Temperature and voltage responses of a molten carbonate fuel cell in the presence of a hydrogen fuel leakage

Transcription

1 IOP Conference Series: Materials Science and Engineering PAPER OPEN ACCESS Temerature and voltage resonses of a molten carbonate fuel cell in the resence of a hydrogen fuel leakage To cite this article: M C Law et al 2015 IOP Conf. Ser.: Mater. Sci. Eng View the article online for udates and enhancements. This content was downloaded from IP address on 14/10/2018 at 21:35

2 Temerature and voltage resonses of a molten carbonate fuel cell in the resence of a hydrogen fuel leakage M C Law 1, G V Y Liang, V C C Lee, S K Wee 2 1 Deartment of Mechanical Engineering, Faculty of Engineering and Science, Curtin University, Miri, Sarawak, Malaysia 2 Deartment of Chemical Engineering, Faculty of Engineering, Universiti Malaysia Sabah, Kota Kinabalu, Sabah, Malaysia 1 m.c.law@curtin.edu.my, 2 weesiawkhur@ums.edu.my Abstract. A two dimensional (2-D), dynamic model of a molten carbonate fuel cell (MCFC) was develoed using COMSOL Multi-hysics. The model was used to investigate the dynamic behaviour of the MCFC in the resence of hydrogen fuel leakage. A leakage was modelled as a known outflow velocity at the anode gas channel. The effects of leakage velocity and the leakage location were investigated. The simulations show that anode electrode temerature increases as the leakage velocity increases. The voltage generated is shown to decrease at the start of the leakage occurrence due to loss of hydrogen gas. Later the voltage increases as the anode temerature increases. The results also show that the changes of temerature and voltage are more significant if a leakage occurs nearer to the inlet comared to that at the outlet of anode gas channel. 1. Introduction Fossil fuels have been the main energy source for centuries. However, due to the issues of environmental ollution and scarcity of the fossil fuels, the demand for greener energy sources is increasing. A molten carbonate fuel cell (MCFC) aears to be one of the strong energy alternatives in this resect. A MCFC stack consists of a number of anode and cathode electrodes which are searated by an electrolyte as shown in the figure 1. Figure 1. Schematic of a tyical MCFC. 1 To whom any corresondence should be addressed. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

3 At the cathode electrode, the following reaction occurs, 1 2O2 CO2 2e CO 3 (R1) At the anode electrode, hydrogen gas is used as a fuel, H2 CO3 H2O CO2 2e (R2) The overall reaction can be written as, H2 1 2O2 H2O (R3) which roduces water as a by-roduct. As hydrogen is a flammable gas which could cause exlosion, it is imortant to take recautionary measures to revent the hydrogen leakage. If the leakage does occur, it is imortant to ensure that the leakage can be detected as early as ossible. From the literature conducted, there is a limited amount of work which has been done on the hydrogen leakage detection. From the study of Ashraf Khorasani et al. [1], the location of the leakage in the fuel cell could be determined based on the data received from the voltage-time grahs of the stack under hydrogen sudden sto condition. The authors suggested an aroximate exression for the cells voltage loss due to a decrease in artial ressure of anode and cathode reactants. Tian et al.[2] resented data of a failed fuel stack due to the internal gas leakage. With the hel of statistical analysis tools, it enables the detection of failed cells inside the fuel cell stack in two different cases, namely anode/cathode crossover and anode/cooling comartment leak. Voltage and the ressure readings of the cell were analysed to detect the cell with leakages. Husar, Serra and Kunusch [3] reorted that cross-over in olymer membrane fuel cell resulted in unexected changes in system variables such as temerature, ressure and voltage. Kim et al.[4] redicted the hydrogen disersion in a residential fuel cell systems by using comutational fluid dynamics (CFD) simulations. A three dimensional geometric model of the system was constructed and the flow attern and hydrogen concentration in the residential fuel cell system was studied. The objective of the current work is to investigate the erformance changes of a MCFC in the resence of hydrogen leakages. The result is helful in develoing a model-based hydrogen leakage detection method in future. 2. Model develoment 2.1. Governing equations The governing equations of the MCFC model are as follows: For the gases in the channels, Navier- Stokes equation is used, u 0 t (1) u 2 T t u u u u 3 u (2) where, and u are the gas density, dynamic viscosity and velocity vector resectively. For the gas hase momentum conservation of orous anode and cathode electrodes is described by Darcy equation, T 2 u u u I u u u I u / (3) t 3 where and are the orosity and ermeability of the orous media. 2

4 For the transort of concentrated gas secies i, it can be described as, i Ji u i Ri (4) t J i is the mass flux, i relative to the average mass velocity. For the water gas shift (WGS) reaction, the equation that takes lace in the anode is as follows [5], and R k K (5) WGS WGS, f H O CO H CO, WGS k WGS, f ex( ) (6) RT 3 2 u g K, WGS ex( Z 0.635Z Z ) (7) 1000 Z 1 (8) T T g is the gas hase temerature and i is the artial ressure of a gas secies, i. For the cathode electrode, oxygen and carbon dioxide are consumed. Thus, the rates of consumtion of the gas secies are as follows, RO ict M O / 4 Fcat (9) g ct 2 RCO i M CO / 2 Fcat (10) where the sulied current density is denoted as i ct. The molecular weight of a gas secies i is denoted as M i and F and are Faraday s constant and thickness of an electrode resectively. For the anode electrode, carbon dioxide and water vaour are roduced and hydrogen gas is consumed. Thus, the rates of consumtion of the gas secies are as follows [6], RH M 2 H / (2 ) 2 RWGS ict F an (11) RH 2O M H2O ict / 2Fan R WGS (12) RCO M 2 CO / (2 ) 2 RWGS ict F an (13) It is assumed that the electrodes of the MCFC are orous structures and thermal equilibrium. Hence, the heat transfer of the MCFC is governed by convection, conduction and radiation mechanisms. Although the effect of the thermal radiation is found to be negligible by Koh et al [7], it is still be consider in the resent work with case study. Hence, the temerature of the fuel cell, T is calculated according to, c T / t c u T ( k T ) Q Q Q (14) eff, eff eff, eff eff Ohm r rad where c, eff and keff are the secific heat caacity and the thermal conductivity resectively. The reaction heat source for the anode electrode, Qr HWGS RWGS H f,h2o M HOct i (2 F ) 2 a n (15) whilst it is zero at the cathode electrode. The enthalies of reaction for water-gas shift and water formation are denoted as H and resectively. WGS Heat transfer by thermal radiation ( Q rad H f,h O 2 ) is described as, 4 4 rad Bs S sur Q ( T T ) (16) where B is the Stefan-Boltzmann constant. TS, T sur and s are cell searator temerature, surrounding temerature and emissivity of the searator lates, resectively. 3

5 The Ohmic heat source Q Ohm is defined as, QOhm Vcell ict / k (17) where δ k is the thickness of anode, cathode electrodes or current collector. The temerature of the gas domain in the gas hase is governed by the following equations, c T / t c u T ( k T ) Q (18) where Qr HWGS RWGS, g g, g g g g r for the anode gas hase and Q 0 for the cathode gas hase. The imact of electrochemical dynamics of cell voltage is accounted for in the MCFC model. The voltage of the MCFC can be calculated via [8]: V E i (19) where E eq cell eq ct act conc Ohm is calculated by Nernst equation and the voltage losses due to activation, concentration and Ohmic overotentials are calculated as follows, 6435 (20) act ex( ) H 2 CO 2 H2O T conc ex O 2 CO2 T 4 Ohm ex T Detailed discussion on the modeling can be found in [9] Comutational domain A number of leakages are introduced at the searator of the anode gas channels as shown in figure 2. r (21) (22) 0.12 m 0.02 m 0.07 m Leakage A Leakage B Leakage C Inlet Inlet Anode gas channel Anode electrode Electrolyte Cathode electrode Cathode gas channel Outlet Outlet Figure 2. Comutational domain of the MCFC Leakage modelling and boundary / oerating conditions The leakage modelling is carried in two stages. The first stage is to assume no leakage in the fuel cell. The above-mentioned governing equations are solved to obtain the steady-state solution. The leakage 5 domains A, B and C are assumed to be a orous media with a low orosity ( 10 ) and the boundary conditions of the leakages are described as walls. The second stage involves solving the transient governing equations (1) to (22). The initial conditions are taken from the reviously obtained stead-state solution. This is imortant because it imroves the convergence of the transient solution. The leakage domains A, B and C are now treated as art of the gas channel ( 1). The boundary condition of the leakage is rescribed by a known flow velocity. The leakage is not rescribed by atmosheric ressure because it avoids the need to generate a high number of meshes around a small leakage region. 4

6 The inuts of the model [10] are summarized in table 1. Table 1. Oerating conditions and geometry details of the model. Inut Values Anode gas velocity (m/s) 1.24 Cathode gas velocity (m/s) 3.73 Molar fraction in the anode gas channel, H 2:CO 2:H 2O 0.4:0.4:0.2 Molar fraction in the cathode gas channel, O 2:CO 2:N :0.296:0.556 Inlet gas temerature at the anode and cathode gas channel (K) 873 Current density demand (A/m 2 ) 1400 MCFC length (m) 0.14 Anode / cathode gas channel width (m) Emissivity of the searator late Results and discussions Two case studies are conducted using the current model. These include: The effects of leakage flow rates and the locations of leakage The effects of leakage flow rates In the first case study, a leakage is assumed to occur at the middle of the MCFC, i.e. leakage B as in figure 2. Figure 3 shows the velocity magnitude of the anode gas channel for the case when the leakage velocity is 1.5m/s. It can be observed that uon entering the gas channel, the velocity increases due to the entrance effect. The anode gas velocity remains constant before it encounters the leakage. The velocity is reduced after it asses by leakage as a ortion of the gas is lost to the environment through the leakage. Figure 3. Velocity magnitude of the anode gas channel of the MCFC Figure 4 shows the temerature of anode electrode at the outlet. It shows that as the anode temerature increases leakage velocity increases. This is because a larger leakage velocity causes the in-channel gas flow to decrease. A lower channel gas temerature is unable to dissiate heat quickly which causes an increase of the anode temerature. Figure 5 shows the changes of the voltage of MCFC when the leakage occurs. It can be observed that there is a voltage dro initially due to the lower electrochemical reaction owing to the smaller convective mass flux at the electrodes. After a while, the voltages increase. The grah shows 5

7 that the voltage becomes higher if the leakage velocity increases. This observation is due to the increasing electrode temerature which causes the reduction in the olarization voltage, i.e. activation, concentration and Ohmic olarizations. The changes of these olarizations are shown in figure 6 to figure 8. These grahs show that as the electrode temerature increases, the overotentials decrease. The overotentials dro more noticeably as the leakage velocity increases from 0.5 m/s to 1.5 m/s. From equation (19), the voltage generated increases as the overotentials decrease. Among the three overotentials, activation overotential is relatively small comared to the other two. Figure 6 and figure 7 show that the change of overotential decreases gradually as the temerature increases. On the other hand, a drastic dro can be observed for the activation overotential at the onset of leakage at t = 0 s. The decrease is more ronounced for a higher leakage which is due to the decrease of gas ressure at the anode Temerature, K Figure 4. The temerature of the anode electrode at the outlet of the gas channel for the case of middle leakage Voltage, V Figure 5. The voltage generated by the MCFC during various leakage conditions for the case of middle leakage 6

8 Concentration overotential, Ω Figure 6. The concentration overotential of the MCFC during various leakage conditions for the case of middle leakage Activation overotential, Ω Figure 7. The activation overotential of the MCFC during various leakage conditions for the case of middle leakage Ohmic overotential, Ω Figure 8. The Ohmic overotential of the MCFC during various leakage conditions for the case of middle leakage 7

9 3.2. The effects of leakage locations The model was used to study the effects of leakage locations on the erformances of the MCFC. Figures 9 and 10 show the anode electrode temeratures at the outlet for the cases of front and end leakages. By comaring figures 4, 9 and 10, it can be seen that the temerature rise is larger as the leakage is nearer to the inlet. This is because gas loss from the front leakage will result in smaller gas velocity in majority of the gas channel. On the other hand, much smaller temerature rise is observed as it is nearer to the outlet as lower gas velocity is available only at a small section of the gas channel after the leakage. The voltages generated by both front and end leakages are also comared as shown in figure 11. It can be observed that the changes of voltage by end leakage are insignificant. By comaring figure 5 and figure 11, it can be concluded that front leakage will cause a noticeable change in voltage reading comared to the mid and end leakage conditions. The results can be used to develo a model-based hydrogen leakage detection system. Data collected from sensors will be used to comare with the simulation data. If the difference of both data is within a redefined tolerance, alarm will be triggered to inform the resence of hydrogen leakage from a MCFC Temerature, K Figure 9. The temerature of the anode electrode at the outlet of the gas channel for the cases of front leakage Temerature, K Figure 10. The temerature of the anode electrode at the outlet of the gas channel for the case of end leakage 8

10 0.82 Front End Voltage, V End End Front Front Figure 11. The voltage generated by the MCFC under front and end leakage conditions 4. Conclusions In the current study, a dynamic model of a MCFC was develoed. The model was used to investigate the dynamic erformance of the MCFC in the resence of hydrogen gas leakage. The study indicates that when a leakage occurs, the in-channel anode gas velocity will decrease. This in turn causes the anode electrode temerature to increase. The increase of anode electrode temerature is roortional to the leakage velocity. The simulation also shows that when a leakage occurs, the MCFC voltage decreases initially due to a lower electrochemical reaction rate. After that, the voltage increases due to the electrode temerature rise which encourages a higher reaction rate. In addition, it has been shown that a leakage location has a noticeable effect on the temerature and voltage of a MCFC. For the same leakage flow rate, a leakage nearer to the inlet of the gas channel results in larger temerature / voltage increase comared to that at the end of the gas channel. This is because the gas velocity in the majority section of the gas channel becomes smaller after it asses by the leakage in the former case. The current result is useful to develo a model-based hydrogen leakage detection system. 5. References [1] Ashraf Khorasani M R, Asghari S, Mokmeli A, Shahsamandi M H and Faghih Imani B 2010 Int. J. Hydrogen Energ [2] Tian G, Wasterlain S, Candusso D, Harel F, Hissel D and François X 2010 Int. J. Hydrogen Energ., [3] Husar A, Serra M, and Kunusch C 2007 J. Power Sources, [4] Kim Y, Nam J H, Shin D, Chung T Y, Kim Y G 2010 Current Alied Physics, 10 S81-S85 [5] Haberman B A, Young J B 2004 Int. J. Heat Mass Tran, [6] Akhtar N, Decent S P, Loghin D, Kendall K 2009 Int. J. Hydrogen Energ [7] Koh J H, Kang B S, Lim H C 2000 J. Power Sources [8] Brouwer J, Jabbari F, Leal E M, Orr T 2006 J. Power Sources [9] Law M C, Lee V C C, Tay C L 2015 Alied Thermal Engineering (acceted for ublication) [10] Yu L J, Ren G P, Jiang X M 2008 Energ. Convers. Manage