Oscillating Water Energy
WAVE ENERGY ASSIGNMENT
A REPORT
ON
Oscillating Water Column Optimization Techniques
Preface
This document is submitted as a partial evaluation requirement towards the Ocean Engineering Unit (OENA 8550) following the cancellation of the original group submission. In recognition of the dynamic nature of this field of study, most references have been taken from lecture notes and journals which are considered more up-to-date than text books on the subject.
Table of Contents
Preface 2
Table of Contents 3
1.0 Executive Summary 4
2.0 Introduction 4
3.0 Oscillating Water Column (OWC)-Principles and theory of operation 5
4.0 Efficiency of an OWC 6
5.0 Efficiency improvement of Oscillating Water Column (OWC) using aerodynamic controls. 9
6.0 Conclusion 11
7.0 References 11
1.0 Executive Summary
Oscillating water column wave energy converter presents a very high potential in wave energy conversion technology but there is a need to make changes in the basic design so as to optimize the overall efficiency of power output from this device. This paper talks about some of the available optimization techniques with more focus on the aerodynamics control of the air flow across wells turbines.
2.0 Introduction
Waves are formed through the interaction of wind with water over an extensive area called the fetch. The wind is derivable from the solar energy of the earth and the transfer of solar energy to wave is greatest in areas with most powerful wind currents generally regions within the 30o and 60o latitudes. In general, the storm winds create complex and irregular waves which travel great distances in deep water and in a regular and smooth pattern, such as swells, which retain much of the energy of the original. On approaching the shallow water, the energy level generally drops to a lower level than it had in the deep water. (Stappenbelt 2008).
It is a common knowledge among the science and energy community that wave energy conversion has been demonstrated to be technically feasible, however, the extraction technology is still far from being commercially viable in comparison to the fossil fuels. Oscillating water column is one of the several methodologies currently being developed for the extraction of wave energy.
Wave energy conversion takes place in three stages namely primary energy conversion where the oscillating component interact directly or indirectly with the wave, secondary energy conversion where a turbine or similar transducer converts the wave energy to rotational energy and the tertiary stage where electric generators are used to derive electricity from the rotational energy. Also important is the information about the sea states as this determines the properties and hence, the energy content of the incident waves (Stappenbelt 2008).
Improvement in energy conversion efficiency can be gained through the utilization of optimisation techniques in any of the above three stages. The following sections will focus on few possible optimization techniques available to improve the power conversion efficiency in an Oscillating water column wave energy converter.
3.0 Oscillating Water Column (OWC)-Principles and theory of operation
As depicted in Fig 1, the oscillating water column is a form of terminator wave energy device that allows water to enter through a subsurface opening into a chamber with air trapped over it. When the wave approaches the device, it forces the air in the chamber to oscillate up and down like a piston. The air chamber is connected to a turbine at the top of the structure, which rotates with respect to the air movement and, thus, indirectly converts the wave energy to rotational kinetic energy. The turbine is further connected to a generator, which does the final conversion into electricity (Călina 2008).
According to Harris et al as shown in Fig 2, oscillating water column is suitable for nearshore, onshore or offshore installation. When installed offshore, vortex induced vibration becomes important in the analysis of the performance of the device especially as it affects the deep sea mooring systems. According to Harris et al, design for station keeping in terms of the ability of the mooring systems to withstand the environmental loads and its cost effectiveness are the two major requirements for any wave energy converter installed in offshore. These have impacts on the overall efficiency of the OWC wave energy devices. Other important considerations in the mooring design include maintainability, reliability and availability.
4.0 Efficiency of an OWC
Using values averaged over a period of time, the electrical power output from an OWC energy conversion device could be fundamentally written as P = Ww - L where Ww is the power absorbed from the waves and L represent all the losses that occur in the three energy conversion chain described above with the most significant losses being due to viscous fluid effects of the water, aerodynamic losses in the turbine (including connecting ducts) and valves, bearing friction losses and the electrical losses in the generator (Falca˜o 2002).
From the energy production efficiency point of view, the plant should be controlled to maximize the value of P in every sea state by, for example, controlling the turbine rotational speed (or alternately the torque applied by the electrical generator upon the turbine) and also possibly by controlling the air flow using valves arrangement or by variable geometry turbine such as variable pitch Wells turbine (Falca˜o 2002).
According to Michael et al, the hydrodynamic efficiency of an OWC can be defined in terms of the capture width ratio, which is the ratio of the time-averaged energy flux generated across the interior free-surface of the chamber and the time-averaged rate of energy input into the system per unit wave crest width, W where and Pw is the energy flux of the incident waves averaged over a period of time, which, by employing the principles of linear wave theory, can be defined by following equation.
Where the wave amplitude is denoted by A, water density by ρ, gravitational acceleration by g and vg is the group velocity of the incident wave defined by vg=dW/dk.
It is understood that the dispersion relation, W=ktanhkh, where K=W2/g, relates the wave frequency W and wave number k. The energy flux of plane progressive waves (of small amplitude) can also be described by
Where h is the water depth. The energy flux transferred across the interior free-surface of the chamber can be written as:
where Si represent the cross-sectional area of the internal free-surface. P and Vn respectively represent the hydrodynamic pressure and normal velocity of interior free-surface. We can easily see that while the expression for Po is in units of Js−1, the energy flux of the incident waves is in units of Js−1m−1. Therefore, ratio of the capture width is in the units of meter.
The capture width ratio is useful in experiments involving wave tank of finite width l as one can simply multiply the incident waves' energy flux by the width of the wave tank. Substituting this into the capture width ratio equation gives the hydrodynamic efficiency of the OWC device as η=W/l with a boundary. A value of η=1 implies that the device has effectively captured the total energy contained in the incident wave.
Under this condition, the free-surface of the interior of the OWC is said to be in resonant mode. To remain at optimal efficiency, the rate of energy extraction and rate of radiation damping and the interior free-surface must remain in a state of resonance. The physical meaning of this is that the scattered waves have been superposed and cancelled by the radiated waves, resulting from the oscillatory motion of the interior free-surface of the OWC. In this instance the device has thereby, theoretically, absorbed all of the incident wave energy.
According to Falcao and Justino (1999) the turbine hydrodynamic efficiency depends on air flow-rate and rotational speed while variations in the turbine's rotational speed affect the relationship between the air flow rate and the pressure-head curves that is, alter the turbine damping. These concepts modify the hydrodynamic radiation, which in turn affects the amount of energy absorbs from the waves. For these reasons, the turbine aerodynamic performance and the hydrodynamics properties of the wave energy absorption are related through the turbine speed. These two factors have to be jointly considered in any optimization efforts on the plant control algorithm. They further postulated that if the flow is assumed incompressible and the effects due to Reynolds number and Mach number variation are neglected, the instantaneous value of the aerodynamic efficiency of a given turbine is solely characterized by the parameter Tt/N2; where Tt is the aerodynamically produced torque on the turbine rotor and N is rotational speed. If the bearing friction torque is ignored, the values of Tt and Te (electromagnetic torque on the generator rotor) averaged over a period of time must be equal.
There are several methods available in the market through which the energy capture efficiency of an OWC can be improved. These methods include the following:
Use of Uni-directional Wells turbine with variable blade angle.
Use of adaptive (or predictive) control strategy that predicts the wave height and other properties of the incoming waves and prepares the OWC for optimum interaction between the device and the waves.
Use of reflector arms to focus the waves thereby maximizing energy extraction
Aerodynamic controls of the air pressure and flow
The following chapter will discuss the aerodynamic options further
5.0 Efficiency improvement of Oscillating Water Column (OWC) using aerodynamic controls.
According to Falcão (1998) the efficiency of oscillating water column (OWC) wave energy devices are particularly affected by flow oscillations basically for two reasons namely:
(1) Unsteady (reciprocating) flow of air as it is being displaced by the oscillating water free surface.
(2) Increment of the air flow, above a limit depending the rotational speed of the turbine. This gives rise to a rapid drop in the aerodynamic efficiency and, hence in the power output of the turbine.
Gato and Falca˜o (1989) proposed a method to partially circumvent this problem. The method involves controlling the pitch of the turbine rotor blades in order to limit the instantaneous angle of incidence of the relative flow below the critical value above which stalling of the rotor blades occurs. An alternative method to prevent the flow rate through the turbine from becoming excessive is through the use of air valves. This second method could be implemented in two different ways by either mounting the valves at the boundary between the chamber and the atmosphere parallel to the turbine (relief valves installed on the upper part of the air chamber structure) and are made to open (either actively or by passive control mechanism) in order to prevent the overpressure (or the under pressure) in the chamber from exceeding aerodynamic limits set by the characteristics of the turbine at its instantaneous speed or by mounting a valve in a series arrangement with turbine in the duct connecting the chamber to the atmosphere. Modulating the valves through some form of control mechanisms then prevents excessive flow through the turbines.
Both schemes achieve control of air flow through the turbine by dissipating energy at the valves. The two methods are equivalent in terms of limiting the flow rate through the turbine but the resulting pressure changes in the chamber area consequently leads to modification of the hydrodynamic process of energy extraction from the waves (Gato and Falca˜o, 1989).
The above arrangement is depicted in Fig 3 below, where the OWC device is considered fixed when compared to the bottom of the ocean. Parameters pa, Ta, ρa represents the atmospheric conditions and pa + pt is the pressure of air inside the chamber. With mt being the mass of air contained inside the chamber, the mass flow rate M= ρV where V is the volume of air inside the chamber.
6.0 Conclusion
In view of the need to bring the WEC devices to a stage where they could favourably compete with the popular Biofuels as an alternative energy resource, a number of methods to optimize the efficiency of an OWC wave energy converter device were mentioned. The use of control or relief valves to optimise the aerodynamic efficiency of the wells turbine was discussed. The control valves function to achieve a match between the air flow rate and the aerodynamic properties of the turbine in order to prevent stalling of the turbine and, hence maximise power output.
7.0 References
STAPPENBELT, B. 2008, WEC, lecture notes distributed in Ocean Engineering (OENA 8550), The University of Western Australia, Stirling in July 2008.
A.F.DE O. FALCÃO, J. P. A. P. (1998) OWC Wave Energy Devices with Air Flow Control. Estrada do Paco do Lumiar, 1699 Lisbon Codex, Portugal.
HARRIS ROBERT E., JOHANNING LARS & WOLFRAM JULIAN Mooring systems for wave energy converters: A review of design issues and choices.
CĂLINA OANA, L. A., CÂLNICEAN SILVIANA (2008) Wave Power Station.
GATO, L. M. C. & FALCÃO, A. F. (1989) de O., 1989, Aerodynamics of the Wells Turbine: Control by Swinging Rotor Blades,. Int. J. Mech. Sci, 31, 425-434.
TREVOR, W. (2006) Energy 2100 Wave Power Technology. Belfast, Queen's University.
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