin-lattice relaxation. Type-VIII Ba8Ga16Sn30 exhibits an enhanced dynamics-related term at low temperature, but no rattling response as observed for the type-I structure. Type-I Ba8In16Ge30 was compared with the type-I Ba8Ga16Sn30 because their cage structures are similar. No strong anharmonic contribution was found in the NMR T1 behavior of Ba8In16Ge30, however the T2 showed behavior characteristic of atomic motion. In all cases, the magnetic relaxation was used to characterize the electron structures, and n- type Ba8Ga16Ge30 exhibited a spin-lattice relaxation behavior which is characteristic of impurity band structures near the Fermi surface. Also, a series of Ba8CuxGe46-x clathrates were investigated and showed much more insulating like behavior. In related work, the layered BaGa4 and BaGa3Sn have shown interesting NMR spin-spin relaxation behavior that indicates atomic fluctuations. This is similar to the situation found in type-I Ba8In16Ge30. The influence of atomic motion on the NMR and also the atomic structures of these alloys is further discussed in this work.

Thesis (doctoral)--Tallinn University of Technology, 2009. / Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Vita. Includes bibliographical references.

Thesis (Ph. D. in Interdisciplinary Materials Science)--Vanderbilt University, Aug. 2007. / Title from title screen. Includes bibliographical references.

Η παρούσα διπλωματική εργασία πραγματεύεται την ανάλυση της λειτουργίας των θερμοηλεκτρικών στοιχείων, τη συγκριτική μελέτη μεθόδων απομάστευσης μέγιστης ισχύος από αυτά καθώς και την κατασκευή διάταξης για την πειραματική σύγκριση των μεθόδων αυτών στο εργαστήριο. Η εργασία αυτή εκπονήθηκε στο Εργαστήριο Ηλεκτρομηχανικής Μετατροπής Ενέργειας του Τμήματος Ηλεκτρολόγων Μηχανικών και Τεχνολογίας Υπολογιστών της Πολυτεχνικής Σχολής του Πανεπιστημίου Πατρών. Κύριος στόχος είναι η επιλογή ορισμένων μεθόδων εντοπισμού του σημείου απομάστευσης μέγιστης ισχύος, που μπορούν να εφαρμοστούν σε διάταξη θερμοστοιχείων και η μελέτη αυτών ώστε να εξαχθούν συγκριτικά αποτελέσματα για τις μεθόδους αυτές. Στην παρούσα διπλωματική εργασία διεξήχθη η απαραίτητη βιβλιογραφική αναζήτηση, η μελέτη των μεθόδων απομάστευσης μέγιστης ισχύος, η μελέτη των ιδιαίτερων χαρακτηριστικών των θερμοστοιχείων και η μοντελοποίηση τους ως γεννήτριες ηλεκτρικής ενέργειας. Αρχικά έγινε η αναζήτηση στη βιβλιογραφία των ιδιοτήτων και των χαρακτηριστικών λειτουργίας των θερμοστοιχείων για κατανόηση της λειτουργίας τους. Έγινε μελέτη των ιδιαιτεροτήτων τους και των διαφορών τους από τα άλλα συστήματα που έχουν εφαρμοστεί μέθοδοι απομάστευσης μέγιστης ισχύος, καθώς και αναζήτηση για εύρεση μοντέλου κατάλληλου για την προσομοίωση τους. Στη συνέχεια έγινε βιβλιογραφική αναζήτηση για τις μεθόδους απομάστευσης μέγιστης ισχύος που έχουν εφαρμοστεί σε θερμοστοιχεία, αλλά και σε άλλα συστήματα, όπως ανεμογεννήτριες και φωτοβολταϊκά συστήματα. Μελετήθηκε η φιλοσοφία της κάθε μίας και μεταξύ αυτών έγινε επιλογή των καταλληλότερων για την εφαρμογή τους σε θερμοστοιχεία. Το επόμενο βήμα ήταν η προσομοίωση τόσο του θερμοστοιχείου βάσει ενός εκ των μοντέλων που υφίστανται στη βιβλιογραφία, σε περιβάλλον Matlab/Simulink, όσο και των μεθόδων απομάστευσης μέγιστης ισχύος που επιλέχθηκαν. Στόχος είναι η καλύτερη κατανόηση των χαρακτηριστικών των θερμοστοιχείων και η διεξαγωγή πρώιμης σύγκρισης βάσει των αποτελεσμάτων των προσομοιώσεων. Τέλος μελετήθηκε και κατασκευάστηκε πειραματική διάταξη, κατάλληλη για την εφαρμογή των μεθόδων απομάστευσης μέγιστης ισχύος. Βάσει των μετρήσεων που λήφθηκαν με αυτή τη διάταξη έγινε η τελική σύγκριση μεταξύ των μεθόδων που επιλέχθηκαν. / This diploma thesis deals with the analysis of the operation of thermoelectric elements, and with a comparative study on methods of maximum power bleed and constructing a provision for the experimental comparison of these methods. This work was conducted in the Laboratory of Electromechanical Energy Conversion, Department of Electrical and Computer Engineering, Faculty of Engineering, University of Patras. The main objective is to choose between certain maximum power point tracking methods that can be applied on thermocouples and study them in order to draw comparative results for these methods. In this diploma thesis the necessary literature search was conducted. Also a study on maximum power point tracking methods, and a study on the characteristics of thermocouples was conducted for modeling them as generators of electricity. Initially, a literature search on the properties and operating characteristics of the thermocouples was done in order to understand their function. We investigated their specificities and their differences from other systems where maximum power point tracking methods have been applied, in order to find a suitable model for their simulation. Then a literature search, for maximum power point tracking methods that are applied to thermocouples and other systems such as wind generators and photovoltaic systems, was done. The philosophy of each one of them was studied and were selected the most suitable for application on thermocouples. The next step was to simulate both the thermocouple, under one of the existing models in the literature, and the methods for maximum power point tracking that were selected, on Matlab / Simulink. The objective is to better understand the characteristics of thermocouples and to conduct an early comparison based on the simulation results. Finally an experimental device suitable for implementing the methods was studied and built. Based on the measurements obtained with this device the final comparison between the methods was made.

La nanostructuration des matériaux thermoélectriques semble être la solution pour palier au faible rendement généralement rencontré chez ces matériaux. Bien qu’il y ait eu de nets progrès en manipulant de nouvelles formulations et en nano-structurant les composés existants, peu de travaux ont été entrepris sur la fabrication de dispositifs permettant la micro-génération ou le micro-refroidissement thermoélectrique. Dans ce contexte, les travaux de recherche ont été en premier lieu l’optimisation et la maitrise de la croissance par voie électrochimique de réseaux auto-ordonnés de nanofils thermoélectriques à base de tellurure de bismuth de type n (Bi2Te3-xSex) et de type p (BixSb2-xTe3) dans des membranes d’alumine nanoporeuses. Les propriétés structurales et thermoélectriques de ces nanofils ont été investigués. Concernant les caractérisations électriques, une nouvelle méthode de mesure sur nanofil unique dans sa membrane a été développée. Elle permet de s’affranchir des problèmes d’oxydation quand le fils est libéré de sa membrane dans les méthodes de mesure classiques. La conductivité thermique a aussi été estimée sur des fils dans leurs membranes avec la méthode 3-omega. La deuxième partie de la thèse a été consacré à des premiers essais d'assemblage des deux types pour évaluer la performance d’un prototype de thermogénérateur (TEG) fonctionnant à une température proche de l’ambiante. Et en parallèle une étude des problèmes de résistance de contact entre le métal et le thermoélectrique a été réalisée. / Nanostructuration of thermoelectric materials seems to be the solution for their low efficiency generally encountered. Even if fair progress have been realized in manipulation of new formulations and nanostructuration of existing materials, few studies have been undertaken to develop devices for thermoelectric micro-generation or micro-cooling. In this context, bismuth antimony telluride (BixSb2-xTe3) and bismuth tellurium selenide (Bi2Te3-xSex) nanowires, have been potentiostatically electrodeposited using anodic alumina membrane. Nanowires structure, stoichiometry and thermoelectric properties have been investigated. A new method for electrical characterization have been developed, consisting in direct electrical transport measurement on a single nanowire embedded in his matrix. This method avoid oxidation effects and time-consuming processing as nanowires need no preliminary preparation and remain in their template during measurement. Thermal conductivity was also investigated using 3-omega method. The second part of this work was dedicated to first tests on n-type and p-type assembly in order to study the efficiency of TEG prototype working at ambient temperature. In parallel, a study of contact resistance (thermoelectric / metal) have been conducted.

abstract: Thermoelectric devices (TED's) continue to be an area of high interest in both thermal management and energy harvesting applications. Due to their compact size, reliable performance, and their ability to accomplish sub-ambient cooling, much effort is being focused on optimized methods for characterization and integration of TED's for future applications. Predictive modeling methods can only achieve accurate results with robust input physical parameters, therefore TED characterization methods are critical for future development of the field. Often times, physical properties of TED sub-components are very well known, however the "effective" properties of a TED module can be difficult to measure with certainty. The module-level properties must be included in predictive modeling, since these include electrical and thermal contact resistances which are difficult to analytically derive. A unique characterization method is proposed, which offers the ability to directly measure all device-level physical parameters required for accurate modeling. Among many other unique features, the metrology allows the capability to perform an independent validation of empirical parameters by measuring parasitic heat losses. As support for the accuracy of the measured parameters, the metrology output from an off-the-shelf TED is used in a system-level thermal model to predict and validate observed metrology temperatures. Finally, as an extension to the benefits of this metrology, it is shown that resulting data can be used to empirically validate a device-level dimensionless relationship. The output provides a powerful performance prediction tool, since all physical behavior in a performance domain is captured using a single analytical relationship and can be plotted on a singe graph. / Dissertation/Thesis / M.S. Mechanical Engineering 2011

Depuis les cinquante dernières années, les préoccupations d’ordre énergétique sont au cœur de l’actualité. Or, une grande partie de l’énergie produite est rejetée et perdue sous forme de chaleur. Ainsi, la récupération d’énergie par des générateurs thermoélectriques apparaît comme une solution pour le mixe énergétique de demain.La thermoélectricité est la conversion directe et réciproque entre énergie thermique et électrique. Les générateurs thermoélectriques sont constitués d’un assemblage de plots de semi-conducteurs de type n et p. Un gradient de température appliqué entre les deux faces du générateur entraîne une migration des charges du matériau qui génère un courant électrique.Les systèmes thermoélectriques ont attiré l’attention du monde scientifique grâce à leurs avantages comparativement aux moyens de récupération d’énergie plus conventionnels. Ce sont des dispositifs compacts, statiques, silencieux et fiables, qui possèdent une longue durée de vie sans nécessiter de maintenance et impactant peu l’environnement.Pour la récupération d'énergie, le challenge actuel est la perte d’énergie thermique des automobiles et des camions ainsi que la chaleur perdue générée dans les industries de la métallurgie ou du nucléaire, par exemple. Ces deux segments nécessitent l’utilisation de modules thermoélectriques ayant un rendement optimum dans la gamme de température 300-600 °C.La performance d’un matériau thermoélectrique est exprimée par le facteur de mérite ZT, donné par l’expression : ZT=S2σT/к. Un ZT élevé peut être obtenu en optimisant les propriétés de transport du matériau. Le coefficient de Seebeck (S), et la conductivité électrique (σ), doivent être le plus élevé possible, alors que la conductivité thermique (κ) doit rester faible.Afin d’être viable pour une production industrielle, un matériau thermoélectrique doit répondre à un certain nombre de critères. Premièrement, ses composants doivent être non toxiques, peu chers et abondants. Ensuite, la voie de fabrication doit être robuste et compatible avec une production en grand volume. Enfin, les matériaux élaborés doivent posséder des propriétés thermoélectriques satisfaisantes dans la gamme de température de l’application visée. Ils doivent également être stables selon les environnements liés à l’application et avoir une bonne tenue mécanique.Les matériaux de type half-Heusler apparaissent comme prometteurs pour la génération de puissance thermoélectrique dans la gamme de température 300-600 °C. En effet, ils possèdent un coefficient de Seebeck et une conductivité électrique élevés. Cependant, leur conductivité thermique est relativement haute comparée aux autres matériaux thermoélectriques.Ce travail de thèse s’est donc focalisé sur l’étude des relations microstructure-propriétés thermoélectriques de matériaux half-Heusler de composition générique (Hf,Zr,Ti)Ni(Sb,Sn) et (Hf,Zr,Ti)Co(Sb,Sn) et de leurs possibles variantes. Les compositions testées ont toutes été synthétisées de la même manière : une fusion par induction permet d’obtenir des lingots qui sont ensuite réduits en poudre par broyage, celle-ci est ensuite frittée par frittage SPS (spark plasma sintering) afin d’obtenir une pastille dense et polycristalline. Les propriétés thermoélectriques et la microstructure de ces échantillons sont ensuite caractérisées et discutées.Un des objectifs de ce travail de thèse était également de réduire coût au kilogramme de ces matériaux half-Heusler, sans impacter de manière négative leurs propriétés thermoélectriques. Nous y sommes parvenus, d’une part, en réduisant la concentration en hafnium incorporé dans les formulations, et d’autre part, en simplifiant le processus de fabrication. En effet, nous avons observé qu’une synthèse sous air des poudres half-Heusler permettait la formation in-situ de précipités d’oxydes, agissant comme source de diffusion des phonons et donc favorisant la diminution de la conductivité thermique. / The search for alternative energy technologies has taken an accelerated pace in the last 50 years due to an increasing concern about climate change. In this quest to find new energy sources, it is interesting to point out that a lot of energy is wasted as heat released into the environment. As a potential solution, thermoelectric power generators could be used to transform the waste heat into useful electrical energy.Thermoelectric generators are converting directly heat into electricity and vice versa. They consist in an assembly of n and p-type semiconducting legs connected electrically in series and thermally in parallel. An applied temperature difference between n and p-sides drives charge carriers displacement in the material from the hot side to the cold one. Therefore a current flow is generated through the circuit. Thermoelectric devices have attracted interest because of their advantages over conventional power generator: no moving part, no liquid involved, reliability, noiseless, long life time without maintenance and also low environmental impact.Over the last several decades, the increased energy demand combined to the environmental concerns, leads to another potential use of thermoelectricity as an alternative energy source by recovering the huge amount of heat lost in industrial or domestic applications. Presently, wasted-heat recovery in cars and trucks and wasted-heat in industry (metallurgy/nuclear…) are becoming a major concern. Both recovery problematics may be addressed using thermoelectric devices efficient in the 300-500 °C temperature range.Numerous thermoelectric materials couples have been investigated and developed over the last 20 years. Most of the already known class of thermoelectric materials have been improved and new classes have been developed, leading to a significant improvement of ZT values being optimum in different temperature ranges. In order to be efficient and to be viable for large scale manufacturing of power generators, a thermoelectric material has to fulfill several requirements. First, the raw materials chosen have to be non-toxic, cheap and abundant. Secondly, the manufacturing process should be robust and compatible to the production of a high volume of materials per day. Last but not least, the elaborated materials have to exhibit acceptable thermoelectric properties in the temperature range of interest for the final application. They must also have a long-term thermal stability in different kinds of environments and good mechanical properties.Half-Heusler materials have been shown to be good candidates in the 300 to 600 °C temperature range. Indeed, due to their semiconductor like band structure, they exhibit a large Seebeck coefficient and high electrical conductivity. Unfortunately, half-Heusler’s thermal conductivity is rather high when compared to other thermoelectric materials. Therefore, the main research efforts on half-Heusler formulations, devoted to be used for thermoelectric applications, have been focused on decreasing the thermal conductivity, while keeping a good electronic transport.Accordingly the main objective of the PhD thesis was to investigate the link between the microstructure and the thermoelectric properties of n and p-type half-Heusler alloys from the generic compositions MNiSn (n-type) and MCoSb (p-type), with M being Ti, Zr and Hf. All investigated compositions have been elaborated by a three step process: (i) ingots synthesis using cold crucible levitation melting, (ii) subsequent ball milling to obtain a calibrated powder and (iii) sintering by spark plasma sintering to obtain dense polycrystalline pellets that are characterized regarding their microstructure and thermoelectric properties from room temperature to 500-600 °C.