Numerical investigation into hydrogen supply stability and I-V performance of PEM fuel cell system with passive Venturi ejector

Journal Pre-proofs Numerical investigation into hydrogen supply stability and I-V performance of PEM fuel cell system with passive Venturi ejector Jenn-Kun Kuo, Wei-Zhe Jiang, Chun-han Li, Tzu-Hsuan Hsu PII: DOI: Reference:

S1359-4311(19)34522-3 https://doi.org/10.1016/j.applthermaleng.2020.114908 ATE 114908

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

1 July 2019 9 December 2019 5 January 2020

Please cite this article as: J-K. Kuo, W-Z. Jiang, C-h. Li, T-H. Hsu, Numerical investigation into hydrogen supply stability and I-V performance of PEM fuel cell system with passive Venturi ejector, Applied Thermal Engineering (2020), doi: https://doi.org/10.1016/j.applthermaleng.2020.114908

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Numerical investigation into hydrogen supply stability and I-V performance of PEM fuel cell system with passive Venturi ejector Jenn-Kun Kuoa*, Wei-Zhe Jianga, Chun-han Lib, Tzu-Hsuan Hsub aDepartment

of Greenergy, National University of Tainan, Tainan, 70005, Taiwan Energy and Environment Research Laboratories, Industrial Technology Reasearch Institute, Taiwan

bGreen

*Corresponding

author. Tel.: +886-6-2605021; Fax: +886-6-2602205 E-mail address: [email protected]

Abstract In traditional PEM fuel cells (PEMFCs), the unreacted hydrogen gas at the anode outlet is recovered using a mechanical pump or centrifuge and returned to the inlet side for reuse. However, the energy consumed by the pump reduces the system efficiency. Accordingly, the present study considers a 3-kW PEMFC in which the unreacted hydrogen gas is recovered passively from the exhaust stream using a Venturi ejector. An integrated simulation framework is constructed consisting of a COMSOL model of the ejector unit and a MATLAB/Simulink model of the hydrogen recovery system. The simulation framework is used to examine the temperature, pressure and velocity distributions within the ejector for various values of the inlet hydrogen pressure. The stability of the hydrogen supply in PEMFC systems incorporating a passive ejector system and a traditional mechanical hydrogen recovery system, respectively, is then investigated and compared. It is shown that the ejector stabilizes the hydrogen supply more 1

quickly than the traditional mechanical system. As a result, it not only improves the hydrogen utilization rate and environmental safety of the PEMFC system, but also reduces the overall hydrogen consumption. The simulation results for the I-V performance of the PEMFC with the Venturi ejector are shown to be in good qualitative agreement with the experimental results. Overall, the numerical framework constructed in the present study provides a useful tool for exploring the effects of the ejector design parameters on the hydrogen recovery performance and determining the operating conditions which maximize the performance of the PEMFC system. Keywords: proton exchange membrane fuel cell, hydrogen recovery technology, Venturi passive recovery, active pump, efficiency

1. Introduction As the world continues to develop, the global energy demand has increased dramatically. Consumer and industrial demand for energy has traditionally been met using fossil fuels, such as coal, oil and natural gas. However, the world’s remaining supplies of these fuels are being rapidly consumed. Moreover, the combustion of fossil fuels contributes towards many adverse environmental effects, including global warming, climate change, rising sea levels, and so on. Consequently, the problem of developing alternative green energy sources has attracted great

2

interest in recent years. Among the various technologies which have been proposed, including solar power, wind power, geothermal power, and so on, fuel cells are one of the most promising [1]. Fuel cells have many advantages for practical applications, including good portability, low noise, a relatively high efficiency, and a reasonably low operating temperature. Furthermore, they can operate on many different fuels, including methanol, ethanol, methane, hydrogen, and others. They are thus considered to be one of the most attractive solutions for future power generation [2]. In practical PEMFC systems, the fuel cell is provided with an over stoichiometric ratio of hydrogen in order to ensure the availability of sufficient hydrogen to meet the power load demand under dynamic operating conditions [3]. As a consequence, the anode exhaust stream contains a small amount of unreacted hydrogen. If this hydrogen is simply discharged to the atmosphere, the efficiency of the fuel cell system is seriously degraded. Furthermore, in confined spaces, there is a real risk of explosion if the exhaust gas encounters an accidental spark or fire source. As a result, it is necessary to recover the unreacted gas from the exhaust in some way in order to improve both the efficiency of the system and its operational safety. Broadly speaking, existing gas recovery systems for fuel cells comprise two main types, namely active and passive [4]. Active systems generally take the form of recirculation pumps or highspeed centrifuges, which extract the unreacted hydrogen from the flue gas mechanically and return it to the anode inlet side for reuse. However, the energy consumed by the pump or

3

centrifuge in recycling the hydrogen reduces the overall system efficiency. In addition, the need for additional hardware increases the system cost. Accordingly, the use of passive devices such as Venturi vacuum pumps is generally preferred. Venturi ejectors have many advantages for hydrogen recovery purposes, including a small size, a light weight, a simple design, a straightforward operation, and a lack of maintenance. Furthermore, they have a low cost and consume no additional power [5]. As a result, the design and application of ejector systems has attracted significant interest in the literature. Yan et al. [6] showed that the ejector reflow efficiency increased significantly as the inlet area of the hydrogen recovery end-tube increased. Moreover, for a constant inlet area, the cycle recovery ratio increased with a smaller pressure in the mixing chamber. Besagni et al. [7] presented a comprehensive review of the ejectors used in refrigeration systems. Kim et al. [8] designed and evaluated an ejector system for hydrogen recovery in a submarine PEMFC. He et al. [9] proposed a hybrid hydrogen recirculation system for PEMFCs comprising both an ejector and a blower. Bao et al. [10] presented a control model for a PEMFC system consisting of a hydrogen recycling model and an ejector model. The model was used to examine the dynamic characteristics of a typical PEMFC system under real-world operating conditions. Brunner et al. [11] developed a two-dimensional (2D) ejector model to investigate the design and control of a PEMFC injector. Maghsoodi et al. [12] combined a 2D ejector model with an artificial neural network (ANN) and a genetic algorithm (GA) to evaluate the optimal

4

ejector design. Nikiforow et al. [13] designed a hydrogen circulation ejector for a 5-kW static PEMFC system using a 2D axisymmetric ejector model. The experimental results showed that the boundary conditions at the secondary inlet and the pressure at the ejector outlet had a significant effect on the ejector performance; particularly at lower pressures. In a later study [14], the same group performed numerical simulations to investigate the power strain assist capability of a hydrogen recovery system in a 5-kW PEMFC attached to the grid. It was found that the output power could be increased from 2 kW to 3.7 kW in 1 second provided that the system was provided with sufficient fuel and had a low anode side recovery pressure. As described above, various geometric models and control techniques have been proposed for the design and optimization of vacuum ejectors. However, all of the studies employ only 2D models. Thus, it is difficult to validate the simulation results by reference to experimental observations. Although the literature contains many 3D models for solid oxide fuel cell (SOFC) systems [15-20], a detailed 3D analysis of the performance and stability of PEMFCs with a passive hydrogen recovery capability is still lacking. Accordingly, the present study constructs an integrated 3D simulation framework consisting of COMSOL and MATLAB/Simulink models to examine the performance of a 3-kW PEMFC incorporating a Venturi ejector for hydrogen recovery purposes. The simulations focus particularly on the temperature, pressure and velocity distributions within the ejector given various hydrogen inlet pressures and the stability of the hydrogen supply following system startup. The validity of the

5

proposed framework is demonstrated by comparing the hydrogen mass flow rate within the ejector under various inlet pressures with the experimental results. The simulated I-V response of the PEMFC is also compared with the experimental response. In general, the results show that the use of the passive ejector to perform hydrogen recovery reduces the overall hydrogen supply to the stack and results in a more stable hydrogen supply than that obtained using a traditional mechanical pump or centrifuge system.

2. System Module Development

Figure 1 presents a schematic illustration of the passive Venturi ejector used in the present study for hydrogen recovery purposes. The main components include a primary flow inlet, a secondary flow inlet, a mixing chamber, a diffuser and an outlet. During operation, a high-pressure gas stream is ejected from the outlet end of the ejector and the resulting vacuum effect within the chamber / diffuser draws additional fluid into the ejector through the secondary inlet [15]. For the PEMFC system considered in the present study, hydrogen is supplied to the ejector from a storage tank through the main inlet and is mixed with unreacted hydrogen recovered from the anode outlet side and flowed into the mixing chamber through the secondary inlet.

6

Figure 1. Schematic illustration of Venturi ejector. Figure 2 illustrates the passive hydrogen recovery system considered in the present study. As shown, the ejector replaces the active pump system used in traditional PEMFCs, and hence increases the overall energy efficiency of the PEMFC while simultaneously reducing the cost. The main items of the hydrogen supply and recovery system on the anode inlet side include a hydrogen storage tank, a manual valve, a solenoid valve, a regulator valve, a hydrogen mass flow meter / controller, and the ejector. On the anode outlet side, the system additionally contains a purge solenoid valve and manual valve to exhaust the anode outlet flow stream to the environment to adjust the system pressure during startup.

Figure 2. Passive hydrogen recovery system.

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During operation, hydrogen is supplied to the fuel cell stack from the hydrogen tank by manually opening the supply valve. The in-pipe pressure within the system is adjusted using the purge valve on the anode outlet side until the nominal system pressure is obtained. The mass flow controller (MFC) is then used to adjust the inlet flow rate into the ejector in such a way as to compensate for the unreacted hydrogen recovered from the anode outlet side. Figure 3 shows the simulation model of the entire passive hydrogen recovery system constructed in MATLAB/Simulink. Note that the detailed temperature, pressure and velocity conditions within the ejector are modeled separately using COMSOL.

Figure 3. Detailed Simulink model of passive hydrogen recovery system.

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2.1 Ejector system and principle Figure 4 shows the four main regions of interest in the ejector in the present system, namely (A) the primary fuel inlet, (B), the mixed fuel outlet, (C) the secondary (recovery) fuel inlet, and (D), the mixing chamber inlet nozzle.

Figure 4. Sectional view of Venturi ejector. From the principles of mass conservation, the mass flow rate at A must equal that at D, i.e., (1)

mＡ＝mD where m is the mass flow rate, and is given as

(2)

𝑚＝𝜌𝐴𝑉

where ρ is the gas density, A is the area of the inlet section of the ejector, and V is the average gas flow velocity. Assuming the gas flow to be incompressible, the density ρ can be ignored, and Eq. (1) can be simplified as (3)

𝐴 𝐴𝑉 𝐴 = 𝐴 𝐷𝑉 𝐷 Furthermore, since 𝑉𝐷 ≫ 𝑉𝐴 , Bernoulli's principle applies, and hence

9

1

1

(4)

𝑃𝐴 + 2𝜌𝑉𝐴 +𝜌𝑔ℎ𝐴 = 𝑃𝐷 + 2𝜌𝑉𝐷 +𝜌𝑔ℎ𝐷

where P is the pressure, g is the gravity acceleration force, and h is the height. Since A and D are located at the same height (i.e., the ejector is installed horizontally), Eq. (4) can be simplified to 1

1

(5)

𝑃𝐴 + 2𝜌𝑉𝐴 = 𝑃𝐷 + 2𝜌𝑉𝐷

where PA is the inlet pressure and PD is the pressure at the nozzle exit and is less than the input pressure. Gas flows naturally from high-pressure regions to low-pressure regions. Thus, when PD falls below a certain critical value, a suction force is developed within the ejector which draws unreacted hydrogen from the anode outlet side of the PEMFC into the ejector throughout the secondary fuel inlet (C).

2.2 Governing equations of hydrogen recovery system The continuity equation for the control volume has the form ∂𝑃

(6)

∇ ∙ ρ𝑉 + ∂𝑇 = 0 The momentum conservation equations can be expressed as [21-23]: 𝐷𝑢

∂𝑝

∂

[(

∂𝑢

2

)]

∂

[(

𝜌 𝐷𝑡 = 𝜌𝑔𝑥 ― ∂𝑥 + ∂𝑥 𝜇 2∂𝑥 ― 3∇ ∙ 𝑉 + ∂𝑦 𝜇

∂𝑢 ∂𝑦

)]

∂𝑣

∂

[(

+ ∂𝑥 + ∂𝑧 𝜇

∂𝑤 ∂𝑥

)]

∂𝑢

+ ∂𝑧

(7)

10

𝐷𝑣

∂𝑝

∂

[(

𝜌 𝐷𝑡 = 𝜌𝑔𝑦 ― ∂𝑦 + ∂𝑥 𝜇

∂𝑢 ∂𝑦

)]

∂𝑣

∂

[(

∂𝑢

2

)]

∂

[(

+ ∂𝑥 + ∂𝑦 𝜇 2∂𝑦 ― 3∇ ∙ 𝑉 + ∂𝑧 𝜇

∂𝑣 ∂𝑧

)]

∂𝑤

+ ∂𝑦

(8)

𝐷𝑤

∂𝑝

∂

[(

𝜌 𝐷𝑡 = 𝜌𝑔𝑧 ― ∂𝑧 + ∂𝑥 𝜇

∂𝑤 ∂𝑥

)] + ∂𝑦∂ [𝜇(∂𝑣∂𝑧 + ∂𝑤∂𝑦)] + ∂𝑧∂ [𝜇(2∂𝑤∂𝑧 ― 23∇ ∙ 𝑉)]

∂𝑢

+ ∂𝑧

(9)

The steady-state dynamic properties of the fluid can be expressed using the following NavierStokes equation for incompressible flow: 𝜌(𝑢 ∙ ∇)𝑢 = ∇ ∙ [ ―𝑝𝑙 + 𝜇(∇𝑢 + (∇𝑢)𝑇)] +𝐹

(10)

where u is the velocity of the fluid and μ is the viscosity coefficient. When the boundary condition of the model inlet is set as a constant pressure, the Navier-Stokes equation reduces to 𝜇(∇𝑢 + (∇𝑢))𝑇 = 0 , P = P0

(11)

where P0 is the constant pressure. Meanwhile, when the boundary condition of the model entrance is set as the mass flow rate, then (12)

𝒹𝑧∫𝜌𝑢𝒹S = 𝑚 where 𝒹𝑧 ∙ ∫𝒹S is the cross-sectional area of the pipeline.

In the present study, the working fluid is hydrogen gas, and the density and viscosity are thus given respectively as [24] (13)

𝜌𝐻2 = 0.0899𝑘𝑔/𝑚^3 ,𝜇𝐻2 = 8.76𝑃𝑎 ∙ 𝑠

11

2.3 Numerical method The temperature, pressure and velocity conditions within the ejector were modeled using 3D COMSOL Multiphysics 5.2a finite element software using an unstructured grid system with triangle elements. The momentum, energy and mass transport within the ejector were obtained using the SPOOLES solver, while the energy conservation fields were obtained using the GMRES (Generalized Minimum RESidual) solver. In the SPOOLES solver, the governing equations were expressed in matrix form using the multi-frontal method, and were solved by LU (upper-triangular and lower-triangular) factorization, followed by nested dissection. In the GMRES solver, the LU factorization method was replaced by a direct iteration method. To improve the convergence speed, a geometric multigrid method was used as a preconditioner [25]. In performing the calculations, the damping constants of the momentum and energy equations were appropriately defined and the spatial resolution was set as 25,058 meshes in order to obtain an acceptable tradeoff between the numerical accuracy and the computational cost. The iterative computation process was terminated once the residues fell to a value of less than 10−8.

2.4 Hydrogen supply subsystem

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The hydrogen supply subsystem comprised a hydrogen storage tank and a hydrogen flow regulation module. The hydrogen flow rate required to satisfy the load demand (i.e., the electrical current) was calculated in accordance with 6000𝑅𝑇𝐻2𝑁𝑓𝑐𝐼

(14)

𝑉𝐻2 = 𝑍𝐻 𝐹𝑃𝐻 𝜂𝑢𝑛𝑡𝑖𝑙,𝐻 𝜂𝑝𝑢𝑟𝑖𝑡𝑦,𝐻 2

2

2

2

where R is a constant, T is the temperature, Nfc is the number of stacks, I is the current, P is the pressure, ηutil is the hydrogen consumption rate, and ηpurity is the hydrogen purity. In the Simulink model, the hydrogen storage tank and flow regulation module were combined into a single hydrogen supply subsystem, as shown in Fig. 5.

Figure 5. Simulation model for hydrogen supply subsystem.

2.5 Air supply subsystem The air supply subsystem comprised an air source, an air flow adjustment module, a blower, and a controller, as shown in Fig. 6. For given pressure and temperature conditions, the air flow rate was determined as

𝑉 𝑂2 =

6000𝑅𝑇𝑎𝑖𝑟𝑁𝑓𝑐𝐼 𝑍𝑂2𝐹𝑃𝑎𝑖𝑟𝜂𝑢𝑡𝑖𝑙,𝑂2𝜂𝑝𝑢𝑟𝑖𝑡𝑦,𝑂2

13

(15)

where R is a constant, T is the temperature, Nfc is the number of stacks in the fuel cell, I is the current, P is the pressure, ηutil is the oxygen consumption rate, and ηpurity is the oxygen purity. Furthermore, referring to Fig. 6, Pmechanical is the blower output power and Output is the outlet fluid state (including the mass).

Figure 6. Simulation model for air supply subsystem.

2.6 Venturi ejector supply subsystem The flow rate and pressure conditions in the Venturi ejector were modeled as follows [26]:

𝑞1 =

𝐴𝑛 1 + 𝐾𝑛

2(𝑝1 ― 𝑝0) 、 𝜌

𝑞2 =

2(𝑝2 ― 𝑝0)

𝐴𝑛 ∙ 𝑐 1 + 𝐾𝑒𝑛

(16)

𝜌

(2𝑏 + 1 ―2 𝑏𝑀2 ― (1 + 𝑀)2 ∙ (1 + 𝐾𝑡ℎ + 𝐾𝑑𝑖 + 𝑎2))

𝑝𝑑 ― 𝑝0 = 𝑍𝑏2 where 𝑎 =

𝐴𝑡ℎ

𝐴𝑛

𝐴𝑑 , 𝑏 = 𝐴𝑡ℎ , 𝑐 =

1―𝑏 𝑏 ,

𝑍=𝜌

𝑉𝑛2 2

𝑞12

(17)

𝑞2

= 𝜌2𝐴 2, 𝑀 = 𝑞1, and K is the loss coefficient. 𝑛

Furthermore, q1 is the main gas flow rate (see Fig. 7), q2 is the recovery gas flow rate, qd is the outlet flow rate, p1 is the main inlet end pressure, p2 is the recovery inlet end pressure, p0 is the throat inlet pressure, pd is the outlet pressure, and An is the nozzle area. Finally, Ath is the throat area, Ad is the outlet diffusion area, Kn is the nozzle fluid loss rate, Ken is the throat inlet fluid loss rate, Kth is the throat fluid loss rate, Kdi is the diffusion fluid loss rate, and ρ is the fluid density.

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Figure 7. Geometry and simulation parameters for ejector. The basic settings of the main simulation parameters in the Simulink model are shown in Table 1. Note that the reaction area for each cell stack was set as 0.5 m2 and the heat transfer rate was specified as 8.36x103𝐽 ∙ 𝐾 ―1. In addition, the simulation time was set as 36000 seconds and the system was assumed to be in a transient condition (i.e., system startup). Table 1. Main simulation parameters in Simulink model of hydrogen recovery system Experiment/Simulation PEM Fuel Cell Current 15

30

60

120

190

240

300

H2 inlet(bar)

1.50

1.50

1.50

1.46

1.90

1.90

1.90

H2 stack in let(bar)

1.43

1.43

1.43

1.42

1.70

1.70

1.70

Air inlet(bar)

1.13

1.13

1.13

1.26

1.43

1.43

1.43

cell stack

25

25

25

25

25

25

25

15

3. Simulation and Experimental Results for PEMFC Hydrogen Recovery System The simulations commenced by examining the temperature, pressure and velocity fields within the ejector for various values of the hydrogen inlet pressure. The stability of the hydrogen supply rate was then examined in two 3-kW PEMFC systems with an ejector mechanism and a traditional mechanical pump, respectively. Finally, the I-V response of the PEMFC system with an ejector unit was numerically derived. The validity of the numerical framework (COMSOL and Simulink) was confirmed by comparing the simulation results for the hydrogen mass flow rate in the ejector under various inlet pressures with the experimental results obtained using an equivalent experimental setup. The validity was further confirmed by comparing the simulated I-V response of the PEMFC with the experimental response.

3.1. Simulation results for vacuum ejector device Figures 8, 9, 10 and 11 show the COMSOL simulation results for the temperature, pressure and velocity distributions within the ejector give inlet hydrogen pressures of 1.75 bar, 2.25 bar, 3.25 bar and 4.5 bar, respectively. Overall, the four figures show that the pressure and velocity within the ejector body increase with an increasing inlet pressure, while the temperature reduces. From inspection, the minimum temperature in the ejector is equal to just 140 K (see Fig. 11). From an operational perspective, a low temperature is undesirable since it is unable to remove the moisture content of the hydrogen recovered from the PEMFC.

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Consequently, in practical applications, the pressure conditions at the ejector inlet must be carefully controlled in order to ensure an adequate temperature within the ejector tube.

Figure 8. Temperature, pressure and velocity distributions in ejector given hydrogen inlet pressure of 1.75 bar.

Figure 9. Temperature, pressure and velocity distributions in ejector given hydrogen inlet pressure of 2.25 bar.

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Figure 10. Temperature, pressure and velocity distributions in ejector given hydrogen inlet pressure of 3.25 bar.

Figure 11. Temperature, pressure and velocity distributions in ejector given hydrogen inlet pressure of 4.5 bar.

Figure 12 compares the simulation and experimental results for the variation of the hydrogen mass flow rate within the ejector given hydrogen inlet pressures of 1.2 ~ 4.5 bar. In general, the results show that the secondary flow rate (i.e., the recovery flow rate) increases

18

with an increasing inlet pressure. For an inlet pressure of less than 1.75 bar, the primary mass flow rate is non-linear. As a result, the ejector throat portion is not blocked, and the mixed hydrogen flow exits the ejector at a subsonic speed. However, for a higher inlet pressure of 2.25 bar, the primary mass flow rate is greater than the secondary mass flow rate, and the ejector throat portion thus begins to block. Nonetheless, the mixed mass flow rate is non-linear, and hence the recovery end flow is still not blocked. For an inlet pressure of 3.25 bar, the primary and secondary flows are blocked in the mixing chamber. As the inlet pressure increases further, the exit flow rate also increases, and reaches a simulated value of approximately 50 NL *min-1 at an inlet pressure of 4.5 bar. It is noted that this value is in good qualitative agreement with the experimental value. Thus, the basic validity of the numerical model is confirmed.

Figure 12. Experimental and simulation results for variation of hydrogen mass flow rate in ejector with hydrogen inlet pressure. 19

3.2. Simulation results for stability of hydrogen recovery system Figure 13 shows the Simulink results for the initial (10 hours) hydrogen mass flow rate to the fuel stack of a traditional PEMFC system with an active (mechanical) hydrogen recovery loop. As shown, the system is supplied with hydrogen from both the main reservoir tank and the recovery tank. Following system startup, both hydrogen supplies exhibit an oscillatory behavior with a large amplitude as a result of pressure imbalances between the two tanks and the fuel stack. However, as the fuel stack warms and reaches its operational pressure, the amplitudes of both fuel supplies reduce and approach a more settled behavior.

Tank Recovery H2 Consumed

0.10

250000

200000 0.05 150000

0.00

Pressure (pa)

Mass Flow (g·s-1)

Tank Pressure

100000

0

4000

8000

12000 16000 20000 24000 28000 32000 36000

Time (sec) Figure 13. Variation of mass flow rate and pressure in hydrogen supply system of PEMFC with mechanical hydrogen recovery loop.

20

Figure 14 shows the variation of the hydrogen supply mass flow rate in a PEMFC system with and without a passive ejector, respectively. For the case where the ejector is not employed, the hydrogen supply remains stable at 0.1 g s-1. However, the hydrogen consumption is just 0.08 g s-1, and hence the residual hydrogen in the anode flu gas must be exhausted to the atmosphere. When the ejector is employed, a large amount of hydrogen is required initially to achieve a pressure balance within the system. However, after around 1000 seconds, the hydrogen supply mass flow rate reduces to just 0.02 g s-1 and remains approximately constant thereafter. In other words, the use of the ejector reduces the overall hydrogen supply, increases the hydrogen utilization rate, and improves the environmental safety of the fuel cell compared to a traditional PEMFC with a mechanical hydrogen recovery loop.

21

0.6

Hydrogen supply(Recovery) Hydrogen supply(Non-recovery) H2 consumed

Mass Flow (g·s-1)

0.5 0.4 0.3 0.2 0.1 0.0 0

600

1200

1800

2400

3000

3600

Time (sec) Figure 14. Variation of hydrogen supply mass flow rate with and without passive Venturi ejector device, respectively. Figure 15 shows the variation of the mass flow rate of the hydrogen supplied from the recovery tank in the PEMFC system with a mechanical hydrogen recovery loop. As described above, the hydrogen supply rate oscillates markedly following system startup due to pressure imbalances within the system. Consequently, a minimum warmup time of across 20000 second is required to achieve a stable flow rate, which is far greater than the time required for the system with a Venturi ejector device (1000 s).

22

0.10

Tank recovery H2 consumed

Flow Mass (g·s-1)

0.08

0.06

0.04

0.02

0.00

0

4000

8000

12000 16000 20000 24000 28000 32000 36000

Time (sec) Figure 15. Variation of hydrogen mass flow rate from recovery tank in PEMFC system with mechanical recovery loop.

Figure 16 shows the experimental and simulation results for the I-V response of the 3-kW PEMFC system with a Venturi ejector hydrogen recovery loop. The experimental voltage is slightly higher than the simulated voltage since the exhaust function used in the experimental setup to maintain the system efficiency and stability is not implemented in the simulation model. However, a good qualitative agreement is nevertheless observed between the two sets of results. Consequently, by applying a suitable calibration factor to the simulation values, reliable estimates of the I-V response can be obtained. In other words, the proposed simulation model provides an effective tool for the design and optimization of the PEMFC system and its various components. 23

30 27 24

Simulation Experiment

E (V)

21 18 15 12 9 6 0

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320

I (A) Figure 16. Comparison of experimental and simulation results for I-V response of PEMFC system with Venturi ejector hydrogen recovery system.

4. Conclusion This study has constructed an integrated simulation framework consisting of COMSOL and Simulink models to analyze the hydrogen supply stability and recovery performance of a PEMFC system incorporating a passive Venturi ejector device. The validity of the proposed framework has been demonstrated by comparing the simulation results for the hydrogen mass flow rate in the ejector and the I-V response of the PEMFC with the experimental observations. It has been shown that the pressure and velocity within the ejector increase with an increasing hydrogen inlet pressure. By contrast, the temperature reduces as the pressure increases. A lower 24

ejector temperature is disadvantageous in removing the moisture content of the hydrogen gas recovered from the anode outlet. Consequently, in practical applications, the hydrogen inlet pressure must be carefully controlled to ensure an adequate temperature within the ejector body. Most significantly, the results have shown that the use of a passive Venturi ejector to perform hydrogen recovery significantly reduces the time required to obtain a stable hydrogen supply compared to that in a traditional mechanical recovery system. Furthermore, the steady-state hydrogen mass flow rate is also lower than that in a traditional system. Consequently, the passive ejector not only reduces the cost of the system and improves its overall energy efficiency, but also reduces the hydrogen consumption, increases the hydrogen utilization efficiency and improves the environmental safety. Overall, the present results have shown that the proposed simulation framework provides a useful tool for investigating the performance of the PEMFC system and identifying the optimal geometry and working parameters of the various components within it.

Acknowledgements The authors gratefully acknowledge the financial support provided to this study by the Ministry of Science and Technology (MOST) of Taiwan under Grant No. 106-2221-E-024010. The experimental data provided by the Green Energy and Environment Research

25

Laboratories of Tainan, Industrial Technology Research Institute of Taiwan is also much appreciated.

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Highlights  Integrated simulation framework consisting of COMSOL and Simulink models.  Hydrogen supply stability and recovery performance of a PEMFC system.  A lower ejector temperature is disadvantageous in removing the moisture content.  Passive Venturi ejector to perform hydrogen recovery significantly reduces the time required.

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Editorial board Applied Thermal Engineering

Dear Editor Thank you for the Reviewer’s comments, I have revised the reviewer’s comments for Revised 2. Enclosed please find three complete copies of the paper entitled “Numerical investigation into

hydrogen supply stability and I-V performance of PEM fuel cell system with passive Venturi ejector” which is authored by J.K. Kuo. This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. All study participants provided informed consent, and the study design was approved by the appropriate ethics review boards. All the authors have approved the manuscript and agree with submission to your esteemed journal. There are no conflicts of interest to declare. I look forward to hearing from you at your earliest convenience. Yours Sincerely, Jenn-Kun Kuo Professor Department of Greenergy National University of Tainan Tainan, Taiwan E-mail: [email protected] Tel:+886-6-2605021 Fax.: 886-6-2602205

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Editorial board Applied Thermal Engineering

Dear Editor Enclosed please find three complete copies of the paper entitled “Numerical investigation into

hydrogen supply stability and I-V performance of PEM fuel cell system with passive Venturi ejector” which is authored by J.K. Kuo. This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. All study participants provided informed consent, and the study design was approved by the appropriate ethics review boards. All the authors have approved the manuscript and agree with submission to your esteemed journal. There are no conflicts of interest to declare. I look forward to hearing from you at your earliest convenience. Yours Sincerely, Jenn-Kun Kuo Professor Department of Greenergy National University of Tainan Tainan, Taiwan E-mail: [email protected] Tel:+886-6-2605021 Fax.: 886-6-2602205

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