- Email: [email protected]
Performance assessment of the proton exchange membrane fuel cell - chemical heat pump hybrid system
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
ScienceDirect ScienceDirect
Energy Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online atatwww.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy (2018) 000–000 125–131 EnergyProcedia Procedia144 00 (2017) www.elsevier.com/locate/procedia
2018 The Fourth International Symposium on Hydrogen Energy, Renewable Energy and 2018 The Fourth International Hydrogen Energy, Renewable Materials, HEREMSymposium 2018, 13-15onJune 2018, Bangkok, Thailand Energy and Materials, HEREM 2018, 13-15 June 2018, Bangkok, Thailand
Performance assessment of the proton exchange membrane fuel cell Performance of Symposium the proton exchange membrane Theassessment 15th International on District Heating and Cooling fuel cell - chemical heat pump hybrid system - chemical heat pump hybrid system Assessing the feasibility of using the heat demand-outdoor Emin Açıkkalpaa, Hakan Caliskanb,b,* Emin , Hakan district Caliskan heat * temperature function forAçıkkalp a long-term demand forecast Department of Mechanical Engineering, Faculty of Engineering, Bilecik S.E. University, 11230, Bilecik, Turkey a a
b Department of Mechanical Engineering, Faculty of Engineering, Bilecik S.E. University, 11230, Bilecik, Turkey Department of Mechanical Engineering, Faculty of Engineering, Usak University, 64200, Usak, Turkey
a,b,c a b c Department Mechanical Faculty Engineering, Usak University, 64200, Usak, I. Andrić *,ofA. PinaaEngineering, , P. Ferrão , J.of Fournier ., B. Lacarrière , O.Turkey Le Correc b
a
IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract
Abstract In this paper, a hybrid system that consists of proton exchange membrane fuel cell and chemical heat pump is investigated. In terms 2. In In paper, a hybrid that consists of proton membrane fuel cell and chemical pump is investigated. In terms of this power outputs, thesystem maximum points and higherexchange power outputs are obtained at the currentheat densities between 1-2 A/cm 2. In ofAbstract powerhigher outputs, the maximum and higher power outputs are obtained at the current densities between A/cm contrast, energy efficienciespoints are provided at low current densities. According to the results, the higher power1-2 output can be contrast, higher energy efficiencies providedatatthese low current to the theenergy higherefficiency. power output be provided at higher current densities.are However, currentdensities. densities According hybrid system hasresults, smaller Forcan better District and heating networks are commonly addressed incurrent the literature one the most solutions for idecreasing the provided at higher current densities. However, at these system has effective smaller Forcurrent better efficient environmental utilization of the hybrid system, it densities should as behybrid usedofunder condition of ienergy (iη and η
E-mailaddress:[email protected] Keywords: Heat demand; Forecast; Climate change 1876-6102© 2018 The Authors. Published by Elsevier Ltd. 1876-6102© The Authors. by Elsevier Ltd. Selection and2018 peer-review under Published responsibility of the scientific committee of the 2018 The Fourth International Symposium on Hydrogen Energy, Selection peer-review under responsibility of the scientific committee of the 2018 The Fourth International Symposium on Hydrogen Energy, Renewableand Energy and Materials. Renewable Energy and Materials. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 Copyright © 2018 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the 2018 The Fourth International Symposium on Hydrogen Energy, Renewable Energy and Materials 10.1016/j.egypro.2018.06.017
126 2
Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131 E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000
energy sources due to fossil fuel depletion and environmental pollution. In this perspective, fuel cells become very important to solve these problems. Fuel cells generally generate electricity by means of electrochemical reaction. Also, they produce heat (generally known as waste heat) that may be converted to useful work. Producing electricity with fuel cells is an environmentally friendly process since the main source of this devices is hydrogen which is renewable [1,2]. The advantages of fuel cell can be explained as follows: (i) cleaner electrical energy production technology, (ii) elevated energy efficiency, (iii) producing heat, (iv) reliability and (v) low noise. The PEM fuel cell is the most commonly used fuel cell type due to its high power density, compact design, low operating temperature, and quick response. There are generally two types of PEM fuel cell as low temperature (below 100°C) and high temperature (over 100°C) [3]. There are various utilizations of PEM fuel cells with heat and power systems in the open literature [4-10]. In this study, the PEM fuel cell is used with chemical heat pump (HP) as a hybrid system and its power output and efficiency are investigated. The current density is also used to assess the power outputs and energy efficiencies of the hybrid system, PEM fuel cell and chemical HP. The difference of this paper from the previous studies can be considered as the type of the heat pump (chemical) and the operation condition of the fuel cell. 2. System Description and Analysis A hybrid combination, which includes PEM fuel cell - chemical HP combination, is considered as a system. The schematic layout of the proton exchange membrane fuel cell - chemical heat pump hybrid system is illustrated in Figure 1. In this system, fuel cell uses hydrogen gas (H2) and oxygen gas (O2) as fuel. Fuel cell converts the chemical energy to electricity and heat which can be used for additional processes. The main reaction occurred in the fuel cell is H2 + O2 H2O + electricity + heat. The rejected heat from the PEM fuel cell can be used in the chemical heat pump which is alternative way to utilize the low temperature heat source, waste heat and the renewable energy. Contrast to conventional steam compressed heat pumps, chemical heat pumps generally do not involve mechanical compression. In this paper, i-propanol-acetone-hydrogen chemical heat pump is chosen. They have capacity to provide heat to the environment up to 150oC-200oC and operation pressure is about 1-2 bar [5]. The PEM fuel cell is low temperature type, and the operation temperature is considered as 90oC. This heat is rejected at 90oC and utilized by the chemical HP in the dehydrogenation process where heat is absorbed. As it is seen in Figure 1, heat rejected by the PEM fuel cell is transferred to the dehydrogenation reactor. Dehydrogenation reaction is an endothermic reaction where hydrogen is separated from the i-propanol–acetone–hydrogen and then it is sent to hydrogenation reactor and ipropanol–acetone–hydrogen is composed, which is an exothermic reaction. Heat released in the exothermic reaction is transferred to the ambient which is heated. By means of this system, electricity generation and heating process can be accomplished at the same time.
Fig. 1. Schematic layout of the proton exchange membrane fuel cell - chemical heat pump hybrid system.
E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000 Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131
3 127
In this section, thermodynamic analysis of the hybrid system is carried out. The reversible voltage of the PEM fuel cell ( Vo ) is found by [11]:
Vo 1.229 8.5 104 T To 4.3085 105 T ln PH2 PO2
(1)
Activation overpotential ( Vact ), concentration overpotential ( Vcon ), ohmic overpotential ( Vohm ) and exchange
current density ( io ) are written as follows [11,12]:
C RT i Vact A ln A C ne F io i Vcon i 1 im it Vohm mem A mem
(2)
2
(3) (4)
io 1.08 1021 e 0.086T
(5)
where, i is current density, T is operation temperature of the fuel cell, To is ambient temperature. Also, PH 2 and PO2 are partial pressures of the hydrogen and oxygen (atm), λA and λC are charge transfer coefficients of the anode and cathode, respectively; while R is universal gas constant, F is Faraday constant, and β1 and β2 are constants. On the other hand, tmem, σmem and A are membrane thickness, membrane conductivity and polar plate area, respectively. Voltage of the fuel cell (V) is written to be;
V Vo Vact Vcon Vohm
(6)
Power output ( Pf ) and efficiency ( f ) of the fuel cell can be calculated as;
Pf iVA
f
(7)
Pf H
where A is polar plate area, be described as [13];
(8)
H is enthalpy change or total energy provided to fuel cell per unit time. H can
iAh H ne F
(9)
where Δh is molar enthalpy change at the operating temperature. Heat transfer occurred in the regenerator ( Qr ) is given by [14]; (10) Qr r 1 r T To where θ is heat conductance, εr is regenerator effectiveness and To is ambient temperature. The heat released from the fuel cell ( QH , f ) is:
E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000 Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131
4 128
QH , f H Pf Qr
(11)
Secondly, the equations for the chemical heat pump should be calculated. Heat addition to the chemical heat pump ( QL ,c ) is written as follows [15]:
y AL 1 y AL
QL ,c mac H dr H vap( AD / K ) H vap( AL )
(12)
where, mac is molar flow rate of the acetone, H dr is enthalpy change of the dehydrogenation reaction, H vap( AD / K ) is enthalpy change of the vaporization of aldehyde or keton, H vap( AL ) is enthalpy change of the alcohol and yAL is alcohol fraction in vapor phase. Heat rejection to the environment for heating process ( QH ,c ) can be described to be [15];
QH ,c mac H hr
(13)
where, H hr is enthalpy change of the hydrogenation reaction. Hydrogenation ( H hr ) and dehydrogenation ( H dr ) equations are expressed as [15]:
H dr 82261.638 241.734T 1.30314T 2 2.6713 103 T 3 1.866941106 T 4 2
2
5
3
8
H hr 53139.911 1.011T 4.0687 10 T 6.7723 10 T 3.015875 10 T
(14) 4
(15)
Enthalpy change in the vaporization process ( H vap ) is given to be [15];
H vap ,2( AC / P )
T2 1 Tcr H vap ,1( AC / P ) T 1 1 Tcr
n
(16)
where, T1 is initial temperature (K), T2 is final temperature (K) and Tcr is critical temperature. Also, H vap ,2( AC / P ) is final enthalpy change of the vaporization process and H vap ,1( AC / P ) is initial enthalpy change of the vaporization process. The rejected heat from the condenser is defined to be [16];
QL ,c QH ,c QCON ,c
(17)
Mass flow rate of the acetone ( mac ) can be calculated by; mac
0.5QH , f y AL H dr H vap( AD / K ) H vap( A ) 1 y AL
COP of the chemical heat pump ( COPc ) is described as follows:
(18)
E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000 Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131
COPc
QH ,c
5 129
(19)
QL ,c
Equivalent power output of the chemical heat pump (Pc) is described by [11,17,18];
Pc QH ,c 1
To
(20)
TH ,c
Equivalent efficiency of the chemical heat pump (ηc) is written as [11,17,18];
c
Pc QH ,c
(21)
Finally, the power output ( Ph ) and efficiency ( h ) of the hybrid system are expressed by; Ph Pf Pc
h
(22)
Pf Pc
(23)
H
3. Results and Discussion The results of the system are presented and discussed for the hybrid system and its equipment. The system is investigated in terms of power density and energy efficiency. Variations of these parameters (Power outputs and energy efficiencies) are shown in Figure 2; while the power-efficiency change of the hybrid system is given in Figure 3. The values used in calculations can be seen in Table 1. Table 1. Values used in calculations [11,135,17,18]. Parameter Unit ne
-
Value
Parameter
Unit
Value
2
Δh @90 C
J/mol
-283900
atm
0.8046
atm
0.4061
o
PH 2
@90 oC
-237300
PO2
@90oC
0.018
σmem@90 C
Cm/Ω 0.177
J/molK 8.314
β1@90 C
-
0.3299
λA
-
1
yA@90 C
-
0.369
λC
-
1
To
K
313.15
im
A/cm
2.5
TCON,c
K
303.15
β2
-
2
F Δgo tmem R
C/mol J/mol cm
2
96485
o
o
o
Current density is the main parameter affecting the all system performance. As it is seen in Figure 2a, the power outputs of the PEM fuel cell, chemical heat pump and hybrid system are investigated in terms of current density. For the hybrid system, the optimum point is found at 373.998 W power density and i= 1.89 A/cm2 current density, while the optimum point for the fuel cell is provided at i= 1.63 A/cm2 current density and 266.263 W power density. The
Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131 E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000
130 6
power equivalent of the chemical HP has no optimum point. The chemical HP has nearly linear tendency and it increases continuously. 400
0.5
Ph
0.60
Pc
0.55
Pf
350
0.4
c
0.50
250
hf
0.45
200 150
0.3
0.40 0.2
0.35
100
0.30
50
0.25
0
f
c
Ph, Pf, Pc (W)
300
h
0.1
0.20
0.0
0.5
1.0
1.5
2.0
0.0
2
0.5
1.0
1.5
2 i (A/cm ) i (A/cm ) (2a) (2b) Fig. 2. Power outputs (2a) and energy efficiencies (2b) of the hybrid system, PEM fuel cell and chemical HP.
2.0
0.0
The energy efficiencies of the hybrid system, PEM fuel cell and chemical HP are shown in Figure 2b. The hybrid system reaches its optimum point at lower current density values. It reaches the optimum value when i= 0.04 A/cm2 and it is equal to η=0.566. The efficiency of the PEM fuel cell decreases with current density and this change is nearly linear. The chemical HP has constant efficiency and the reason of this is that efficiency of the chemical HP is the function of the temperature. Because, y parameter is constant for any given temperature, and the mass flow rate of the acetone is same for the heat rejection from the chemical HP and heat absorption to the chemical HP. 0.60 max
0.55
0.50 0.45 0.40 P
0.35
Pmax
0.30 0 P
50
100
150
200
250
300
350
400
P (W) Fig.3. Power-efficiency (P-η) change of the hybrid system.
Finally, the power-efficiency (P-η) curve of the hybrid system is illustrated in Figure 3. In this figure, Pmax and ηmax are maximum power output and maximum efficiency, respectively; while Pη and ηP are power output at the maximum efficiency and efficiency at the maximum output, respectively. As is seen in this figure, Pη is equal to only 3.5% of the maximum power density. In addition, ηP is 59% of the maximum efficiency. When the results are assessed, the difference of the power output is 83% and energy efficiency is 45%. According to these results, the current density
Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131 E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000
131 7
for the hybrid system is chosen as iη
ScienceDirect ScienceDirect
Energy Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online atatwww.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy (2018) 000–000 125–131 EnergyProcedia Procedia144 00 (2017) www.elsevier.com/locate/procedia
2018 The Fourth International Symposium on Hydrogen Energy, Renewable Energy and 2018 The Fourth International Hydrogen Energy, Renewable Materials, HEREMSymposium 2018, 13-15onJune 2018, Bangkok, Thailand Energy and Materials, HEREM 2018, 13-15 June 2018, Bangkok, Thailand
Performance assessment of the proton exchange membrane fuel cell Performance of Symposium the proton exchange membrane Theassessment 15th International on District Heating and Cooling fuel cell - chemical heat pump hybrid system - chemical heat pump hybrid system Assessing the feasibility of using the heat demand-outdoor Emin Açıkkalpaa, Hakan Caliskanb,b,* Emin , Hakan district Caliskan heat * temperature function forAçıkkalp a long-term demand forecast Department of Mechanical Engineering, Faculty of Engineering, Bilecik S.E. University, 11230, Bilecik, Turkey a a
b Department of Mechanical Engineering, Faculty of Engineering, Bilecik S.E. University, 11230, Bilecik, Turkey Department of Mechanical Engineering, Faculty of Engineering, Usak University, 64200, Usak, Turkey
a,b,c a b c Department Mechanical Faculty Engineering, Usak University, 64200, Usak, I. Andrić *,ofA. PinaaEngineering, , P. Ferrão , J.of Fournier ., B. Lacarrière , O.Turkey Le Correc b
a
IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract
Abstract In this paper, a hybrid system that consists of proton exchange membrane fuel cell and chemical heat pump is investigated. In terms 2. In In paper, a hybrid that consists of proton membrane fuel cell and chemical pump is investigated. In terms of this power outputs, thesystem maximum points and higherexchange power outputs are obtained at the currentheat densities between 1-2 A/cm 2. In ofAbstract powerhigher outputs, the maximum and higher power outputs are obtained at the current densities between A/cm contrast, energy efficienciespoints are provided at low current densities. According to the results, the higher power1-2 output can be contrast, higher energy efficiencies providedatatthese low current to the theenergy higherefficiency. power output be provided at higher current densities.are However, currentdensities. densities According hybrid system hasresults, smaller Forcan better District and heating networks are commonly addressed incurrent the literature one the most solutions for idecreasing the provided at higher current densities. However, at these system has effective smaller Forcurrent better efficient environmental utilization of the hybrid system, it densities should as behybrid usedofunder condition of ienergy (iη and η
E-mailaddress:[email protected] Keywords: Heat demand; Forecast; Climate change 1876-6102© 2018 The Authors. Published by Elsevier Ltd. 1876-6102© The Authors. by Elsevier Ltd. Selection and2018 peer-review under Published responsibility of the scientific committee of the 2018 The Fourth International Symposium on Hydrogen Energy, Selection peer-review under responsibility of the scientific committee of the 2018 The Fourth International Symposium on Hydrogen Energy, Renewableand Energy and Materials. Renewable Energy and Materials. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 Copyright © 2018 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the 2018 The Fourth International Symposium on Hydrogen Energy, Renewable Energy and Materials 10.1016/j.egypro.2018.06.017
126 2
Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131 E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000
energy sources due to fossil fuel depletion and environmental pollution. In this perspective, fuel cells become very important to solve these problems. Fuel cells generally generate electricity by means of electrochemical reaction. Also, they produce heat (generally known as waste heat) that may be converted to useful work. Producing electricity with fuel cells is an environmentally friendly process since the main source of this devices is hydrogen which is renewable [1,2]. The advantages of fuel cell can be explained as follows: (i) cleaner electrical energy production technology, (ii) elevated energy efficiency, (iii) producing heat, (iv) reliability and (v) low noise. The PEM fuel cell is the most commonly used fuel cell type due to its high power density, compact design, low operating temperature, and quick response. There are generally two types of PEM fuel cell as low temperature (below 100°C) and high temperature (over 100°C) [3]. There are various utilizations of PEM fuel cells with heat and power systems in the open literature [4-10]. In this study, the PEM fuel cell is used with chemical heat pump (HP) as a hybrid system and its power output and efficiency are investigated. The current density is also used to assess the power outputs and energy efficiencies of the hybrid system, PEM fuel cell and chemical HP. The difference of this paper from the previous studies can be considered as the type of the heat pump (chemical) and the operation condition of the fuel cell. 2. System Description and Analysis A hybrid combination, which includes PEM fuel cell - chemical HP combination, is considered as a system. The schematic layout of the proton exchange membrane fuel cell - chemical heat pump hybrid system is illustrated in Figure 1. In this system, fuel cell uses hydrogen gas (H2) and oxygen gas (O2) as fuel. Fuel cell converts the chemical energy to electricity and heat which can be used for additional processes. The main reaction occurred in the fuel cell is H2 + O2 H2O + electricity + heat. The rejected heat from the PEM fuel cell can be used in the chemical heat pump which is alternative way to utilize the low temperature heat source, waste heat and the renewable energy. Contrast to conventional steam compressed heat pumps, chemical heat pumps generally do not involve mechanical compression. In this paper, i-propanol-acetone-hydrogen chemical heat pump is chosen. They have capacity to provide heat to the environment up to 150oC-200oC and operation pressure is about 1-2 bar [5]. The PEM fuel cell is low temperature type, and the operation temperature is considered as 90oC. This heat is rejected at 90oC and utilized by the chemical HP in the dehydrogenation process where heat is absorbed. As it is seen in Figure 1, heat rejected by the PEM fuel cell is transferred to the dehydrogenation reactor. Dehydrogenation reaction is an endothermic reaction where hydrogen is separated from the i-propanol–acetone–hydrogen and then it is sent to hydrogenation reactor and ipropanol–acetone–hydrogen is composed, which is an exothermic reaction. Heat released in the exothermic reaction is transferred to the ambient which is heated. By means of this system, electricity generation and heating process can be accomplished at the same time.
Fig. 1. Schematic layout of the proton exchange membrane fuel cell - chemical heat pump hybrid system.
E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000 Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131
3 127
In this section, thermodynamic analysis of the hybrid system is carried out. The reversible voltage of the PEM fuel cell ( Vo ) is found by [11]:
Vo 1.229 8.5 104 T To 4.3085 105 T ln PH2 PO2
(1)
Activation overpotential ( Vact ), concentration overpotential ( Vcon ), ohmic overpotential ( Vohm ) and exchange
current density ( io ) are written as follows [11,12]:
C RT i Vact A ln A C ne F io i Vcon i 1 im it Vohm mem A mem
(2)
2
(3) (4)
io 1.08 1021 e 0.086T
(5)
where, i is current density, T is operation temperature of the fuel cell, To is ambient temperature. Also, PH 2 and PO2 are partial pressures of the hydrogen and oxygen (atm), λA and λC are charge transfer coefficients of the anode and cathode, respectively; while R is universal gas constant, F is Faraday constant, and β1 and β2 are constants. On the other hand, tmem, σmem and A are membrane thickness, membrane conductivity and polar plate area, respectively. Voltage of the fuel cell (V) is written to be;
V Vo Vact Vcon Vohm
(6)
Power output ( Pf ) and efficiency ( f ) of the fuel cell can be calculated as;
Pf iVA
f
(7)
Pf H
where A is polar plate area, be described as [13];
(8)
H is enthalpy change or total energy provided to fuel cell per unit time. H can
iAh H ne F
(9)
where Δh is molar enthalpy change at the operating temperature. Heat transfer occurred in the regenerator ( Qr ) is given by [14]; (10) Qr r 1 r T To where θ is heat conductance, εr is regenerator effectiveness and To is ambient temperature. The heat released from the fuel cell ( QH , f ) is:
E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000 Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131
4 128
QH , f H Pf Qr
(11)
Secondly, the equations for the chemical heat pump should be calculated. Heat addition to the chemical heat pump ( QL ,c ) is written as follows [15]:
y AL 1 y AL
QL ,c mac H dr H vap( AD / K ) H vap( AL )
(12)
where, mac is molar flow rate of the acetone, H dr is enthalpy change of the dehydrogenation reaction, H vap( AD / K ) is enthalpy change of the vaporization of aldehyde or keton, H vap( AL ) is enthalpy change of the alcohol and yAL is alcohol fraction in vapor phase. Heat rejection to the environment for heating process ( QH ,c ) can be described to be [15];
QH ,c mac H hr
(13)
where, H hr is enthalpy change of the hydrogenation reaction. Hydrogenation ( H hr ) and dehydrogenation ( H dr ) equations are expressed as [15]:
H dr 82261.638 241.734T 1.30314T 2 2.6713 103 T 3 1.866941106 T 4 2
2
5
3
8
H hr 53139.911 1.011T 4.0687 10 T 6.7723 10 T 3.015875 10 T
(14) 4
(15)
Enthalpy change in the vaporization process ( H vap ) is given to be [15];
H vap ,2( AC / P )
T2 1 Tcr H vap ,1( AC / P ) T 1 1 Tcr
n
(16)
where, T1 is initial temperature (K), T2 is final temperature (K) and Tcr is critical temperature. Also, H vap ,2( AC / P ) is final enthalpy change of the vaporization process and H vap ,1( AC / P ) is initial enthalpy change of the vaporization process. The rejected heat from the condenser is defined to be [16];
QL ,c QH ,c QCON ,c
(17)
Mass flow rate of the acetone ( mac ) can be calculated by; mac
0.5QH , f y AL H dr H vap( AD / K ) H vap( A ) 1 y AL
COP of the chemical heat pump ( COPc ) is described as follows:
(18)
E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000 Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131
COPc
QH ,c
5 129
(19)
QL ,c
Equivalent power output of the chemical heat pump (Pc) is described by [11,17,18];
Pc QH ,c 1
To
(20)
TH ,c
Equivalent efficiency of the chemical heat pump (ηc) is written as [11,17,18];
c
Pc QH ,c
(21)
Finally, the power output ( Ph ) and efficiency ( h ) of the hybrid system are expressed by; Ph Pf Pc
h
(22)
Pf Pc
(23)
H
3. Results and Discussion The results of the system are presented and discussed for the hybrid system and its equipment. The system is investigated in terms of power density and energy efficiency. Variations of these parameters (Power outputs and energy efficiencies) are shown in Figure 2; while the power-efficiency change of the hybrid system is given in Figure 3. The values used in calculations can be seen in Table 1. Table 1. Values used in calculations [11,135,17,18]. Parameter Unit ne
-
Value
Parameter
Unit
Value
2
Δh @90 C
J/mol
-283900
atm
0.8046
atm
0.4061
o
PH 2
@90 oC
-237300
PO2
@90oC
0.018
σmem@90 C
Cm/Ω 0.177
J/molK 8.314
β1@90 C
-
0.3299
λA
-
1
yA@90 C
-
0.369
λC
-
1
To
K
313.15
im
A/cm
2.5
TCON,c
K
303.15
β2
-
2
F Δgo tmem R
C/mol J/mol cm
2
96485
o
o
o
Current density is the main parameter affecting the all system performance. As it is seen in Figure 2a, the power outputs of the PEM fuel cell, chemical heat pump and hybrid system are investigated in terms of current density. For the hybrid system, the optimum point is found at 373.998 W power density and i= 1.89 A/cm2 current density, while the optimum point for the fuel cell is provided at i= 1.63 A/cm2 current density and 266.263 W power density. The
Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131 E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000
130 6
power equivalent of the chemical HP has no optimum point. The chemical HP has nearly linear tendency and it increases continuously. 400
0.5
Ph
0.60
Pc
0.55
Pf
350
0.4
c
0.50
250
hf
0.45
200 150
0.3
0.40 0.2
0.35
100
0.30
50
0.25
0
f
c
Ph, Pf, Pc (W)
300
h
0.1
0.20
0.0
0.5
1.0
1.5
2.0
0.0
2
0.5
1.0
1.5
2 i (A/cm ) i (A/cm ) (2a) (2b) Fig. 2. Power outputs (2a) and energy efficiencies (2b) of the hybrid system, PEM fuel cell and chemical HP.
2.0
0.0
The energy efficiencies of the hybrid system, PEM fuel cell and chemical HP are shown in Figure 2b. The hybrid system reaches its optimum point at lower current density values. It reaches the optimum value when i= 0.04 A/cm2 and it is equal to η=0.566. The efficiency of the PEM fuel cell decreases with current density and this change is nearly linear. The chemical HP has constant efficiency and the reason of this is that efficiency of the chemical HP is the function of the temperature. Because, y parameter is constant for any given temperature, and the mass flow rate of the acetone is same for the heat rejection from the chemical HP and heat absorption to the chemical HP. 0.60 max
0.55
0.50 0.45 0.40 P
0.35
Pmax
0.30 0 P
50
100
150
200
250
300
350
400
P (W) Fig.3. Power-efficiency (P-η) change of the hybrid system.
Finally, the power-efficiency (P-η) curve of the hybrid system is illustrated in Figure 3. In this figure, Pmax and ηmax are maximum power output and maximum efficiency, respectively; while Pη and ηP are power output at the maximum efficiency and efficiency at the maximum output, respectively. As is seen in this figure, Pη is equal to only 3.5% of the maximum power density. In addition, ηP is 59% of the maximum efficiency. When the results are assessed, the difference of the power output is 83% and energy efficiency is 45%. According to these results, the current density
Emin Açıkkalp et al. / Energy Procedia 144 (2018) 125–131 E. Açıkkalp and H. Caliskan / Energy Procedia 00 (2018) 000–000
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for the hybrid system is chosen as iη