Power Generation

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Power Generation Wave power generation plants are of either the fixed or floating types. Fixed generating devices are located along the shore or are fastened to the seabed and are generally simpler to maintain and operate. Floating devices are installed on floating platforms. Examples of fixed energy conversion systems are oscillating water columns, tapered channel systems, and underwater turbines. Examples of floating systems are the Archimedes Wave Swing and the Salter’s Duck. These devices work directly by activating a generator or pushing a working fluid (water or air) to drive a turbine and generator. No large-scale commercial wave power plants have been built yet, although major research is underway and several prototype systems have been built in Norway, Japan, India, and Scotland. Onshore Systems Most wave power machines use the Oscillating Water Column. They work by trapping air over the surface of water in a chamber that then moves a piston up and down. This can be achieved by the up and down motion of water in open seas, or by the back and forth motion of waves as they slam on the shoreline. In one design, waves coming toward the shore Figure 4-9 Schematic of an Oscillating Water Column. Image courtesy of Fujita Research Power of a Wave Mathematical Interlude ... The power delivered by a wave is obtained by dividing the total energy transported by its period: P = ρg2TH232π(v) In this equation: P is power of wave front [W/m] g is the local gravitational acceleration [m/s2] r is the water density (kg/m3) T is period (time it takes two successive crests or troughs pass the same point [s], and; H is the wave height (amplitude) [m] Example: A deep-water wave with a wavelength of 20 m and amplitude of 3 m travels at a speed of 5 m/s. Calculate the maximum power that can be exploited from such a wave. Solution: Taking water density r = 1030 kg/m3, g=9.81 m/s2, and a period of T = 20/5 = 4 s, we have: P = (1030)(9.81)2(4)(32)32π= 35,500 W/m = 35.5 kW/m of wave front (v) 75 Chapter 4 - Hydro Energy Figure 4-12 TapChan wave energy device. A demonstration plant has been installed off a remote Norwegian island and operating since 1985. Raised reservoir Converging inclined channel Return to the sea Cli� face Turbine House Figure 4-11 The Limpet 500. Image Courtesy of Wavegen Corporation 15 Islay LIMPET Wave Power Plant, Company website at http://www.wavegen.co.uk. 16 Japan Marine Science and Technology Center, http://www.jamstec.go.jp/jamstec/MTD/Whale/. push water through a channel and act as a pump to compress a column of air that is then passed through a turbine-generator system to produce electricity. As the waves recede, air is sucked back, causing the turbine to continue its operation (Figure 4-9). Wells turbines are the key to the successful operation of the oscillating wave column system (Figure 4-10). These turbines always turn in one direction, independent of the direction of flow. This feature allows them to provide continuous operation whether waves are moving toward the coastlines or moving away from them. The first operational wave power station, called LIMPET (for Land-Installed Marine-Powered Energy Transformer), was installed on the Scottish island of Islay and generates 500 kW of electric power (Figure 4-11).15 Another onshore wave technology device is the Tapered Channel, which consists of a tapered channel and a reservoir constructed on a cliff a few meters above sea level (Figure 4-12). Due to the narrowing of the channel, waves rise and water pours into and fills a reservoir. The reservoir provides the necessary head to run a turbine and generate power. Tapered channel systems are especially suitable during peak demands since they store energy in the reservoir until it is needed. Offshore Systems In an effort to develop practical viable floating wave power devices for offshore applications, a prototype floating-type version of the oscillating water column called the Mighty Whale was designed by the Japan Marine Science and Technology Center. The 50 x 50 m platform was anchored to the bottom of the sea near Japan, operated from 1998-2002, and produced about 110 kW of electricity (Figure 4-13).16 Mighty Whale also acts as a wave breaker to calm water for the fisheries. The Archimedes Wave Swing (AWS) consists of a number of air-filled chambers submerged below the sea surface and is connected by movable floats that oscillate up and down as waves move over them. A series of linkages convert the vertical oscillation of the platform into rotational motion, which is in turn used to generate electricity. A pilot project off Power from Underwater Currents Mathematical Interlude ... The power of a current follows the same formulation that was given for wind power: where: P is delivered power in watts, r is the density of seawater, d is the turbine diameter, and V is the current speed. For tidal currents close to the shoreline in estuaries is a sinusoidal function in time, and h is the turbine efficiency. P (vi) e = ρ d 2V 3η π8 Figure 4-10 The Wells Turbine continues to rotate in one direction even as the direction of flow reverses Photo courtesy of Wavegen Corporation (http://www.wavegen.co.uk) 76 the coast of Portugal is constructed and produces 8 MW of electricity. Salter’s Duck (Figure 4-14) operates on a similar principle with the exception that it is installed on a floating chamber connected to a fixed platform that swings up and down as a wave passes. The bobbing motion is converted to rotational motion (for example, using it to pump a hydraulic fluid through the blades of a turbine) which then runs a generator.17 Underwater turbines are similar to wind turbines, except that the kinetic energy in water is converted into rotational energy by underwater axial turbines (Figure 4-15). In contrast to wind velocity, currents in deep waters have relatively steady speeds, and therefore no energy storage system is necessary.


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