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View the PDF document Design, fabrication, installation, and analysis of a closed cycle demonstration OTEC plant
Author:Faizal, Mohammed
Institution: University of the South Pacific.
Award: M.Sc.
Date: 2012
Call No.: pac In Process
BRN: 1188653
Copyright:20-40% of this thesis may be copied without the authors written permission

Abstract: Ocean water covers a vast portion of the earth’s surface and is also the world’s largest solar energy collector. It plays an important role in maintaining the global energy balance as well as in preventing the earth’s surface from continually heating up due to solar radiation. The ocean also plays an important role in driving the atmospheric processes. The heat exchange processes across the ocean surface are represented in an ocean thermal energy budget, which is important because the ocean stores and releases thermal energy. The solar energy absorbed by the ocean heats up the surface water, despite the loss of heat energy from the surface due to backradiation, evaporation, conduction and convection, and the seasonal change in the surface water temperature is less in the tropics. The cold water from the higher latitudes is carried by ocean currents along the ocean bottom from the poles towards the equator, displacing the lower density water above and creating a thermal structure with a large reservoir of warm water at the ocean surface and a large reservoir of cold water at the bottom, with a temperature difference of 22ºC to 25ºC between them. The available thermal energy, which is the almost constant temperature water at the beginning and end of the thermocline, in some areas of the oceans, is suitable to drive ocean thermal energy conversion (OTEC) plants. These plants are basically heat engines that use the temperature difference of the surface and deep ocean water to drive turbines to generate electricity. An overview of the heat energy budget of the ocean is presented taking into consideration all the major heat inputs and outputs. The theoretical analysis of the closed cycle OTEC system is also presented. Experimental studies were performed on a corrugated plate heat exchanger for small temperature difference applications. Experiments were performed on a single corrugation pattern on twenty plates arranged parallelly, with a total heat transfer area of 1.16298 m2. The spacing, X, between the plates was varied (X = 6 mm, 9 mm, and 12 mm) to experimentally determine the configuration that gives the optimum heat transfer. Water was used on both the hot and the cold channels with the flow being parallel and entering the heat exchanger from the bottom. The hot water flowrates were varied. The cold side flowrate and the hot and cold water inlet temperatures were kept constant. It is found that for a given X, the average heat transfer between the two liquids increases with increasing hot water flowrates. The corrugations on the plates enhance turbulence at higher velocities, which improves vi the heat transfer. The optimum heat transfer between the two streams is obtained for the minimum spacing of X = 6 mm. The pressure losses are found to increase with increasing flowrates. The overall heat transfer coefficients, U, the temperature difference of the two stream at outlet, and the thermal length are also presented for varying hot water flowrates and X. The findings from this work would enhance the current knowledge in plate heat exchangers for small temperature difference applications and also help in the validation of CFD codes. A closed cycle demonstration OTEC plant was designed, fabricated, and installed in the Thermo-fluids Lab, The University of the South Pacific. An experimental study was carried out on the demonstration plant with the help of temperature and pressure readings before and after each component. An increase in the warm water temperature increases the heat transfer between the warm water and the working fluid, thus increasing the working fluid temperature, pressure, and enthalpy before the turbine. The performance is better at larger flowrates of the working fluid and the warm water. It is found that the thermal efficiency and the power output of the system both increases with increasing operating temperature difference (difference of warm and cold water inlet temperature). The performance of the system improves with increasing pressure drop across the turbine. Increasing turbine inlet temperatures also increase the efficiency and the work done by the turbine. A maximum efficiency of about 1.5 % was achieved in the system.
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