Thermodynamic analysis of a Proton Exchange Membrane fuel cell

Thermodynamic analysis of a Proton Exchange Membrane fuel cell

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Thermodynamic analysis of a Proton Exchange Membrane fuel cell € ¨ r, Ali Cem Yakaryilmaz* Tayfun Ozgu Department of Automotive Engineering, C¸ukurova University, Adana, 01330, Turkey

article info

abstract

Article history:

In this study, energy and exergy analyses of a 1 kW Horizon H-1000 XP Proton Exchange

Received 24 May 2018

Membrane (PEM) Fuel Cell has been investigated. A testing apparatus has been established

Received in revised form

to analyze the system efficiencies based on the first and second laws of thermodynamics.

22 June 2018

In this mechanism pure hydrogen has been directly used as a fuel in compressed gas

Accepted 25 June 2018

formation. Purity of hydrogen was above 99.99%. The system performance was investi-

Available online xxx

gated through experimental studies on energy and parametric studies on exergy by changing the operating pressure and operation temperature. The results showed that the

Keywords:

energy efficiency of PEM fuel cell is 45.58% for experimental study and 41.27% for para-

PEM fuel cell

metric study at full load. Also, 2.25% and 4.2% performance improvements were obtained

Energy

by changing the operating temperature ratio (T/T0) from 1 to 1.2 and operating pressure

Exergy

ratio (P/P0) from 1 to 2, respectively.

Efficiency

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Performance

Introduction All over the world fossil fuels and fossil fuels-based energy systems have been using extremely high levels with growing world population and energy demands of the population. These fuels and systems have unique importance in every part of life such as transportation of goods and passengers, heating etc. But the most used field of fossil fuels is naturally in vehicles. Fossil fuels cause air pollution and damage the world. Recently, these negative impacts of fossil fuels have been seen more clearly than ever before. Also, fossil fuel resources diminish rapidly and soon or later will vanish. This situation and energy demands of the human will appear as a big problem for the future. That problem and the policies of the countries direct the researchers to study out new alternative energy source and fuels that emit less emissions to the environment. Sun, wind, geothermal, hydroelectric can be

count as natural alternative energy sources. But none of these sources can be a direct alternative for the most critic field which is using in vehicles. In the middle of the 1800s fuel cell researches had been conducted. However, after the discovering of the internal combustion engines, the fuel cells fell behind of these engines which use fossil-based fuels. For the last 20 years, applications and the utilizations of the fuel cells have mostly been used instead of internal combustion engine due to decrease of fossil fuels and the negative impacts of these fuels to the environment such as ozone depletion, greenhouse gas effect, global warming etc. for providing power in portable and stationary application. The fuel cells are clean energy conversion devices which convert the chemical energy of hydrogen to the electricity with zero emission. There are several types of fuel cell which are conducted in the studies. Nevertheless, PEM fuel cells step forward with fast start-up, high efficiency, high power density, low operating

* Corresponding author. E-mail address: [email protected] (A.C. Yakaryilmaz). https://doi.org/10.1016/j.ijhydene.2018.06.152 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. € ¨ r T, Yakaryilmaz AC, Thermodynamic analysis of a Proton Exchange Membrane fuel cell, InPlease cite this article in press as: Ozgu ternational Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.152

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temperature, easy and safe handling. When considered from this point of view many researchers have studied the performance of a PEM fuel cell and the parameters that affects the PEM fuel cell performance [1e5]. Benmouiza and Cheknane [6], Ozen et al. [7] and Eshef and Hamid [8] investigated the effects of operating temperature on performance of single cell fuel cell. Benmouiza and Cheknane [6] were tested the voltage changes that were applied to a single cell fuel cell like Taner [9]. Besides, Santarelli and Torchio [10], Yin et al. [11] and Kim et al. [12] studied the effect of operating pressure of PEM fuel cell. In PEM fuel cell, it was found out that the operating parameters such as operating pressure and temperature and also current density, relative humidity etc. had quite importance. When the effects of operating parameters realised, the studies about energy and exergy terms which are on the basis of thermodynamics first and second law gradually increased [13e17]. Miansari et al. [18] studied the effects of operating temperature and pressure on performance of PEM fuel cell and concluded that operating pressure and temperature could improve performance of the cell. Midilli et al. [19] parametrically investigated the effects of irreversibilities on the thermodynamic performance of PEM fuel cell based on thermodynamics at different operating conditions. It was found that higher current density and membrane thickness caused decrement in efficiency of exergy of PEM fuel cell, and exergy efficiency of PEM fuel cell raised with a increment of cell operation pressure and with a decrement of current density for the same membrane thickness. There are several types hybrid system in the literature which are the combination of PEM fuel cell and other various systems. One of them PEM fuel cell unit in solar-hydrogen system. Yilanci et al. [20] made an investigation about energy and exergy analysis of a 1.2 kW PEM fuel cell. PEM fuel cell fed by hydrogen which was produced from solar based production system. Another investigation is in a marine application. Leo et al. [21] investigated the mechanism which consists of a PEM fuel cell and hydrogen production system. Hydrogen was produced by reforming of methanol and used in PEM fuel cell. The possible integration of this system to the ships and submarines was analyzed. Lastly, the most promising hybrid systems are PEM fuel cell and cogeneration systems. PEM fuel cells are highly efficient systems and thanks to combination of cogeneration system it can become more efficient. Ahmadi and Ehyaei [22] studied exergy and design optimization of a 5 kW PEM fuel cell with cogeneration. The performance investigation showed that for the maximum efficiency and the temperature of fuel cell and voltage should be as high as possible. Today most of vehicles or most of energy conversion units use fossil fuels in order to obtain energy or power. However, these vehicles and conversion units emit emissions which are harmful for population and nature. Especially, PEM fuel cells have zero emission using pure H2 and the efficiency of these devices are higher. Although fuel cells are eco-friendly energy conversion devices, there is an important drawback which is cost. The aim of this study is to identify the best operating conditions for the PEM fuel cell in order to operate the fuel cell highly efficient to ignore this drawback. In this study energy and exergy analysis were carried out to by use of a 1 kW selfhumidified PEM fuel cell. 1st and 2nd laws of thermodynamic

efficiencies related with quantity and quality were determined and compared with each other for different operating temperature and operating pressure conditions.

Material and methods First and second law of thermodynamics Thermodynamics could be defined as the scientific study of the energy. Even though everyone has an idea in the aspect of energy, it is difficult task to make a completely precise definition for that term. The ability of causing changes can be thought as energy. These changes could be occurred by means of work and heat. The first law of thermodynamics (FLT) also known as the conservation of energy principle indicates that there are two ways to change the energy of a closed system; which are energy transfer by heat change or by work [23]. The conservation principles are not always sufficient, but the second law of thermodynamics (SLT) combines the conservation of energy and mass principles together with property relations. The second law is also considered, including performance limits for thermodynamic cycles. This can also be defined as energy has quality as well as quantity [24].

Energy and exergy analysis Energy is the basic term of thermodynamics and it is very important subject for engineering analysis. Thermodynamics studies let to be known of efficiency, behavior and performance characteristics of any system that is in interaction with heat and work. Thermodynamics analyses are generally based on energy conservation (FLT) and the calculations are made on the inlet and outlet quantities of energy and masses [25]. Exergy is the useful work potential that can be achieved by bringing a system into equilibrium with its surroundings. It is also called available energy or availability. Every system has a quantity of exergy, in the case of no equilibrium with its environment, and oppositely, in the case of the equilibrium with its environment it has zero exergy means it has no ability to do work with respect to its environment [26]. Energy efficiency can be deficient on the evaluating the performance of the system and how it is close to its ideal efficiency. Moreover, the energy analyses may not give the accurate results which are spoiled by the thermodynamic losses within the system and cause to deviate from ideality. The exergy analysis gives precise results considering those thermodynamic losses. The basic inefficiencies can be shown by the energy analysis results within the wrong parts of the system. This means a realistic efficiency definition is required. This can only be conducted by exergy efficiency since exergy analysis allows many of the limitations of energy analysis to get over. It is useful in identifying the magnitudes, locations, and causes of process inefficiencies thanks to the second law [27]. For the performing of the energy and exergy analysis a 1 kW Horizon H-1000 XP PEM fuel cell is used (Fig. 1.). The technical specifications of the PEM fuel cell are listed in Table 3.

€ ¨ r T, Yakaryilmaz AC, Thermodynamic analysis of a Proton Exchange Membrane fuel cell, InPlease cite this article in press as: Ozgu ternational Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.152

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Fig. 1 e Schematically representation of PEM fuel cell test rig.

Table 1 e Chemical exergy of products and reactants of PEM fuel cell.

Table 3 e Technical specifications of Horizon H-1000 XP PEM fuel cell.

Chemical exergy, exch (kJ/kg) Reactant/ product PEM fuel cell

Type

Reactant air

Reactant H2

Product H2O

Product Air

0

159138

2.5

8.58

Performance Fuel

Number of cells Peak power Rated current DC voltage Reactants H2 pressure

Table 2 e Properties at standard condition. Property Air stoichiometry, l Temperature of standard, T0 Pressure of standard, P0 Hydrogen, specific heat in average, cp Hydrogen, specific heat in average, cv Air and hydrogen, specific heat ratio, k ¼ cp/cv Air, average specific heat, cp Water entropy at standard condition, s0 Water enthalpy at standard condition, h0

Operation

Value 3 298 K 1 atm 14.3 kJ/kg K 10.16 kJ/kg K 1.4 1.005 kJ/kg K 0.3674 kJ/kg K 104.88 kJ/kg

A chemical reaction was considered which occurs theoretically when there was a combustion of hydrogen and oxygen mixture in PEM fuel cell in order to perform energy and exergy analyses. The reaction equation is given as below: 2H2 þ O2 /2H2 O

 For the calculation the value of chemical exergy of reactant and product gases of a PEM fuel cell are taken from literature and listed in Table 1 [15,28].  Properties at operating conditions are also taken from literature and listed in Table 2 [15,29]. Energy efficiency (first law efficiency) is calculated as the _ to energy input rate of fuel (Efuel) from: ratio of work rate (W)

(1)

Some assumptions made in the energy and exergy analysis are listed below:  Considered control volume is whole PEM fuel cell and it runs as a steady state open system.  The potential and kinetic energy effects of all reactant and product gases are neglected.

Ambient temperature Max. temperature of stack Humidification Cooling Relative humidity

PEM 50 1100 W 0e33.5 A @ 30 V 25e48 V Hydrogen and oxygen 7.2e9.4 psi (0.5e0.7 bar) 5e35  C 65  C Self-humidified Air non-condensing (10%e95% RH)

·



W Efuel

(2)

Energy input rate (Efuel) can be found using mass flow rate of fuel (m_ fuel) (kg/s) and lower heating value of the fuel which is hydrogen LHVH2 (kJ/kg) as follows: ·

Efuel ¼ mH2 LHVH2

(3)

€ ¨ r T, Yakaryilmaz AC, Thermodynamic analysis of a Proton Exchange Membrane fuel cell, InPlease cite this article in press as: Ozgu ternational Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.152

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Electrical Power ðExergyÞR  ðExergyÞP

(4)

·

j¼

·

·

Ex air;R þ Ex H2 ;R



W  ·  ·  Ex air;P þ Ex H2 O;P

(5)

P/P0=1 Physical exergy (kJ/kg)

The exergy efficiency (exergetic efficiency) of a fuel cell _ divided by the exergy differequals to the power output W, ences of reactants and products and it can be calculated by the following formula:

P/P0=2

1000 800 600 400 200 0 1

Other detailed equations related to exergy analysis were given in Appendix.

1.05

1.1 T/T0

1.15

1.2

Fig. 3 e Physical exergy of hydrogen.

Results and discussion to 75.3349 kJ/kg at 1 atm and 358 K. Moreover, as temperature raises from 298 K to 358 K, the physical exergy of hydrogen ranges from zero at 1 atm reference pressure to 920.115 kJ/kg at operating pressure of 2 atm. Fig. 4 demonstrates the physical exergy of entering air of PEM fuel cell at different operating temperature and pressure ratios. Physical exergy of entering air increases from zero at reference pressure and temperature condition (1 atm, 298 K) to 64.6645 kJ/kg at pressure of 2 atm and temperature of 358 K. It can be clearly seen from the figure; pressure changes rather affect the physical exergy of entering air. Fig. 5 shows the physical exergy of product water for different operating pressure and temperature ratios. It can be said that the increment in pressure ratio does not affects

P/P0=1 Physical exergy (kJ/kg)

Fuel cell performance experiments were conducted on a 1 kW self-humidified PEM fuel cell which takes place in Hydrogen Laboratories of Department of Automotive Engineering in C ¸ ukurova University. The fuel cell gives the maximum 1 kW electrical power and it was operated 900 s for each power ranges to implement both exergy and energy analysis. Fig. 2 shows efficiency of energy of PEM fuel cell at different power (work) output. It can be seen from the figure that both experimental and parametrical energy efficiency of PEM fuel cell decreases with raising the power demand. The efficiency of first law of PEM fuel cell decreases from 62.81% to 45.58% for experimental calculation and decreases from 62.81% to 41.27% for parametric calculation. Also, energy efficiencies of experimental and parametrical analysis decrease about 27% and 35%, respectively. The reduction of efficiency can be explained by the losses which raises with increasing the power demand. The energy efficiency of experiment was about 10% higher than the energy efficiency of parametric results at full load condition that was 1000 W. Fig. 3 shows the physical exergy of hydrogen at different operating pressure and temperature ratios. It is clearly seen that the specific physical exergy of hydrogen can be increased with raising the operating pressure and operating temperature ratios. Also, it can be said that the effects of operating temperature ratio are lower than the effects of operating pressure ratios. The physical exergy of hydrogen ranges from zero at reference pressure of 1 atm and temperature of 298 K

P/P0=2

70 60 50 40 30 20 10 0 1

1.05

1.1 T/T0

1.15

1.2

Fig. 4 e Physical exergy of entering air. Experimental

Parametric P/P0=1

60.00 50.00 40.00 30.00 100 200 300 400 500 600 700 800 900 1000

Power (W) Fig. 2 e Energy efficiency of PEM fuel cell at different power output.

Physical exergy (kJ/kg)

Efficiency (%)

70.00

P/P0=2

40 30 20 10 0 1

1.05

1.1 T/T0

1.15

1.2

Fig. 5 e Physical exergy of product water.

€ ¨ r T, Yakaryilmaz AC, Thermodynamic analysis of a Proton Exchange Membrane fuel cell, InPlease cite this article in press as: Ozgu ternational Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.152

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Physical exergy (kJ/kg)

P/P0=1

P/P0=2

200 150 100 50 0 1

1.05

1.1 T/T0

1.15

1.2

Fig. 6 e Physical exergy of exit air. P/P0=1

P/P0=2

Exergy efficiency (%)

40.00 38.00 36.00 34.00 32.00 30.00 1

1.05

1.1 T/T0

1.15

1.2

Fig. 7 e Exergy efficiency for different operating conditions at full load.

P/P0=1

Exergy efficiency (%)

significantly the physical exergy of product water. As contrast to increment in pressure ratio, increment in temperature ratio from 1 to 1.2 increases the physical exergy of product water. It can be easily seen from figure the maximum physical exergy of water is about 35 kJ/kg. Fig. 6 shows the physical exergy of exit air for different operating temperature and pressure ratios. The exit air was acted like a waste product of the fuel cell. The physical exergy of the exit air which is not used in the system ranges from zero at the relevant pressure and temperature ratios of P/P0 ¼ 1 and T/T0 ¼ 1e170.2 kJ/kg at P/P0 ¼ 2 and T/T0 ¼ 1.2. Additionally, a remarkable increment in the air physical exergy can be seen from the figure just by increasing the operating pressure ratio from 1 to 2. Fig. 7 shows the variation of exergy efficiency for different operating conditions at full load. Exergy efficiency can be found by dividing the power output to the differences of exergy of reactants and products. The efficiency of exergy of PEM fuel cell ranges from 34.57% to 35.99% at relevant temperature of 298 K to 35.35% and 36.86% at operating temperature of 358 K for P/P0 ¼ 1 and P/P0 ¼ 2, respectively. About 2.5% performance improvement was obtained due to increasing the operating temperature from 298 K to 358 K. Generally, increment in operating temperature and pressure affects the fuel cell performance positively. Fig. 8 shows the exergy efficiency of the PEM fuel cell at relevant operating temperature of 298 K and different pressure ratio for different power output. The efficiency of exergy of PEM fuel cell ranges from 47.61% to 49.35% at relevant

P/P0=2

55.00 50.00 45.00 40.00 35.00 30.00 100 200 300 400 500 600 700 800 900 1000

Power (W) Fig. 8 e Exergy efficiency for different power output at different operating pressure ratio.

temperature of 298 K to 34.57% and 35.99% at P/P0 ¼ 1, P/P0 ¼ 2, respectively for power output variations. It can be seen from the figure that exergy efficiency of PEM fuel cell more efficient at P/P0 ¼ 2 than P/P0 ¼ 1 about 4.2%. Since increased power demand, exergy efficiency decreases about 27% for both pressure ratio values.

Conclusion This study was carried out in order to find out the performance of a PEM fuel cell at various operating temperature and operating pressure ratio. During the study the operating temperature ratio (T/T0) is ranged from 1 to 1.2 and operating pressure ratio (P/P0) is ranged from 1 to 2, also, power output is ranged from 100 W to 1000 W. According to test results, the following can be summarized;  The increment in power output decreased energy efficiency from 62.81% to 45.58% for experimental results.  The increment in power output decreased energy efficiency from 62.81% to 41.27% for parametric results.  The energy efficiency of experiment was about 10% higher than the energy efficiency of parametric results at full load condition that was 1000 W.  Increasing operating pressure and operating temperature enhances the specific physical exergy of all reactants and products.  Also, increment in operating pressure and operating temperature raised the exergy efficiency of the PEM fuel cell.  Higher temperature means higher reaction rate and high reaction rate means high efficiency.  Due to increased operating temperature, exergy performance of PEM fuel cell was raised from 34.57% to 35.35% for operating pressure ratio of 1, from 35.99% to 36.86% for operating pressure ratio of 2.  About 4.2% performance improvement was obtained due to changing the operating pressure ratio from 1 to 2.  With the increasing of power output, the efficiency of exergy of fuel cell decreased due to irreversibilities of PEM fuel cell such as heat losses.  More feasible and exact process assessment was obtained from exergy analysis rather than energy analysis.

€ ¨ r T, Yakaryilmaz AC, Thermodynamic analysis of a Proton Exchange Membrane fuel cell, InPlease cite this article in press as: Ozgu ternational Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.152

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·

Acknowledgements

·

mair;P ¼ 3:57x10

The authors would like to thank the C¸ukurova University Scientific Research Project Coordination (FBA-2018-9994) for financial support.

In Appendix equations of exergy analysis were given detailed for the PEM fuel cell.

·

Ex air;R þ Ex H2 ;R



W  ·  ·  Ex air;P þ Ex H2 O;P

(1)

_ (kJ) represents the total exergy of species and where Ex subscript R and P represent the reactant and products, respectively. The total exergy transfer of a stream for any matter can be expressed as the sum of the specific physical (exph), chemical (exch), kinetic (exkn) and potential exergies (expt). ·  ·  Ex ¼ m exph þ exch þ exkn þ expt

(2)

However, the kinetic and potential exergies are not taken into consideration for the exergy calculation in a PEM fuel cell. Under these circumstances, the specific exergy (kJ/kg) of each chemical component throughout PEM fuel cell operation consists of only physical and chemical exergies and defined as: ex ¼ exph þ exch

(3)

exph ¼ h  h0  T0 ðs  s0 Þ

(4)

where h is the enthalpy (kJ/kg), s is the entropy (kJ/kg K) and the subscript zero indicates properties at the restricted dead state of P0 ¼ 1 atm and T0 ¼ 298 K. The physical exergy of an ideal gas with constant specific heat cp and specific heat constant ratio k can be written as: "

ph

ex

#    ðk1 k Þ T T P ¼ cp T0  1  ln þ ln T0 T0 P0

(5)

The chemical exergy is associated with the departure of the chemical composition of a system from that environment. The chemical exergy can be determined from: exch ¼ xn exch n þ RT0 xn ln xn

(6) ch

where xn is the mass fraction of the species n, ex is the standard chemical exergies (kJ/kg) of the species n evaluated at dead state. ·

·

lW Vcell

·

W Vcell

mair;R ¼ 3:57x107

·

mH2 ;R ¼ 1:05x108

·

·

8

mH2 O;P ¼ 9:34x10

!

W Vcell

·

8

 8:29x10

W Vcell

! kg=s

(9)

! kg=s

(10)

·   · · Ex air;R ¼ mair;R exair;R ¼ mair;R exph þ exch air;R kW

(11)

·   · · Ex H2 ;R ¼ mH2 ;R exH2 ;R ¼ mH2 ;R exph þ exch H

(12)

2 ;R

·

·

lW Vcell

As a result, the total exergy of the reactants and the products can be calculated through the following equations:

Appendix. Equations of exergy analysis of PEM fuel cell

j¼

7

! kg=s

(7)

kg=s

(8)

!

kW

·   · · Ex air;P ¼ mair;P exair;P ¼ mair;P exph þ exch air;P kW ·   · · Ex H2 O;P ¼ mH2 O;P exH2 O;P ¼ mH2 O;P exph þ exch H2 O;P kW

(13)

(14)

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€ ¨ r T, Yakaryilmaz AC, Thermodynamic analysis of a Proton Exchange Membrane fuel cell, InPlease cite this article in press as: Ozgu ternational Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.152