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Author's personal copy international journal of hydrogen energy 38 (2013) 1795e1805 1801 Table 3 e Parameter values resulting from energy and exergy analyses of the system. Parameters Value Net power output, W_ net (kW) 101.96 Exergy efficiency, J (%) 22.70 Energy efficiency, h (%) 3.60 Sustainability index, SI 1.29 $ Total exergy destruction rate, ExD;tot (kW) 42.12 Hydrogen production rate, m_ H2 (kg/h) 1.20 PEM electrolyzer exergy efficiency, JPEM (%) 56.34 Warm surface pump power, W_ WS (kW) 1.30 Cold surface pump power, W_ CS (kW) 3.13 Working fluid pump power, W_ WF (kW) 0.88 Fig. 3 e Comparison of present model with experimental data. Three main parts are first individually modelled including the each exergy flow rate. Engineering equation solver (EES) is linked to Matlab to calculate the properties of the different working fluids (i.e., water and ammonia) such as pressure, temperature, enthalpy and entropy. Several simplifying assumptions are made here to render the analysis more tractable, while retaining adequate accuracy to illustrate the principal points of the study All processes operate at steady state. The thermodynamic cycle of the integrated system in Fig. 1 is an ideal saturated Rankine cycle using pure ammonia as the working fluid. All the components are adiabatic. Pressure drops in ORC cycle are negligible. State 5 is saturated vapour. Heat losses from piping and other auxiliary components are negligible. To conduct the simulation, input data are required. For each subsystem some reliable data are input to the simulation code in order to determine the outputs. Table 1 lists the input parameters for the OTEC system simulation. Also, Table 2 lists the parameter used to simulate the PEM electrolyzer. 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1000 Fig. 4 e Variation of electrolyzer overpotentials at various current densities. To ensure the accuracy and validity of the developed computer simulation code, the PEM electrolyzer is validated with experimental data from the literature. Specifically, the electrochemical model is used to simulate experiments pub- lished in the literature and the modeling results and experi- mental data are compared. The electrolyte used in the experiments [36,37] is Nafion, a polymer widely used as electrolyte in fuel cells and electrolyzers. The thicknesses of the electrolytes tested by Ioroi et al. [36] and Millet et al. [37] were 50 mm and 178 mm, respectively. Platinum was used as the electrode catalyst. The simulation code for the JeV char- acteristics for PEM electrolysis are compared with experi- mental data of Ioroi et al. [36] as shown in Fig. 3. The modeling results agree well with the experimental data, supporting the accuracy of the present model. It is found that the cell potential increases rapidly when current density is less than 300 A/m2. When J exceeds 300 A/m2, the cell potential increases slightly with J. To enhance the understanding of the electrochemical performance of the PEM electrolyzer, ohmic and activation overpotentials are examined and shown individually in Fig. 4. This figure shows that the ohmic over- potential is very small and increases slightly with current density. This observation is attributable to the fact that the membrane ionic conductivity (sPEM) is significantly high for typical values of l as well as the operating temperature, which leads to a lower RPEM (see Eqs. (25) and (27)), which in turn, lowers the overall ohmic resistance and the ohmic overpotential. Ohmic activation Cathode activation Anode activation 2000 3000 Current density, J (A/m2) 4000 5000 6000 7000 8000 Fig. 5 e Exergy destruction rate and dimensionless exergy destruction ratio for each component of the ocean thermal energy conversion (OTEC) system for hydrogen production. Overpotential(V)PDF Image | H2 Energy
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