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Installation in SwedenOTEC Power Plant in HawaiiSustainable Energy Science and Engineering CenterReference: Renewable Energy by Godfrey Boyle, Oxford University Press, 2004.Ocean EnergySustainable Energy Science and Engineering CenterOcean EnergyOceans cover most of the (70%) of the earth’s surface and they generate thermal energy from the sun and produce mechanical energy from the tides and waves. The solar energy that is stored in the upper layers of the tropical ocean, if harnessed can provide electricity in large enough quantities to make it a viable energy source. World ocean temperature difference at a depth of 1000 mThis energy source is available throughout the equatorial zone around the world or about 20 degrees north and south of the equator - where most of the world's population lives.Sustainable Energy Science and Engineering CenterTidesThe ocean tides are caused by the gravitational forces from the moon and the sun and the centrifugal forces on the rotating earth. These forces tend to raise the sea level both on the side of the earth facing the moon and on the opposite side. The result is a cyclic variation between flood (high) and ebb (low) tides with a period of 12 hours and 25 minutes or half a lunar day. Additionally, there are other cyclic variations caused by by the combined effect of the moon and the sun. The most important ones are the 14 days spring tide period between high flood tides and the half year period between extreme annual spring tides. Low flood tides follow similar cycles. The ocean bottom topography has pronounced effect on the local tides. The tides are accurately predictable.Sustainable Energy Science and Engineering CenterTidal Power Potential The world potential for tidal power exceeds 450 TWhIn the open ocean, the maximum amplitude of the tides is about one meter. Tidal amplitudes are increased substantially towards the coast, particularly in estuaries. This is mainly caused by shelving of the sea bed and funneling of the water by estuaries. In some cases the tidal range can be further amplified by reflection of the tidal wave by the coastline or resonance. This is a special effect that occurs in long, trumpet-shaped estuaries, when the length of the estuary is close to one quarter of the tidal wave length. These effects combine to give a mean spring tidal range of over 11 m in the Severn Estuary (UK). Tidal energy is highly predictable in both amount and timing. The available energy is approximately proportional to the square of the tidal range. Extraction of energy from the tides is considered to be practical only at those sites where the energy is concentrated in the form of large tides and the geography provides suitable sites for tidal plant construction. Such sites are not commonplace but a considerable number have been identified in the UK, France, eastern Canada, the Pacific coast of Russia, Korea, China, Mexico and Chile. Other sites have beenidentified along the Patagonian coast of Argentina, Western Australia and western India.Sustainable Energy Science and Engineering CenterTidal PowerSustainable Energy Science and Engineering CenterTidal EnergyTide mills were in use as far back as 1100 AD on the coasts of Spain, France and the UK. They consisted of a pond filled through a sluice during the flood or high tide and emptied on the ebb or low tide through an undershot water wheel.Today, a tide mill consists of a semi-permeable barrage built across an estuary, allowing flood waters to fill a basin via a series of sluices. At high water the sluice gates are closed, creating a head of water on the low tide. Electricity is generated by releasing water through a series of conventional water turbines. The La Rance tidal power station generates 240 MW power. The tides are caused by the Moon and the Sun moving around the Earth and so in many places the tides happen twice a day without fail. This means that it is a more reliable way of generating electricity than using wind or sunshine. Generating electricity this way doesn't produce any greenhouse gases that cause climate change.Sustainable Energy Science and Engineering CenterLa Rance Barrage24 Reversible pump turbinesMaximum tidal range : 12 mTypical water head = 5 mNet output = 480 GWh per yearTypical electricity cost: 10 - 15 ¢ per kWhSustainable Energy Science and Engineering CenterTidal Energy TypesTypes of tidal energy: The potential energy of sea level differences associated with the tidesDams close off sea basins at flood or high tide and low head turbines through which the trapped water are released at low tide.The kinetic energy of the tidal currentsWater mills submerged in the tidal stream - tidal stream turbinesThe fluid power of the flow is given by: P =12ρV3AW/m21025 kg/m3Area swept by turbineTidal turbineSustainable Energy Science and Engineering CenterTidal PowerConsider a rectangular basin with a constant surface area of A. Tidal range: RTotal volume of basin water = ARMass = ρARPotential energy available = ρArg (R/2)Tidal Period = TAverage potential energy extracted = ρAR2g/2TThe center of gravity for the mass of water will be at R/2 above the lower tide level.Sustainable Energy Science and Engineering CenterSihwa Tidal Power PlantSustainable Energy Science and Engineering CenterWave EnergyWaves, particularly those of large amplitude, contain large amounts of energy. Wave energy is in effect a stored and concentrated form of solar energy, since the winds that produce waves are caused by pressure differences in the atmosphere arising from solar heating. The strong winds blowing across the oceans create large waves, making many coastal regions around the globe ideally suited to wave energy schemes. The global wave power resource is estimated to be about 2 TW with electricity generation potential of about 2000 TWh annually.Air flowing over the sea exerts a tangential stress on the water surface resulting in the formation and growth of wavesTurbulent air flow close to the water surface creates rapidly varying shear stresses causing ripples, known as capillary waves. Capillary waves create more water surface increasing the friction between water and wind. This adds more energy, which increases the size of the waves, making them larger and larger.When the winds slow down or stop, the waves continue their journey, gradually but very slowly losing their energy. Waves may travel thousands of km before rolling ashore. This predictability of waves is one of the


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FSU EML 4450 - EML 4450L22 Lecture Notes

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