Manabu Takao

A review of impulse turbines for wave energy conversion

Oscillating Water Column based wave energy plants convert wave energy into low pressure pnuematic power in the form of bi-directional air flows. Air turbines which are capable of rotating uni-directionally in bi-directional air flow, otherwise also known as self-rectifying turbines, are used to extract mechanical shaft power which is further converted into electrical power by a generator. This paper reviews the state of the art in self-rectifying impulse air turbines. New results on optimum parameters for the fixed-guide-vane impulse turbine are also presented. Starting characteristics and conversion efficiencies of two types of impulse turbines are compared with the well known Wells turbine.

A modified Wells turbine for wave energy conversion

The method of wave energy conversion utilises an oscillating water column (OWC). The OWC converts wave energy into low-pressure pneumatic energy in the form of bi-directional airflow. Wells turbine with its zero blade pitch setting has been used to convert this pneumatic power into uni-directional mechanical shaft power. Measurements in OWC based wave energy plants in India and Japan show that the airflow velocity is not equal in both directions. The velocity is more when the airflows out to the atmosphere (exhalation) than in the reverse direction. It may be advantageous to set the rotor blade pitch asymmetrically at a positive pitch so as to achieve a higher mean efficiency in a wave cycle. Towards this objective, performance characteristics of a turbine with different blade setting angles in steady flow were found by experimentation. Quasi-steady analysis was then used to predict the mean efficiency for a certain variation of air velocity with time. This variation with time was taken as pseudo-sinusoidal wherein the positive part of the cycle was taken as a half sine-wave whose amplitude is greater than that of the negative half sine-wave. Such a variation is representative of what happens in reality. For exhalation velocity amplitude to inhalation velocity ratios 0.8 and 0.6, a rotor blade setting angle of 2° was found to be optimum.

Effect of guide vane shape on the performance of a Wells turbine

A Wells turbine has inherent disadvantages: lower efficiency and poorer starting characteristics. Providing guide vanes on either side of the rotor could be one of the most effective ways of improving its performance. Several papers have demonstrated the usefulness of 2D guide vanes so far. In order to achieve further improvement in the performance of the Wells turbine, the effect of 3D guide vanes has been investigated experimentally by testing a model under steady flow conditions. Then, the running and starting characteristics under irregular flow conditions have been obtained by a computer simulation using quasi-steady analysis. It is found that the running and starting characteristics of the Wells turbine with 3D guide vanes are superior to those with 2D guide vanes.

Performance of an impulse turbine with fixed guide vanes for wave power conversion

A simple fixed geometry impulse turbine has been studied as a suitable power converter in Oscillating Water Column based wave power plants. Comparison with the Wells turbine, which is the commonly used self-rectifying turbine in such applications, shows it to be superior in performance under irregular flow conditions. Optimum guide vane angle for maximum efficiency has been arrived at based on the five angles tested.

Air Turbines for Wave Energy Conversion

This paper describes the present status of the art on air turbines, which could be used for wave energy conversion. The air turbines included in the paper are as follows: Wells type turbines, impulse turbines, radial turbines, cross-flow turbine, and Savonius turbine. The overall performances of the turbines under irregular wave conditions, which typically occur in the sea, have been compared by numerical simulation and sea trial. As a result, under irregular wave conditions it is found that the running and starting characteristics of the impulse type turbines could be superior to those of the Wells turbine. Moreover, as the current challenge on turbine technology, the authors explain a twin-impulse turbine topology for wave energy conversion.

Performance comparison of turbines for wave power conversion

Abstract The objective of this paper is to compare the performances of bi-directional turbines under irregular wave condition, which could be used for wave power conversion in the near future. The overall performances in connection with the behavior of oscillating water columns have been evaluated numerically. The types of turbine included in the paper are as follows: (a) Wells turbine with guide vanes (WTGV); (b) turbine with self-pitch-controlled blades (TSCB); (c) biplane Wells turbine with guide vanes (BWGV); (d) impulse turbine with self-pitch-controlled guide vanes (ISGV), and (e) impulse turbine with fixed guide vanes (IFGV). In the study, the experimental investigations were carried out to clarify the performance under steady flow condition and then the numerical simulation was used for predicting the performance of the turbine under irregular wave condition, which typically occurs in the sea. As a result, it is found that the running and starting characteristics of the impulse type turbines could be superior to those of the Wells turbine under irregular wave condition.

Hysteresis on Wells Turbine Characteristics in Reciprocating Flow

The Wells turbine for a wave power generator has hysteretic characteristics in a reciprocating flow, which results in the inaccurate prediction of performance in a quasisteady analysis. In order to clarify the hysteretic characteristics, an experimental investigation has been made by use of a newly devised turbine equipment in which a sinusoidal flow condition is simulated. The results have shown that hysteresis becomes more pronounced as solidity and blade thickness become larger, but it is insensitive to the Reynolds number and blade surface roughness. By means of pressure measurements on the blade surface, it is found that the hysteresis occurs due to different behavior of wakes between an accelerating flow and a decelerating flow.

A performance study of a radial turbine for wave energy conversion

Abstract Bidirectional flow air turbines are used in association with oscillating water columns to convert wave energy available in the oceans into mechanical power. Radial turbines, among others, were studied for such an application. The efficiency of radial turbines using reaction blading was found to be extremely low. On the other hand, the efficiency of impulse-type radial turbines was found to be higher. However, detailed performance studies on impulse-type radial turbines are not found in the literature. This paper presents detailed performance characteristics as well as some parametric studies based on laboratory experiments.

Effect of blade profile on the performance of a large-scale Wells turbine for wave-energy conversion

The aim of this study is to clarify the effect of rotor blade profile on the performance of the Wells turbine operated at high Reynolds number. In the study, four kinds of blade profile were selected with regard to the blade profile of the Wells turbine. The types of blade profile are as follows: NACA0020, NACA0015, CA9, and HSIM 15-262123-1576. In order to determine the optimum rotor blade profile of the turbine, experimental investigations have been performed for two solidities by model testing and numerical simulation. As a result, it has been concluded that a suitable choice, namely the preferable rotor geometry, is the blade profile of NACA0015. Furthermore, it has been found that the critical Reynolds number of the turbine is around 4×105.

Effects of Compressibility on the Performance of a Wave-Energy Conversion Device with an Impulse Turbine Using a Numerical Simulation Technique

This article presents work carried out to predict the behavior of a 0.6 m impulse turbine with fixed guide vanes as compared with that of a 0.6 hub-to-tip ratio turbine under real sea conditions. In order to predict the true performance of the actual oscillating water column (OWC), the numerical technique was fine-tuned by incorporating the compressibility effect. Water surface elevation versus time history was used as the input data for this purpose. The effect of compressibility inside the air chamber and the turbines performance under unsteady and irregular flow conditions were analyzed numerically. Considering the quasi-steady assumptions, the unidirectional steady-flow experimental data was used to simulate the turbines characteristics under irregular unsteady flow conditions. The results showed that the performance of this type of turbine is quite stable and that the efficiency of the air chamber and the mean conversion efficiency are reduced by around 8% and 5%, respectively, as a result of the compressibility inside the air chamber. The mean efficiencies of the OWC device and the impulse turbine were predicted for 1 month, based on the Irish wave climate, and it was found that the total time period of wave data used is one of the important factors in the simulation technique.