Global Hierarchical Control of a Park of Spar Buoy-type Floating Oscillating Water Column Wave Energy Converters

Converting energy from ocean waves is a challenging area for control theory application because of the nonlinear dynamics in various time scales. Generally, wave energy converter (WEC) control is applied in order to maximize power absorption, in the most common wave conditions, and subject to the devices’ physical constraints. Commonly, researchers and designers prescribe the type of control algorithm based on the WEC archetype and its actuators. However, due to the nonlinear response to the constantly varying ocean waves, a single controller is unlikely to fit all operating conditions. This dissertation presents a global hierarchically controlled park of floating oscillating water column (OWC) spar-buoy type WECs for grid-scale power production. The controller is composed of a hierarchical controller and a subset of local controllers. The supervisory controller, which is based on the discrete event systems dynamics, ensures a safe and power-improving behavior, by enabling different local controllers depending on the current operating regime. The knowledge incorporated in the supervisor is based on a detailed state-space model of a park of seven OWCs, developed from scratch. This control-oriented wave-to-wire model considers hydrodynamic interactions, nonlinear forces, air compressibility, and a shared mooring configuration in six degrees of freedom. The novel local controllers include a robust second-order sliding mode controller, for reference following between a server and client WEC array and an algorithm for power shedding when needed. The results show good potential for the application of the standard supervisory control approach in Wave Energy, due to its adaptability to different WEC types and incorporation of safety mechanisms

Modelling of an Array of Floating Oscillating Water Column Wave Energy Converters with Full Hydrodynamic Coupling and Nonlinear Power Take Off Dynamics

Wave energy is one of the most promising renewable energy sources. Thus, research in this field has increased substantially over the last decade. However, significant costs associated with development and maintenance have so far impeded the implementation of commercially viable projects. Furthermore, the hydrodynamic interactions between water and prototypes meant to convert the energy in wave tanks are not trivial to scale up for the deployment in the ocean. Relief can be produced by using detailed full-scale mathematical models to predict the power generation capabilities. The objective of this work is the derivation of a full scale model of three floating oscillating water column wave energy converters, taking all hydrodynamic interactions between the devices into account. The kinetic, and potential, energy transferred to the air chamber causes a pressure difference that maintains the running of novel biradial turbines. This can be accurately modeled using pressure dependent turbine characteristics, whilst air compressibility into consideration. The description of these analytical relationships is followed by a numerical simulation of the hydrodynamic coefficients, for the specifically used geometries, with the aid of the boundary element method solver (ANSYS Aqwa). Subsequently the implementation that puts together the analytical and numerical parts is presented, and the resulting simulation model is validated based on regular and irregular sea states. This work is concluded with a discussion of possible control approaches to improve power generation.Wellenenergie ist eine der vielversprechendsten Quellen für erneuerbare Energien, weshalb der Forschungsaufwand über die letzten Jahre stetig stieg. Dennoch erschwe-ren hohe Entwicklungs- sowie Instandhaltungskosten die Umsetzung kommerzieller Projekte. Zusätzlich sind die hydrodynamischen Interaktionen, die bei kleineren Pro-totypen in Wellentanks ermittelt werden, nicht ohne weiteres für den Einsatz im Meer skalierbar. Abhilfe kann durch die Verwendung maßstabsgetreuer Modelle zur Vorhersage der Energieerzeugung gescha˙en werden. Das Ziel dieser Arbeit ist die Herleitung eines Simulationsmodells im Maßstab 1:1 von drei schwimmenden, oszillie-renden Wassersäulen Wellenenergie Umwandlern, unter Beachtung aller hydrodyna-mischen Interaktionen zwischen den Apparaten. Die auf die Luftkammer übertragene kinetische und potentielle Energie, führt zu einer Druckänderung, wodurch eine bira-diale Turbine angetrieben wird, die mit Hilfe der druckabhängigen Turbinencharakte-ristika detailliert beschrieben wird, unter Berücksichtigung der Kompressibilität der Luft. Der Beschreibung dieser analytischen Zusammenhänge folgt die numerische Si-mulation der hydrodynamischen Koeÿzienten für die spezifischen Abmessungen der verwendeten Endgeräte mit dem Boundary Element Method Solver ANSYS Aqwa. Anschließend wird die Implementierung, welche die analytischen und numerischen Ergebnisse vereint, präsentiert. Das resultierende Simulationsmodell wird anhand von regelmäßigem und unregelmäßigem Seegang validiert. Die Arbeit wird mit einer Betrachtung von möglichen Regelungsansätzen zur Verbesserung der Energiegewin-nung abgeschlossen

Design concepts for the Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy

Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA

Modeling and predicting power from a WEC array

This study presents a numerical model of a WEC array. The model will be used in subsequent work to study the ability of data assimilation to support power prediction from WEC arrays and WEC array design. In this study, we focus on design, modeling, and control of the WEC array. A case study is performed for a small remote Alaskan town. Using an efficient method for modeling the linear interactions within a homogeneous array, we produce a model and predictionless feedback controllers for the devices within the array. The model is applied to study the effects of spectral wave forecast errors on power output. The results of this analysis show that the power performance of the WEC array will be most strongly affected by errors in prediction of the spectral period, but that reductions in performance can realistically be limited to less than 10% based on typical data assimilation based spectral forecasting accuracy levels

Introducing the CTA concept

The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project