Wave Energy Converter Farm Research Articles

Analyzing the Near-Field Effects and the Power Production of an Array of Heaving Cylindrical WECs and OSWECs Using a Coupled Hydrodynamic-PTO Model

The Power Take-Off (PTO) system is the key component of a Wave Energy Converter (WEC) that distinguishes it from a simple floating body because the uptake of the energy by the PTO system modifies the wave field surrounding the WEC. Consequently, the choice of a proper PTO model of a WEC is a key factor in the accuracy of a numerical model that serves to validate the economic impact of a wave energy project. Simultaneously, the given numerical model needs to simulate many WEC units operating in close proximity in a WEC farm, as such conglomerations are seen by the wave energy industry as the path to economic viability. A balance must therefore be struck between an accurate PTO model and the numerical cost of running it for various WEC farm configurations to test the viability of any given WEC farm project. Because hydrodynamic interaction between the WECs in a farm modifies the incoming wave field, both the power output of a WEC farm and the surface elevations in the ‘near field’ area will be affected. For certain types of WECs, namely heaving cylindrical WECs, the PTO system strongly modifies the motion of the WECs. Consequently, the choice of a PTO system affects both the power production and the surface elevations in the ‘near field’ of a WEC farm. In this paper, we investigate the effect of a PTO system for a small wave farm that we term ‘WEC array’ of 5 WECs of two types: a heaving cylindrical WEC and an Oscillating Surge Wave Energy Converter (OSWEC). These WECs are positioned in a staggered array configuration designed to extract the maximum power from the incident waves. The PTO system is modelled in WEC-Sim, a purpose-built WEC dynamics simulator. The PTO system is coupled to the open-source wave structure interaction solver NEMOH to calculate the average wave field η in the ‘near-field’. Using a WEC-specific novel PTO system model, the effect of a hydraulic PTO system on the WEC array power production and the near-field is compared to that of a linear PTO system. Results are given for a series of regular wave conditions for a single WEC and subsequently extended to a 5-WEC array. We demonstrate the quantitative and qualitative differences in the power and the ‘near-field’ effects between a 5-heaving cylindrical WEC array and a 5-OSWEC array. Furthermore, we show that modeling a hydraulic PTO system as a linear PTO system in the case of a heaving cylindrical WEC leads to considerable inaccuracies in the calculation of average absorbed power, but not in the near-field surface elevations. Yet, in the case of an OSWEC, a hydraulic PTO system cannot be reduced to a linear PTO coefficient without introducing substantial inaccuracies into both the array power output and the near-field effects. We discuss the implications of our results compared to previous research on WEC arrays which used simplified linear coefficients as a proxy for PTO systems.

Wake effect assessment of a flap type wave energy converter farm under realistic environmental conditions by using a numerical coupling methodology

Ocean Energy Europe has estimated that 100 GW of ocean energy capacity (wave and tidal) could be deployed in Europe by 2050. Along with the European targets it is expected that large farms of Wave Energy Converters (WECs) will be installed in the sea and, as part of the consenting process for their installation, it will be necessary to quantify their impact on the local environment. The objective of this study is to improve the assessment of WEC farms impact on the surrounding wave field (wake effect) through the use of a numerical coupling methodology. The methodology consists of a Boundary Element Method (BEM) solver to obtain the wave perturbation generated by the WEC farm for the near-field accounting for the wave-body interactions within the farm whilst a Wave Propagation Model (WPM) based on the mild-slope equations determines the wave transformation in the far-field. The near-field solution obtained from the BEM solver is described as an internal boundary condition in the WPM and then it is propagated throughout the WPM numerical domain. The internal boundary is described by imposing the solution of the surface elevation and velocity potential at the free-surface at each instant of time along a line surrounding the WEC farm.As a case study the methodology was applied to flap type WECs that are deployed in shallow water conditions. The validation of the technique was done first for a single flap and then for a farm of 5 flaps. Once validated, a realistic scenario was assessed by quantifying the impact of irregular sea states composed of long crested waves on a large WEC farm composed of 18 flaps and located on a real bathymetry. The irregular waves were obtained by superposing the regular wave field solutions for all wave frequencies represented in the considered sea state based on the linear water wave theory. Within the limits of this theory these simulations demonstrate the versatility of the methodology to accurately represent the impact of a WEC farm on the surrounding wave climate. The influence of the peak period and the spacing between flaps on the WEC farm wake effect was assessed as well.

Assessment of the Power Output of a Two-Array Clustered WEC Farm Using a BEM Solver Coupling and a Wave-Propagation Model

One of the key challenges in designing a Wave Energy Converter (WEC) farm is geometrical layout, as WECs hydrodynamically interact with one another. WEC positioning impacts both the power output of a given wave-energy project and any potential effects on the surrounding areas. The WEC farm developer must seek to optimize WEC positioning to maximize power output while minimizing capital cost and any potential deleterious effects on the surrounding area. A number of recent studies have shown that a potential solution is placing WECs in dense arrays of several WECs with space between individual arrays for navigation. This innovative arrangement can also be used to reduce mooring and cabling costs. In this paper, we apply a novel one-way coupling method between the NEMOH BEM model and the MILDwave wave-propagation model to investigate the influence of WEC array separation distance on the power output and the surrounding wave field between two densely packed WEC arrays in a farm. An iterative method of applying the presented one-way coupling to interacting WEC arrays is used to compute the wave field in a complete WEC farm and to calculate its power output. The notion of WEC array ‘independence’ in a farm from a hydrodynamic point of view is discussed. The farm is modeled for regular and irregular waves for a number of wave periods, wave incidence angles, and various WEC array separation distances. We found strong dependency of the power output on the wave period and the wave incidence angle for regular waves at short WEC array–array separation distances. For irregular wave operational conditions, a large majority of WEC array configurations within a WEC farm were found to be hydrodynamically ‘independent’.

Multivariate analysis of the reliability, availability, and maintainability characterizations of a Spar\u2013Buoy wave energy converter farm

Quantitative reliability, availability, and maintainability (RAM) assessments are of fundamental importance at the early design stages, as well as planning and operation of marine renewable energy systems. This paper presents an RAM framework adaptable to different offshore renewable technologies, conceived to provide support in the choice of the device components and subsequent planning of the O&M strategies. A case study, characterizing a pilot farm of oscillating water column (OWC) wave energy converters (WECs), is illustrated together with the method used to obtain reliable estimate of its key performance indicators (KPIs). Based on a fixed feed-in-tariff for the project, economic figures are estimated, showing a direct relationship with the availability of the farm and the cost of maintenance interventions. Consequently, the probability distributions of the most relevant output variables are presented, and the mutual correlations between them investigated using principal components analysis (PCA) with the aim of discovering the relationships influencing the performance of the offshore farm. In this way, the contributions of the individual factors on the profitability of the project are quantified, and generic guidelines to support the decision-making process are derived. It is shown how this type of analysis provides important insights not only to ocean energy farm operators after the deployment of the devices, but also to device developers at the early design stage of wave energy concepts.

Feasibility of WEC installations for domestic and public electrical supplies: A case study off the Calabrian coast

Abstract This paper presents a performance evaluation of Wave Energy Converters (WECs) in terms of electricity production for domestic and public supplies of coastal towns located near hot spot areas. The study was conducted off the coast of Calabria (Southern Italy), a Mediterranean region characterized by a mild wave climate which is quite representative of mean sea states in the Mediterranean basin. The wave energy potential was assessed in deep waters by means of ECMWF wave data validated against RON buoys and UKMO data. Thirteen types of offshore WECs were considered, comparing the performances of full-scale and downscaled devices in terms of yearly and seasonal mean electricity production and operating days. Based on the domestic electricity supplies and those related to the public plants of some selected coastal cities, the number of differently sized devices was calculated in order to satisfy the above needs. To preserve the wave energy production level, the spatial arrangement of wave energy farms was determined in order to minimize possible park effects. The obtained results highlight that the occurrence of offshore WEC farms in a typical Mediterranean zone can lead to a significant number of WECs for domestic and public electricity supplies with a progressive reduction for more energetic sites.

The role of wave energy converter farms on coastal protection in eroding deltas, Guadalfeo, southern Spain

Many worldwide coasts are under erosion with climate projections indicating that damages will rise in future decades. Specifically, deltaic coasts are highly vulnerable systems due to their low-lying characteristics. This paper investigates the role of wave energy converter (WEC) farms on the protection of an eroding gravel-dominated deltaic coast (Guadalfeo, southern Spain). Eight scenarios with different alongshore locations of the wave farm were defined and results were compared with the present (no farm) configuration of the coast. Assuming that storm conditions drive the main destruction to the coast, we analysed the impact of the most energetic storm conditions and quantified the effects of the location of the farm. Significant wave heights in the lee of the farm were calculated by means of a calibrated wave propagation model (Delft3D-Wave); whereas wave run-up and morphological changes in eight beach profiles were quantified by means of a calibrated morphodynamic model (XBeach-G). The farm induces average reductions in significant wave heights at 10 m water depth and wave run-up on the coast down to 18.3% and 10.6%, respectively, in the stretch of beach most affected by erosion problems (Playa Granada). Furthermore, the erosion of the beach reduces by 44.5% in Playa Granada and 23.3% in the entire deltaic coast. Combining these results with previous works at the study site allowed selecting the best alternative of wave farm location based not only on coastal protection but also on energetic performance criteria. This work, whose methodology is feasibly extensible to other coasts worldwide, provides insights into the role of the alongshore location of WEC farms on wave propagation, run-up and morphological storm response of deltaic coasts.

Modelling of a flap‐type wave energy converter farm in a mild‐slope equation model for a wake effect assessment

It is expected that large farms of wave energy converters (WECs) will be installed and as part of the consenting process it will be necessary to quantify their impact on the local environment. The objective of this study is to assess the impact a WEC farm has on the incoming wave field through the use of a novel methodology. This methodology assesses the changes of the significant wave height surrounding a flap-type WEC farm with a special focus on the lee of the farm. A time-dependent mild-slope equation model is employed to solve the propagation of surface waves and their interaction with the devices. The model represents the devices as obstacle cells with attributed absorption coefficients tuned against near-fields obtained from a boundary element method (BEM) solver. The wake effect of the farm is determined by using a step-by-step approach starting first with an assessment of one device and progressively incrementing to a larger number of flaps. The effect of incident sea states, device separations and water depth changes on the wake effect of the farm is also investigated. This work shows the potential of a WEC farm to reduce significant wave heights on the leeside.

Stakeholder requirements for commercially successful wave energy converter farms

In this study, systems engineering techniques are applied to wave energy to identify and specify stakeholders’ requirements for a commercially successful wave energy farm. The focus is on the continental scale utility market. Lifecycle stages and stakeholders are identified. Stakeholders’ needs across the whole lifecycle of the wave energy farm are analyzed. A list of 33 stakeholder requirements are identified and specified. This list of requirements should serve as components of a technology performance level metric that could be used by investors and funding agencies to make informed decisions when allocating resources. It is hoped that the technology performance level metric will accelerate wave energy conversion technology convergence.

Experimental Measurement of Wave Field Variations around Wave Energy Converter Arrays

Wave energy converters (WECs) inherently extract energy from incident waves. For wave energy to become a significant power provider in the future, large farms of WECs will be required. This scale of energy extraction will increase the potential for changes in the local wave field and coastal environment. Assessment of these effects is necessary to inform decisions on the layout of wave farms for optimum power output and minimum environmental impact, as well as on potential site selection. An experimental campaign to map, at high resolution, the wave field variation around arrays of 5 oscillating water column WECs and a methodology for extracting scattered and radiated waves is presented. The results highlight the importance of accounting for the full extent of the WEC behavior when assessing impacts on the wave field. The effect of radiated waves on the wave field is not immediately apparent when considering changes to the entire wave spectrum, nor when observing changes in wave climate due to scattered and radiated waves superimposed together. The results show that radiated waves may account for up to 50% of the effects on wave climate in the near field in particular operating conditions.

Comparison Between Centralized and Decentralized Storage Energy Management for Direct Wave Energy Converter Farm

This paper compares the sizing of distributed energy storage systems (ESS) with two control types in order to smooth a direct wave energy converter farm production, namely a centralized one that deals with the point of common coupling (PCC) power with aggregated information and a decentralized one that deals with each unit with only local information. The main objective is to compare the two controls on the basis of their life cycle cost. The ESS is necessary for grid integration in the case considered here due to the flicker constraint. The co-optimization strategies for both the sizing and the management of an ESS are based on a rule-based energy management and a sequential approach to deal with the power quality constraint. This management strategy has been optimized for each size in order to reduce aging speed while respecting the flicker criterion. The final design is expected to minimize total system cost. The centralized control clearly allowed smaller capacity. However, it may lead to an increase in cables losses compared with the decentralized case, although it is expected to be negligible under the conditions considered in this paper. The confirmation of this hypothesis will be the objective of future work.

Wave farm flicker severity: Comparative analysis and solutions

This paper proposes a flicker severity study for Wave Energy Converter farms. The flicker severity is introduced and the reason why it is an important constraint for a wave farm is explained. A new representation called intrinsic flicker severity is introduced which describes the flicker severity independently of the grid. The influence of device type, its control and the sea-state on average production, flicker severity and on the ratio between flicker and production are studied with three types of devices: an Oscillating Water Column and two Direct Wave Energy Converters (two point absorbers: a Heaving Buoy and the SEAREV). The influence of the size and the placement of each unit in the wave farm is presented with a farm-unit flicker ratio, compared with the square-root of unit hypothesis (noise behavior), as a function of wave direction by taking into account wave direction dispersion. Finally, solutions are presented to reduce the flicker produced to comply with grid code requirements in order to allow grid integration of wave farms.

Harnessing wave power in open seas

We discuss some of the latent potential for harnessing wave power in open seas. Large farms of wave-energy converters in the open sea may extract energy several times over the course of an ocean basin, allowing the waves to grow under the influence of the wind, and capturing energy otherwise dissipated. Our calculations show that such an approach results in increasing the wave power potential by an order of magnitude compared to coastal capture alone. To carry out such calculations one needs the captured, reflected, and transmitted energy transfer functions of the farms. Here we simulate the functioning of the farms by one simplified two-dimensional converter consisting of two vertical floating plates, for which explicit transfer functions are calculated. Our main goals are to increase the awareness of the scientific community to the importance of harvesting wave power in open seas, and to provide a preliminary picture for the geometry and size of wave energy farms in open seas.

Sea-state modification and heaving float interaction factors from physical modelling of arrays of wave energy converters

Wave energy converters (WECs) extract energy from ocean waves and have the potential to produce a significant amount of electricity from a renewable resource. However, large “WEC farms” or “WEC arrays” (composed of a large number of individual WECs) are expected to exhibit “WEC array effects”. These effects represent the impact of the WECs on the wave climate at an installation site, as well as on the overall power absorption of the WEC array. Tests have been performed in the Shallow Water Wave Basin of DHI (Denmark) to study such “WEC array effects”. Large arrays of up to 25 heaving point absorber type WECs have been tested for a range of geometric layout configurations and wave conditions. Each WEC consists of a buoy with a diameter of 0.315 m. Power take-off was modeled by realizing friction based energy dissipation through damping of the WECs' motion. The produced database is presented: WEC response, wave induced forces on the WECs, and wave field modifications have been measured. A first understanding of WEC array effects is obtained. This unique experimental set-up of up to 25 individual WEC units in an array layout, placed in a large wave tank, is at present the largest set-up of its kind studying the important WEC array effects. The data obtained from these experimental tests will be very useful for validation and extension of numerical models. This model validation will enable optimization of the geometrical layout of WEC arrays for realistic wave farm applications and reduction of the cost of energy from wave energy systems.

Investigation of suitable sites for wave energy converters around Sicily (Italy)

Abstract. An analysis of wave energy along the coasts of Sicily (Italy) is presented with the aim of selecting possible sites for the implementation of wave energy converters (WECs). The analysis focuses on the selection of hotspot areas of energy concentration. A third-generation model was adopted to reconstruct the wave data along the coast over a period of 14 years. The reconstruction was performed using the wave and wind data from the European Centre for Medium-Range Weather Forecasts. The analysis of wave energy allowed us to characterise the most energetic zones, which are located on the western side of Sicily and on the Strait of Sicily. Moreover, the estimate of the annual wave power on the entire computational domain identified eight interesting sites. The main features of the sites include relatively high wave energy and proximity to the coast, which makes them possible sites for the implementation of WEC farms.

Effects of hydrodynamic interactions and control within a point absorber array on electrical output

A significant role is envisaged for ocean wave energy to meet the different renewable energy targets set by various governments and world bodies. To make use of this potential, the industry will soon be moving from sea testing of individual wave energy converters (WECs) to the deployment of arrays and farms of WECs. The total power extracted by an array of WECs is influenced by the hydrodynamic interactions between them, especially when the WECs are spaced very closely. By control of the power take-off (PTO) forces and moments acting on the WECs within the array, the hydrodynamic interactions between the WECs and the total power extracted by the array can be modified. In this paper, different resistive and reactive PTO control strategies, applied to a time-domain wave-to-wire model of a three-float Danish Wavestar device, are compared. The time-domain modelling approach, as opposed to the frequency-domain, allows the use of constraints on the maximum PTO moment to be applied in order to make the study realistic. In this paper, the effects that PTO control has on the hydrodynamic interactions between the floats and on the total power generated by the device, when placed in a range of irregular sea states, are studied. It was found that the performance of the three-float device improved as the sophistication of the PTO control strategy and the level of hydrodynamic interactions taken into account in the control problem increased. From among the different control strategies tested in this work, fully-coordinated global array control (matrix control) was found to maximise the time-averaged power generated by the array. Fully-coordinated control potentially enables wave farm developers and device designers to explore the opportunities of connecting and maximising energy yields from installations that will be necessary to contribute to meeting the 2020 and 2050 targets for offshore renewable energy.

MODELLING OF WAVE ATTENUATION INDUCED BY MULTI-PURPOSE FLOATING STRUCTURES USED FOR POWER SUPPLY AND COASTAL PROTECTION

Experiments have been performed in the Shallow Water Wave Basin of DHI (Horsholm, Denmark) within the EU FP7 Hydralab Programme, on large farms of up to 25 heaving point absorber type Wave Energy Converters (WECs). For a range of geometric lay-out configurations and wave conditions (regular, polychromatic, long- and short-crested irregular waves), WEC response and modification of the wave field have been measured to provide data for the understanding of WEC farm interactions and for the evaluation of farm interaction numerical models. A first extensive wave farm database is established. The experimental arrangement and the obtained database are presented, as well as results for wave height attenuation downwave of the farms. For long-crested irregular waves, up to 18.1 % and 20.8 % reduction in significant wave height is observed downwave of the 5x5-WEC rectilinear and staggered farm, respectively. Wave height attenuation is expected to be larger, since in practical wave farm applications WECs will be controlled to extract a large amount of power from the waves, and therefore the array will cause larger wave height dissipation. These findings present the ability to combine the harvesting of energy from sea waves with coastal defence systems, resulting in cost reduction for both applications when WECs operate as multi-purpose devices.

Power Collection from Wave Energy Farms

Most Wave Energy Converters (WECs) produce highly distorted power due to thereciprocal motion induced by ocean waves. Some WEC systems have integrated energystorage that overcomes this limitation, but add significant expenses to an already costlysystem. As an alternative approach, this article investigates the direct export option thatrelies on aggregate smoothing among several WECs. By optimizing the positioning of theWEC devices with respect to the incoming waves, fluctuations may be mutually canceledout between the devices. This work is based on Fred. Olsen’s WEC system Lifesaver, anda WEC farm consisting of 48 devices is designed in detail and simulated. The major costdriver for the electrical export system is the required oversize factor necessary for transferof the average power output. Due to the low power quality, this number can be as high as20 at the entry point of the electrical system, and it is thus crucial to quickly improve thepower quality so that the downstream power system is efficiently utilized. The simulationsundertaken in this work indicate that a high quality power output can be achieved at the farmlevel, but that a significant oversize factor will be required in the intermediate power systemwithin the farm.

Theory and Experiment on an Elastically Moored Cylindrical Buoy

In this paper, we compare simulated forces and accelerations for a moored floating buoy with full-scale experimental results in real ocean waves. The buoy is moored with a wire connected by springs to a concrete foundation situated at the seafloor. This study aims to develop a computer model using potential theory with a linearized free-surface boundary condition to describe the motion of such a system. The intention is to use the model for future study of wave-energy absorption and design of converters. Another objective is to see how complex a model is required to get accurate results. The method used is computationally fast and makes it possible to couple linear buoy wave interaction with nonlinear generator models, so that different loads and latching can be studied. A computationally fast method is required to model farms of wave-energy converters