Tony Lewis

Wave energy in Europe: current status and perspectives

The progress in wave energy conversion in Europe during the past ten years is reviewed and current activities and initiatives in the wave energy sector at National and Union level are described. Other important activities worldwide are summarized. The technical and economical status in wave energy conversion is outlined and important wave energy developments are presented.

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”.

Dimensioning the equipment of a wave farm: Energy storage and cables

Still largely untapped, wave energy is particularly abundant and may represent an important share in the energy mix of many countries in the future. However, the power fluctuations generated by most wave energy converters with little to no storage means or without suitable control strategies may deteriorate the power quality of the local network to which wave farms will be connected. They may in particular generate an excessive level of flicker. The minimum amount of storage required for a wave farm to be grid compliant with respect to typical flicker requirements was investigated and is presented in the first part of this study. Besides giving rise to power quality issues, the rapid and high amplitude power peaks generated by wave devices may also render more complex the optimal dimensioning of the wave farm electrical components, whose cost is highly dependent on their power rating. This statement applies also to submarine cables, as the maximum current flowing potentially through them seems to be no longer a relevant criterion for determining their optimal current rating. Hence, the second part of the presented study focuses on the minimum current rating required from a submarine cable to avoid its thermal overloading.

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.

Weather Window Analysis of Irish and Portuguese Wave Data With Relevance to Operations and Maintenance of Marine Renewables

This paper presents the results of a weather window analysis of wave data from the west coast of Ireland and the Atlantic coast of Portugal in order to quantify the levels of access to ocean energy renewables, which may be deployed there, for operation and maintenance activities. In order to operate and maintain offshore marine renewables, a device will have to be accessible for a certain period of time. This will require a weather window consisting of a consecutive period of wave heights low enough and long enough for the device to be accessed. It is important to quantify what the levels of access are off the Irish west coast and Portuguese Atlantic coast given their high wind and wave resource.Wave data from two wave buoys, the M3 buoy located 56km off the west coast of Ireland and the Leixoes buoy located 19km off the Portuguese coast, are analysed to quantify the levels of access that exist. The data is used to quantify the general regimes at both sites by presenting the wave energy resource, the mean annual exceedance and the wave height frequency at both sites. The levels of access are quantified at operations and maintenance (O/M) access limits of Hs 1.0, 1.5, 2.0 and 2.5m wave height, by presenting the number of windows and the percentage of the year that these windows make up as well as the total number of hours, monthly and annual, that the wave heights are below these limits. Also presented are the waiting periods between windows by showing both the longest individual waiting periods between windows in a year and also the total intervals between windows in a year. The levels of access observed off Ireland and Portugal are then compared to levels of access observed at other marine renewable locations, namely the North Sea, Irish East Coast and Pacific North-western US coast.The results indicate that the levels of access off Ireland and Portugal are far below those observed at other marine renewable locations, and at the lower wave height access limits, there are very few suitable weather windows and considerable winter waiting periods between these windows. The implications of these low levels of access suggest that maintaining wave energy converters, off the west coast, may not be feasible and devices will need to be brought ashore for O/M activities.Copyright

Wave energy resource characterization and the evaluation of potential Wave Farm sites

In theory, the energy that could be extracted from ocean waves is in excess of any current, or future, human requirements. Methods to evaluate and compare the wave energy resource at different locations are required in order to inform the developers of Wave Energy Converter (WEC) projects and allow them to select the most favorable sites for achieving optimal power capture and economic performance from their devices as the wave energy industry begins to approach the commercial deployment of Wave Farms, arrays of full-scale WECs.

Dimensioning the Equipment of a Wave Farm: Energy Storage and Cables

Still largely untapped, wave energy may represent an important share in the energy mix of many countries in the future. However, the power fluctuations generated by most wave energy devices with little to no storage means, or without suitable control strategies, may cause power quality issues that must be solved before large wave energy farms are allowed to connect to a network. For instance, large power fluctuations may induce an excessive level of flicker in the distribution networks to which they are currently envisaged to be connected. Although storage appears to be a technically feasible solution, the minimum amount of storage required for a wave farm to become grid compliant with respect to typical flicker requirements is still unknown and is therefore investigated. This study constitutes the first part of this paper. Another issue, on which the second part of this paper focuses, concerns the optimal dimensioning of wave farm electrical components, which is traditionally performed assuming steady-state conditions (i.e., a constant current level), and is thus irrelevant in the case of wave farms outputting power fluctuations of significant amplitude. Hence, a second study, the results of which are presented in this paper, focuses on the minimum current rating for which a submarine cable may be safely operated without thermal overloading. Addressing both these issues is essential to the economic viability of a wave farm as the cost of both storage means and electrical components is highly dependent on their rating and may represent a significant percentage of the capital expenditure.

The status of ocean energy development in europe and some current research questions

European researchers and development companies are involved with wave and tidal energy systems which are, in many cases, reaching the pre-commercial stage. This paper describes the fundamental research and development stages which are necessary for a technology developer to achieve a successful outcome. The progress of a number of developers with devices that are being tested at sea is described as well as other systems which are at an earlier stage of development.

Numerical Assessment on Primary Wave Energy Conversion of Oscillating Water Columns

Oscillating water column (OWC) wave energy converters (WECs) are probably the simplest and most promising wave energy converters due to their good feasibility, reliability and survivability in practical wave energy conversions and also regarded as the most studied and developed when compared to other types of the wave energy converters.This research aims to develop a reliable numerical tool to assess the performance of the OWC wave energy converters, particularly in the primary wave energy conversion. In the numerical assessment tool, the hydrodynamics of the device and thermodynamics of the air chamber can be studied separately. However, for the complete dynamic system when a power takeoff (PTO) system is applied, these two dynamic systems are fully coupled in time-domain, in which the PTO can have a simple mathematical expression as the relation between the pressure difference across the PTO (the chamber pressure) and its flowrate through the PTO. And the application of a simple PTO pressure-flowrate relation very much simplifies the complicated aerodynamics and thermodynamics in the air turbine system so the whole dynamic system can be simplified.The methodology has been applied to a generic OWC device and the simulation results have been compared to the experimental data. It is shown that the developed numerical method is reliable in and capable of assessing the primary wave energy conversion of oscillating water columns.Copyright

Chapter 8 – Wave Energy

Publisher Summary This chapter gives an overview of the wave energy sector and some of its fundamental principles, and an understanding of the challenges and possible solutions. The focus of the wave energy sector, though, is the conversion of ocean wind waves. These wind waves are formed by winds blowing across large areas of ocean, with the surface friction transforming the energy. There are two types of wave that the wave energy converter is interested in: swell waves and local wind seas. Swell waves are generated from distant storms, whereas local wind seas are generated much closer to the point of interest. The forecasting of waves is useful for estimating energy output in the longer term. Using prediction allows an even more accurate short-term estimate. Both of these factors could combine to allow wave energy to become a dispatchable energy resource. The wave energy resource can be measured at a particular location using a number of different instruments. The industry standard is usually a small floating buoy which follows the sea surface and records its own vertical displacement. This is then recorded onboard and also transmitted ashore via a suitable telemetry system. These devices usually record for a period of around 20 minutes to get a representative sample of the wave conditions. The drivers and context for energy are now diversity of generation and security of supply. Once wave energy devices can show their commercial potential, their integration into the mainstream energy market will follow swiftly.