Thèse de doctorat : Sciences physiques : Paris, Faculté des sciences : 1875. / Titre provenant de l'écran-titre.

Thesis (doctoral)--Göteborgs, universitet, 1985. / Summary in English. Added t.p. with thesis statement inserted. Includes bibliographical references (p. 274-280).

Thesis--University of Pennsylvania.

Thesis (Ph. D.)--Ohio State University, 2003. / Title from first page of PDF file. Document formatted into pages; contains xiii, 303 p.; also includes graphics (some col.). Includes abstract and vita. Advisor: Lei Bao, Dept. of Physics. Includes bibliographical references (p. 299-303).

Driven by resource politics and climate change, the transition from conventional fossil fuel based and centralized energy generation to distributed renewables is increasing rapidly. Wind and solar power generation offer carbon dioxide neutral electricity but also present some integration difficulties for energy system operators and planners due to intermittent power output. A promising way of dealing with the intermittency from renewables is energy storage. The method of storing energy in the electricity grid, especially by the means of electrochemical storage, has gained a lot of attention over the last years in the energy sector. While most utilities and energy market stakeholders have the basic understanding of energy storage, a more profound knowledge of grid storage applications is often lacking. This thesis aims to highlight and explain possible energy storage applications with focus on renewables integration. Battery energy storage can allow higher amounts of renewable electricity generation to be integrated by smoothening power output, time shifting generated energy to follow demand and increase hosting capacities through peak shaving. Power quality related issues due to intermittency can be mitigated by controlling the storage’s charging patterns to respond to grid variables. For optimal utilization and maximum storage value, several applications should be within the operational repertoire of the storage unit. Other applications including arbitrage, grid investment deferral and load following are discussed. Several battery technologies which have been developed and tested for such applications including lead acid, sodium sulfate and lithium-ion are presented. The most promising battery energy storage technology is lithium-ion with exceptional storage characteristics and most importantly a favorable near term price development. Two case studies on two of Umeå Energy’s low voltage networks simulating high penetrations of solar generation have been carried out to demonstrate mitigation of overvoltage and peak shaving with battery energy storage systems. The simulations show that energy storage systems can successfully be used to aid the integration of renewables in the electricity grid. Present capital costs are still too high for energy storage to be feasible but falling pricing and a developing market is foreseen to lower the hurdles.  The main obstacle for energy storage at grid scale besides high capital costs are, in principle, non-existing legal frameworks regulating the ownership of energy storage systems and system technology standardization. Further discussions on the matter in combination with testing and pilot projects are needed to gain national and international experience with battery energy storage for the successful high share integration of renewables. / Sinande naturresurser och växthuseffekten driver på övergången från centraliserad kraftproduktion baserad på fossila bränslen till distribuerad förnyelsebar energiproduktion i rask takt. Vind- och solkraft levererar koldioxidneutral el men ställer samtidigt balansansvariga och elnätsplanerare inför en rad problem på grund av periodiskt återkommande och tidvis ostabil effektgenerering. Energilager presenteras som en lovande lösning på problemen orsakade av förnyelsebara energikällor Att lagra energi i elnätet, i synnerhet med batterier, har fått en hel del uppmärksamhet de senaste åren i energibranschen. De flesta elnätsbolag och intressenter på energimarknaden har en grundläggande förståelse kring energilagring i elnätet men saknar ofta mer djupgående kunskap. Detta examensarbete syftar att belysa och förklara användningsområden och potentialer för energilagring med fokus på integreringen av förnyelsebara energikällor. Teorin beskriver hur batterilager kan användas för tillåta integreringen av en hög andel förnyelsebar elproduktion. Några tillämpningar är; effektutjämning, lagring av producerad energi för senare bruk samt ökad nätkapacitet genom att kapa toppar. Problem relaterade till försämrad elkvalité orsakad av varierande kraftproduktion visas kunna pareras med hjälp av programmerbara energilagringssystem som läser av storheter på elnätet såsom spänning och frekvens. För att utnyttja energilagret optimalt och komma åt dess maximala värde bör flera användningsområden kombineras. Därför diskuteras även andra användningsområden såsom arbitrage, lagringskapacitet för att skjuta upp eller undvika förstärkning av elnätet och lastföljning. Ett flertal batteriteknologier aktuella för de diskuterade användningsområdena såsom bly-, natriumsulfat- och litium-jonbatterier presenteras. Den mest lovande teknologin är litium-jon tack vare dess utmärkta egenskaper och framförallt mycket gynnsamma förväntade prisutveckling. Två fallstudier av två av Umeå Energi´s nätområden med hög simulerad andel solenergiproduktion har utförts för att demonstrera utnyttjandet av energilager för reglering av överspänning och kapning av toppar. Simuleringarna visar att energilagringssystem med framgång kan underlätta integreringen av förnyelsebara energikällor. Dagens kapitalkostnader är fortfarande för höga för att energilagring ska vara ekonomiskt försvarbart men fallande priser och en växande marknad väntas verka till teknikens fördel. Det visar sig att regelverk gällande ägandeskapet och standardiseringen av energilager är i det närmaste obefintliga vilket utgör ytterligare hinder för tekniken. Fortsatta diskussioner gällande dessa punkter i kombinationen med test- och pilotanläggningar för att införskaffa erfarenhet av energilagring i elnätet krävs.

This dissertation examines single-phase nonlinear power electronic loads. The transient response of power electronic loads is unlike traditional linear loads. Therefore, a composite power electronic transient load model is developed. The load model dynamics are validated with actual utility voltage sag response data, laboratory controlled load response testing, and power electronic load dynamic simulations. The power electronic load model is applied in the University of Texas at Austin power system. The system transient response is unique and considerably different from what it would be with traditional linear loads. The power electronic load can be friendly or unfriendly to the system depending on the fault and system configuration. / text

Since its first successful growth in 1996, the ferromagnetic semiconductor (Ga,Mn)- As has had a great inuence on the research field of semiconductor spintronics. Among the outstanding characteristics of this material the large spin-orbit interaction for the holes in the valence band plays a major role, since it is responsible for some of the most interesting properties of (Ga,Mn)As, like the magnetocrystalline anisotropy, the magnetoelastic coupling and the extraordinary contributions to the magnetotransport. Furthermore, the combination of large magnetic anisotropies, large spin stiffness and relatively small magnetic moments renders (Ga,Mn)As a hard ferromagnetic system with excellent micromagnetic properties, including mean-field like magnetization and macroscopic single-domain characteristics, that can be described both phenomenologically and microscopically, thanks to the relatively simple band structure. Finally, the interplay between ferromagnetism and semiconductivity, arising from the hole-mediated nature of the ferromagnetic interaction in (Ga,Mn)As, allows for the remarkable possibility of manipulating its magnetic properties by varying the state of the holes using non-magnetic parameters like electric fields, electric currents, light or strain. This circumstance could in principle be very useful to improve the process of writing information in magnetic memories, which is currently performed, not very efficiently, with magnetic fields. However, it does seem unlikely that (Ga,Mn)As will become a relevant material for technological applications since the highest Curie temperature so far obtained for (Ga,Mn)As is still well below room temperature. Nonetheless the study of (Ga,Mn)As remains a fervent research area since it allows to explore a variety of novel functionalities and spintronics concepts that could in future be implemented in other systems. For this reason (Ga,Mn)As is often referred to as a test bench material for semiconductor spintronics. This Thesis presents the results of a series of experimental investigations showing how dfferent approaches can be used to manipulate the magnetic anisotropy in (Ga,Mn)As thin films. In Chapter 4 the properties of the ferromagnetic semiconductor (Ga,Mn)(As,P) are investigated through structural, magnetometry, transport and magnetotransport measurements. By varying the amount of phosphorus incorporated it is possible to vary the sign of the in-built growth strain, to which the magnetic anisotropy in (Ga,Mn)As is extremely sensitive. It is in fact shown that samples with large enough phosphorus concentrations are characterized by a perpendicular-to-plane magnetic easy axis, which is an extremely useful property since it allows to detect the orientation of the magnetization via anomalous Hall effect and polar magneto-optical Kerr effect. Furthermore, it is demonstrated that by varying the temperature or the post-growth annealing time it is possible to obtain a reorientation of the magnetic easy axis from an in-plane direction to the perpendicular-to-plane direction in some samples, which is another interesting aspect of this material. Chapter 5 consists of a study exploring the effects of piezoelectric-induced strain on the magnetic anisotropy of a highly-doped annealed (Ga,Mn)As sample bonded to a piezoelectric actuator. It is shown that large and reversible rotations of the magnetic easy axis can be achieved in this sample by varying the voltage applied to the piezoelectric actuator, thus demonstrating that strain-mediated electric control of ferromagnetism is effective even in the limit of high doping levels and high Curie temperatures, where direct electric control of ferromagnetism via carrier manipulation is not possible. Furthermore, the results obtained from magnetotransport and SQUID magnetometry measurements are compared, extracting the dependence of the piezo-induced uniaxial magnetic anisotropy constant upon strain in both cases and discussing why the magnetotransoport measurements are believed to be more accurate than SQUID magnetometry measurements in evaluating the inverse magnetostriction effects in (Ga,Mn)As-piezoelectric actuator hybrid systems. Finally, Chapter 6 contains the results of an investigation attempting to use ultrashort strain pulses to switch the magnetization direction in a (Ga,Mn)(As,P) sample on fast time scales. These pulses are generated by femtosecond optical excitation of a metal transducer film deposited on the back of the substrate and travel ballistically through it until they reach the sample under investigation. Despite demonstrating that this method can indeed be used to induce a fast irreversible switching of the magnetization orientation in the (Ga,Mn)(As,P) sample, time-resolved magnetotransport measurements show that the switching is not triggered by the strain pulse, but rather by the transverse heat pulse, the latter being generated with the strain pulse during the optical excitation of the metal film. It is shown that the switching occurs through domain-related processes and the possible mechanisms behind its cause are speculated.

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