In the past twenty years, the use of wave energy systems has significantly increased, generally depending on the oscillating water column (OWC) concept. Wells turbine is one of the most efficient OWC technologies. This article provides an updated and a comprehensive account of the state of the art research on Wells turbine. Hence, it draws a roadmap for the contemporary challenges which may hinder future reliance on such systems in the renewable energy sector. In particular, the article is concerned with the research directions and methodologies which aim at enhancing the performance and efficiency of Wells turbine. The article also provides a thorough discussion of the use of computational fluid dynamics (CFD) for performance modeling and design optimization of Wells turbine. It is found that a numerical model using the CFD code can be employed successfully to calculate the performance characteristics of W-T as well as other experimental and analytical methods. The increase of research papers about CFD, especially in the last five years, indicates that there is a trend that considerably depends on the CFD method.

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Wells turbine for wave energy conversion: a review

Performance analysis of wells turbine blades using the entropy generation minimization method

In developing a wave energy converter (WEC), assessing and rating the device is a difficult, but important issue. Conventionally, a large scaled device (maybe large enough for accommodating a power takeoff (PTO) system) or prototype device is needed to be tested in wave tanks or in seas in different wave conditions so that a power matrix for the device can be defined using scaling or interpolation/extrapolation methods. Alternatively, a pure numerical simulation in time-domain may be used for assessing the power capture capacities of wave energy devices. For the former, it is convincing, but can be especially difficult in the early stages of development, when small scaled models are normally used; and for the latter, the pure numerical simulation may not be very reliable and convincing, especially when the dynamic problem is very complicated. In this paper, a method for assessing the captured wave power for a device from its power capture response is presented. In the proposed method, a measured or calculated linear power capture response of the device is combined with wave spectrum to compute the average captured power function. Once the average captured power function is obtained, the overall average captured power corresponding to the wave state can be easily calculated. If a linear power capture response is obtained from a model test, the power assessment based on this proposed method can be very convincing and reliable. To illustrate the application of the proposed method, an example of a fully linear dynamic system, including the linear hydrodynamics of the floating structure and a linear power takeoff, is considered. For such a system, the frequency-domain analysis can be employed to obtain the performance of the floating device under waves and the power takeoff system. The hydrodynamic performance of the wave energy converter is then used to define the power capture response and to calculate the average captured power functions in different sea states. Then, the captured power of the device in different sea states, i.e, the power matrix, can be calculated, and accordingly, the device can be assessed and rated. To validate the proposed method, a time-domain analysis is also performed for a cross-check. In the time-domain analysis, the hydrodynamic coefficients and responses are first assessed in frequency-domain, and then transformed into the relevant terms by means of impulse response functions for establishing the time-domain (TD) equation. By comparing the results from frequency-domain and time-domain analyses of irregular waves, it can be concluded that the proposed wave energy capture assessment method can be used in assessing or rating the device.

Assessment of Wave Energy Extraction From Seas: Numerical Validation

On improving wave energy conversion, part I: Optimal and control technologies

The wave energy devices currently being built in the United Kingdom and elsewhere have adopted the Wells air turbine for converting pneumatic energy in the device to mechanical energy of rotation. A monoplane (single-plane) Wells turbine can operate efficiently over a certain maximum flowrate and pneumatic pressure drop due to blade stall and tip speed limitations respectively. The knowledge base on the monoplane Wells turbine is reviewed. An interactive methodology for an aerodynamic design of this turbine is presented.

A Methodology for Wells Turbine Design for Wave Energy Conversion

This paper describes the optimization of the power conversion chain and the engineering design considerations of an oscillating water column wave power device which would form part of a 2-GW power station. Novel features of the principal device described include the multi-resonant concept, which considerably widens the frequency bandwidth response, and the use of the simple highly efficient Wells self-rectifying air turbine in the secondary power conversion stage. It is concluded that using established technology wave power stations comprising sea bed mounted reinforced concrete structures could produce power for as little as 1.3p per kW-hr after the initial capital repayment period and thus the economics are similar to that of hydro-electric power.

A. E. Long

Papers

The Q.U.B. Axisymmetric and Multi-Resonant Wave Energy Convertors