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Analysis of optimization in an OTEC plant using ORC

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Analysis of optimization in an OTEC plant using ORC ( analysis-optimization-an-otec-plant-using-orc )

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Renewable Energy 68 (2014) 25e34 Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene Analysis of optimization in an OTEC plant using organic Rankine cycle Min-Hsiung Yang a, *, Rong-Hua Yeh b a Department of Naval Architecture and Ocean Engineering, National Kaohsiung Marine University, Taiwan, ROC b Department of Marine Engineering, National Kaohsiung Marine University, Taiwan, ROC articleinfo abstract Article history: Received 29 August 2013 Accepted 26 January 2014 Available online 16 February 2014 Keywords: OTEC ORC Optimal Evaporation Condensation Performance 1. Introduction Saving energy and reducing carbon dioxide emissions have become increasingly critical aspects of energy usage because of concerns regarding energy shortage, global warming, and envi- ronmental pollution. Thus, researchers have extensively investi- gated approaches to use renewable and sustainable energy sources effectively. The ocean thermal energy conversion (OTEC) process uses the disparity in temperature between the warm seawater on the ocean surface and the deep cold seawater to operate a Rankine cycle system for producing electrical power without consuming fuel or emitting carbon [1e3]. Although using ocean thermal en- ergy has enormous potential and the OTEC plants have a small environment impact, the low net efficiency of OTEC resulting from the lower temperature differences between surface seawater and cold seawater restricts the implementation of this technology [4]. To improve the thermal efficiency of OTEC, suitable working fluids for use in the Rankine cycle must be identified. Ammonia, which is named R717, was used in the OTEC system widely in the past because of its excellent thermodynamic properties. Uehara * Corresponding author. No.142, Haizhuan Rd., Nanzi Dist., Kaohsiung City 81157, Taiwan, ROC. Tel.: þ886 7 3617141x3404; fax: þ886 7 3656481. E-mail address: mhyang@webmail.nkmu.edu.tw (M.-H. Yang). 0960-1481/$ e see front matter ! 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2014.01.029 This study quantified the effects of evaporation temperature, condensation temperature, and the inlet- and outlet-temperature differences of deep cold seawater and warm seawater on the performance of an ocean thermal energy conversion (OTEC) plant using an organic Rankine cycle (ORC), and also investi- gated the optimal operations required for the performance. A finite-temperature-difference heat transfer method is developed to evaluate the objective parameter, which is the ratio of net power output to the total heat transfer area of heat exchanger in the system, and R717, R600a, R245fa, R152a, and R134a were used as the working fluids. The optimal evaporation and condensation temperatures were obtained under various conditions for maximal objective parameters in an OTEC system. The results show that R717 performed optimally in objective parameter evaluation among the five working fluids, and that R600a performed better than other fluids in thermal efficiency analysis. The optimal seawater temperature differences between the inlet and outlet of the evaporator and condenser are proposed. Furthermore, the influences of inlet temperatures of warm and cold seawater in the ORC are presented for an OTEC plant. The simulation results should enable the performance of an ORC system to be compared when using various organic working fluids. ! 2014 Elsevier Ltd. All rights reserved. et al. [5,6] investigated the major components of an OTEC plant theoretically and experimentally. In their simulation results, the temperatures of warm and cold seawater were 26 C and 4 C, respectively, for optimizing a 100-MW OTEC system. Uehara et al. also reported that R717 was one of the suitable working fluids for a closed-Rankine-cycle OTEC plant. Using R717 as the working fluid, Yeh et al. [7] studied theoretically the effects of temperature and flow rate of cold seawater on the net output of an OTEC plant. They concluded that the network has a maximal output at a specific cold seawater flow rate. To improve efficiency and to reduce system costs, the organic Rankine cycle (ORC) is used with working liquids that have a low boiling temperature. To identify suitable working fluids for an ORC, Chen et al. [8] and Wang et al. [9] investigated the thermodynamic performances of ORC by using various working fluids to convert low-grade heat. Sun et al. [10] optimized numerically the design for an ORC in OTEC with R717 and R134a. The exergy-analysis mode was used to evaluate the maximal net output under various warm water mass-flow rates and evaporation temperatures. However, in the calculation, the overall heat transfer coefficient remained un- changed and the work consumed in pumping seawater was neglected. From the first law of thermodynamics, a high heat- source temperature would increase the pressure and enthalpy of the working fluid at the turbine inlet. To increase the thermal ef- ficiency of the OTEC system, solar heat energy [11,12] and the waste

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