Content uploaded by Jørgen Hals Todalshaug
Author content
All content in this area was uploaded by Jørgen Hals Todalshaug on Apr 30, 2015
Content may be subject to copyright.
A preview of the PDF is not available
The NumWEC project. Numerical estimation of energy delivery from a selection of wave energy converters – final report
Abstract and Figures
In order to provide a benchmark for the income potential of wave energy
converters, the project described in this document was carried out in order
to estimate and analyse the energy conversion for a representative selection of
converter designs. Eight different wave energy converter designs and their operation in five different locations in European waters were studied. For each design the yearly energy delivery, including its distribution with time is reported.
Figures - uploaded by Jørgen Hals Todalshaug
Author content
All figure content in this area was uploaded by Jørgen Hals Todalshaug
Content may be subject to copyright.
... The diameter is used for circular devices while the characteristic dimension is based on the WECs maximum horizontal crosssectional area for non-circular devices. Duration Curves (Babarit et al., 2011) Distribution of output power in function of fractions of the year. Energy per Wave Force (Babarit et al., 2011) Yearly energy output per unit characteristic excitation force, in kWh/kN. ...
... Duration Curves (Babarit et al., 2011) Distribution of output power in function of fractions of the year. Energy per Wave Force (Babarit et al., 2011) Yearly energy output per unit characteristic excitation force, in kWh/kN. Energy p/ Device Mass (Babarit et al., 2011) Yearly energy output per characteristic mass, in MWh/ton. ...
... Energy per Wave Force (Babarit et al., 2011) Yearly energy output per unit characteristic excitation force, in kWh/kN. Energy p/ Device Mass (Babarit et al., 2011) Yearly energy output per characteristic mass, in MWh/ton. Energy per Wet Surface (Babarit et al., 2011) Yearly energy output per characteristic wetted surface area, in MWh/m 2 . ...
As for any novel technology, the need to consider, identify and formulate performance requirements and related assessment criteria has been an important subject in the development of Wave Energy Converters (WECs). These allow the characterisation of each technology through techno-economic indicators, which in turn allow comparisons between different technologies, and an assessment of alternative solutions throughout all the development stages. Such assessment is ideally carried out through the application of metrics, which should comply with several attributes, such as being objective, quantitative, specific, measurable, repeatable, and independent. In the present work, more than 50 metrics to monitor the development of WECs are compiled, explained, and discussed. These metrics are divided in the following evaluation areas: 1) Performance; 2) Reliability; 3) Survivability; 4) Techno-economics. In addition, the important evaluation area of Environmental Impact is briefly discussed concerning the need for common metrics. The compilation summarised in this paper and its discussion aim to provide a practical reference source concerning metrics for WEC development, which is currently unavailable in the published literature in terms of broadness and condensed presentation. Such compilation includes multiple formulations from the wave energy sector and other relatable industries (e.g. wind energy) that are typically diluted among specialist literature, standards, guidelines and recommendations, scientific papers, and project reports. The paper is concluded with a reflection of any salient gaps that are not addressed by current metrics, in a context of accelerating the development of WEC technologies.
... As a directional device, PeWEC aligns with the incoming waves thanks to a mooring system based on a spreading catenary. The OSWC is a pitching flap, hinged at its base to a fixed axis, secured to the seabed, and primarily operates in shallow to intermediate waters [45]. The oscillatory movement of the OWSC drives two hydraulic cylinders connected to the flap and the fixed sub-frame, generating electricity through an electrical generator placed onshore. ...
... OSWC's schematic design and main technical characteristics, inspired from[45]. ...
... Assuming rigid-body modes and planar motion of the floating platform and three WECs, the wind-wave system has six independent degrees of freedom: two translational modes (surge and heave) and one rotational mode (pitch) for the platform and one translational mode corresponding to the motion of each WEC relative to the platform. A very detailed derivation of the equations of motion for a similar problem can be found in [53]. However, due to the increased number of WECs, the authors took a different approach, in particular, the equations of motion are derived for each body (platform and three WECs) separately, and coupled through the power take-off system and hydrodynamics. ...
Hybrid offshore renewable energy platforms have been proposed to optimise power production and reduce the levelised cost of energy by integrating or co-locating several renewable technologies. One example is a hybrid wave-wind energy system that combines offshore wind turbines with wave energy converters (WECs) on a single floating foundation. The design of such systems involves multiple parameters and performance measures, making it a complex, multi-modal, and expensive optimisation problem. This paper proposes a novel, robust and effective multi-objective swarm optimisation method (DMOGWA) to provide a design solution that best compromises between maximising WEC power output and minimising the effect on wind turbine nacelle acceleration. The proposed method uses a chaotic adaptive search strategy with a dynamic archive of non-dominated solutions based on diversity to speed up the convergence rate and enhance the Pareto front quality. Furthermore, a modified exploitation technique (Discretisation Strategy) is proposed to handle the large damping and spring coefficient of the Power Takeoff (PTO) search space. To evaluate the efficiency of the proposed method, we compare the DMOGWA with four well-known multi-objective swarm intelligence methods (MOPSO, MALO, MODA, and MOGWA) and four popular evolutionary multi-objective algorithms (NSGA-II, MOEA/D, SPEA-II, and PESA-II) based on four potential deployment sites on the South Coast of Australia. The optimisation results demonstrate the dominance of the DMOGWA compared with the other eight methods in terms of convergence speed and quality of solutions proposed. Furthermore, adjusting the hybrid wave-wind model's parameters (WEC design and PTO parameters) using the proposed method (DMOGWA) leads to a considerably improved power output (average proximate boost of 138.5%) and a notable decline in wind turbine nacelle acceleration (41%) throughout the entire operational spectrum compared with the other methods. This improvement could lead to millions of dollars in additional income per year over the lifespan of hybrid offshore renewable energy platforms.
... The DBPA, a surface located and selfreferenced two-body heaving WEC, was generated by scaling down (based on Froude's similitude) the WEC model used in the Reference Model Project [11] [12], to align with the constraints indicated in Table I. With the same purpose, also the OWSC, a seabed-mounted andreferenced, surface-piercing oscillating flap, was generated by scaling down the B-OF device of the NumWEC project [13] [14]. While SPD and DBPA assumed to integrate a linear PTO, the OWSC is the only device modelled using a rotary PTO. ...
This paper presents the design process and application of a dual hardware hardware-in-the-loop (DHIL) testing platform targeting components and subsystems of wave energy conversion devices. The DHIL testing methodology combines two HIL-equipped test rigs allowing to simultaneously test components mounted on each rig and connected to the same simulation loop. The platform was therefore developed combining a HIL test rig for drivetrains with a HIL test rig for structural components.The DHIL testing platform has the capability to address multiple types of tests on critical subsystems and components within a WEC: characterization tests, for defining key performances of the equipment under test; accelerated tests, to assess qualitative and quantitative reliability features; and ultimate load testing, for survivability purposes. The overall aim of these tests is to identify weaknesses in an early design phase of device design or as a qualification activity prior the deployment of an ocean prototype. Additionally, HIL and DHIL tests can be used to assess the influence of a design update on the overall WEC model and to track failure interdependencies at a relevant scale. Finally, all of the above-mentioned activities can inform the development of a more accurate global numerical model, potentially at sub-system / critical component level, to be validated based on the test results.The definition of the test rig mechanical and electrical input specifications is dependent on the understanding of the load envelope each subsystem / component will be subject to during its lifetime. To define such envelope, a research activity modelling three different device types, three deployment sites and multiple design situations (e.g. power production, parked, shut-down etc.) led to the creation of a load database that was combined with information from WEC developers.After defining the input specifications, the rigs were designed by identifying the optimal layout, the key components and the setup of the overall test area. Analyses on mechanical, electrical and hydraulic parameters were completed to ensure the required performance of each rig could be achieved, while guaranteeing the safety during their operation.The signal processing of each rig was also analysed, with the aim of defining the minimum rig latency to allow HIL and DHIL tests to be performed. This analysis took into account the possible interface characteristics of the simulator, the sensors and the main features of the models used for real-time tests.An overview of the testing activities to be conducted within the IMPACT project will be presented in this paper. Such activities aim to de-risking the technologies at early stages and increasing their maturity through a structured process. The proposed approach has the final goal of demonstrating that the DHIL testing platform is capable to reduce capital-intensive activities, often associated with the development of large-scale prototypes, which is a critical factor for the successful development and time-to-market of a WEC.
... The viscous drag coefficient can be derived either from numerical simulation [16]- [18], on the basis of the Reynolds number and device geometry [19], or from experimental measurements [20]. Here, the drag coefficient in heave is obtained by curve fitting of the decay test response of experimental measurement to the numerical WEC-Sim one by means of the least square method. ...
The survivability of wave energy converters (WECs) is one of the challenges that have a direct influence on their cost. To protect the WEC from the impact of extreme waves, it is often to over-dimension the components or adopt survivability modes e.g. by submerging or lifting the WEC if it is applicable. Here, a control strategy for adjusting the system damping is developed based on deep neural networks (DNN) to minimize the line (mooring) force exerted on a 1:30 scaled WEC. This DNN model is then implemented in a control system of a numerical WEC-Sim model to find the optimal power take-off (PTO) damping for every zero up-crossing episode of surface elevation which minimizes the peak line force. The WEC-Sim model was calibrated based on a 1:30 scaled wave tank experiment that was designed to investigate the WEC response in extreme sea states with a 50-year return period. It is shown that this survival strategy reduces the peak forces when compared with the response of a system that has been set to a constant PTO damping for the entire duration of the sea state.
This paper presents a design methodology for integrating an electrical energy storage unit into a hardware-in-the-loop (HIL) test rig for wave energy converters (WECs). Typically, the power production from WECs is characterised by pronounced fluctuations at low frequency and high peaks compared to the average. Wave energy test rigs should be able to reproduce these variations to impose realistic conditions to the device under test. Thus, the grid connection of the rig must be sized to cope with high peaks, and additional measures may be required to avoid disturbances on nearby loads and negative effects on voltage quality. The integration of electrical energy storage can smoothen power fluctuations and mitigate these drawbacks, while resulting in lower installation and operating costs. The design methodology indicates how to effectively size the storage unit and which technology to favour based on the type and duration of test campaigns. Numerical simulation results are presented for a dual HIL test rig and operational profiles of three different WEC technologies. For designs with energy storage lifetime shorter than the calendar life, sensitivity analyses indicate that the rig's annual utilisation rate and the level of accelerated testing have a significant effect on the storage energy requirements.
To build efficient and portable wave energy conversion devices, energy-dense power takeoff (PTO) systems are required. Contemporary PTOs developed for wave power stations are focused on the year-round averaged sea condition, leading to undesirable performance under low wave frequencies. Facing the challenge, a multicoil electromagnetic wave power takeoff (ME-PTO) with a pneumatic velocity upconversion mechanism is introduced. The new architecture features a pneumatic cylinder connected to a coiled tube to house a reciprocating permanent magnet for electricity generation, and the designed cross-sectional area differential between the magnet and piston allows for boosting the magnet speed, to deliver more capturable power. Dynamic models for the ME-PTO were established, and a numerical solution from the wave–buoy interaction to the electricity output was developed. A compact 23-mm diameter by 190 mm long prototype was built in the lab and an experimental test platform was constructed for model validation. Initial results of operation between 0.2 and 1 Hz have shown that the power density of ME-PTO can reach 72.4 W/m
3
at 0.4 Hz, and is capable of extracting more wave energy compared to a baseline 50-W mechanical PTO when the incident wave is below 0.2 Hz. Under an incident wave with a peak frequency of 0.31 Hz and amplitude below 0.5 m, the cumulative work density of the ME-PTO can be three times higher than the baseline. Even at higher wave frequencies when the three-phase motor is more efficient, the ME-PTO can still exhibit comparable work density. In addition, performance indicators, such as the yearly energy output density of the prototype, have also been compared to contemporary devices, exhibiting advantages in decimeter-level wave energy extraction.
This paper is concerned with advances in research and developments on offshore aquaculture and renewable energy production. We first discuss the motivation and challenges for moving offshore in these two blue industries. This is followed by a summary of recent advances and research needs in offshore fish farming, seaweed cultivation, and harvesting energy from offshore wind, solar, wave and tidal current.
A linearised frequency domain numerical model of small seabed-mounted bottom-hinged wave energy converters is developed that accounts for vortex shedding at body edges and decoupling at large angles of rotation. The numerical model is verified and calibrated using data from wave-tank experiments. It is found that in general the device capture factor increases with both the device width and wave frequency due to increasing wave force. The model also indicates that for typical flap dimensions and incident wave amplitudes the peak in capture factor at the body's natural pitching frequency is suppressed due to viscous losses and motion constraints. The effect of viscous losses and motion constraints are also responsible for limiting the increase in performance that is obtainable with phase control. Three cost functions, power per unit displaced volume, power per unit structural task and power per unit surge force are produced and applied to the results of a parametric analysis. Three distinct regions of the design space are identified; EB Frond and BioWave are found to sit in one region, WaveRoller in another region and Oyster in the final region. Characteristics are identified for each region and related to the distinct designs of the commercial systems identified.
This short paper, structured in 3 distinct sections will touch on some of the key features of the Oyster wave energy device and its recent development. The first section discusses the nature of the resource in the nearshore environment, some common misunderstandings in relation to it and its suitability for exploitation of commercial wave energy. In the second section a brief description of some of the fundamentals governing flap type devices is given. This serves to emphasise core differences between the Oyster device and other devices. Despite the simplicity of the design and the operation of the device itself, it is shown that Oyster occupies a theoretical space which is substantially outside most established theories and axioms in wave energy. The third section will give a short summary of the recent developments in the design of the Oyster 2 project and touch on how its enhanced features deal with some of the key commercial and technical challenges present in the sector
The theoretical potential for maximising energy output of wave-energy converters by means of optimum control is quantified for a heaving semi-submerged sphere on deep water. The heave amplitude is constrained to not exceed 0.6 units of the radius. Sinusoidal incident waves of amplitudes up to 3 m and period in the range of 6 s to 12 s are considered, when the sphere radius is 5 m. Optimum reactive control, contrary to sub-optimal latching phase control, requires ability for reversing the energy flow through the power take-off machinery. Computed results show that, for a typical wave of 0.5 m amplitude and 9 s period, the maximum absorbed power is 24, 137, and 172 kW for the cases of no phase control (passive system), sub-optimal latching control, and ideally optimal reactive control, respectively. The ratio between the maximum/minimum instantaneous power and the average absorbed power is 2/0, 4.1/0 and 11.0/-9.0, respectively, for the three different strategies.
The smoothing effect on power production in an array of SEAREV wave energy converters due to the summation of each single converter?s production was studied. The strong standard deviation of the instantaneous power in regard with the mean power for a single device can be reduced by a large factor when considering the power production of an array. The distance between wave energy converters appeared to have no large influence on the smoothing effect, neither the characteristics of the sea state. This smoothing effect in arrays of wave energy converter should allow to decrease the need for energy storage, enhancing the economical reliability of the wave energy conversion. This is an abstract of a paper presented at the Eighteenth International Offshore and Polar Engineering Conference (Vancouver, Canada 7/6-11/2008).
This paper discusses some of the key design drivers for the next generation of the Oyster wave energy converter which is being developed by Aquamarine Power Ltd. The paper presents a general overview of the Oyster technology including the nearshore wave energy resource, the power capture characteristics of bottom-hinged flap type oscillators and the hydroelectric power take-off system. A status update is then provided for the full-scale proof of concept device which was successfully installed at the European Marine Energy Centre (EMEC) in Orkney, Scotland in 2009. The final section, and main body of the paper, concerns the next generation of Oyster device, Oyster 2, which is currently being developed for deployment in 2011. The paper provides an introduction to the fundamental tenets which have guided the design as well as an overview of the ensuing features and the resultant step-change improvement in the performance of the device.
Several numerical integration methods are compared in order to search out the most effective method for the Kramers-Kronig transformation, using the analytical formula of the Kramers-Kronig transformation of a Lorentzian function as a reference. The methods to be compared involve the use of (1) Maclaurin's formula, (2) trapezium formula, (3) Simpson's formula, and (4) successive double Fourier transform methods. It is found that Maclaurin's formula, in which no special approximation is necessary for the pole part of the integration, gives the most accurate results, and also that its computation time is short. Successive Fourier transform is less accurate than the other methods, but it takes the least time when used without zero-filling. These results have important relevance for programs used to obtain optical constant spectra and to analyze spectral data.
A commonly used method of simulating ocean waves from a specified frequency spectrum is shown to be incorrect. The method consists of adding numerous sine curves with random phases; and the error arises from assuming that the amplitudes of these component sine waves are deterministic, when they are in fact random variables. Methods of using random amplitudes are described and only one is found to be satisfactory. In this method the number of random values simulated — and then transformed with an inverse FFT — equals the required number of simulated data points. So simulation in the frequency domain can only give relatively short runs; it is necessary to work in the time domain if arbitrarily long runs are required.Errors in wave group statistics derived from the incorrect simulation method are discussed and related to discrepancies reported between groupiness in simulated data and ocean measurements.
This paper describes a numerical wave-to-wire model of the second-generation wave energy converter called SEAREV. Governing equations are given in the time domain for the motion of the masses involved in the device and for the hydraulic power take-off (PTO) used to convert the motion into electricity. The hydrodynamic forces are derived using the standard linear potential theory. The memory term in the radiation force is replaced by additional states using the Prony method in order to change the equation of motion into the ordinary differential equation form. The PTO is composed of hydraulic rams, an accumulator, and a hydraulic generator, which delivers electricity when there is enough energy stored in the accumulator.
Using the MATLAB Simulink tool, the equation of motion is solved to simulate the full device (including the power take-off) from the incident wave to the electricity delivered to the grid. Simulation results are presented in the paper and comparisons are made with a simpler PTO: a linear damper. They show that the torque applied to the hydraulic PTO must exceed a threshold to start absorbing energy, unlike the linear damping model. They also show that the power production can be very discontinuous, depending on the level of the incident wave power. This is due to the fact that the generator considered can transform the energy stored in the accumulator faster than the energy transmitted by the rams into the accumulator. It could therefore be interesting to use several generators to adapt the electrical energy production to the level of incident wave power, or a generator that could work efficiently at part load in order to achieve continuous energy production.