Scale Wave Energy Converter Research Articles

A small scale wave energy converter development by experimental test

A small scale wave energy converter development by experimental test

Degrees of Freedom Effects on a Laboratory Scale WEC Point Absorber

The number of rigid bodies and operational degrees of freedom of a wave energy converter (WEC) have significant impacts on power production magnitude and efficiency across wave periods, but this concept is not well studied. Investigating the effect of the number of bodies and degrees of freedom will help WEC developers design devices that most efficiently meet power and cost needs. This paper presents experimental and numerical results for a point absorber WEC across three modes of operation in regular waves. The three modes of operation are ‘one body heave only’, ‘two body heave only’, and ‘two body six degrees of freedom’. The Laboratory Upgrade Point Absorber (LUPA) was used as the test article. This work builds on the 2021 EWTEC paper [1] by completing fabrication and undergoing wave testing. The LUPA experiment took place in the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University in Corvallis, Oregon, USA; the installation and a wave test are shown in Figure 1. LUPA is constructed as a 1:20 scale two body point absorber with a buoyancy driven float, and a spar with a heave plate to act as a reactionary body. The spar is 3.7 meters tall and the float has a 1-meter diameter. Figure 1. (Left) Mooring line installation. (Right) Wave tests in the Large Wave Flume. The ‘one body heave only’ mode locks the LUPA spar so only the float is moving, and only in heave (vertical). The ‘two body heave only’ mode allows both the float and spar to move but both bodies are restricted to heaving. The ‘six degrees of freedom’ mode has taut mooring lines connected to the spar and LUPA can translate and rotate in all directions. A damping optimization of the power take off was performed experimentally and numerically by sweeping damping values across a wide range of regular wave periods. The optimal damping value is used to compare the power generation of the three modes of operation. The LUPA project is an open-source, lab-scale wave energy converter that acts as a testing platform for students and researchers. The data from this testing along with numerical models and engineering design files will be publicly available here: https://github.com/PMEC-OSU/LUPA. This paper presents how increasing complexity of WECs through the number of bodies and degrees of freedom affects power production. It also provides a baseline case for LUPA which other researchers can compare against when applying their research to the LUPA project.

A Detailed Investigation Into the Wave Energy Resource at a Small-Scale Ocean Energy Test Site in China

During the R&D of marine renewable energy converters, scale models are usually used in the early stages of development of marine renewable energy converters. China’s National Ocean Integrated Test Site (NOITS) is being developed by the National Ocean Technology Center (NOTC) in Weihai, Shandong Province, to facilitate testing of scaled wave and tidal energy converters in an open sea environment. This research aims to gain a detailed understanding of the wave characteristics and the wave energy resource at this site. A nested modeling system has been implemented using Simulating WAves Nearshore (SWAN) to provide a 10-year high-resolution wave hindcast between 2009 and 2019 with an approximately 60 m resolution covering the test site. Analysis of the spatio-temporal distribution of wave energy resource in NOITS reveals its strong seasonality, with the mean wave power density of >1.5 kW/m during winter and <0.2 kW/m during summer. We also performed a statistical analysis of the high and extreme wave conditions and their occurrence. By applying scaling methods to wave resources, four WECs were selected to demonstrate the testing at different scale ratios in NOITS. The results of this study provide engineering references for the design of scale wave energy converter models which target to perform open sea trials in NOITS.

LMI‐based passivation of LTI systems with application to marine structures

Due to the inherent relevance of passive (physically representative) models for control, state-estimation, and motion simulation in the field of marine systems, in this paper, an optimisation-based approach to passivation of linear time-invariant (LTI) systems is proposed with application to physically consistent dynamical modelling of marine structures. In particular, the presented strategy is based upon the introduction of a suitably designed perturbation, computed via minimisation of a linear objective subject to a specific set of linear matrix inequalities (LMIs). The performance of the passivation technique is showcased in terms of two case studies: An offshore platform, with a frequency-domain response computed by means of hydrodynamic codes, and a 1:20 scale wave energy converter (WEC), is characterised in terms of real experimental data.

Experimental Analysis of a Novel Adaptively Counter-Rotating Wave Energy Converter for Powering Drifters

Nowadays, drifters are used for a wide range of applications for researching and exploring the sea. However, the power constraint makes it difficult for their sampling intervals to be smaller, meaning that drifters cannot transmit more accurate measurement data to satellites. Furthermore, due to the power constraint, a modern Surface Velocity Program (SVP) drifter lives an average of 400 days before ceasing transmission. To overcome the power constraint of SVP drifters, this article proposes an adaptively counter-rotating wave energy converter (ACWEC) to supply power for drifters. The ACWEC has the advantages of convenient modular integration, simple conversion process, and minimal affection by the crucial sea environment. This article details the design concept and working principle, and the interaction between the wave energy converter (WEC) and wave is presented based on plane wave theory. To verify the feasibility of the WEC, the research team carried out a series of experiments in a wave tank with regular and irregular waves. Through experiments, it was found that the power and efficiency of the ACWEC are greatly influenced by parameters such as wave height and wave frequency. The maximum output power of the small scale WEC in a wave tank is 6.36 W, which allows drifters to detect ocean data more frequently and continuously.

Wave Excitation Force Estimation for Wave Energy Converters of the Point-Absorber Type

Advanced control strategies play a crucial role in increasing the energy extraction capacity of wave energy converters (WECs). So far, the most promising control schemes have predominantly been studied in simulation, based on the idealized assumption of the wave excitation force availability in real time. In practical WEC implementations, this is not the case, since this force cannot be measured directly when the WECs are running. Hence, the force has to be estimated via the measurement of other quantities. Two approaches are presented in this brief to fulfill this objective. The first approach is based on a Kalman filter coupled with a random-walk model of the wave excitation force, while a receding horizon-unknown input estimation approach-is employed for the second one. The proposed estimation methods are evaluated by using real measurements from a laboratory scale WEC.

Hydrodynamic analysis of a heaving wave energy converter

In the present paper, the development of a model scale Wave Energy Converter (WEC) and an experimental WEC test rig are presented, and results of numerical simulations and experimental measurements are shown. The presented point absorber WEC is coupled to a generic power take-off (PTO) and is restricted to pure heave motion in regular waves. Experiments were carried out at the Universidad Austral de Chile (UACh) Wave Tank and results from responses and efficiencies were compared with data from the BEM (boundary element method) code WAMIT. Numerical and experimental results showed good agreement. Finally, results were extrapolated and superposed with typical wave energy spectra found in different Chilean regions, providing a first performance estimation for a wave energy converter in Chile. Results are discussed and compared with an existing technology and give an insight of the potential for wave energy technologies in Chile. Further investigation is proposed for an analysis in irregular waves and the use of a more advanced PTO (power take-off) in the future.

An empirical evaluation of the sea depth effects for various wave characteristics on the performance of a point absorber wave energy converter

In this paper, the effects of sea depth on the performance of a modified point absorber wave energy converter (WEC) are examined using an experimental study. For experimental evaluation of WEC, the general cubic shape is used. For obtaining the final shape of this novel WEC, 3 stages are considered including, still water depth optimization, optimization of geometrical specifications and shape optimization. The focus of this article is on the first stage. In this context, by creating waves with different amplitudes and periods, evaluation is conducted in three different still water depths. WEC oscillation in the directions of heave, surge and pitch are measured and compared. Furthermore, an appropriate scale for building a full scale WEC for operating in the Caspian Sea environment is studied and proposed. Experimental test results show that increasing the water depth causes an increment on the WEC oscillation on the directions of heave and pitch. However, this process is different in the case of surge oscillations; increasing the depth leads to a reduction in WEC oscillation. The analysis of these results shows that the use of point absorber WEC with the dominant motion of pitch in deep water can provide better conditions for energy extraction.

Proposition and experiment of a sliding angle self-tuning wave energy converter

The hydraulic power-take-off mechanism (HPTO) is one of the most popular methods in wave energy converters (WECs). However, the conventional HPTO with a fixed direction motion has some drawbacks which limit its power capture capability. This paper proposes a sliding angle self-tuning wave energy converter (SASTWEC) to find the optimal sliding angle automatically, with the purpose of increasing the power capture capability and energy efficiency. Furthermore, a small scale WEC test rig was fabricated and a wave making source has been employed to verify the sliding angle performance and efficiency of the proposed system throughout experiments.

Effects of non-vertical linear motions of a hemispherical-float wave energy converter

The hydraulic power-take-off mechanism (HPTO) is one of the most popular methods in wave energy converter (WECs). However, the conventional HPTO with only one direction motion has some drawbacks which limit its power capture capability. This paper proposes an adjustable slope angle wave energy converter (ASAWEC) and investigates the effect of slope angle on the performance of the proposed wave energy converter to find the optimal slope angle with the purpose to increase the power capture capability as well as energy efficiency. A mathematical model of components from a floating buoy to a hydraulic motor was modeled. A small scale WEC test rig was fabricated to verify the power capture capability and efficiency of the proposed system throughout experiments.

파력발전기의 동력인출장치의 회전각도가 효율에 미치는 영향 분석

The hydraulic power-take-off mechanism (HPTO) is one of the most popular methods in wave energy converters (WECs). However, the conventional HPTO with only one direction motion has a number of drawbacks that limit its power capture capability. This paper proposes an adjustable moving angle wave energy converter (AMAWEC) and investigates the effect of the moving angle on the performance of the wave energy converter to find the optimal moving angle in order to increase the power capture capability as well as energy efficiency. A mathematical model of components from a floating buoy to a hydraulic motor was modeled. A small scale WEC test rig was fabricated to verify the power capture capability and efficiency of the proposed system through experiments.

Characteristics of the Operational Noise from Full Scale Wave Energy Converters in the Lysekil Project: Estimation of Potential Environmental Impacts

Wave energy conversion is a clean electric power production technology. During operation there are no emissions in the form of harmful gases. However there are unsolved issues considering environmental impacts such as: electromagnetism; the artificial reef effect and underwater noise. Anthropogenic noise is increasing in the oceans worldwide and wave power will contribute to this sound pollution in the oceans; but to what extent? The main purpose of this study was to examine the noise emitted by a full scale operating Wave Energy Converter (WEC) in the Lysekil project at Uppsala University in Sweden. A minor review of the hearing capabilities of fish and marine mammals is presented to aid in the conclusions of impact from anthropogenic sound. A hydrophone was deployed to the seabed in the Lysekil research site park at distance of 20 and 40 m away from two operational WECs. The measurements were performed in the spring of 2011. The results showed that the main noise was a transient noise with most of its energy in frequencies below 1 kHz. These results indicate that several marine organisms (fish and mammals) will be able to hear the operating WECs of a distance of at least 20 m.

Scaled Development of a Novel Wave Energy Converter Including Numerical Analysis and High-Resolution Tank Testing

This paper presents the scaled development of a novel point absorber wave energy converter (WEC) developed by Columbia Power Technologies (COLUMBIA POWER, Corvallis, OR, USA), including numerical analysis and scaled wave tank testing. Four hydrodynamic modeling tools are employed to evaluate and optimize the performance of the WEC, including WAMIT, Garrad Hassan's GH WaveDyne, OrcaFlex, and ANSYS AQWA. Performance and mooring estimates at full scale are evaluated and optimized, followed by the development of 1 : 33, 1 : 15, and 1 : 7 scale physical models. The physical tests of the 1 : 33 and 1 : 15 scale WECs were conducted in the wave basins of Oregon State University's O.H. Hinsdale Wave Research Laboratory, in conjunction with the Northwest National Marine Renewable Energy Center (NNMREC). This paper concludes with the development of the 1 : 7 scale WEC and the associated field testing.

Wave energy resource characterisation of the Atlantic Marine Energy Test Site

The Atlantic Marine Energy Test Site (AMETS), a grid connected test area for the deployment of full scale Wave Energy Converters (WECs), is being developed by the Sustainable Energy Authority of Ireland near Belmullet in Co. Mayo, Ireland. In this paper measured data provided by two wave buoys, positioned at a deepwater location (100m depth) and an offshore location (50m depth), are analysed in order to characterise the wave resource at the site. In particular, a distinction is made between which sea states occur with the most regularity and which wave conditions are the most significant for the capture of power. The spatial variation in the occurrence of important summary statistics between the deepwater and offshore sites is examined and the difference in incident wave power calculated. Finally, this paper compares conditions at the Belmullet site with those measured at the quarter scale test site in Galway Bay. An assessment on the degree of scalability between resource parameters relevant to WEC power production experienced at the two locations, as recommended by development protocols, is made and methods for comparing benign and exposed sites proposed.

Laboratory experiment on using non-floating body to generate electrical energy from water waves

This paper describes an innovative method of using a non-buoyant body to harness ocean waves. All the point absorbers are buoyant in nature and move up due to buoyancy and come down because of gravity. The point absorbers are designed to move along the waves to make the device efficient. These devices face excessive stress during the rough weather on account of the extreme motion of waves and cause the total device failure. The present study shows that using a non-buoyant body for conventional point absorber principle is much efficient and safer than any other device proposed till today. A small scale wave energy converter with non-buoyant body was designed, fabricated and tested in small scale wave maker. An electrical generator was coupled with the device to generate electrical energy from harnessed waves. The generator was electrically loaded and the generated power was measured. It was found from the experiments that the proposed device showed a significant improvement in electricity generation and safety during extreme conditions. In addition to the electricity generation, the characteristics of the device were also studied by using various wave and device parameters.

Design and testing of a non-linear power take-off simulator for a bottom-hinged plate wave energy converter

Eddy current brakes provide a versatile way of simulating the power take-off system (PTO) in model testing of small scale wave energy converters (WECs). This type of PTO simulator is based on the principle that a conductive material moving in a magnetic field generates a braking force proportional to its velocity. A bottom-hinged pitching plate WEC model has been designed using an eddy current brake as a PTO simulator. A dedicated electric current source unit was developed to provide a controllable and reliable level of DC current intensity to feed the magnetic field generating coils. Using a real-time data acquisition and control, this unit can be used to impose non-linear damping PTO characteristic curves in several types of WEC models based on eddy current brakes. In the present case, this current source has been used to simulate a constant damping PTO on a small scale pitching WEC model that has been tested in the IST wave flume. Two different cases were considered: one corresponding to a surface piercing plate and another to a fully submerged plate. Experimental results are presented for plate motion and for non-dimensional capture width.

Fundamental Research on Multi-Hull-Type Floating Wave Energy Converter (2nd Report)

In this paper, model experiments were conducted in regular and irregular waves for the twin-hull-type floating wave energy converter equipped with some electric generators.The results of generated electric power, motion of the converter and transmitting wave obtained by the experiments were compared with the ones predicted by theoretical method.It was found that the results obtained by the experiments were in agreement with the predicted values.Further, the average values of generated electric power and the rate of operation of the full scale wave energy converter (Δ=1, 430 ton) were estimated by using the experimental data and actual wave data for some coastal areas of Japan. The average electric power and the rate of operation were respectively estimated to have about 40 kW and 97 percent throughout the year at Habu, which was located on the Pacific side of Japan.