Monthly Archives: June 2013

Numerical Simulation of a Straight-Bladed Vertical-Axis Water Turbine Operating in a 2 m/s Current


Marco Raciti Castelli and Ernesto Benini – Applied Mechanics and Materials, June 2013

Abstract

The present work proposes a full campaign of simulation of a Darrieus-type Vertical-Axis Water Turbine (VAWaterT) operating in an open flow-field. After describing the computational model and the relative validation procedure, a complete campaign of simulations based on full RANS unsteady calculations is presented for a three-bladed rotor architecture, characterized by a NACA 0025 blade profile. Flow field characteristics are investigated for several values of tip speed ratio and for a constant unperturbed free-stream water velocity of 2 m/s. Finally, the torque coefficient generated from the three blades is determined for each simulated angular velocity, allowing the calculation of the rotor power-curve. Keywords: Vertical-Axis Water Turbine, hydrokinetic technology, CFD, NACA 0025.

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Assessment of wave energy variation in the Persian Gulf


B. Kamranzad, A. Etemad-shahidi, and V. Chegini – Ocean Engineering, September 2013

Abstract

Since wave energy has the highest marine energy density in the coastal areas, assessment of its potential is of great importance. Furthermore, long term variation of wave power must be studied to ensure the availability of stable wave energy. In this paper, wave energy potential is assessed along the southern coasts of Iran, the Persian Gulf. For this purpose, SWAN numerical model and ECMWF wind fields were used to produce the time series of wave characteristics over 25 years from 1984 till 2008. Moreover, three points in the western, central and eastern parts of the Persian Gulf were selected and the time series of energy extracted from the modeled waves were evaluated at these points. The results show that there are both seasonal and decadal variations in the wave energy trends in all considered points due to the climate variability. There was a reduction in wave power values from 1990 to 2000 in comparison with the previous and following years. Comparison of wind speed and corresponding wave power variations indicates that a small variation in the wind speed can cause a large variation in the wave power. The seasonal oscillations lead to variation of the wave power from the lowest value in summer to the highest value in winter in all considered stations. In addition, the seasonal trend of wave power changed during the decadal variation of wave power. Directional variations of wave power were also assessed during the decadal variations and the results showed that the dominant direction of wave propagation changed in the period of 1990 to 2000 especially in the western station.

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Filed under Modeling, Resource Assessment

Modeling of a Point Absorber for Energy Conversion in Italian Seas


Silvia Bozzi, Adrià Moreno Miquel, Alessandro Antonini, Giuseppe Passoni, and Renata Archetti – Energies, June 2013

Abstract

In the present paper, we investigate the feasibility of wave electricity production in Italian seas by the deployment of the Seabased wave energy converter (WEC). A numerical model of the coupled buoy-generator system is presented, which simulates the behavior of the wave energy converter under regular waves of different wave heights and periods. The hydrodynamic forces, including excitation force, radiation impedance and hydrostatic force, are calculated by linear potential wave theory, and an analytical model is used for the linear generator. Two buoys of different radii are considered to explore the effect of buoy dimension on energy conversion and device efficiency. The power output is maximized by adding a submerged object to the floating buoy, in order to bring the system into resonance with the typical wave frequencies of the sites. The simulation results show a very good agreement with the published data on the Seabased WEC. The model is used to estimate energy production at eight Italian offshore locations. The results indicate that the degree of utilization of the device is higher than 20% at the two most energetic Italian sites (Alghero and Mazara del Vallo) and that it can be considerably increased if the floating body is connected to a submerged object, thanks to the resonant behavior of the WEC. In this case, the degree of utilization of the device would be higher than 40% at most of the study sites, with the highest value at Mazara del Vallo. The work enlarges the perspective, to be confirmed by experimental tests and more accurate numerical modeling, on clean electric power production from ocean waves in the Italian seas.

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A wave energy resource assessment case study: Review, analysis and lessons learnt


Helen C.M. Smith, David Haverson, and George H. Smith – Renewable Energy, December 2013

Abstract

A case study of the development of an overall resource assessment for the Wave Hub site in the southwest UK is presented. Wave Hub is one of the earliest large-scale wave farms planned. Several resource assessment studies have been performed for the site, but the published results are high-level and predicted power availability varies significantly. This paper provides a detailed overview and re-analysis of the multiple datasets used in the original studies, which consisted of a combination of physical measurement and numerical modelling. The quality of the datasets is assessed, and reasons for the discrepancies between predicted resource levels investigated. Results from a SWAN model for the region illustrate significant levels of spatial variability in the resource due to the complexity of the local bathymetry, and examination of long-term global model datasets shows notable inter-annual variability. It is thus concluded that a resource assessment methodology utilising datasets from multiple locations and of short duration significantly reduces the accuracy of the predicted levels of resource. From these results, key learnings for future developments are discussed.

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Influence of control strategy on the global efficiency of a direct wave energy converter with electric power take-off


Thibaut Kovaltchouk, Bernard Multon, Hamid Ben Ahmed, François Rongère, Judicaël Aubry, and Alain Glumineau – 2013 8th International Conference and Exhibition on Ecological Vehicles and Renewable Energies (EVER), June 2013

Abstract

The choice of control strategy for Direct Wave Energy Converters (DWEC) is often discussed without taking into account the limitations of electric Power Take-Off (PTO): limits of torque or force and power, as well as losses in the electric chain. These assumptions leads to large and expensive electric systems, that prevent leading to a global minimization of the per-kWh cost. We propose herein a simple loss model in order to design a better control strategy.

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An integrated methodology for the optimal thermal design of an ocean turbine pressure vessel: A soft-computing approach


Nikolaos I. Xiros and Khaled Kaise – Proceedings of IMarEST – Part A – Journal of Marine Engineering and Technology, April 2013

Abstract

This paper presents a generally applicable approach to numerical thermal design. A novel thermal design procedure for the prediction of heat transfer inside a pressure vessel of an ocean current turbine using integrated procedure by finite element method of heat transfer analysis, artificial neural network and genetic algorithms is presented. Numerical heat transfer analysis was done using commercial software ANSYS for two-dimensional heat transfer in simplified domains. The calculation was limited to only heat conduction. The ANSYS simulations results were used for training and approximating the unknown functional behaviour of heat transfer by using artificial neural networks (ANN). The trained ANN serves as the nonlinear objective function of the optimisation procedure. Genetic algorithms (GA) were used as the optimisation tool. The optimum results obtained from the GA were verified against ANSYS and ANN results. Both the ANN and GA were implemented in MATLAB, while the overall methodology proposed could be applied to other engineering design problems as well.

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Grid Connection of Wave Power Farm Using an N-Level Cascaded H-Bridge Multilevel Inverter


Rickard Ekström and Mats Leijon – Journal of Electrical and Computer Engineering, May 2013

Abstract

An N-level cascaded H-bridge multilevel inverter is proposed for grid connection of large wave power farms. The point-absorber wave energy converters are individually rectified and used as isolated DC-sources. The variable power characteristics of the wave energy converters are discussed, and a method of mitigating this issue is demonstrated. The complete power control system is given in detail and has been experimentally verified for a single-phase setup of the 9-level inverter. Theoretical expressions of the power sharing between multilevel cells are derived and show good correspondence with the experimental results.

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Power measurement of hydrokinetic turbines with free-surface and blockage effect


Amir Hossein Birjandi, Eric Louis Bibeau, Vijay Chatoorgoon, and Anurag Kumar – Ocean Engineering, September 2013

Abstract

Vertical hydrokinetic turbines in an array that extends from one side of a channel or river to the other side of it experience a fixed blockage effect as a result of the adjacent turbines and a variable free-surface effect due to water level changes above turbines. For tidal applications, the water level above turbine blades changes continuously throughout the day; for river applications, the water level changes on a seasonal basis. In this study, a vertical turbine operating in an array of turbines with one diameter lateral distance between two adjacent turbines is modeled. The model turbine is tested in a water tunnel at various water levels. Results show that the water level reduction improves the power coefficient of the turbine when the turbine is fully submerged—the power coefficient increases due to the free-surface effect, with trends in agreement with the one-dimensional actuator-disc flow theory. However, the power coefficient decreases significantly when the turbine is only partially submerged. In this particular condition, the entrained air into the water by turbine blades separates the water from the blade surface. A high-speed camera visualizes the flow separation while a transducer measures the instantaneous torque of the turbine.

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A New Way for Access and Maintenance of Offshore Wind Farms: The Use of Cableway to Reduce Cost and Improve Accessibility


Massimo Grecchi, Luigi Meroni, and Piergiorgio Betteto – Wind Engineering, June 2013

Abstract

This work proposes a new method in order to deploy an affordable and reliable way for maintenance when it is necessary to access to offshore wind farms. The new system, based on cableway infrastructure is aimed to allow a lower cost of maintenance compared to other system for the same kind of operation but also to improve the reactivity and accessibility compared to them. Maintenance activities are taken into consideration in term of cost, period, nature, accessibility of wind farm, kind of maintenance we want/need to use in order to ameliorate the availability/reliability of the wind farm. Five different configurations with related cableway system are analyzed in order to demonstrate that facing a reasonable increasing of Capital Cost, sizable saving and extra revenue can be obtained if projected on the expected life of wind farms.

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Performance of Combined Water Turbine with Semielliptic Section of the Savonius Rotor


Kaprawi Sahim, Dyos Santoso, and Agus Radentan – International Journal of Rotating Machinery, May 2013

Abstract

The Darrieus turbine is a suitable power generation in free stream flow because it is simple in construction, but it has the disadvantage of its small starting torque. The Savonius turbine has a high starting torque but the efficiency is smaller than that of Darrieus turbine. To improve the starting torque of Darrieus turbine, the Savonius buckets are introduced into the Darrieus turbine and the combined turbine is called Darrieus-Savonius turbine. In this study, three semielliptic sections of aspect ratio 0.8 were used for Savonius bucket while the Darrieus blade used three wings of airfoil NACA 0015. The Darrieus-Savonius turbine’s performances were studied experimentally in an irrigation canal of South Sumatera, Indonesia. The results show that the distance of Savonius buckets from the shaft centre influences performance of combined turbine, and the attachment angle of Savonius rotor made important variation of turbine performance.

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