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Performance assessment of a direct formic acid fuel cell system through exergy analysis
i n t e r n a t 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 4 0 ( 2 0 1 5 ) 1 2 7 7 6 e1 2 7 8 3
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Performance assessment of a direct formic acid fuel cell system through exergy analysis Alper Reis, Suha Orc¸un Mert* Yuzuncu Yıl University, Faculty of Engineering and Architecture, Department of Chemical Engineering, Van, 65080, Turkiye
article info
abstract
Article history:
In this study, performance assessment of a direct formic acid fuel cell system is conducted,
Received 4 June 2015
including evaluation of a broad set of operating parameters. The fuel cell system is
Received in revised form
modeled and simulated by using sub-routine developed by Matlab. The system compo-
23 July 2015
nents, such as compressor, humidifier and heat exchangers, are also taken into consid-
Accepted 24 July 2015
eration. Effects of varying operating temperature, pressure, membrane thickness, current
Available online 14 August 2015
density, anodeecathode stoichiometry and reference properties on the system are evaluated to predict the performance as close to real-cases as possible. It is found that increasing
Keywords:
the temperature greatly favors the performance of the direct formic acid fuel cell system,
Formic acid fuel cell
low pressure and high reference environment temperature values have the same effect on
Exergy
the cell performance. The maximum exergetic efficiency by broad parametric investigation
Performance
is estimated as 24%, where the power production is 10 kW.
Efficiency
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The basic properties expected from an alternative fuel may be summarized in terms of having high power production, being non-toxic, environmentally benign, compatible with various energy producers and being economic. One alternative fuel that may be considered to having all these different important features while also being potentially limitless is, hydrogen. Hydrogen energy usage has an increasing trend worldwide with the enormous research on fuel cells which is the most efficient way of converting its chemical energy to electrical energy. Fuel cells are relatively new type of energy generator that has developed quickly, especially in the second half of 20th century. Nowadays the “fuel chemical energy-heat energy-
mechanical energy” transformation scheme in the thermal systems leaves the stage rapidly to “fuel chemical energyelectric energy-mechanic energy” systems in the fuel cells with increasing research and development. In a fuel battery, electricity is produced through an electro-chemical transformation, without combustion. The produced electricity can be used for any desired purpose [1,2]. Thus, the fuel cell is a generator that turns the fuel's chemical energy directly into electric energy. After the fuel battery reaction, which may also be described as the reverse reaction of electrolysis, DC electricity is produced. When studies about the performance of the fuel cell systems in the literature is investigated, it is found that various studies deal with the electrochemical modeling and parametric investigation of single or stack fuel cells and systems [3e9]. There is numerous research, in the literature,
* Corresponding author. Tel.: þ90 505 2356022; fax: þ904322251730. E-mail addresses: [email protected] (A. Reis), [email protected] (S.O. Mert). http://dx.doi.org/10.1016/j.ijhydene.2015.07.131 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t 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 4 0 ( 2 0 1 5 ) 1 2 7 7 6 e1 2 7 8 3
conducted on energetic performance. Limited studies focused on exergoeconomic studies [6,9e13]. Similarly, thermal management is another dynamic topic that is covered [14]. However, most of these studies tend to consider fuel cell systems other than DFAFC's, such as Polymer Electrolyte Fuel Cells (PEMFC), Direct Methanol Fuel Cells (DMFC) and Solid Oxide Fuel Cells (SOFC). The general studies in the literature about electrochemical modeling and performance investigation cases [15e19] regarding DFAFCs systems seem to be focused on performance improvements on the DFAFC's, and more from catalysts' and electrodes' point of views [2,20,21]. Rejal et al. [15], suggest that the overall DFAFC performance was primarily determined by the rate-determining steps, such as fuel and oxygen supply/feed, and electrode reaction. The study of Yu and Pickup [22] indicates that peak power density reached about 375 mW/cm2 with an operating temperature of 50 C, using Pd catalyst. Adding to the literature, this study looks the performance, of a Direct Formic Acid Fuel Cell (DFAFC) system through exergy analysis. This is done through using a wide variety of operating parameters, which also include equipment used in the system as a separate parameter of performance. Since there is currently lack of studies in the literature about the subject, the real performance of the DFAFC system is revealed and this study is believed to contribute to the research on the evaluation of the DFAFC system.
volumetric energy density. This can be remedied by the use of a high concentration of formic acid feed. The electrode and cell reactions that occur in the formic acid fuel cell is as follows [18]: Anode reaction : HCOOH/CO2 þ 2e þ 2Hþ 1 O2 þ 2Hþ þ 2e /H2 O 2 1 Cell reaction : HCOOH þ O2 þ Hþ /CO2 þ H2 O 2 Cathode reaction :
(1)
System description A direct formic acid supplied energy generation system (Fig. 2) is evaluated in this study considering storage to heat integration for revealing the exergetic performance as being a candidate to renewable and sustainable energy producer. Formic acid is supplied from a storage tank and air is supplied by the compressor to the system. The formic acid is initially sent from the tank to the heat exchanger by the utilization of a pump. This is done to increase the temperature of the formic acid to temperatures for the reaction before it arrives into the fuel cell. While this process is occurring on one side of the system, the air received from the compressor on the other end of the system first needs to be humidified to sustain the water management in the fuel cell. Water, CO2 and the unreacted components leave the system as a result of these reactions. . For each of the specified currents below, exergy analysis was conducted and the change of efficiency based alterations on various parameters was analyzed. During the modeling some assumptions were made for the sake of ease in calculations. These were:
Direct formic acid fuel cell system In Fig. 1, a generalized anode cathode structure of a fuel cell is shown. Fuel and other reactants are supplied to the prevailing section where the reaction occurs. DFAFCs have a great advantage compared to other fuel cell types as they have a relatively high electromotive force (theoretical open circuit potential 1.48 V) [22]. Formic acid fuel cell systems generally have higher cell voltages when compared to the other popular fuel cell systems [3,23]. Another supplementary advantage is the limited fuel crossover and relatively high power densities. However, the main disadvantage, on the other hand, is having a low
DFAFC system cell was taken as steady state. The heat loss during production was taken as zero [24] with proper insulation. Relative humidity content of air fed to the system was taken as 50%. Isentropic efficiency of the pumps are taken as 85%. Reference condition is taken as 298 K and 1 atm.
Oxidant (e.g.,Oxygen, Air….)
Reaction Products (e.g., H2O, CH2....) & Heat
Cathode Load
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Electrolyte Anode -
e
Fuel (e.g., H2, CH4, CO, CH3OH) Fig. 1 e General fuel cell mechanics.
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Fig. 2 e Flow chart of a formic acid fuel cell system (DFAFC).
Fuel cell stack was taken as single cell. Area of fuel cell was taken as 25 cm2. Isentropic efficiency of the compressor is taken as 85%.
Exergetic aspects Exergy analysis is utilized to detect and reduce the inadequacies in the design of a system regarding the results of investigation with a second law based approach to harness a higher level of efficiency [25]. The notion of exergy states that it is relative to the environmental conditions and the magnitude of useful energy changes depending on these conditions without neglecting the first law of thermodynamics [26]. In order to be able to define “exergy” accurately, some fundamental thermodynamics should first be clarified. One clarification is that theoretically, to get the “maximum possible work” out of a system, two conditions should first be met: to get the work done in a “completely reversible change of state” in the system -from beginning to the end- and to have the final state of the system “in complete balance with the environment it is within”. From that, we get the term ‘irreversibility’. There is zero “irreversibility” in a completely reversible state change, and the maximum amount of useful work that can be harnessed from a system is defined as “reversible work” [27]. That being said, the maximum amount of work that can be harnessed from a system is also dependent on environmental conditions, along with aforementioned reversible changes in states. A system that is thermodynamically in equilibrium with the environment it is defined to be at “dead state”. In a dead state, the system also has zero relative kinetic and potential energy to its environment and a chemical balance also exists in this state. Under these conditions, it is impossible for a system to have any kind of interaction with the environment. In other words, no work can be harnessed from a system in a dead state. Exergy analysis mainly identifies and underlines the sources of inefficiencies through the system by revealing the irreversibilities and exergy destruction that gives chance to engineers to focus on these areas. _ phy ), Exergy has various components such as physical (Ex _ _ _ chemical (Exch ), kinetic (Exk ) and potential (Exp ) exergies.
_ ¼ Ex _ phy þ Ex_ch þ Ex_k þ Ex_p ¼ m* _ exphy þ exch þ exk þ exp Ex
(2)
Physical exergy is in direct relation with environmental conditions exphy ¼ ðh h0 Þ T0 *ðs s0 Þ
(3)
The benefits of exergy analysis can be summarized as follows: Determines the real value of thermal losses in a system, quantifies thermodynamic defects of a system. From a thermodynamics perspective, it is a tool to define energy quality. It enables comparison between systems with equal efficiency. Shows possibilities to reduce existing inefficiencies in a system. Gives opportunity to evaluate the performance of systems and processes in which the first law of thermodynamics is insufficient to determine. Since exergy is the parameter that defines real amount of energy used in thermal systems, exergy analysis is naturally convenient in preliminary design phases, feasibility surveys and optimization studies of systems [28].
Modeling of the DFAFC system Electrochemical modeling While modeling a fuel cell, initial step should be calculation of overpotential losses that will take place. The “net potential” will be deduced based on these irreversibilities. Potential sources for these losses are described below: - Reversible cell voltage Cell voltage provides maximum efficiency in the absence of irreversibilities. By applying Nernst Equation to the formic acid fuel cell, the equation below is formulated [3]: 8:314*T Erev ¼ 1000* 1:45 2*F
(4)
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The equation shows that local pressures and environmental temperatures have great effect on the amount of losses. Here, T represents the temperature of the fuel cell, F stands for Faraday constant.
lmem ¼
Activation loss is the energy loss that takes place initially in the operation which initiates the electron transfer through anode and cathode. It is caused by slow reaction rates on anode and cathode [29].
; ;
0
(12) a¼
- Activation Overpotential
0:143 þ 17:81a 39:85a2 þ 39:85a3 14:1 þ 1:4ða 1Þ
xH2 O P Psat
(13)
where a is the membrane water activity.
Exergetic modeling
i0a ¼ 0:0035*eð0:0132*TÞ
(5)
The general exergy balance equation for the direct formic acid fuel system is derived as:
i0c ¼ 0:0032*eð0:0124*TÞ
(6)
_ fc;in SEx _ fc;out SEx _ fc;heat SEx _ fc;work _ fc;D ¼ SEx Ex
Eact ¼ Aa *logði þ in Þ=i0a Þ þ Ac *logði þ in Þ=i0c Þ
(7)
In the equations above, in represents equipotent current density of fuel, Aa and Ac represent Tafel reflections on anode and cathode, respectively, i0a and i0c represent local current densities in anode and cathode, respectively. - Concentration Overpotential
In this equation the exergy destruction is tired to be calculated considering inlet and outlet exergies of the streams (fuel and exhaust) in addition to the heat loss and the electrical power produced by the fuel cell system. Power generated by the fuel cell system is evaluated by taking the cell active area and number of cells to the consideration and the amount of power produced by the system is revealed as: Wstack ¼ EiAcell ncell =1000
With the increase in current density, difficulties in mass transfer and some overpotential in anode and cathode begin to appear. As the reaction progresses, concentration shifts towards the surface reactions, which causes the emergence of “concentration gradient”. High current density and concentration gradient are limiting factors for the reaction rate [29]. i þ in i þ in þ Bc *log 1 Econ ¼ Ba *log 1 i1a i1c
In the equation above Ba and Bc represent mass transfer parameters for anode and cathode, respectively, i1a and i1c represent limiting current density in anode and cathode, respectively. - Ohmic Overpotential
Rohm ¼
tmem smem
(9) (10)
smem is the membrane conductivity (1/U cm) and the fuel cell operating temperature and the relative humidity are effective on the membrane conductivity as given in the flowing equations: 1 1 smem ¼ ð0:005139lmem 0:00326Þexp 1268 303 TFC where, lmem is the membrane water content [30];
(16)
The power consumption of the system equipment are calculated and evaluated as total used power ðWused Þ in the system. By deducing the consumed power by system components, the net power is found: Wnet ¼ Wstack Wused
(17)
The amount of fed formic acid evaluates the total exergetic performance of the system:
The electrical resistance on the MEA components arises. Ohmic overpotential is dependent on the current density. The resistance to the flow of protons in the membrane is much more effective than the resistance of the electrodes and resistance of the bipolar plates [11]. vohm ¼ i Rohm
(15)
The model used for the exergetic calculations of the fuel cell system includes the actual consumption of the compressor and pumps in the system considering the parametric investigation operating parameters and dynamically recalculates the values dependently. Wused ¼ Wactcomp þ Wpump;2 þ Wpump;1
(8)
(14)
(11)
nsystem ¼
Wnet Ex_FA
(18)
Parametric study and simulation The current study covers the parametric investigation of the performance of the DFAFC system. The investigated operating parameters are formic acid concentration, temperature, current density and reference temperature (Table 1). The effect of these parameters on energy and exergy efficiency, power production and emission values are calculated and investigated. Through this research, the real behavior of the DFAFC system is revealed. In addition to this, operating conditions are proposed for the efficient and environmentally friendly operation of the fuel cell system. The exergetic fuel cell performance model is developed using Matlab and the performance characteristics such as efficiency, polarization curve and power production values are
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1.00
Table 1 e Parametric investigation ranges. Base case
Formic acid concentration [M] Temperature [K] Current density [mA/cm2] Reference temperature [K]
2e20 323e393 0e1000 263e313
10 323 850 298
plotted and data are gathered using an in-house developed mfile that solves the model in a sequential oriented structure. The algorithm of the derived program is given in Fig. 3.
294 K 306 K 294 K Exp. 306 K Exp
0.90
0.80 0.70 0.60 0.50 0.40 0.30 0.20 0
200
400 i [mA/cm2]
Model verification
The parametric investigation shows that produced power and efficiency of the fuel cell increase as the operating temperature increases. It seems that the effect of the activation overpotential (Fig. 5) is larger when compared to the other fuel cell systems [32,33] as there is a 30% activation loss from total potential of the FAFC system, whereas it is seen that this value is around 20% in PEM Fuel Cell [3], 15% in SOFC systems [32]. Increase in the current density along with the temperature, however, causes an efficiency and potential drop. This can be seen in Fig. 6. Increase in produced power will enable usage of the fuel cell in various systems, along with potential reductions in cost. Fig. 7 summarizes that as the fuel cell working pressure increases, the efficiency decreases proportionally. Along with efficiency, produced power also declines. It is also noticeable that when the pressure is increased to 3 atm from 1 atm, the produced power value falls to 5.4 kW from its peak point of 6.2 kW. The increase in the pressure causes power consumption as well as the fuel crossover values to increase. These are the phenomena which lead to total decrease in
I/V
•Revealing the polariza on curve for selected opera ng parameters for the fuel cell stack
System Analysis
•Calcula on of the flow proper es and power needs of system equipment
1000 323 K 343 K 363 K 383 K
900 800 Cell Voltage[V]
Results and discussion
•Applica on of exergy analysis considering system equipment and fuel cell
Fig. 3 e Algorithm of the developed program.
333 K 353 K 373 K 393 K
700 600 500 400 300 0
100
200
300
400
500
600
700
800
900
Current Density [mA/cm2]
Fig. 5 e Polarization curve for DFAFC for various temperatures.
power production and efficiency with the increment in pressure increment. In Fig. 8, the relation between power and efficiency values of the fuel cell and environmental temperatures are
30
12
Efficiency
Power
25
10
20
8
15
6
10
4
5
323 K 363 K
0
Exergy Analysis
800
Fig. 4 e Comparison of present model with experimental data [29,31].
Efficiency[%]
The developed models validation is issued using experimental studies results in the same operating conditions [29,31]. Ten molal formic acid was used at a flow rate of 1 ml min1 in these experiments and dry air was supplied to the cathode at a flow rate of 390 sccm. It is seen that there is a small gap between the values of polarization in two different operating temperatures (Fig. 4). This error is in an acceptable range and the model is thought as validated for investigation of the performance characteristics of formic acid fuel cell systems.
600
0
200
333 K 373 K
343 K 383 K
400 600 Current Density [mA/cm2]
353 K 393 K 800
Power [kW]
Ranges
Cell Voltage[V]
Parameters
2
0 1000
Fig. 6 e Efficiency and Power with respect to temperature and current density.
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10
3 2
5
1 atm
1.5 atm
2.5 atm
3 atm
2 atm
Efficiency[%]
4
Power[kW]
Efficiency[%]
5
15
400
600
273 K 293 K 313 K
2 1 0
500
600
400
500
600
700
800
900
Fig. 9 e Efficiency and Power with respect to anode stoichiometry and current density.
power. (Fig. 10). This expected situation is a result of the supplied air's exergy value which is zero since it fed to the system in reference conditions. It is well known that increasing the membrane thickness reduces the fuel crossover effect [34], but also has a negative effect due to the increase of resistance it creates on the transport of electrons. As Fig. 11 shows, lower thickness increases the produced power, and exergetic efficiency increases as an expected result of this. A 1 cm increase in the thickness causes 3% drop in efficiency and 2 kW drop in produced power. Lower membrane thicknesses are shown to be more favorable, even though they have some adverse effects.
Conclusion In conclusion, the performance assessment of a selected direct formic acid fuel cell system is applied in this study. The investigation covers a broad parametric study considering operating temperature, pressure, membrane thickness, anodeecathode stoichiometry and reference temperature. It is found that increasing operating temperature may lead to a performance increment as 5% in efficiency and 2 kW in power production. Moreover low pressures favor system performance as it affects the efficiency by an average of 3% in the range of 1e3 atm pressure. Using small membrane thickness
700
800
900
Current Density [mA/cm2]
Fig. 8 e Efficiency and Power with respect to references temperature and current density.
7 Efficiency
Power
6
20 Efficiency[%]
3
400
300
25 Power[kW]
Efficiency[%]
4
10
300
200
8
5
200
0
100
9
Power
6
100
2
i [mA/cm2]
15
0
4
0
7
0
5
0
800
20
5
6
1
investigated. As it can be seen, when the temperature rises from 263 K to 313 K, a 10% increase in efficiency also emerges. These results support the principle that fuel cell systems operated in warmer climate areas are more efficient [3]. The figure also shows the system can sustain the power production even with a low efficiency value of being under 0 C degrees. Meanwhile, the environmental pressure parameter was also inspected and its effect on the system efficiency was found out to be less than 1%. Thus it was assumed as having no significant effect on the performance and was omitted from the graph and its effects were ignored in later calculations. In parametrical evaluation of anode stoichiometry, the lowest possible excess fuel amount was found to have the most favorable effect on the system. Increasing the stoichiometry also causes a drop in the produced power. This situation, which is represented in Fig. 9, shows us that after some point, further increase in the supplied fuel rate stops having any positive effect on the reaction speed. On the contrary, it creates unnecessary consumption of fuel, and as it affects the overall system pressure and fuel crossover, it starts to have negative effects. When the cathode stoichiometry of the system was parametrically inspected between the values of 2e3, it was found out to have negligible effects on efficiency and produced
263 K 283 K 303K 323 K
7
5
Fig. 7 e Efficiency and Power with respect to pressure and current density.
Efficiency
8
3
Current Density [mA/cm 2]
25
Power
10
0 200
Efficiency
15
1
0 0
Sto-ano 1.5 Sto-ano 2 Sto-ano 2.5 Sto-ano 3
20
6
Power[kW]
7
Power
5 15
4
10
3 2
5
Cat. Stoich. 2 Cat. Stoich. 3
0 0
100
200
300
400
Cat. Stoich. 2.5
1 0
500
600
700
800
900
Current Density [mA/cm2]
Fig. 10 e Efficiency and Power with respect to cathode stoichiometry and current density.
Power [kW]
Efficiency 20
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8
25
Efficiency
Power
7
6 5
15
4
10
3 0.01 mm 0.014 mm 0.018 mm
5
2
0.012 mm 0.016 mm 0.02 mm
1
0 0
200
400
600
Power[kW]
Efficiency[%]
20
800
0 1000
Current Density [mA/cm2]
Fig. 11 e Efficiency and Power with respect to membrane thickness and current density.
is generally effective on the higher current densities as it reaches up to a 4% effect with 2 kW power production increment. It is seen that cathode stoichiometry and reference pressure has negligible effect on the performance of the system where anode stoichiometry is excessively dominant in the efficiency. The environment temperature proportionally favors the fuel cell production, and it can be concluded that high climate zones are more efficient to use the DFAFC system. Considering the complex structure of the fuel cells and systems the need for proper modeling and simulation is crucial. Combining electrochemical modeling and exergy as a tool, the performance of a fuel cell system reveals greatly if a broad investigation is made. This study shows researchers a view point for evaluating the limits and performance characteristics of a DFAFC system. Following to this study conducting a thermo-economic (exergoeconomic) analysis as well as an optimization and multi-objective optimization also will completely identify the performance tendencies of the DFAFC system.
Acknowledgment The authors gratefully acknowledge the financial support provided by the Scientific and Technological Research Council _ ¨ BITAK-MAG-114M156) of Turkey (TU
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Nomenclature A: Area, cm2 a: Membrane activity E: Cell Voltage, V _ Exergy, W, kW Ex: F: Faraday's Constant, C/mole h: Enthalpy, kJ/kg i: Current Density, mA/cm2 m: Molar Flowrate, mole/s ncell: Number of Fuel Cells P: Pressure, atm, bar R: Resistance, 1/ohm
s: Entropy, kJ/kgK T,Temp: Temperature, K tmem: Membrane thickness, mm x: Mole Fraction, _ Power, kW, W W: Greek Letters h: Efficiency r: resistivity, ohm lmem: Membrane water content smem: Membrane conductivity, 1/U cm Subscripts 0: Reference State A: Anode act: Activation C: Cathode con: Concentration comp: Compressor fc: Fuel Cell ohm: Ohmic mem: Membrane rev: Reversible W: Power Acronyms DC: Direct Current DFAFC: Direct Formic Acid Fuel Cell MEA: Membrane Electrode Assembly PEM: Polymer Electrolyte Membrane
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Performance assessment of a direct formic acid fuel cell system through exergy analysis Alper Reis, Suha Orc¸un Mert* Yuzuncu Yıl University, Faculty of Engineering and Architecture, Department of Chemical Engineering, Van, 65080, Turkiye
article info
abstract
Article history:
In this study, performance assessment of a direct formic acid fuel cell system is conducted,
Received 4 June 2015
including evaluation of a broad set of operating parameters. The fuel cell system is
Received in revised form
modeled and simulated by using sub-routine developed by Matlab. The system compo-
23 July 2015
nents, such as compressor, humidifier and heat exchangers, are also taken into consid-
Accepted 24 July 2015
eration. Effects of varying operating temperature, pressure, membrane thickness, current
Available online 14 August 2015
density, anodeecathode stoichiometry and reference properties on the system are evaluated to predict the performance as close to real-cases as possible. It is found that increasing
Keywords:
the temperature greatly favors the performance of the direct formic acid fuel cell system,
Formic acid fuel cell
low pressure and high reference environment temperature values have the same effect on
Exergy
the cell performance. The maximum exergetic efficiency by broad parametric investigation
Performance
is estimated as 24%, where the power production is 10 kW.
Efficiency
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The basic properties expected from an alternative fuel may be summarized in terms of having high power production, being non-toxic, environmentally benign, compatible with various energy producers and being economic. One alternative fuel that may be considered to having all these different important features while also being potentially limitless is, hydrogen. Hydrogen energy usage has an increasing trend worldwide with the enormous research on fuel cells which is the most efficient way of converting its chemical energy to electrical energy. Fuel cells are relatively new type of energy generator that has developed quickly, especially in the second half of 20th century. Nowadays the “fuel chemical energy-heat energy-
mechanical energy” transformation scheme in the thermal systems leaves the stage rapidly to “fuel chemical energyelectric energy-mechanic energy” systems in the fuel cells with increasing research and development. In a fuel battery, electricity is produced through an electro-chemical transformation, without combustion. The produced electricity can be used for any desired purpose [1,2]. Thus, the fuel cell is a generator that turns the fuel's chemical energy directly into electric energy. After the fuel battery reaction, which may also be described as the reverse reaction of electrolysis, DC electricity is produced. When studies about the performance of the fuel cell systems in the literature is investigated, it is found that various studies deal with the electrochemical modeling and parametric investigation of single or stack fuel cells and systems [3e9]. There is numerous research, in the literature,
* Corresponding author. Tel.: þ90 505 2356022; fax: þ904322251730. E-mail addresses: [email protected] (A. Reis), [email protected] (S.O. Mert). http://dx.doi.org/10.1016/j.ijhydene.2015.07.131 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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conducted on energetic performance. Limited studies focused on exergoeconomic studies [6,9e13]. Similarly, thermal management is another dynamic topic that is covered [14]. However, most of these studies tend to consider fuel cell systems other than DFAFC's, such as Polymer Electrolyte Fuel Cells (PEMFC), Direct Methanol Fuel Cells (DMFC) and Solid Oxide Fuel Cells (SOFC). The general studies in the literature about electrochemical modeling and performance investigation cases [15e19] regarding DFAFCs systems seem to be focused on performance improvements on the DFAFC's, and more from catalysts' and electrodes' point of views [2,20,21]. Rejal et al. [15], suggest that the overall DFAFC performance was primarily determined by the rate-determining steps, such as fuel and oxygen supply/feed, and electrode reaction. The study of Yu and Pickup [22] indicates that peak power density reached about 375 mW/cm2 with an operating temperature of 50 C, using Pd catalyst. Adding to the literature, this study looks the performance, of a Direct Formic Acid Fuel Cell (DFAFC) system through exergy analysis. This is done through using a wide variety of operating parameters, which also include equipment used in the system as a separate parameter of performance. Since there is currently lack of studies in the literature about the subject, the real performance of the DFAFC system is revealed and this study is believed to contribute to the research on the evaluation of the DFAFC system.
volumetric energy density. This can be remedied by the use of a high concentration of formic acid feed. The electrode and cell reactions that occur in the formic acid fuel cell is as follows [18]: Anode reaction : HCOOH/CO2 þ 2e þ 2Hþ 1 O2 þ 2Hþ þ 2e /H2 O 2 1 Cell reaction : HCOOH þ O2 þ Hþ /CO2 þ H2 O 2 Cathode reaction :
(1)
System description A direct formic acid supplied energy generation system (Fig. 2) is evaluated in this study considering storage to heat integration for revealing the exergetic performance as being a candidate to renewable and sustainable energy producer. Formic acid is supplied from a storage tank and air is supplied by the compressor to the system. The formic acid is initially sent from the tank to the heat exchanger by the utilization of a pump. This is done to increase the temperature of the formic acid to temperatures for the reaction before it arrives into the fuel cell. While this process is occurring on one side of the system, the air received from the compressor on the other end of the system first needs to be humidified to sustain the water management in the fuel cell. Water, CO2 and the unreacted components leave the system as a result of these reactions. . For each of the specified currents below, exergy analysis was conducted and the change of efficiency based alterations on various parameters was analyzed. During the modeling some assumptions were made for the sake of ease in calculations. These were:
Direct formic acid fuel cell system In Fig. 1, a generalized anode cathode structure of a fuel cell is shown. Fuel and other reactants are supplied to the prevailing section where the reaction occurs. DFAFCs have a great advantage compared to other fuel cell types as they have a relatively high electromotive force (theoretical open circuit potential 1.48 V) [22]. Formic acid fuel cell systems generally have higher cell voltages when compared to the other popular fuel cell systems [3,23]. Another supplementary advantage is the limited fuel crossover and relatively high power densities. However, the main disadvantage, on the other hand, is having a low
DFAFC system cell was taken as steady state. The heat loss during production was taken as zero [24] with proper insulation. Relative humidity content of air fed to the system was taken as 50%. Isentropic efficiency of the pumps are taken as 85%. Reference condition is taken as 298 K and 1 atm.
Oxidant (e.g.,Oxygen, Air….)
Reaction Products (e.g., H2O, CH2....) & Heat
Cathode Load
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Electrolyte Anode -
e
Fuel (e.g., H2, CH4, CO, CH3OH) Fig. 1 e General fuel cell mechanics.
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Fig. 2 e Flow chart of a formic acid fuel cell system (DFAFC).
Fuel cell stack was taken as single cell. Area of fuel cell was taken as 25 cm2. Isentropic efficiency of the compressor is taken as 85%.
Exergetic aspects Exergy analysis is utilized to detect and reduce the inadequacies in the design of a system regarding the results of investigation with a second law based approach to harness a higher level of efficiency [25]. The notion of exergy states that it is relative to the environmental conditions and the magnitude of useful energy changes depending on these conditions without neglecting the first law of thermodynamics [26]. In order to be able to define “exergy” accurately, some fundamental thermodynamics should first be clarified. One clarification is that theoretically, to get the “maximum possible work” out of a system, two conditions should first be met: to get the work done in a “completely reversible change of state” in the system -from beginning to the end- and to have the final state of the system “in complete balance with the environment it is within”. From that, we get the term ‘irreversibility’. There is zero “irreversibility” in a completely reversible state change, and the maximum amount of useful work that can be harnessed from a system is defined as “reversible work” [27]. That being said, the maximum amount of work that can be harnessed from a system is also dependent on environmental conditions, along with aforementioned reversible changes in states. A system that is thermodynamically in equilibrium with the environment it is defined to be at “dead state”. In a dead state, the system also has zero relative kinetic and potential energy to its environment and a chemical balance also exists in this state. Under these conditions, it is impossible for a system to have any kind of interaction with the environment. In other words, no work can be harnessed from a system in a dead state. Exergy analysis mainly identifies and underlines the sources of inefficiencies through the system by revealing the irreversibilities and exergy destruction that gives chance to engineers to focus on these areas. _ phy ), Exergy has various components such as physical (Ex _ _ _ chemical (Exch ), kinetic (Exk ) and potential (Exp ) exergies.
_ ¼ Ex _ phy þ Ex_ch þ Ex_k þ Ex_p ¼ m* _ exphy þ exch þ exk þ exp Ex
(2)
Physical exergy is in direct relation with environmental conditions exphy ¼ ðh h0 Þ T0 *ðs s0 Þ
(3)
The benefits of exergy analysis can be summarized as follows: Determines the real value of thermal losses in a system, quantifies thermodynamic defects of a system. From a thermodynamics perspective, it is a tool to define energy quality. It enables comparison between systems with equal efficiency. Shows possibilities to reduce existing inefficiencies in a system. Gives opportunity to evaluate the performance of systems and processes in which the first law of thermodynamics is insufficient to determine. Since exergy is the parameter that defines real amount of energy used in thermal systems, exergy analysis is naturally convenient in preliminary design phases, feasibility surveys and optimization studies of systems [28].
Modeling of the DFAFC system Electrochemical modeling While modeling a fuel cell, initial step should be calculation of overpotential losses that will take place. The “net potential” will be deduced based on these irreversibilities. Potential sources for these losses are described below: - Reversible cell voltage Cell voltage provides maximum efficiency in the absence of irreversibilities. By applying Nernst Equation to the formic acid fuel cell, the equation below is formulated [3]: 8:314*T Erev ¼ 1000* 1:45 2*F
(4)
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The equation shows that local pressures and environmental temperatures have great effect on the amount of losses. Here, T represents the temperature of the fuel cell, F stands for Faraday constant.
lmem ¼
Activation loss is the energy loss that takes place initially in the operation which initiates the electron transfer through anode and cathode. It is caused by slow reaction rates on anode and cathode [29].
; ;
0
(12) a¼
- Activation Overpotential
0:143 þ 17:81a 39:85a2 þ 39:85a3 14:1 þ 1:4ða 1Þ
xH2 O P Psat
(13)
where a is the membrane water activity.
Exergetic modeling
i0a ¼ 0:0035*eð0:0132*TÞ
(5)
The general exergy balance equation for the direct formic acid fuel system is derived as:
i0c ¼ 0:0032*eð0:0124*TÞ
(6)
_ fc;in SEx _ fc;out SEx _ fc;heat SEx _ fc;work _ fc;D ¼ SEx Ex
Eact ¼ Aa *logði þ in Þ=i0a Þ þ Ac *logði þ in Þ=i0c Þ
(7)
In the equations above, in represents equipotent current density of fuel, Aa and Ac represent Tafel reflections on anode and cathode, respectively, i0a and i0c represent local current densities in anode and cathode, respectively. - Concentration Overpotential
In this equation the exergy destruction is tired to be calculated considering inlet and outlet exergies of the streams (fuel and exhaust) in addition to the heat loss and the electrical power produced by the fuel cell system. Power generated by the fuel cell system is evaluated by taking the cell active area and number of cells to the consideration and the amount of power produced by the system is revealed as: Wstack ¼ EiAcell ncell =1000
With the increase in current density, difficulties in mass transfer and some overpotential in anode and cathode begin to appear. As the reaction progresses, concentration shifts towards the surface reactions, which causes the emergence of “concentration gradient”. High current density and concentration gradient are limiting factors for the reaction rate [29]. i þ in i þ in þ Bc *log 1 Econ ¼ Ba *log 1 i1a i1c
In the equation above Ba and Bc represent mass transfer parameters for anode and cathode, respectively, i1a and i1c represent limiting current density in anode and cathode, respectively. - Ohmic Overpotential
Rohm ¼
tmem smem
(9) (10)
smem is the membrane conductivity (1/U cm) and the fuel cell operating temperature and the relative humidity are effective on the membrane conductivity as given in the flowing equations: 1 1 smem ¼ ð0:005139lmem 0:00326Þexp 1268 303 TFC where, lmem is the membrane water content [30];
(16)
The power consumption of the system equipment are calculated and evaluated as total used power ðWused Þ in the system. By deducing the consumed power by system components, the net power is found: Wnet ¼ Wstack Wused
(17)
The amount of fed formic acid evaluates the total exergetic performance of the system:
The electrical resistance on the MEA components arises. Ohmic overpotential is dependent on the current density. The resistance to the flow of protons in the membrane is much more effective than the resistance of the electrodes and resistance of the bipolar plates [11]. vohm ¼ i Rohm
(15)
The model used for the exergetic calculations of the fuel cell system includes the actual consumption of the compressor and pumps in the system considering the parametric investigation operating parameters and dynamically recalculates the values dependently. Wused ¼ Wactcomp þ Wpump;2 þ Wpump;1
(8)
(14)
(11)
nsystem ¼
Wnet Ex_FA
(18)
Parametric study and simulation The current study covers the parametric investigation of the performance of the DFAFC system. The investigated operating parameters are formic acid concentration, temperature, current density and reference temperature (Table 1). The effect of these parameters on energy and exergy efficiency, power production and emission values are calculated and investigated. Through this research, the real behavior of the DFAFC system is revealed. In addition to this, operating conditions are proposed for the efficient and environmentally friendly operation of the fuel cell system. The exergetic fuel cell performance model is developed using Matlab and the performance characteristics such as efficiency, polarization curve and power production values are
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1.00
Table 1 e Parametric investigation ranges. Base case
Formic acid concentration [M] Temperature [K] Current density [mA/cm2] Reference temperature [K]
2e20 323e393 0e1000 263e313
10 323 850 298
plotted and data are gathered using an in-house developed mfile that solves the model in a sequential oriented structure. The algorithm of the derived program is given in Fig. 3.
294 K 306 K 294 K Exp. 306 K Exp
0.90
0.80 0.70 0.60 0.50 0.40 0.30 0.20 0
200
400 i [mA/cm2]
Model verification
The parametric investigation shows that produced power and efficiency of the fuel cell increase as the operating temperature increases. It seems that the effect of the activation overpotential (Fig. 5) is larger when compared to the other fuel cell systems [32,33] as there is a 30% activation loss from total potential of the FAFC system, whereas it is seen that this value is around 20% in PEM Fuel Cell [3], 15% in SOFC systems [32]. Increase in the current density along with the temperature, however, causes an efficiency and potential drop. This can be seen in Fig. 6. Increase in produced power will enable usage of the fuel cell in various systems, along with potential reductions in cost. Fig. 7 summarizes that as the fuel cell working pressure increases, the efficiency decreases proportionally. Along with efficiency, produced power also declines. It is also noticeable that when the pressure is increased to 3 atm from 1 atm, the produced power value falls to 5.4 kW from its peak point of 6.2 kW. The increase in the pressure causes power consumption as well as the fuel crossover values to increase. These are the phenomena which lead to total decrease in
I/V
•Revealing the polariza on curve for selected opera ng parameters for the fuel cell stack
System Analysis
•Calcula on of the flow proper es and power needs of system equipment
1000 323 K 343 K 363 K 383 K
900 800 Cell Voltage[V]
Results and discussion
•Applica on of exergy analysis considering system equipment and fuel cell
Fig. 3 e Algorithm of the developed program.
333 K 353 K 373 K 393 K
700 600 500 400 300 0
100
200
300
400
500
600
700
800
900
Current Density [mA/cm2]
Fig. 5 e Polarization curve for DFAFC for various temperatures.
power production and efficiency with the increment in pressure increment. In Fig. 8, the relation between power and efficiency values of the fuel cell and environmental temperatures are
30
12
Efficiency
Power
25
10
20
8
15
6
10
4
5
323 K 363 K
0
Exergy Analysis
800
Fig. 4 e Comparison of present model with experimental data [29,31].
Efficiency[%]
The developed models validation is issued using experimental studies results in the same operating conditions [29,31]. Ten molal formic acid was used at a flow rate of 1 ml min1 in these experiments and dry air was supplied to the cathode at a flow rate of 390 sccm. It is seen that there is a small gap between the values of polarization in two different operating temperatures (Fig. 4). This error is in an acceptable range and the model is thought as validated for investigation of the performance characteristics of formic acid fuel cell systems.
600
0
200
333 K 373 K
343 K 383 K
400 600 Current Density [mA/cm2]
353 K 393 K 800
Power [kW]
Ranges
Cell Voltage[V]
Parameters
2
0 1000
Fig. 6 e Efficiency and Power with respect to temperature and current density.
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10
3 2
5
1 atm
1.5 atm
2.5 atm
3 atm
2 atm
Efficiency[%]
4
Power[kW]
Efficiency[%]
5
15
400
600
273 K 293 K 313 K
2 1 0
500
600
400
500
600
700
800
900
Fig. 9 e Efficiency and Power with respect to anode stoichiometry and current density.
power. (Fig. 10). This expected situation is a result of the supplied air's exergy value which is zero since it fed to the system in reference conditions. It is well known that increasing the membrane thickness reduces the fuel crossover effect [34], but also has a negative effect due to the increase of resistance it creates on the transport of electrons. As Fig. 11 shows, lower thickness increases the produced power, and exergetic efficiency increases as an expected result of this. A 1 cm increase in the thickness causes 3% drop in efficiency and 2 kW drop in produced power. Lower membrane thicknesses are shown to be more favorable, even though they have some adverse effects.
Conclusion In conclusion, the performance assessment of a selected direct formic acid fuel cell system is applied in this study. The investigation covers a broad parametric study considering operating temperature, pressure, membrane thickness, anodeecathode stoichiometry and reference temperature. It is found that increasing operating temperature may lead to a performance increment as 5% in efficiency and 2 kW in power production. Moreover low pressures favor system performance as it affects the efficiency by an average of 3% in the range of 1e3 atm pressure. Using small membrane thickness
700
800
900
Current Density [mA/cm2]
Fig. 8 e Efficiency and Power with respect to references temperature and current density.
7 Efficiency
Power
6
20 Efficiency[%]
3
400
300
25 Power[kW]
Efficiency[%]
4
10
300
200
8
5
200
0
100
9
Power
6
100
2
i [mA/cm2]
15
0
4
0
7
0
5
0
800
20
5
6
1
investigated. As it can be seen, when the temperature rises from 263 K to 313 K, a 10% increase in efficiency also emerges. These results support the principle that fuel cell systems operated in warmer climate areas are more efficient [3]. The figure also shows the system can sustain the power production even with a low efficiency value of being under 0 C degrees. Meanwhile, the environmental pressure parameter was also inspected and its effect on the system efficiency was found out to be less than 1%. Thus it was assumed as having no significant effect on the performance and was omitted from the graph and its effects were ignored in later calculations. In parametrical evaluation of anode stoichiometry, the lowest possible excess fuel amount was found to have the most favorable effect on the system. Increasing the stoichiometry also causes a drop in the produced power. This situation, which is represented in Fig. 9, shows us that after some point, further increase in the supplied fuel rate stops having any positive effect on the reaction speed. On the contrary, it creates unnecessary consumption of fuel, and as it affects the overall system pressure and fuel crossover, it starts to have negative effects. When the cathode stoichiometry of the system was parametrically inspected between the values of 2e3, it was found out to have negligible effects on efficiency and produced
263 K 283 K 303K 323 K
7
5
Fig. 7 e Efficiency and Power with respect to pressure and current density.
Efficiency
8
3
Current Density [mA/cm 2]
25
Power
10
0 200
Efficiency
15
1
0 0
Sto-ano 1.5 Sto-ano 2 Sto-ano 2.5 Sto-ano 3
20
6
Power[kW]
7
Power
5 15
4
10
3 2
5
Cat. Stoich. 2 Cat. Stoich. 3
0 0
100
200
300
400
Cat. Stoich. 2.5
1 0
500
600
700
800
900
Current Density [mA/cm2]
Fig. 10 e Efficiency and Power with respect to cathode stoichiometry and current density.
Power [kW]
Efficiency 20
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8
25
Efficiency
Power
7
6 5
15
4
10
3 0.01 mm 0.014 mm 0.018 mm
5
2
0.012 mm 0.016 mm 0.02 mm
1
0 0
200
400
600
Power[kW]
Efficiency[%]
20
800
0 1000
Current Density [mA/cm2]
Fig. 11 e Efficiency and Power with respect to membrane thickness and current density.
is generally effective on the higher current densities as it reaches up to a 4% effect with 2 kW power production increment. It is seen that cathode stoichiometry and reference pressure has negligible effect on the performance of the system where anode stoichiometry is excessively dominant in the efficiency. The environment temperature proportionally favors the fuel cell production, and it can be concluded that high climate zones are more efficient to use the DFAFC system. Considering the complex structure of the fuel cells and systems the need for proper modeling and simulation is crucial. Combining electrochemical modeling and exergy as a tool, the performance of a fuel cell system reveals greatly if a broad investigation is made. This study shows researchers a view point for evaluating the limits and performance characteristics of a DFAFC system. Following to this study conducting a thermo-economic (exergoeconomic) analysis as well as an optimization and multi-objective optimization also will completely identify the performance tendencies of the DFAFC system.
Acknowledgment The authors gratefully acknowledge the financial support provided by the Scientific and Technological Research Council _ ¨ BITAK-MAG-114M156) of Turkey (TU
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Nomenclature A: Area, cm2 a: Membrane activity E: Cell Voltage, V _ Exergy, W, kW Ex: F: Faraday's Constant, C/mole h: Enthalpy, kJ/kg i: Current Density, mA/cm2 m: Molar Flowrate, mole/s ncell: Number of Fuel Cells P: Pressure, atm, bar R: Resistance, 1/ohm
s: Entropy, kJ/kgK T,Temp: Temperature, K tmem: Membrane thickness, mm x: Mole Fraction, _ Power, kW, W W: Greek Letters h: Efficiency r: resistivity, ohm lmem: Membrane water content smem: Membrane conductivity, 1/U cm Subscripts 0: Reference State A: Anode act: Activation C: Cathode con: Concentration comp: Compressor fc: Fuel Cell ohm: Ohmic mem: Membrane rev: Reversible W: Power Acronyms DC: Direct Current DFAFC: Direct Formic Acid Fuel Cell MEA: Membrane Electrode Assembly PEM: Polymer Electrolyte Membrane
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