Modeling and Simulation of a PEM Fuel Cell System Under Various Temperature Conditions

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1 nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 6-8, 008 Modelg and Simulation of a EM Fuel Cell System Under Various Temperature Conditions A. A. SALAM, A. MOHAMED, M A HANNAN Department of Electrical, Electronic and Systems Engeerg Universiti Kebangsaan Malaysia Bangi, Selangor MALAYSIA Abstract: - This paper describes the modelg, simulation and analysis of the dynamic model of a 5kW EM system. The model cludes hydrogen fuel reformer and the EM stack. The model is used to vestigate the effect of temperature on the voltage and power flow at various temperatures. Simulation results show that the temperature has a significant impact on output voltage and output power. Key-Words: - Dynamic Model, EM, Hydrogen fuel reformer, Fuel cell Stack, Temperature, ower. Introduction The application of s and other renewable energy sources like photovoltaic and wd power for both stationary and mobile power applications is beneficial for the sustaable energy conversion. In residential and vehicular application, a new kd of commercially viable, efficient and reliable is to be fully developed. roton Exchange Membrane (EM) has the advantage of low operatg temperature []. Several models of EM s have been developed and simulated [-3]. Models that have been reported the literature for the EM range from stationary to dynamic models [3-7]. In this paper, the proposed EM model is based on the model given [], which is modified to account for temperature variations. The EM system must be accurately modeled order to be able to apply a suitable nonlear control scheme. Fuel cells are widely regarded as one of the most promisg energy sources [3 due to their high energy efficiency, extremely low emission of oxides of nitrogen and sulfur, very low noise, and clean energy production. Fuel cells are classified based on their operatg temperature. Details of s classification based on the type of electrolyte, operatg temperature and fuel are shown Table. The EM technology is the best amongst other technologies because of its low operatg temperature, highpower density, and relatively short startup time [6]. This paper discusses the modelg, simulation and vestigates the effect of temperature on the EM system. The simulation model of the was developed based on the mathematical relationship for EM and subsystems usg the SimowerSystems toolbox of Matlab-Simulk. Table Type of electrolyte, operatg temperature and fuel for different s Type Electrolyte Fuel Operatg Temp. 0 C Alkale KOH H hosphoric hosphoric acid fuel acid cell H ~0 roton exchange membrane Molten carbonate Solid oxide Solid polymer Lithium and potassium carbonate Solid oxide electrolyte pure H H, CO, CH 4 other hydrocarbons H, CO, CH 4 other hydrocarbons ~ EM The EM consists of a solid polymer electrolyte sandwiched between two electrodes (anode and cathode). In the electrolyte, only ions can exit and electrons are not allowed to pass through. So, the flow of electrons needs a path like an external circuit from the anode to the cathode to produce electricity because of a potential difference ISSN: ISBN:

2 nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 6-8, 008 between the anode and cathode. The overall electrochemical reactions for EM fed with a hydrogen-contag anode gas and an oxygencontag cathode gas are as follows: At anode raection, + H = 4H + 4e () At cathode reaction, + O + 4 H + 4e = H O () Overall reaction H O = H O + electricity + heat (3) + The output stack voltage V is defed as a function of the stack current, reactant partial pressures, temperature, and membrane humidity. The potential difference between the anode and cathode is calculated usg the Nernst s euation and Ohm s law and can be written as follows: 0.5 RT H O V = N 0 E0 + ln B ln CI F H O The relationship between molar gas flow through the valve is proportional to its partial pressure and can be expressed as, H K an = = K (5) H M H H H O an = H O H O R t I (4) K = K (6) H O M where, H : Molar flow of hydrogen H O : Molar flow of water H : artial pressure of hydrogen H O : artial pressure of water O : artial pressure of oxygen kh : Hydrogen valve molar constant kh O : Water valve molar constant k an : Anode valve molar constant MH : Molar mass of hydrogen MH O : Molar mass of water Accordg to the basic electrochemical relationships, the relationship between the stack current and the molar flow of reacted hydrogen can be expressed as, r N0I H = = KrI (8) F Substitutg euation (8) to (7), the hydrogen partial pressure can be written as, d dt H RT = ( H out H Kr ) (9) V I an 3 Modelg Descriptions The model used this paper to simulate a 5kW EM is given Fig.. Usg the euations derived for the EM model, a simulation model is developed usg the SimowerSystems toolbox of Matlab-Simulk. The overall model of the EM consists of 5 subsystems such as partial pressures, ideal voltages, activation losses, ohmic losses and reformer model. Figs. -6 show the various subsystems of the dynamic simulation model of the EM. The parameters of the simulation model are given as Table. For the hydrogen molar flows (kmol/s), there are three major factors such as the output flow, the put flow and the flow that takes part the reaction. These flows of hydrogen molar are represented as, d dt RT out r = ( ) (7) V H H H H an Fig. Overall model of EM ISSN: ISBN:

3 nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 6-8, 008 Table arameters used EM simulation Representation Value Faraday s constant (F) C/mol Universal gas constant (R) J/kmol - K No load voltage, E 0 (V).9 V Number of cells (N) 4 Constant (K=N/4F),kmol/(s - A).0883 x 0-7 Valve molar constant for hydrogen (KH ), kmol/(s - atm) 4. x 0-5 Valve molar constant for. x 0-5 oxygen (KO ), kmol/(s - atm) Valve molar constant for 7.76 x 0-6 water (KH O), kmol/(s atm) Response time for hydrogen flow (T H ) 3.37 sec Response time for water flow (T HO ), sec 8.48 Response time for oxygen flow (T O ), sec 6.74 Utilization factor, U 0.8 Reformer time constant, τ Reformer time constant, τ Conversion Factor, CV Activation voltage constant, B (A - ) Activation voltage constant, C (V) Stack ternal resistance, R t I ga constants K 5 and K 6 0 Ratio of hydrogen to oxygen (r HO ).68 Methane reference signal, Q methref artial ressures Usg euation (5) to replace the output flow and performg Laplace transform of both sides of euation (9) the hydrogen partial pressure can be expressed as: KH ( H = H K I) (0) r Van + s K RT H Similar analysis were carried out reference [4], to derive the partial pressure of oxygen ( ) O and water ( H O ) of euations () and (), respectively. Considerg the euations for the pressures, the model that represents the water, hydrogen and oxygen pressure is given Fig. Oxygen partial pressure: KO O = ( O K + τ s O Water partial pressure: K H O H ( K I) O = r + τ s H O r I) () () Fig. Hydrogen, Water and Oxygen pressures model 3. Nernst Voltages The voltage can be calculated usg the Nernst voltages (E) and considerg the activation (N act ) and ohmic losses (N ohmic ) the system. The voltage is given by, V = E + N + N (3) cell act ohmic The variation temperature affects the ideal voltage at different temperatures. Considerg the variation of temperature, the Nernst voltage from references [,3] can be expressed as, H O E = ( Δg + Δs[ T + Tref ] + RTop ln( ) V (4) F H O where R is the gases universal constant (8.34 J/ºK.mol), F is the Faraday constant (96485 C), Δg is the variation of Gibbs free energy, Δs is the variation entropy (-63. J/K) the reaction, T ref (98ºK) and T op are the reference and operational temperatures ºK, respectively. Applyg the constants to the open circuit voltage (Nernst maximum voltage) and cludg the number of series cell the stack, the euation can be simplified as shown references [3,7] as, E = N ( H O [ T 98] T ln( V 3 op op ) HO (5) ISSN: ISBN:

4 nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 6-8, 008 where, T op is the operatg temperature of the stack. The model that is represented by euation (5) is shown Fig 3. The number of cells connected series with 4 cells which formed the total stack voltage for the output voltage is simulated. losses voltage with temperature variation and the ternal resistance can be expressed as, R 5 5 = T + 8 I (7) t 0 where, I is the current. Therefore, the ohmic losses are given by, N ohmic = R I (8) t The model that represents the ohmic losses is shown Fig 5. Fig.3 Model to represent voltage 3.3 Activation Losses The activation losses of the relates to the slowness of the reactions that take place on the surface of the electrodes. A portion of the voltage generated is lost drivg the chemical reaction at the electrodes as mentioned previously. In this paper, the activation over voltage is considered as constant and is expressed references [, 3 and 7] as, N act = B ln C I (6) ( ) where, I is the stack current, B = V and C = A -. The model used to represent the activation loss is shown the Fig. 4. Here, U represents the current, B and C are constants, respectively. Fig.4 Activation losses model 3.4 Ohmic Losses The ohmic losses occur due to the polymeric membrane resistance to ionic flux, the resistance due to electrodes, resistance of separatg plate to flux of electrons and gas spreaders, respectively. An ohmic Fig.5 Ohmic losses model 3.5 Reformer Model A reformer model that produces hydrogen through a reformg process is troduced references [, 3, 5]. This is a second order reformer function which can be expressed as, H CV = + ( + ) (9) τ τ s τ τ s + menthanol where, CV is a conversion factor (kmol of hydrogen kmol - of methanol), methanol is the methanol flow rate (kmols - ) and τ,τ are time constants. Stack current feed back [, 3, 4] and the relationship between the crease stack current and the reuired hydrogen can be expressed as, N I re H = 0 FU (0) where, U is the utilization rate (coefficient) and re H is the reuired amount of hydrogen flow to meet load demand (kmols ). To control the methanol flow rate, the amount of hydrogen reuired to meet the load demand is, k k3 N 0I + τ 3s FU methanol = 3 H () ISSN: ISBN:

5 nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 6-8, 008 where, τ 3 is the time constant and k 3 is the ga of the roportional Integral (I) controller which is used to control the flow rate of methanol the reformer. The oxygen flow is controlled usg the ratio r h o. The reformer model is shown Fig 6. rated power at about.5s. The model is sub seuentially simulated from the temperature of 5ºC-80ºC (98ºK-353ºK) and the results of the simulation are shown Table EM Fuel Cell ower Ouput ower(w) Results Fig.6 Reformer model In the simulation, it was assumed that the hydrogen, oxygen and water pressures are considered as ideal, hence no variation of the pressures are to be expected. The stack current is considered as 95A based on the power ratg of 5kW and voltage of 50V DC. The EM output voltage and output power with respect to the simulation run time of 0 seconds are plotted and shown Fig. 7 and Fig. 8, respectively at a temperature of 5ºC (98ºK) and constant current of 95A. Voltage(Volts) EM Fuel Cell Voltage Time(Seconds) Fig.7 Fuel Cell Stack Voltage output The simulation results show that the output voltage creases from about 5.5 to a steady state value of about 5.5 V. From Fig. 8, the output power shows the start to produce the Time(Seconds) Fig.8 Fuel Cell Stack Output ower From Table 3, it can be seen that the Nernst voltage decreases with the crease temperature and this fact is illustrated graphically as Fig 9. The decrease ideal voltage therefore reduces the voltage. The stack voltage and power decreases as the temperature creases as shown Figs. 0 and, respectively. Table 3 Effect of varyg temperature Temp Voltages (Volts) ower K Eo Ohmic V fc x The ohmic losses decrease with creasg temperature as shown Fig. This is accordance with the theory which as the temperature creases, the losses due to ternal resistance will reduce. It is noted Fig. that the ohmic losses can be reduced by operatg the EM ISSN: ISBN:

6 nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 6-8, 008 at a higher temperature than the standard temperature of 98K. Nernst Voltage Stack Voltage Fuel Cell ower Stack Temperature Fig.9 Nernst Voltage Variation with Temperature Temperature(K) Fig.0 Stack Voltage Variation with Temperature Temperature(K) Fig. Fuel Cell ower Variation with Temperature Ohmic Losses voltage losses. However, other factors like overall efficiency of the system would be reduced. 4 Conclusions Dynamic behavior of a 5kW EM system is studied and evaluated to vestigate the effect of temperature on the output voltage and output power at various temperatures. Simulation results show that the stack voltage and its output power decrease with crease temperature. The developed dynamic model of a 5kW EM fuel cell is useful for future studies such as grid connected system. References [] M.Y. El-Sharkh, A. Rahman, M.S. Alam,.C. Byrne, A.A. Sakla, T.Thomas, A dynamic model for a stand-alone EM power plant for residential application, Journal of ower Source, 38, 004, pp [] M.Uzunoglu and M.S.Alam,Dynamic Modellg, Design and Simulation of a Combed EM Fuel Cell and Ultra capacitor System for Stand- Alone Residential Application, IEEE Transactions on energy conversion, Vol., No. 3, 006. [3] J. adulles, G. W. Ault, and J. R. McDonald,An Approach to the Dynamic Modelg of Fuel Cell Characteristics for Distributed Generation Operation, IEEE- ES Wter Meetg, Vol., 000, pp [4] S. asricha, and S. R. Shaw, A Dynamic EM Fuel Cell Model, IEEE Trans. Energy Conversion, Vol., Issue, 006, pp [5] M.Y.El-Sharkh,N.S.Sisworahardjo, Uzunoglu, O.Onar, M.S. Alam, Dynamic behaviour of EM and Microturbe power plants, Journal of ower Source,64, 007, pp [6] J. urkrushpan, A. G. Stefanopoulou, and H. eng, Modelg and control for EM stack system, roc. Amer. Control Conf., Anchorage, AK, 00, pp.37 3 [7] C.Wang, M. H. Nehrir, and S. R. Shaw, Dynamic model and model validation for EM s usg electrical circuits, IEEE Trans. Energy Convers., Vol. 0, No., 005, pp Stack Temperature Fig. Ohmic losses due to temperature variation The simulation results show an improvement i n th e ohmic losses as the stack temperature creases from 5ºC to 80 ºC (300K-355K). It seems that the higher the operatg temperature, the lower ISSN: ISBN:

H are designed for these purposes. The fuzzy logic control scheme is employed for the design of the two controllers.

H are designed for these purposes. The fuzzy logic control scheme is employed for the design of the two controllers. An Intelligent Control of Solid oxide Fuel cell voltage Kanhu Charan Bhuyan and Kamalakanta Mahapatra Dept of ECE, National Institute of Technology-Rourkela, India-7698 Email: kanhu6@gmail.com, kmaha@rediffmail.com