Showing papers on "Wave power published in 2011"

Constrained Optimal Control of a Heaving Buoy Wave-Energy Converter

Abstract: Current prognoses are that, unless counteracted by very strong political measures, the world will meet both energy shortage and climate crisis within a horizon of a few decades, both of which are strongly related to our dependence on fossil fuels. Renewable energy sources may be harvested sustainably, and developing technology for their exploitation therefore forms an obvious part of strategies to reduce emissions and secure energy supply. Wave energy is a resource with relatively high power density, readily available along the coasts, and thus coinciding with the areas where industry and people tend to be accumulated. In some regions this resource is large enough to form a significant part of the energy mix. The technology for harnessing the power of ocean waves is today still on the research and development stage. The challenge is to make a design where the costs of investment, operation and maintenance (in terms of money, resources and energy) can be justified by the availability and potential earnings. This thesis focuses on two aspects of systems for wave energy conversion: How to model such systems, which is important for understanding and design, and how to control their motion, which is crucial for the primary power conversion – the inevitable step that forms the basis for revenues and energy output from such a device. The dissertation is based on articles published in scientific conferences and journals, as well as an account for background of the undertaken research and the methods used. The bond graph modelling language has been chosen as a promising aid for the modelling of the power converter dynamics. It enables a systematic and transparent approach to the path from drawing board to mathematical equations. Examples show how energy conversion systems may be modelled and simulated within this framework. These include heave-motion models for a semi-submerged sphere, a platform/buoy two-body system and a smallscale oscillating water column (OWC), as well as wave-to-wire models of two made-up systems. The OWC model was also studied by laboratory experiments. A range of control strategies has been studied and compared by numerical simulation, and in one case also by laboratory experiments. These strategies include phase control by latching and by clutching, approximations to complex-conjugate control, and model predictive control (MPC). Constraint handling and real-time parameter tuning are discussed, too. The constrained optimal power absorption is investigated, and for the example of a semi-submerged heaving sphere in irregular waves it is found that MPC in combination with a Kalman filter predictor is able to provide an absorbed power in excess of 90% as compared to the non-causal (and hence not completely realisable) constrained optimum. Other causal controller implementations gives an absorbed power ranging from 10 to 90% of that achieved with MPC. The best performing control strategies, however, involve a large flow of reactive power through the machinery, which in normal irregular-wave operation may give peak-to-average power ratio as high as 25 and above. This represents a challenge to the design of machinery and controller. An interesting observation from the numerical simulations is the possibility of increased absorbed power in irregular waves as compared to regular waves having about the same wavelength characteristics and the same wave power level. An explanation is suggested for this phenomenon.

A Comparison of Selected Strategies for Adaptive Control of Wave Energy Converters

Abstract: Current prognoses are that, unless counteracted by very strong political measures, the world will meet both energy shortage and climate crisis within a horizon of a few decades, both of which are strongly related to our dependence on fossil fuels. Renewable energy sources may be harvested sustainably, and developing technology for their exploitation therefore forms an obvious part of strategies to reduce emissions and secure energy supply. Wave energy is a resource with relatively high power density, readily available along the coasts, and thus coinciding with the areas where industry and people tend to be accumulated. In some regions this resource is large enough to form a significant part of the energy mix. The technology for harnessing the power of ocean waves is today still on the research and development stage. The challenge is to make a design where the costs of investment, operation and maintenance (in terms of money, resources and energy) can be justified by the availability and potential earnings. This thesis focuses on two aspects of systems for wave energy conversion: How to model such systems, which is important for understanding and design, and how to control their motion, which is crucial for the primary power conversion – the inevitable step that forms the basis for revenues and energy output from such a device. The dissertation is based on articles published in scientific conferences and journals, as well as an account for background of the undertaken research and the methods used. The bond graph modelling language has been chosen as a promising aid for the modelling of the power converter dynamics. It enables a systematic and transparent approach to the path from drawing board to mathematical equations. Examples show how energy conversion systems may be modelled and simulated within this framework. These include heave-motion models for a semi-submerged sphere, a platform/buoy two-body system and a smallscale oscillating water column (OWC), as well as wave-to-wire models of two made-up systems. The OWC model was also studied by laboratory experiments. A range of control strategies has been studied and compared by numerical simulation, and in one case also by laboratory experiments. These strategies include phase control by latching and by clutching, approximations to complex-conjugate control, and model predictive control (MPC). Constraint handling and real-time parameter tuning are discussed, too. The constrained optimal power absorption is investigated, and for the example of a semi-submerged heaving sphere in irregular waves it is found that MPC in combination with a Kalman filter predictor is able to provide an absorbed power in excess of 90% as compared to the non-causal (and hence not completely realisable) constrained optimum. Other causal controller implementations gives an absorbed power ranging from 10 to 90% of that achieved with MPC. The best performing control strategies, however, involve a large flow of reactive power through the machinery, which in normal irregular-wave operation may give peak-to-average power ratio as high as 25 and above. This represents a challenge to the design of machinery and controller. An interesting observation from the numerical simulations is the possibility of increased absorbed power in irregular waves as compared to regular waves having about the same wavelength characteristics and the same wave power level. An explanation is suggested for this phenomenon.

DIRECT IMAGING OF QUASI-PERIODIC FAST PROPAGATING WAVES OF ∼2000 km s–1 IN THE LOW SOLAR CORONA BY THE SOLAR DYNAMICS OBSERVATORY ATMOSPHERIC IMAGING ASSEMBLY

Abstract: Quasi-periodic, propagating fast mode magnetosonic waves in the corona were difficult to observe in the past due to relatively low instrument cadences. We report here evidence of such waves directly imaged in EUV by the new SDO AIA instrument. In the 2010 August 1 C3.2 flare/CME event, we find arc-shaped wave trains of 1–5% intensity variations (lifetime ∼200 s) that emanate near the flare kernel and propagate outward up to ∼400 Mm along a funnel of coronal loops. Sinusoidal fits to a typical wave train indicate a phase velocity of 2200 ± 130kms −1 . Similar waves propagating in opposite directions are observed in closed loops between two flare ribbons. In the k–! diagram of the Fourier wave power, we find a bright ridge that represents the dispersion relation and can be well fitted with a straight line passing through the origin. This k–! ridge shows a broad frequency distribution with indicative power at 5.5, 14.5, and 25.1 mHz. The strongest signal at 5.5 mHz (period 181 s) temporally coincides with quasi-periodic pulsations of the flare, suggesting a common origin. The instantaneous wave energy flux of (0.1–2.6)× 10 7 ergs cm −2 s −1 estimated at the coronal base is comparable to the steady-state heating requirement of active region loops. Subject headings: Sun: activity—Sun: corona—Sun: coronal mass ejections—Sun: flares—Sun: oscillations—waves

Direct Imaging by SDO AIA of Quasi-periodic Fast Propagating Waves of ~2000 km/s in the Low Solar Corona

Abstract: Quasi-periodic, propagating fast mode magnetosonic waves in the corona were difficult to observe in the past due to relatively low instrument cadences. We report here evidence of such waves directly imaged in EUV by the new SDO AIA instrument. In the 2010 August 1 C3.2 flare/CME event, we find arc-shaped wave trains of 1-5% intensity variations (lifetime ~200 s) that emanate near the flare kernel and propagate outward up to ~400 Mm along a funnel of coronal loops. Sinusoidal fits to a typical wave train indicate a phase velocity of 2200 +/- 130 km/s. Similar waves propagating in opposite directions are observed in closed loops between two flare ribbons. In the k-$\omega$ diagram of the Fourier wave power, we find a bright ridge that represents the dispersion relation and can be well fitted with a straight line passing through the origin. This k-$\omega$ ridge shows a broad frequency distribution with indicative power at 5.5, 14.5, and 25.1 mHz. The strongest signal at 5.5 mHz (period 181 s) temporally coincides with quasi-periodic pulsations of the flare, suggesting a common origin. The instantaneous wave energy flux of $(0.1-2.6) \times 10^7 ergs/cm^2/s$ estimated at the coronal base is comparable to the steady-state heating requirement of active region loops.

WorldWaves wave energy resource assessments from the deep ocean to the coast

Abstract: WorldWaves is a global wave and wind climate package developed through EU and industry sponsorship over many years. The offshore data incorporates global hindcast and operational wave and wind data from ECMWF, validated and calibrated with independent satellite and buoy data worldwide. These data, which may comprise full directional wave spectra time series, are used as boundary conditions to the latest version of the SWAN model for calculation of nearshore wave climate parameter and spectral time series and statistics. The WorldWaves methodology was originally developed in the late 1980s as part of a large wave energy resource mapping project being performed by OCEANOR at that time for SOPAC (South Pacific Geoscience Commission) in Fiji for many South Pacific island nations. Based on the WorldWaves global database, Fugro OCEANOR have created various high precision offshore wave energy resource and variability maps. In this paper some of the peculiarities of the global wave energy climate are discussed. Further, areas worldwide exhibiting a stable energy-rich wave climate are pinpointed as are areas with a favourable ratio of extreme to mean annual wave power density, a rough indicator of the economic potential of a site. Use of shallow water models such as SWAN together with short-term in-situ wave measurements (buoys) is generally needed at the feasibility stage for a proposed wave farm. At the pre-feasibility stage, the nearshore mapping of coastal wave energy resources is often required over larger areas (e.g., a country or state) and full SWAN modelling is usually too expensive. An alternative, utilising the offshore WorldWaves data together with nearshore satellite observations is a cost-effective alternative. This method is described © Proceedings of the 8th European Wave and Tidal Energy Conference, Uppsala, Sweden, 2009 and validated against nearshore buoy data on the US West Coast. The package will also be demonstrated live at the conference exhibition to interested parties.

Diffuse auroral scattering by whistler mode chorus waves: Dependence on wave normal angle distribution

Abstract: [1] Using the statistical CRRES measurements of the electric field intensities of lower band chorus (LBC) and upper band chorus (UBC) around L = 6 under geomagnetically moderate conditions, we evaluate the variations in modeled magnetic field spectral intensity and the resultant changes in resonant scattering rates of plasma sheet electrons caused by different choices of the wave normal distribution. UBC scattering rates inferred from electric field measurements show a common trend of decreasing scattering with increasing peak wave normal angle, θm, for the plasma sheet electrons at all resonant pitch angles. This trend is mainly due to the lower power of magnetic field as derived from the electric field measurements for oblique waves. The LBC resonant diffusion inferred from electric field measurements shows a considerable increase in scattering rates with increasing θm for ∼1 keV electrons at all resonant pitch angles and for 3–30 keV electrons over certain ranges of pitch angles, which is contrary to the decrease in wave magnetic field amplitude and results mainly from the decrease in resonant energy and redistribution of the majority of wave power at large wave normal angles for increased peak wave normal angle. LBC-induced scattering rates of 3–10 keV electrons decrease with increasing θm at low pitch angles, consistent with the decrease in wave magnetic field amplitude when θm increases. Our investigation demonstrates that the knowledge of the wave normal distribution of LBC and UBC is essential for an accurate quantification of the net resonant scattering rates and loss timescales of the plasma sheet electrons for an improved global simulation of diffuse auroral precipitation and the evolution of plasma sheet electron pitch angle distribution if only measurements of wave electric field intensity are available. In contrast, the diffuse auroral scattering rates calculated from magnetic field measurements are much less sensitive to the assumption on wave normal angle distribution. While UBC scattering with constant magnetic field power is roughly insensitive to the assumed wave normal distribution, LBC scattering with constant magnetic field power becomes more dependent on the assumed wave normal angle distribution, especially for ∼1 keV electrons.

Offshore wave power measurements—A review

Abstract: The first wave power patent was filed in 1799. Since then, hundreds of ideas for extraction of energy from ocean waves have surfaced. In the process of developing a concept, it is important to lear ...

Wave power generator

Abstract: The present invention relates to an offshore wave power generator having a wireless power transmission function, which comprises: a guide post installed upwardly vertically from an ocean floor; a floating plant which is comprised to be elevated along the guide post, and ascends according to up and down wave movement; a power generator to generate electricity while the floating plant ascends, by being installed in the floating plant; a wireless power transmission unit comprising a power transmission coil to transmit generated electricity of the power generator; and a wireless power receiving unit comprising a pickup coil where electromotive force is induced by magnetic flux generated in the power transmission coil. According to the present invention, the offshore wave power generator can secure flexibility without exposing a power cable to the seawater, while the electricity generated by driving the power generator is transmitted and received wirelessly using the primary power transmission coil and the secondary pickup coil when the floating plant elevated according to the up and down wave movement ascends along the guide post installed vertically upwardly on the ocean floor.

Prediction of ocean wave energy from meteorological variables by fuzzy logic modeling

Abstract: Research highlights? A new approach based on an expert system of fuzzy logic modeling was intoduced in ocean wave energy prediction. ? It is found that the fuzzy model for wave energy prediction performs better than classical approaches. ? By this proposed approach, it is possible to determine potential wave power in any area from meteorological measurements in the absence of spectral wave measurements. Ocean wave energy which is one of the promising renewable energy types has a direct relationship with the wave climate. The purpose of this study is to investigate the relationship between ocean wave energy and meteorological variables such as wind speed, air temperature, and sea temperature. It was shown that fuzzy logic modeling of these variables provides the possible non-linear relationship between them and consequently the wave energy can be predicted including the possible uncertainties in the system behavior. Compared to traditional approaches, fuzzy logic is more efficient in linking the multiple inputs to a single output in a non-linear manner. Here Takagi-Sugeno (TS) type fuzzy inference system was employed to predict wave energy amount from meteorological variables in the absence of wave records. For the application of the proposed approach the offshore stations located in the Pacific Ocean were used. The results were compared with the classical wave energy equation.

Global energy transfer during a magnetospheric field line resonance

Abstract: [1] Field line resonances (FLRs) are important for transferring energy from fast mode waves to shear Alfven waves in the Earth's magnetosphere. Using simultaneous multi-satellite observations from THEMIS and the IMAGE ground magnetometer array, we report on the transfer of energy from compressional magnetopause undulations through an FLR to the ionosphere. Energy diversion from the magnetosphere to the ionosphere took place at the FLR: we find net energy flux there to have comparable values in the radial and the field-aligned directions. The field-aligned energy flux, when mapped to the ionosphere, was 0.70 mW/m2 and consistent with the inferred Joule dissipation rate at that time. IMAGE's regional monitoring of wave activity reveals that the temporal evolution of the FLR wave power and energy transfer were correlated with the amplitude profile of magnetopause undulations, confirming these waves to be the FLR driver.

Status and potentials of offshore wave energy resources in Chahbahar area (NW Omman Sea)

Abstract: Chahbahar area is located at the southern coasts of Iran in the Omman Sea. This paper examines possible examples of offshore wave power installations at Chahbahar area in the Omman Sea. The study aims at showing the physical possibilities of wave energy and electric power generation based upon point-absorbers and attenuator devices in the selected site. This site has been chosen to represent a range of offshore wave climates around Chahbahar area. Hindcasting data is used allowing estimations of wave energy generated and results show promising conditions in this area. Wave climate power density, or incident wave power per meter of WEC device reach a maximum value 24 kW/m with monthly maximum of 9.70 kW/m and annual average equals to 4.14 kW/m. We study power recoverable possibility for three different wave energy devices, based on their published power matrices; 750 kW Pelamis device, hypothetical modified 1500 kW Pelamis device and hypothetical 750 kW Single Point Absorber (SPA). Results show corresponding annual electric energy generation for this devices are 0.32 GWh, 4.9 GWh and 2 GWh respectively. Finally, we determine appropriate WEC device for selected site. Also, we propose a solution for some environmental problem.

Ocean wave measurement and wave climate prediction of peninsular Malaysia

Abstract: This paper presents wave measurement and wave climate prediction within Peninsular Malaysia. Rayleigh and Weibull density functions were used to predict wave heights. The total wave energy density was found to be 17.69 MWh/m within an average year, whereas average wave power density varied from 0.15 to 6.49 kW/m. Furthermore, more than 60% of the annual wave energy was caused by wave heights between 0.2 to 1.2 m. Waves with peak periods between 2 and 8 s accounted for more than 70% of the total wave energy. The extreme significant wave heights were predicted, using Gumbel, Weibull and Generalised Pareto distributions, as having return periods of 10 to 200 years for the same locations. The extreme significant wave heights varied from 2.6 to 3.4 m for the aforementioned return periods. The results of the present study will contribute greatly to the design of ocean engineering projects.

On the latitudinal extent of chorus emissions as observed by the Polar Plasma Wave Instrument

Abstract: [1] The statistical distribution of chorus wave power in the off-equatorial region is evaluated using data from the Plasma Wave Instrument (PWI) Sweep Frequency Receiver (SFR) on board the Polar spacecraft. Maps of average wave power in the meridional plane divided into four local time sectors are presented. The geomagnetic dependence of wave power is examined, and substorm activity and enhanced solar wind speed result in distinctly different wave distributions. The maximum latitudinal extent of chorus as a function of latitude and L* is estimated within the orbit constraints of the spacecraft, and on the basis of this the corresponding minimum resonant energy for first-order relativistic cyclotron resonance is calculated using a realistic magnetic field model.

Wake effects behind a farm of wave energy converters for irregular long-crested and short-crested waves

Abstract: The contribution of wave energy to the renewable energy supply is rising. To extract a considerable amount of wave power, Wave Energy Converters (WECs) are arranged in several rows or in a ’farm’. WECs in a farm are interacting (e.g. the presence of other WECs influence the operational behaviour of a single WEC) and the overall power absorption is affected. In this paper wake effects in the lee of a single WEC and multiple WECs of the overtopping type, where the water volume of overtopped waves is first captured in a basin above mean sea level and then drains back to the sea through hydro turbines, are studied using the time-dependent mild-slope equation model MILDwave. The wake behind a single WEC is investigated for long-crested and short-crested incident waves. The wake becomes wider for larger wave peak periods. An increasing directional spreading results in a faster wave regeneration and a shorter wake behind the WEC. The wake in the lee of multiple WECs is calculated for two different farm lay-outs, i.e. an aligned grid and a staggered grid, with varying lateral and longitudinal spacing. The wave power redistribution in and behind each farm lay-out is studied in detail using MILDwave. In general, the staggered grid results in the highest overall wave power absorption.

Joint exploitation of wave and offshore wind power

Abstract: This paper presents some preliminary analysis of three European marine energy test sites in terms of their potential suitability for deployment of renewable energy platforms combining more than one technology. An understanding of the resources and their correlation in time is developed, followed by an analysis of the different power production characteristics from some hypothetical combinations of wind and wave energy devices. Finally, a case study is presented, demonstrating how the addition of wave power to an offshore wind site could offer some advantages in relation to meeting consumer demand. Keywords— Wave, offshore wind, combined platforms, correlation, demand.

Modelling Wave Energy Resources in the Irish West Coast

Abstract: In order to assess the potential wave energy extraction, a study is made to validate a model that can be used to characterize Ireland’s wave climate in a more extensive study. The target area is the Irish West Coast, known for having the highest average wave power in Europe. The wave conditions in the coastal area were characterized by coupling the wave models SWAN and WAVEWATCH III. Validation tests are carried out with buoy data so that the model’s performance can be evaluated. The wave parameters considered for the comparisons in the time domain are significant wave height and mean period, and the spatial distribution of wave energy is examined in a case study. Theoretical values of wave power are obtained for sites close to the coast and in particular for the two tests sites of Galway and Belmullet.Copyright © 2011 by ASME

Wave Energy Resources Assessment in the China Sea During the Last 22 Years by Using WAVEWATCH-III Wave Model

Abstract: The third-generation wave model WAVEWATCH-Ⅲ was used to simulate the wave field of the China Sea from January,1988 to December,2009,with wind input of CCMP wind field.In order to evaluate the wave energy resources reasonably,the seasonal characteristics,frequency of wave energy scale,long-term trend,and stability of wave energy density were analyzed.The result showed that the dominant areas of wave energy resources in the China Sea were found,which could apply guidance to wave power plant location.

The slow-mode nature of compressible wave power in solar wind turbulence

Abstract: We use a large, statistical set of measurements from the Wind spacecraft at 1 AU, and supporting synthetic spacecraft data based on kinetic plasma theory, to show that the compressible component of inertial range solar wind turbulence is primarily in the kinetic slow mode. The zero-lag cross correlation C(delta n, delta B_parallel) between proton density fluctuations delta n and the field-aligned (compressible) component of the magnetic field delta B_parallel is negative and close to -1. The typical dependence of C(delta n,delta B_parallel) on the ion plasma beta_i is consistent with a spectrum of compressible wave energy that is almost entirely in the kinetic slow mode. This has important implications for both the nature of the density fluctuation spectrum and for the cascade of kinetic turbulence to short wavelengths, favoring evolution to the kinetic Alfven wave mode rather than the (fast) whistler mode.

2D Numerical Simulation of Ocean Waves

Abstract: As fossil energy is depleting and global warming effect is worsening rapidly, developing renewable energies become the top priority on most developed and some developing countries. Among different kinds of renewable energies, wave energy attracts more and more attention in recent years due to its high energy density and enormous global amount. However, some technical difficulties still need to be overcome for extracting wave power. In designing a wave energy converter, it is important to develop an efficient method to determine the wave load and predict its response. In this paper, a n umerical investigation of ocean waves is presented. Commercial software code FLUENT is used as a computational platform in this study. Based on the NavierStokes equations for viscous, incompressible fluid and Volume of fluid (VOF) method, a two dimensional numerical wave tank is established. Dynamic meshing method is used to simulate the wave maker, and GeoReconstruct scheme is used to capture the free surface. A wave-absorbing method employing porous media model is proposed, which can absorb the wave energy efficiently. Moving boundary, wall boundary and pressure-inlet boundary are used to construct the computational domain. Linear regular waves are simulated accurately using the proposed numerical model. The numerical results matched with the theoretical calculation.

Wave power generating system with floating-body-based rope pulley

Abstract: The invention provides a wave power generating system with a floating-body-based rope pulley, relating to a wave energy power generating system and comprising a wave energy collecting part, a power generator and a base, wherein the wave energy collecting part has the mechanism of: pushing a floating body by waves, tightening a rope, further driving a rotating wheel arranged on the floating body to apply work, and outputting mechanical energy to realize the collection of wave energy; and in a wave dropping step, reversely rotating the rotating wheel by using small force, withdrawing the rope, pulling and withdrawing in the way, and continuously circulating. The wave energy collecting part is used for supplying power to the power generator to generate power. The wave power generating system can adapt to most shapes of waves and has the advantages of strong wind and wave resistance, low cost and easy maintenance.