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Nanostructured magnets are essential materials in numerous key applications, such as energy conversion, automotive industry, data storage and medical imaging. These developing technologies address critical climate change and sustainability issues and demand better magnetic materials. In order to design better magnets, it is crucial to understand the connection between the magnetic performance and nanoscale microstructure. Particularly important are grain and grain-boundary architecture, where complex spin structures may form due to competing magnetic interactions. So far, technological limitations in imaging nanoscopic spin structures have hindered progress in the field. In this thesis, Lorentz transmission electron microscopy (LTEM) and off-axis electron holography are combined with micromagnetic simulations to characterize complex spin structures and understand their connection to the nanoscale microstructure and macroscopic magnetic performance. Complex spin textures arise in chiral magnets due to the competition of the Heisenberg exchange and Dzyaloshinskii-Moriya interactions, which favor collinearity and curling of neighboring spins, respectively. Particle-like magnetic skyrmions form if the symmetry is broken by an external magnetic field or internal perpendicular magnetic anisotropy. Skyrmions have topologically non-trivial spin winding, meaning that they cannot be unwound without a phase transition. This stability makes them appealing for future spintronic applications, such as data storage. It is therefore important to understand how skyrmions behave in the presence of defects. In this thesis, it is numerically predicted that in chiral magnetic films without defects skyrmion lattices undergo a first-order phase transition in which they invert their magnetization. The inversion is induced by an external magnetic field and mediated by a temporary formation of antiskyrmions, which have an opposite spin winding to that of skyrmions. In the presence of defects, the inversion is replaced by a second-order phase transition of defect-induced melting, where the skyrmion lattice is gradually annihilated around the defects. This is experimentally observed in chiral FeGe thin films, where the coexistence of original and inverted skyrmion lattices is also found. These results signify the importance of defect analysis for the development of future spintronic materials. Contrary to what was previously believed, it is shown in this thesis that complex spin structures, such as topologically non-trivial domain walls and magnetic vortices, can also form in Sm(Co,Fe,Cu,Zr) permanent magnets due to their complex microstructure, which comprises SmCo5 cells with Cu-rich Sm2Co17 cell walls and a Zr-rich Z phase. These spin structures play a crucial role in the magnetization reversal by inducing spin curling and domain-wall nucleation where the Z phase intersects the cells and cell walls. The Z-phase thickness should therefore be minimized to increase the coercivity. It is further demonstrated by in situ heating experiments that the magnetization reversal in Sm(Co,Fe,Cu,Zr) magnets is dominated by domain-wall pinning at Sm2Co17 cell walls at low temperatures and domain-wall nucleation at high temperatures. The transition temperature between pinning and nucleation, as well as the coercivity, increase with increasing the slope of the Cu gradient in the cell walls. However, the coercivity also decreases with increasing the total Cu concentration in the cell walls. The Cu gradient should therefore be carefully designed to maximize the coercivity. These findings give clear guidelines on how to improve the performance of Sm(Co,Fe,Cu,Zr) magnets and reveal the complexity of magnetic structures and processes in permanent magnets. Magnetic imaging is generally performed in the highly specialized LTEM mode, where the objective lens is turned off to protect the sample from its magnetic field and replaced by a Lorentz lens below the sample. In this thesis, it is shown that such a modification is not required for permanent magnets because their magnetic state is not affected by the objective lens due to their high coercivity. Specifically, the domain-wall width parameter and the magnetic induction-thickness product are measured by conventional and Lorentz TEM with similar precision and accuracy. This demonstrates that magnetic imaging experiments on permanent magnets are accessible to a much wider scientific community.

Denne Ph.d.-afhandling omhandler forskning i innovative design- og optimeringsmetoder, og teknikker til avancerede kraftværkssystemer, der skal installeres på nye og eksisterende offshore olie- og gasplatforme. Systemet som studeres er generaliseret ved kraftværkssystemet som benyttes ombord på olie- og gasplatformen Draugen i Kristiansund, Norge. De nævnte metoder anvendes til at adressere fem forskningsmæssige mål: i) Definition af den optimale varmegenvindingsteknologi, ii) identificering af de bedste arbejdsmedier til effektive, kompakte og økonomisk attraktive varmegenvindingskedler, iii) integration af dynamiske kriterier i projekteringsfasen, for at identificere uhensigtsmæssige designs, iv) udvikling af en kontrolstrategi, som sikrer optimal systemeffektivitet, og v) forbedring af den dynamiske fleksibilitet, ved hjælp af model predictive control algoritmer. Den optimale teknologi bliver defineret ved hjælp af en nyudviklet optimeringsalgoritme, som kan identificere optimale Rankine- og Braytonprocesdesigns. Målene for optimeringerne er reducerede CO2 emissioner og komponentvægt, samt det økonomiske potentiale. Optimeringsalgoritmen benytter validerede designmodeller af varmevekslere, som giver et umiddelbart estimat på varmegenvindingsmodulets kompakthed, og på betydningen af trykfald i fht. systemets eektivitet. Økonomiske modeller og dellastsimuleringer gør det muligt at kvantificere emissionsreduktioner og investeringsafkast. Resultaterne indikerer at den organiske Rankineproces har det største potentiale for at reducere CO2-emissioner, mens dampkraftsprocessen præsenterer det største økonomiske potentiale. Braytonprocessen giver de letteste moduler, og deres anvendelse anbefales i forbindelse med eftermontering på eksisterende offshore platforme, hvis levetid er forlænget. Dampkraftsprocessen og den organiske Rankineproces er konkurrerende alternativer for nye installationer. Resultaterne indikerer at den organiske Rankineproces er fordelagtig i situationer hvor brændstoexibilitet er vægtet højt. For denne proces anbefales det at koncentrere de tekniske bestræbelser omkring at minimere produktionsomkostningerne for ekspanderen og vægten af varmevekslere. Benzen og cyklopentan er, i denne proces, arbejdsmedier som fremviser de bedste præstationer, og de giver mulighed for at designe kompakte og økonomisk effektive kraftværksmoduler. Den dynamiske analyse viser sig som et værdifuldt værktøj i designprocessen, idet den muliggør identifikationen af procesdesigns som ikke opfylder de specifikke krav som findes til offshore elnettet. I den forbindelse viser analyser at brugen af kompakte enheder bør frarådes pga. uacceptable frekvensudsving og procesustabilitet. Derudover kan det præsenterede simuleringsværktøj identificere systemdesigns, som udsætter arbejdsmediet for en uacceptabel risiko for kemisk nedbrydning. Den dynamiske analyse er udvidet yderliggere ved udviklingen af et innovativt kontrolsystem, som har til formål at styre gasturbinen og det organiske Rankineproces kraftværksmodul, på den mest effektive måde. Kontrollogikken anvender en model predictive control algoritme, til at opretholde kvaliteten i elforsyningen og samtidigt at sikre den maksimale effektivitet af anlægget. Pumpens hastighed varieres således at driften altid sker med maksimal virkningsgrad. Kontrollogikken kan ydermere verificere om det optimale driftspunkt er opnået, givet de operationelle begrænsninger. Resultaterne indikerer, at maksimal effektivitet er mulig under laster fra 40% op til nominel effekt, med potentielle brændstofbesparelser på omkring 3% og en forøgelse af den økonomiske indkomst på op til 10 %. Den foreslåede kontrollogik er også påvist at kunne føre til vigtige forbedringer i den dynamiske fleksibilitet, i forhold til standardkontrolleren. Feedbacksystemet kan i følge beregningerne lede til en 20 - 40% reduktion af frekvensudsving for gasturbinen alene, og mere end 60% reduktion med det organiske Rankineproces kraftværksmodul. Analyse af virkningen af ikke-målbare faktorer (såsom tilsmudsning i varmevekslerne) verificerer, at den dynamiske respons er mere stabil med det nyudviklede kontrolsystem. På den anden side viser resultaterne, at der er behov for en realtidsopdatering af de interne modeller, f.eks. vha. intelligente automatiserede adaptions- og læringsteknikker. This doctoral thesis is devoted to the research of innovative design methods and control strategies for power systems supplying future and existing oshore oil and gas facilities.The author uses these methods to address five research challenges: i) the definitionof the optimal waste heat recovery technology, ii) the identification of the best working fluid to design ecient, light and cost-competitive waste heat recovery units, iii) the integration of dynamic criteria in the project phase to discard infeasible designs, iv) the development of a novel control strategy to optimally operate the power system, and v) the enhancement of its dynamic flexibility using the model predictive control. The case study of this work is the power system of the Draugen oil and gas platform (Kristiansund,Norway), where the possibility of equipping one of the gas turbines with abottoming cycle unit is investigated.The optimal technology is determined by programming a multi-objective optimization procedure, capable of optimizing the design of Rankine and Brayton engines. The objective functions are the daily carbon dioxide emissions, the weight of the componentsand the economic revenue. The optimization routine is interfaced with validated models sizing the heat transfer equipment. This software integration provides an initialestimate of the module compactness and of the impact of the pressure drops on thesystem performance. Finally, part-load and economic models quantify the spared emissionsand the feasibility of the investment. The organic Rankine cycle technology hasthe largest potential to decrease the carbon dioxide emissions (10 - 15 %). On the other hand, the steam Rankine engine is more competitive from an economic perspective.The air Bryton cycles give the lightest modules (10 - 60 t). Therefore, this technology should be employed to retrofit existing oshore facilities with extended lifetime.Steam and organic Rankine engines are competing alternatives for new installations.The findings suggest to lean toward the use of organic Rankine cycle turbogenerators,if fuel-flexibility is a main priority for the platform operator. Engineering eorts should focus on cutting the production cost of the expander and on minimizing the core volume of the primary heat exchanger. In the selection of the working fluid, benzene and iv Abstractcyclopentane are the organic compounds giving the highest performance, compactness and economic revenue. Considering the use of organic Rankine turbogenerators, dynamic analyses simulating critical scenarios (i.e. trip of one gas turbine) are shown tovaluably complement the steady-state design procedure. The use of dynamic criteria can help identifying those candidates which do not meet the requirements of oshoreelectric grids. Specifically, the use of low-weight units is discouraged. It entails frequency fluctuations outside the specified tolerance (i.e. 4 %) and instabilities in the process variables. Additionally, the proposed simulation tool can detect the system designs which expose the working fluid to unacceptable risk of chemical decomposition. Accounting for such phenomenon is of paramount importance as it may compromise the performance and lifetime of the components.This work pays further attention to the design of an innovative controller to optimally operate one gas turbine connected to an organic Rankine cycle unit. The regulator uses the linear model predictive control to maintain the quality of the power supply, and, at the same time, to track the maximum performance of the plant. The speed of the pump of the organic Rankine cycle unit is varied to maximize the energy conversion e-ciency of the plant. The controller can also verify real-time the actual feasibility of the optimal working condition with respect to operational constraints, i.e., the acid formation at the outlet of the primary heat exchanger and the decomposition of the working fluid. The results demonstrate that the activities at the peak eciency are practicable from 40% load to nominal power. The potential fuel savings are around 3 %. The increment of the final revenue can be up to 10 %. More conservative control strategies are advised at low-power activities (especially when burning combustibles with a highsulfur content), or when the thermal stresses on the working fluid should be minimized. Additionally, the controller is demonstrated to improve the dynamic flexibility of the plant compared to the reference controller designed by the gas turbine manufacturer.The model predictive control can reduce the frequency fluctuations in the range of 20- 40 %, considering the gas turbine alone. The reduction increases up to 60 %, connecting the engine to the organic Rankine cycle module. The analysis on the effect of unmeasured disturbances (i.e. fouling in the heat transfer equipment) confirms thestability of the dynamic response. On the other hand, the results pinpoint the need forreal-time upgrading of the internal models. This task is achievable adopting intelligent adaptation and learning techniques.

A hybrid Solid Oxide Fuel Cell (SOFC) and Steam Turbine (ST) plant is integrated with a gasification plant. Wood chips are fed to the gasification plant to produce biogas and then this gas is fed into the anode side of a SOFC cycle to produce electricity and heat. The gases from the SOFC stacks enter into a burner to burn the rest of the fuel. The offgases after the burner are now used to generate steam in a Heat Recovery Steam Generator (HRSG). The generated steam is expanded in a ST to produce additional power. Thus a triple hybrid plant based on a gasification plant, a SOFC plant and a steam plant is presented and studied. The plant is called as IGSS (Integrated Gasification SOFC Steam plant). Different systems layouts are presented and investigated. Electrical efficiencies up to 56% are achieved which is considerably higher than the conventional integrated gasification combined cycles (IGCC). Plants characteristics are discussed while the plants sizes are defined form the available steam turbine as well as cultivation area.

Offshore wind technology is rapidly developing and a wind farm can be integrated with offshore power stations. This paper considers as case study a futuristic platform powered by a wind farm and three combined cycle units consisting of a gas turbine and an ORC (organic Rankine cycle) module. The first aim of this paper is to identify the maximum amount of wind power that can be integrated into the system, without compromising the electric grid balance. The stability of the grid is tested using a dynamic model of the power system based on first principles. Additionally, the dynamics of the system is compared with a simplified plant consisting of three gas turbines and a wind farm, in order to identify benefits of the installation of the ORC system. The maximum allowable wind power is 10 MW for a nominal platform load of 30 MW. The results show that the presence of the ORC system allows decreasing frequency oscillations and fuel consumptions of the platform, with respect to the simplified configuration. On the other hand, the dynamic response of the combined cycle units is slower due to the thermal inertia of the heat transfer equipment.

Gas Turbines (GT) are widely used for power generation in offshore oil and gas facilities, due to their high reliability, compactness and dynamic response capabilities. Small heavy duty and aeroderivative units in multiple arrangements are typically used to offer larger load flexibility, but limited efficiency of such machines is the main drawback. A solution to enhance the system performance, also in Combined Heat and Power (CHP) arrangement, is the implementation of Organic Rankine Cycle (ORC) systems at the bottom of the gas turbines. Moreover, the resulting GT-ORC combined cycle could be further integrated with additional renewable sources. Offshore wind technology is rapidly developing and floating wind turbines could be combined with offshore GT-ORC based power plants to satisfy the platform load. The pioneering stand alone power system, for an oil and gas platform, examined in this paper comprises a 10MW offshore wind farm and three gas turbines rated for 16.5MW, each one coupled with an 4.5MW ORC module. The ORC main parameters are observed under different wind power fluctuations. Due to the non-programmable availability of wind and power demand, the part-load and dynamic characteristics of the system should be investigated. A dynamic model of the power system based on first principles is used, developed in the Modelica language. The model is integrated with a time series-based model of two offshore wind mills. Various thermodynamic indexes, available in the literature, are identified and evaluated to compare the actual combined heat and power performances of single components and of the overall integrated system in the considered wind scenarios.