Raymond Alcorn

Impact of a Medium-Size Wave Farm on Grids of Different Strength Levels

Power fluctuations generated by most oscillating wave energy converters may have a negative impact on the power quality of the local grid to which the wave farms will be connected. Hence, assessing their impact is an important step in the selection process of a suitable deployment location. However, site-specific grid impact assessment studies are relatively time-consuming and require a high level of detail on the local network. Both of these constraints mean that grid impact studies are usually not performed in the preliminary stages of the site selection process, despite the extremely negative consequences resulting from poor power quality. This paper details a comprehensive study based on a relatively typical wave farm design connected to networks of different strength levels. The study was performed using experimental electrical power time series of an oscillating water column (OWC) device generated under the framework of the European FP7 project “CORES”. Simulations were performed using DIgSILENT power system simulator “PowerFactory”.

On thermodynamics in the primary power conversion of oscillating water column wave energy converters

The paper presents an investigation to the thermodynamics of the air flow in the air chamber for the oscillating water column wave energy converters, in which the oscillating water surface in the water column pressurizes or de-pressurises the air in the chamber. To study the thermodynamics and the compressibility of the air in the chamber, a method is developed in this research: the power take-off is replaced with an accepted semi-empirical relationship between the air flow rate and the oscillating water column chamber pressure, and the thermodynamic process is simplified as an isentropic process. This facilitates the use of a direct expression for the work done on the power take-off by the flowing air and the generation of a single differential equation that defines the thermodynamic process occurring inside the air chamber. Solving the differential equation, the chamber pressure can be obtained if the interior water surface motion is known or the chamber volume (thus the interior water surface motion) i...

Assessment of primary energy conversions of oscillating water columns. I. Hydrodynamic analysis

This is an investigation on the development of a numerical assessment method for the hydrodynamic performance of an oscillating water column (OWC) wave energy converter. In the research work, a systematic study has been carried out on how the hydrodynamic problem can be solved and represented reliably, focusing on the phenomena of the interactions of the wave-structure and the wave-internal water surface. These phenomena are extensively examined numerically to show how the hydrodynamic parameters can be reliably obtained and used for the OWC performance assessment. In studying the dynamic system, a two-body system is used for the OWC wave energy converter. The first body is the device itself, and the second body is an imaginary “piston,” which replaces part of the water at the internal water surface in the water column. One advantage of the two-body system for an OWC wave energy converter is its physical representations, and therefore, the relevant mathematical expressions and the numerical simulation can be straightforward. That is, the main hydrodynamic parameters can be assessed using the boundary element method of the potential flow in frequency domain, and the relevant parameters are transformed directly from frequency domain to time domain for the two-body system. However, as it is shown in the research, an appropriate representation of the “imaginary” piston is very important, especially when the relevant parameters have to be transformed from frequency-domain to time domain for a further analysis. The examples given in the research have shown that the correct parameters transformed from frequency domain to time domain can be a vital factor for a successful numerical simulation.

Assessment of primary energy conversions of oscillating water columns. II. Power take-off and validations

This is the second part of the assessment of primary energy conversions of oscillating water columns (OWCs) wave energy converters. In the first part of the research work, the hydrodynamic performance of OWC wave energy converter has been extensively examined, targeting on a reliable numerical assessment method. In this part of the research work, the application of the air turbine power take-off (PTO) to the OWC device leads to a coupled model of the hydrodynamics and thermodynamics of the OWC wave energy converters, in a manner that under the wave excitation, the varying air volume due to the internal water surface motion creates a reciprocating chamber pressure (alternative positive and negative chamber pressure), whilst the chamber pressure, in turn, modifies the motions of the device and the internal water surface. To do this, the thermodynamics of the air chamber is first examined and applied by including the air compressibility in the oscillating water columns for different types of the air turbine ...

Investigation to Air Compressibility of Oscillating Water Column Wave Energy Converters

It has been suggested that for full scale oscillating water column (OWC) devices, the pressure in and the volume of the air chambers can be large to create air compressibility in the air chamber. Due to compressibility, its density and temperature are different from those in atmosphere. When in exhalation, the pressurized air is driven out of the air chamber and mixes into the atmosphere outside the air chamber; whilst in inhalation, the atmosphere is sucked through the power take-off (PTO) system into the air chamber, and mixes with the de-pressurized air in the chamber.This paper presents a study on air compressibility in OWC air chambers by theoretical analyses and the relevant experimental studies. The theoretical analysis is based on the first-order differential equation for the flowrate and the chamber pressure, which has been derived for the air flow under the assumptions of the isentropic process and the known power take-off characteristics. In the study, an orifice type of PTO and a porous membrane type PTO, which are supposed to represent a typical nonlinear and linear PTO for small models, respectively, are both investigated. The investigation has shown the feasibility of the theoretical method on the air compressibility and the possible power loss due to the air compressibility.Copyright

Experimental Studies of a Floating Cylindrical OWC WEC

Oscillating water column (OWC) wave energy converters (WECs) are a popular type of wave energy devices, due to their advantages over many other WECs. For example, OWC WECs normally have no moving components in sea water, and have a small torque and a high rotational speed for a certain power take-off. Practically, some foundation-type pioneer plants of OWC WECs have been very successful in generating electricity to grids continuously. In order to obtain higher yields of wave energy production, it is proposed to move the OWC WECs to open and deep water regions, and for the purposes of economics and reliability, the OWC WECs are designed to be floating devices, with a potential of utilizing the device motions to improve wave energy conversion capacity. To further understand the OWC WEC performances in waves, a floating cylindrical OWC has been designed and tested in an ocean wave tank. In the model test, five different size orifices are designed to represent different damping levels of the air flow. In the experimental study, a systematic series of tests in both regular and irregular waves has been conducted to help understand the hydrodynamics and aerodynamics of the generic OWC device.In the model test, the interior water surface motion and the pressure in the air chamber are measured and based on them the primary power take-off by the device can be calculated. Alternatively, the power take-off can be calculated by the pressure measurement only or by the interior water surface measurement only due to the unique relation of the pressure drop and the airflow passing through the orifices. In addition, in the experiment, the motions of the floating structure have also been measured, from which it is possible to correlate the motions and the wave energy extraction.As expected, the orifices exhibit a quadratic non-linear relation between pressure and the flowrate. Though simple, the orifice power take-off system may exhibit a similar flow feature to that of an impulse turbine, thus an appropriate model to the impulse turbine.Copyright

Effect of Wave Farm Aggregation on Power System Stability

There is a large and varied range of wave energy technologies presently under development, in varying stages of progress from model scale to pre-commercial prototypes. Similarly to offshore wind farms, it is anticipated that when these wave energy converters (WECs) ultimately reach commercial stage, they will be implemented in a farm layout, in order to improve economic feasibility and power quality. Transmission System Operator (TSOs) carry out power system dynamic simulations in order to investigate the effect of renewable energy farms on the stability and reliability of the grid under varying operating conditions. A detailed dynamic model of each individual WEC in the wave farm would be complex and computationally intensive; therefore aggregating the wave energy farm into a smaller number of models may be desirable for the TSO. This paper describes the methodology for simulating arrays of WECs in a power systems simulation tool, and examines the effects of aggregation on the load flow studies and stability analysis of the farm. The methodology is validated using DIgSILENT Power Factory simulations.

A novel method for estimating the flicker level generated by a wave energy farm composed of devices operated in variable speed mode

The output power of wave energy farms may be very fluctuating, which may give rise to power quality issues such as flicker. However, although there existed a method for estimating the flicker level generated by a wave energy farm in relation to its short-circuit ratio (as described in IEC standard 61400-21), until recently, no method had been defined yet regarding the two other major parameters on which flicker level is highly dependent: the impedance angle at the point of connection and the rated power of the farm. In a previous work, the authors had presented a method for estimating the level of wave farm-induced flicker as a function of these latter parameters. They had identified two relationships regarding the impedance angle and the rated power in the case where the wave energy devices composing the farm are operated in fixed speed mode. This article presents the results of a follow-on work regarding the generalization of this method in the case of wave energy devices operated in variable speed mode.

Challenges and lessons learned in the deployment of an offshore oscillating water column

Purpose – The purpose of this paper is to disseminate the lessons learned from the successful deployment of a wave energy converter (WEC) and accelerate growth in the field of ocean energy. Design/methodology/approach – A thorough, well structured, documented, industrial approach was taken to the deployment because of the depth and scale of the task required. This approach is shown throughout the paper, which reflects the importance of a comprehensive project plan in success as well as failure. Findings – The findings demonstrate the viability of the use of off shore WEC to generate electricity and that such a project can be completed on time and on budget. Research limitations/implications – The research implications of the paper include the importance of an enhanced, integrated supervisory system control in terms of efficiency, operation and maintenance, and long-term viability of WECs. This paper can be used to help guide the direction of further research in similar areas. Practical implications – The ...

Numerical Studies on Hydrodynamics of a Floating Oscillating Water Column

The oscillating water column (OWC) is one of the more successful wave energy converters so far due to its mechanical and structural simplicity; there are no components for power take-off in seawater. Though there are some successful practical developments in bottom-fixed OWCs, floating OWCs are still in different stages of development. A specific oscillating water column, the OE Buoy (i.e. backward-bent duct device, ‘B2D2’), developed by OceanEnergy (Ireland), has recently attracted much attention. A 1:2.5 scale device has finished a sea-trial in Galway Bay (Ireland) for a period over two years during which period the device has gone through a severe storm. Thus its survivability has been confirmed to some extent. In this research, numerical simulations to the floating wave energy device are performed using a boundary element method code WAMIT. To consider the motions of the internal water in the column for energy extraction, a “numerical lid” is placed on the free surface in the column. In WAMIT, the motions of the “numerical lid” can be calculated by introducing relevant generalized modes to the conventional 6-DOF motions of the floating structure. For wave energy extraction, the “piston effect” of the internal water must be considered. To include the effect of the mooring system to the motions of floating structure, the mooring forces have been linearised, and their equivalent spring coefficients have been input to WAMIT for analysis of the moored floating structure. For the numerical simulation, the first case is to tune the damping coefficients based on wave tank results since in WAMIT, only hydrodynamic damping is included in calculation. In reality, larger damping may be needed to limit the large responses in heave of floating structure and the motion of the internal water surface. The tuned damping coefficients are then applied to the modified OWCs of different duct length, in which it is hoping that the corresponding responses of the internal free surface structure are used to assess the performance of the floating OWC. The aim of the research is to explore the relation between the OWC size and its performance so that it may provide a reference for optimizing the design of a floating OWC in the future.Copyright