Monthly Archives: December 2012

Modeling and simulation of oscillating water column wave energy generator


Q. Xu, Y. Ju, and H. Ge – Advanced Materials Research, December, 2012

Abstract

This paper focuses on the oscillating water column (OWC) wave energy generator. An overall mathematical model is established comprising of the wave energy capture, drive system, permanent magnet synchronous generators (PMSG), vector control, maximum power point tracking (MPPT), and low voltage ride through (LVRT) control. With this mathematical model, an OWC wave energy generator based on PMSG simulation model is set up in Matlab/Simulink environment. A simulation analysis of the model is carried out which is connected to the grid under the condition of wave changes and power system faults. The simulation facilitates the MPPT and the decoupling control of power for OWC wave energy generator. Results show that the system with back-to- back PWM converter operates in a satisfying way and the model established works correctly and effectively.

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A new time domain analysis of wave power


M.X. Wei and Z.G. Bai – Applied Mechanics and Materials, December, 2012

Abstract

The present frequency domain method of calculating wave power may not be accurate enough for calculating the incident wave power of a specific site, which is primary measurement for evaluating the efficiency of wave energy converters (WECs) and an alternative measure, the time domain method, is proposed. Three sites including two nearshore sites and one deepwater site at Chengshantou sea area were selected, and a sample wave parameters data set was obtained from wave model SWASH to demonstrate the application of these two methods. A comparison of the results of each method was also performed and two influential parameters used in calculation were analyzed. The results show that frequency domain method is very likely to overestimate the wave power at both deepwater and nearshore site. The time domain method proposed in this paper is believed to be more superior in calculating the incident wave power during a short term.

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Polycrystalline diamond bearing testing for marine hydrokinetic application


B.A. Lingwall, T.N. Sexton, C.H. Cooley – Technical Report submitted to DOE EERE Wind & Water Power Program, December, 2012

Abstract

Polycrstalline diamond (PCD) bearings were designed, fabricated and tested for marine-hydro-kinetic (MHK) application. Bearing efficiency and life were evaluated using the US Synthetic bearing test facility. Three iterations of design, build and test were conducted to arrive at the best bearing design. In addition life testing that simulated the starting and stopping and the loading of real MHK applications were performed. Results showed polycrystalline diamond bearings are well suited for MHK applications and that diamond bearing technology is TRL4 ready. Based on life tests results bearing life is estimated to be at least 11.5 years. A calculation method for evaluating the performance of diamond bearings of round geometry was also investigated and developed. Finally, as part of this effort test bearings were supplied free of charge to the University of Alaska for further evaluation. The University of Alaska test program will subject the diamond bearings to sediment laden lubricating fluid.

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Optimal active control of a wave energy converter


E. Abraham and E. Kerrigan – 51st Annual Conference on Decision and Control (CDC), December, 2012

Abstract

This paper investigates optimal active control schemes applied to a point absorber wave energy converter within a receding horizon fashion. A variational formulation of the power maximization problem is adapted to solve the optimal control problem. The optimal control method is shown to be of a bang-bang type for a power take-off mechanism that incorporates both linear dampers and active control elements. We also consider a direct transcription of the optimal control problem as a general nonlinear program. A variation of the projected gradient optimization scheme is formulated and shown to be feasible and computationally inexpensive compared to a standard NLP solver. Since the system model is bilinear and the cost function is non-convex quadratic, the resulting optimization problem is not a convex quadratic program. Results will be compared with an optimal command latching method to demonstrate the improvement in absorbed power. Time domain simulations are generated under irregular sea conditions.

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Tidal effect compensation system for point absorbing wave energy converters


V. Castellucci, R. Water, M. Eriksson, M. Leijon – Renewable Energy, March, 2013

Abstract

Recent studies show that there is a correlation between water level and energy absorption values for the studied wave energy converters: the absorption decreases when the water levels deviate from average. The situation appears during tides when the water level changes significantly. The main objective of the paper is to present a first attempt to increase the energy absorption during tides by designing and realizing a small-scale model of a point absorber equipped with a device that is able to adjust the length of the rope connected to the generator. The adjustment is achieved by a screw that moves upwards in the presence of low tides and downwards in the presence of high tides. Numerical results as well as experimental tests suggest that the solution adopted to minimize the tidal effect on the power generation shows potential for further development.

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Experimental studies on a closed cycle demonstration OTEC plant working on small temperature difference


M. Faizal and M.R. Ahmed – Renewable Energy, March, 2013

Abstract

Ocean thermal energy conversion (OTEC) technology utilizes the temperature difference between the warm surface water and deep coldwater of the ocean to operate a heat engine to generate electricity. An experimental study was carried out on a newly designed closed cycle demonstration OTEC plant with the help of temperature and pressure measurements before and after each component. An increase in the warm water temperature increases the heat transfer between the warm water and the working fluid, thus increasing the working fluid temperature, pressure, and enthalpy before the turbine. The performance is better at larger flowrates of the working fluid and the warm water. It is found that the thermal efficiency and the power output of the system both increase with increasing operating temperature difference (difference between warm and cold water inlet temperature). Increasing turbine inlet temperatures also increase the efficiency and the work done by the turbine. The efficiency and the power output increase with increasing ratio of warm water to coldwater flowrates. A maximum efficiency of about 1.5% was achieved in the system. The findings from this work can contribute to the development of OTEC technologies.

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Keeping ship hulls free of marine organisms


Manfred Füting – Fraunhofer-Gesellschaft, December, 2012

If a ship is at anchor for longer periods algae, shells and barnacles will colonize it. Every year, this so-called biofouling causes economic losses of billions of Dollar. Biological growth on the underwater surface promotes corrosion. The deposits increase the roughness of the hull below the waterline which has a braking effect as the ship travels. Depending on the roughness of the basified bio layer, the consumption of fuel can increase by up to 40 percent. In the case of a large container ship this can result in additional annual costs of several millions.

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Optimal design of a tidal turbine


J.L. Kueny, T. Lalande, J.J. Herou and L. Terme – IOP Conference Series: Earth and Environmental Science, 2012

Abstract

An optimal design procedure has been applied to improve the design of an open-center tidal turbine. A specific software developed in C++ enables to generate the geometry adapted to the specific constraints imposed to this machine. Automatic scripts based on the AUTOGRID, IGG, FINE/TURBO and CFView software of the NUMECA CFD suite are used to evaluate all the candidate geometries. This package is coupled with the optimization software EASY, which is based on an evolutionary strategy completed by an artificial neural network. A new technique is proposed to guarantee the robustness of the mesh in the whole range of the design parameters. An important improvement of the initial geometry has been obtained. To limit the whole CPU time necessary for this optimization process, the geometry of the tidal turbine has been considered as axisymmetric, with a uniform upstream velocity. A more complete model (12 M nodes) has been built in order to analyze the effects related to the sea bed boundary layer, the proximity of the sea surface, the presence of an important triangular basement supporting the turbine and a possible incidence of the upstream velocity.

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Device for passive flow control around vertical axis marine turbine


C.I. Coşoiu, A.M. Georgescu, M. Degeratu, L. Haşegan and D. Hlevca – IOP Conference Series: Earth and Environmental Science, 2012

Abstract

The power supplied by a turbine with the rotor placed in a free stream flow may be increased by augmenting the velocity in the rotor area. The energy of the free flow is dispersed and it may be concentrated by placing a profiled structure around the bare turbine in order to concentrate more energy in the rotor zone. At the Aerodynamic and Wind Engineering Laboratory (LAIV) of the Technical University of Civil Engineering of Bucharest (UTCB) it was developed a concentrating housing to be used for hydro or aeolian horizontal axis wind turbines, in order to increase the available energy in the active section of turbine rotor. The shape of the concentrating housing results by superposing several aero/hydro dynamic effects, the most important being the one generated by the passive flow control devices that were included in the housing structure. Those concentrating housings may be also adapted for hydro or aeolian turbines with vertical axis. The present paper details the numerical research effectuated at the LAIV to determine the performances of a vertical axis marine turbine equipped with such a concentrating device, in order to increase the energy quantity extracted from the main flow. The turbine is a Darrieus type one with three vertical straight blades, symmetric with respect to the axis of rotation, generated using a NACA4518 airfoil. The global performances of the turbine equipped with the concentrating housing were compared to the same characteristics of the bare turbine. In order to validate the numerical approach used in this paper, test cases from the literature resulting from experimental and numerical simulations for similar situations, were used.

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A study on the design assessment of 50kW ocean current turbine using fluid structure interaction analysis


B.S. Kim, S.Y. Bae, W.J. Kim, S.L. Lee and M.K. Kim – IOP Conference Series: Earth and Environmental Science, 2012

Abstract

This research represents the results of performance prediction and structural safety evaluation of 50 kW ocean current turbine rotor assembly. Unsteady CFD simulation on the rotor assembly was performed to predict the performance of rotor assembly, and a cavitation model was applied with consideration of underwater operating conditions. Flow analysis result showed that the average power output was 47 kW at an extreme flow velocity of 6 m/s and cavitation phenomena which is repeatedly occurred around the rotor assembly was also observed. The structural safety of the rotor assembly was evaluated by unidirectional FSI analysis. The analysis results showed that the minimum safety factor of the rotor assembly was 3.8. From the result, it was concluded that the rotor assembly had sufficient structural safety at an extreme operating condition.

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