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Author's personal copy 1804 international journal of hydrogen energy 38 (2013) 1795e1805 which lowers the exergy efficiency. Since the exergy destruction rate in the condenser is much higher than in other components, any changes in the input exergy flow to the condenser leads to an increase in its exergy destruction, so an increase in condenser temperature leads to an increase in exergy flow at point 2 keeping other parameters constant. 5. Conclusions The comprehensive thermodynamic modeling and exergy analysis of an integrated OTEC system for hydrogen produc- tion has provided useful insights. A complete thermodynamic simulation is performed for an OTEC cycle equipped with a flat plate solar collector and PEM electrolysis for hydrogen production. Also, an exergy analysis is conducted and the results show that the exergy efficiency of the integrated OTEC system is about 22%. Other significant conclusions follow: An increase in solar radiation intensity decreases the total exergy destruction rate of the system and raises the exergy efficiency of the cycle and the hydrogen production rate. An increase in solar intensity results in a drastic reduction in condenser exergy destruction rate, which ultimately leads to a reduction in total exergy destruction rate of the cycle. An increase in ambient temperature has two different effects on exergy efficiency: an increase in both exergy efficiency and sustainability index below T0 1⁄4 292 K, and a reduction of both exergy efficiency and SI above T0 1⁄4 292 K. Increasing the condenser temperature reduces the exergy efficiency of the cycle because this increase raises the enthalpy of the turbine outlet which reduces the net power output. Acknowledgement The authors acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada. Nomenclature A surface area (m2) CP specific heat at constant pressure (kJ/kg K) d diameter (m) Deq equivalent diameter (m) DP depletion number Eact;i activation energy in cathode or anode (kJ) $ Ex exergy flow rate (kW) $ ExD exergy destruction rate (kW) $ Exelectric electric exergy input rate (kW) F Faraday constant (C/mol) g gravitational acceleration (m/s2) G Gibbs free energy (J/mol) h specific enthalpy (kJ/kg) I solar radiation intensity (W/m2) J current density (A/m2) J0 Jref i L m_ N_ P Q_ R RPEM s S T V V0 Vact Vact;a Vact;c W_ W_ G W_ CS W_ WF W_ WS exchange current density (A/m2) pre-exponential factor (A/m2) length (m) mass flow rate (kg/s) molar mass flow rate (mol/s) pressure (kPa) heat transfer rate (kW) gas constant (kJ/kg K) proton exchange membrane resistance (U) specific entropy (kJ/kg K) radiation flux (W/m2) temperature ( C or K) velocity (m/s) reversible potential (V) activation overpotential (V) anode activation overpotential (V) cathode activation overpotential (V) work rate (kW) turbine generator power (kW) cold seawater pumping power (kW) working fluid power (kW) warm seawater pumping power (kW) Greek letters h energy efficiency hT turbine efficiency hG generator efficiency hWFP working fluid pump efficiency hCSP cold surface pump efficiency hWSP warm surface pump efficiency la water content at the anode-membrane interface (U1) lc water content at the cathode-membrane interface (U1) lðxÞ water content at location x in the membrane (U1) r density (kg/m3) sPEM proton conductivity in PEM (s/m) sðxÞ local ionic PEM conductivity (s/m) J exergy efficiency Subscripts act activation G generator CS cold surface C condenser D destruction E evaporator eq equivalent in inlet condition net net power ohm ohmic out outlet condition PEM polymer exchange membrane T turbine p pump WF working fluid WS warm surface Superscripts $ ratePDF Image | H2 Energy
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