Effects of Rotor Solidity on the Performance of Impulse Turbine for OWC Wave Energy Converter

Impulse turbine, working as a typical self-rectifying turbine, is recently utilized for the oscillating water column（OWC） wave energy converters, which can rotate in the same direction under the bi-directional air flows. A numerical model established in Fluent is validated by the corresponding experimental results. The flow fields, pressure distribution and dimensionless evaluating coefficients can be calculated and analyzed. Effects of the rotor solidity varying with the change of blade number are investigated and the suitable solidity value is recommended for different flow coefficients.

A RANS-VoF Numerical Model to Analyze the Output Power of An OWC-WEC Equipped with Wells and Impulse Turbines in A Hypothetical Sea-State

Wave energy is a renewable source with significant amount in relation to the global demand. A good concept of a device applied to extract this type of energy is the onshore oscillating water column wave energy converter(OWC-WEC). This study shows a numerical analysis of the diameter determination of two types of turbines, Wells and Impulse, installed in an onshore OWC device subjected to a hypothetical sea state. Commercial software FLUENT?,which is based on RANS-VoF(Reynolds-Averaged Navier-Stokes equations and Volume of Fluid technique), is employed. A methodology that imposes air pressure on the chamber, considering the air compressibility effect, is used. The mathematical domain consists of a 10 m deep flume with a 10 m long and 10 m wide OWC chamber at its end(geometry is similar to that of the Pico's plant installed in Azores islands, Portugal). On the top of the chamber, a turbine works with air exhalation and inhalation induced by the water free surface which oscillates due to the incident wave. The hypothetical sea state, represented by a group of regular waves with periods from 6 to 12 s and heights from 1.00 to 2.00 m(each wave with an occurrence frequency), is considered to show the potential of the presented methodology. Maximum efficiency(relation between the average output and incident wave powers) of46% was obtained by using a Wells turbine with the diameter of 2.25 m, whereas the efficiency was 44% by an Impulse turbine with the diameter of 1.70 m.

Experimental Investigation on a Fixed Oscillating Water Column with an Impulse Turbine for Wave Energy Conversion

A fixed oscillating water column(OWC)-type wave energy converter consists of an air chamber,an air turbine and a generator.The energy conversion processes are the primary conversion in an air chamber and the secondary conversion of the turbine.For the practical use,it is necessary to develop a design method which can consider the incident wave motion,the motion of the internal free surface affected in the structure such as a partly submerged wall,the fluctuation of air pressure in an air chamber,and the rotation of the air turbine.At here,the authors carried out the wave tank tests using the model OWC equipped with the impulse turbine and a generator to obtain the experimental data needed to make this design method.As the result,the efficiencies of the three cases with different speed ratio between generator and turbine,and the effects of the curtain wall depth and the wave length on the energy conversion performance were clarified.

Effect of Fluidic Diode on Performance of Unidirectional Impulse Turbine

A pair of unidirectional turbines(UT)can operate in oscillatory airflow without additional units.However,this arrangement suffers from poor flow rectification.A fluidic diode(FD)offers variable hydrodynamic resistance based on the flow direction,and this can be coupled with UT to improve flow rectification.In this work,a numerical investigation on the effect of FD with UT is presented using the commercial fluid dynamics software ANSYS Fluent 16.1 with k-ωSST turbulence closure model.Periodic domains of UT and FD are numerically validated individually with experimental results.Later,both are coupled to obtain the combined effect,and these results are compared with the analytical approach.It was observed that coupling FD with UT improved the unit's performance at the lower flow coefficient(<1),but its performance decreased as the flow coefficient increased.Due to the diode's presence,fluid leaving the turbine experiences higher resistance at a higher flow coefficient,which decreases the overall performance of the combined unit.

A Twin Unidirectional Impulse Turbine for Wave Energy Conversion

A twin unidirectional impulse turbine has been proposed in order to enhance the performance of wave energy plant. This turbine system uses two unidirectional impulse turbines and their flow direction is different from each other. However, the turbine characteristics have not been clarified to date. The performances of a unidirectional impulse turbine under steady flow conditions were investigated experimentaUy by using a wind tunnel with large piston/cylinder in this study. Then, efficiency of the twin impulse turbine have been estimated by a quasi-steady analysis using experimental results.

Pneumatic Performance of Staggered Impulse Turbine for OWC Wave Energy Convertor

A staggered impulse turbine is proposed for asymmetric air flows in Oscillating Water Column wave energy plants, which is expected to enhance the pneumatic power output in a wave cycle. The setting angle of rotor blades is set as 5°. The 3D numerical simulations were conducted under steady conditions using MRF and Mixing Plane model based on CFD software Fluent 12.0. Its mean efficiencies under different velocity amplitude ratios are studied using quasi-steady analysis, which derive corresponding data from the numerical simulation. It is found that the staggered turbine shows better performance than the conventional one under the asymmetrical air flows. Furthermore, its mean efficiency and output-work in a wave period are compared with another unsymmetrical twin impulse turbine system. The results show that the staggered turbine shows better output-work performance than the twin turbine system over the high flow coefficient domain(φ>0.7), which provides more choices to future research on turbine's optimization.

Impulse Turbine with 3D Guide Vanes for Wave Energy Conversion

In this study, in order to achieve further improvement of the performance of an impulse turbine with fixed guide vanes for wave energy conversion, the effect of guide vane shape on the performance was investigated by experiment. The investigation was performed by model testing under steady flow condition. As a result, it was found that the efficiency of the turbine with 3D guide vanes are slightly superior to that of the turbine with 2D guide vanes because of the increase of torque by means of 3D guide vane, though pressure drop across the turbine for the 3D case is slightly higher than that for the 2D case.

Application of Numerical Simulation Method to Predict the Performance of Wave Energy Device with Impulse Turbine

This paper presents the work carried out to predict the behavior of a 0.6m impulse turbine with fixed guide vanes with 0.6hub-to-tip(H/T) ratio under real sea conditions.In order to predict the true performance of the actual Oscillating Water Column(OWC),the numerical technique has been fine tuned by incorporating the compressibility effect.Water surface elevation verses time history based on Pierson Moskowitz Spectra was used as the input data,Standard numerical techniques were employed to solve the non-linear behavior of the sea waves.The effect due to ompressibility inside the air chamber and turbine performance under unsteady and irregular flow condition has been analyzed numerically,Considering the quasi-steady assumptions unidirectional steady flow experimental data was used to simulate the turbine characteristics under irregular unsteady flow conditions.The results show that the performance of this type of turbine is quite stable and efficiency of air chamber and the mean conversion efficiency is reduced around 8% and 5% respectively,due to compressibility inside air chamber.

Effect of partial blockage of air duct outlet on performance of OWC device

The use of a Savonius rotor as turbine for an oscillating water column(OWC) is demonstrated.The effect of tuning the OWC using turbine duct blockage is also studied for different wave conditions.A horizontal turbine section OWC employing a Savonius rotor was tested by varying the opening of OWC exit(0%,25%,50%,75% and 100%) to study the behavior and performance of the device.The OWC model was tested at water depth of 0.29 m at frequencies of 0.8,0.9 and 1.0 Hz while the exit openings are varied.The static pressure,dynamic pressure,rotational speed of the Savonius rotor and the coefficient of power are presented as results.The OWC with exit opening of 25% showed greater performance in terms of rotational speed and CP compared to OWC with other exit opening percentages.This proves the ability of the OWC to be tuned by regulating flow in the turbine duct.

Parametric optimisation of two Pelton turbine runner designs using CFD

This paper aims to develop a generic optimisation method for Pelton turbine runners using computational fluid dynamics （CFD）. Two different initial runners are optimised to achieve more generic results. A simple bucket geometry based on existing bibliography is parameterised and initially optimised using fast Lagrangian solver （FLS）. It is then further optimised with a more accurate method using ANSYS Fluent. The second geometry is a current commercial geometry with good initial performance and is optimised using ANSYS CFX. The analytical results provided by CFX and Fluent simulations are used to analyse the characteristics of the flow for different runner geometries.

Pelton turbine: Identifying the optimum number of buckets using CFD

A numerical case study on identifying the optimum number of buckets for a Pelton turbine is presented. Three parameters： number of buckets, bucket radial position and bucket angular position are grouped since they are found to be interrelated. By identifying the best combination of the radial and angular position for each number of buckets it is shown that reduction in the number of buckets beyond the limit suggested by the available literature can improve the efficiency and be beneficial with regard to the manufacturing complexity and cost perspective. The effect of this reduction in the number of buckets was confirmed experimentally.

Experimental and numerical investigation of design optimization of a partial admitted supersonic turbine

To obtain a high specific work output,the large pressure ratios across the turbine are required.This can be achieved using a supersonic turbine.When the fluid mass flow is low,the impulse kind of one or two stages supersonic turbine is employed.To prevent losses due to low blade aspect ratio and issues related to manufacturing and industrial problems,the turbine is used in partial admission conditions.Studies show that the turbine efficiency is highly dependent on the amount of partial admission coefficient.The turbine efficiency in full admission is high,but the use of partial admission lowers the additional losses.Therefore,there will be a degree of partial admission in which the turbine will have the highest efficiency.The aim of this work is to achieve the optimum partial admission for a special impulse turbine as a case study.Therefore,in the beginning,an appropriate model of losses is presented.Then,using a nonlinear design optimization code,the partial admission of an impulse supersonic turbine is optimized.This code is written using a genetic algorithm.Then,using three-dimensional numerical analysis,the optimal model will be selected.In the optimization problem,the turbine efficiency is the objective function.The amount of design parameters and constraints used in this process are ten and eight,respectively.After the optimization process,prototypes of designed and modified turbines are made and tested.Test results were compared and analyzed.The results showed that the turbine efficiency is improved between 2.5%and 3%depending on various operation conditions.

Comparative Study of Turbines for Wave Energy Conversion

The objective of this paper is to compare the performances of the themes, which could be used for wave energy conversion in the near future, under various irregular wave conditions. The turbines included in the paper are as follows: (a) Wells turbine with guide vanes; (b) impulse turbine with self-pitch-controlled guide vanes; (c) impulse turbine with fixed guide vanes. In this study, experimental investigations were carried out to clarify the performances of the turbines under steady flow conditions, and then a numerical simulation was used for predicting the performances under irregular wave conditions with various significant wave heights. As a result it was found that the running and starting characteristics of the impulse turbines could be superior to those of the Wells turbine.

Performance estimation of bi-directional turbines in wave energy plants

Oscillating water column （OWC） based wave energy plants have been designed with several types of bidirectional turbines for converting pneumatic power to shaft power. Impulse turbines with linked guide vanes and fixed guide vanes have been tested at the Indian Wave Energy plant. This was after initial experimentation with Well＇s turbines. In contrast to the Well＇s turbine which has a linear damping characteristic, impulse turbines have non-linear damping. This has an important effect in the overall energy conversion from wave to wire. Optimizing the wave energy plant requires a turbine with linear damping and good efficiency over a broad range of flow coefficient. This work describes how such a design can be made using fixed guide vane impulse turbines. The Indian Wave Energy plant is used as a case study.

Acoustic characteristics of bi-directional turbines for thermoacoustic generators

Bi-directional turbines combined with rotary motors may be a feasible option for developing high power thermoacoustic generators with low cost.A general expression for the acoustic characteristics of the bidirectional turbine was proposed based on theoretical derivation,which was validated by computational fluid dynamics modeling of an impulse turbine with fixed guide vanes.The structure of the turbine was optimized primarily using steady flow with an efficiency of near 70%(the shaft power divided by the total energy consumed by the turbine).The turbine in the oscillating flow was treated in a lumped-parameter model to extract the acoustic impedance characteristics from the simulation results.The key acoustic impedance characteristic of the turbine was the resistance and inertance due to complex flow condition in the turbine,whereas the capacitance was treated as an adiabatic case because of the large-scale flow channel relative to the heat penetration depth.Correlations for the impedance were obtained from both theoretical predictions and numerical fittings.The good fit of the correlations shows that these characteristics are valid for describing the bi-directional turbine,providing the basis for optimization of the coupling between the thermoacoustic engine and the turbine using quasi-one-dimensional theory in the frequency domain.