Year 2040 the goal is to have a 100 % renewable Swedish energy system. Svenska kraftnät (Svk) predicts fully decommissioned nuclear power plants and an increased amount of connected wind power plants, especially offshore, year 2045. These kind of renewable power plants are non-synchronous and do not provide the grid with the same system stability services naturally as synchronous generators, such as nuclear power plants. With the increased number of renewables connected, one future challenge is to maintain the stability of the power grid. Grid stability can be divided into voltage-, frequency- and rotor angle stability.This thesis has investigated how large-scale offshore wind power plants (OWPPs) can contribute with reactive power compensation and in turn voltage stability to a nearby onshore power grid in Sweden. The evaluation has been done from the perspective of the TSO and the OWPP owner interests, with a focus on grid codes, economic incentives and technical capabilities.This project has been made in three parts. First, a comparison of voltage stability control requirements in different European grid codes was made. Secondly, static power flow simulations of a case study of a 1000 MW OWPP have been performed in PowerWorld Simulator, testing the OWPP’s reactive power outputs under different circumstances. Thirdly, a market opportunity analysis has been completed, analyzing reactive power market opportunities for OWPPs as well as for TSOs.The study shows that the reactive power capabilities of the simulated OWPP is considerable higher than the Swedish grid codes requires. Thus, an opportunity is to make the grid codes stricter, in combination with economic incentives. The case study showed that the distance offshore has an impact of the reactive power reaching the grid onshore. Though, the OWPP’s contribution to local voltage stability onshore is considered as good. Finally, with short- and long-term contracts, a reactive power market can be favorable for both the OWPP owner and the TSO.

Cost efficient operation and maintenance strategies are crucial for reducing cost of wind energy. Since the regime change from feed-in tariffs to an auction-based bidding system for capacity in most European wind projects, levelized cost of energy is challenged constantly. Therefore, new technologies such as new controllers are developed to improve operation and to increase profit. Previous research studies demonstrated the advantage of increased power output of wake redirection control. However, understanding and quantifying the impact of wind farm control technologies on operation and maintenance strategies is inevitable to evaluate the economic feasibility of such new technologies. Thus, an event-based O&M simulation tool has been developed. Besides general modules, such as the wind turbine model, the weather forecasting model and a model for simulating corrective and planned maintenance, the developed tool also takes wake effects into account. This allows considering different power productions for each individual turbine and a failure rate distribution within the wind farm which is based on altering loads on the different components. Both aspects are driven by changes in operation when applying a new controller technology. Exemplarily, the economic feasibility of a closed-loop active wake steering control has been analysed. Main achievements of this study are the possibility to quantify the impact of the active wake steering control on O&M related KPIs. Results show that additional loads caused by applying yaw-misalignment and redirecting wake, lead to an increase in OPEX. However, the achieved energy production gain and thus related additional revenue exceeds additional cost in the case study. Nonetheless, the study reveals that the profitability of the controller is highly dependent on the electricity price which can be acquired during the wind farm’s lifetime.

Durch die Zunahme von Offshorewindparks wird der anthropogene Lärmpegel im Meer weiter ansteigen. Deshalb ist es notwendig, mögliche negative Effekte von Windturbinenschall auf Fische zu erkennen und zu minimieren. Um mögliche Effekte des Turbinenschalls auf Kabeljau (Gadus morhua) und Scholle (Pleuronectes platessa) zu untersuchen, wurden die Fische in Laborversuchen verschiedenen tiefen Frequenzen und Schallstärken, wie sie von Windturbinen emittiert werden ausgesetzt. Ein ringförmiges Betonversuchsbecken mit 10 Metern Durchmesser wurde mit Schallisolationwänden in verbundene Viertel unterteilt, wodurch im Becken eine Schalldruckdifferenz von 32-52 dB, abhängig von Frequenz und produziertem Schalldruckpegel erzeugt wurde. Die Fische, die sich frei im Becken bewegen und so, wenn sie wollten, den höchsten Schalldruckpegeln ausweichen konnten, wurden monofrequenten Tönen im Frequenzbereich zwischen 25 und 250 Hz in Schalldruckpegeln von 130 und 140 dB re 1 µPa ausgesetzt. Die Position der Fische im Becken wurde in den 24-Stunden-Phasen vor, während und nach Beschallung verglichen und zeigte in den meisten Versuchen während Beschallung signifikant weniger Kabeljau in der Nähe der Schallquelle, als in den Phasen vor und nach Beschallung. Scholle zeigte uneinheitliche Reaktionen, die teilweise auf Meideverhalten, in anderen Versuchen auf Attraktion durch den Schall hindeuteten. Die Ergebnisse lassen wenigstens kurzzeitige, durch Schall verursachte Verteilungsänderungen von Kabeljau in Offshorewindparks erwarten. / The growing number of offshore wind farms will lead to an increase of anthropogenic noise in the sea. Research is therefore necessary to identify and minimize any potentially negative effects of offshore wind turbine noise on fish. To examine the potential impact of offshore wind farm noise emissions on cod (Gadus morhua) and plaice (Pleuronectes platessa), tank experiments were carried out exposing the fishes to different low frequencies at sound levels as emitted by wind turbines An annular concrete tank of 10 m in diameter was divided in connected quarters by sound barriers, so that sound pressure differences of 32 to 52 dB were achieved, depending on frequency and produced sound levels. Pure tones in the frequency range between 25 and 250 Hz were artificially emitted in sound levels of 130 and 140 dB re 1µPa. The experimental fish were free to move around in the tank and therefore could avoid the highest sound levels if they chose to. The location of fish in the 24 hour periods before, during and after sound production were compared and showed significantly lower numbers of cod in the vicinity of the sound source during sound presentation in the majority of the experiments. Reactions of plaice were ambiguous, suggesting both avoidance of, or attraction to the sound. From the results at least short-term changes in distribution of cod related to sound in the vicinity of offshore wind farms could be expected.

The aim of this thesis is to evaluate different repowering alternatives from the viewpoint of increasing power production from existing offshore wind farms (OWF), as some of the first commissioned OWFs are approaching the end of their expected lifetime. The thesis presents a literature review of components and financial aspects that are of importance for repowering of OWFs. In the literature review, risks and uncertainties regarding repowering are also lifted and analysed. The thesis contains a case study on Horns Rev 1 OWF, where three different repowering scenarios are evaluated by technical and financial performance, aiming to compare the cost of repowering alternatives. The design of the case study is based around previous studies of offshore repowering having focused mainly on achieving the lowest possible levelized cost of energy (LCoE) and highest possible capacity factor, often resulting in suggested repowering utilizing smaller wind turbines than the existing ones. In order to evaluate the financial viability of repowering alternatives, the software RETScreen Expert was used to estimate the annual energy production (AEP) after losses and calculate the net present value (NPV) and LCoE for lifetime extension and full repowering utilizing different capacity wind turbines. Input values from the literature as well as real wind resource measurements from the site was utilized to achieve as accurate results as possible. The result of the case study shows that repowering of OWFs have the possibility of providing a very strong business case with all scenarios resulting in a positive NPV as well as lower LCoE than the benchmarked electricity production price. Although the initial investment cost of the different repowering alternatives presented in this thesis still are uncertain to some extent, due to the lack of reliable costs for repowering alternatives, this thesis provides a base for further research regarding the repowering of OWFs.

Mathematical modeling and simulations of a submerged renewable storage system integrated to a wind farm, chosen based on the geographical and wind conditions at the Baltic Sea, gives insight on the feasibility of the submerged renewable storage and an approximation of the payback period and profits that could be generated. Genetic Algorithms were used to obtain the optimal number of spheres for a certain depth, based on 2 objective functions I.e. Minimum Life Cycle Cost (LCC) and maximum reduction in wind curtailment. The new arrangement concept shows that the Initial Capital Cost (ICC) could be decreased by 25% to 60% depending upon the number of sphere employed. Based on the inputs considered in the study, the results prove that the submerged renewable storage system would be feasible, and the profits ranging from 15 Million Euro to 29 Million Euro can be achieved at the chosen location, towards the Swedish side of the Baltic sea. Although, in a real life scenario it is assumed that only up to half of the profits obtained in the results would be achievable. The results also show that, the Pump/Turbine with a high turbine efficiency and lower pump efficiency, generated better profits, compared to a Pump/Turbine running with a higher pump efficiency and lower turbine efficiency. An attempt to increase the round-trip efficiency by adding a multi stage submersible pump, resulted in additional ICC and LCC, which saw a decrease in profits.

L’intégration massive de la production d’énergie éolienne intermittente au niveau des réseaux électriques pose un problème de stabilité du système électrique. En effet les caractéristiques sont très différentes de celles de sources conventionnelles maîtrisées par les gestionnaires de réseau. Par conséquent, l’injection de cette énergie induit de nouveaux challenges pour les gestionnaires de réseaux électriques. De plus, les conditions de raccordement évoluent et tendent à ce que toutes les sources participent aux services rendus aux systèmes électriques. Les travaux de cette thèse sont focalisés sur la proposition d’une nouvelle stratégie de commande non-linéaire basée sur la commande par « platitude à une boucle » d’un système de conversion d’énergie éolienne. Cette stratégie de contrôle vise la gestion des interactions au point de connexion par la génération et le suivi de trajectoires de références. De par le fait que toutes les variables du système sont liées à la « sortie plate » de ce système, cette commande procure une rapidité de réponse et une bonne maitrise des régimes transitoires. La mise en œuvre de cette nouvelle stratégie de contrôle pour la constitution d’un parc éolien offshore, capable de satisfaire différentes conditions de raccordement a été simulée avec succès. Plus particulièrement, l’évaluation de l’impact de différents défauts du réseau sur les services proposés (Régulation de fréquence et de tension, la tenue aux creux de tension.) a été réalisée. / The massive integration of intermittent production of wind energy in electrical networks creates an electrical system stability problem. Indeed, its characteristics are very different from those of conventional sources controlled by Grid managers. Therefore, the injection of this energy makes new challenges for power Grid operators. Moreover, the connection conditions are evolving and go towards the situations that all different sources participate into services of electrical systems. This thesis proposes a new nonlinear control strategy based on a « one loop flatness control » of a wind energy conversion system. This control strategy has aim of energy interactions management at the connection point through generation and tracking of reference trajectories. As all system variables are functions of the « flat output » of the system, this control provides fast response and good control in transient state. The application of this new control strategy into an offshore wind farm which is able to satisfy different connection conditions was simulated successfully. Specifically, effects of various grid faults on the proposed ancillary services (frequency and voltage regulation, and low-voltage ride through capabilities) were performed.

L’installation des énergies marines renouvelables s’effectue dans le respect des législations environnementales françaises. La séquence Eviter-Réduire-Compenser (ERC) est ainsi appliquée pour aboutir à une non-perte nette de biodiversité. L’objectif de la thèse est de questionner l’efficacité de cette séquence, et plus particulièrement celle de la compensation écologique, en tant qu’instrument d’internalisation et de lutte contre l’érosion de la biodiversité marine. Une approche empirique qualitative a ainsi été mise en oeuvre pour (i) identifier les facteurs écologiques et sociétaux ainsi que leurs caractéristiques théoriques qui doivent permettre à la compensation d’atteindre l’objectif de non-perte nette de biodiversité ; et (ii) contrôler si ces conditions sont vérifiées en pratique dans le cas de l’éolien en mer en Europe et en France. L’analyse met en avant les enjeux juridiques, institutionnels, méthodologiques et sociétaux à relever pour permettre à la compensation écologique d’atteindre son objectif. Sur la base de ce constat, une évaluation multicritères est proposée afin de renforcer les étapes d’évitement et de réduction pour finalement mieux définir les besoins de compensation écologique en mer. L’analyse met par ailleurs en évidence un glissement d’une compensation basée sur une équivalence écologique stricte à une compensation fondée sur une équivalence écologique relâchée. Les actions de compensation tendent à être plus généralistes et/ou davantage dirigées vers les services écosystémiques que sur les composantes des écosystèmes. Associées aux mesures d’accompagnement, elles peuvent contribuer à faciliter l’acceptabilité sociale d’un projet d’aménagement. / The installation of marine renewable energies is carried out in compliance with French environmental legislation. The mitigation hierarchy is thus applied to achieve an objective of no net loss of biodiversity. This thesis aims at questioning the effectiveness of the mitigation hierarchy and more specifically biodiversity offsetting as an internalization instrument to halt the erosion of marine biodiversity. We use a qualitative empirical approach to (i) identify the ecological and societal factors as well as their theoretical characteristics that are supposed to enable the offsets achieving the objective of no net loss of biodiversity; and (ii) control whether these conditions are verified in practice for the case of offshore wind farms in Europe and France. The analysis highlights the legal, institutional, methodological and societal issues to be addressed in order to enable biodiversity offsetting to achieve the no net loss priority. On the basis of this observation, a multi-criteria assessment is carried out to reinforce the avoidance and reduction steps of the mitigation hierarchy in order to better define offsetting needs. Ultimately, the analysis shows a shift in biodiversity offsetting based on a strict ecological equivalence to a biodiversity offsetting based on a released ecological equivalence. Offsetting actions tend to be more generalist and / or more directed to ecosystem services than to ecosystem components. Associated with accompanying measures, offsetting actions can help to increase the social acceptability of a development project.

This study aims to explore the ability of a multi-criteria decision making with analytical hierarchy process (MCDM-AHP) model to emulate the results of a cost benefit analysis (CBA) model in the context of offshore wind farm siting within the Swedish exclusive economic zone (EEZ). The research question addressed is whether the MCDM-AHP analysis produces similar results to the CBA analysis. In addition to this, the strengths and weaknesses of each model is explored. The MCDM-AHP model employs the spatial criteria in a more basic manner compared to the CBA model, simplifying the evaluation process while still explaining 89.5% of the variation in the CBA model and defining similar areas as suitable. Thus, it can be concluded that the MCDM-AHP model adequately emulates the CBA model within the context of offshore wind farm siting within the Swedish EEZ. However, it is crucial to note that the two models produce outputs on different scales. While the CBA model provides levelized cost of energy (LCOE) values that can be thresholded for investment viability comparisons, the suitability score generated by the MCDM-AHP model remains a relative and arbitrary score within the model. Both models entail uncertainties, limiting their usage beyond making general assumptions or identifying areas of interest. The findings reveal that the CBA model demonstrates greater robustness when confronted with changes in spatial input parameters compared to the MCDM-AHP model. This discrepancy is attributed to the iterative computation process and consideration of flat cost inputs in the CBA model, whereas the MCDM-AHP model represents a linear combination of various spatial parameters. However, the calculated LCOE values in the CBA model are highly sensitive to changes in modeling assumptions regarding external parameters, resulting in significant linear variations. The LCOE values obtained from the CBA model baseline case fall within a range of 52.1 - 98.9 EUR/MWh, which aligns with similar studies, validating the CBA model. Nonetheless, caution should be exercised when considering these results as an accurate representation of the real world due to inherent uncertainties in cost inputs and the LCOE measure. The strengths of the MCDM-AHP model lie in its robustness when the order of relative importance remains stable for key spatial evaluators. It is sensitive to significant changes in water depth and wind speed, which heavily influence its output. The model's simplicity allows for a quick overview of the problem, but it requires assumptions that introduce uncertainties. Validation of the MCDM-AHP model using existing and planned offshore wind farms within the Swedish EEZ was possible but limited by the arbitrary scale and limited validation areas. The comparison between the two models could be enhanced with more comprehensive spatial and economic data for an in-depth CBA model, which could serve as a ground truth for the MCDM-AHP model. Nevertheless, the comparison made in this study considers the CBA model to be closer to the truth, acknowledging the underlying assumptions that should be considered during evaluation. In conclusion, within the context of offshore wind farm siting, the MCDM-AHP model produces outputs that are similar to the CBA model.