- Email: [email protected]
Performance study of a PEM fuel cell
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 3 7 ( 2 0 1 2 ) 1 4 7 5 7 e1 4 7 6 0
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Performance study of a PEM fuel cell A.E. Gonnet a,*, S. Robles b, L. Moro a a b
Facultad Regional Bahı´a Blanca UTN, 11 de Abril 461, 8000 Bahı´a Blanca, Argentina Departamento de Ingenierı´a, Universidad Nacional del Sur, Av. Alem 1253, Bahı´a Blanca, Argentina
article info
abstract
Article history:
This paper presents the installation, maintenance and the efficiency of a Polymer Elec-
Received 13 September 2011
trolyte Membrane (PEM) fuel cell, Ballard Trade Mark that use pure hydrogen as fuel and air
Accepted 12 December 2011
as an oxidant. A study of the overall efficiency, considering the co-generation of electrical
Available online 4 January 2012
and thermal energies, is performed. The system consists of the cell, a CC/CC converter, a battery, a DC/AC inverter and the load. The behavior of the system is experimentally
Keywords:
analyzed for different load states (cases) by measuring and controlling all the parameters
Fuel cell
registered by the communication software of the cell. The software can adjust limit values
Hydrogen
for current intensity, hydrogen flow, pressure and the temperature.
Energy efficiency
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Due to the limited and decreasing supply of fossil fuels and the environmental damage caused by combustion exhaust, such as global climate change and air pollution, polymer Electrolyte Membrane (PEM) fuel cells, which convert the chemical energy stored in hydrogen fuel directly and efficiently to electrical energy, with water as the only byproduct, are regarded as an alternative power source for transportation, stationary, and portable applications. Fuel cells efficiency can reach as high as 60% in electrical energy conversion and overall 80% in co-generation of electrical and thermal energies with 90% reduction in major pollutants [1]. Phenomena involved in PEM fuel cell operation are complex; they involve heat transfer, species and charge transport, multi-phase flows, and electrochemical reactions. Fundamentals of these multi-physics phenomena during fuel cell operation and their relevance to material properties are critically important to overcome the two major barriers durability and cost. Even today these factors still remain as the major barriers to fuel cell commercialization [2].
In the present study the main purpose is to investigate the performance of a fuel cell model Nexa comparing it with the theoretical modeling.
2.
Cell description
Basically the fuel cell consists of a battery of single cells and an ancillary equipment to provide hydrogen, and air for both the reaction and cooling. It has embedded sensors to monitor performance, and a control board with a microprocessor to achieve fully automatic operation. It also has a security system that allows operating indoors. Fig. 1 shows a schematic diagram of the studied cell. Dotted lines indicate the limit of the components that comprise it, in addition the most important connections that serve as interfaces are shown [3]. The output of the cell is connected to a power conditioning equipment. Furthermore, communication with a computer through a serial port can be established. The cell provides a nominal power of 1200 W. The output voltage varies with current, ranging from 43 V to 26 V (no load e full load).
* Corresponding author. E-mail addresses: [email protected] (A.E. Gonnet), [email protected] (S. Robles), [email protected] (L. Moro). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.12.076
14758
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 3 7 ( 2 0 1 2 ) 1 4 7 5 7 e1 4 7 6 0
Fig. 1 e Diagram of the fuel cell components.
For safety, during operation, the voltage of the cell battery is checked as part of a control and diagnostic. At the same time each cell is checked by a voltage called cell voltage (CVC) to detect anomalies. The cell is turned off under failures or conditions of unsafe operation detected. The equipment is designed to operate with pure hydrogen gas. The humidifying is not required the fuel. Hydrogen can be supplied at pressures ranging from 70 to 1720 KPa. Fig. 2 shows the installation of the storage cylinder of hydrogen to supply the cell. A solenoid valve is installed in the cylinder. This valve can be commanded manually, or via the control board of the cell.
Over the cell is located a hydrogen sensor to make the system safer indoors. Otherwise the system for the conditioning of the cell output power, according to the scheme in Fig. 3, are comprise by the following elements: a CC/CC BSZ PG 1200 converter; two rechargeable batteries 12 V, 18 Ah; a norm converter RS232/ RS485 for PC serial port communication; an investor CC/CA 1500 W and a Relay for load connection and disconnection. The output voltage of the cell, which varies with the load, is conditioned to a constant voltage of 24 V, value that is independent of the load, with a CC/CC converter. The use of rechargeable batteries connected to the converter output,
Fig. 2 e Scheme of the hydrogen supply.
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 3 7 ( 2 0 1 2 ) 1 4 7 5 7 e1 4 7 6 0
So that, at operating temperature of 80 C has a value of 0.80. In theoretical operation, the voltage of 1.18 per cell, should be maintained for any state of load, however due to the polarization and ohmic losses, the voltage drops when current increases. In a real cell, the voltage can reach 1.1 V in the unloaded condition and may decrease to a value of 0.5 to 0.6 V, operating normally. Is convenient to express the actual efficiency of a cell based on the relationship between present and ideal voltage. Then, replacing [6]:
Fig. 3 e Scheme of conditioning the power output.
ensure a dynamic and stable operation, on the other hand the battery control system keeps them charged [4]. The system delivers power to the load through the investor DC/AC. If the battery voltage decreases, the converter turns the cell to charge the batteries, while delivering power to the load. Using a control panel allows turned the cell on or off manually as well as read the parameters of the converter and the cell.
hreal ¼ 0:68 Vpresent
(5)
In practice not all the fuel supplied to the cell is consumed in the reaction, thus defining a utilization coefficient [5]. m¼
mass of fuel consumedin the reaction mass of fuel ssuplied to the cell
Theorical formulation
One of the main advantages of fuel cells is its efficiency. It should be keep in mind that there is a difference between the efficiency of the electrochemical reaction that occurs and the efficiency of the entire set, that is the fuel cell, including auxiliary systems and power conditioning system. The overall reaction for a fuel cell fed with hydrogen and oxygen is H2 þ 1=2 O2 /H2 O
(1)
The amount of energy released with this reaction is obtained through the reaction enthalpy ΔH0, and has a value of 285.8 kJ mol1 of water, at 25 C and 1 atm. This value is more affected by changes in temperature than in pressure. The amount of this energy available as useful work is given by the change in Gibbs free energy (ΔG)0, and has a value of 237.1 kJ mol1 of water at 25 C and 1 atm. Taking into account the effects of temperature, its value at the operating temperature of 80 C is 228.2 kJ mol1 of water [5]. The theoretical voltage of a single PEM fuel cell is related to Gibbs free energy (DG) by: Vtheoretical ¼
DG nF
(6)
Thus the cell efficiency expressed: Efficiency ¼ mf hreal ¼ mf 0:68 Vpresent
3.
14759
(7)
Estimating a value of 0.95 for the utilization coefficient [5]. Another useful way to evaluate efficiency is relating the output electric power necessary ( p) and the voltage (Vpresent), to refer to hydrogen as fuel consumption. Then the hydrogen consumption is calculated considering that the cell needs 0.037605 kg of hydrogen per hour for an electric current of 1000 A [6]: p 0:037605 Vpresent $1000
mh ¼
(8)
Finally this result should be corrected by the coefficient of utilization and the consumer of auxiliary system of the fuel cell. Then, the efficiency of the fuel cell is calculated as: Efficiency of cell ¼
Electric energy generated Chemical energy absorbed
(9)
And the total efficiency, are considered the performance of total system, to be the efficiency of the CC/CC converter (hCC) and the CC/CA investor (hCA) are both 95% [6]: EfficiencyTotal ¼ Efficiencycell $hCC $hCA
(10)
The total efficiency that is to be account when comparing with other traditional systems. This efficiency can be
(2)
where n is the number of electrons involved in the reaction expressed in mole of electrons and F is the Faraday’s constant (96,500 Coulombs/mole of electrons). Since DG, n and F are all known constants, the theoretical voltage of a single hydrogen/ oxygen fuel cell at 25 C and atmospheric pressure is 1.182 V. Thermodynamic efficiency of a fuel cell is defined as the ratio of useful energy produced and the enthalpy of the product (water) and reactants (hydrogen and oxygen). h¼
Produced Energy DH
(3)
Therefore, the theoretical efficiency of the cell operating reversibly is: htheoretical ¼
DG DH
(4)
Fig. 4 e Hydrogen consumption versus currents intensity.
14760
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 3 7 ( 2 0 1 2 ) 1 4 7 5 7 e1 4 7 6 0
b- The air filter of the cell requires replacement every 500 h operation, although this will depend on the environmental conditions. c- The fuel cell incorporates a humidity exchanger which requires replacement every 800 h. The study of all factors affecting the efficiency of a complete system is complex and depends on the fuel source, processing method and the difficulties of handling and transport. Comparing this method with an others should be has the advantage of minimal environment impact and the potential for cogeneration. Given the danger that the escape of hydrogen, was considered important to provide the compound of a system for monitoring the presence of hydrogen with visible an audible alarm [8]. The cell maintenance requires are easy to operate, the only difficulty is the replacement of humidity exchanger for which needs certain previous knowledge.
Fig. 5 e Efficiency theoretical and experimental versus currents intensity.
increased with the use of heat and water produced during the reaction [7]. The amount of water produced is calculated as: 8
mH2 O ¼ 9:34$10
4.
p
Vpresent
(11)
Results and discussion
In Figs. 4e6 are represented the values of the hydrogen consumption, the efficiency and the water produced for different currents intensities values. The hydrogen consumption and the water produced increase while the efficiency decreasing when the currents intensities increase. The installation requires periodically perform for maintenance, the task listed bellow: a- The fuel cell incorporates an automatic rejuvenation by which losses are corrected. To maintain the maxim performance we recommend that the process is carried out each two or three month or after a prolonged storage.
Fig. 6 e Water produced versus currents versus currents intensity.
5.
Conclusions
The efficiency of the fuel cell found experimentally reflects the behavior expected from theoretical modeling. The amount of water produced by the fuel cell, and theoretically estimated, are approximate results. The difference is attributed that in the case theoretically were not taken into account the consumptions of auxiliary equipment. The calculation of the performance of the devices used in the equipment the fitting out of the power output, it is possible to find the overall performance of the generating plant. Comparing this method with an others should be has the advantage of minimal environment impact and the potential for cogeneration. The efficiency of the fuel cell varies from a value of 0,53 to 0,38, because with increase the currents intensities the efficiency decreasing.
references
[1] Wang Y, Chen KS, Mishler J, Chan Cho S, Cordobes Adroher X. A review of polymer electrolyte membrane fuel cells technology, applications, and needs on fundamental research. Applied Energy 2011;88(4):981e1007. [2] Wu J, Zi Yuan X, Martin JJ, Wang H, Zhang J, Shen J, et al. A review of PEM fuel cell durability: degradation mechanisms and mitigation strategies. Journal of Power Sources 2008;184: 104e19. [3] Ballard Power Systems Inc Nexa. Power module user’s manual; 2003. [4] BSZ PG1200 Technical Description, Isle Gmbh, Limenou, Germany, version 1.2. [5] Larminie JE, Dicks A. Fuel cell system explained. Chichester, U.K: Wiley; 2003. cap. 2, pp. 33. [6] Fuel cell handbook. 7th ed. Morgantown, West Virginia: U.S. Department of Energy; 2004. cap. 9 y 8. [7] Larminie JE, Dicks A. Fuel cell system explained. Chichester, U. K: Wiley; 2003. Appendix2 pp. 399. [8] Scott Healt & Safety, Freedom 5000, Universal Analog gas Transmitter.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Performance study of a PEM fuel cell A.E. Gonnet a,*, S. Robles b, L. Moro a a b
Facultad Regional Bahı´a Blanca UTN, 11 de Abril 461, 8000 Bahı´a Blanca, Argentina Departamento de Ingenierı´a, Universidad Nacional del Sur, Av. Alem 1253, Bahı´a Blanca, Argentina
article info
abstract
Article history:
This paper presents the installation, maintenance and the efficiency of a Polymer Elec-
Received 13 September 2011
trolyte Membrane (PEM) fuel cell, Ballard Trade Mark that use pure hydrogen as fuel and air
Accepted 12 December 2011
as an oxidant. A study of the overall efficiency, considering the co-generation of electrical
Available online 4 January 2012
and thermal energies, is performed. The system consists of the cell, a CC/CC converter, a battery, a DC/AC inverter and the load. The behavior of the system is experimentally
Keywords:
analyzed for different load states (cases) by measuring and controlling all the parameters
Fuel cell
registered by the communication software of the cell. The software can adjust limit values
Hydrogen
for current intensity, hydrogen flow, pressure and the temperature.
Energy efficiency
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Due to the limited and decreasing supply of fossil fuels and the environmental damage caused by combustion exhaust, such as global climate change and air pollution, polymer Electrolyte Membrane (PEM) fuel cells, which convert the chemical energy stored in hydrogen fuel directly and efficiently to electrical energy, with water as the only byproduct, are regarded as an alternative power source for transportation, stationary, and portable applications. Fuel cells efficiency can reach as high as 60% in electrical energy conversion and overall 80% in co-generation of electrical and thermal energies with 90% reduction in major pollutants [1]. Phenomena involved in PEM fuel cell operation are complex; they involve heat transfer, species and charge transport, multi-phase flows, and electrochemical reactions. Fundamentals of these multi-physics phenomena during fuel cell operation and their relevance to material properties are critically important to overcome the two major barriers durability and cost. Even today these factors still remain as the major barriers to fuel cell commercialization [2].
In the present study the main purpose is to investigate the performance of a fuel cell model Nexa comparing it with the theoretical modeling.
2.
Cell description
Basically the fuel cell consists of a battery of single cells and an ancillary equipment to provide hydrogen, and air for both the reaction and cooling. It has embedded sensors to monitor performance, and a control board with a microprocessor to achieve fully automatic operation. It also has a security system that allows operating indoors. Fig. 1 shows a schematic diagram of the studied cell. Dotted lines indicate the limit of the components that comprise it, in addition the most important connections that serve as interfaces are shown [3]. The output of the cell is connected to a power conditioning equipment. Furthermore, communication with a computer through a serial port can be established. The cell provides a nominal power of 1200 W. The output voltage varies with current, ranging from 43 V to 26 V (no load e full load).
* Corresponding author. E-mail addresses: [email protected] (A.E. Gonnet), [email protected] (S. Robles), [email protected] (L. Moro). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.12.076
14758
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 3 7 ( 2 0 1 2 ) 1 4 7 5 7 e1 4 7 6 0
Fig. 1 e Diagram of the fuel cell components.
For safety, during operation, the voltage of the cell battery is checked as part of a control and diagnostic. At the same time each cell is checked by a voltage called cell voltage (CVC) to detect anomalies. The cell is turned off under failures or conditions of unsafe operation detected. The equipment is designed to operate with pure hydrogen gas. The humidifying is not required the fuel. Hydrogen can be supplied at pressures ranging from 70 to 1720 KPa. Fig. 2 shows the installation of the storage cylinder of hydrogen to supply the cell. A solenoid valve is installed in the cylinder. This valve can be commanded manually, or via the control board of the cell.
Over the cell is located a hydrogen sensor to make the system safer indoors. Otherwise the system for the conditioning of the cell output power, according to the scheme in Fig. 3, are comprise by the following elements: a CC/CC BSZ PG 1200 converter; two rechargeable batteries 12 V, 18 Ah; a norm converter RS232/ RS485 for PC serial port communication; an investor CC/CA 1500 W and a Relay for load connection and disconnection. The output voltage of the cell, which varies with the load, is conditioned to a constant voltage of 24 V, value that is independent of the load, with a CC/CC converter. The use of rechargeable batteries connected to the converter output,
Fig. 2 e Scheme of the hydrogen supply.
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 3 7 ( 2 0 1 2 ) 1 4 7 5 7 e1 4 7 6 0
So that, at operating temperature of 80 C has a value of 0.80. In theoretical operation, the voltage of 1.18 per cell, should be maintained for any state of load, however due to the polarization and ohmic losses, the voltage drops when current increases. In a real cell, the voltage can reach 1.1 V in the unloaded condition and may decrease to a value of 0.5 to 0.6 V, operating normally. Is convenient to express the actual efficiency of a cell based on the relationship between present and ideal voltage. Then, replacing [6]:
Fig. 3 e Scheme of conditioning the power output.
ensure a dynamic and stable operation, on the other hand the battery control system keeps them charged [4]. The system delivers power to the load through the investor DC/AC. If the battery voltage decreases, the converter turns the cell to charge the batteries, while delivering power to the load. Using a control panel allows turned the cell on or off manually as well as read the parameters of the converter and the cell.
hreal ¼ 0:68 Vpresent
(5)
In practice not all the fuel supplied to the cell is consumed in the reaction, thus defining a utilization coefficient [5]. m¼
mass of fuel consumedin the reaction mass of fuel ssuplied to the cell
Theorical formulation
One of the main advantages of fuel cells is its efficiency. It should be keep in mind that there is a difference between the efficiency of the electrochemical reaction that occurs and the efficiency of the entire set, that is the fuel cell, including auxiliary systems and power conditioning system. The overall reaction for a fuel cell fed with hydrogen and oxygen is H2 þ 1=2 O2 /H2 O
(1)
The amount of energy released with this reaction is obtained through the reaction enthalpy ΔH0, and has a value of 285.8 kJ mol1 of water, at 25 C and 1 atm. This value is more affected by changes in temperature than in pressure. The amount of this energy available as useful work is given by the change in Gibbs free energy (ΔG)0, and has a value of 237.1 kJ mol1 of water at 25 C and 1 atm. Taking into account the effects of temperature, its value at the operating temperature of 80 C is 228.2 kJ mol1 of water [5]. The theoretical voltage of a single PEM fuel cell is related to Gibbs free energy (DG) by: Vtheoretical ¼
DG nF
(6)
Thus the cell efficiency expressed: Efficiency ¼ mf hreal ¼ mf 0:68 Vpresent
3.
14759
(7)
Estimating a value of 0.95 for the utilization coefficient [5]. Another useful way to evaluate efficiency is relating the output electric power necessary ( p) and the voltage (Vpresent), to refer to hydrogen as fuel consumption. Then the hydrogen consumption is calculated considering that the cell needs 0.037605 kg of hydrogen per hour for an electric current of 1000 A [6]: p 0:037605 Vpresent $1000
mh ¼
(8)
Finally this result should be corrected by the coefficient of utilization and the consumer of auxiliary system of the fuel cell. Then, the efficiency of the fuel cell is calculated as: Efficiency of cell ¼
Electric energy generated Chemical energy absorbed
(9)
And the total efficiency, are considered the performance of total system, to be the efficiency of the CC/CC converter (hCC) and the CC/CA investor (hCA) are both 95% [6]: EfficiencyTotal ¼ Efficiencycell $hCC $hCA
(10)
The total efficiency that is to be account when comparing with other traditional systems. This efficiency can be
(2)
where n is the number of electrons involved in the reaction expressed in mole of electrons and F is the Faraday’s constant (96,500 Coulombs/mole of electrons). Since DG, n and F are all known constants, the theoretical voltage of a single hydrogen/ oxygen fuel cell at 25 C and atmospheric pressure is 1.182 V. Thermodynamic efficiency of a fuel cell is defined as the ratio of useful energy produced and the enthalpy of the product (water) and reactants (hydrogen and oxygen). h¼
Produced Energy DH
(3)
Therefore, the theoretical efficiency of the cell operating reversibly is: htheoretical ¼
DG DH
(4)
Fig. 4 e Hydrogen consumption versus currents intensity.
14760
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 3 7 ( 2 0 1 2 ) 1 4 7 5 7 e1 4 7 6 0
b- The air filter of the cell requires replacement every 500 h operation, although this will depend on the environmental conditions. c- The fuel cell incorporates a humidity exchanger which requires replacement every 800 h. The study of all factors affecting the efficiency of a complete system is complex and depends on the fuel source, processing method and the difficulties of handling and transport. Comparing this method with an others should be has the advantage of minimal environment impact and the potential for cogeneration. Given the danger that the escape of hydrogen, was considered important to provide the compound of a system for monitoring the presence of hydrogen with visible an audible alarm [8]. The cell maintenance requires are easy to operate, the only difficulty is the replacement of humidity exchanger for which needs certain previous knowledge.
Fig. 5 e Efficiency theoretical and experimental versus currents intensity.
increased with the use of heat and water produced during the reaction [7]. The amount of water produced is calculated as: 8
mH2 O ¼ 9:34$10
4.
p
Vpresent
(11)
Results and discussion
In Figs. 4e6 are represented the values of the hydrogen consumption, the efficiency and the water produced for different currents intensities values. The hydrogen consumption and the water produced increase while the efficiency decreasing when the currents intensities increase. The installation requires periodically perform for maintenance, the task listed bellow: a- The fuel cell incorporates an automatic rejuvenation by which losses are corrected. To maintain the maxim performance we recommend that the process is carried out each two or three month or after a prolonged storage.
Fig. 6 e Water produced versus currents versus currents intensity.
5.
Conclusions
The efficiency of the fuel cell found experimentally reflects the behavior expected from theoretical modeling. The amount of water produced by the fuel cell, and theoretically estimated, are approximate results. The difference is attributed that in the case theoretically were not taken into account the consumptions of auxiliary equipment. The calculation of the performance of the devices used in the equipment the fitting out of the power output, it is possible to find the overall performance of the generating plant. Comparing this method with an others should be has the advantage of minimal environment impact and the potential for cogeneration. The efficiency of the fuel cell varies from a value of 0,53 to 0,38, because with increase the currents intensities the efficiency decreasing.
references
[1] Wang Y, Chen KS, Mishler J, Chan Cho S, Cordobes Adroher X. A review of polymer electrolyte membrane fuel cells technology, applications, and needs on fundamental research. Applied Energy 2011;88(4):981e1007. [2] Wu J, Zi Yuan X, Martin JJ, Wang H, Zhang J, Shen J, et al. A review of PEM fuel cell durability: degradation mechanisms and mitigation strategies. Journal of Power Sources 2008;184: 104e19. [3] Ballard Power Systems Inc Nexa. Power module user’s manual; 2003. [4] BSZ PG1200 Technical Description, Isle Gmbh, Limenou, Germany, version 1.2. [5] Larminie JE, Dicks A. Fuel cell system explained. Chichester, U.K: Wiley; 2003. cap. 2, pp. 33. [6] Fuel cell handbook. 7th ed. Morgantown, West Virginia: U.S. Department of Energy; 2004. cap. 9 y 8. [7] Larminie JE, Dicks A. Fuel cell system explained. Chichester, U. K: Wiley; 2003. Appendix2 pp. 399. [8] Scott Healt & Safety, Freedom 5000, Universal Analog gas Transmitter.