lable at ScienceDirect Applied Thermal Engineering xxx (2014) 1e5 Contents lists avai Applied Thermal Engineering journal homepage: www.elsevier .com/locate/apthermeng Performance characteristics of a high-efficiency R717 OTEC power cycle Jung-In Yoon a, Chang-Hyo Son a, *, Seung-Moon Baek a, Byung Hyo Ye a, Hyeon-Ju Kim b, Ho-Saeng Lee b a Department of Refrigeration and Air-Conditioning Engineering, College of Engineering, Pukyong National University, San 100, Yongdang-dong, Nam-gu, Pusan 608-739, South Korea b Deep Ocean Water Application Research Center, Korea Ocean Research & Development Institute, 245-7 Oho-ri, Jukwang-myeon, Goseong-gun, Gangwon-do 219-822, South Korea h i g h l i g h t s � The efficiency of Uehara, Kalina and this OTEC cycle is 2.379, 2.361 and 2.401, respectively. � The proposed OTEC cycle is the highest efficiency among these cycles. � The proposed OTEC power cycle requires the optimal control of the system. a r t i c l e i n f o Article history: Received 25 November 2013 Accepted 29 May 2014 Available online xxx Keywords: R717 (ammonia) OTEC power cycle Expansion valve Cooler Performance analysis * Corresponding author. Tel.: þ82 51 629 6802; fax E-mail address:
[email protected] (C.-H. Son). http://dx.doi.org/10.1016/j.applthermaleng.2014.05.10 1359-4311/© 2014 Elsevier Ltd. All rights reserved. Please cite this article in press as: J.-I. Yoon Engineering (2014), http://dx.doi.org/10.101 a b s t r a c t This paper proposes a high-efficiency R717 Ocean Thermal Energy Conversion (OTEC) power cycle with an expansion valve and a cooler. To provide the basic design data for the operating variables of this high- efficiency R717 OTEC power cycle, the cycle performance was analyzed in terms of the evaporation heat capacity, condensation heat capacity, total work and efficiency using the HYSYS program. The operating variables considered in this study included the high-stage turbine outlet pressure, the expansion valve outlet pressure, the turbine efficiency, the pressure drop of the evaporator and the cooler outlet vapor quality of the R717 OTEC power cycle. The main results can be summarized as follows. The high- efficiency R717 OTEC power cycle was most affected by the turbine efficiency and the cooler outlet vapor quality. Therefore, these effects must be considered when the R717 OTEC power cycle is designed. Furthermore, the actual application of the proposed OTEC power cycle requires the development of a highly efficient turbine, the optimal control of the cooler outlet vapor quality and the optimization of the system. The efficiency of the proposed OTEC system is compared with that of Uehara and Kalina one. The efficiency of Uehara, Kalina and proposed OTEC system is 2.379, 2.361 and 2.401, respectively. This OTEC system is the highest efficiency among other systems. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Ocean Thermal Energy Conversion (OTEC) refers to the acqui- sition of power from a turbine by applying the temperature dif- ference between the high-temperature surface water and the low- temperature deep water to the conventional steam power cycle, and the production of electricity by using this power to drive a generator [1,2]. This OTEC power generation involves the production of elec- tricity by using the temperature difference between the high- : þ82 51 611 6368. 3 , et al., Performance characte 6/j.applthermaleng.2014.05.1 temperature surface water (25 �C) and the low-temperature deep water (5 �C). It is an excellent resource because it uses clean harmless ocean energy, is a stable energy source that allows power production during the day and night and enables premeditated power generation by considering seasonal changes in advance [3]. However, the cycle efficiency is very low because the temperature difference between surface water and deep water is small. Various studies [4e10] have been performed to complement the low efficiency of the OTEC power generation cycle. To improve its efficiency, the installations of a regenerator, a reheater, a multi- stage turbine, etc. to the cycle or increasing the temperature of the surface water through solar heat or the heated effluent from a power plant have been suggested. To examine the related previous ristics of a high-efficiency R717 OTEC power cycle, Applied Thermal 03 mailto:
[email protected] www.sciencedirect.com/science/journal/13594311 http://www.elsevier.com/locate/apthermeng http://dx.doi.org/10.1016/j.applthermaleng.2014.05.103 http://dx.doi.org/10.1016/j.applthermaleng.2014.05.103 http://dx.doi.org/10.1016/j.applthermaleng.2014.05.103 Nomenclature fp compression ratio, [/] h specific enthalpy, [kJ/kg] m mass flow rate, [kg/s] P pressure, [kPa] Q heat capacity, [kW] T temperature, [�C] W power, work, [kW] x vapor quality, [/] h thermal efficiency, [/] DP pressure drop, [kPa] DT temperature difference, [�C] Subscripts c condenser, cooler ds deep seawater e evaporator, evaporating ex expansion H high-stage in inlet L low-stage m middle-stage out outlet p pump r refrigerant s seawater ss surface seawater t turbine v valve J.-I. Yoon et al. / Applied Thermal Engineering xxx (2014) 1e52 studies, Kalina [8] proposed a high-efficiency OTEC power cycle that can decrease the average temperature difference inside the heat exchanger (the evaporator and the condenser) using an ammonia-water mixture as the working fluid and improve the cycle efficiency through a reheater and a gaseliquid separator. Furthermore, Uehara et al. [11] proposed a high-efficiency OTEC power cycle that improves efficiency by installing a reheater and a multi-stage turbine. It is similar to the existing Kalina power cycle because it uses an ammonia-water mixture as the working fluid. Kim et al. [7] used the heated effluent of a power plant to improve the efficiency of the OTEC power cycle and comparatively analyzed the performance characteristics of the cycle. As a result, the effi- ciency of the OTEC power cycle could be improved using the heated effluent of a power plant instead of the surface water of the ocean, and the efficiency of the OTEC power cycle with a regenerator was higher than that of the Kalina cycle. To summarize the results of these previous studies, the existing high-efficiency OTEC power cycles showed a low efficiency of approximately 4e6%, and the heated effluent of the power plant was used to complement this shortcoming. The Freon refrigerant that has been applied to the OTEC power generation systems can no longer be used due to global warming and ozone layer destruction. Therefore, new refrigerants that can replace it must be developed and applied. One refrigerant that is being cited as capable of solving the global environmental problem with the Freon refrigerant is R717 (ammonia) [11]. From the before mentioned references about Uehara and Kalian, the feature of a new high-efficiency R717 OTEC system is as follows. The main difference is the used refrigerant. Mixed refrigerant is used at Uehara and Kalina for their study but for this proposed system, pure refrigerant is used. Because of this feature, the structure of system is different respectively. Therefore, this study was conducted to simulate a proposed high-efficiency R717 OTEC power generation system with an expansion valve and a cooler using the HYSYS [12] program, and to provide the basic design data for the OTEC power generation system. Fig. 1. Schematic of the proposed R717 OTEC power cycle. 2. System modeling 2.1. System prescription Fig. 1 shows the schematic of the high-efficiency R717 OTEC power cycle that is proposed in this study, and Fig. 2 shows the state Please cite this article in press as: J.-I. Yoon, et al., Performance characte Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.05.1 points of this OTEC power cycle on a Peh diagram. As shown in Fig. 1, this OTEC cycle consists of two turbines, one pump, four heat exchangers, one gaseliquid separator and one expansion valve. For the working fluid, R717 (ammonia) with a high latent heat of vaporization is used. Furthermore, the high-temperature surface seawater and the low-temperature deep seawater are used for the evaporation and condensation of the working fluid that circulates inside the evaporator and the condenser, respectively. To examine the flow of the working fluid that circulates inside this OTEC power cycle, the working fluid with a high temperature and pressure that evaporated from the evaporator enters the high- stage turbine and produces storedwork around it by expanding to a medium pressure before cooling while passing through the regenerator and the cooler. The cooled working fluid (point 5) is separated into vapor (point 6) and liquid (point 11) in the vaporeliquid.separator. The sepa- rated liquid (point 11) expands through the expansion valve and cools the working fluid (point 4) inside the cooler by becoming a low-temperature and low-pressure working fluid (point 12). The separated gas (point 6) enters the low-stage turbine and produces work output before mixing with the working fluid (point 13) that exited the cooler, and enters the condenser. Then the working fluid (point 8) is condensed by exchanging heat with the low- temperature deep water. The fluid (point 9) leaving the ristics of a high-efficiency R717 OTEC power cycle, Applied Thermal 03 Fig. 2. Peh diagram of the high-efficiency R717 OTEC power cycle that is proposed in this study. Table 1 Analysis range of the high-efficiency R717 OTEC power cycle. Variable Value Refrigerant R717 (ammonia) Pe,in [kPa] 915 Pc,in [kPa] 535 DPe [kPa] 5a, 15, 25, 35, 45 DPc [kPa] 5a, 15, 25, 35 m [kg/h] 1560 ht [/] 0.75, 0.85a, 0.95, 0.99 hp [/] 0.65, 0.75, 0.85a, 0.95 Tss,in [�C] 25a Tds,in [�C] 4a mss [kg/h] 232,300 mds [kg/h] 221,900 xout [/] 0.95a, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1 Tss,ineTe,out [�C] 2a Tds,ineTc,out[�C] 2a a Reference value. Fig. 3. Performance characteristics of the high-efficiency R717 OTEC power cycle with high turbine outlet pressures. J.-I. Yoon et al. / Applied Thermal Engineering xxx (2014) 1e5 3 evaporator is pressurized in the pump and enters a regenerator where its temperature increases by exchanging heat with the high temperature working fluid (point 3) exiting the turbine 1 (high- stage turbine). The working fluid (point 1) enters the evaporator where its temperature increases by exchanging heat with surface seawater. The superheated vapor (point 2) expands in turbine 1. The vapor (point 3) leaving turbine 1 enters the regenerator, and is cooled by cold liquid working fluid from the pump (point 10). The working fluid (point 4) enters the cooler to be cooled by cold stream fluid (point 12) from the expansion valve. The fluid (point 5) returns to the vaporeliquid separator to complete the OTEC cycle. As stated, the OTEC power cycle that is proposed in this study reduces the discharged heat in the condenser by adopting a cooler and a gaseliquid separator, decreases the absorbed heat using a regenerator and increases the output work and the system effi- ciency through a two-stage turbine with high and low stages. 2.2. Mathematical analysis model For the thermodynamic properties of the R717 refrigerant and the equation of state that is required for the analysis of the per- formance of this proposed OTEC power cycle, the Lee-Kesler- Plocker and Peng-Robinson equations were used from among the various equations of state provided by the HYSYS program. The equations required for the analysis of the performance of the OTEC power cycle for R717 are described as follows. First, the efficiency of this OTEC power system is calculated with Equation (1): h ¼ W Qe (1) wherein Qe is the evaporation heat capacity andW is the total work, which is the high- and low-stage turbine work (Wt) minus the pump work (Wp): W ¼ Wt �Wp (2) Table 1 shows the range of the analysis of the performance of the OTEC power system that was used in this study. The perfor- mance analysis range in Table 1 spans the general operation con- ditions of the R717 OTEC power system. Using the results of the calculation from the analysis conditions in Table 1, such factors as Please cite this article in press as: J.-I. Yoon, et al., Performance characte Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.05.1 the high-stage turbine outlet pressure, the expansion valve outlet pressure, the turbine and pump efficiency, the pressure drop of the evaporator and condenser and the cooler outlet vapor quality, which affect the evaporation heat capacity, the condensation heat capacity, the turbine work and the total work, were examined. 3. Results and discussion 3.1. Types and characteristics of working fluids 3.1.1. Effects of the high-stage turbine outlet pressure To find the medium pressure (the high-stage turbine outlet pressure) at which the efficiency of the proposed high-efficiency OTEC power cycle peaks, the changes in the evaporation heat ca- pacity, the condensation heat capacity and the total work were examined while the outlet pressure (PH,t,out) of the high-stage tur- bine were examined. In other words, the following equation was used to find the outlet pressure of the high-stage turbine (PH,t,out ¼ Pm) at which the efficiency of the OTEC power cycle peaked while the compression ratios of the high-stage and low- stage turbines were varied: Pm ¼ Plow þ fp � Phigh � Plow � (3) Consequently, as shown in Fig. 3, as the outlet pressure of the high-stage turbine decreased, the system efficiency peaked and then decreased again. This is because as shown in Fig. 2, as the ristics of a high-efficiency R717 OTEC power cycle, Applied Thermal 03 Fig. 5. Performance characteristics of the high-efficiency R717 OTEC power cycle with pressure drops in the evaporator. J.-I. Yoon et al. / Applied Thermal Engineering xxx (2014) 1e54 outlet pressure of the high-stage turbine (Pm) decreases, the high- stage turbine work increases but the low-stage turbine work de- creases. Therefore, the existence of a turbine outlet pressure that shows the highest efficiency was confirmed. That is, the high- efficiency OTEC power cycle has the highest efficiency of 4.29% at the high-stage turbine outlet pressure of 650 kPa. Furthermore, as shown in Fig. 3, as the high-stage turbine outlet pressure increases, the refrigerant flow becomes constant but the evaporation heat capacity increases because of the difference in the enthalpy between the inlet and the outlet of the evaporator. In addition, as the high-stage turbine outlet pressure increases, the condensation heat capacity increases because the enthalpy of the evaporator inlet increases. The total work of the high-efficiency OTEC power cycle increases because the high-stage turbine work increases. 3.1.2. Effect of the expansion valve outlet pressure Fig. 4 depicts the evaporation heat capacity, the condensation heat capacity, the total work and the efficiency of the R717 OTEC power cycle with respect to the changes in the outlet pressure of the expansion valve (Pex,v,out). In Fig. 4, as the expansion valve outlet pressure increases, the evaporation and condensation heat capacity values become con- stant. This is because the expansion valve outlet pressure does not affect the evaporator or the condenser. Furthermore, the total work of the cycle decreases because the low-stage turbine work decreases. As the expansion valve outlet pressure increases, the system efficiency decreases. This is because as the expansion valve outlet pressure decreases, the total work decreases but the evaporation heat capacity and the condensation heat capacity are constant. Therefore, the system efficiency peaks at 4.28%when the expansion valve exit pressure of this cycle is lowest at 540 kPa. 3.1.3. Effect of the evaporator pressure drop Fig. 5 represents the evaporation heat capacity, the condensa- tion heat capacity, the total work and the efficiency of the R717 OTEC power cycle according to the changes in the pressure drop (DPe) of the refrigerant in the evaporator in the analysis range in Table 1. As shown in Fig. 5, even though the pressure drop in the evaporator increased, the evaporation and condensation heat ca- pacity did not change. This is because the pressure drop of the evaporator or the condenser does not affect the enthalpy of the inlet and the outlet of the evaporator or the condenser. Further- more, the increase in the refrigerant pressure drop in the evapo- rator decreases the turbine work, so the total work also decreases. Fig. 4. Performance characteristics of the high-efficiency R717 OTEC power cycle with the expansion valve outlet pressures. Please cite this article in press as: J.-I. Yoon, et al., Performance characte Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.05.1 From these results, it follows that the total efficiency of the high- efficiency OTEC power cycle decreases. 3.1.4. Effects of the turbine efficiency Fig. 6 displays the evaporation heat capacity, the condensation heat capacity, the total work and efficiency of the R717 OTEC power cycle with respect to the changes in the turbine efficiency (ht). As shown in Fig. 6, as the turbine efficiency increased, the high- and low-stage turbine works increased, which increased the total work. Furthermore, the evaporation heat capacity became constant because it is unrelated to the turbine efficiency. The condensation heat capacity decreases as the turbine efficiency increases because the condenser inlet enthalpy decreases. Therefore, the total effi- ciency of the OTEC power system increases. 3.1.5. Effect of the cooler outlet vapor quality The efficiency of the new efficient OTEC power cycle proposed in this study was compared with that of Uehara and Kalina cycle using HYSYS software at the same saturation temperature of evaporator and condenser inlet, respectively, as shown in Table 2. Thus, Table 2 depicts the comparison conditions of each cycle. From the com- parison results in Table 2, the power of three cycles is 20 kWequally and the evaporation capacity, the mass flow rate of refrigerant and seawater of the proposed OTEC cycle are lowest among three cycles. So, the efficiency of the proposed OTEC cycle is the highest among three cycles. (Fig. 7) Fig. 6. Performance characteristics of the high-efficiency R717 OTEC power cycle with turbine efficiencies. ristics of a high-efficiency R717 OTEC power cycle, Applied Thermal 03 Table 2 Comparison conditions of each OTEC cycle. Variable Cycle name Uehara cycle Kalina cycle This cycle Te,in [�C] (Pe,in [kPa]) 21 (817.9) 21 (821) 21 (880) Te,in [�C] (Pc,in [kPa]) 12.28 (600) 12.28 (602.5) 12.28 (661.5) xc,out [kPa] e e 0.95 Tss,in [�C] 26 26 26 Tds,in [�C] 5 5 5 Tss,ineTe,out [�C] 2 2 2 W [kW] 20 20 20 ht [/] 0.8 0.8 0.8 hp [/] 0.65 0.65 0.65 mr, [kg/h] 2810 2800 2710 mss [kg/h] 232,800 234,400 230,500 mds [kg/h] 118,100 118,300 116,400 DPr [kPa] 10 10 10 DPs [kPa] 50 50 50 Refrigerant H2O þ NH3 H2O þ NH3 NH3 Mass fraction of NH3 [kg/kg] 0.955 0.958 e System efficiency [%] 2.379 2.361 2.401 Fig. 7. Performance characteristics of the high-efficiency R717 OTEC power cycle with the vapor qualities of the cooler outlet. J.-I. Yoon et al. / Applied Thermal Engineering xxx (2014) 1e5 5 4. Conclusions Such factors as the high-stage turbine exit pressure, the expansion valve exit pressure, the turbine and pump efficiency, the pressure drop of the evaporator and the condenser, and the cooler exit vapor quality, which affect the evaporation heat capacity, the condensation heat capacity, the turbine work, the total work and the efficiency of the proposed R717 OTEC power system, were examined under the operating conditions in Table 2. It was found that the evaporation heat capacity, the condensa- tion capacity, the turbine work, the total work and the efficiency of Please cite this article in press as: J.-I. Yoon, et al., Performance characte Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.05.1 the high-efficiency R717 OTEC power cycle were affected by the high-stage turbine outlet pressure, the expansion valve outlet pressure, the turbine efficiency, the evaporator pressure drop and the cooler exit quality. Among them, the turbine efficiency and the cooler outlet vapor quality had the greatest influence on the effi- ciency of the OTEC power cycle. Therefore, these effects must be identified and considered in the design of the R717 OTEC power cycle. Furthermore, the actual application of the OTEC power cycle that is proposed in this paper requires the development of a highly efficient turbine, the optimal control of the cooler outlet vapor quality and the optimization of the system. In comparison of efficiency of three OTEC cycles including Uehara and Kalina cycle, the efficiency of Uehara, Kalina and this OTEC cycle is 2.379, 2.361 and 2.401, respectively. The proposed OTEC cycle is the highest efficiency among these cycles. Acknowledgements This work was financially supported by the National R&D project of the “Development of Energy utilization technology with Deep Ocean Water” supported by the Korean Ministry of Land, Transport and Maritime Affairs (C-D-2013-0641). References [1] J.I. Yoon, C.H. Son, S.M. Baek, H.J. Kim, H.S. Lee, Performance characteristics of the R744 OTEC power cycle with its operation parameters, J. Korean Soc. Mar. Eng. 36 (2010) 13e20. [2] J.I. Yoon, C.H. Son, S.M. Baek, H.J. Kim, H.S. Lee, Efficiency comparison of subcritical OTEC power cycle using various working fluids, Heat Mass Transfer 50 (2014) 985e996. [3] H.S. Lee, H.J. Kim, D.H. Jung, D.S. Moon, A study on the improvement of the cycle efficiency of the closed-type OTEC, J. Korean Soc. Mar. Eng. 35 (2011) 46e52. [4] J.I. Yoon, C.H. Son, S.M. Baek, H.J. Kim, H.S. Lee, Exergy analysis of the R744 OTEC power cycle with its operation parameters, J. Korean Soc. Mar. Eng. 36 (2012) 1036e1043. [5] Z. Shengjun, W. Huaixin, G. Tao, Performance comparison and parametric optimization of the subcritical organic rankine cycle (ORC) and the tran- scritical power cycle system for low-temperature geothermal power genera- tion, Appl. Energy 88 (2011) 2740e2754. [6] A. Etemadi, A. Emdadi, O. AsefAfshar, Y. Emami, Electricity generation with ocean thermal energy, Energy Procedia 12 (2011) 936e943. [7] N.J. Kim, C.N. Kim, W. Chun, Using the condenser effluent from a nuclear power plant for ocean thermal energy conversion (OTEC), Int. Commun. Heat Mass Transfer 36 (2009) 1008e1013. [8] A.I. Kalina, Combined-cycle system with a novel bottoming cycle, J. Eng. Gas Turbines Power 106 (1984) 737e742. [9] J. Bao, L. Zhao, Exergy analysis and parameter study on a novel auto-cascade rankine cycle, Energy 48 (2012) 539e547. [10] X. Li, C. Zhao, X. Hu, Thermodynamic analysis of the organic rankine cycle with an ejector, Energy 42 (2012) 342e349. [11[ H. Uehara, Y. Ikegami, T. Nishida, Performance analysis of OTEC system using a cycle with absorption and extraction processes, Trans. Jpn. Soc. Mech. Eng. (Part B) 64 (1998) 384e389. [12] Aspen HYSYS, Version 8.0, Aspen Technsology Inc., 2013. ristics of a high-efficiency R717 OTEC power cycle, Applied Thermal 03 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref1 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref1 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref1 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref1 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref2 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref2 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref2 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref2 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref3 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref3 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref3 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref3 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref4 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref4 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref4 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref4 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref5 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref5 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref5 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref5 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref5 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref6 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref6 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref6 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref7 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref7 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref7 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref7 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref8 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref8 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref8 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref9 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref9 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref9 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref10 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref10 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref10 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref11 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref11 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref11 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref11 http://refhub.elsevier.com/S1359-4311(14)00500-6/sref13 Performance characteristics of a high-efficiency R717 OTEC power cycle 1 Introduction 2 System modeling 2.1 System prescription 2.2 Mathematical analysis model 3 Results and discussion 3.1 Types and characteristics of working fluids 3.1.1 Effects of the high-stage turbine outlet pressure 3.1.2 Effect of the expansion valve outlet pressure 3.1.3 Effect of the evaporator pressure drop 3.1.4 Effects of the turbine efficiency 3.1.5 Effect of the cooler outlet vapor quality 4 Conclusions Acknowledgements References