On the park effect in arrays of oscillating wave energy converters

This paper aims to provide guidelines for designing the layout of arrays of oscillating Wave Energy Converters (WECs) based on a review of the literature of wave interactions and park effect in WEC arrays that has been published over the past 30 years.First, the fundamentals of wave energy absorption by oscillating bodies are summarised, and the principal differences between the park effect in arrays of wave energy converters and wind turbines are highlighted. Then, the numerical approaches commonly used to deal with WEC arrays are outlined briefly and their limitations are discussed. It is argued that, at present, only Boundary Element Methods (BEM) are capable of the appropriate analysis. Finally, previous work on wave interactions and park effect in WEC arrays is reviewed. Similar trends are found in these studies, which allow conclusions to be drawn regarding the significance of the park effect as a function of the number of WECs in the array and their spacing. Based on these conclusions, the following tentative guidelines are proposed:For small arrays of conventional devices (fewer than 10 devices of typical dimension 10–20 m) with usual layouts (regular or shifted grids with separating distance of order 100–200 m), the park effect appears to be negligible. For larger arrays (more than 10 devices), a negative park effect seems to be increasingly important with increasing number of rows (the lines of WECs perpendicular to the incident wave direction). Therefore, the number of rows should remain as small as possible, with a separating distance as large as possible. For arrays of non-conventional WECs (WECs of typical dimensions much larger than 10–20 m), no information has been found. However, trends similar to the previous cases could be expected, provided that aspect ratios are maintained.     

A methodology for production and cost assessment of a farm of wave energy converters

To generate a substantial amount of power, Wave Energy Converters (WECs) are arranged in several rows or in a ‘farm’. Both the power production and cost of a farm are lay-out dependent.In this paper, the wave power redistribution in and around three farm lay-outs in a near shore North Sea wave climate, is assessed numerically using a time-dependent mild-slope equation model. The modelling of the wave power redistribution is an efficient tool to assess the power production of a farm. Further, for each lay-out an optimal (low cost) submarine cable network is designed. The methodology to assess the power production and cost of a farm of WECs is applied to the Wave Dragon Wave Energy Converter (WD–WEC). The WD–WEC is a floating offshore converter of the overtopping type, which captures the water volume of overtopped waves in a basin above mean sea level and produces power when the water drains back to the sea through hydro turbines.It is observed that the cable cost is relatively small compared to the cost of the WD–WECs. As a result, WD–WECs should be installed in a lay-out to increase power production rather than decrease cable cost, taking spatial and safety considerations into account. WD–WECs arranged in a single line produce the highest amount of power, but require an available sea area with a large width (51 km). Installing a single line of WD–WECs in front of a farm of wind turbines increases the time window for accessing the wind farm (applied to Horns Rev II – significant wave height smaller than 1–2 m during 8 h at minimum) by 9–14%.     

Model predictive control of sea wave energy converters – Part II: The case of an array of devices

This paper addresses model predictive control (MPC) of highly-coupled clusters of sea wave energy converters (WECs). Since each WEC is not only a wave absorber but also a wave generator, the motion of each WEC can be affected by the waves generated by its adjacent WECs when they are close to each other. A distributed MPC strategy is developed to maximize the energy output of the whole array and guarantee the safe operation of all the WECs with a reasonable computational load. The system for an array is partitioned into subsystems and each subsystem is controlled by a local MPC controller. The local MPC controllers run cooperatively by transmitting information to each other. Within one sampling period, each MPC controller performs optimizations iteratively so that a global optimization for the whole array can be approximated. The computational burden for the whole array is also distributed to the local controllers. A numerical simulation demonstrates the efficacy of the proposed control strategy. For the WECs operating under constraints explored, it is found that the optimized power output is an increasing function of degree of WEC–WEC coupling. Increases in power of up to 20% were achieved using realistic ranges of parameters with respect to the uncoupled case.     

Improving the assessment of wave energy resources by means of coupled wave-ocean numerical modeling

Sea waves energy represents a renewable and sustainable energy resource, that nevertheless needs to be further investigated to make it more cost-effective and economically appealing. A key step in the process of Wave Energy Converters (WEC) deployment is the energy resource assessment at a sea site either measured or obtained through numerical model analysis. In these kind of studies, some approximations are often introduced, especially in the early stages of the process, viz. waves are assumed propagating in deep waters without underneath ocean currents. These aspects are discussed and evaluated in the Adriatic Sea and its northern part (Gulf of Venice) using locally observed and modeled wave data. In particular, to account for a “state of the art” treatment of the Wave–Current Interaction (WCI) we have implemented the Simulating WAves Nearshore (SWAN) model and the Regional Ocean Modeling System (ROMS), fully coupled within the Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) system. COAWST has been applied to a computational grid covering the whole Adriatic Sea and off-line nested to a high-resolution grid in the Gulf of Venice. A 15-year long wave data set collected at the oceanographic tower “Acqua Alta”, located approximately 15 km off the Venice coast, has also been analyzed with the dual purpose of providing a reference to the model estimates and to locally assess the wave energy resource. By using COAWST, we have quantified for the first time to our best knowledge the importance of the WCI effect on wave power estimation. This can vary up to 30% neglecting the current effect. Results also suggest the Gulf of Venice as a suitable testing site for WECs, since it is characterized by periods of calm (optimal for safe installation and maintenance) alternating with severe storms, whose wave energy potentials are comparable to those ordinarily encountered in the energy production sites.     

Adaptability of a generic wave energy converter to different climate conditions

This study evaluates the influence of wave climate tunability on the performance of a generic Wave Energy Converter (WEC) for different climate scenarios. The generic WEC is assumed to be composed of an array of heaving, floating cylinders. In this study, two natural periods for the cylinders of 4 s and 8 s (typical of enclosed seas and the mean Atlantic swell, respectively) and a location-tunable cylinder are considered to evaluate the influence of tuning on the power performance of the cylinder. The WEC power matrix is computed using a frequency domain model, and the performance of the WEC is evaluated along the global coasts; the met-ocean data originated from the global reanalysis database (GOW) from Reguero et al. (2012). The performance of the WEC is evaluated using two parameters: the capture width ratio (CWR), which evaluates the efficiency of the converter at each location, and the kW/Ton (KWT) parameter, which evaluates the efficiency of the converter using “economic” terms. Tuning a converter for each location displayed a positive CWR; however, the KWT was low after WEC tuning because of the weight of the structures required to tune the converter that experiences high peak periods.     

Application of the time-dependent mild-slope equations for the simulation of wake effects in the lee of a farm of Wave Dragon wave energy converters

Time-dependent mild-slope equations have been extensively used to compute wave transformations near coastal and offshore structures for more than 20 years. Recently the wave absorption characteristics of a Wave Energy Converter (abbreviated as WEC) of the overtopping type have been implemented in a time-dependent mild-slope equation model by using numerical sponge layers. In this paper the developed WEC implementation is applied to a single Wave Dragon WEC and multiple Wave Dragon WECs. The Wave Dragon WEC is a floating offshore converter of the overtopping type. Two wave reflectors focus the incident wave power towards a ramp. The focussed waves run up the ramp and overtop in a water reservoir above mean sea level. The obtained potential energy is converted into electricity when the stored water drains back to the sea through hydro turbines. The wave reflectors and the main body (ramp and reservoir) are simulated as porous structures, exhibiting the same reflection, respectively absorption characteristics as obtained for the prototype Wave Dragon WEC. The wake effects behind a single Wave Dragon WEC are studied in detail for uni- and multidirectional waves. The shadow zone indicating the wake effect is decreasing with increasing directional spreading. The wake in the lee of a farm of five Wave Dragon WECs, installed in a staggered grid (3 WECs in the first row and 2 WECs in the second row), is calculated for three in-between distances of respectively D, 2D and 3D, with D the distance between the tips of the wave reflectors of a single WEC. As a result, a farm of five Wave Dragon WECs installed in a staggered grid with an in-between distance of 2D is preferred, when taking cost and spatial considerations into account.     

Wave electricity production in Italian offshore: A preliminary investigation

In this paper the feasibility of wave energy exploitation off the Italian coasts is investigated. At this aim, the energy production and the performance characteristics of three of the most promising and documented wave energy converters (AquaBuOY, Pelamis and Wave Dragon) are estimated for two of the most energetic Italian locations. The sites are Alghero, on the western coast of Sardinia and Mazara del Vallo, on the Sicily Strait and they have respectively an average annual wave power of 10.3 kW/m and 4 kW/m, and an available annual wave energy of 90 MWh/m and 35 MWh/m.The energy production of the hypothetical wave farms is calculated based on the performance matrices of the wave energy converters (WECs) and on 21 years of wave buoy records, covering the period from 1990 to 2011. The estimated capacity factors are low (between 4% and 9%) compared to the ones obtained for the same wave energy converters in other locations and are affected by a strong seasonal variability. This indicates that the considered WECs are oversized with respect to the local wave climate and that a more efficient energy conversion would be obtained if they were downscaled according to the typical wave height and period of the study sites. As a consequence of the optimization of the device scale, at Alghero the deployment of 1:2.5 AquaBuOY, Pelamis or Wave Dragon devices would result in capacity factors around 20% and in a quite constant energy production throughout the year. In fact, the size reduction of the wave energy converters allows to capture the energy of the small waves which would otherwise be lost with the original WECs.The results of the present work suggest that deploying classic wave energy converters in Italian seas would not be cost effective but if the devices could accommodate a proper downscaling, their performance in energy conversion would become economically attractive also for some Italian locations.     

Wave energy potential along the western Portuguese coast

An assessment of nearshore wave energy resource along the Portuguese coast is presented, focusing on identify appropriate locations for testing and developing Wave Energy Converter (WEC) for commercial exploit. The analysis covers the whole west seaside, to which a partition defined by 7 linear sections parallel to the coastline at 50 m depth was considered. Available wave energy at each linear sector was calculated from nearshore wave parameters, using as input the offshore wave conditions provided by a 15-year ocean wind-wave model simulation and considering a simplified but well-established analytical procedure for shoreward wave transformation. Two alternative measures of the nearshore wave energy resource were considered, the standard omni-directional wave power density and the more restricted normally-directed wave energy flux.Offshore wave direction combine to shoreline orientation proved to be determinant on the evaluation of the wave energy resource in each section, since sectors of the shoreline directly facing the offshore annual average wave direction have limited reduction in available wave energy as compare to offshore values. Independently of the wave energy measured criteria used, the analysis suggests that the sector from Peniche to Nazaré is the more suitable location for nearshore wave energy exploitation, with annual wave energy around 200 MWh m⁻¹, closely followed by the adjacent sector from Nazaré to Figueira da Foz.     

Nonlinear 2D analysis of the efficiency of fixed Oscillating Water Column wave energy converters

This paper reports on the development of a two-dimensional, fully nonlinear Computational Fluid Dynamics (CFD) model to analyse the efficiency of fixed Oscillating Water Column (OWC) Wave Energy Conversion (WEC) devices with linear power take off systems. The model was validated against previous experimental, analytical and numerical results of others. In particular, the simulation results show excellent agreement with the analytical results obtained by Sarmento and Falcão [1] for linear waves in a 2D channel and with previous experiments by others on the interaction between nonlinear waves and a fixed barge. Results are presented for linear waves on the influence of the seaward wall draft and thickness of the OWC device on the resonant frequency and the capture efficiency of the OWC. The key outcome of the present work is that for fully nonlinear waves a substantial decrease in the hydrodynamic capture efficiency of the OWC device was observed with increasing wave height, which represents a significant departure from the linear wave case. The optimal pneumatic damping coefficient for the OWC was also found to be dependent on the wave height. By analysing the magnitude of the first and higher order components of the incident nonlinear waves and the response of the OWC it was found that the first order capture efficiency decreases with increasing wave height, which in turn implies that the OWC hydrodynamic system is fully nonlinear and that the behaviour of an OWC in a nonlinear wave train cannot be accurately represented by the superposition of the linear response to a number of component linear waves. These results have significant implications for the design and operation of practical OWC systems.     

An investigation of the impacts of climate change on wave energy generation: The Wave Hub,Cornwall, UK

In this paper a generic methodology is presented that allows the impacts of climate change on wave energy generation from a wave energy converter (WEC) to be quantified. The methodology is illustrated by application to the Wave Hub site off the coast of Cornwall, UK. Control and future wave climates were derived using wind fields output from a set of climate change experiments. Control wave conditions were generated from wind data between 1961 and 2000. Future wave conditions were generated using two IPCC wind scenarios from 2061 to 2100, corresponding to intermediate and low greenhouse gas emissions (IPCC scenarios A1B and B1 respectively). The quantitative comparison between future scenarios and the control condition shows that the available wave power will increase by 2–3% in the A1B scenario. In contrast, the available wave power in the B1 scenario will decrease by 1–3%, suggesting, somewhat paradoxically, that efforts to reduce greenhouse gas emissions may reduce the wave energy resource. Meanwhile, the WEC energy will yield decrease by 2–3% in both A1B and B1 scenarios, which is mainly due to the relatively low efficiency of energy extraction from steeper waves by the specific WEC considered. Although those changes are relatively small compared to the natural variability, they may have significance when considered over the lifetime of a wave energy farm. Analysis of downtime under low and high thresholds suggests that the distribution of wave heights at the Wave Hub will have a wider spread due to the impacts of climate change, resulting in longer periods of generation loss. Conversely, the estimation of future changes in joint wave height-period distribution provides indications on how the response and power matrices of WECs could be modified in order to maintain or improve energy extraction in the future.     

Prototype testing of the wave energy converter wave dragon

The Wave Dragon is an offshore wave energy converter of the overtopping type. It consists of two wave reflectors focusing the incoming waves towards a ramp, a reservoir for collecting the overtopping water and a number of hydro turbines for converting the pressure head into power.In the period from 1998 to 2001 extensive wave tank testing on a scale model was carried at Aalborg University. Then, a 57×27 m wide and 237 tonnes heavy (incl. ballast) prototype of the Wave Dragon, placed in Nissum Bredning, Denmark, was grid connected in May 2003 as the world's first offshore wave energy converter.The prototype is fully equipped with hydro turbines and automatic control systems, and is instrumented in order to monitor power production, wave climate, forces in mooring lines, stresses in the structure and movements of the Wave Dragon.In the period May 2003 to January 2005 an extensive measuring program has been carried out, establishing the background for optimal design of the structure and regulation of the power take off system. Planning for deployment of a 4 MW power production unit in the Atlantic by 2007 is in progress.     

Multi–chamber oscillating water column wave energy converters and air turbines: A review

The oscillating water column (OWC) is a more common type of wave energy converter (WEC) that has been the subject of the study and development for several decades. Multi–chamber oscillating water column (MC–OWC) devices or arrays have the advantage of being more efficient in energy extraction compared to a single chamber system, particularly in more chaotic sea states. A variety of single and array OWC devices have been proposed and studied on a small–scale, whereas few large–scale devices have been tested under ocean wave conditions. This paper provides a concise review of the current state of MC–OWC device development in laboratory conditions. The review highlights explicitly the main stages of MC–OWC device development for one ongoing study as an example. This review was based on the available information in the literature from 2003 to 2012, in addition, further work is presented as part of the current study at the University of Technology Sydney. This study is from 2015 to 2018. The discussion shows the challenges that a device needs to overcome to be more competitive with other WECs in the global of wave energy converter area.     

Further analysis of change in nearshore wave climate due to an offshore wave farm: An enhanced case study for the Wave Hub site

This paper addresses the use of numerical wave models for assessing the impact of offshore wave farms on the nearshore wave climate. Previous studies have investigated the effect of energy extraction by wave energy devices through the use of spectral models such as SWAN, representing a wave farm as one or more barriers within the model domain and applying a constant wave energy transmission percentage across the whole wave spectrum incident at the barrier. However, this is an unrealistic representation of the behaviour of real wave energy converters. These will exhibit frequency-dependent energy absorption characteristics that will correspond to the spectral response of the device, and may reflect its ability to be tuned to extract energy at particular frequencies. This study describes a modification of the SWAN source code to enable frequency-dependent wave energy transmission through a barrier. A detailed analysis of the wave climate at the Wave Hub wave farm site is also presented, with a particular focus on the occurrence of bimodal sea states. The modified SWAN code is used to assess how impact predictions for typically occurring sea states may differ when using frequency-dependent rather than constant wave energy transmission, with reference to a previous study using the unmodified code (Millar, Smith and Reeve, 2007 [1]). The results illustrate the dependence of the magnitude of the impact on both the response function of the devices and the spectral sea state in which they are operating.     

Offshore wave energy resource assessment in the East China Sea

The offshore wave energy resource in the East China Sea (ECS) off the coast of the southern East China is assessed using wave buoy data covering the period of 2011−2013. It is found that the averaged offshore wave power was approximately 13 kW m⁻¹ in the region of interest. Most of the offshore wave energy in the ECS is contributed by the sea states with significant wave heights between 1.5 m and 3.5 m and with wave energy periods between 6 s and 8 s. Seasonal variations are detected in the wave characteristics of significant wave height and wave power. The predominant wave directions are mainly from the II quadrant and the IV quadrant, respectively, in winter and summer, in accordance with the monsoon characteristics in the ECS. Wave heights, periods and power are generally higher in winter and autumn, and weaker in spring and summer; however, extreme values occur in some summer and autumn months due to the extreme conditions caused by typhoons passing over this region. These extreme sea states do not contribute much to the total annual energy, mainly because of their low occurrence, but may bring risks to the wave energy converters.     

Increases in the average power output of wave energy converters using quiescent period predictive control

The potential of controlling wave energy converters, (WEC), by deterministic prediction of large damaging waves is introduced and shown to offer very substantial increases in the annual average power output of such devices. Results obtained for idealised WEC models show that the potential exists for this increase to be at least a factor of two. Numerical simulations of actual dynamical models for both point absorbers and directionally sensitive devices employing practical control strategies show that most of this potential can actually be realised. The control of large scale wave farms using quiescent period predictive control is likely to be most cost effective using master/slave WEC systems. To achieve the computational savings that will allow this strategy analytic approximations are required for the response of WECs with time varying coefficients, preliminary forms of these have also been introduced.     

Multi-criteria evaluation of wave energy projects on the south-east Australian coast

In the last decade, multiple studies focusing on national-scale assessments of the ocean wave energy resource in Australia identified the Southern Margin to be one of the most energetic areas worldwide suitable for the extraction of wave energy for electricity production. While several companies have deployed single unit devices, the next phase of development will most likely be the deployment of parks with dozens of units, introducing the risk of conflicts within the marine space.This paper presents a geo-spatial multi-criteria evaluation approach to identify optimal locations to deploy a wave energy farm while minimizing potential conflicts with other coastal and offshore users. The methodology presented is based around five major criteria: ocean wave climatology, nature of the seabed, distance to key infrastructure, environmental factors and potential conflict with other users such as shipping and fisheries.A case study is presented for an area off the south-east Australian coast using a total of 18 physical, environmental and socio-economic parameters. The spatial restrictions associated with environmental factors, wave climate, as well as conflict of use, resulted in an overall exclusion of 20% of the study area. Highly suitable areas identified ranged between 11 and 34% of the study area based on scenarios with varying criteria weighting. By spatially comparing different scenarios we identified persistence of a highly suitable area of 700 km² off the coast of Portland across all model domains investigated. We demonstrate the value of incorporation spatial information at the scale relevant to resource exploitation when examining multiple criteria for optimal site selection of Wave Energy Converters over broad geographic regions.     

基于风电场海域海况的波能浮子阵列发电功率优化

[目的]为了响应国家集约用海,发展清洁能源,助力碳中和,对海上风电-波浪能装置多能融合模式进行初步分析,对波能浮子进行优化设计,以获得更高的功率输出。[方法]依据势流理论,对漂浮式风机平台-波能浮子阵列进行仿真计算,分析浮子的外形尺寸和固有周期对浮子的输出功率的影响。[结果]仿真结果表明：同一固有周期下,波能浮子越扁平,波能浮子阵列的总发电功率越大,且浮子的经济性差异很小。对于海况下,不同固有周期的波能浮子阵列经济性差异较大,因此要综合分析考虑。[结论]在已知海域海况条件下,可以通过对波能浮子固有周期和外形尺度进行优化设计,使波能浮子获得更高的功率输出,提高单位海域能量产出。     

Electricity generation from wave power in Canada

The performance of three different types of wave energy converters (WECs) is evaluated at hundreds of Canadian locations using wave activity data made available by the Marine Environmental Data Service of Canada. Two Atlantic and three Pacific locations are found where at least one of these devices operates with a capacity factor of greater than 20%, while also being located close to urban/industrial centers. The economics of a nominal 25 GWh wave power plant are investigated at these five locations and compared among the three WEC types using two indicators: the 25-year life-cycle cost, and the required price of electricity for a 10-year simple payback period. The lowest required electricity price for a 10-year payback is $0.089/kWh, and occurs at a location near the Hibernia Oil Platform using the AquaBuOY WEC. The highest annual capacity factor is 32.1%, which occurs near the Hibernia Oil Platform when using the WaveDragon WEC. The 25-year life-cycle cost evaluations suggest that wave power plants at locations near Ucluelet, St. John's, and the Hibernia Oil Platform could all be profitable using either the AquaBuOY or the WaveDragon if a price of electricity between $0.10 and $0.15/kWh can be secured, depending on location and device.     

From wave to jet and from jet to hydrogen: A promising hybrid system

One of the main challenges that our society must overcome in this century is that of finding alternative energy sources to fossil fuels. These, ideally, must be inexpensive, less polluting than current fuels and available for a substantial time. One promising alternative is hydrogen, which has the great advantage that it can be produced by coupling renewable energy devices with water electrolysis. Several projects devoted to connecting photovoltaic and wind systems with electrolysis devices have been successful; however, little research has been done into the coupling of ocean wave energy converters with water electrolysis. The work here proposes a basic system that stores the energy from waves in the form of hydrogen. The WEC considered is a novel design known as a Blow-Jet, which captures waves and converts them into a water jet. The performance of the Blow-Jet is found to depend more on wavelength than on wave height. The electrolyser results show, at 0.200 A and 1.88 V, that the electrolysis of water produces 0.082 Nl h⁻¹ of hydrogen and a current efficiency (η_I) of 90.58%.     

Assessments of wave energy in the Bohai Sea,China

Wave fields in the Bohai Sea are continuously simulated by the third-generation wave model SWAN in order to determine the wave energy resources from 1985 to 2010. The wind parameters used to simulate waves are obtained by the Regional Atmospheric Modeling System (RAMS). Comparisons of significant wave heights between simulations and observations show good agreement. The spatial distributions of mean monthly and annual averaged significant wave height and wave power flux are presented. Wave energy roses and temporal variations of average wave power density at five typical points in the Bohai Sea are calculated. Furthermore, the correlations between significant wave height and wave energy period are studied in scatter and energy diagrams.