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1.
A linear hydrodynamic model is used to assess the sensitivity of the performance of a wave energy converter (WEC) array to control parameters. It is found that WEC arrays have a much smaller tolerance to imprecision of the control parameters than isolated WECs and that the increase in power capture of WEC arrays is only achieved with larger amplitudes of motion of the individual WECs. The WEC array radiation pattern is found to provide useful insight into the array hydrodynamics. The linear hydrodynamic model is used, together with the wave climate at the European Marine Energy Centre (EMEC), to assess the maximum annual average power capture of a WEC array. It is found that the maximum annual average power capture is significantly reduced compared to the maximum power capture for regular waves and that the optimum array configuration is also significantly modified. It is concluded that the optimum configuration of a WEC array will be as much influenced by factors such as mooring layout, device access and power smoothing as it is by the theoretical optimum hydrodynamic configuration. 相似文献
2.
Wave energy fluctuating a great deal endangers the security of power grid especially micro grid in island. A DC nano grid supported by batteries is proposed to smooth the output power of wave energy converters (WECs). Thus, renewable energy converters connected to DC grid is a new subject. The characteristics of WECs are very important to the connection technology of HPTO type WECs and DC nano grid. Hydraulic power take-off system (HPTO) is the core unit of the largest category of WECs, with the functions of supplying suitable damping for a WEC to absorb wave energy, and converting captured wave energy to electricity. The HPTO is divided into a hydraulic energy storage system (HESS) and a hydraulic power generation system (HPGS). A primary numerical model for the HPGS is established in this paper. Three important basic characteristics of the HPGS are deduced, which reveal how the generator load determines the HPGS rotation rate. Therefore, the connector of HPTO type WEC and DC nano grid would be an uncontrollable rectifier with high reliability, also would be a controllable power converter with high efficiency, such as interleaved boost converter-IBC. The research shows that it is very flexible to connect to DC nano grid for WECs, but bypass resistance loads are indispensable for the security of WECs.. 相似文献
3.
Measurements of static and dynamic mooring line damping and their importance for floating WEC devices 总被引：4，自引：0，他引：4
The dynamic response of the mooring line will be a dominant factor to consider in their use for the station keeping of a wave energy converter (WEC). Due to the relatively small size of WECs and their being moored in relatively shallow waters the effect of waves, tide and current can be of greater significance than for other floating offshore systems. Axial line stretching and high-frequency ‘top-end’ dynamics can importantly modify damping and top-end loading.If a ‘farm’ of devices is to be considered then limitations in sea space may necessitate that the devices be relatively densely packed. This will mean that the ‘footprint’ of the mooring should be constrained, to ensure that the moorings from each device do not interfere and this will have great significance for the loading experienced by the line. One must also consider how the mooring system might change the response of the WEC and so alter its ability to extract power from the waves. Unlike a typical offshore system, the design of moorings for a WEC device must consider reliability and survivability, and the need to ensure efficient energy conversion.The design and operation of a chain mooring for a WEC is considered here. Generic experimental measurements of mooring line damping were conducted in the Heriot-Watt University wave basin at a scale of 1:10. The measurements were conducted on a single mooring line for surge motions and include the study of axial stretching and high top-end dynamics. The laboratory procedures were designed to resemble tests undertaken earlier at ‘full’ scale in 24 m water depth. The measurements were also compared with numerical studies. The experimental findings for WEC devices, supports the conclusion that dynamic mooring line motion will be an important variable, needing to be considered carefully within the design. 相似文献
4.
Several Wave Energy Converters (abbreviated as WECs) have intensively been studied and developed during the last decade and currently small farms of WECs are getting installed. WECs in a farm are partly absorbing, partly redistributing the incident wave power. Consequently, the power absorption of each individual WEC in a farm is affected by its neighbouring WECs. The knowledge of the wave climate around the WEC is needed to predict its performance in the farm. In this paper a technique is developed to implement a single and multiple WECs based on the overtopping principle in a time-dependent mild-slope equation model. So far, the mild-slope equations have been widely used to study wave transformations around coastal and offshore structures, such as breakwaters, piles of windmills and offshore platforms. First the limitations of the WEC implementation are discussed through a sensitivity analysis. Next the developed approach is applied to study the wave height reduction behind a single WEC and a farm. The wake behind an isolated WEC is investigated for uni- and multidirectional waves; it is observed that an increase of the directional spread leads to a faster wave redistribution behind the WEC. Further 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, by adapting the performance of each WEC to its incident wave power. The evolved technique is a fast tool to find the optimal lay-out of WECs in a farm and to study the possible influence on surrounding activities in the sea. 相似文献
5.
《African Journal of Marine Science》2013,35(1):959-974
Historical and contemporary data on Benguela seabirds assumed to be responsive to changes in food availability are reviewed as potential indicators and predictors of change in the ecosystem. Cyclic variations in the annual yields of seabird products evident at decadal long time-scales may reflect fluctuations in prey populations at similar periods. However, no such relationship has been demonstrated and consequently the value of such annual yields as indicators is not proved. More recently, interannual variation in diets, foraging effort, breeding success and chick growth of two of the three most abundant species of seabird in the ecosystem have been examined in relation to changes in prey abundance. Although the relative abundance of important forage species in the diets of both species of seabird have reflected regional changes in the absolute abundance of these prey, changes in the values of other parameters thought to reflect food availability were not consistently correlated between and within species. This indicates poor potential for their use as indicators of change at least over the range of abundances experienced in this study. Under present management protocol, data from seabirds do not nor are likely to provide abundance indices that may be used directly in stock assessment of commercially important species. To date, insights into distribution and migration of pelagic fish provided by seabird-derived data probably represent the most valuable contribution of birds to fisheries scientists studying the Benguela system. 相似文献
6.
Both foreign and domestic pelagic longline fishing vessels operate in South Africa’s Exclusive Economic Zone and adjacent international waters where they kill hundreds of seabirds each year as bycatch. To update assessments of the impact of the pelagic longline fishery on seabirds off South Africa, information on necropsied seabirds and national fisheries observer bycatch records were summarised for 2006–2013. Foreign-flagged (Asian) vessels had 100% observer coverage throughout the study period, whereas only 6% of the fishing effort by South African-flagged vessels was observed (with no coverage in 2011–2013). Vessels with observers caught seabirds at a rate of 0.132 birds per 1 000 hooks, resulting in an estimated mortality of 2 851 individuals (356 per year) comprising 14 species. Extrapolation of the observed fishing sets to the unobserved fishing sets by the South African domestic longline fleet suggested that approximately 750 additional birds were likely killed during the study period, therefore a combined 450 birds were killed per year. White-chinned petrel Procellaria aequinoctialis was the most frequently killed species (66%), followed by ‘shy-type’ albatrosses Thalassarche cauta/steadi (21%), black-browed albatross T. melanophris (7%), Indian yellow-nosed albatross T. carteri (3%), and Cape gannet Morus capensis (2%). The seabird bycatch rates were lower than in 1998–2005. Nationality of the vessel, time of line-setting, moon phase, year, season, fishing area, and seabird bycatch mitigation measures all influenced seabird mortality. Concurrent with 100% observer coverage, significant reductions in the seabird bycatch rate occurred in the Asian fleet in the latter years of the study, and these rates now approximate the national target (0.05 birds per 1 000 hooks). However, seabird bycatch rates remained high in the South African fleet, where no observers were deployed during 2011–2013, highlighting the need for independent observer programmes in fisheries—a matter of global interest. Suggestions are made as to how seabird bycatch by pelagic longline fisheries off South Africa may be further reduced. 相似文献
7.
This study presents an analysis of a wave energy converter (WEC) system consisting of a buoy, a mooring system, and a power cable connected to a hub. The investigated WEC system is currently under full-scale testing near Runde in Norway. The purpose of the study was to investigate the characteristics of the entire system, primarily with regard to energy performance and the fatigue life of the mooring lines and power cable, considering the effects of marine biofouling and its growth on the system’s components. By means of parametric study, the energy performance and fatigue life of the mooring lines and power cable were investigated considering two mooring configurations, three biofouling conditions, four sea states in a scatter diagram, and three wave and current directions. Hydrodynamic and structural response simulations were conducted in a coupled response analysis using the DNV-GL software SESAM. Energy performance analyses and stress-based rainflow counting fatigue calculations were performed separately using an in-house code. The results show that, for a WEC system which has been deployed for 25 years, biofouling can reduce the total power absorption by up to 10% and decrease the fatigue life of the mooring lines by approximately 20%. 相似文献
8.
Compared with solar and wind energy, wave energy is a kind of renewable resource which is enormous and still under development. In order to utilize the wave energy, various types of wave energy converters (WECs) have been proposed and studied. And oscillating-body WEC is widely used for offshore deployment. For this type of WEC, the oscillating motion of the floater is converted into electricity by the power take off (PTO) system, which is usually mathematically simplified as a linear spring and a damper. The linear PTO system is characteristic of frequency-dependent response and the energy absorption is less powerful for off resonance conditions. Thus a nonlinear snap through PTO system consisting of two symmetrically oblique springs and a linear damper is applied. A nonlinear parameter γ is defined as the ratio of half of the horizontal distance between the two oblique springs to the original length of both springs. JONSWAP spectrum is utilized to generate the time series of irregular waves. Time domain method is used to establish the motion equation of the oscillating-body WEC in irregular waves. And state space model is applied to replace the convolution term in the time domain motion equation. Based on the established motion equation, the motion response of both the linear and nonlinear WEC is numerically calculated using 4th Runge–Kutta method, after which the captured power can be obtained. Then the influences of wave parameters such as peak frequency, significant wave height, damping coefficient of the PTO system and the nonlinear parameter γ on the power capture performance of the nonlinear WEC is discussed in detail. Results show that compared with linear PTO system, the nonlinear snap through PTO system can increase the power captured by the oscillating body WEC in irregular waves. 相似文献
9.
Top predators, particularly seabirds, have repeatedly been suggested as indicators of marine ecosystem status. One region currently under pressure from human fisheries and climate change is the North Sea. Standardized seabird monitoring data have been collected on the Isle of May, an important seabird colony in the northwestern North Sea, over the last 10-20 years. Over this period oceanographic conditions have varied markedly, and between 1990 and 1999 a major industrial fishery for sandlance (Ammodytes marinus), the main prey of most seabird species, was prosecuted nearby. Sandlance fishing grounds close to seabird colonies down the east coast of the UK were closed in 2000 in an attempt to improve foraging opportunities for breeding seabirds, particularly black-legged kittiwakes (Rissa tridactyla). Initially this closure seemed to be beneficial for kittiwakes with breeding success recovering to pre-fishery levels. However, despite the ban continuing, kittiwakes and many other seabird species in the North Sea suffered severe breeding failures in 2004. In this paper, we test the predictive power of four previously established correlations between kittiwake breeding success and climatic/trophic variables to explain the observed breeding success at the Isle of May in 2004. During the breeding season, kittiwakes at this colony switch from feeding on 1+ group to 0 group sandlance, and results up until 2003 indicated that availability of both age classes had a positive effect on kittiwake breeding success. The low breeding success of kittiwakes in 2004 was consistent with the late appearance and small body size of 0 group sandlance, but at odds with the two variables likely to operate via 1 group availability (lagged winter sea surface temperature and larval sandlance cohort strength in 2003). The reason for the discrepancy is currently unknown, but analysis of 1 group sandlance body composition indicated that lipid content in 2004 was extremely low, and thus fish eaten by kittiwakes during pre-breeding and early incubation were likely to be of poor quality. Monitoring of reproductive success of kittiwakes, although useful, was clearly not sufficient to tease apart the complex causation underlying the 2004 event. Monitoring programs such as this, therefore, need to be complemented by detailed research to identify the mechanisms involved, and to attribute and predict the effects of natural and human-induced environmental change. 相似文献
10.
Ocean wave energy converters (WECs) are obtaining more and more attentions in the world. So far, many types of converters have been invented. Oscillating body systems are a major class of WECs, which typically have one degree of freedom (DOF), and the power absorption efficiency is not quite satisfactory. In this paper, a 3-DOF WEC is proposed and a simplified frequency-domain dynamic model of the WEC depending on the linear potential theory is conducted. The performances of three geometries of the oscillating body including the cone, the cylinder and the hemisphere have been compared, and the results show that the hemisphere is more suitable for the 3-DOF WEC. Subsequently, the relationship among the parameters of the hemisphere is established based on the equal natural frequencies of the heave and pitch (or roll) motions, and the results show that lowering the center of gravity leads to the better power absorption in the pitch (or roll) motion. In the end, the power matrixes of different sizes of the hemispheres under different irregular waves are obtained, which can give a size design reference for engineers. 相似文献
11.
SW Evans N Cole H Kylin NS Choong Kwet Yive V Tatayah J Merven 《African Journal of Marine Science》2016,38(3):317-327
A survey of seabirds and turtles at St Brandon’s Rock, 400 km north of Mauritius, was undertaken in 2010. We estimated that 1 084 191 seabirds comprising seven breeding species and excluding non-breeders were present at the archipelago and we counted 279 turtle tracks and nesting pits of green turtles Chelonia mydas. Hawksbill turtles Eretmochelys imbricata were also present. Analyses of 30 different islets that make up the atoll showed that the seabird species mostly partitioned their use of islets based on islet size, with four species preferring larger islets and two species preferring smaller islets. Alien species introduced historically are still present and other threats, such as shipwrecks, remain. We propose conservation and other measures that should adequately protect the birds, turtles and coral reef by treating the atoll as a system. 相似文献
12.
《African Journal of Marine Science》2013,35(1):125-137
The distribution and the abundance of flying seabirds are described in relation to the marine environment near Gough Island in the south Atlantic Ocean. Results from a combined seabird, hydrographic and hydro-acoustic survey are assessed in order to highlight possible biotic-abiotic associations. In accordance with recently published work, the macroscale abundance and species composition of avifauna are explained in terms of Gough Island's position between two major biogeographic zones and by the island's utilization as a breeding site for pelagic seabirds. On the mesoscale, seabird abundance was greatest in areas of hydrographical variability, where results from acoustic transects and net hauls suggested concentration of potential food items (i.e. zooplankton and micronekton). 相似文献
13.
《Deep Sea Research Part I: Oceanographic Research Papers》2007,54(7):1129-1145
Our objective was to understand how marine birds respond to oceanographic variability across the Southern Indian Ocean using data collected during an 16-day cruise (4–21 January 2003). We quantified concurrent water mass distributions, ocean productivity patterns, and seabird distributions across a heterogeneous pelagic ecosystem from subtropical to sub-Antarctic waters. We surveyed 5155 km and sighted 15,606 birds from 51 species, and used these data to investigate how seabirds respond to spatial variability in the structure and productivity of the ocean. We addressed two spatial scales: the structure of seabird communities across macro-mega scale (1000 s km) biogeographic domains, and their coarse-scale (10 s km) aggregation at hydrographic and bathymetric gradients. Both seabird density and species composition changed with latitudinal and onshore–offshore gradients in depth, water temperature, and chlorophyll-a concentration. The average seabird density increased across the subtropical convergence (STC) from 2.4 birds km−2 in subtropical waters to 23.8 birds km−2 in sub-Antarctic waters. The composition of the avifauna also differed across biogeographic domains. Prions (Pachyptila spp.) accounted for 57% of all sub-Antarctic birds, wedge-tailed shearwaters (Puffinus pacificus) accounted for 46% of all subtropical birds, and Indian Ocean yellow-nosed albatross (Thallasarche carteri) accounted for 32% of all birds in the STC. While surface feeders were the most abundant foraging guild across the study area, divers were disproportionately more numerous in the sub-Antarctic domain, and plungers were disproportionately more abundant in subtropical waters. Seabird densities were also higher within shallow shelf-slope regions, especially in sub-Antarctic waters, where large numbers of breeding seabirds concentrated. However, we did not find elevated seabird densities along the STC, suggesting that this broad frontal region is not a site of enhanced aggregation. 相似文献
14.
This paper is part of a comprehensive review of the oceanography of the eastern tropical Pacific, the oceanic region centered on the eastern Pacific warm pool, but also including the equatorial cold tongue and equatorial current system, and summarizes what is known about oceanographic influences on seabirds and cetaceans there. The eastern tropical Pacific supports on the order of 50 species of seabirds and 30 species of cetaceans as regular residents; these include four endemic species, the world’s largest populations for several others, three endemic sub-species, and a multi-species community that is relatively unique to this ecosystem. Three of the meso-scale physical features of the region are particularly significant to seabirds and cetaceans: the Costa Rica Dome for blue whales and short-beaked common dolphins, the Equatorial Front for planktivorous seabirds, and the countercurrent thermocline ridge for flocking seabirds that associate with mixed-species schools of spotted and spinner dolphins and yellowfin tuna. A few qualitative studies of meso- to macro-scale distribution patterns have indicated that some seabirds and cetaceans have species-specific preferences for surface currents. More common are associations with distinct water masses; these relationships have been quantified for a number of species using several different analytical methods. The mechanisms underlying tropical species–habitat relationships are not well understood, in contrast to a number of higher-latitude systems. This may be due to the fact that physical variables have been used as proxies for prey abundance and distribution in species–habitat research in the eastern tropical Pacific.Though seasonal and interannual patterns tend to be complex, species–habitat relationships appear to remain relatively stable over time, and distribution patterns co-vary with patterns of preferred habitat for a number of species. The interactions between seasonal and interannual variation in oceanographic conditions with seasonal patterns in the biology of seabirds and cetaceans may account for some of the complexity in species–habitat relationship patterns.Little work has been done to investigate effects of El Niño-Southern Oscillation cycles on cetaceans, and results of the few studies focusing on oceanic seabirds are complex and not easy to interpret. Although much has been made of the detrimental effects of El Niño events on apex predators, more research is needed to understand the magnitude, and even direction, of these effects on seabirds and cetaceans in space and time. 相似文献
15.
This paper presents a nonlinear frequency domain model and uses this to assess the performance of a wave energy converter (WEC) array with a nonlinear power take-off (PTO). In this model, the nonlinear PTO forces are approximated by a truncated Fourier series, while the dynamics of the WEC array are described by a set of linear motion equations in the frequency domain, and the hydrodynamic coefficients are obtained with the boundary element method. A single heave absorber is firstly investigated to establish the accuracy of the new model in capturing the nonlinear behaviour of the pumping system. Subsequently, simulations of a 2D array with 18 WECs and a pillar in the centre (representing the tower of a wind turbine) are carried out to understand wave interference effects. Several optimisation strategies are proposed to improve the overall performance of the WEC array. These results demonstrate a computationally effective method for accounting for nonlinear effects in large WEC arrays. The proposed approach may potentially be applied for developing control algorithms for the adaptability of a 2D array to incoming wave excitation. 相似文献
16.
A novel floating pendulum wave energy converter (WEC) with the ability of tide adaptation is designed and presented in this paper. Aiming to a high efficiency, the buoy''s hydrodynamic shape is optimized by enumeration and comparison. Furthermore, in order to keep the buoy''s well-designed leading edge always facing the incoming wave straightly, a novel transmission mechanism is then adopted, which is called the tidal adaptation mechanism in this paper. Time domain numerical models of a floating pendulum WEC with or without tide adaptation mechanism are built to compare their performance on various water levels. When comparing these two WECs in terms of their average output based on the linear passive control strategy, the output power of WEC with the tide adaptation mechanism is much steadier with the change of the water level and always larger than that without the tide adaptation mechanism. 相似文献
17.
《中国海洋工程》2017,(5)
A novel floating pendulum wave energy converter(WEC) with the ability of tide adaptation is designed and presented in this paper.Aiming to a high efficiency,the buoy's hydrodynamic shape is optimized by enumeration and comparison.Furthermore,in order to keep the buoy's well-designed leading edge always facing the incoming wave straightly,a novel transmission mechanism is then adopted,which is called the tidal adaptation mechanism in this paper.Time domain numerical models of a floating pendulum WEC with or without tide adaptation mechanism are built to compare their performance on various water levels.When comparing these two WECs in terms of their average output based on the linear passive control strategy,the output power of WEC with the tide adaptation mechanism is much steadier with the change of the water level and always larger than that without the tide adaptation mechanism. 相似文献
18.
Effect of mooring-line stiffness on the performance of a dual coaxial-cylinder Wave-Energy Converter
A point-absorber-type Wave-Energy Converter (WEC) consisting of a floating vertical inner cylinder and an annular outer cylinder that slides along the inner one is considered. The two cylinders heave differently under wave excitation, and wave energy can be harnessed from the relative heave motion between the two cylinders using a Permanent Magnet Linear Generator (PMLG) as the Power Take-Off unit. A mooring cable is attached to the bottom of the inner cylinder. This paper aims to examine the effect of the stiffness of the mooring cable on the performance of the coaxial-cylinder WEC system. The two limiting cases of no mooring cable (freely floating inner and outer cylinders) and an infinitely stiff mooring cable (fixed inner cylinder) were also considered. To perform the analysis, hydrodynamic and interference coefficients of the two heaving cylinders were computed semi-analytically using the method of matched eigenfunction expansions. Experimentally determined viscous corrections on damping were also included in the model in order to have more realistic predictions. The performance of the system in terms of motion responses and capture width were predicted and discussed for both regular and irregular waves. The results of the analysis indicate that both the freely floating design and the design with rigidly moored inner cylinder are viable. The two limiting cases show similar optimal performances, albeit with very different optimal generator damping. However, an ill-chosen mooring-cable stiffness may cause the inner and the outer cylinders to have the same resonance frequency, eliminating the relative heave motion and leading to almost no energy extraction. This situation needs to be avoided when designing the mooring system for a coaxial-cylinder WEC. 相似文献
19.
Several control methods of wave energy converters (WECs) need prediction in the future of wave surface elevation. Prediction of wave surface elevation can be performed using measurements of surface elevation at a location ahead of the controlled WEC in the upcoming wave. Artificial neural network (ANN) is a robust data-learning tool, and is proposed in this study to predict the surface elevation at the WEC location using measurements of wave elevation at ahead located sensor (a wave rider buoy). The nonlinear autoregressive with exogenous input network (NARX NN) is utilized in this study as the prediction method. Simulations show promising results for predicting the wave surface elevation. Challenges of using real measurements data are also discussed in this paper. 相似文献
20.
《African Journal of Marine Science》2013,35(3-4):553-560
There are 21 and 15 species of seabirds that breed in the Humboldt and Benguela upwelling systems respectively. Only two species of gull are common to both systems, one as an endemic subspecies to the Benguela system. Eleven species and two subspecies are endemic (or nearly so) to the Humboldt system; seven species and one subspecies to the Benguela system. Each system has an endemic penguin, sulid, cormorant and tern that feed mainly on anchovy Engraulis spp., sardine Sardinops sagax or both these fish. The Peruvian pelican Pelecanus thagus also feeds primarily on these prey items. A plentiful availability of food has resulted in many of these seabirds attaining high levels of abundance. For the four pairs of species that feed on anchovy and sardine, those in the Humboldt system all have a biology that enables them to increase more rapidly than their Benguela counter-parts. This reflects the higher frequency of environmental perturbations that depress seabird populations in the Humboldt system. In addition, both systems have a small endemic cormorant that feeds near the coast and a small endemic tern that breeds in the adjacent mainland desert and feeds at the sea surface. Several seabirds endemic to a system have no obvious ecological equivalent in the other system: the pelican, a diving-petrel, four storm-petrels and a gull in the Humboldt system; a cormorant and a gull in the Benguela system. Some species with tropical or subantarctic affinities breed at the boundaries of the systems. Others breed also in freshwater systems. The grey gull Larus modestus, which feeds in the Humboldt system, breeds in montane deserts. 相似文献