A study by Kim et al. [8][9] was referred to in the selection of working fluids for this study. Working fluids were selected by referring to NIST Refprope. CFC and HCFC affiliated refrigerants that have caused environmental problems, such as ozone depletion and global warming, were excluded due to environmental regulations. Working fluids demanding that evaporation is above 3 MPa or critical pressure in design condition were additionally excluded. All the working fluids selected have an Atmosphere Life Time (ALT) less than 1,000 years, a Global Warming Potential (GWP) less than 1,500, and an Ozone Depletion Potential (ODP) of 0. The American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) safety group of the fluids was also considered, and fluids in groups Al, A2L, and A2 were chosen as they have low toxicity and flammability. Seven working fluids, shown in Table 1, were selected. They can be classified into dry, isentropic, and wet fluid categories.

Pinch Point Analysis (PPA) is a thermodynamic cycle analysis method that uses Pinch Point Temperature Difference (PPTD) [14]. The PPTD ensures that the temperature and pressure in an evaporator and a condenser are constantly maintained, so that a comparison analysis of each cycle and the various working fluids can be performed. The design parameters of this study, shown in Table 2, were based on previous research that used PPA [8][9]. The PPTD of the heat exchangers is the same value as was used in the research of Aydin et al. [15]. The temperature increase in the surface seawater due to the solar collector was assumed to be 20°C, as chosen by Yamada et al. [6]. Aspen HYSYS, a thermal process and cycle design program, was used as an analysis tool. A SOTEC system was replicated using this program. The selected working fluids were applied, and the thermodynamic performance was calculated in each cycle.

Since the ideal gas equation has limited use in practical applications, the real gas state equations were used to precisely indicate the state of the substance [10]. In this study, the Peng-Robinson equation was used as well as the real gas state equations, and they can be expressed as follows.

Figure 5 shows the exergy efficiency of the simple Rankine cycle in a SOTEC system, referring to the study by Kim et al. [8][9]. RE245fa2 showed the highest exergy efficiency (64.76%) and R1234yf showed the lowest exergy efficiency (62.23%). These results are similar to those of the study by Kim et al. [8][9] that performed thermodynamic comparison analysis of various working fluids in an OTEC system. Therefore, RE245fa2 showed the best thermodynamic performance among the selected working fluids, and has the potential to be a working fluid in a SOTEC system as well as an OTEC system.

Figure 5: 
Exergy efficiency of the simple Rankine cycle

Figure 6 shows cycle irreversibility of the simple Rankine cycle in a SOTEC system, by referring to the study by Kim et al. [8][9]. R1234yf showed the highest cycle irreversibility (62.57 kW), and RE245fa2 showed the lowest cycle irreversibility (58.24 kW). Cycle irreversibility is the difference between reversible work and useful work [10]. The cycle irreversibility of the working fluids was inversely proportional to exergy efficiency. The reason for this phenomenon is that the recovered exergy decreases as the exergy is destroyed, which means cycle irreversibility increases [10]. Exergy destruction is greatly influenced by the entropy generation of a cycle and the ambient temperature.

Figure 6: 
Cycle irreversibility of the simple Rankine cycle

Figure 7 shows exergy efficiency of the Rankine cycle with an open feedliquid heater vs. the pressure ratio in a SOTEC system. This was calculated using Equation (13). All of the selected working fluids showed peak thermal performance during this cycle when the pressure ratio was 0.4. This means that the quantity of vapor bled from the turbine should provide this pressure ratio for maximum efficiency.

Figure 7: 
Exergy efficiency of the Rankine cycle with an open feedliquid heater vs. the pressure ratio

Figure 8 shows exergy efficiency of the Rankine cycle with an open feedliquid heater, when the pressure ratio is 0.4, compared to the simple Rankine cycle in a SOTEC system. RE245fa2 showed the highest exergy efficiency (67.79%). R1234yf, which had the lowest exergy efficiency, had the greatest increase in exergy efficiency (3.78%), and R152a showed the smallest increase of 2.96%. The selected working fluids showed different thermodynamic characteristics, such as mass flow and enthalpy, depending on the design conditions.

Figure 8: 
Exergy efficiency of the Rankine cycle with an open feedliquid heater compared to the simple Rankine cycle

Figure 9 shows cycle irreversibility of the Rankine cycle with an open feedliquid heater vs. the pressure ratio in a SOTEC system. This was calculated using Equation (11). All of the selected working fluids showed the lowest cycle irreversibility during this cycle when the pressure ratio was 0.4. This is because the cycle irreversibility of working fluids is inversely proportional to exergy efficiency, as mentioned previously.

Figure 9: 
Cycle irreversibility of the Rankine cycle with an open feedliquid heater vs. the pressure ratio

Figure 10 shows the cycle irreversibility of the Rankine cycle with an open feedliquid heater when the pressure ratio is 0.4, compared to the simple Rankine cycle. R1234yf showed the highest cycle irreversibility (58.07 kW) and the greatest decrease in cycle irreversibility (4.50 kW). R152a showed the lowest decrease (3.17 kW). The cycle irreversibility of all the working fluids is affected by the entropy generation in this cycle.

Figure 10: 
Cycle irreversibility of the Rankine cycle with an open feedliquid heater compared to the simple Rankine cycle

All of the selected working fluids in this cycle showed an improvement in thermodynamic performance, as seen in the results of the studies by Kim et al. [8][9], Shin et al. [12], and Kim et al. [13]. Therefore, to further improve thermodynamic performance, the installation of multiple regenerators (open or closed type of feedliquid heaters) has been proposed [10].