CFD Modeling of SOFC Cogeneration System for Building Application

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 109 (2017) 361 – 368

International Conference on Recent Advancement in Air Conditioning and Refrigeration, RAAR 2016, 10-12 November 2016, Bhubaneswar, India

CFD Modeling of SOFC Cogeneration System for Building Application Tushar Choudharya*, Mithilesh kumar Sahub, Sanjayc a,b,c

National Institute of Technology,Jamshedpur,831014,India

Abstract This paper focuses on CFD modeling of Solid oxide fuel cell (SOFC) which is coupled with co-generation system. The SOFC cogeneration system is a low emission and energy efficient combined heat and power (CHP) is a most promising candidate in the field of electric and thermal energy generation technology for execution in future commercial buildings. Using finite volume approach CFD modeling of SOFC has been carried out by using commercial software COMSOL 4.3.1. The performance characteristic of an SOFC has been examined by performing parametric analysis. The effect of fuel utilization factor, recirculation ratio, operating temperature significantly affects the cell performance. It has been observed that, internal reforming is advantageous over the external reforming system in terms of thermo-economics and power supply. © 2017 2017The TheAuthors. Authors.Published Published Elsevier Ltd. © byby Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility ofthe organizing committee of RAAR 2016. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of RAAR 2016. Keywords: SOFC; Energy efficiency; CFD; Building Application

1. Introduction The use of cogeneration system to lower the electrical and thermal demands in buildings has gotten expanding consideration among the last few years. Although it is still in the developing stages and the possibilities for energy sparing in buildings are not altogether comprehended to date. Among different innovations accessible for building relevance’s, in buildings regarding thermal energy and power generation fuel cells are considered as a most promising energy conversion device. On comparing conventional power plants with SOFC, SOFC have higher power generation and efficiency. * Corresponding author. Tel.: +91-9752005705; fax: +91 657 2382246 E-mail address: [email protected] (T.Choudhary), [email protected] (Sanjay)

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of RAAR 2016. doi:10.1016/j.egypro.2017.03.087

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Tushar Choudhary et al. / Energy Procedia 109 (2017) 361 – 368

The utilization of fuel cell likewise brings about the much lower release of green-house gasses and other pollutants [1]. Among various types of fuel cell, SOFC is consider for this work, as in small and large scale building (commercial and private) the need of power is meeting by cogeneration system, where SOFC is integrated with cogeneration system due to having wide scope of operation. Nomenclature Active surface area, cm2 A Daeff Anode effective gaseous diffusivity, cm2/s Dceff Cathode effective gaseous diffusivity, cm2/s Voltage, V E Faraday constant, C F ഥ Specific molar enthalpy, J/mol ࢎ Enthalpy flow rate, W H icd,a Anode exchange current density , A/cm2 Current density, A/cm2 i icd,c Cathode exchange current density, A/cm2 limiting current density of anode, A/cm2 ias limiting current density of cathode, A/cm2 ics Current, A I Equilibrium constant K Thickness of a cell component, ȝm ȉ LHV Lower heating value, J/mol Molecular weight, g/mol M Number of Electrons ne Recirculation ratio R Pressure, bar P Universal gas constant, J/molK R

T UF Ua Wfc X ȡ Ș ഥ ǻࡳ

an cat conc elec F ohm O

Temperature, K Fuel utilization ratio Air utilization ratio Power output of cell, W Molar concentration Greek Letter Resistivity of the cell components Cell Efficiency Gibbs free energy j/mol Subscripts Anode Cathode Concentration Electrolyte Fuel Ohmic Superscripts Standard State

On comparing SOFC with fossil-fuels technology, SOFC facilitates higher combined thermal and electrical efficiency of about 90% and are not restricted by Carnot efficiency, as it does not have any moving parts. For distributed power generation and even for stationary, SOFC-stacks are appropriate means; since it has high operating temperature of about 600–1000ƕC makes it possible for hydrocarbon based fuel with internal reforming and produce a high–quality exhaust waste simultaneously for cogeneration. In the case of working with natural gas as fuel, SOFCs can wipe out all NOx, SOx, and particulate matter outflows, and decrease the carbon content in discharges by up-to 53% in contrast with conventional fossil-energized power plants. These characteristics of SOFCs make them important for power generation situations where high power consistency is required, emissionminimization is needed and where natural waste-gasses are accessible for fuel [2]. It's important that the working temperature of SOFC is sufficiently high to supply adequate heat to the reforming of fuel [3]. This permits SOFCs to change fuel inside, which expels the requirement for costly outer reformers used to create hydrogen (H2) in SOFCs. The high working temperature additionally empowers SOFCs to utilize the remaining thermal energy for space and water heating, permitting SOFC to be utilized as a cogeneration system. Numerous analytical model has been proposed by various researchers for the single and stack model, such models are costamagna et al. [4], Achenbach et al. [3], Bessette et al [5], Massardo et al [6], Xue et al. [7], Bove et al. [8], Aguiara [9], Bove et al. [10] and Hosseini et al. [11]. But there are various shortcomings in their models i.e. The models are oversimplified, 1 dimensional, the mechanism inside a fuel cell are not well illustrated, assume uniform temperature distribution along PEN structure, ignore actual chemical kinetics. Later, advancement in modelling and simulation is done by introducing of matured 2 and 3 dimensional modelling. As various physical and chemical mechanisms take place inside the SOFCs, these phenomena can be well understood by computational approach. Computational fluid dynamic (CFD) analysis has been carried out as a tool


of analysis by various researchers, such as Hosseini et al. [11] perform CFD analysis of CH4 fuelled single cell SOFC-stack and examines the effect of micro/macro structural parameters on cell performance. Huangfu et al. [12] conduction 1D transient state analysis of SOFC under varying temperature and pressure and found that increasing temperature and pressure cell performance increases. Wei et al. [13] modelled the flow channel design and investigates the thermal stress within the fuel and proposed and optimum design of current collector. Al-Masri et al. [14] model can predict the temperature distribution of SOFC short stack under transient operating conditions, but not account for different flow configuration. Zhang et al. [15] develop a 3D model to examine the transport phenomena within the fuel cell for different flow configuration, but ignored the impact of thermal radiation within the cell during the electrochemical transport phenomenon. The constraints of these models significantly affect the precision and consistency of the result outcome. On the basis of above literature survey research gap has been identified that they all are based on cell level model and don’t have enough potential to estimate fuel cell stacks outside processes and phenomenon. In the entire life, buildings have to encounter several condition, discussion about the system execution assessment under the arrange of conditions is beneficial. Taking into account this study, the aim of the present work, made out of two sections, in first section CFD modelling of SOFC has been carried out using commercial software COMSOL 4.3.1. The effect of various parameters such as fuel utilization ratio, operating temperature, recirculation ratio has been examined. While in the second section overall effect of each parameter has been observed for various configurations of the SOFC cogeneration system 2. System Description assumptions This subsection examines the displaying calculations of the fuel cell stack and the BOP (Balance of Plant) segments. A power module of fuel-cell stack includes two significant parts: first the thermal model and second the electrochemical model. Since various complex chemical and physical processes occur within the SOFC system, a few presumptions were made for analysis purposes [12-18]: x x x x x x x x x

Gasses inside the system are taken as ideal gas. Every cell provides a constant electrical voltage output. The temperature within fuel cell stack remains uniform. At the outlet of the fuel cell stack, the temperature of fuel and air should be same as the channels temperature. Each cell of the stack is consistently supplied with fuel and air. Methane gas is supplied to the reformer as a fuel. The composition of inlet gas at anode: CH4, H2O (steam), H2, CO and CO2. The composition of inlet gas at cathode: O2=21%, N2=79%. The water-gas moving response happens at harmony, since the response achieves the balance condition rapidly.

In the generic SOFC cogeneration system several possible configuration has been examined. It include recycling of anode, cathode, both anode-cathode gas stream and basic design configuration.

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Figure 1. SOFC cogeneration system. Table 1 Simulation parameters [18, 19] Geometrical Parameter

Physical Properties of the cell Components

Thickness of anode

500ȝm

Anode exchange current density (ioa)

0.65 A/cm2

Thickness of cathode Thickness of electrolyte Active Surface Area Height of fuel channel Height of air channel

50ȝm 10ȝm 10000mm2 1mm 1mm

Cathode Exchange current density (ioc) Porosity of anode Porosity of cathode Tortuosity of anode Tortuosity of cathode Thermal conductivity of electrolyte

0.25 A/cm2 50% 50% 3 3 2W/mK

Operating Parameters Pressure Inlet fuel temperature Inlet air temperature Condition of wall Fuel utilization percentage Fuel used Composition of air Operating voltage of cell

1 bar 1023K 1023K Adiabatic 85% Syn Gas 21% O2, 79% N2 0.7V

Pressure drops Afterburner Air pre-heater Water pre-heater Pre-reformer

20 mbar 100 mbar 15 mbar 50 mbar

2.1 Electrochemical model The motivation behind electrochemical-model is to figure the real cell voltage and the resultant force yield from the fuel-cell stack by checking the synthetic responses occurring inside the stack. During operation electrochemical reaction takes place within the fuel cell. Where H2 is derived from the reforming reaction of methane and water-gas shift reaction and then electric power is produced by oxidation reaction of H2. The electrochemical reactions are given below: CH4 H2O U CO 3H2 (Steam reforming Reaction)

(1) CO H2O U H2 CO2 (Water Gas Shift Reaction) (2) Overall Cell Reaction 1 H2 O2 o H2O 2 (3) Using Nernst equation the developed cell potential at open circuit condition is calculated by equation (4), which is function of partial pressure and Gibbs potential Go which is a component of the temperature ENernst

0.5 'GT0 RT § X H2 X O2 · 1 RT § P · ln ¨ ln ¨ ¸ ¸ ne F ne F ¨© X H2O ¸¹ 2 ne F © Po ¹

The ENernst varies between 0.99 V-1.01V.

(4)

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Conventionally in the cell, the actual voltage decreases due to presence of irreversibility. This irreversibility is contribution of three forms of polarization (losses) namely activation, concentration and ohmic polarization which are mentioned in equation (5)-(8). § i · RT § i · RT (5) E sinh 1 sinh 1 E E act , an

act

F

§ W an W elec ¨ © V an V elec

Rohm i

Eohm Econc

act , cat

Econc,an Econc ,cat

¨¨ ¸¸ ¨¨ ¸¸ © 2icd , an ¹ F © 2icd ,cat ¹ n W · § · cat ¸ i ¨ Rcontact ¦ U k Lk ¸ i V cat ¹ k © ¹

ª RT § i ln ¨1 « «¬ ne F © ias

X H 2 i P · º ª RT § · RT § i ·º o ¸» « ln ¨1 ln ¨1 ¸ » ¸ ¹ ne F ¨© X H 2O ias P ¸¹ »¼ ¬ ne F © ics ¹ ¼

(6) (7)

Where ias ics

ne F X H 2 Daeff P RT W a

(8)

ne F X O2 Dceff P § X O2 P ¨1 ¨ X O2 P 0 ©

· ¸ RT W c ¸ ¹

(9) Bossel [16] proposed the equation of electrical conductivity of SOFC that is a function of solid structure temperature and is given by equation (10): E EN Eact Eohm Econc (10) The generated current of fuel cell is calculated as follow: I

Wfc

m Hf 2 1 r r U F

2 F

Ai

(11)

F 2c

I .E

(12)

The efficiency (electrical) of fuel cell stack is calculated as under: Kcell

n

Wfc m f .LHV

(13)

Where ݊ത is number of cell stack.

Fig. 2 Validation of V-I characteristics 3. Validation of model In order to validate the proposed model the present work has been contrasted with the experimental as well as analytical work of Tao et al. [17] and Colpan et al. [16] respectively. It has been observed that the obtained result

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shows good agreement with the available literature and they are within acceptable limits as shown in figure 2. The variation in result is mainly due to having varying model assumptions and operating condition. 4. Results and Discussion 4.1Effect of Fuel utilization and recirculation ratio

Fig. 3 (a) Influence of Fuel utilization ratio, (b) recirculation ratio on the power density and voltage Fig. 3(a) shows the influence of fuel utilization on the cell performance. It has been seen that with the increase in fuel utilization ratio both cell voltage and power density decreases. This is because of high concentration of activation polarization at the anode side, whereas the influence of ohmic polarization at the cathode side is not noteworthy. During the process, fuel flows through a fuel channel, carbon dioxide, hydrogen and methane are utilized within fuel channel during electrochemical reaction, which results in depletion of fuel stream at the channel exit. Therefore, the concentration-polarization in this section becomes more noteworthy. Fig. 3(b) demonstrates the influence of recirculation ratio on cell voltage and power density. It was found that the cell performance adversely gets affected as recirculation ratio increases. This mainly due to depletion of fuel stream which mainly have rich content of steam and CO2. As these CO2 and Steam is re-circulated through recirculation line, which lower the concentration of H2 and CO at the inlet of fuel channel which eventually leads to decrease in cell performance. 4.2 Effect of flow configuration on temperature distribution

Fig. 4 (a) Comparison of temperature profile for co flow and counter flow throughout the cell, (b) Temperature profile of throughout the cell


Fig. 4a illustrates the temperature profile for co flow and counter flow throughout the cell. The cell is heated up due to electrochemical processes as heat is generated. In order to maintain uniform temperature with the cell recirculation and Excess air flow along the anode and cathode side is one of the measures to minimize the temperature rise across the PEN structure. At high temperature internal reforming of fuel takes place via reforming reaction (Eq.1). Which is endothermic in nature leads to temperature drop of 100K near the inlet of the fuel channel as evident from figure 4a. The same can be observed from Fig. 4(b).

(a)

(b)

(c) (d) Fig. 5 (a) (a) Mole fraction distribution of H2 in counter-flow; (b) Mole fraction distribution of H2 in Co-flow; (c) Mole fraction distribution of CH4 in Co-flow; (d) Mole fraction distribution of H2O within the cell length Fig. 5 demonstrates the mole-fraction distribution of fuel-species across the co-flow and counter-flow arrangement. It has been found that the concentration of H2 increases 20% along the cell length from the inlet Fig. 5(a, b), this is because of CH4 reforming reaction and H2O gas shift reaction. While H2O concentration initially reduces and then starts increasing as shown in Fig. 5(d). From (Fig.5c) about 33% from the inlet CH4 is completely been utilized within the fuel channel of counter-flow and similarly for co-flow it gets consumed at across 67% from the inlet. Due to endothermic nature of CH4 reforming reaction, temperature dip of 71K occurs near the fuel inlet. Consequently, at the same simulation parameters counter-flow arrangement yields superior Nernst potential as compared the co-flow. On comparing the present mole distribution with finite difference result of Aguiar et al. [9] the result shows same trend.

Fig. 6 (a, b) Effect of fuel utilization in electrical and cogeneration efficiency and effect of cell voltage Vs heat recovery for different co-generation system

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Fig. 6(a) illustrates the influence of fuel utilization in the cogeneration and electrical efficiency. It has been found that cogeneration and electrical efficiency for all configuration of SOFC cogeneration system increases significantly as percentage of fuel utilization increases. The trend of electrical efficiency is linear and quite differs as compared to cogeneration efficiency. Fig. 6 (b) shows the effect on heat recovery as cell voltage increases heat recovery for all for all configurations of SOFC cogeneration system decreases linearly this is because of increase in recirculation ratio the heat recovery increases significantly. 5. Conclusions In this paper (section 1), CFD modelling of SOFC has been carried out and in section 2 the effect of parametric analysis has been done for the SOFC cogeneration system. On the basis above analysis the following conclusions have been drawn: x x x x

The CFD model of SOFC has been compared with the analytical and experimental work and the obtained results are within acceptable range. On the basis of CFD analysis comparing counter-flow and co-flow arrangement, counter-flow gives superior performance. As the fuel utilization ratio increases cell performance decreases, but the cogeneration efficiency for different cogeneration system increases significantly. Increase in recirculation ratio cell performance decreases while the cogeneration system efficiency along with heat recovery eventually increases.

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