Gas distribution inside an MCFC

ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

33 (2008) 3173 – 3177

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

Gas distribution inside an MCFC Danilo Marra Department of Civil, Environmental and Architectural Engineering (DICAT), Faculty of Engineering, University of Genoa, Via Opera Pia, 15– 16145 Genoa, Italy

art i cle info

ab st rac t

Article history:

Of all the fuel cells currently available or under investigation, molten carbonate fuel cells

Received 11 December 2007

(MCFCs) are one of the most technologically suitable for the production of electrical energy.

Received in revised form

However, their study and design needs to be developed on two different scales: at the plant

28 February 2008

scale for the optimisation of the entire system and the integration of the different

Accepted 4 March 2008

components and at the cell scale for the analysis of the electro-chemical reactions and the

Available online 7 May 2008

determination of the local operating constraints. For this reason the author, in his PhD

Keywords: MCFC current collector MCFC FEM analysis MCFC fluid-dynamic

thesis, is analysing the optimisation of a 1 MW MCFC energy plant system and examining the fluid-dynamic characterisation of the anodic and cathodic gas distributions. In this work, the finite element model developed for the simulation of the fuel cell current collector will be explained from a fluid-dynamic point of view. Due to the geometrical complexity of this component and the exhaustive computational resources necessary for the study, it was essential to identify a symmetrical module on which to focus. The results of this simulation, in terms of non-dimensional head losses as a function of the Reynolds number, will be presented and discussed. The FEM analysis was carried out with Comsol Multiphysics

r

software.

& 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Today one way to meet the growing demand for energy while respecting the environment is represented by molten carbonate fuel cell (MCFC) systems. At the moment MCFC hybrid plants for the distributed production of electrical energy are at a pre-commercial stage. The study and development of this innovative technology have to be carried out at two different scales:  at a plant scale, in order to analyse the optimisation of the entire system and the integration of the different components;  at a cell scale, where the electro-chemical reactions and the local operating constrains are taken into account.

These two stages are not independent but, on the contrary, are closely connected: in fact the results of the local analysis are essential for the optimisation and the integration of the entire system; moreover, the plant scale analysis could underline the need for certain modifications or further investigations into local behaviour. The author’s PhD thesis is based on the study described above. The first phase of the work considered the process analysis of a 500 kW MCFC hybrid plant (called ‘‘Serie 2TW’’) developed by Ansaldo Fuel Cells S.p.A (AFCo) [1]. Following that it took into consideration international studies [2,3], focusing on the development and optimisation of an MCFC system with a nominal power of 1 MW [4]. The analysis was carried out using Aspen Plusr software that not only allows the study of all the standard components of the plant, but also the integration of innovative components modelled

Fax: +39 0103532589.

E-mail address: [email protected] 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.03.005

ARTICLE IN PRESS 3174

I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

Nomenclature l m p

friction factor, – dynamic viscosity, kg=ðm sÞ pressure, Pa

using specific Fortran language codes, such as MCFC stacks simulated using the detailed proprietary 3D model MCFCD3Sr [5]. Thanks to this code, developed by the Department of Civil, Environmental and Architectural Engineering (DICAT) of the University of Genoa in collaboration with AFCo, it was possible to take into account both the local behaviour of the cells and the operating constraints of the entire stack. The results demonstrated that one of the trickiest aspects of the analysis is the thermal management of the cell: in fact one of the main restrictions on the utilisation of the MCFC unit is that the maximum temperature inside the stack must be lower than 6802690  C [6,7]. Furthermore, the temperature distribution on the cell plane must be as uniform as possible. In order to analyse these phenomena it was decided to study the fluid-dynamics of the feeding anodic and cathodic gases on the cell plane. To do this the fluiddynamic characteristics of a fuel cell current collector (FCCC), typically adopted as a gas distributor in MCFCs, was analysed. This paper will firstly present a finite elements model (developed using Comsol Multiphysicsr software) of an elementary module of the considered FCCC; then the results obtained for the non-dimensional head losses (friction factor, l) will be shown as a function of the Reynolds number (Re).

2.

R r Re u¯ U

33 (2008) 3173 – 3177

hydraulic radius, m density, kg=m3 Reynolds number, – ðu; v; wÞ, velocity vector, m/s mean velocity, m

sectional view (b) of the collector plate, respectively, as reported in the patent [10]. Looking at the figures we can note that the considered geometry does not force the gas to flow in the principal longitudinal direction only, but also allows a transversal flow. This fact could be important not only for understanding the correct behaviour of the fuel cell, but also for an optimisation phase, particularly from a thermal point of view. The FCCC geometry shown in Fig. 2(a) may sometimes need modifying: a typical change, necessary when a thin electrode is used, is to add a number holes between the arches to increase the open contact area between the gas and electrode. Fig. 2(c) shows a planar view of a portion of this modified FCCC; the following analysis will be carried out considering this modified collector plate configuration.

2.1.

The finite elements model

In order to study the gas-flow distribution through the previously described FCCC, it was decided to model

The fuel cell current collector

A molten carbonate fuel cell stack is composed of a number of individual fuel cells stacked in electrical series. Each cell, consisting of an anode, a matrix that includes the electrolyte (molten carbonates), and a cathode, is separated from the next one by a separator plate. The fuel cell stack is subjected to an axial compressive load in order to maintain the appropriate cell contacts. A current collector is located between the electrode (anode or cathode) and the separator plate in each cell. This component has to satisfy three important requirements: 1. to provide the distribution of the anodic and cathodic gases; 2. to maintain the electrical contact; 3. to resist the compression load. MCFC simulation models usually consider the FCCC as composed of many flow-streets that are parallel to each other [8,9]. Fig. 1 shows two sketches of an MCFC with a cross-flow feed: on the left the configuration with a real gas distributor and on the right the configuration with the simplified one. This assumption does not match the real geometry of the typical FCCC adopted [10]. Fig. 2 shows a planar view (a) and a

Fig. 1 – Sketches of an MCFC with a cross-flow feed: on the top the configuration with a real gas distributor and on the bottom the configuration with the simplified one.

ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

33 (2008) 3173 – 3177

3175

Fig. 2 – Fuel cell current collector (FCCC)—planar view (a), sectional view (b), planar view of a portion of the modified collector (c) [10].

It was assumed that the flow was incompressible inside the analysed module. Taking into account the symmetries that mark the chosen module, the following boundary conditions were set:

Fig. 3 – Basic 3D elementary module chosen for the FEM analysis of the FCCC.

this component using the finite element method (FEM). For this purpose Comsol Multiphysicsr software was adopted. The first step of this analysis was to select an elementary module for the collector plate to reduce the geometrical dimensions by using the symmetries and the anti-symmetries of the problem. Fig. 3 shows the basic 3D module chosen for the FEM analysis of the FCCC. This geometry, after being inscribed in a prism with a rectangular base, was opportunely imported to the Comsol Multiphysics environment. The second step was to define the governing equations of the problem: it was decided to use the complete stationary 3D Navier–Stokes equations: rð~ u  rÞ~ u ¼ r  ½pI¯ þ mðr~ u þ ðr~ uÞT Þ, ~ r  u ¼ 0,

(1)

 no-slip conditions on the body of the module and on the top face where the module will be in contact with the separator plate in the real cell;  conditions of symmetry on the face parallel to the main flow direction;  periodic boundary conditions on the inlet and the outlet faces: this point was important for studying the fully developed flow;  firstly a no-slip condition was considered on the ‘‘open’’ faces (under the arch and on the bottom of the hole, where the gas will be in contact with the electrode in the real cell); then a vertical component of the velocity ðwÞ, corresponding to the entering or exiting mass flow (depending on whether we are analysing the anode or the cathode) was added. A pressure gradient ðDpÞ that guaranteed the gas motion between the inlet face and the outlet face was imposed. After that the module was discretised with a mesh made of tetrahedral elements of the second order for the velocity field and of the first order for the pressure field. Here it should be emphasised that the total number of elements in the mesh is 30,465, corresponding to the 156,684th degree of freedom (dof): due to this large dof, and the numerical complexity in solving the Navier–Stokes equations, the computational time for solving the problem is very high and a very large resource memory is necessary.

where:

3.  r; m are, respectively, the density ðkg=m3 Þ and the dynamic viscosity of the gas ðkg=ðm sÞ;  p is the pressure (Pa);  u¯ ¼ ðu; v; wÞ is the velocity vector (m/s).

Results obtained

The model was solved considering the typical properties ðr; mÞ of both the anodic and cathodic gases used in the plant analysis [4] (Table 1).

ARTICLE IN PRESS 3176

I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

33 (2008) 3173 – 3177

λ = f (Re)

Table 1 – Properties of the anodic and cathodic gases

100



m ðkg=ðm sÞÞ r ðkg=m3 Þ

3:75  10 0.9

5

w = 0 [m/s]

Cathodic gas 3:2  10 1.35

w = 1.5*10-3 [m/s]

10

5

λ [-]

Anodic gas

1 0.1 0.1 In order to obtain the results in a non-dimensional form, the relationships between the head losses and the velocity were expressed as l ¼ f ðRe; wÞ,

λ [-] 4URr . m

w = -1.1*10-3 [m/s]

10 1 0.1 0.1

(3)

with:  DL: the geometrical module dimension parallel to the principal flow (module length);  R: the hydraulic radius of the inlet section of the module;  U the mean inlet velocity module;  Re: Reynolds Number, defined as follows: Re ¼

1000

w = 0 [m/s]

After this analysis the same study was carried out for a complete transverse flow: the boundary conditions were changed in order to take into account a stream that only flowed perpendicularly to the main flow. This step was very important because it made it possible to also analyse the behaviour of the FCCC in the case of transversal fluxes: this is the starting point for overstepping the hypothesis of complete parallel flow along the main direction. In this analysis the

100

1000

100 10 1

w: vertical component on the open faces of the module.

 the curves are almost back straight: this fact is very important because it underlines that the flow is laminar;  the difference between the curves for w ¼ 0 and wa0 is almost negligible: this suggests that the presence of a vertical velocity component in the contact sections of the FCCC and the electrodes does not notably change the head losses of the collector.

10 Re [-] λ = f (Re)

(4)

Two graphs were drawn (with logarithmic co-ordinates) for the above relationship, one for the anode (Fig. 4(a)) and one for the cathode (Fig. 4(b)). There are two curves in each graph: the lighter one is for the case of w equal to zero; the darker is for the highest achievable value of w (positive for the anodic gas and negative for the cathodic gas). These maximum values of w have been evaluated, on the basis of the plant analysis, by considering the difference between the flow that enters (and exits) both the anode and cathode sides. From the graphs we can note that:

1

1000

λ [-]

DP 4R DL ð1=2Þr  U2

100

λ = f (Re)

100

 l: friction factor, defined as follows: l¼

10 Re [-]

1000

(2)

where:

1

0.1 0.10

1.00

10.00

100.00

Re [-] Fig. 4 – k ¼ fðReÞ for the anodic gas (a), for the cathodic gas (b), for a completely transversal flow (c).

vertical component of the velocity on the ‘‘open’’ faces was neglected ðw ¼ 0Þ: this assumption made it possible to obtain a unique graph valid for both the anode and cathode. Fig. 4(c) presents the graph of l ¼ f ðReÞ for a completely transversal flow. We can note that the motion could be considered as laminar also for the transversal flow (back straight trend of the curve in Fig. 4(c). The relationships obtained with the FEM analysis (graphically described in Fig. 4) are remarkable because they could be used to describe the FCCC fluid-dynamic behaviour in a concise way.

4.

Conclusions and future perspectives

The aim of this work was to analyse the FCCC from a fluiddynamic point of view. Having identified a basic module of the gas distributor, thanks to the Finite Elements Method and r using Comsol Multiphysics software, it was possible to obtain two relationships that correlated the friction factor l and the Reynolds number Re for both completely longitudinal

ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

and transversal flows. Future analyses will be aimed at the integration of the fluid-dynamic relationships for the entire cell plane to obtain the complete gas distribution for both the anodic and cathodic sides.

Acknowledgements The author wish to thank Elisabetta Arato, Barbara Bosio and Paolo Costa of DICAT for fruitful discussions and suggestions. R E F E R E N C E S

[1] hwww.ansaldofuelcells.comi. [2] Kivisaari K. Studies of natural gas and biomass fuelled MCFC systems. Electrochem Soc Proc 1997;97(4):179–90. [3] Ishikawa T, Hiroo Y. Start-up, testing and operation of 1000 kW class MCFC power plant. J Power Sources 2000;86:145–50.

33 (2008) 3173 – 3177

3177

[4] Marra D, Bosio B. Process analysis of 1 MW MCFC plant. Int J Hydrogen Energy 2007;32:809–18. [5] Bosio B, Arato E. Molten carbonate fuel cell system simulation tools. In: Zhang XW, editor. Advances in fuel cells. Trivandrum, Kerala, India, 2005. p. 277–90. [6] Scattolini A, Bosio B. Theoretical and experimental investigation of MCFC performance. In: Proceedings of the 1st World Congress of young scientists on hydrogen energy systems (HysyDays), Torino, Italy. 2005. p. 473–8. [7] Marra D, Bosio B. Simulation of multi-MW MCFC systems. In: Proceedings of the 1st World Congress of young scientists on hydrogen energy systems (HysyDays), Torino, Italy. 2005. p. 461–6. [8] De Simon G, Parodi F, Fermeglia M, Taccani R. Simulation of process for electrical energy production based on molten carbonate fuel cells. J Power Sources 2003;115:210–8. [9] Liu SF, Chu HS, Yuan P. Effect of inlet flow maldistribution on the thermal and electrical performance of a molten carbonate fuel cell unit. J Power Sources 2006;161:1030–40. [10] Katz M, Bonk SP, Maricle DL, Abrams M. Fuel cell current collector. United States Patent, Patent Number: 4983472. International Fuel Cells Corporation; 1991.