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Unformatted text preview: 11-1 Chapter 11 OCEAN ENERGY SYSTEMS 11.1 Introduction Oceans cover about 71% of earth surface. They receive, store and dissipate energy through various physical processes. There are three basic ways under active consideration to tap the oceans for their energy. These are: (i) using the ocean's high and low tides, (ii) using ocean's waves and (iii) using temperature differences between surface water and deep water. The first two resources exist in the form of mechanical energy while the third exists as thermal energy. Tidal energy is relatively more developed compared to the other two, which are still undergoing evaluation and initial stages of development. Main disadvantages common to all of them are: (i) low energy density and (ii) in general the potential occurs remote from the consumption center. 11.2 Tidal Energy The level of water in large oceans of the earth rises and falls according to a predictable pattern. Main periods of these tides is semidiurnal (twice each day) at about 12 h 25 min. The highest level of tidal water is known as flood tide orhigh tide. The lowest level is known as low tideor ebb. The level difference between two successive high and low tides is known as tidal range, R. Exploiting the potential energy of water caused due to level difference of tides is known as tidal range energy system. Tidal range varies significantly from one location to other due to local factors. Only sites with large tidal ranges (about 5 m or more) are considered suitable for power generation. The combined global potential at these sites is estimated as 120,000 MW. The movement of tidal water produces tidal currents, which may reach speed of 2-3 m/s in coastal and inter-island channels. The kinetic energy in the tidal currents may be harnessed in a manner similar to wind energy. This is called tidal stream energyor tidal current energy. Thus ocean tides are responsible for producing two types of energy resources: (i) tidal range energy (or simply tidal energy) and (ii) tidal current energy. Tidal energy is not a new concept. Tidal mills were in use on the coast of Spain, France, UK and China during medieval period, around 1100 AD. They remained in common use for many centuries, but were gradually replaced by more convenient and cheaper sources made available due to industrial revolution. The principle used for harnessing the energy consisted of a pond filled through sluice(rapid controlled gates) when tides are high and emptying it during low tides via an undershot waterwheel, producing mechanical power. Still same basic principle with improvements in the design, material and operating techniques is being used to generate electricity, in the same manner as in a hydroelectric plant. 11-2 The tidal behavior at most costal regions is well documented and analyzed because of the demand of navigation and oceanography. The nature may be predicted accurately within an uncertainty of 4%, and so the tidal energy presents a very reliable and assured form of renewable energy. so the tidal energy presents a very reliable and assured form of renewable energy....
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