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Techno-economic analysis of polymer electrolyte membrane fuel cell system configurations
Renewable Energy Focus Volume 19–20, Number 00 June 2017
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ORIGINAL RESEARCH ARTICLE
Techno-economic analysis of polymer electrolyte membrane fuel cell system configurations B. Mukhtar*, S. Ayuba, S.M. Waziri and A.S. Olawale Department of Chemical Engineering, Ahmadu Bello University, Zaria, Kaduna State, Nigeria
The major constraints restricting wide-spread commercialization of fuel cell systems are both technical and economical. Many researchers have designed polymer electrolyte membrane (PEM) fuel cell system configurations, which are different in terms of energy utilization and cost. Therefore, it is imperative to determine the best system configuration in order to produce efficient and economically viable PEM fuel cell systems. This paper reports an exergy and economic analyses conducted on five different PEM fuel cell system configurations with a view to assess their performance. Thermolib – a tool box for MATLAB/ Simulink designed for modelling and simulation of energy systems was used to model and simulate the operation of each of the systems (5 kW stack power, 40 cells and 0.25m2 active membrane area) and data obtained was used for the exergy analysis. It was found that largest exergy loss occurred in the fuel cell stack (over 90%). The overall exergy and energy efficiencies of the studied systems were between 24.23% to 30.18% and 47.77% to 59.48%, respectively. A hybrid PEM fuel cell system configuration was proposed and analyzed. It was found to have an overall exergy and energy efficiencies of 31.95% and 62.97%, respectively. The return-on-investment evaluated for each of the five studied PEM fuel cell system configurations as well as the proposed system configuration were 1–27% while the payback periods were 3–13 yrs. The proposed system configuration was found to have the best performance in terms of energy utilization and had the lowest cost per kilowatt net power. Introduction Owing to the growing concerns on the negative effects of emissions from the conventional energy conversion technologies on the environment, there has been intense interest in research to develop more efficient alternative energy conversion technologies. Fuel cells, which are electrochemical devices that convert chemical energy stored in fuels (such as hydrogen, methanol and natural gas) directly to electrical energy, have attracted a lot of interest in recent years because of their relatively high energy conversion efficiency and very low harmful emissions. Fuel cells are mostly classified according to the type of electrolyte or fuel used. Polymer electrolyte membrane (PEM) fuel cell also known as proton exchange membrane fuel cell uses a proton conductive *Corresponding author. Mukhtar, B. ([email protected])
polymer membrane electrolyte. PEM fuel cell is among the most pursued fuel cell technologies due to its simplicity, high power density, low operating temperature and various applications. It has been demonstrated in almost any conceivable power application from powering a cell phone to a locomotive [1]. However, its most important application is in transportation and stationary power generation. There are many research and development efforts on different aspects of PEM fuel cell technology. These include individual components development such as electrocatalysts [2,3], membranes [4,5], membrane electrode assembly [6,7], system design, modelling and simulation [8,9,10], and issues related to the fuel (hydrogen), that is, production, storage and distribution [11,12,13]. Several researchers have worked on the modelling, simulation, optimization and design of PEM fuel cell components 1755-0084/ß 2017 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ref.2017.05.002
38
and complete system in order to achieve the best components and system performance. Modelling and simulation of electrochemical systems is an effective tool of getting better insights and understanding of the systems and helps in determining some important properties and characteristics of a system in order to make it more efficient and cost effective [14]. Migliardini et al. [15] analyzed the performance of a 6 kW PEM fuel cell system using two fuel feeding procedures (dead-end or flow-through) with an aim to highlight effect of the anode operation mode on stack efficiency and durability. They found that the flow-through mode gave a better stack efficiency and reduced the risks of fast degradation due to reactant starvation during transient operative phases. Rahimi-Esbo et al. [16] studied the influence of flow fields design on fuel cell performance at the optimum channel to rib ratio. They reported that optimization of the flow-field design helps to prevent problems such as flooding or drying of the membrane, which may reduce the life-time of the membrane electrode assembly. Mohammad et al. [17] examined model order reduction of cascade-type PEM fuel cell stack with integrated humidifiers and water separators. They developed a reducedorder model for continuous and discrete form of the cascade-type PEM fuel cell, which was verified using a full nonlinear model. There was close agreement between the results predicted by the developed reduced-order model on one hand and the full nonlinear model and experimental data on the other hand, which implied that the developed model can be tested for real-time control and diagnostic purposes. However, there are many design and operating variables that affect the overall performance of a PEM fuel cell system [18]. Various research groups have reported different PEM fuel cell system configurations in the literature with a tendency of having varying performance in terms of energy utilization and cost. To the best of our knowledge, all the reported works did not carry out energy and economic analyses and compare their proposed system configuration with other reported configurations in the literature in order to ascertain its level of performance. Thus, it is important to carry out both energy utilization and cost analyses of the different configurations in order to determine the most efficient and economical design. In this paper, Five different PEM fuel cell system configurations that can be modelled using Thermolib software were selected and identified as System 1 [19], System 2 [20], System 3 [21], System 4 [22] and System 5 [23]. In addition, a new PEM fuel cell system configuration was proposed and analyzed. The most commonly used method for analysis of energy systems is the first law of thermodynamics. However, a new approach to process analysis uses exergy analysis, which provides a more realistic view of the process and a useful tool for engineering evaluation. Therefore, exergy and economic analyses of the different PEM fuel cell system configurations were carried out in order to investigate their performance. Data required for the exergy analysis were obtained by simulation using the Thermolib software.
Simulation of the selected PEM fuel cell system configurations Table 1 shows the main components and features of the selected PEM fuel cell system configurations. It can be observed that there are variations in the number and arrangement of components.
ORIGINAL RESEARCH ARTICLE
The variations will invariably affect the system efficiency and cost [24]. The selected PEM fuel cell system configurations were modelled (as shown in Figures 1–5) and simulated using the Thermolib software. From the simulation results, data such as operating temperatures, pressures, enthalpies, entropies, molar flow rates and molar fractions of components in each stream were obtained and used in the exergy analysis. Table 2 shows the specified values of parameters used for the simulation. In system 1, pure hydrogen from the storage cylinder mixes with the recycled stream and goes into the fuel cell stack. The fuel cell stack also receives a supply of air through the compressor and humidifier. Cooling water from the water tank is pumped through the heat exchanger into the stack and back through the humidifier. In system 2, pure hydrogen from a cylinder enters the fuel cell stack through the humidifier. The exit stream goes into the first condenser. Air goes into the fuel cell stack through the compressor and humidifier and the exit air goes into the second condenser. The condensate from both condensers is used to humidify both the air and hydrogen steams. The hydrogen is recycled through the blower. Cooling water is pumped into the fuel cell stack, preheater, and radiator. In system 3, pure hydrogen from a tank goes into the fuel cell stack through a humidifier and pre-heater. The exit stream is recycled. The fuel cell stack also receives air through the compressor, air humidifier and pre-heater while the exit stream goes out through a water separator and expander. In system 4, pure hydrogen from a tank goes into the fuel cell stack through ejector and a membrane humidifier. The exit stream is recycled through the recirculation blower ejector while air goes into the fuel cell stack through the compressor and air humidifier. The exit air goes into the membrane humidifier, enthalpy wheel, expander and out through the dilution mixer. In system 5, pure hydrogen from a tank goes into the fuel cell stack through the humidifier. Air also goes into the fuel cell stack through the compressor and air humidifier and exits from the fuel cell stack. Cooling water is pumped into the fuel cell stack from the reservoir tank. The exit hot water is passed through the air humidifier and radiator and back into the reservoir tank. The results obtained from the exergy analysis indicated that system 1 has the best performance compared to the other systems studied. Table 3 shows its stream properties. However, the performance of system 1 could be further improved by replacing the air compressor with a Compressor Expander Module (CEM). This will decrease the parasitic power and hence increase efficiency of the system by recovering power from expansion of the exit air stream, which is used to offset the electrical power required for air compression. In addition, it was observed that the hydrogen blower in the hydrogen recirculation line can be removed since the pressure drop across the stack is insignificant. This will reduce the capital cost of the system. Based on these modifications on system 1, a new system configuration shown in Figure 6 was designed, modelled, simulated and analyzed. It is expected to have better energy utilization and lower cost. Table 4 shows its stream properties. The proposed PEM fuel cell system configuration is composed of the fuel cell stack, Compressor Expander Module (CEM), air 39
ORIGINAL RESEARCH ARTICLE
Renewable Energy Focus Volume 19–20, Number 00 June 2017
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Renewable Energy Focus Volume 19–20, Number 00 June 2017
TABLE 1
Main components and features of the selected PEM fuel cell system configurations.
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System
Major Components
Distinct Features
System 1
1. 2. 3. 4. 5. 6.
Stack Air compressor Air humidifier Hydrogen recirculation blower Radiator Cooling water pump
- No hydrogen humidification - Stack cooling water is used for humidification - Hydrogen recirculation with blower
System 2
1. Stack 2. Air compressor 3. Air humidifier 4. Hydrogen humidifier 5. Hydrogen recirculation blower 6. Heat exchanger 7. Radiator 8. Cooling water pump 9. Recirculation water pump 10. Cathode exit gas condenser 11. Anode exit gas condenser
- Recirculation water is used for humidification. - Both air and hydrogen are humidified - Hydrogen recirculation with blower
System 3
1. Stack 2. Compressor Expander Module (CEM) 3. Air humidifier 4. Hydrogen humidifier 5. Radiator 6. Cooling water pump 7. Recirculation water pump 8. Cathode inlet gas heater 9. Anode inlet gas heater 10. Water separator
-
System 4
1. 2. 3. 4. 5. 6. 7.
Stack Compressor Expander Module (CEM) Air humidifier Hydrogen humidifier Hydrogen recirculation blower Radiator Cooling water pump
- Compressor Expander Module (CEM) used for air compression - Hydrogen recirculation with blower - Both air and hydrogen are humidified
System 5
1. 2. 3. 4. 5. 6. 7.
Stack Air compressor Air humidifier Hydrogen humidifier Radiator Cooling water pump Two humidification water pump
- Both air and hydrogen are humidified - No hydrogen recirculation
humidifier, radiator and cooling water pump. Pure hydrogen from a tank goes into the fuel cell stack. The exit stream is recycled. Air goes into the fuel cell stack through the CEM and humidifier. The exit air from the fuel cell stack is discharged through the CEM.
Recirculation water is used for humidification Compressor Expander Module (CEM) used for air compression Two heaters used to heat anode and cathode inlet streams Hydrogen recirculation without blower
Exergy analysis The results of the simulations were used to carry out the exergy analysis. This was done by calculating the exergy content (Exi) of each stream using the following relations [25]: Exi ¼ f i ci
TABLE 2
ci ¼
Specified (supplied) data for the simulation. Component
Parameter
Value
Stack
Power (kW) Number of cells Maximum hydrogen conversion rate Total active membrane area (m2) Reference environment temp. (8C) Operating temperature (8C) Operating pressure (bar) Efficiency (%) Efficiency (%)
5.00 40 0.8 0.25 25 80 1.5 70 80
Compressor Pump 40
ph ci
cch i
ph ci
þ cch i
¼ ðhi ho;i ÞT o ðsi so;i Þ X X ¼ xk;i cch xk;i lnxk;i o;k;i þ RT o
(1) (2) (3) (4)
where fi, total molar flow of stream i; xk,i, mole fraction of component k, in stream i; ci, total specific exergy of stream i; ph ci ; specific physical exergy of stream i; cch i ; specific chemical exergy of stream i; si, specific entropy of stream i at operating conditions; cch o;k;i ; standard specific chemical exergy of component k in stream i, R, gas constant; ho,i, specific enthalpy of stream i at
ORIGINAL RESEARCH ARTICLE
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Renewable Energy Focus Volume 19–20, Number 00 June 2017
FIGURE 1
Thermolib model of system 1.
FIGURE 2
Thermolib model of system 2. 41
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ORIGINAL RESEARCH ARTICLE FIGURE 3
Thermolib model of system 3.
FIGURE 4
Thermolib model of system 4. 42
Renewable Energy Focus Volume 19–20, Number 00 June 2017
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Renewable Energy Focus Volume 19–20, Number 00 June 2017
FIGURE 5
Thermolib model of system 5.
TABLE 3
Stream properties of system 1. Stream
S01 S02 S03 S04 S05 S06 S07 S08 S09 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21
T (K)
298 335 336 336 300 300 311 311 302 302 298 298 302 353 353 353 341 353 353 353 300
H (w)
968 744 8429 1,015,690 3 3 29 1,015,000 1,030,440 1,033,640 1,034,690 1,142,920 1,141,890 24,946 26 26 1,023,270 18,710 6136 18710 0
ho (J/mol) 4620 4620 41,347 285,731 4 4 4 4 285,731 285,731 285,731 285,731 285,731 104,324 4 4 285,731 88,649 285,731 88649 285731
S (W/K)
42 42 49 284 6 8 10 0 258 258 254 281 284 52 2 2 290 50 2 51 0
so (J/mol K) 199 199 184 70 131 131 131 131 70 70 70 70 70 152 131 131 70 159 70 159 70
x H2
O2
H2O
N2
– – – – 1 1 1 1 – – – – – – 1 1 – – – – 1
0.2060 0.2060 0.1787 – – – – – – – – – – 0.0391 – – – 0.0424 – 0.0424 –
0.0191 0.0191 0.1490 1 – – – – 1 1 1 1 1 0.3683 – – 1 0.3137 1 0.3137 –
0.7749 0.7749 0.6723 – – – – – – – – – – 0.5927 – – – 0.6439 – 0.6439 –
f (mol/s)
cph (J/mol)
cch (J/mol)
Total exergy (W)
0.2095 0.2095 0.2414 3.5893 0.0649 0.0649 0.0811 0.0000 3.6110 3.6212 3.6212 4.0000 4.0000 0.2739 0.0162 0.0162 3.6212 0.2521 0.0218 0.2521 0.0000
0.00 1065.42 1361.73 131.61 11,961.61 1005.67 1012.86 38,966.93 4.46 6.54 4.67 7.60 6.53 1796.70 1137.33 1137.33 204.73 2320.13 331.48 1319.16 306670.34
115.35 115.35 810.32 3120.00 238,500.00 238,500.00 238,500.00 238,500.00 3120.00 3120.00 3120.00 3120.00 3120.00 2899.77 238,500.00 238,500.00 3120.00 2368.70 3120.00 2368.63 3120.00
24.16 247.34 524.43 11,670.84 16,255.16 15,544.11 19,430.72 0.00 11,282.53 11,321.88 11,315.11 12,510.42 12,506.13 1286.33 3888.16 3888.16 12,039.57 1182.10 75.19 929.72 0.00 43
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Renewable Energy Focus Volume 19–20, Number 00 June 2017
ORIGINAL RESEARCH ARTICLE FIGURE 6
Thermolib model of the proposed PEM fuel cell system configuration.
TABLE 4
Stream properties of the proposed PEM fuel cell system configuration. Stream
S01 S02 S03 S04 S05 S06 S07 S08 S09 S10 S11 S12 S13 S14 S15 S16 S17 44
T (K)
298 335 333 333 300 300 311 353 343 353 300 336 300 298 333 300 300
H (w)
968 744 7021 1180500 3 3 29 23172 23441 26 0 1186680 1190620 1142920 1132850 1190500 1197930
ho (J/mol) 4620 4620 35178 285,731 4 4 4 100299 100299 4 4 285,731 285,731 285,731 285,731 285,731 285,731
S (W/K)
42 42 47 326 6 8 10 52 52 2 0 332 294 281 313 295 296
so (J/mol.K) 199 199 187 70 131 131 131 154 154 131 131 70 70 70 70 70 70
x H2
O2
H2O
N2
– – – – 1 1 1 – – 1 1 – – – – – –
0.2060 0.2060 0.1832 – – – – 0.0399 0.0399 – – – – – – – –
0.0191 0.0191 0.1275 1 – – – 0.3542 0.3542 – – 1 1 1 1 1 1
0.7749 0.7749 0.6893 – – – – 0.6058 0.6058 – – – – – – – –
f (mol/s)
cph (J/mol)
cch (J/mol)
Total exergy (W)
0.2095 0.2095 0.2355 4.1685 0.0649 0.0649 0.0811 0.2679 0.2679 0.0162 0.0000 4.1945 4.1685 4.0000 4.0000 4.1685 4.1945
0.00 1065.42 1267.60 134.68 11961.38 1005.67 1012.86 2202.71 1080.27 1137.33 38966.93 185.00 2.81 7.60 104.42 4.35 4.62
115.35 115.35 659.01 3120.00 238500.00 238500.00 238500.00 2760.28 2760.28 238500.00 238500.00 3120.00 3120.00 3120.00 3120.00 3120.00 3120.00
24.1628 247.3393 453.7115 13566.9615 16255.1439 15544.1098 19430.7201 1329.8239 1029.0691 3888.1636 0.0000 13862.7408 13017.2783 12510.4161 12897.6967 13023.6832 13,106.1301
Renewable Energy Focus Volume 19–20, Number 00 June 2017
ORIGINAL RESEARCH ARTICLE
Evaluation of irreversibilies The irreversibility of each system component was determined using the following relations [26]: I Stack ¼ PStack þ ðf i ci Þinlet ðf i ci Þoutlet cathode þ ðf i ci Þinlet ðf i ci Þoutlet anode (5) h i ph ph þ ðf i ci Þinlet ðf i ci Þoutlet cooling
where PStack, stack power. For other units, Eqn 6 was used. X X ph ph f i ci f i ci Ij ¼ in
out
(6)
where j represents each unit.
Overall system exergy and energy efficiency The overall exergy efficiency of each system is defined as the ratio of the net power generated and the exergetic potential of the primary fuel (standard exergy of hydrogen). It is expressed as [26]: P jWj hExergy ¼ (7) ch f i;in co hydrogen
P
where jWj is the net power generated by the system. On the other hand, the energy efficiency was calculated based on the Lower Heating Value (LHV) of the fuel (hydrogen): P jWj hEnergy ¼ (8) ðf i;in LHVÞhydrogen
TABLE 5
Values of economic parameters. Parameter
Value
M c1 ($/m2) c2 ($) cs ($/kg) cp ($/troy ounce) Lp (mg/cm2) Pd (W/cm2) d (%/year) N (years)
1.1 454.45 428.51 5.244 1100 0.25 0.5 6.5 15
assumed. Table 5 shows the values of economic parameters used for estimation of the costs of the fuel cell stack. The costs of ancillary components were estimated from the costs of similar equipment reported by Direct Technologies, Inc. (DTI) and TIAX in 2009. The 2009 costs estimates were scaled up for time to obtain the 2016 estimates using Eqn 11: I2 C2 ¼ C1 (11) I1 where C1, purchase cost in 2009; C2, purchase cost in 2016; I1, chemical engineering plant cost index in 2009; I2, chemical engineering plant cost index in 2016.
Evaluation of economic indicators The economic indicators used were payback period and return on investment expressed in Eqns 12 and 13, respectively. Fixed capital investment Anual cash income
Economic analysis
Payback period ðPBPÞ ¼
The capital cost of each system was estimated by using relations and costs of similar equipment obtained from the literature. The economic indicators – return on investment (ROI) and payback period (PBP) were evaluated and used to compare the economic performances of the selected systems.
Return on investment ðROIÞ Income after tax 100 ¼ Fixed capital investment ðFCIÞ
Cost estimation The capital cost of a complete system was calculated using Eqn 9. Csyst ¼ Cstack þ Canci
(9)
where Csyst, capital cost of complete system; Cstack, estimated cost of fuel cell stack; Canci, estimated cost of ancillary components. The capital cost of the fuel cell stack was estimated using Eqn 10 [27]: " # C1 105:4 Lp Cp PStack ð1 þ dÞN þ þ c2 CStack ¼ M (10) 10 21:64 Pd where M, fixed cost markup (1.1 default); c1, cost parameter ($/m2); Lp, fuel cell platinum loading for both electrodes (mg/cm2); cp, platinum cost ($/troy ounce); PStack, fuel cell stack power (kW); Pd, fuel cell power density (W/cm2); d, annual fuel cell degradation (%/year); N, assumed fuel cell useful lifetime (years); c2, stack fixed cost parameter ($). The fuel cell stack cost depends on the two cost parameters c1 and c2 which have been developed for five different production volumes [27]. A production volume of 10,000 units per year was
(12)
(13)
The following assumptions were made: 1. Capacity – Annual operation time 6480 h/yr. 2. Analysis period – Analysis was carried out over a period of 15 years (assumed life cycle of equipment), after which it is assumed that all equipment have zero value and will be replaced. 3. Price of electricity from fuel cell source $0.185/kWh. 4. Installation cost is 20% of system purchase price. 5. Fuel cell system maintenance cost is 9% of purchase price. 6. Hydrogen price is 1.2$/kg.
Results and discussions Exergy analysis results Figures 7 and 8 show the irreversibility and exergy efficiency of the stages or components of system 1, respectively while Figures 9 and 10 show the irreversibility and exergy efficiency of the stages or components of the proposed system configuration, respectively. It was found that for the five different PEM fuel cell system configurations considered in this study as well as the proposed PEM fuel cell system configuration the largest exergy loss occurs in the fuel cell stack (over 90% of the total losses). This is due to the 45
ORIGINAL RESEARCH ARTICLE
reference conditions; hi, specific enthalpy of stream i at operating conditions; so,i, specific entropy of stream i at reference conditions.
Renewable Energy Focus Volume 19–20, Number 00 June 2017
100.00
86.39
86.06
80.00
66.00
72.56
54.15
60.00 40.00 20.00 0.00
FIGURE 7
Irreversibility of components of system 1.
Exergy Efficiency (%)
98.14
100.00
83.06 74.39
80.00 60.00
79.67 79.56
68.09 64.97
54.66
System Components FIGURE 10
Exergy efficiency of components of the proposed system.
40.00 20.00 0.00
System Components FIGURE 8
Exergy efficiencies of components of system 1.
efficiencies of some of the other components are up to 95%, which is excellent. The overall exergy efficiencies of the five selected PEM fuel cell system configurations as well as the proposed PEM fuel cell system configuration lie between 24.23% and 31.95% while their energy efficiencies lie between 47.77% and 62.97% as shown in Figure 11. Energy analysis is based on the first law of thermodynamics, which does not take cognizance of the loss in quality of energy and all forms of energy are considered to be similar. However, exergy analysis is based on the first and second laws of thermodynamics and it considers the thermodynamic imperfections of a process, which comprises of all energy losses and irreversibilities [29]. The proposed PEM fuel cell configuration had the highest exergy and energy efficiencies as shown in the figure. The energy efficiency from a fuel cell stack can be as high as 80% but for the total system the efficiency is usually lower. The system efficiency depends on the number and kinds of components that are used.
Economic analysis results The economic analysis was carried out by estimating the cost of each system from the costs estimates of its individual components.
70.00
FIGURE 9
Irreversibility of components of the proposed system.
significant contribution of the chemical exergy change involved because of the electrochemical processes taking place at the electrodes in the fuel cell stack. The losses in the fuel cell stack are caused by activation polarization, ohmic and concentration polarization [28]. Other components do not involve any chemical changes; only physical changes take place due to changes in temperature and pressure. Besides, the temperatures and pressures involved are low so the losses observed are very much lower. In addition, the exergy analysis revealed that the exergy efficiency of the stack in the selected PEM fuel cell system configurations lies between 53% and 56%. This means the losses in the fuel cell stack account for about 44–47%, which is quite high. The performance of the systems will improve if these losses are reduced. The exergy 46
62.97
59.48 60.00
Efficiency (%)
ORIGINAL RESEARCH ARTICLE
Exergy Efficiency (%)
ORIGINAL RESEARCH ARTICLE
57.79
55.88
51.52
47.77
50.00 40.00
30.18
30.00
26.14
29.32
28.35
31.95 24.23
20.00 10.00 0.00 System 1
System 2
System 3
System 4
System 5
Systems Exergy Efficiency (%)
Energy Efficiency (%)
FIGURE 11
Overall exergy and energy efficiencies of each system.
Proposed Syst.
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TABLE 6
Economic analysis results. System 2
System 3
System 4
System 5
Proposed syst.
4416.20 4.671 945.42 0.471 3709.61 3967.91 30,269.11 397.46 4365.36 5599.79 1234.42 247.31 987.11 740.34 987.64 4697.25 4 5 20
5329.60 4.654 1145.17 0.542 4476.86 4564.12 30,157.92 479.66 5043.79 5579.22 535.43 298.46 236.97 177.73 476.19 4953.05 9 2 4
4464.39 4.539 983.65 0.471 3750.09 3967.91 29,410.04 401.80 4370.35 5440.86 1070.51 250.01 820.50 615.38 865.38 4615.47 4 5 16
5517.93 4.941 1116.71 0.530 4635.06 4468.60 32,019.17 496.61 4965.21 5923.55 958.34 309.00 649.33 487.00 796.00 5431.07 6 3 11
4140.60 4.689 882.96 0.589 3478.10 4960.49 30,387.82 372.65 5333.14 5621.75 288.60 231.87 56.73 42.55 274.42 3752.53 13 2 1
4327.64 4.945 875.13 0.471 3635.22 3967.91 32,044.57 389.49 4357.39 5928.25 1570.85 242.35 1328.50 996.38 1238.73 4873.94 3 7 27
1145.17
1200.00
1116.71 983.65
945.42
882.96
875.13
System 5
Proposed Syst.
900.00 600.00 300.00 0.00 System 1
System 2
System 3
System 4
ORIGINAL RESEARCH ARTICLE
System 1
Esmated cost per kW of net power ($/kW)
Systems Estimated cost ($) Net power (kW) Cost per kW of net power ($/kW) Fuel consumption (kg/h) Fixed capital investment ($) Annual fuel cost ($) Annual output (kWh) Annual maintenance cost ($) Annual operating cost ($) Annual income (sales) ($) Annual operating income ($) Annual depreciation ($) Income before tax ($) Income after tax ($) Annual cash income ($) Annual cash flow ($) Pay back period (yrs) Proceeds per dollar ($) ROI (%)
Figure 13 shows the summary for the pay-back period and return on investment for the studied systems while Table 6 shows the detailed economic analysis results. The proposed fuel cell system configuration had the lowest payback period of about 3 yrs and the highest ROI of 27% while system 5 had the highest pay-back period of about 13 yrs and the lowest ROI of 1%.
Conclusion
Systems FIGURE 12
Estimated costs per kilowatt of net power of each system.
Figure 12 shows the cost estimates per kilowatt of net power for each of the system. It can be observed from Figure 12 that system 2 has the highest cost estimate per kilowatt net power followed by system 4, system 3, system 1, system 5 and the proposed system, in that order. The proposed system recorded the least cost of $875.13 among the six systems investigated due to its choice and number of components.
27
An exergy analysis carried out on five selected PEM fuel cell system configurations revealed that the largest exergy loss occurs in the fuel cell stack (over 90%) for each of the systems. The overall exergy and energy efficiencies of the systems lie between 24.23% and 30.18% and between 47.77% and 59.48%, respectively. A hybrid PEM fuel cell system configuration was proposed, modelled and analyzed. The exergy and energy efficiencies of the proposed PEM fuel cell system configuration were found to be 31.95% and 62.97%, respectively. The proposed system recorded the least cost of $875.13 with a pay-back period of about 3 years and ROI of 27%. In terms of both energy utilization and cost, the proposed PEM fuel cell system configuration is the best followed by system 1, system 3, system 4, system 2 and system 5. Thus, this study has proposed a PEM fuel cell system configuration that has very good performance in terms of energy utilization with relatively lower cost.
20 16
Acknowledgement 12.7
3.8
The authors are grateful to EUtech Scientific Engineering GbmH, Germany for providing the Thermolib software used in this work.
11
9.4 4
4.3
5.8 1
System 1
System 2
System 3
System 4
Pay Back Period (yrs)
System 5
2.9
References Proposed Syst.
ROI (%)
FIGURE 13
Economic indicators for the PEM fuel cell system configurations studied.
[1] F. Barbie, PEM Fuel Cells: Theory and Practice, Elsevier Inc., 2005. [2] R. Sakamoto, K. Omichi, T. Furuta, M. Ichikawa, J. Power Sources 269 (2014) 117–123. [3] B. Chen, G. Ma, Y. Zhu, J. Wang, W. Xiong, Y. Xi, J. Power Sources 334 (2016) 112–119. [4] K. Ketpang, B. Son, D. Lee, S. Shanmugam, J. Membr. Sci. 488 (2015) 154–165. [5] K.-J. Peng, J.-Y. Lai, Y.-L. Liu, J. Membr. Sci. 514 (2016) 86–94.
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[6] H. Guo, M.H. Wang, J.X. Liu, Z.H. Nie, F. Ye, C.H. Ma, J. Power Sources 273 (2015) 775–783. [7] S. Vierrath, M. Breitwieser, M. Klingele, B. Britton, S. Holdcroft, R. Zengerle, S. Thiele, J. Power Sources 326 (2016) 170–175. [8] A. Beicha, J. Power Sources 205 (2012) 335–339. [9] K. Simmons, Y. Guezennec, S. Onori, J. Power Sources 246 (2014) 736–746. [10] M. Guarnieri, P. Alotto, F. Moro, J. Power Sources 297 (2015) 23–32. [11] N.A.A. Rusman, M. Dahari, Int. J. Hydrogen Energy 41 (2016) 12108–12126. [12] O. Sekwon, K. MinJoong, J.K. Kwangsupeom, K. Dohyang, C. Eunae, K. Hyuksang, Int. J. Hydrogen Energy 41 (2016) 5296–5303. [13] F.V. Vazquez, P. Simell, J. Pennanen, J. Lehtonen, Int. J. Hydrogen Energy 41 (2016) 924–935. [14] K. Roman, D. Monika, P. Petr, N. Michal, S. Dalimil, B. Karel, Electrochim. Acta 179 (2015) 538–555. [15] F. Migliardini, C. Capasso, P. Corbo, Int. J. Hydrogen Energy 39 (2014) 21746– 21752. [16] M. Rahimi-Esbo, A.A. Ranjbar, A. Ramiar, E. Alizadeh, M. Aghaee, Int. J. Hydrogen Energy 41 (2016) 3023–3037. [17] M.B. Mohammad, D. Morteza, A. Ebrahim, R. Abas, Energy 113 (2016) 683–692. [18] B. Mukhtar, S.M.J. Zaidi, N.M. Faqir, ECS Trans. 26 (1) (2010) 77–88. [19] F. Barbir, in: N. Sammes (Ed.), Fuel Cell Technology Reaching Towards Commercialization, Springer Inc., 2006, pp. 27–51.
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[20] S. Gurski, Cold Start Effects on Performance and Efficiency for Vehicle Fuel Cell System, Virginia Polytechnic Institute and State University, United States, 2002, Master of Science Thesis. [21] J. Pukrushpan, Modeling and Control of Fuel Cell Systems and Fuel Processors, University of Michigan, United States, 2003, PhD Dissertation. [22] E.J. Carlson, P. Kopf, J. Sinha, S. Sriramulu, Y. Yang, Cost Analysis of PEM Fuel Cell Systems for Transportation, 2005 Subcontract Report NREL/SR-560-39104 https://pdfs.semanticscholar.org/7a08/ 9fcb2c55b0713af410dc87c0670788c129b6.pdf. [23] K. Haraldsson, K. Wipke, J. Power Sources 126 (2004) 88–97. [24] H. Ma, W. Cheng, F. Fang, C. Hsu, C. Lin, Energies 7 (2014) 2498–2514. [25] H.K. Yilanci, O.A. Ozturk, I. Dincer, in: Proceedings of Third International Energy, Exergy and Environment Symposium, University of Ontario Institute of Technology, L1H 7K4, Canada, 2007. [26] Y. Casas, L.E. Arteaga, M. Morales, E. Rosa, L.M. Peralta, J. Dewulf, Energy and Exergy Analysis of an Ethanol Fueled Solid Oxide Fuel Cell Power Plant, 2010 https://biblio.ugent.be/publication/985811/file/985812. [27] F.A. Christos, G.N. Lambros, Energy 31 (2006) 1501–1519. [28] E.R. Delsman, C.U. Uju, M.H.J. de Croon, J.C. Schouten, K.J. Ptasinski, Energy 31 (2006) 3300–3330. [29] A. Rabbani, Dynamic Performance of a PEM Fuel Cell System, Technical University of Denmark, Kongens Lyngby, 2013, PhD Dissertation.
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ORIGINAL RESEARCH ARTICLE
Techno-economic analysis of polymer electrolyte membrane fuel cell system configurations B. Mukhtar*, S. Ayuba, S.M. Waziri and A.S. Olawale Department of Chemical Engineering, Ahmadu Bello University, Zaria, Kaduna State, Nigeria
The major constraints restricting wide-spread commercialization of fuel cell systems are both technical and economical. Many researchers have designed polymer electrolyte membrane (PEM) fuel cell system configurations, which are different in terms of energy utilization and cost. Therefore, it is imperative to determine the best system configuration in order to produce efficient and economically viable PEM fuel cell systems. This paper reports an exergy and economic analyses conducted on five different PEM fuel cell system configurations with a view to assess their performance. Thermolib – a tool box for MATLAB/ Simulink designed for modelling and simulation of energy systems was used to model and simulate the operation of each of the systems (5 kW stack power, 40 cells and 0.25m2 active membrane area) and data obtained was used for the exergy analysis. It was found that largest exergy loss occurred in the fuel cell stack (over 90%). The overall exergy and energy efficiencies of the studied systems were between 24.23% to 30.18% and 47.77% to 59.48%, respectively. A hybrid PEM fuel cell system configuration was proposed and analyzed. It was found to have an overall exergy and energy efficiencies of 31.95% and 62.97%, respectively. The return-on-investment evaluated for each of the five studied PEM fuel cell system configurations as well as the proposed system configuration were 1–27% while the payback periods were 3–13 yrs. The proposed system configuration was found to have the best performance in terms of energy utilization and had the lowest cost per kilowatt net power. Introduction Owing to the growing concerns on the negative effects of emissions from the conventional energy conversion technologies on the environment, there has been intense interest in research to develop more efficient alternative energy conversion technologies. Fuel cells, which are electrochemical devices that convert chemical energy stored in fuels (such as hydrogen, methanol and natural gas) directly to electrical energy, have attracted a lot of interest in recent years because of their relatively high energy conversion efficiency and very low harmful emissions. Fuel cells are mostly classified according to the type of electrolyte or fuel used. Polymer electrolyte membrane (PEM) fuel cell also known as proton exchange membrane fuel cell uses a proton conductive *Corresponding author. Mukhtar, B. ([email protected])
polymer membrane electrolyte. PEM fuel cell is among the most pursued fuel cell technologies due to its simplicity, high power density, low operating temperature and various applications. It has been demonstrated in almost any conceivable power application from powering a cell phone to a locomotive [1]. However, its most important application is in transportation and stationary power generation. There are many research and development efforts on different aspects of PEM fuel cell technology. These include individual components development such as electrocatalysts [2,3], membranes [4,5], membrane electrode assembly [6,7], system design, modelling and simulation [8,9,10], and issues related to the fuel (hydrogen), that is, production, storage and distribution [11,12,13]. Several researchers have worked on the modelling, simulation, optimization and design of PEM fuel cell components 1755-0084/ß 2017 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ref.2017.05.002
38
and complete system in order to achieve the best components and system performance. Modelling and simulation of electrochemical systems is an effective tool of getting better insights and understanding of the systems and helps in determining some important properties and characteristics of a system in order to make it more efficient and cost effective [14]. Migliardini et al. [15] analyzed the performance of a 6 kW PEM fuel cell system using two fuel feeding procedures (dead-end or flow-through) with an aim to highlight effect of the anode operation mode on stack efficiency and durability. They found that the flow-through mode gave a better stack efficiency and reduced the risks of fast degradation due to reactant starvation during transient operative phases. Rahimi-Esbo et al. [16] studied the influence of flow fields design on fuel cell performance at the optimum channel to rib ratio. They reported that optimization of the flow-field design helps to prevent problems such as flooding or drying of the membrane, which may reduce the life-time of the membrane electrode assembly. Mohammad et al. [17] examined model order reduction of cascade-type PEM fuel cell stack with integrated humidifiers and water separators. They developed a reducedorder model for continuous and discrete form of the cascade-type PEM fuel cell, which was verified using a full nonlinear model. There was close agreement between the results predicted by the developed reduced-order model on one hand and the full nonlinear model and experimental data on the other hand, which implied that the developed model can be tested for real-time control and diagnostic purposes. However, there are many design and operating variables that affect the overall performance of a PEM fuel cell system [18]. Various research groups have reported different PEM fuel cell system configurations in the literature with a tendency of having varying performance in terms of energy utilization and cost. To the best of our knowledge, all the reported works did not carry out energy and economic analyses and compare their proposed system configuration with other reported configurations in the literature in order to ascertain its level of performance. Thus, it is important to carry out both energy utilization and cost analyses of the different configurations in order to determine the most efficient and economical design. In this paper, Five different PEM fuel cell system configurations that can be modelled using Thermolib software were selected and identified as System 1 [19], System 2 [20], System 3 [21], System 4 [22] and System 5 [23]. In addition, a new PEM fuel cell system configuration was proposed and analyzed. The most commonly used method for analysis of energy systems is the first law of thermodynamics. However, a new approach to process analysis uses exergy analysis, which provides a more realistic view of the process and a useful tool for engineering evaluation. Therefore, exergy and economic analyses of the different PEM fuel cell system configurations were carried out in order to investigate their performance. Data required for the exergy analysis were obtained by simulation using the Thermolib software.
Simulation of the selected PEM fuel cell system configurations Table 1 shows the main components and features of the selected PEM fuel cell system configurations. It can be observed that there are variations in the number and arrangement of components.
ORIGINAL RESEARCH ARTICLE
The variations will invariably affect the system efficiency and cost [24]. The selected PEM fuel cell system configurations were modelled (as shown in Figures 1–5) and simulated using the Thermolib software. From the simulation results, data such as operating temperatures, pressures, enthalpies, entropies, molar flow rates and molar fractions of components in each stream were obtained and used in the exergy analysis. Table 2 shows the specified values of parameters used for the simulation. In system 1, pure hydrogen from the storage cylinder mixes with the recycled stream and goes into the fuel cell stack. The fuel cell stack also receives a supply of air through the compressor and humidifier. Cooling water from the water tank is pumped through the heat exchanger into the stack and back through the humidifier. In system 2, pure hydrogen from a cylinder enters the fuel cell stack through the humidifier. The exit stream goes into the first condenser. Air goes into the fuel cell stack through the compressor and humidifier and the exit air goes into the second condenser. The condensate from both condensers is used to humidify both the air and hydrogen steams. The hydrogen is recycled through the blower. Cooling water is pumped into the fuel cell stack, preheater, and radiator. In system 3, pure hydrogen from a tank goes into the fuel cell stack through a humidifier and pre-heater. The exit stream is recycled. The fuel cell stack also receives air through the compressor, air humidifier and pre-heater while the exit stream goes out through a water separator and expander. In system 4, pure hydrogen from a tank goes into the fuel cell stack through ejector and a membrane humidifier. The exit stream is recycled through the recirculation blower ejector while air goes into the fuel cell stack through the compressor and air humidifier. The exit air goes into the membrane humidifier, enthalpy wheel, expander and out through the dilution mixer. In system 5, pure hydrogen from a tank goes into the fuel cell stack through the humidifier. Air also goes into the fuel cell stack through the compressor and air humidifier and exits from the fuel cell stack. Cooling water is pumped into the fuel cell stack from the reservoir tank. The exit hot water is passed through the air humidifier and radiator and back into the reservoir tank. The results obtained from the exergy analysis indicated that system 1 has the best performance compared to the other systems studied. Table 3 shows its stream properties. However, the performance of system 1 could be further improved by replacing the air compressor with a Compressor Expander Module (CEM). This will decrease the parasitic power and hence increase efficiency of the system by recovering power from expansion of the exit air stream, which is used to offset the electrical power required for air compression. In addition, it was observed that the hydrogen blower in the hydrogen recirculation line can be removed since the pressure drop across the stack is insignificant. This will reduce the capital cost of the system. Based on these modifications on system 1, a new system configuration shown in Figure 6 was designed, modelled, simulated and analyzed. It is expected to have better energy utilization and lower cost. Table 4 shows its stream properties. The proposed PEM fuel cell system configuration is composed of the fuel cell stack, Compressor Expander Module (CEM), air 39
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TABLE 1
Main components and features of the selected PEM fuel cell system configurations.
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System
Major Components
Distinct Features
System 1
1. 2. 3. 4. 5. 6.
Stack Air compressor Air humidifier Hydrogen recirculation blower Radiator Cooling water pump
- No hydrogen humidification - Stack cooling water is used for humidification - Hydrogen recirculation with blower
System 2
1. Stack 2. Air compressor 3. Air humidifier 4. Hydrogen humidifier 5. Hydrogen recirculation blower 6. Heat exchanger 7. Radiator 8. Cooling water pump 9. Recirculation water pump 10. Cathode exit gas condenser 11. Anode exit gas condenser
- Recirculation water is used for humidification. - Both air and hydrogen are humidified - Hydrogen recirculation with blower
System 3
1. Stack 2. Compressor Expander Module (CEM) 3. Air humidifier 4. Hydrogen humidifier 5. Radiator 6. Cooling water pump 7. Recirculation water pump 8. Cathode inlet gas heater 9. Anode inlet gas heater 10. Water separator
-
System 4
1. 2. 3. 4. 5. 6. 7.
Stack Compressor Expander Module (CEM) Air humidifier Hydrogen humidifier Hydrogen recirculation blower Radiator Cooling water pump
- Compressor Expander Module (CEM) used for air compression - Hydrogen recirculation with blower - Both air and hydrogen are humidified
System 5
1. 2. 3. 4. 5. 6. 7.
Stack Air compressor Air humidifier Hydrogen humidifier Radiator Cooling water pump Two humidification water pump
- Both air and hydrogen are humidified - No hydrogen recirculation
humidifier, radiator and cooling water pump. Pure hydrogen from a tank goes into the fuel cell stack. The exit stream is recycled. Air goes into the fuel cell stack through the CEM and humidifier. The exit air from the fuel cell stack is discharged through the CEM.
Recirculation water is used for humidification Compressor Expander Module (CEM) used for air compression Two heaters used to heat anode and cathode inlet streams Hydrogen recirculation without blower
Exergy analysis The results of the simulations were used to carry out the exergy analysis. This was done by calculating the exergy content (Exi) of each stream using the following relations [25]: Exi ¼ f i ci
TABLE 2
ci ¼
Specified (supplied) data for the simulation. Component
Parameter
Value
Stack
Power (kW) Number of cells Maximum hydrogen conversion rate Total active membrane area (m2) Reference environment temp. (8C) Operating temperature (8C) Operating pressure (bar) Efficiency (%) Efficiency (%)
5.00 40 0.8 0.25 25 80 1.5 70 80
Compressor Pump 40
ph ci
cch i
ph ci
þ cch i
¼ ðhi ho;i ÞT o ðsi so;i Þ X X ¼ xk;i cch xk;i lnxk;i o;k;i þ RT o
(1) (2) (3) (4)
where fi, total molar flow of stream i; xk,i, mole fraction of component k, in stream i; ci, total specific exergy of stream i; ph ci ; specific physical exergy of stream i; cch i ; specific chemical exergy of stream i; si, specific entropy of stream i at operating conditions; cch o;k;i ; standard specific chemical exergy of component k in stream i, R, gas constant; ho,i, specific enthalpy of stream i at
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FIGURE 1
Thermolib model of system 1.
FIGURE 2
Thermolib model of system 2. 41
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Thermolib model of system 3.
FIGURE 4
Thermolib model of system 4. 42
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FIGURE 5
Thermolib model of system 5.
TABLE 3
Stream properties of system 1. Stream
S01 S02 S03 S04 S05 S06 S07 S08 S09 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21
T (K)
298 335 336 336 300 300 311 311 302 302 298 298 302 353 353 353 341 353 353 353 300
H (w)
968 744 8429 1,015,690 3 3 29 1,015,000 1,030,440 1,033,640 1,034,690 1,142,920 1,141,890 24,946 26 26 1,023,270 18,710 6136 18710 0
ho (J/mol) 4620 4620 41,347 285,731 4 4 4 4 285,731 285,731 285,731 285,731 285,731 104,324 4 4 285,731 88,649 285,731 88649 285731
S (W/K)
42 42 49 284 6 8 10 0 258 258 254 281 284 52 2 2 290 50 2 51 0
so (J/mol K) 199 199 184 70 131 131 131 131 70 70 70 70 70 152 131 131 70 159 70 159 70
x H2
O2
H2O
N2
– – – – 1 1 1 1 – – – – – – 1 1 – – – – 1
0.2060 0.2060 0.1787 – – – – – – – – – – 0.0391 – – – 0.0424 – 0.0424 –
0.0191 0.0191 0.1490 1 – – – – 1 1 1 1 1 0.3683 – – 1 0.3137 1 0.3137 –
0.7749 0.7749 0.6723 – – – – – – – – – – 0.5927 – – – 0.6439 – 0.6439 –
f (mol/s)
cph (J/mol)
cch (J/mol)
Total exergy (W)
0.2095 0.2095 0.2414 3.5893 0.0649 0.0649 0.0811 0.0000 3.6110 3.6212 3.6212 4.0000 4.0000 0.2739 0.0162 0.0162 3.6212 0.2521 0.0218 0.2521 0.0000
0.00 1065.42 1361.73 131.61 11,961.61 1005.67 1012.86 38,966.93 4.46 6.54 4.67 7.60 6.53 1796.70 1137.33 1137.33 204.73 2320.13 331.48 1319.16 306670.34
115.35 115.35 810.32 3120.00 238,500.00 238,500.00 238,500.00 238,500.00 3120.00 3120.00 3120.00 3120.00 3120.00 2899.77 238,500.00 238,500.00 3120.00 2368.70 3120.00 2368.63 3120.00
24.16 247.34 524.43 11,670.84 16,255.16 15,544.11 19,430.72 0.00 11,282.53 11,321.88 11,315.11 12,510.42 12,506.13 1286.33 3888.16 3888.16 12,039.57 1182.10 75.19 929.72 0.00 43
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ORIGINAL RESEARCH ARTICLE FIGURE 6
Thermolib model of the proposed PEM fuel cell system configuration.
TABLE 4
Stream properties of the proposed PEM fuel cell system configuration. Stream
S01 S02 S03 S04 S05 S06 S07 S08 S09 S10 S11 S12 S13 S14 S15 S16 S17 44
T (K)
298 335 333 333 300 300 311 353 343 353 300 336 300 298 333 300 300
H (w)
968 744 7021 1180500 3 3 29 23172 23441 26 0 1186680 1190620 1142920 1132850 1190500 1197930
ho (J/mol) 4620 4620 35178 285,731 4 4 4 100299 100299 4 4 285,731 285,731 285,731 285,731 285,731 285,731
S (W/K)
42 42 47 326 6 8 10 52 52 2 0 332 294 281 313 295 296
so (J/mol.K) 199 199 187 70 131 131 131 154 154 131 131 70 70 70 70 70 70
x H2
O2
H2O
N2
– – – – 1 1 1 – – 1 1 – – – – – –
0.2060 0.2060 0.1832 – – – – 0.0399 0.0399 – – – – – – – –
0.0191 0.0191 0.1275 1 – – – 0.3542 0.3542 – – 1 1 1 1 1 1
0.7749 0.7749 0.6893 – – – – 0.6058 0.6058 – – – – – – – –
f (mol/s)
cph (J/mol)
cch (J/mol)
Total exergy (W)
0.2095 0.2095 0.2355 4.1685 0.0649 0.0649 0.0811 0.2679 0.2679 0.0162 0.0000 4.1945 4.1685 4.0000 4.0000 4.1685 4.1945
0.00 1065.42 1267.60 134.68 11961.38 1005.67 1012.86 2202.71 1080.27 1137.33 38966.93 185.00 2.81 7.60 104.42 4.35 4.62
115.35 115.35 659.01 3120.00 238500.00 238500.00 238500.00 2760.28 2760.28 238500.00 238500.00 3120.00 3120.00 3120.00 3120.00 3120.00 3120.00
24.1628 247.3393 453.7115 13566.9615 16255.1439 15544.1098 19430.7201 1329.8239 1029.0691 3888.1636 0.0000 13862.7408 13017.2783 12510.4161 12897.6967 13023.6832 13,106.1301
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ORIGINAL RESEARCH ARTICLE
Evaluation of irreversibilies The irreversibility of each system component was determined using the following relations [26]: I Stack ¼ PStack þ ðf i ci Þinlet ðf i ci Þoutlet cathode þ ðf i ci Þinlet ðf i ci Þoutlet anode (5) h i ph ph þ ðf i ci Þinlet ðf i ci Þoutlet cooling
where PStack, stack power. For other units, Eqn 6 was used. X X ph ph f i ci f i ci Ij ¼ in
out
(6)
where j represents each unit.
Overall system exergy and energy efficiency The overall exergy efficiency of each system is defined as the ratio of the net power generated and the exergetic potential of the primary fuel (standard exergy of hydrogen). It is expressed as [26]: P jWj hExergy ¼ (7) ch f i;in co hydrogen
P
where jWj is the net power generated by the system. On the other hand, the energy efficiency was calculated based on the Lower Heating Value (LHV) of the fuel (hydrogen): P jWj hEnergy ¼ (8) ðf i;in LHVÞhydrogen
TABLE 5
Values of economic parameters. Parameter
Value
M c1 ($/m2) c2 ($) cs ($/kg) cp ($/troy ounce) Lp (mg/cm2) Pd (W/cm2) d (%/year) N (years)
1.1 454.45 428.51 5.244 1100 0.25 0.5 6.5 15
assumed. Table 5 shows the values of economic parameters used for estimation of the costs of the fuel cell stack. The costs of ancillary components were estimated from the costs of similar equipment reported by Direct Technologies, Inc. (DTI) and TIAX in 2009. The 2009 costs estimates were scaled up for time to obtain the 2016 estimates using Eqn 11: I2 C2 ¼ C1 (11) I1 where C1, purchase cost in 2009; C2, purchase cost in 2016; I1, chemical engineering plant cost index in 2009; I2, chemical engineering plant cost index in 2016.
Evaluation of economic indicators The economic indicators used were payback period and return on investment expressed in Eqns 12 and 13, respectively. Fixed capital investment Anual cash income
Economic analysis
Payback period ðPBPÞ ¼
The capital cost of each system was estimated by using relations and costs of similar equipment obtained from the literature. The economic indicators – return on investment (ROI) and payback period (PBP) were evaluated and used to compare the economic performances of the selected systems.
Return on investment ðROIÞ Income after tax 100 ¼ Fixed capital investment ðFCIÞ
Cost estimation The capital cost of a complete system was calculated using Eqn 9. Csyst ¼ Cstack þ Canci
(9)
where Csyst, capital cost of complete system; Cstack, estimated cost of fuel cell stack; Canci, estimated cost of ancillary components. The capital cost of the fuel cell stack was estimated using Eqn 10 [27]: " # C1 105:4 Lp Cp PStack ð1 þ dÞN þ þ c2 CStack ¼ M (10) 10 21:64 Pd where M, fixed cost markup (1.1 default); c1, cost parameter ($/m2); Lp, fuel cell platinum loading for both electrodes (mg/cm2); cp, platinum cost ($/troy ounce); PStack, fuel cell stack power (kW); Pd, fuel cell power density (W/cm2); d, annual fuel cell degradation (%/year); N, assumed fuel cell useful lifetime (years); c2, stack fixed cost parameter ($). The fuel cell stack cost depends on the two cost parameters c1 and c2 which have been developed for five different production volumes [27]. A production volume of 10,000 units per year was
(12)
(13)
The following assumptions were made: 1. Capacity – Annual operation time 6480 h/yr. 2. Analysis period – Analysis was carried out over a period of 15 years (assumed life cycle of equipment), after which it is assumed that all equipment have zero value and will be replaced. 3. Price of electricity from fuel cell source $0.185/kWh. 4. Installation cost is 20% of system purchase price. 5. Fuel cell system maintenance cost is 9% of purchase price. 6. Hydrogen price is 1.2$/kg.
Results and discussions Exergy analysis results Figures 7 and 8 show the irreversibility and exergy efficiency of the stages or components of system 1, respectively while Figures 9 and 10 show the irreversibility and exergy efficiency of the stages or components of the proposed system configuration, respectively. It was found that for the five different PEM fuel cell system configurations considered in this study as well as the proposed PEM fuel cell system configuration the largest exergy loss occurs in the fuel cell stack (over 90% of the total losses). This is due to the 45
ORIGINAL RESEARCH ARTICLE
reference conditions; hi, specific enthalpy of stream i at operating conditions; so,i, specific entropy of stream i at reference conditions.
Renewable Energy Focus Volume 19–20, Number 00 June 2017
100.00
86.39
86.06
80.00
66.00
72.56
54.15
60.00 40.00 20.00 0.00
FIGURE 7
Irreversibility of components of system 1.
Exergy Efficiency (%)
98.14
100.00
83.06 74.39
80.00 60.00
79.67 79.56
68.09 64.97
54.66
System Components FIGURE 10
Exergy efficiency of components of the proposed system.
40.00 20.00 0.00
System Components FIGURE 8
Exergy efficiencies of components of system 1.
efficiencies of some of the other components are up to 95%, which is excellent. The overall exergy efficiencies of the five selected PEM fuel cell system configurations as well as the proposed PEM fuel cell system configuration lie between 24.23% and 31.95% while their energy efficiencies lie between 47.77% and 62.97% as shown in Figure 11. Energy analysis is based on the first law of thermodynamics, which does not take cognizance of the loss in quality of energy and all forms of energy are considered to be similar. However, exergy analysis is based on the first and second laws of thermodynamics and it considers the thermodynamic imperfections of a process, which comprises of all energy losses and irreversibilities [29]. The proposed PEM fuel cell configuration had the highest exergy and energy efficiencies as shown in the figure. The energy efficiency from a fuel cell stack can be as high as 80% but for the total system the efficiency is usually lower. The system efficiency depends on the number and kinds of components that are used.
Economic analysis results The economic analysis was carried out by estimating the cost of each system from the costs estimates of its individual components.
70.00
FIGURE 9
Irreversibility of components of the proposed system.
significant contribution of the chemical exergy change involved because of the electrochemical processes taking place at the electrodes in the fuel cell stack. The losses in the fuel cell stack are caused by activation polarization, ohmic and concentration polarization [28]. Other components do not involve any chemical changes; only physical changes take place due to changes in temperature and pressure. Besides, the temperatures and pressures involved are low so the losses observed are very much lower. In addition, the exergy analysis revealed that the exergy efficiency of the stack in the selected PEM fuel cell system configurations lies between 53% and 56%. This means the losses in the fuel cell stack account for about 44–47%, which is quite high. The performance of the systems will improve if these losses are reduced. The exergy 46
62.97
59.48 60.00
Efficiency (%)
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Exergy Efficiency (%)
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57.79
55.88
51.52
47.77
50.00 40.00
30.18
30.00
26.14
29.32
28.35
31.95 24.23
20.00 10.00 0.00 System 1
System 2
System 3
System 4
System 5
Systems Exergy Efficiency (%)
Energy Efficiency (%)
FIGURE 11
Overall exergy and energy efficiencies of each system.
Proposed Syst.
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TABLE 6
Economic analysis results. System 2
System 3
System 4
System 5
Proposed syst.
4416.20 4.671 945.42 0.471 3709.61 3967.91 30,269.11 397.46 4365.36 5599.79 1234.42 247.31 987.11 740.34 987.64 4697.25 4 5 20
5329.60 4.654 1145.17 0.542 4476.86 4564.12 30,157.92 479.66 5043.79 5579.22 535.43 298.46 236.97 177.73 476.19 4953.05 9 2 4
4464.39 4.539 983.65 0.471 3750.09 3967.91 29,410.04 401.80 4370.35 5440.86 1070.51 250.01 820.50 615.38 865.38 4615.47 4 5 16
5517.93 4.941 1116.71 0.530 4635.06 4468.60 32,019.17 496.61 4965.21 5923.55 958.34 309.00 649.33 487.00 796.00 5431.07 6 3 11
4140.60 4.689 882.96 0.589 3478.10 4960.49 30,387.82 372.65 5333.14 5621.75 288.60 231.87 56.73 42.55 274.42 3752.53 13 2 1
4327.64 4.945 875.13 0.471 3635.22 3967.91 32,044.57 389.49 4357.39 5928.25 1570.85 242.35 1328.50 996.38 1238.73 4873.94 3 7 27
1145.17
1200.00
1116.71 983.65
945.42
882.96
875.13
System 5
Proposed Syst.
900.00 600.00 300.00 0.00 System 1
System 2
System 3
System 4
ORIGINAL RESEARCH ARTICLE
System 1
Esmated cost per kW of net power ($/kW)
Systems Estimated cost ($) Net power (kW) Cost per kW of net power ($/kW) Fuel consumption (kg/h) Fixed capital investment ($) Annual fuel cost ($) Annual output (kWh) Annual maintenance cost ($) Annual operating cost ($) Annual income (sales) ($) Annual operating income ($) Annual depreciation ($) Income before tax ($) Income after tax ($) Annual cash income ($) Annual cash flow ($) Pay back period (yrs) Proceeds per dollar ($) ROI (%)
Figure 13 shows the summary for the pay-back period and return on investment for the studied systems while Table 6 shows the detailed economic analysis results. The proposed fuel cell system configuration had the lowest payback period of about 3 yrs and the highest ROI of 27% while system 5 had the highest pay-back period of about 13 yrs and the lowest ROI of 1%.
Conclusion
Systems FIGURE 12
Estimated costs per kilowatt of net power of each system.
Figure 12 shows the cost estimates per kilowatt of net power for each of the system. It can be observed from Figure 12 that system 2 has the highest cost estimate per kilowatt net power followed by system 4, system 3, system 1, system 5 and the proposed system, in that order. The proposed system recorded the least cost of $875.13 among the six systems investigated due to its choice and number of components.
27
An exergy analysis carried out on five selected PEM fuel cell system configurations revealed that the largest exergy loss occurs in the fuel cell stack (over 90%) for each of the systems. The overall exergy and energy efficiencies of the systems lie between 24.23% and 30.18% and between 47.77% and 59.48%, respectively. A hybrid PEM fuel cell system configuration was proposed, modelled and analyzed. The exergy and energy efficiencies of the proposed PEM fuel cell system configuration were found to be 31.95% and 62.97%, respectively. The proposed system recorded the least cost of $875.13 with a pay-back period of about 3 years and ROI of 27%. In terms of both energy utilization and cost, the proposed PEM fuel cell system configuration is the best followed by system 1, system 3, system 4, system 2 and system 5. Thus, this study has proposed a PEM fuel cell system configuration that has very good performance in terms of energy utilization with relatively lower cost.
20 16
Acknowledgement 12.7
3.8
The authors are grateful to EUtech Scientific Engineering GbmH, Germany for providing the Thermolib software used in this work.
11
9.4 4
4.3
5.8 1
System 1
System 2
System 3
System 4
Pay Back Period (yrs)
System 5
2.9
References Proposed Syst.
ROI (%)
FIGURE 13
Economic indicators for the PEM fuel cell system configurations studied.
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ORIGINAL RESEARCH ARTICLE
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