2nd Paper - Tidal Current Turbines (1)

7/30/2019 2nd Paper - Tidal Current Turbines (1)

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TIDAL CURRENT TURBINES

NASIR MEHMOOD, SHENG QIHU, WANG XIAOHANG, ZHANG LIANG

College of Shipbuilding EngineeringHarbin Engineering University Harbin, China

ABSTRACT

Today, the world is heavily dependent on fossil fuels, as most of the energy

requirements are being met through conventional methods of burning these

fuels. The energy demand is increasing day with growing population.

Consequently, fossil fuel reserves are depleting continuously and will soon run

out in coming years. Therefore, renewable energy resources have gained

enormous attention in recent years. The growing interest in exploring tidal

current technologies has many compelling reasons such as its renewable nature,tidal energy is cleaner than fossil fuels, intermittent but predictable, security and

diversity of supply, and limited social and environmental impacts. Tidal current

technologies are still in development phase, yet need some time to mature to

prove their full potential. Tidal current turbine is an important tidal current

technology. The purpose of this paper is to present a comprehensive review of

tidal current turbine, its potential and associated challenges. The paper discusses

general theoretical background of fluid flow in a tidal stream and forces

governing the flow behavior. The author will also discuss the core issues and

challenges faced in research and development such as unforgiving marineenvironment, corrosion, cavitation phenomena and extreme structural loads.KEY WORDS

Ocean power, Tidal power, Tidal current turbines, Tidal current devices

1.INTRODUCTIONThe primary thirst of worlds energy requirement has always been fulfilled

by fossil fuels. This world of ours is worryingly dependent on fossil fuels, asmost of the energy requirements are met by burning fossil fuels. Dependence on

fossil fuels is swelling with growing population as energy demand is increasing,thus mounting burden on fossil fuel reserves. It is therefore a matter of deepconcern that these reserves will soon run out in coming years. Immensedependence on fossil not only augments the issues like security of supply, butalso harms the environment. Fossil fuels are the main source of CO 2 emission.

The growing interest in exploring tidal current technologies has compellingreasons like its cleaner than fossil fuels, intermittent but predictable, security anddiversity of supply, and limited social and environmental impacts.

In 1980, more than a thousand patents were registered for converting wave

energy into power [1]. However, the concept is very old, one of the earliest

patents was registered by a Frenchman and his son Girard in 1799 [2]. Researchon wave energy is underway around the globe (e.g [3-10]) and this technology

has been tested in many countries around the world [11-12].The purpose of this paper is to provide a comprehensive review of tidal

current turbine, its potential and associated challenges. This paper gives anoverview of some basic fluid dynamics concepts along with the force acting on a


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fluid. It will also present important challenges to the development of thesetechnologies.

2.BASIC FLUID DYNAMICS CONCEPTS2.1 Dynamic and Kinematic Viscosity

Dynamic viscosity, also referred as absolute viscosity, is the fluidsresistance to flow [13]. Water has a dynamic viscosity of about 0.001 Kg/ms.Kinematic viscosity is the ratio of dynamic viscosity to the inertial force.Kinematic viscosity is defined as the resistance by fluid to applied force at amolecular level. Mathematically:

= / (1)where is kinematic viscosity, is dynamic viscosity and is density.2.2 Forces Acting on a Fluid

Different materials respond differently to same stress conditions due todifferent material properties. The results are elastic, plastic, fracture and viscousdeformation. Elastic deformation is recoverable where as others are not. Water, aviscous substance, responds to stress by flowing. The rate of deformation (flow)is a function of the magnitude of stress [14]. Mathematically:

= du/dz (2)where is the shear stress, du/dz is the velocity gradient and is dynamicviscosity. In case of water, stress has a direct relationship with velocity gradient,

thus it is called a Newtonian fluid. In a fluid domain, forces acting on a fluid are

explained below. These forces may be present as a single force or have a

combined effect on a fluid.2.2.1 Inertial Force and Viscous Force

Inertial force is equal to the product of mass and acceleration of the flowing

fluid and acts in the direction opposite to the direction of acceleration. Inertial

forces cause destabilization in fluids behavior. This force is present in all fluidflow conditions.

Viscous force is equal to the product of shear stress () due to viscosity andsurface area of the flow, where viscosity has an important role to play. Viscousforces are responsible for stabilizing effect on a fluid [15].2.2.2 Gravity and Pressure Force

Gravity force is equal to the product of mass and acceleration due to gravityof the flowing fluid. This force is present in case of open surface flow.

Pressure force is equal to the product of pressure intensity and cross-sectional area of the flowing fluid. This force is present in case of pipe flow.2.2.3 Surface Tension and Elastic Force

Surface tension force is equal to the products of surface tension and lengthof surface of the flowing fluid.

Elastic force is the product of elastic stress and area of the flowing fluid.In any case of fluid flow, the above mentioned forces may not always be presentand these forces are not of equal magnitude. There is always one or two forces

which dominates the other forces. These dominating forces govern the flow offluid.2.3 Laminar and Turbulent Flow

Laminar flow is coherent where stabilizing viscous forces dominate thedestabilizing inertial forces. In laminar flow, transfer of momentum through fluidoccurs at molecular level. Equation (2) is also used to describe laminar flow.


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Turbulent flow is chaotic where inertial destabilizing forces dominate thestabilizing viscous forces. In turbulent flow, whole packets of water aretransferred during transfer of momentum [16]. Mathematically:

= Kz du/dz (3)

where Kz is eddy viscosity and du/dz is the velocity gradient.2.4 Steady and Unsteady FlowIn a channel when velocity, pressure, density of fluid remain unchanged with

respect to time, the flow is known as steady flow. If one or more of thesecharacteristics change, then flow is called unsteady flow [17]. Mathematically:

(V/x) x0,y0,z0 = 0(P/x) x0,y0,z0 = 0(/x) x0,y0,z0 = 0

(V/x) x0,y0,z0 0(P/x) x0,y0,z00(/x) x0,y0,z0 0

(4)

Steady Flow Unsteady Flowwhere V is the velocity, P is the pressure, is the density and (x0,y0,z0) is a fixed

point in fluid channel.2.5 Uniform Flow and Nonuniform FlowWhen the depth or the average velocity of flow is constant along the distance

at any given time, the flow is known as uniform flow. If one or more of theseparameters change, the flow is called nonuniform flow [18]. Mathematically itcan be expressed as:

(y/x or u/x)t = t0 = 0(y/x or u/x)t = t0 0

Uniform flowNon uniform flow

(5)

where y is depth and u is average velocity.2.6 Critical, Supercritical and Subcritical Flow

Speed of the surface wave depends on the square root of the product ofacceleration due to gravity and the water depth [19]. Thus ability of the surfacewave to travel upstream, against the current, depends upon the water depth andgravity. Critical, supercritical and subcritical flows occur in open channel flowand is usually defined by Froudes number (Fr). When the surface wave travelsupstream with same celerity as current it is called critical flow (F r=1). Insupercritical flow, surface wave is unable to progress upstream against current(Fr>1). While when surface wave travels upstream against current it is known assubcritical flow (Fr1), critical (Fr=1) or subcritical (Fr


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3.CLASSIFICATION OF OCEAN POWER AND TIDAL CURRENT DEVICESOcean power, being an embryonic energy solution, has enormous potential for

future energy production. Ocean power technologies are relatively new andapplications are developing at very fast pace. As a result, concrete boundaries for

classification, applications and conversion concepts have yet to be defined. Thissection is devoted to presenting these issues keeping in view current availableliterature and industrial trends.

Ocean power can be categorized based on tidal rise and fall, tidal/oceancurrents, waves, salinity gradient and thermal gradient, shown in Fig 1 [22].Classification of tidal power, also referred as tidal energy is shown in Fig 2. thatincludes tidal current devices, tidal barrage and tidal fence.

Tidal Current Devices

Horizontal Axis Turbines

Vertical Axis Turbines

Cross Flow Turbines

Diffuser Augmented Turbines

Oscillating Hydrofoil

Tidal Power/Tidal Energy

Tidal Barrage Tidal Fence

Fig 1. Classification of ocean power. Fig 2. Classification of tidal power.

4.CHALLENGES TO DEVELOPMENT4.1 CorrosionCorrosion is defined as it is the electrochemical oxidation of a metal [23].Corrosion can be explained as reversion of any metal to its ore form. Marinecorrosion depends on numerous factors such as temperature, galvanicinteractions, alloy surface films, biofouling, water chemistry, alloy composition,microbiological organisms, geometry and surface roughness etc. [24]. It is vital tounderstand how these factors affect marine corrosion to design a robust supportstructure for tidal current turbine. Generally, corrosion in seawater accelerateswith increase in temperature. Other catalysts such as concentration of oxygen andmarine biological activity should also be considered. The solubility of oxygendecreases with the increase in temperature. Corrosion of metals in seawater isalso affected by the turbulent or laminar flow. Corrosion rate may accelerate withfluid flow by taking off the protective film or migration of deleterious species orby enhancing diffusion. On the other hand, increased fluid flow may also helpdecrease corrosion by removing the aggressive ions that begins to accumulate onmetal surface. Generally, cavitation and erosion-corrosion are forms of flow

influenced corrosion.4.2 CavitationCavitation occurs in liquids flowing at high velocity, causing a pressure drop

after a body that leads to formation of vapor bubbles. When the static pressure ofthe liquid falls below the vapor pressure cavitation phenomena starts. The liquidpressure has two components, static and dynamic. Dynamic pressure is due toliquid flow velocity and static pressure is the actual fluid pressure. Formation of


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vapor bubble or boiling is a function of static pressure. Cavitation mostly occursnear the fast moving blades of the turbine. The reason being fast moving blades,local dynamic head increases and thus static pressure falls. Bubble formation incavitation is not the major issue, the major issue is the breakdown of these

bubbles as they generate high frequency pressure wave which is very damaging.The breakdown of these bubbles near the blade surface causes erosion on thesurface, also termed as cavitation erosion. Small sized bubbles only damage theblade surface of the blade and do not cause efficiency drop. Large sized bubblesreduce the efficiency of the turbine, since big size bubbles disturb the fluid flowand cause flow separation.4.3 Extreme Loading conditions

Tidal current turbines are exposed to extreme structural loading conditions inmarine environment. Seawater has a density of approximately 1025 kg/m

3so the

forces acting on the turbine and support structure are enormous. Turbines facing

the flow direction of seawater are exposed to a thrust force while extractingkinetic energy from the flow stream. As the blades of the turbine rotate to extractthe available energy, there is a change in momentum between the upside anddownside of the turbine. This change in momentum exerts a force on the turbineand its support structure. This thrust force has to be absorbed by the supportstructure. The thrust force encountered by tidal current turbines is expressed as:

Tmax= 0.5 ACtV max (8)where is the density of the fluid, A is the cross sectional area, Ct is the thrustcoefficient.

5.CONCLUSIONDue to depleting fossil fuel resources, their rising cost and adverse

environmental effects; the world is obligated to find alternate energy resources.Tidal current technologies are answer to mankind worse fears of energy resourcesdepletion. The author has presented the core issues such as corrosion, cavitationand extreme loading conditions; which pose major challenges as thesetechnologies develop today, and will continue to be vital in recent future. Inaddition, issues such as underwater sealing, deployment and retrieval of thesesystems, optimization of resources involved in installation and retrieval, routinemaintenance and long term impact of presently unseen and minor environmentalfactors are important and these aspects require further exploration.

ACKNOWLEDGEMENTS

This research is financially supported by National Special foundation for Ocean

commonweal (grant 200805040), S&T program (grant 2008BAA15B06) and

for Ocean Renewable Energy (grants GHME2010GC02, GHME2010GC03),

and 111 Project foundation from State Administration of Foreign Experts

Affairs of China and Ministry of Education of China (grant B07019).

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2nd Paper - Tidal Current Turbines (1)