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Analysis of the Effect of Solar Radiation on OTEC

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International Journal of Renewable Energy and Environmental Engineering ISSN 2348-0157, Vol. 02, No. 02, April 2014 Analysis of the Effect of Solar Radiation on Ocean Thermal Energy Conversion (OTEC) in Vietnam NGUYEN VAN HAP1, 2 GEUN SIK LEE1 and NGUYEN MINH PHU2 1School of Mechanical Engineering, University of Ulsan, 93 Daehak-ro. Nam-gu, Korea 2Faculty of Mechanical Engineering, Ho Chi Minh City University of Technology, Vietnam Email: gslee@ulsan.ac.kr Abstract: Ocean thermal energy conversion (OTEC) can be selected as one of the methods to generate electricity from renewable energy and to protect the environment. OTEC produces power from small temperature difference between the warm surface seawater and the cold deep seawater. In this study, simulations were performed to investigate the effect of weather conditions in the Nha Trang region, Vietnam on the efficiency of a solar-heating OTEC (SH-OTEC) system. This system utilizes solar thermal energy as the secondary heat source. Various working fluids are also simulated to select one that is suitable for this system. The results showed that R152A, R600, and R600A, in that order, were the most suitable working fluids. The required effective area of solar collector for 20oC increase in the collector outlet temperature fluctuates from 42.3 to 82.2m2 owing to change in monthly average solar gain. The annual average efficiency of SH-OTEC increases to 8.2%, compared to that of a typical conventional OTEC, which is 2–4%. Keywords: OTEC, Solar energy, working fluids, Organic Rankine Cycle, Sustainable Energy 1. Introduction: The majority of the world’s electricity is generated by burning fossil fuels as coal, natural gas, and oil which are non-renewable resources. The emissions produced in the combustion process play a major role in making the greenhouse effect leading to the global warming. Besides, the earth’s fossil energy resources are being depleted. After the Fukushima nuclear accident in Japan in 2011, Ocean thermal energy has been considered as one of promising ways to generate clean power to supply electricity with non- CO2 emission. The OTEC is a power generation way that generates electricity by using the temperature difference between the surface seawater and deep seawater. In the ocean, the temperature changes from 20-300C at surface to 3-100C at deep sea. Since this available temperature difference is small, the efficiency of the OTEC is also limited. In order to improve the performance of the OTEC system, the secondary heat sources are added by means of solar radiation, waste heat from factories, and so on. The first Mini-OTEC was built in Hawaii, USA in 1979 which generated more than 50kW of gross power. Yeh. et al. [1] studied the effects of the temperature and flow rate of cold seawater on the net efficiency of an OTEC system. Tong et al. [2] studied performance improvement of the closed cycle of OTEC. They used solar energy as added heat to determine a suitable working fluid and found out that the net output power of the system appropriate for the industrial application should be at least of 50kW. Yamada et al. [3] carried out simulation for the effects of solar radiation on the OTEC system in Japan. Straatman and Van Sart [4] described the concept of a unique OTEC system combined with an offshore solar pond which is called hybrid system (OTEC-OSP). Most of the above mentioned studies have focused on discussing the organic Rankine cycle. They fixed the parameters of the added heat. In this study, we carried out the simulations to determine the effects of weather conditions on the efficiency of SH-OTEC in the Nha Trang region, Vietnam by using TRNSYS software ver.16 and EES software ver.6.8. Moreover, we also focused on selecting most suitable working fluid for the SH- OTEC. 2. System description: The system layout and corresponding T-s diagram of a Solar-Heating Ocean Thermal Energy Conversion (SH-OTEC) are depicted on Fig. 1 and 2. As can be seen from Fig.1, the SH-OTEC system consists of a turbine, an evaporator, a condenser, a collector, a working fluid pump, and two seawater pumps. The liquid working fluid in the evaporator is heated and vaporized by the warm seawater from the collector system. The vapor flows into the turbine and its energy is converted into work. The low pressure flow exiting from the turbine is condensed to be liquid state in the condenser. The waste heat from condensing process is discharged to the environment through the cold seawater. The pump supplies the working fluid to the evaporator and makes it possible to perform a thermodynamic cycle of Rankine cycle. The organic Rankine cycle (ORC) adopted in this SH-OTEC is assumed a steady saturated cycle. The heat and work of each component of SH-OTEC can be expressed as follows: The power produced by the turbine is Wt mfh3h4stG (1) IJREEE 020208 Copyright © 2014 BASHA RESEARCH CENTRE. All rights reserved

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