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increase. Second, if the liquid density is high, the net efficiency is decreased due to the increase in the pumping power. 2.2.6. Determination of the working fluid After the evaluation, all of the candidate working fluids were found to be feasible with respect to the electrical properties and toxicity levels. On the other hand, all are green-house gases with high GWP values in spite of their lack of an effect on ozone depletion. R32, R410A, or R125 were evaluated as feasible from an economic perspective. Considering the flammability of the fluids, all working fluid candidates have superior characteristics with respect to flammability except R32 and R143A. Because R32 and R143A were identified as ‘flammable’ and ‘extreme flammable’, respectively, these two fluids were deemed to be unsuitable for use in the C-OTEC system and should be avoided for safe operations to be maintained. This implies that one has to make a selection from among R410A, R125 and R134A. We found that the R410A and R125 would be economical in the design and R134A would be environ- mentally friendly and could be used at a relatively low pressure. Under current municipal law, there are no environmental regulation standards in place affecting the use of these working fluids. The environmental regulation regarding the use of these gases has been solicited for appraisal before an economic feasibility assessment. Therefore, we decided to use R134A as the working fluid. 3. Modeling and analysis 3.1. Heat balance calculation In order to design a prototype of the C-OTEC system, we calculated the heat balance of the cycle through a simulation using EES (Engineering Equation Solver) [24]. The following mathematical models are used to analyze the thermodynamic behavior referring to the notations in Fig. 2: Working fluid pump in C-OTEC: W87 1⁄4 ðh8 h7Þm_ 7 hpmp 1⁄4 h8s h7 h8 h7 h8s 1⁄4fðp8;s7Þ Cold sea water pump: h6s 1⁄4fðp6;s5Þ Secondary condenser: Q761⁄4ðh6h7Þm_6 Q109 1⁄4 ðh10 h9Þm_9 Q76 1⁄4 Q109 TTDcond; SRC 1⁄4 Tsat@p6 T10 Primary condenser and heat exchanger: Q34 1⁄4 ðh3 h4Þm_4 Q12 1⁄4 ðh1 h2Þm_2 Q12 1⁄4 Q34 h31⁄4fðp3;x3Þ h11⁄4fðp1;x1Þ TTDcond; PRC 1⁄4 Tsat@p1 T3 Overall efficiency Dpcsw where Dpcsw is the pressure difference of the cold sea water H. Jung, J. Hwang / Energy 75 (2014) 443e452 447 hgross 1⁄4 W65 cycle Q34 WWW hnet 1⁄4 65 87 cycle Q34 pmp;csw m_ csw Wpmp; csw 1⁄4 h r The design conditions of the C-OTEC prototype included a tur- bine output of 10 kW. The temperature of the steam in the PRC condenser was assumed to be 32.9 C, with quality of 0.85. The cooling side of C-OTEC system is assumed to operate at a deep sea water temperature of 4.0 C considering the location of Yeongdong power plant, which is located along the east coast of South Korea. The terminal temperature differences of primary condenser (TTDcond, PRC) were set to 2.8 C and 1.1 C (typically 5 F and 2 F) considering conservative heat transfer performance of the heat exchanger in the primary condenser. Thus, the respective temper- atures of the working fluid were set to 30.0 C and 31.7 C and the saturated pressure was set to 772.4 kPa and 810.8 kPa respectively. For the secondary condenser, the inlet temperature of the sea water is considered equivalent to 4.0 C and the TTD (terminal tempera- ture difference) is equal to 3.0 C. The isentropic efficiency of a tiny turbine for 10 kW was assumed to be 70% because it is not a commercial but rather a prototype. There are relatively many choices of such a pump from among commercial products; hence 85% isentropic efficiency for the pump is considered feasible. The heat balance and mass flow rate calculations are presented in Tables 6a and 6b, respectively, based on the cycle configuration described in Fig. 2. For a turbine output of 10 kW, the gross efficiency of C-OTEC as modeled in the problem has been estimated to be 4.425% for the TTDcond, PRC 2.8 C and 4.748% for the TTDcond, PRC 1.1 C conditions. A heat exchanger capacity of 226 kW and 211 kW is required based on this efficiency level, which would be linked to the PRC. The steam required to evaporate the working fluid in the primary condenser is 0.1 kg/s, which is about 0.15% of the steam entering the condenser under normal operation conditions. In this paper, the results shown in Tables 6a and 6b are referred to as the reference pmp;csw csw pipeline, Dpcsw 1⁄4 rgDh þ f D 2 þ K 2 L rv2 rv2 hpmp;csw is the efficiency of the cold sea water pump. rcsw is the density of cold sea water. Turbine: W65 1⁄4ðh6 h5Þm_5 1⁄4ðh5 h6Þm_5 hh htur1⁄45 6 h5 h6sPDF Image | Feasibility study of a combined Ocean Thermal Energy Conversion method in South Korea
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