Morten Kramer

AquaBuOY-the offshore wave energy converter numerical modeling and optimization

Ocean energy and offshore wave energy conversion in the United States is at a significant milestone. During the next couple of years, ocean energy technology developers and energy officials have a potential to deploy pilot scale ocean energy conversion installations. This capability comes at a time of increased interest in ocean energy worldwide. The paper outlines AquaEnergys experience in developing the Makah Bay pilot offshore power plant and results of the projected performance data developed using E2I EPRI guidelines. The Makah Bay, WA offshore pilot power plant uses AquaEnergys point absorber wave energy conversion device - AquaBuOY. The device is the next generation of the technology that combines the Swedish Hose-Pump and the IPS Buoy technologies to generate clean energy from ocean waves. Currently the project is undergoing environmental permitting in support of FERC & NOAA licenses. In early 2004 AquaEnergy, US, and RAMBOLL, DK, performed output projections to substantiate the expected performance of the Makah Bay pilot plant in support of the E2I EPRI Offshore Wave Energy Feasibility Demonstration Project. AquaEnergy will conclude its presentation with a brief overview of current legislation affecting the industry. In 2004, ocean scientists, engineers, and developers can continue to lay the groundwork for government spending and interest in ocean energiesThis paper describes development of the mathematical model simulating ocean performance of an offshore wave energy point absorber device-AquaBuOY. The AquaBuOY is the next generation of the technology, based on the IPS point absorber system and the hose pump, both of Sweden. AquaEnergy Group Ltd., USA, is developing the system in cooperation with RAMBOLL, Denmark. In March 2003 the Danish Energy Authority awarded a grant for a design study that includes development of the numerical model for the AquaBuOY operation, experimental testing and design optimisation. The scale model tests will be carried out at Aalborg University, Denmark in order to optimise the device design, operation and installation configuration with the goal of minimising system footprint. The paper provides an overview of the numerical modelling used in establishing system operating characteristics. The experimental results from the model tests on mooring forces under survival conditions will be presented during the conference in conjunction with different footprint configurations and different mooring systems. Finally the performance data based on theoretical and experimental results will be presented for the AquaBuOY in five representative generic sea states. Ocean energy and offshore wave energy conversion in the United States is at a significant milestone. During the next year, ocean energy technology developers and energy officials have the potential to deploy pilot scale ocean power plants and transition to commercial plants in the US. This capability comes at a time of increased interest in ocean energies at the National Academy of Sciences and the US Department of Energy. AquaEnergy will conclude its presentation with a brief overview of current legislation affecting the industry. In 2004, ocean scientists, engineers, and developers can continue to lay the groundwork for government spending and interest in ocean energies.

Control Performance Assessment and Design of Optimal Control to Harvest Ocean Energy

Different approaches exist to design the optimal control strategy to harvest ocean energy with a point absorber. However, the control paradigm changes if the efficiency of the power takeoff (PTO) is taken into account. The motivation of this paper is twofold. The first objective is to develop a framework that includes the PTO efficiency to determine the optimal control strategy. The second objective is to assess the performance of any control strategy given the PTO efficiency. The performance assessment is based on the upper bound of the deliverable electrical energy of the optimal control strategy. As an example, different PTO efficiencies were considered for a given point absorber model, and extensive simulation results show the annual electrical energy delivered to the grid by the optimal control strategy. The performance of a nonoptimal control strategy is assessed with respect to the optimal control strategy.

Oblique Wave Transmission Over Low-Crested Structures

Wave transmission over low-crested structures have often been the subject for research, as the wave field behind these structures determines what will happen in this area. Detached low-crested structures are often parallel to the coastline and in most cases wave attack will be perpendicular to this coastline and therefore, perpendicular to the structure. This situation can be simulated by small scale physical modeling in a wave flume. Recent research, including all data of the above given references and new extensive data sets, has enlarged the insight on the topic. The results from 2D tests are: (1) prediction formulae for the wave transmission coefficient K; and (2) a description of change of spectral shape because of wave transmission. In some situations low-crested structures are not parallel to the coast. T-shaped groynes are an example, but also breakwaters for a harbor where under very extreme storm surge, the structure can be considered as low-crested. In these situations wave attack is very often not perpendicular to the alignment of the structure and in many situations even quite oblique wave attack and transmission occurs. But what will be the difference with perpendicular attack? More in detail: (1) are the prediction formulae for k still valid; (2) is the spectral change (more energy to high frequencies) similar to perpendicular wave attack; (3) is there any influence of short-crestedness of waves; and (4) are wave directions similar in front of the structure and after transmission? Only a three-dimensional investigation in a short-crested wave basin can give answers to these questions. Within the EU-project DELOS these tests have been performed and are the subject of the paper.

Stability of Low-Crested Breakwaters in Shallow Water Short Crested Waves

The paper presents results of 3D laboratory experiments on low-crested breakwaters. Two typical structural layouts were tested at model scale in a wave basin at Aalborg University, Denmark, to identify and quantify the influence of various hydrodynamic conditions (obliquity of short crested waves, wave height and wave steepness) and structural geometries (crest width and freeboard) on the stability of low-crested breakwaters. Results are given in terms of recommendations for design guidelines for structure stability. Damage parameters for the trunk and the roundhead are proposed based on analysis of observed damage. Results for initiation of damage are compared to existing data and a good agreement is found.

Experimental validation of a nonlinear mpc strategy for a wave energy converter prototype

One of the major limitations to the development of advanced wave energy converters (WECs) control strategies are the associated computational costs. For instance, model predictive control (MPC) strategies have the potential to obtain almost optimal performance, provided that the imperfect power conversion in the power take-off (PTO) system is correctly taken into account in the optimization criterion and that the incoming wave force can be estimated and forecast. However, demanding computational requirements as well as the unresolved issue of wave force estimation have so far prevented real-time implementation and validation of such MPC strategies. In this paper, we present the successful experimental results obtained on a scaled-down prototype of the well-known Wavestar machine. Performance comparisons are provided for nonlinear MPC versus a reference PI controller. INTRODUCTION A wave energy converter (WEC) is a device used to produce electricity, or other forms of usable energy, from wave motion. The main challenge faced by the developers of wave energy technologies is the reduction of the levelized cost of energy (LCOE) to a competitive level. A key driver to achieve such a goal is the improvement of “wave-to-wire” efficiency, which, especially for point absorbers of the heaving-buoy type, depends on: • their architecture (geometry, mechanics), • the efficiency of the power take-off (PTO) system • the performance of the PTO control system [1]. ∗Address all correspondence to this author. In order to deal with the naturally narrow-banded frequency response of such dynamic systems, and the continuously changing sea state, flexible PTOs capable of both harvesting and drawing power from the grid (respectively in generator and motor modes) are promising actuator candidates. A flexible PTO along with reactive control allows the absorber to be more often in phase with the incoming waves. It can achieve that, by investing some energy (drawn from the grid) to eventually get a larger energy payback than it would be possible to obtain by just braking the absorber via the PTO force. Indeed, many studies have shown that one of the key aspects for maximizing the energy yield of a WEC is the way of controlling the device. The ProportionalIntegral (PI) velocity feedback controller is the current state of the art for WECs as far as practical implementation is concerned. The integral action is a position feedback implementing a simple form of reactive control, while the proportional velocity feedback provides the basic linear damping which is found in most WEC control system. This strategy is very robust and simple to implement, since it uses only position and velocity measurements to compute the control action. The PI control law shows a reasonable energy conversion rate, but is still far below the theoretical optimum discussed in [2]. Moreover, additional performance loss is to be expected because the sea state changes and the feedback coefficients must be modified online to take into account this variation. Of course, alternatives to PI control do exist. Latching control has been proposed for WECs equipped with a position locking mechanism [3–5]. The basic idea is to lock the point absorber when its velocity is zero, and wait for the most favorable moment to release it again. In this way, the velocity of the point absorber can be brought in phase with the wave excitation force, and the

Comparison Between Numerical Modeling and Experimental Testing of a Point Absorber WEC Using Linear Power Take-Off System

Currently, a number of wave energy converters are being analyzed by means of numerical models in order to predict the electrical power generation under given wave conditions. A common characteristic of this procedure is to integrate the loadings from the hydrodynamics, power take-off and mooring systems into a central wave to wire model. The power production then depends on the control strategy which is applied to the device. The objective of this paper is to develop numerical methods for motion analysis of marine structures with a special emphasis on wave energy converters. Two different time domain models are applied to a point absorber system working in pitch mode only. The device is similar to the well-known Wavestar prototype located in the Danish North Sea. A laboratory model has been set up in order to validate the numerical simulations of the dynamics. Wave Excitation force and the response of the device for regular and irregular waves were measured. Good correspondence is found between the numerical and the physical model for relatively mild wave conditions. For higher waves the numerical model seems to underestimate the response of the device due to its linear fluid-structure interaction assumption and linearized equation of motion. The region over which the numerical model is valid will be presented in terms of non-dimensional parameters describing the different wave states.Copyright

Reliability-based Calibration of Partial Safety Factors for Wave Energy Converters

Wave energy converters (WECs), which harvest energy from the waves and transfer them to electricity, are a new technology, where structural standards need to be developed. An important step towards standardization is the calibration of partial safety factors. A methodology for calibration of partial safety factors for design of welded details for wave energy converter applications is presented in this paper using probabilistic methods. The paper presents an example with focus on the Wavestar device. SN curves and Rainflow counting are used to model fatigue without considering inspections. The influence of inspections is modelled using a fracture mechanics approach, which is calibrated by the SN curve approach. Furthermore, the paper assesses the influence of the inspection quality. The results show that with multiple inspections during the lifetime of the device and by applying a good inspection quality, the safety factor can be significantly reduced.

Wave and Current Flows Around Low-crested Structures

Within the EU-funded project DELOS, 3D tests were performed in the 12.5 m x 9.7 m wave basin at Aalborg University, DK, with the aim of analyzing waves and currents around low-crested structures. This paper presents some experimental results on overtopping of low-crested structures both in a symmetrical layout, composed by two detached breakwaters with a gap in between, and in an oblique layout, composed by a single breakwater oblique to the beach. An original scheme for evaluating overtopping processes with varying structure submergence and wave attacks is provided and discussed in comparison with experimental mass balance.