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The Oscillating Water Column AC 2008-1054: AN OCEAN ENERGY PROJECT: THE OSCILLATING WATER COLUMN Craig Somerton, Michigan State University CRAIG W. SOMERTON Craig W. Somerton is an Associate Professor and Associate Chair of the Undergraduate Program for Mechanical Engineering at Michigan State University. He teaches in the area of thermal engineering including thermodynamics, heat transfer, and thermal design. He also teaches the capstone design course for the department. Dr. Somerton has research interests in computer design of thermal systems, transport phenomena in porous media, and application of continuous quality improvement principles to engineering education. He received his B.S. in 1976, his M.S. in 1979, and his Ph.D. in 1982, all in engineering from UCLA. Page 13.197.1 Page © American Society for Engineering Education, 2008 An Ocean Energy Project: The Oscillating Water Column Introduction Though an important alternative energy source, the topic of ocean energy in an alternative energy class often receives less attention than some of the more popular alternative energy technologies such as solar, wind, fuel cells, and biofuels. To address this concern in the alternative energy course taught at Michigan State University, an ocean energy project is assigned. This paper presents one of those projects, the oscillating water column (OWC). ME 417 Design of Alternative Energy Systems is a senior level design intensive elective course [1]. It is a project based course for which the students carry out three technical projects. In its last offering the projects were: design of a fuel cell system, design of a wave energy system, and design of a solar energy system. The focus of the course is for students to use simple engineering principles in developing predictive models for alternative energy systems. Some of the projects require students to develop their own calculation tool (a spreadsheet or MATLAB program), while other projects use commercially available software, for example spreadsheet programs from RETScreen International [2], which is managed under the leadership and ongoing financial support of Natural Resources Canada’s (NRCan) CANMET Energy Technology Centre - Varennes (CETC-Varennes). However, for some topics in the course, such as ocean energy, software is not readily available. The OWC project presented in this paper uses an in-house MATLAB program that allows the students to perform design studies with respect to power production and energy costs for several design parameters, including location and size. This paper continues with an overview on ocean energy, along with details of the various wave energy systems. The particulars on the thermodynamic models, the project statement, and some typical design analysis results are then be presented. Next, student feedback on the project will be reviewed. Lessons learned conclude the paper Ocean Energy Oceans cover approximately 71% of the earth’s surface [3] and are the largest energy reservoir on the earth’s surface. The United States has a general coastline length of 12,000 miles [4], so there is adequate opportunity to interact with these vast energy reservoirs. One may argue that there are four different forms of ocean energy: tidal energy, current energy, thermal gradient energy, and wave energy. The first three will be briefly reviewed with wave energy being the focus of this section. Tides are created by the gravitational pull of the moon and the sun. Coastal tide heights can very significantly due to local geographical features such as bays and inlets. Current technology requires a five meter difference between high and low tides for the production of electricity [5]. It is estimated that worldwide there are about 40 such sites. Locations for some of the sites with the highest tides is provided in Table 1 Page 13.197.2 Page Table 1 World’s Highest Tides [6] Location Difference between low tide and high tide Bay of Saint-Malo, France 12 m Bay of Fundy, Canada 15 m Puerto Gallegos, Argentina 13.2m Mont Saint Michel, France 12.3m Bristol, United Kingdom 14.6m Bhaunagan, India 12.2m Derby, Australia 11.8 m Cook Inlet, Alaska 11.5 m Two main technologies have been proposed and implemented to harness tidal energy. Dams and barrages capture sea water in a reservoir during the rise to high tide. The reservoir sea water is then released through water turbines as the tide falls. Hence, they function much like a conventional hydroelectric inland dam system. The largest tidal power station in the world (and the only one in Europe) is in the Rance estuary in northern France, built in 1966 [7]. It has a peak rating of 240 Megawatts for its 24 turbines and an annual output of about 600 million kWh resulting in an average power 68 MW. Since 1984, Nova Scotia Power has been generating up to 20 megawatts of power at a tidal barrage in the Bay of Fundy [8]. Tidal turbines operate underwater much in the same way as wind turbine farms. In December 2006, Verdant Power installed two underwater turbines in New York’s East River [9]. Each turbine can capture up to 35 kilowatts of power from the river’s tidal currents. Since the initial installation, four additional turbines have been added. Ocean current energy is analogous to wind energy and, similar to wind energy, is also due to the different rates of natural convection in the ocean arising through different solar heating rates because of the distance from the sun. A typical ocean current map is shown in Figure 1. Figure 1 Ocean Current Map of the Northwest Atlantic Ocean [10] / Page 13.197.3 Page The energy of ocean currents may also be harnessed with the use of underwater turbines that have an advantage over tidal turbines due to more consistent and less periodic flow rates. Thermal energy in the ocean comes from the absorption of solar radiation. This solar energy inflow will heat the surface of the ocean, but does not penetrate very deep, and hence gives rise to a temperature gradient between the water on the surface and that at depth. Very simply, the ocean may be considered as having two heat reservoirs, a high temperature heat reservoir at the surface and a low temperature heat reservoir at some depth. These heat reservoirs can be utilized to run a heat engine operating between them, so as to produce power. This approach is called Ocean Thermal Energy Conversion (OTEC). It is schematically shown in the Fig. 2 Ocean Surface (High Temperature Heat Reservoir) at T H QH Heat Engine Wnet QL Ocean Depth (Low Temperature Heat Reservoir) at T L Figure 2 OTEC Interaction Diagram The potential of OTEC can be seen in the map of Fig. 3 that indicates the temperature difference available in various locations. Figure 3 OTEC Potential [11] Page 13.197.4 Page Waves are primarily produced by the wind, so wave energy is actually a version of solar energy. Figure 4 shows worldwide wave power in terms of kW per meter of wave width. Figure 4: Wave Energy Potential [12] Three strategies have been developed in an attempt to harness wave energy. An onshore reservoir approach is used in the tapered channel system that accelerates the incoming wave by decreasing the flow area, so that it can fill an elevated reservoir. The reservoir water is then released back to sea level, passing through a turbine to extract energy. Mechanical surface systems use the wave motion to move a solid object. The simplest would be a buoy system that rises and falls with the waves and whose tethers turn a generator. The third strategy is the oscillating water column. An oscillating water column system behaves very much like a piston- cylinder system, with the ocean surface acting as the piston, which will compress or expand the air or sea water within the cylinder (or column) as waves rise and fall. The modeling of the oscillating water column system is discussed in detail below. Modeling of Oscillating Water Column For the course project, a series of simple thermodynamic models are used to simulate the operation of the oscillating water column. A two chamber model is used. As shown in Fig. 5, when a wave rises up through the open chamber, its air is compressed and forced through a turbine into the closed chamber, pressurizing it. When the wave falls, the pressure in the open chamber reduces allowing the closed chamber air to return to the open chamber. A five process thermodynamic model is used to simulate this operation. Process 1 Polytropic Compression of Open Chamber as a Closed System Process 2 Isentropic Expansion through the Turbine from Open to Closed Chamber Isotropic, Adiabatic Flow from the Open Chamber as a Transient System Isotropic, Adiabatic Flow into the Closed Chamber as a Transient System Process 3 Polytropic Expansion of Open Chamber as a Closed System Process 4 Isentropic Expansion through the Turbine from Closed to Open Chamber Isotropic, Adiabatic Flow into the Open Chamber as a Transient System Isotropic, Adiabatic Flow from the Closed Chamber as a Transient System Process 5 Venting of Closed and Open Chambers to Atmosphere Page 13.197.5 Page Open Closed Chamber Chamber Turbine Turbine Figure 5 Operation of a Two Chamber Oscillating Water Column A MATLAB program has been developed to facilitate the required calculations and produces performances results such as power produced and efficiency. The monthly wave data for five different coastal locations have been incorporated into the MATLAB program. An economic analysis has been built into the MATLAB program, so that an energy cost ($/kW hr) is calculated and can be used as the objective function in a design optimization. The MATLAB program is available for download at http://www.egr.msu.edu/classes/me417/somerton/#projects The energy cost is calculated by taking the annual energy production predicted by the thermodynamic model (at the coastal location specified) and dividing it by the annual cost of the system.
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