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Wells Turbine for Wave Energy Conversion:
A Review
Ahmed S. Shehata, Qing Xiao, Khalid M. Saqr, Day Alexander
NOTICE: this is the author’s version of a work that was accepted for
publication in the International Journal of Energy Research.
Changes resulting from the publishing process, such as final peer
review, editing, corrections, structural formatting, and other
quality control mechanisms may not be reflected in this document.
Changes may have been made to this work since it was submitted
for publication. This manuscript was accepted for publishing on 29
May 2016 of the Journal. A definitive version was subsequently
published in the International Journal of Energy Research, DOI:
10.1002/er.3583.
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Wells Turbine for Wave Energy Conversion: A Review 1
2
Ahmed S. Shehata
1, 2*
, Qing Xiao
1
, Khalid M. Saqr
3
, Day Alexander
1
3
1) Department of Naval Architecture, Ocean and Marine Engineering, University of Strathclyde, Glasgow 4
G4 0LZ, UK 5
2) Marine Engineering Department, College of Engineering and Technology, Arab Academy for Science 6
Technology and Maritime Transport, P.O. 1029, AbuQir, Alexandria, Egypt 7
3) Mechanical Engineering Department, College of Engineering and Technology, Arab Academy for 8
Science Technology and Maritime Transport, P.O. 1029, AbuQir, Alexandria, Egypt 9
SUMMARY 10
In the past twenty years, the use of wave energy systems has significantly increased, generally 11
depending on the oscillating water column (OWC) concept. Wells turbine is one of the most 12
efficient OWC technologies. This article provides an updated and a comprehensive account of 13
the state of the art research on Wells turbine. Hence, it draws a roadmap for the contemporary 14
challenges which may hinder future reliance on such systems in the renewable energy sector. In 15
particular, the article is concerned with the research directions and methodologies which aim at 16
enhancing the performance and efficiency of Wells turbine. The article also provides a thorough 17
discussion of the use of computational fluid dynamics (CFD) for performance modeling and 18
design optimization of Wells turbine. It is found that a numerical model using the CFD code can 19
be employed successfully to calculate the performance characteristics of W-T as well as other 20
experimental and analytical methods. The increase of research papers about CFD, especially in 21
the last five years, indicates that there is a trend that considerably depends on the CFD method. 22
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Keywords: Wells turbine; CFD; Wave energy; Oscillating water column. 24
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* Corresponding Author: Ahmed S. Shehata 26
E-mail address: ahmed.mohamed-ahmed-shehata@strath.ac.uk 27
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Nomenclature
A The total blade area = (z c b), m
2
Mean rotor radius =
, m
Rotor area =π
, m
2
Rotor radius , m
a Margin distance for the endplate , m
Rotor radius at tip , m
b Blade Span , m
T Time period =
, sec
c Blade chord , m
t Rotor blade thickness , m
Drag force coefficient
Loading torque N m
Lift force coefficient
TSR Tip speed ratio
Power coefficient
Axial velocity =
, m/s
Torque coefficient
Maximum value of axial velocity, m/s
D Drag Force ,N
Resultant air velocity
, m/s
Rotor diameter , m
Output power coefficient
Wave frequency , Hz
Inertia coefficient
F
A
Axial Force
, N
Loading torque coefficient
F
t
Tangential Force
, N
Z Number of blades
g Leading edge offsetting of a blade from an
a axis , m
α Angle of attack- the angle between the chord line
and the direction of the fluid velocity ,degree
I Moment of inertia , kg m
2
η Mean turbine efficiency
K Non-dimensional period
Air specific density , kg/m
3
L lift Force , N
σ Turbine solidity
Δp Pressure difference across
the turbine
,N/m
2
Flow coefficient
Q Flow rate through the rotor area ,m
3
/sec
Rotor angular speed , rad/sec
Rotor radius at hub , m
Non-dimensional angular velocity under irregular
flow condition
List of Abbreviations
AOP An Optimized Profile
AR Aspect Ratio (the ratio of span to chord)
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CFD Computational Fluid Dynamics
NACA National Advisory Committee for
Aeronautics
OWC Oscillating Water Column
TC Tip Clearance
W-T Wells Turbine
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1. Introduction 2
Generating renewable energy has been increasing considerably in the past decade, 3
achieving almost 22% of the global energy production in 2013, compared to 14% in 2005 [1]. 4
The ocean is a potential goldmine for renewable energy generation for several reasons, most 5
important of which is that, unlike wind and solar power, power from ocean waves continues to 6
be produced around the clock [2]. In addition, wave energy varies with the square of wave 7
height, whereas wind energy varies with the cube of air speed. This results in a much higher 8
average power production from waves per unit of time [3]. Moreover, marine waves travel great 9
distances without significant energy losses, so they act as a renewable and an efficient energy 10
transport mechanism across thousands of kilometers. Such renewable energy can be produced 11
through different devices, which produce sufficient work to drive electrical generators that 12
convert such work into electricity.Wave energy extractors can be classified according to the 13
water depth at which they operate. This classification is presented in Figure 1. Another 14
classification based mostly on working principle is presented in Table 1. 15
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Figure 1 Types of wave energy extractors with respect to water depth 2
Most of fixed-structure OWC systems are located on the shoreline or near the shore. 3
Shoreline devices are characterized by relatively easier maintenance and installation, and they do 4
not require deep water moorings and long underwater electrical cables. The floating OWC 5
devices are slack-moored to the sea bed and so are largely free to oscillate, enhancing the wave 6
energy absorption if the device is properly designed for that purpose [4]. 7
Offshore devices are basically oscillating bodies, either floating or fully submerged. They 8
take advantage of the most powerful wave systems available in deep water. Offshore wave 9
energy converters are in general more complex compared to fixed-structure OWC. This, together 10
with additional problems related to mooring and access for maintenance and the need of long 11
underwater electrical cables, has hindered the converters’ development, and only recently have 12
some systems reached, or come close to, the full-scale demonstration stage [5]. 13
Overtopping systems are a different way of converting wave energy to capture the water 14
that is close to the wave crest and introduce it, by over spilling, into a reservoir where it is stored 15
at a level higher than the average free-surface level of the surrounding sea [6, 7]. The potential 16
energy of the stored water is converted into useful energy through more or less conventional low 17
head hydraulic turbines. The hydrodynamics of overtopping devices is strongly nonlinear, and, 18