Fuzzy Logic Controlled Microturbine Generation System for Distributed Generation

AbstractThe microturbine based Distributed Generation (DG) system are becoming the popular source of power industries due to their fuel flexibility, reliability and power quality. The microturbine generation (MTG) system is a complicated thermodynamic electromechanical system with a high speed of rotation, frequency conversion and its control strategy. In spite of several techniques to control high speed of microturbine is not accurate and reliable due to their anti-interference problem. To resolve the anti-interfacing problem, this paper investigates the fuzzy logic based speed governor for a MTG system as an alternative to nominal PI or lead-lag based controller. The development of fuzzy logic based speed governor includes input and output membership function with their respective members. The load variation on MTG system is performed using conventional and fuzzy logic controller, implemented in Matlab/simulink and results are compared with each other. The simulation result shows that, the performance improvement of fuzzy logic governor over a nominal governor based MTG system

A treatise on first-principles studies of ZnO as diluted magnetic semiconductor

Diluted magnetic semiconductors (DMS) are important functional materials that bring together two fundamental aspects of electron, namely the charge and the spin, to derive new properties in materials. Based on mean-field models it was shown that wide band gap diluted semiconductors (with ZnO and GaN) could lead to room temperature ferromagnetism. This triggered much interest in the subject. However, these results are still under intensive debate. The scope of the present thesis is to study the properties of ZnO doped with transition metal elements by the density functional theory (DFT). The DFT is an accurate theory for modeling material properties, which describe the fundamental interactions of electrons with the nucleus and is in principle free from any parameterization. In this thesis, there is extensive study of the electronic structure of ZnO beyond the generalized gradient approximation (GGA) as exchange-correlation functional (Exc), which is well known to be associated with some drawback in predicting the semiconducting properties. One of the method adopted for improvement is by adding extra correlation-energy to GGA (GGA+U) in line with the Hubbard model. We find that with adding U on Zn d-orbitals, the band gap improves with simultaneous shift of the Zn d-bands to lower energies. The band gap nevertheless is still far below the experimental value. Another approach to correct for the Exc is by the treatment of screened hybrid-functionals proposed by Heyd-Scuseria-Ernzerhof (HSE), where the contribution to the exchange energy is through the blend of some percentage of screened Hartree-Fock exchange and rest from the GGA exchange. The correlation energy is derived from GGA. This method also has limitations in describing the electronic structure of ZnO. A simple proposition of mixing the GGA+U and the hybrid-functional treatment may be a solution, which has been investigated in this work. The band structure of ZnO has been compared for various level of theory for Exc, namely, LDA, GGA, GGA+U, and HSE functionals with different screening lengths (μ). A proposal for HSE+U functionals has been been put forward and supported. Using some of the treatments for Exc as stated above, the magnetic properties of TM (Cr, Mn, Fe, Co, and Ni) doped ZnO are studied. Firstly, the electronic structure calculations for TM doped ZnO are done with the Korringa-Kohn-Rostoker (KKR) method with the LDA and using the coherent potential approximation (CPA). The exchange integrals are then calculated using the Liechtenstein's formalism, which are then used in the Monte Carlo simulations to estimate the critical temperature. A case study for Co doped ZnO is discussed, where we have calculated the magnetic phase diagram of the system. Apart from this, supercell calculations with Vienna ab-initio simulation package (VASP) have been done to compare the role of various treatment to the Exc. In case of GGA+U, the U is separately treated on Zn d-orbitals and TM d-orbitals, and then together. From the total energy differences in magnetic states, it is observed that with incorporation of U, all the TM favor antiferromagnetic interactions. This may partially explain why some experimental studies on TM doped ZnO do not show any ferromagnetic correlation. Similar conclusion is also reached for most cases in HSE and HSE+U approach. One of the recent topics under discussion for polar semiconductor materials like ZnO is d0 magnetism. There are several reports in experiments on magnetism due to intrinsic-defect and 2p-block elements doped extrinsic ZnO, the source of magnetism is due to localization of holes and is a topic under active discussion. In present study based on GGA it is shown that Zn-vacancy (VZn) and C substitution on O site (CO) lead to spin-polarized solution. The magnetic energy is mostly below the room-temperature. Related to this, some perspectives of experimental situations which could lead to quenching of such magnetization is also presented. Furthermore, studies on pure ZnO clusters based on GGA and HSE functionals are presented. A general tendency which is observed is that the bond length predicted in HSE calculations are larger than those predicted by GGA. Characteristic arrangement of magnetization density in VZn and CO in clusters as calculated from GGA is presented. A direct comparison of the present results with experiments is difficult because there is wide variety of experimental results for TM doped ZnO which are mostly dependent on sample preparation techniques and for the case of d0-magnetism there are very few element specific experimental characterization. However, the studies presented in the thesis has up-to-date theoretical footing and this is the strength of the work.Verdünnte magnetische Halbleiter (DMS) sind bedeutende funktionale Materialien, die die zwei fundamentalen Aspekte des Elektrons nämlich Ladung und Spin zusammen nutzen, um neue Materialeigenschaften zu generieren. Im Rahmen von Mean-field Modellen wurde gezeigt, dass für verdünnte Halbleiter mit großer Bandlücke (die auf ZnO und GaN basieren) Ferromagnetismus bei Raumtemperatur möglich ist. Dies hat zu einem großen Interesse an diesen Systemen geführt. Allerdings sind diese Ergebnisse immer noch Gegenstand intensiver Diskussion. Das Ziel der vorliegenden Dissertation ist die Untersuchung von ZnO, das mit Übergangsmetallen dotiert wurde im Rahmen der Dichtefunktionaltheorie (DFT). Die DFT ist eine genaue, im Prinzip parameterfreie Methode zur Modellierung von Materialeigenschaften, die die fundamentalen Wechselwirkungen des Elektronensystems mit den Kernen beschreibt. In dieser Arbeit werden intensive Studien von ZnO durchgeführt, die über die verallgemeinerte Gradientenkorrektur (GGA) für das Austausch-Korrelationsfunktional (Exc) hinausgehen, da die GGA den halbleitenden Charakter nicht hinreichend beschreiben kann. Eine Methode besteht darin der GGA im Rahmen des Hubbard-Modells eine extra Korrelationsenergie zuzufügen (GGA+U). Wir finden, dass U angewendet auf die Zn d-Orbitale zu einer besseren Beschreibung der Bandlücke führt und gleichzeitig die Zn d-Bänder zu tieferen Energien verschoben werden. Trotzdem ist die Bandücke nach wie vor deutlich kleiner als der experimentelle Wert. Eine andere Näherung, die verwendet wird, um Exc zu korrigieren, ist das von Heyd-Scuseria-Ernzerhof (HSE) vorgeschlagene abgeschirmte (screened) Hybridfunktional. Dabei wird die Austauschenergie anteilig aus screened Hatree-Fock- und GGA-Austauschenergie zusammengesetzt. Die Korrelationsernergie wird mittels GGA bestimmt. Auch diese Methode hat ihre Grenzen hinlänglich der Beschreibung der elektronischen Struktur von ZnO. Eine Lösung scheint die Kombination von GGA+U und Hybridfunktional zu sein, was im Rahmen dieser Arbeit untersucht wurde. Es wurden die Bandstrukturen von ZnO verglichen, die sich im Rahmen verschiedener Näherungen für Exc nämlich LDA, GGA, GGA+U und HSE mit unterschiedlichen Abschirmlängen (μ) ergaben. Die Untersuchungen lassen die Schlussfolgerung zu, dass die Verwendung von HSE+U die beste Beschreibung liefert. Unter Verwendung der oben genannten Näherungen für Exc wurden die magnetischen Eigenschaften von TM (Cr, Mn, Fe, Co und Ni) dotiertem ZnO untersucht. Zuerst wurde die elektronische Struktur des TM dotierten ZnO im Rahmen der Korringa-Kohn-Rostoker (KKR)-Methode unter Verwendung der LDA und der Coherent Potential Approximation (CPA) bestimmt. Zur Berechnung der Austauschintegrale wurde der Formalismus von Liechtenstein verwendet. In anschließenden Monte Carlo Simulationen wurden die damit bestimmten Austauschintegrale zur Abschätzung der kritischen Temperatur benutzt. In einer Fallstudie wurde für Co dotiertes ZnO die Berechnung des magnetischen Phasendiagramms durchgeführt. Weiterhin wurden Superzellenrechnungen unter Verwendung des Vienna Ab Initio Simulation Package (VASP) gemacht, um den Einfluss verschiedener Behandlungen von Exc zu untersuchen. Im Fall der GGA+U Approximation wurde U separat auf die d-Orbitale von Zn und TM angewendet sowie auch auf beide d-Orbitale gleichzeitig. Aus den Differenzen der Gesamtenergien folgt, dass in Anwesenheit von U alle TM Atome paramagnetisch sind. Dies könnte teilweise erklären, warum in experientellen Untersuchungen TM dotiertes ZnO keine ferromagnetischen Korrelationen gefunden werden. Ähnliche Schluß folgerungen ergeben sich in den meisten Fällen auch im Rahmen der HSE und HSE+U Näherung. Ein aktuelles Thema bezüglich polarer Halbleiter wie ZnO ist der d0-Magnetismus. Obwohl bereits einige experimentelle Arbeiten für ZnO vorliegen, die Magnetismus aufgrund von intrinsischen Defekten und extrinsischen Defekten aufgrund von Dotierung mit Hauptgruppenelementen diskutieren, ist der Ursprung des Magnetismus nicht geklärt und immer noch Gegenstand aktueller Forschung. In der vorliegenden auf der GGA basierenden Arbeit wird gezeigt, dass für Zn-Leerstellen (VZn) und C auf dem Sauerstoffplatz (CO) eine endliche Spinpolarisation auftritt. The magnetische Energie liegt in den meisten Fällen unterhalb der Raumtemperatur. In diesem Zusammenhang wird auch diskutiert welche Bedingungen im Experiment zu einer Auslöschung der magnetischen Moments führen können. Weiterhin wurden ZnO Cluster unter Verwendung von GGA und HSE Fuktionalen untersucht. Dabei zeigt sich, dass die in HSE-Rechnungen vorhergesagte Bindungslänge größer ist, als die mit GGA bestimmte Bindungslänge. Im Fall von GGA wurde auch die Magnetisierungsdichte aufgrund von VZn und CO untersucht. Ein direkter Vergleich der Ergebnisse mit experimentellen Daten ist schwierig, da zwar eine Vielzahl von Ergebnissen für TM dotiertes ZnO vorliegen, diese aber meist von der Probenpräparation abhängen und für den Fall von d0-Magnetismus nur wenige elementspezifische Daten verfügbar sind. Die Stärke dieser Arbeit liegt daher in der Auffindung und Diskussion einer geeigneten theoretischen Beschreibung für magnetische Halbleiter wie ZnO

Anisotropic ferromagnetism in carbon doped zinc oxide from first-principles studies

A density functional theory study of substitutional carbon impurities in ZnO has been performed, using both the generalized gradient approximation (GGA) and a hybrid functional (HSE06) as exchange-correlation functional. It is found that the non-spinpolarized C$_\mathrm{Zn}$ impurity is under almost all conditions thermodynamically more stable than the C$_\mathrm{O}$ impurity which has a magnetic moment of $2\mu_{\mathrm{B}}$, with the exception of very O-poor and C-rich conditions. This explains the experimental difficulties in sample preparation in order to realize $d^{0}$-ferromagnetism in C-doped ZnO. From GGA calculations with large 96-atom supercells, we conclude that two C$_\mathrm{O}$-C$_\mathrm{O}$ impurities in ZnO interact ferromagnetically, but the interaction is found to be short-ranged and anisotropic, much stronger within the hexagonal $ab$-plane of wurtzite ZnO than along the c-axis. This layered ferromagnetism is attributed to the anisotropy of the dispersion of carbon impurity bands near the Fermi level for C$_{\mathrm{O}}$ impurities in ZnO. From the calculated results, we derive that a C$_{\mathrm{O}}$ concentration between 2% and 6% should be optimal to achieve $d^{0}$-ferromagnetism in C-doped ZnO.Comment: 9 pages, 7 figure

Comparison of the Effectiveness of Three Methods of Recanalization in a Model of the Middle Cerebral Artery: Thrombus Aspiration via a 4F Catheter, Thrombus Aspiration via the GP Thromboaspiration Device, and Mechanical Thrombectomy Using the Solitaire Thrombectomy Device

Introduction. This paper compares different approaches to recanalization in a model of the middle cerebral artery (MCA). Methods. An occlusive thrombus (lamb's blood) was introduced into the MCA of a model of the cerebral circulation perfused with Hartmann's solution (80 pulsations/min, mean pressure 90 mm Hg). Three methods of clot retrieval were tested: thrombus aspiration via a 4F catheter (n = 26), thrombus aspiration via the GP thrombus aspiration device (GPTAD) (n = 30), and mechanical thrombectomy via the Solitaire Device (n = 30). Results. Recanalization rate was similar for all 3 approaches (62%, 77%, and 85%). Time to recanalization was faster with aspiration devices (41 SD 42 s for 4F and 61 SD 21 s for GPTAD) than with the Solitaire (197 SD 64 s P < .05 Kruksal-Wallis). Clot fragmentation was the same in the Solitaire (23%) and the GPTAD (23%), but higher with the 4F (53%, P < .05). Conclusion. In this model, thrombus aspiration was faster than mechanical thrombectomy, and similarly effective at recanalization. These results should be confirmed in vivo

Is intraprocedural intravenous aspirin safe for patients who require emergent extracranial stenting during mechanical thrombectomy?

Background: Intraoperative antiplatelet therapy is recommended for emergent stenting during mechanical thrombectomy (MT). Most patients undergoing MT are also given thrombolysis. Antiplatelet agents are contraindicated within 24 hours of thrombolysis. We evaluated outcomes and complications of patients stented with and without intravenous aspirin during MT. Methods: All patients who underwent emergent extracranial stenting during MT at the Royal Stoke University Hospital, UK between 2010 and 2020, were included. Patients were thrombolysed before MT, unless contraindicated. Aspirin 500 mg intravenously was given intraoperatively at the discretion of the operator. Symptomatic intracranial haemorrhage (sICH) and the National Institutes for Health Stroke Scale score (NIHSS) were recorded at 7 days, and mortality and functional recovery (modified Rankin Scale: mRS ≤2) at 90 days. Results: Out of 565 patients treated by MT 102 patients (median age 67 IQR 57-72 years, baseline median NIHSS 18 IQR 13-23, 76 (75%) thrombolysed) had a stent placed. Of these 49 (48%) were given aspirin and 53 (52%) were not. Patients treated with aspirin had greater NIHSS improvement (median 8 IQR 1-16 vs median 3 IQR -9-8 points, p=0.003), but there were no significant differences in sICH (2/49 (4%) vs 9/53 (17%)), mRS ≤2 (25/49 (51%) vs 19/53 (36%)) and mortality (10/49 (20%) vs 12/53 (23%)) with and without aspirin. NIHSS improvement (median 12 IQR 4-18 vs median 7 IQR -7-10, p=0.01) was greater, and mortality was lower (4/33 (12%) vs 6/15 (40%), p=0.05) when aspirin was combined with thrombolysis, than for aspirin alone, with no increase in bleeding. Conclusion: Our findings based on registry data derived from routine clinical care suggest that intraprocedural intravenous aspirin in patients undergoing emergent stenting during MT does not increase sICH and is associated with good clinical outcomes, even when combined with intravenous thrombolysis.</p