• Energy Research

The Oscillating Water Column system (OWC) is an interesting concept for ocean wave energy extraction. Several kinds of air turbines have been used for pneumatic energy conversion to mechanical energy. The Wells turbine has been used widely in OWC plants. However, as an alternative the self-rectifying turbine called Impulse turbine has been studied during the last years. We are interested in the radial version of the Impulse turbine, which was initially proposed by McCormick. A former research work aimed to improve the knowledge of the local flow behaviour and the prediction of the performances for this kind of turbine has been carried out using CFD (FLUENT®). The objectives of that work were connected mainly to the elaboration of a suitable 3D model for air flow simulation in a radial Impulse turbine. Model validation was conducted through a comparison with available experimental results. In the present, the objective is, using the numerical model, to develop a new radial impulse turbine geometry that gets better performances than the original one. This new turbine geometry will be exploited next in a project for an OWC of 250 kW. In this paper we describe the flow behaviour and the performances of this new turbine. For that, we study the Torque and Input coefficients, the losses and flow direction in the turbine elements.

The Double Decker Turbine (DDT) is a recent design introduced for oscillating water column (OWC) devices. Its major contribution is the combination of two typical solutions in just one prototype: a self-rectifying performance, to deal with the bidirectional flow, and the twin-turbine concept, allowing the use of unidirectional turbines. This is achieved by a set of two concentric turbines, called external and internal turbines (ExT—InT). In this work, Computational Fluid Dynamics (CFD) numerical model is developed to study in detail the performance of a DDT, where geometrical components for both turbines have been taken from previous works of the authors. The ANSYS-Fluent code was first executed by means of a URANS simulation with a realizable k-ε turbulence model to obtain the performance curve of the turbine under steady conditions. Results obtained reveal its potential with respect to other solutions in the current state-of-the-art of OWC solutions for Wave Energy Conversion. Following a non-steady analysis, we assumed a sinusoidal input from the chamber which also resulted in promising results. Finally, the flow analysis inside the DDT allowed the authors to envisage geometric improvements that could enhance the DDT efficiency on future works.

Abstract One of the most developed technologies in ocean energy is the OWC concept. It is well-known that the efficiency of the device is closely related to the efficiency of the Power-Take-Off (PTO) which is usually a turbine. Traditionally, self-rectifying turbines are the most widely considered for working in an OWC because unidirectional turbines require a system of valves to rectify the flow. However, another option has been recently proposed: “twin turbine” configuration. This paper is focused on the performance of the turbines used in this configuration. A CFD model has been created in Fluent® software and validated with data from the bibliography. This model has been used to analyze the flow field of the turbine when working in both operation modes: direct and reverse. Flow angles and loss distribution have been analyzed and interesting conclusions can be extracted. The efficiency of the twin turbine configuration has been calculated from the results of the numerical model. The calculations have been made paying attention to the effect of the torque and the flow rate of the turbine which is working in reverse mode. The results obtained are the core of this work. Once the flow field has been analyzed, changes in the turbine geometry are proposed in order to improve the efficiency of the whole system by increasing the blockage made by the turbine in reverse mode. These changes were focused on the solidity of the rotor and guide vanes.

Turbines for wave energy conversion have a special feature to be taken into account in the study of the tip leakage flow: These turbines are self-rectifying, which work inside a cyclically bidirectional flow alternatively as an inflow/outflow turbine. The phenomena at the blade tip will be different in these two situations. Moreover, it is necessary to take into account the tip clearance of the guide vanes because it has a significant influence on the rotor performance. A previously developed numerical model has been used for this study. The geometry proposed by Setoguchi et al. (2002, “A Performance Study of a Radial Impulse Turbine for Wave Energy Conversion,” Journal of Power and Energy, 216, pp. 15–22) is used in the model. Three different tip clearance sizes have been simulated to compare the influence of the tip clearance size on the performance. Results show that changing the size of the tip clearance from 0% to 4% of the blade span reduces the turbine maximum efficiency by up to 8%. However, the efficiency reduction is more pronounced when the turbine works as an inflow turbine because the tip clearance effect is more important in the inner part of the rotor, since flow velocities are higher and the relative casing motion is lower. This study achieves its main aim, which is to improve knowledge about the phenomena related to the tip clearance and its influence on the performance of radial impulse turbines.

One of the most developed technologies in ocean energy is the OWC concept. In this kind of device there is a turbine which plays an essential role, it is one of the factors which determine the efficiency of the system because of its own efficiency and its coupling with the chamber. One of the main characteristics in a turbine for OWC purposes, especially impulse turbines, is to use Guide vanes to optimize the energy extraction. However, they also are the largest source of losses. Improving the Guide vanes performance could reduce the pressure drop and, thus, the efficiency increases and the damping becomes smaller. In this paper the solidity of the guide vanes is analyzed to determine the optimum one. The study has been conducted on a radial impulse turbine with pitch-controlled guide vanes to minimize the incidence losses and, therefore, analyze the effect of the solidity. Experimental tests were carried out to validate a numerical model created in FLUENT®. The numerical model has been used to analyze the same turbine design but with different solidities of the guide vanes. The results have been conclusive: there is an optimal solidity for the guide vanes, which maximize the turbine efficiency by means of improving the guide vanes performance. Moreover, it has been seen that the optimum solidity is different for the inner and outer guide vanes.

Abstract The performance of an oscillating water column (OWC) wave energy converter depends on many factors, among which the incident wave conditions, the tidal level or the coupling between the chamber and the air turbine. In this work a 2D numerical model based on the RANS equations and the VOF surface capturing scheme (RANS–VOF) is implemented in order to study the optimum turbine-chamber coupling for a given OWC. The model represents a numerical wave flume where the OWC is tested under regular and irregular waves and for different damping coefficients, i.e., turbines of different characteristics. First, the numerical model is validated under regular and irregular waves using results from physical model tests. Excellent agreement is obtained between both models, physical and numerical. After the validation, an extensive campaign of computational tests is carried out, studying the performance of the OWC under nine different damping coefficients. The model developed allows, first, to quantify the relevance of the damping coefficient and wave conditions on the performance of an OWC chamber; and second, to define the damping condition which maximizes that performance, determining the characteristics that a turbine must meet to achieve the optimum coupling. In this manner this work contributes to the development of high performance OWCs.

The Spanish Ministry of Economy, Industry and Competitiveness for the R + D Project entitled “Development and Construction of Vertical Axis Wind Turbines for Urban Environments” (DEVTURB) – Ref. ENE2017-89965- P, under the National Plan for Scientific and Technical Research and Innovation. The grant provided by the Principality of Asturias for the support of Research Activities, funded by the Institute for Economic Development (IDEPA) under reference GRUPIN IDI/2018/000205 is also gratefully acknowledged. Additionally, the support given by the University Institute for In- dustrial Technology of Asturias (IUTA) and the City Hall of Gijón, through the financed project SV-18-GIJON-1-05, is also recognized.

Abstract Oscillating Water Column systems (OWC) have been in the spotlight in the last 20 years since these devices are considered one of the most promising devices among wave energy technology. These systems produce electricity by means a generator driven by a turbine, which takes advantage of the bidirectional flow created by the OWC itself. Among these turbines suitable for bidirectional flows, it is possible to find radial impulse turbines, which are the focus of this work. Traditionally, the radial impulse turbines have shown lower efficiencies than their competitors. However, the radial turbines present interesting mechanical features and, recently, some research show that the difference has been reduced. Following this thread, this work deals with another modification in the radial impulse turbine looking for a further improvement. By using a validated CFD model, it has been analysed the influence of the lean angle of the blade. Until now, all the turbines present in the literature are leaned zero degrees, leading to a strong interaction between the guide vanes and the blades. This work shows results of the same turbine, equipped with blades leaning from -5deg to 25deg, in order to determine the influence such a modification on the maximum total-to-static efficiency. Results have revealed a slight improvement in the maximum efficiency for positive leaning angles, whereas negative angles drive the turbine to worse performance.

The authors acknowledge the support provided by "Centro Integrado de F.P. Mantenimiento y Servicios a la Produccion de Langreo”, with special mention to its Director when this work was carried out, Mr. F. Fanjul. Additionally, the Spanish ‘‘Ministerio de Educacion, Cultura y Deporte’’ and its "FPU Program” for the pre-doctoral research scholarship of Manuel García (Grant No. FPU15/04375) is gratefully acknowledged.