WHAT CAUSES THE TIDES? Tides are periodic rises and falls of large bodies of water. Tides are caused by the gravitational interaction between the Earth and the Moon. Tides are the cyclic rising and falling of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the Earth . Tides cause changes in the depth of the sea and produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation. The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone , is an important ecological product of ocean tides.
The Earth and Moon, looking at the North Pole The relative distance of the Moon from the Earth also affects tide heights. When the Moon is at perigee the range is increased and when it is at apogee the range is reduced.
gravitational force The Moon's gravity differential field at the surface of the earth is known as the Tide Generating Force . This is the primary mechanism that drives tidal action and explains two bulges, accounting for two high tides per day. Other forces, such as the Sun's gravity, also add to tidal action.
gravitational force The Moon exerts its gravitational pull differently on different parts of the earth. The farther away the Moon, the weaker its pull. Imagine a shell of the outer Earth , this diagram shows the Moon's gravity differential over the thickness of the shell.
Spring Tides The Sun's Interaction with the Tides Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun , and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon.
Proxigean Spring Tide The eccentricity of the orbit of the moon in this illustration is greatly exaggerated. The Proxigean Spring Tide is a rare, unusually high tide. This very high tide occurs when the moon is both unusually close to the Earth (at its closest perigee , called the proxigee ) and in the New Moon phase (when the Moon is between the Sun and the Earth). The proxigean spring tide occurs at most once every 1.5 years . An artist's conception of spring tide
Neap Tides Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons. An artist'sconception of neap tide
Spring Tides and Neap Tides The Bay of Fundy at low tide The Bay of Fundy at high tide
Tidal range The tidal range is the vertical difference between the highest high tide and the lowest low tide . In other words, it is the difference in height between high and low tides. The most extreme tidal range will occur around the time of the full or new moons , when gravity of both the Sun and Moon are pulling the same way ( new moon ), or exact opposite way ( full ). The typical tidal range in the open ocean is about 0.6 meters (2 feet). As you get closer to the coast, however, this range gets much greater. Coastal tidal ranges vary globally and can differ anywhere from 1.8 meters to 3 meters (6–10 feet).
Tidal cycle time Principal Types of Tides Showing the Moon's declinational effect in production of semidiurnal, mixed, and diurnal tides.
Tidal cycle time The same tidal forcing has differentresults depending on many factors,including coast orientation, continental shelf margin,water body dimensions.
Tidal Energy sometimes called tidal power is the power achieved by capturing the energy contained in moving water currents tides and open ocean currents . There are two types of energy systems that can be used to extracted energy: kinetic energy The moving water of rivers tides and open ocean currents and the rise and fall of the tides that uses the height difference between ebbing and surging tides and potential energy from the difference in height (or head ) between high and low tides.
The former method - generating energy from tidal currents - is considered much more feasible today then building ocean-based dams or barrages that flood eco systems and are expensive to build.
Tidal power is classified as a renewable energy source, because tides are caused by the orbital mechanics of the solar system and to a lesser extent the surface effect of winds and are considered inexhaustible within a human timeframe.
The root source of the energy comes from the slow deceleration of the Earth's rotation. The Moon gains energy from this interaction and is slowly receding from the Earth. Tidal power has great potential for future power and electricity generation because of the total amount of energy contained in this rotation. Tidal power is reliably predictable (unlike wind energy and solar power ).
Tidal power The table shows some significant force values for the Moon: Column 1 : Longitude along any Earth equator passing through the axis, counted from the trans-lunar nodal point. Column 2: The axial gravitational force of the Moon at longitudes from 0° to 180°. Column 3: The tangential component of the gravitational force (value of Col. 2 multiplied by sin n). Column 4: The tangential component of the centrifugal force (33.1 micronewtons multiplied by sin n). Column 5: The tide-raising force. Column 6: The radial centrifugal force, having no effect on the tides, is shown for comparison. 12.10 0 0 0 34.28 180° 21.79 0.56 23.44 24.00 33.94 135° 45.191 0 33.147 33.147 33.147 90° 68.59 -O.54 23.44 22.90 32.38 45° 78.29 0 0 0 32.07 0° 6 5 4 3 2 1
Applycation of tide to generate electricity Tidal Stream Turbines The simplest of all configurations is a rotor on a pole fixed to the seabed: this design acquired top-sides
Applycation of tide to generate electricity Today, this twin turbine design carries two 20m rotors is rated at 1 - 2 MW depending on current speed, and operates in 30 - 50m water depths. Each rotor runs in clean water upstream of its support arm.
Applycation of tide to generate electricity The tidal turbine is shown for comparison against anoffshore wind turbine of the samepower rating and in 25m water depth.
Applycation of tide to generate electricity Another option is to use offshore turbines, rather like an underwater wind farm. This has the advantage of being much cheaper to build, and does not have the environmental problems that a tidal barrage would bring.
Applycation of tide to creat barrage The barrage method of extracting tidal energy involves building a barrage and creating a tidal lagoon . The barrage traps a water level inside a basin. Head ( a height of water pressure) is created when the water level outside of the basin or lagoon changes relative to the water level inside. The head is used to drive turbines. In any design this leads to a decrease of tidal range inside the basin or lagoon, implying a reduced transfer of water between the basin and the sea. This reduced transfer of water accounts for the energy produced by the scheme. The largest such installation has been working on the Rance river The basic elements of a barrage are caissons , embankments, sluices , turbines and ship locks. Sluices, turbines and ship locks are housed in caisson (very large concrete blocks). Embankments seal a basin where it is not sealed by caissons. The sluice gates applicable to tidal power are the flap gate, vertical rising gate, radial gate and rising sector. Barrage systems have been plagued with the dual problems of high civil infrastructure costs associated with what is in effect a dam being placed across two estuarine systems, one for the high water dam storage and the other a low water dam for the release of the storage, and, the environmental problems associated with the flooding of two ecosystems.
Applycation of tide to generate electricity An artistic impression of a tidal barrage, including embankments, a ship lock and caissons housing a sluice and two turbines. The weir at Coburg lake in Victoria (Australia) . A weir is a small overflow-type dam commonly used to raise the level of a river or stream. Weirs have traditionally been used to create mill ponds in such places. Water flows over the top of a weir, although some weirs have sluice gates which release water at a level below the top of the weir. The crest of an overflow spillway on a large dam is often called a weir .
Applycation of tide to generate electricity A manually operated needle dam type weir near Revin on the Meuse River , France A weir in Warkworth , New Zealand
Tidal power schemes being considered In the table, '-' indicates missing information, '?‘ indicates information which has not been decided ? - - Tiburon ? - 6-7 Rio Colorado Mexico - - 4.5 Cheonsu 480 100 4.7 Garolim Korea 7000 1970 6.8 Cambay 900 170 5.3 Kutch India 1800 115 10.0 Shepody 1400 90 10.9 Cumberland 5338 240 12.4 Cobequid Canada ? - 10.9 Secure Bay Australia 6800 - 5.9 San Jose Argentina Maximum capacity (MW) Area of basin (km²) Mean tidal range (m) Place Country
Tidal power schemes being considered ? ? ? Mozambique Channel South Africa 87000 - 6.0 Penzhinskaya Bay 8000 - - Tugur 19200 2300 9.1 Mezen Russia  6501 - 7.5 Turnagain Arm 2900 - 7.5 Knik Arm ? - 5.5 Passamaquoddy Bay United States 33 5.5 5.2 Conwy - - - Strangford Lough 700 61 6.5 Mersey 8640 450 7.8 Severn United Kingdom Maximum capacity (MW) Area of basin (km²) Mean tidal range (m) Place Country
World energy resources and consumption The estimated 15TW total energy consumption of 2004 was divided as follows: 0.5 15 Total 0.006 0.2 Geothermal, wind, solar, wood 0.03 0.9 Nuclear 0.03 0.9 Hydroelectric 0.12 3.8 Coal 0.11 3.5 Gas 0.18 5.6 Oil Energy/year in ZJ Power in TW Fuel type
World energy resources and consumption World energy consumption in TW (= 10 12 Watt) , 1980-2004. Source: Energy Information Administration World energy consumption in TW , 1965-2005. Source: BP 2006 Statistical review
World energy resources and consumption Global energy consumption in successively increasing detail Source: BP 2006 Statistical review and Renewables , Global Status Report 2006 .
Renewable resources World renewable energy in 2005 (except 2004 data for items marked* or **). Source Renewables , Global Status Report 2006 .
Renewable resources Available renewable energy. The volume of the cubes represent the amount of available wind and solar energy. The small red cube shows the proportional global energy consumption. Values are in TW = 10 12 Watt. The amount of available renewable energy dwarfs the global consumption.
Renewable resources Solar energy as it is dispersed on the planet and radiated back to space. Values are in PW = 10 15 Watt. Data to produce this graphic was taken from a NASA publication.
What Causes Waves? The winds cause waves on the surface of the ocean (and on lakes). The wind transfers some of its energy to the water, through friction between the air molecules and the water molecules. Stronger winds (like storm surges) cause larger waves. You can make your own miniature waves by blowing across the surface of a pan of water. Waves of water do not move horizontally, they only move up and down (a wave does not represent a flow of water). You can see a demonstration of this by watching a floating buoy bob up and down with a wave; it does not, however, move horizontally with the wave.
Motion of a particle in a ocean wave Motion of a particle in a ocean wave. A = At deep water. B = At shallow water (ocean floor is now at B). The circular movement of a surface particle becomes elliptical with decreasing depth. 1 = Progression of wave 2 = Crest 3 = Trough
Wave motion Waves are oscillations in the water's surface. For oscillations to exist and to propagate, like the vibrating of a guitar string or the standing waves in a flute, there must be a returning force that brings equilibrium. The tension in a string and the pressure of the air are such forces.
Wave motion These two diagrams show the relationships between wave speed and period for various depths (left), and wave length and period (right), for periodic, progressive surface waves. ( Adapted from Van Dorn, 1974) Note that the term phase velocity is more precise than wave speed . The period of waves is easy to measure using a stopwatch, whereas wave length and speed are not. In the left picture, the red line gives the linear relationship between wave speed and wave period. A 12 second swell in deep water travels at about 20m/s or 72 km/hr. From the red line in the right diagram, we can see that such swell has a wave length between crests of about 250m.
Waves and wind How wind causes water to form waves is easy to understand although many Intricate details still lack a satisfactory theory. On a perfectly calm sea, the wind has practically no grip. As it slides over the water surface film, it makes it move. As the water moves, it forms eddies and small ripples.
how do we measure waves objectively? Scientists do this by introducing a value E which is derived from the energy component of the compound wave. In the left part of the drawing is shown how the value E is derived entirely mathematically from the shape of the wave. Instruments can also measure it precisely and objectively. The wave height is now proportional to the square root of E . The sea state E is two times the average of the sum of the squared amplitudes of all wave samples.
Fully Developoled Sea energy spectrum for various wind speeds When the wind blows sufficiently long from the same direction, the waves it creates, reach maximum size, speed and period beyond a certain distance (fetch) from the shore. This is called a fully developed sea . Because the waves travel at speeds close to that of the wind, the wind is no longer able to transfer energy to them and the sea state has reached its maximum. In the picture the wave spectra of three different fully developed seas are shown. The bell curve for a 20 knot wind (green) is flat and low and has many high frequency components (wave periods 1-10 seconds). As the wind speed increases, the wave spectrum grows rapidly while also expanding to the low frequencies (to the right)
Waves entering shallow water As waves enter shallow water, they slow down, grow taller and change shape. At a depth of half its wave length, the rounded waves start to rise and their crests become shorter while their troughs lengthen. Although their period (frequency) stays the same, the waves slow down and their overall wave length shortens. The 'bumps' gradually steepen and finally break in the surf when depth becomes less than 1.3 times their height. Note that waves change shape in depths depending on their wave length , but break in shallows relating to their height !
Wave groups Part of the irregularity of waves can be explained by treating them as formed by interference between two or more wave trains of different periods, moving in the same direction. It explains why waves often occur in groups. The diagram shows how two wave trains (dots and thin line) interfere, producing a wave group of larger amplitude (thick line).
Wave power Wave power refers to the energy of ocean surface waves and the capture of that energy to do useful work - including electricity generation , desalination , and the pumping of water (into reservoirs). Wave power is a form of renewable energy . Though often co-mingled, wave power is distinct from the diurnal flux of tidal power and the steady gyre of ocean currents . When an object bobs up and down on a ripple in a pond, it experiences an elliptical trajectory.
An Ocean Wave Energy Converter The novel ocean wave energy converter consists of an array of parallel Savonius rotors with elastic blades, which are arranged to form a plane and are mounted on tensioned axes in a rectangular frame. The diameter of the rotors is small compared to their length, and compared to the height of the waves. The rotors are made of rubber or plastic on a core of aluminium and rotate around tensioned axes of carbon fibres or coated steel. At the ends of each rotor sit small dynamos which transform the rotational movement of the rotors into electricity. In order to capture energy from waves the proposed converter must be positioned right beneath the water surface and oriented parallel to it.
Savonius rotor, (2) dynamo,
(3) frame, (4) anchor and
(5) ocean wave, (6) sea ground.
An Ocean Wave Energy Converter The basis of the wave converter is the omni-directional Savonius rotor with elastic blades shown below. Savonius rotors are driven by any local water flow that has a directional component perpendicular to their axis, no matter from which direction the water comes. Under the ocean waves there is an oscillating flow field that locally changes its direction all the time.
An Ocean Wave Energy Converter Using a multitude of small rotors instead of a big one has several advantages. Small rotors can be placed much closer to the water surface, where most of the wave energy is. An array of small rotors covering the same water flux as a big rotor requires much less material to harvest the same flow power.
The ocean wave energy converter can be installed floating offshore as well as be fixed to poles near the coast - invisibly submerged under the water surface. The figure below shows a floating converter and how it is anchored to the ground.
An Ocean Wave Energy Converter The Pelamis Wave Energy Converter (Ocean Power Delivery Ltd.) Pelamis ?prototype (Ocean Power Delivery Ltd.)
R&D ?The Way Forward Roadmap of R&D targets and associated events and activities One effective way of planning future R&D needs is by use of the Roadmap ? a diagram with a timeline, showing the main R&D targets and the associated events and activities, set against the timeline as a high-level plan. It displays the generic issues that must be addressed if wave power is to become commercially realisable in the next few years.