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System model development for a methanol reformed 5 kW high temperature PEM fuel cell system
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System model development for a methanol reformed 5 kW high temperature PEM fuel cell system Simon Lennart Sahlin*, Søren Juhl Andreasen, Søren Knudsen Kær Department of Energy Technology, Aalborg University, Pontoppidanstraede 101, 9220 Aalborg, Denmark
article info
abstract
Article history:
This work investigates the system performance when reforming methanol in an oil heated
Received 21 February 2015
reformer system for a 5 kW fuel cell system. A dynamic model of the system is created and
Received in revised form
evaluated. The system is divided into 4 separate components. These components are the
11 July 2015
fuel cell, reformer, burner and evaporator, which are connected by two separate oil circuits,
Accepted 26 July 2015
one with a burner and reformer and one with a fuel cell and evaporator. Experiments were
Available online 25 August 2015
made on the reformer and measured oil and bed temperatures are presented in multiple working points. The system is examined at loads from 0 to 5000 W electric and the
Keywords:
response time and efficiency of the system are evaluated. The efficiency is estimated to be
Methanol
around 28e30% during load. Startup of the system is estimated to be around 45 min.
Dynamic model
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
HTPEM Reformer Steam reforming
Introduction The proton exchange membrane fuel cell (PEMFC) has received much attention in recent years. In the field of PEM fuel cells there are primarily two types, low temperature PEM fuel cells (LTPEM) and High temperature fuel cells (HTPEM). The most common PEM fuel cell is the LTPEM (Nafion based) that operates at about 50e80 C and requires a clean H2 supply. The HTPEM fuel cells use a PBI (polybenzimidazole) membrane and can operate at a much higher temperature 125e200 C [1e3]. High temperature PEM (HTPEM) fuel cells have shown stable results in high ionic conductivity and are able to tolerate impurities in the gas feed, such as Carbon Monoxide (CO) and methanol (CH3OH) [4,5]. This opens up for the
possibility to use reformate gas without the use of H2 cleaning processes, but it does, however, increase the complexity of the system, by adding additional components such as heat exchangers, evaporators and chemical reactors. Work done on HTPEM by Li et al. [6] has shown that CO levels up to 3% can be tolerated at current density up to 0.8 A/ cm2 at 200 C, and 0.1% CO at 125 C with a current density lower than 0.3 A/cm2. This corresponds well with work done by Korsgaard et al. [7] who shows successful operation using synthetic gas to simulate reformat gas at the anode from 0 to 5% CO and with CO2 contents ranging from 20% to 25%. This study also shows that higher amounts of CO require increased operating temperatures in the fuel cell. To avoid hydrogen under high pressure the examined system uses methanol as a primary fuel. Methanol is an
* Corresponding author. E-mail address: [email protected] (S.L. Sahlin). http://dx.doi.org/10.1016/j.ijhydene.2015.07.145 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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excellent hydrogen carrier for steam reforming (SR) because of the high hydrogen to carbon ratio (4:1), low boiling point and availability. Methanol can be produced from renewable resources thus reducing the production of greenhouse gases [8,9]. A combined methanol reformer and burner is shown by A. Lotric et al. [10] and shows promising results though requires novel HTPEM to be available. The current method of reforming, that is being heavily used today, is steam reforming. This method uses high temperature steam that reacts with a hydrogen rich fuel [11]. The governing SR reaction for methanol is as follows CH3 OH þ H2 O/3H2 þ CO2
(1)
This endothermic reaction can, with a suitable catalyst, reform methanol and water to hydrogen and CO2 at about 220e300 C. At these temperatures the decomposition of methanol can also occur. This reaction is shown in Eq. (2). CH3 OH/2H2 þ CO
(2)
CO in the hydrogen gas has a negative effect on the fuel cell performance and the extent of this influence is highly dependent on the fuel cell temperature [7,12]. In addition to the methanol decomposition a water gas shift (WGS) reaction is also taking place which reduces the CO concentration. CO þ H2 O#H2 þ CO2
(3)
All these reactions are dependent on the temperature of the reformer, methanol flow rate and the Steam-to-Carbon ratio (the water/methanol ratio in this case) [13,14]. As the temperature in the reformer is not uniform it is assumed that the actual temperature of the reformation is dependent on the flow rate. This means that the temperature is higher at low flow rate and lower at high flow rate. A study by Purnama et al. [15] on methanol steam reformers suggests that low contact time also gives a lower amount of CO.
The goal of this work is to create a usable model in Matlab Simulink. The purpose of the model is to investigate control techniques and to get an insight into the initial parameters for a methanol reforming system.
System description The system model is designed with four main components: burner, evaporator, reformer and a fuel cell stack. This system utilizes a thermal oil to transfer heat in the system. In the main operating configuration the system is connected with two oil circuits; one with the burner and reformer and one with the fuel cell and evaporator. These two circuits are separated because of the temperature difference at about 160 C in the fuel cell circuit and about 260 C in the burner circuit. The design layout can be seen in Fig. 1. The first oil circuit is the burner and reformer. The burner transfers hot exhaust gas into a heat exchanger that heats up the oil. The oil is pumped through the burner heat exchanger and is then passed into the reformer. The second oil circuit is for cooling in the fuel cell stack and is about 160 C. When the fuel cell is operating, the excess heat is used to evaporate the methanol/water input fuel before it reaches the reformer. The reformer examined in this work is sized for a 5 kW fuel cell and uses a CuZn based reforming catalyst. A model is used to give some initial parameters for the experimental setup. This includes an estimation on the minimal size of the cooler for the fuel cell system and how the dynamics in the system are affecting performance and stability. This can also give some insight into the effects of changing the load of the fuel cell in an open loop configuration. The model is also used to examine situations when emergency stops are introduced. This system is initially
Burner Air fan
Reformer
Fuel Burner
Reformer
H2 syngas Fuel flow(Liquid) Fuel flow(Gas) Oil circuit Air flow
Reformer pump
Fuel pump
Fuel Evaporator Methanol / H2O
Fuel Cell Fuel Cell pump FC Air fan
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Cooler
Fig. 1 e System layout in the operating configuration.
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Reformer
Air fan
E-23
Fuel Burner
Reformer pump
H2 syngas Fuel flow(Liquid) Oil circuit Air flow
Heat exchanger
Fuel pump Startup heat exchanger
Fuel Evaporator
Methanol / H2O
Fuel Cell
Fuel Cell pump FC Air fan
Cooler
Fig. 2 e System layout in the startup configuration.
designed with no physical safety mechanism, which means that this model can give an indication on the needed controlling requirements, for stable and reliable operation in all operating points. The experiments were carried out on the different parts of the system to verify the model and the work done on the reformer is presented below. To investigate the start-up of the system an alternative configuration is used. This configuration can be seen in Fig. 2. When the system is in this configuration it utilizes the burner to heat up the reformer and fuel cell to operational temperatures with a heat exchanger between the two circuits. When the temperature is reached in the fuel cell and reformer, the system is changed to the operational configuration, as shown in Fig. 1, and the heat exchanger is bypassed. The burner is connected to the methanol fuel flow of the system and is able to heat up the system by passing air and methanol/water into the burner. This section is closed when the operating configuration is used, unless a situation occurs where the temperature of the burner or reformer is too low. The different parts of the system are described below.
Evaporator Before the methanol/water mixture reaches the reformer it is led through a stainless steel heat exchanger. The heat input to this evaporator is supplied from the exhaust air from the fuel cell, the exhaust air from the burner and the heated oil from the oil circuit. This means that during the startup phase the evaporator is heated by the exhaust air from the burner and the oil that is being heated up. This setup can be seen in Fig. 2.
Reformer The reformer is a cylindrical metal tube about 50 cm in height, filled with catalyst pellets which are cylindrical 1 mm high and 1 mm in diameter. The reformer is fitted with several temperature sensors in the center of the catalyst bed starting
Fuel cell The stack modeled in this system is a 5 kW oil cooled high temperature PEM fuel cell with 120 cells from Serenergy A/S. This stack can be seen in Fig. 3 and is rated at a net power of 6 kW on pure H2 and 5 kW on reformate gas. The fuel cell can be run on a minimum H2 stoichiometry of 1.35 and an air stoichmetry of 2e3 [16]. The weight of the stack is 20.5 kg and 227 148 453 mm in dimensions where each cell has an active area of 165 cm2. As the temperature of the exhaust air from the fuel cell is up to 160 C, it would be ideal to be used to heat up the methanol/water fuel mixture in the evaporator.
Fig. 3 e S165L HTPEM stack from Serenergy A/S.
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from the inlet to the outlet. Each sensor is located throughout the reformer with about 6 cm in between. This arrangement of temperature sensors makes it possible to monitor the heat generated in the activation state and gives a more detailed temperature profile of the reformer. When the reformer is being activated the measurement of the temperature is highly important to avoid damaging the catalyst. The outer shell of the reformer is covered by a 5 cm layer of isolation and the methanol fuel is introduced from the bottom. A sketch of the reformer can be seen in Fig. 4.
Air output
Oil input
Heat exchanger
Oil output
Burner Air input
A catalytic burner is used in this system to utilize the anode waste gas from the fuel cell and heat the catalytic reforming process. This is done by directing a flow of hydrogen and air through the burner catalyst. The ratio of the air and hydrogen is highly important, because in the case of a low airflow an active flame can occur. This can both be dangerous and create an unstable system. If the ratio of air is too high, the hydrogen will not react and the burner will cool down. These two situations are to be avoided and a controller for the air blower is needed. The mass flow of hydrogen is difficult to physically measure in the system, so an estimation on the amount of hydrogen is calculated. This estimation is based on the methanol fuel flow, temperature of the reformer and the fuel cell load. A sketch of the burner used in this work can be seen in Fig. 5. The burner is designed to work with an air/h2 mix which is fed in at the bottom and is directed up through a mesh with the catalytic material. At this stage the hot air travels through a heat exchanger, thereby transferring heat to the oil. The exit air will be directed to the evaporator to help heat up the input methanol fuel. The model of these systems and their interaction is presented below.
H2-rich gas
H2-rich gas Oil output
Reformer
Oil output
Oil input
Oil input
Methanol feed
Methanol feed
Fig. 4 e Reformer. left shows the design of the circular tube reformer. right figure shows how the flow of oil and methanol is directed inside the reformer.
Catalytic burner
Hydrogen input
Fig. 5 e Catalytic burner and heat exchanger. Hydrogen and air input at the bottom.
Modeling The purpose of modeling this system is to create an efficient and fast way to evaluate different control strategies and give insight into the dimensions of the cooler, methanol pump and blowers. The model can also be used as a method to obtain better system performance or to enhance the overall system response during load changes or a change in operating conditions. These operating conditions could be ambient temperatures, humidity or changes in pressure. One of the challenges in these kind of systems is the delay of gas throughout the system when a change in the fuel cell load is introduced. Two negative consequences of this fuel delay can occur. The first situation can happen if the system load is changed faster than the hydrogen can be supplied and this can result in a starvation and thereby faster degradation of the fuel cell [17]. The second problem can occur when the load is lowered or halted in the system and the amount of hydrogen that would have been used in the fuel cell is directed into the burner, thereby increasing the temperature rapidly which can result in damage of the burner catalyst or overall system instability. The model is designed to represent the dynamics of the system and to be as simple as possible, so that it can be run on hardware with limited resources. This gives the possibility to use parts of the model for predictive control in an operating system. For each system the input and output temperature of the oil is simulated and transferred to the next system. The model is also used to simulate the temperature of the different components, the fuel cell performance on reformate gas and an estimate on the gas composition from the reformer. The model for the fuel cell voltage and the reformer gas composition is based on experimental data. Each component is modeled as a lumped thermal mass and the
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model is simulated in Matlab Simulink. The model can be run on a normal sized personal computer (1 core at 2701 MHz) and 10 h simulation is computed in 733 s. This is including various controllers and gas composition observers. The following sections will give an overview of the important model equations.
Model input A series of load changes are introduced as the model input. The system starts from an idle ambient temperature and is run through a startup sequence until it reaches the operational system temperature. After the startup sequence the system will experience a stepup and stepdown sequence running from 0 to 5 kW electric output. The whole sequence will run over 30.000 s and the load changes will happen with (at minimum) 300 s intervals. The fuel input is directly coupled to the fuel cell load to investigate the impact on the fuel cell and reformer. The model input can be seen in Fig. 6 which also shows the relation between the current density of the fuel cell and the flow of the methanol/water mix. This relationship is shown in Eq. (12).
Fuel cell model The temperature of the fuel cell is varied by both the oil temperature, heat generated by the fuel cell, heat loss from conduction and heat loss from convection in the airflow. The temperature of the fuel cell is calculated by the following energy balance Q_ stack ¼ Q_ fc þ Q_ Conduction þ Q_ Convection þ Q_ oil
(4)
The different heat transfer contributions are summed up to represent the heat transfer of the system Q_ stack and is calculated by Eq. (4). The power generated inside the fuel cell stack (Q_ fc ), the airflow convection (Q_ convection ), the conduction (Q_ conduction ) and the heat from the oil circuit (Q_ oil ) is used to calculate the total heat in the stack. The temperature of the fuel cell stack is modeled by 5. The calculation integrates over the power and changes the temperature as the time changes from 0 to a given time t.
DTfc ¼
1 , mfc ,Cpfc
Zt
Q_ stack dt
(5)
0
Where Tfc is the stack temperature, mfc is the mass of the fuel cell stack, Cpfc is the specific heat capacity, and Q_ stack is the power transferred to or from the system. The heat generated in the fuel cell, shown as Q_ fc , is calculated from the difference in the cell voltage compared to the thermoneutral cell voltage (VTN) multiplied with the current density (i) and the cell area Acell. This is shown in Eq. (6). Q_ fc ¼ i,ðVTN Vcell Þ,Acell
(6)
The heat removed from the fuel cell is estimated by calculating the difference in oil exit temperature Te and the oil input temperature. This can be seen in eq. 7 m_ oil ,Cpoil ,ðTe Ti Þ ¼ hfc;oil ,Afc;oil , Tfc Ti
(7)
This equation uses the fuel cell temperature Tfc, the input _ the specific heat capacity temperature Ti, the oil mass flow m, Cpoil of the oil, the active area Afc,oil is the inside wall area and the heat transfer coefficient hfc,oil for the oil. As the temperature of the oil changes during the length of the fuel cell a logarithmic mean temperature difference (LMTD) should be used. This would increase the computational time significantly of the model and a mean temperature is used instead though this is an approximation. As the exit oil temperature is now calculated, it is possible to estimate the amount of heat that is added or removed from the system. Q_ oil ¼ m_ oil ,Cpoil ,ðTi Te Þ
(8)
The heat removed by conduction from the fuel cell is calculated by Eq. 9 kfc;cond Q_ conduction ¼ Tambient Tfc ,Afc , Dxfc
(9)
where Tfc is the fuel cell temperature, Tambient is the ambient temperature, Afc is the outer area of the fuel cell, Dx is the isolation thickness and kfc,cond is the thermal conductivity of the isolation. The cooling effect of the air blower in the fuel cell is calculated by the change in enthalpy hinlet of the air multiplied with the airflow vair. The air is assumed to be the same temperature as the fuel cell at the exit houtlet. The density of the air is shown as rair Q_ convection ¼ ðhinlet houtlet Þ,rair ,vair
(10)
The enthalpy is calculated using EES (Engineering Equation calculator) where the data is imported into the model.
Fuel cell voltage
Fig. 6 e Fuel cell simulation input.
The fuel cell voltage is estimated with a 5-dimensional lookup table based on experimental data. The inputs for the table are FC stack temperature, current density, CO gas fraction, H2 gas fraction and the air stoichiometry. The only external inputs in the fuel cell model are the CO and H2 fractions which are estimated in the reformer model.
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Reformer The reformer temperature and the steam-to-carbon ratio of the input feed are used to estimate the gas composition from the steam reformer. The temperature in the reformer is assumed uniform, although this might not be the case, it is considered acceptable in this work. The heat required in the steam reformation process is H ¼ 49.4 [kJ/mol]. A verification on the reformer temperature is presented in the results part of this work. The temperature of the reformer is considered to be the average of the bed temperature.
Burner Similar to the reformer and fuel cell stack, the burner temperature is modeled as a the fuel cell and reformer with the exception that the anode exhaust gas is burned. The burner is heated by the combustion of the excess hydrogen from the fuel cell anode and the methanol fuel under the startup sequence. This is expressed as Q_ heat Q_ heat ¼ DH+c;CH3OH ,n_CH3OH þ DH+c;H2 ,n_H2
(11)
Where DH+c;ch3oh is the heat of combustion of methanol and DH+c;H2 is that of hydrogen. Both DH+c;CH3OH and DH+c;H2 are both the lower heating value(LHV). n_CH3OH and n_H2 are the molar flows of methanol and hydrogen which are fed into the burner. The evaporator is a 3-stream heat exchanger with both hot air from the fuel cell and burner, an oil circuit heated by the fuel cell and a methanol feed line. This is to ensure the methanol feed is evaporated before it reaches the reformer. The temperature is modeled as the fuel cell as presented in Eq. (5). If there is not sufficient heat from the fuel cell to heat up the fuel mix it needs to come from the burner, which means that a higher stoichiometry is needed, which then leads to a lower overall system efficiency.
Pump model To estimate the fuel flow such that a desired H2 stoichiometry can be achieved at the fuel cell Eq. (12) is used. Flowfuel ½ml=hr ¼
i,ncell ,Acell lH2 1000 , ,r , 2,F kgas ,XH2 meoh=h2o 3600
(12)
Where i is the current density, ncell is the number of cells in the FC, Acell is the area of the cell and F is the Faraday constant. lH2 is the desired h2 stoichiometry. This equation assumes that the molar fraction of hydrogen, XH2, does not vary much from 65% under normal operating conditions. This fraction is assumed constant as long as the reformer is within the operating temperature. The ratio of 65% is also set conservatively low to ensure that the initial pump flow is always delivering a higher stoichiometry at the fuel cell. This stoichiometry can then be lowered with a controlling system where the temperature of the reformer and burner is regulated. The ratio kgas is the ratio between the methanol gas and hydrogen and is assumed to be 1.8. This ratio is based on the
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conservation of mass of the methanol fuel mix at the reformer inlet and the hydrogen output from the reformer. The ratio is estimated from the molar flow of methanol and hydrogen going through the reformer and is dependent on the reformer temperature. rmeoh=h2 o is the density of the methanol/water fuel mix. This flow estimation can be used as a feed-forward part of the controller as it estimates the pump flow. This means that the assumptions made in Eq. (12) requires the system to be in a stable operating condition. To model the transfer of liquid and gas in the system a delay is implemented in two steps. The first delay is introduced in fuel flow in the reforming phase of the fuel system line and the second is the transport delay from the fuel cell exhaust to the burner.
Model results Based on the input sequence shown in Fig. 6 it can be seen how the component temperatures change. The constants used in the model can be seen in Table 1. The system is started at ambient temperature and when the temperature in the main parts of the system reaches a minimum operational temperature, the system will change to the normal operation. This temperature is set to 150 C at the fuel cell and 180 C for the reformer. Based on Fig. 7a it can be seen how the temperature varies during the load sequence, shown in Fig. 7c. A total efficiency can be seen from Fig. 7b which varies depending on the temperature of the components and the fuel flow. After the warmup the input steps up to about 3500[ml/hr] at around 5.000 s and it can be seen that the temperature in the
Table 1 e Constants used in model. Reformer mreformer Cpoil Lengthreformer Cpmix roil SCratio AReformer Ar,oil kr,cond Burner DH+c;ch3oh Cpair mburner Aburner Ab,oil Fuel cell mfc hfc,oil lair ncell rair Afc Evaporator mevaporator Aevaporator hevaporator ke,cond
12 [kg] 0.5 [kJ/kg/K] 0.450 [m] 3.28 [kJ/kg/K] 936 [kg/m3] 1.5[] 0.51[m2] 0.4240[m2] 0.15 [W/m/K]
hr,oil Rreformer m_ oil
Dxreformer hreformer
140 [W/m2/K] 0.180 [m] 0.023 [kg/s] 1.96 [kJ/kg/K] 293.15 [K] 10 [l/min] 0.03 [m] 0.3 [W/m2/K]
681 [kJ/mol] 1.0214 [kJ/kg/K] 10 [kg] 0.5[m2] 0.25[m2]
DHc,H2 hburner cpburner Dxburner kb,cond
242 [kJ/mol] 100 [W/m2/K] 3 [kJ/kg/K] 0.02 [m] 0.15 [W/m/K]
14[kg] 978 [W/m2/K] 2.5 [] 120 [cells] 1.28 [kg/m3] 0.41 [m2]
Cpfc Acell lh2 Afc,oil kfc,cond Dxfc
0.970 [kJ/kg/K] 162.5[cm2] 1.35 [] 0.9945 [m2] 0.15 [W/m/K] 0.01 [m]
2 [kg] 0.2 [m2] 0.15 [W/m2/K] 0.15 [W/m/K]
he,oil Dxevaporator Ae,oil
200 [W/m2/K] 0.02 [m] 0.21 [m2]
Cpmix,gas Tambient m_ oil
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Fig. 8 e Heat contributions in the FC during startup and operational configuration.
Fig. 7 e (a): Components temperature during operation (b): System efficiency during operation (c): Pump flow during operation based on the fuel cell load.
reformer and burner is still not at a stable level. At 10.000 s the input fuel flow is about 6500[ml/hr] and the temperature in all components are reaching a steady-state at about 12.000 s. At 15.000 s the load is stepped down to 0 during 2000 s and it can be seen how the temperature in the burner drops to the reformer temperature as there is no fuel to heat the burner. At 17.000 s the input flow is increased to 6500[ml/hr] again and the burner regains its operating temperature and the system is operational again. This means that this system is capable to regain the operational states if the temperature in the reformer is kept above the lowest reforming temperature. Based on this model the start-up time is estimated to be about 45 min. The heat contributions in the FC can be seen in Fig. 8 and this shows how the oil circuit is the only part of the heating during the startup sequence. After the startup heating the oil circuit will be used as a cooling of the FC. As shown in Eq. (7) the main contribution to heating is from the electrochemical reaction in the fuel cell. The conduction of heat from the fuel cell is also significantly lower compared to the convection of air. Due to limitations of the heat transfer fluid, the temperature of the oil should not exceed above 316 C. This fluid also has a film temperature of 343 C which is set to be the maximum limit of the burner temperature [18]. As previously mentioned in Section 3.1 the input to the system is directly coupled to the load of the fuel cell as an open loop controller. The fuel delay from the methanol pump through the evaporator and reformer to the fuel cell is
therefore able to be evaluated and modeled. Because of this delay the fuel cell can either be affected by a fuel starvation when the load increases or the burner will experience a fuel increase when the load decreases. These spikes are modeled and can be seen in Fig. 9. To increase the efficiency and the dynamic response it is beneficial to have a knowledge of how the reformer is performing. This can give some insight into what the actual lh2 is and this can be used as a feedback input to the pump equation Eq. (12). At steady state and constant current density it is possible to get closer to the 1.35, though increases in current needs to be controlled with an increase of flow before the current is drawn from the fuel cell. So to increase the efficiency of the system, the hydrogen stoichiometry needs to be as low as possible without starving the fuel cell. From Fig. 9 the stoichiometry is about 1.4 and this corresponds to an estimated electric efficiency as shown in Fig. 7b. The efficiency is calculated using the lower heating value and is at these operating conditions about 27e30%. There is still room for improvement on the efficiency and this
Fig. 9 e Hydrogen stoichiometry in the Fuel Cell.
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Fig. 10 e System setup.
will be investigated in later work. Other work with similar system design is reporting a power efficiency at about 23e33% which corresponds well with the presented results [19]. Nicola Zuliani et al. show systems working on natural gas which shows efficiencies at about 26% with a HTPEM system and from 25 to 27% on LTPEM systems [20]. A similar methanol reforming commercial HTPEM system is analyzed by Justesen et al. and show an efficiency up to 32% [21]. To verify the model a series of experiments are made on the reformer.
Experimental work The different parts of the system are tested individually and the results from the reformer system are presented below. A setup with two electric heaters is used in this setup. One for the evaporator and one for the reformer system. This setup can be seen in Fig. 10. The model is validated against a variety of parameters on both the input temperature of the oil and the flow rate of the
Fig. 11 e (a) Reformer oil exit temperature during load stepup (b) Temperature difference between model and experiment.
Fig. 12 e (a) Comparison of different heat flows in reformer. (b) Fuel flow into the reformer.
methanol. The temperature output of the oil can be seen in Fig. 11a and describes the relationship between the input flow rate of the methanol flow and the temperature output of the oil from the reformer. Here it can be seen that the temperature decreases as the methanol flow increases. It can be seen from Fig. 11b how the predicted temperature differs from the experimental data. The agreement is good both in steady state and transient operation. The heat flows in the reformer can be seen from Fig. 12. The relationship between the methanol flow and the gas convection in the reformer can be identified around t ¼ 8000 s. This can also be seen on the steam reforming process. The conduction is based only on the temperature of the reformer and is not related directly to the methanol flow. The temperature of the oil at the exit of the reformer can be seen from Fig. 13 where the model and experiment is plotted against each other during a ramp up in methanol flow. The
Fig. 13 e Reformer oil exit temperature compared to input flow.
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temperature of the reformer starts at ambient temperature (about 20 C) and it takes about 2500 s to heat up the reformer to the operational temperature. The duration of this startup sequence can be decreased but because the reformer is heated by an oil circuit the temperature of the oil is limited to the maximum possible temperature of the catalyst (about 300 C). The methanol feed is increased in steps to investigate the dynamics of the system. As the simulation is simplified the results can deviate up to 3 C from the experimental data though the dynamics of the system show acceptable results for this application. During these experiments it was measured how the methanol flow increase was affecting the flow in the exit gas from the reformer and a delay of about 10 s was found. More work needs to be done in investigating the dynamics of the fuel flow throughout the system as this will affect the dynamics of the system significantly. The model is configured with a constant delay and this is assumed sufficient. Based on a stoichiometry of 1.4 a change ramp of 0.3% percentage points per second is the limit to avoid hydrogen fuel starvation. If a lower stoichiometry is introduced would also result in a lower possible ramp. If the system is used as a steady-state application this will not be a problem, though if the application needs a higher degree of transients this will need to be improved.
Conclusion The model was created to investigate some of the dynamics of the system and it shows how an initial system design can be operated. This can provide a good base for the control and optimization of future reformer systems focusing on the most important components of the plant. It was shown how the temperature of the system can be controlled and it shows that an efficiency of 27e30% is possible before improvements are implemented. The experiments showed that with a methanol/water mix it was possible to startup the reformer in about 45 min. The experiments also gave some insight in the dynamics of the fuel flow through the system and a delay of about 10 s from methanol pump to fuel cell was observed. The stoichiometry on the fuel cell was observed and considerations on the delay in the fuel supply is considered a main factor when designing a controller for this system. The model was sufficient in predicting the temperature of the components and how the interaction in the oil circuit affects the components. This shows a good initial base for future work with testing of startup strategies, reformer controls and control strategies with better performance during part load operation.
Future work Fuel cell characterization and durability on reformate gas Variable reformer temperature based on constant methanol slip Control system design to increase efficiency Control system based on model predictable control
Acknowledgments We gratefully acknowledge the financial support of the Danish EUDP programme and through the COBRA-II project.
Nomenclature DH+c;CH3OH heat of combustion of methanol, J/kmol heat of combustion of hydrogen, J/kmol DH+c;H2 Dxburner thickness of isolation on burner, m Dxevaporator thickness of isolation on evaporator, m isolation thickness on fuel cell, m Dxfc Dxreformer thickness of isolation on reformer, m mass flow of oil, kg/s m_ oil molar flow of methanol, kmol/s n_CH3OH molar flow of hydrogen, kmol/s n_H2 Q_ Conduction conduction in FC, J/s Q_ Convection air convection in FC, J/s Q_ fc heat generated in FC, J/s total heat from combustion in burner, J/kmol Q_ heat Q_ oil heat or cooling from oil circuit, J/s Q_ stack total heat transfer in FC stack, J/s hydrogen stoichiometry lH2 density of air, kg/m3 rair rmeoh=h2o density of methanol and water mix, kg/m3 Ab,oil active oil area in burner, m2 Aburner outer area of burner, m2 Acell cell area in fuel cell, m2 Ae,oil active oil area in evaporator, m2 Aevaporator outer area of evaporator, m2 Afc,oil active oil area in fc, m2 Afc outer area of FC, m2 Ar,oil active oil area in reformer, m2 Areformer outer area of reformer, m2 Cpfc specific heat capacity of stack, kJ/kg/K Cpoil specific heat of oil, kJ/kg/K F Faraday constant, C mol1 Flowfuel fuel flow, ml/hr hevaporator heat transfer coefficient of evaporator isolation, W/ m2/K heat transfer coefficient of oil surface in fc, W/m2/K hfc,oil heat transfer coefficient, W/m2/K hfc,oil enthalpy at FC inlet, J hinlet enthalpy at FC outlet, J houtlet i current density, A/cm2 kb,cond thermal conductivity of burner isolation, W/m/K ke,cond thermal conductivity of evaporator isolation, W/m/K the material's conductivity, W/m/K kfc,cond molar ratio before and after reformation kgas thermal conductivity of reformer isolation, W/m/K kr,cond mass of stack, kg mfc number of cells in stack ncell Rreformer radius of reformer, m t time, s Tambient ambient temperature, K oil exit temperature, K Te stack temperature, K Tfc oil inlet temperature, K Ti vair air flow, m3/s
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Vcell VTN XH2
cell voltage, V thermoneutral voltage, V hydrogen ratio [11]
references
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System model development for a methanol reformed 5 kW high temperature PEM fuel cell system Simon Lennart Sahlin*, Søren Juhl Andreasen, Søren Knudsen Kær Department of Energy Technology, Aalborg University, Pontoppidanstraede 101, 9220 Aalborg, Denmark
article info
abstract
Article history:
This work investigates the system performance when reforming methanol in an oil heated
Received 21 February 2015
reformer system for a 5 kW fuel cell system. A dynamic model of the system is created and
Received in revised form
evaluated. The system is divided into 4 separate components. These components are the
11 July 2015
fuel cell, reformer, burner and evaporator, which are connected by two separate oil circuits,
Accepted 26 July 2015
one with a burner and reformer and one with a fuel cell and evaporator. Experiments were
Available online 25 August 2015
made on the reformer and measured oil and bed temperatures are presented in multiple working points. The system is examined at loads from 0 to 5000 W electric and the
Keywords:
response time and efficiency of the system are evaluated. The efficiency is estimated to be
Methanol
around 28e30% during load. Startup of the system is estimated to be around 45 min.
Dynamic model
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
HTPEM Reformer Steam reforming
Introduction The proton exchange membrane fuel cell (PEMFC) has received much attention in recent years. In the field of PEM fuel cells there are primarily two types, low temperature PEM fuel cells (LTPEM) and High temperature fuel cells (HTPEM). The most common PEM fuel cell is the LTPEM (Nafion based) that operates at about 50e80 C and requires a clean H2 supply. The HTPEM fuel cells use a PBI (polybenzimidazole) membrane and can operate at a much higher temperature 125e200 C [1e3]. High temperature PEM (HTPEM) fuel cells have shown stable results in high ionic conductivity and are able to tolerate impurities in the gas feed, such as Carbon Monoxide (CO) and methanol (CH3OH) [4,5]. This opens up for the
possibility to use reformate gas without the use of H2 cleaning processes, but it does, however, increase the complexity of the system, by adding additional components such as heat exchangers, evaporators and chemical reactors. Work done on HTPEM by Li et al. [6] has shown that CO levels up to 3% can be tolerated at current density up to 0.8 A/ cm2 at 200 C, and 0.1% CO at 125 C with a current density lower than 0.3 A/cm2. This corresponds well with work done by Korsgaard et al. [7] who shows successful operation using synthetic gas to simulate reformat gas at the anode from 0 to 5% CO and with CO2 contents ranging from 20% to 25%. This study also shows that higher amounts of CO require increased operating temperatures in the fuel cell. To avoid hydrogen under high pressure the examined system uses methanol as a primary fuel. Methanol is an
* Corresponding author. E-mail address: [email protected] (S.L. Sahlin). http://dx.doi.org/10.1016/j.ijhydene.2015.07.145 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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excellent hydrogen carrier for steam reforming (SR) because of the high hydrogen to carbon ratio (4:1), low boiling point and availability. Methanol can be produced from renewable resources thus reducing the production of greenhouse gases [8,9]. A combined methanol reformer and burner is shown by A. Lotric et al. [10] and shows promising results though requires novel HTPEM to be available. The current method of reforming, that is being heavily used today, is steam reforming. This method uses high temperature steam that reacts with a hydrogen rich fuel [11]. The governing SR reaction for methanol is as follows CH3 OH þ H2 O/3H2 þ CO2
(1)
This endothermic reaction can, with a suitable catalyst, reform methanol and water to hydrogen and CO2 at about 220e300 C. At these temperatures the decomposition of methanol can also occur. This reaction is shown in Eq. (2). CH3 OH/2H2 þ CO
(2)
CO in the hydrogen gas has a negative effect on the fuel cell performance and the extent of this influence is highly dependent on the fuel cell temperature [7,12]. In addition to the methanol decomposition a water gas shift (WGS) reaction is also taking place which reduces the CO concentration. CO þ H2 O#H2 þ CO2
(3)
All these reactions are dependent on the temperature of the reformer, methanol flow rate and the Steam-to-Carbon ratio (the water/methanol ratio in this case) [13,14]. As the temperature in the reformer is not uniform it is assumed that the actual temperature of the reformation is dependent on the flow rate. This means that the temperature is higher at low flow rate and lower at high flow rate. A study by Purnama et al. [15] on methanol steam reformers suggests that low contact time also gives a lower amount of CO.
The goal of this work is to create a usable model in Matlab Simulink. The purpose of the model is to investigate control techniques and to get an insight into the initial parameters for a methanol reforming system.
System description The system model is designed with four main components: burner, evaporator, reformer and a fuel cell stack. This system utilizes a thermal oil to transfer heat in the system. In the main operating configuration the system is connected with two oil circuits; one with the burner and reformer and one with the fuel cell and evaporator. These two circuits are separated because of the temperature difference at about 160 C in the fuel cell circuit and about 260 C in the burner circuit. The design layout can be seen in Fig. 1. The first oil circuit is the burner and reformer. The burner transfers hot exhaust gas into a heat exchanger that heats up the oil. The oil is pumped through the burner heat exchanger and is then passed into the reformer. The second oil circuit is for cooling in the fuel cell stack and is about 160 C. When the fuel cell is operating, the excess heat is used to evaporate the methanol/water input fuel before it reaches the reformer. The reformer examined in this work is sized for a 5 kW fuel cell and uses a CuZn based reforming catalyst. A model is used to give some initial parameters for the experimental setup. This includes an estimation on the minimal size of the cooler for the fuel cell system and how the dynamics in the system are affecting performance and stability. This can also give some insight into the effects of changing the load of the fuel cell in an open loop configuration. The model is also used to examine situations when emergency stops are introduced. This system is initially
Burner Air fan
Reformer
Fuel Burner
Reformer
H2 syngas Fuel flow(Liquid) Fuel flow(Gas) Oil circuit Air flow
Reformer pump
Fuel pump
Fuel Evaporator Methanol / H2O
Fuel Cell Fuel Cell pump FC Air fan
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Cooler
Fig. 1 e System layout in the operating configuration.
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Reformer
Air fan
E-23
Fuel Burner
Reformer pump
H2 syngas Fuel flow(Liquid) Oil circuit Air flow
Heat exchanger
Fuel pump Startup heat exchanger
Fuel Evaporator
Methanol / H2O
Fuel Cell
Fuel Cell pump FC Air fan
Cooler
Fig. 2 e System layout in the startup configuration.
designed with no physical safety mechanism, which means that this model can give an indication on the needed controlling requirements, for stable and reliable operation in all operating points. The experiments were carried out on the different parts of the system to verify the model and the work done on the reformer is presented below. To investigate the start-up of the system an alternative configuration is used. This configuration can be seen in Fig. 2. When the system is in this configuration it utilizes the burner to heat up the reformer and fuel cell to operational temperatures with a heat exchanger between the two circuits. When the temperature is reached in the fuel cell and reformer, the system is changed to the operational configuration, as shown in Fig. 1, and the heat exchanger is bypassed. The burner is connected to the methanol fuel flow of the system and is able to heat up the system by passing air and methanol/water into the burner. This section is closed when the operating configuration is used, unless a situation occurs where the temperature of the burner or reformer is too low. The different parts of the system are described below.
Evaporator Before the methanol/water mixture reaches the reformer it is led through a stainless steel heat exchanger. The heat input to this evaporator is supplied from the exhaust air from the fuel cell, the exhaust air from the burner and the heated oil from the oil circuit. This means that during the startup phase the evaporator is heated by the exhaust air from the burner and the oil that is being heated up. This setup can be seen in Fig. 2.
Reformer The reformer is a cylindrical metal tube about 50 cm in height, filled with catalyst pellets which are cylindrical 1 mm high and 1 mm in diameter. The reformer is fitted with several temperature sensors in the center of the catalyst bed starting
Fuel cell The stack modeled in this system is a 5 kW oil cooled high temperature PEM fuel cell with 120 cells from Serenergy A/S. This stack can be seen in Fig. 3 and is rated at a net power of 6 kW on pure H2 and 5 kW on reformate gas. The fuel cell can be run on a minimum H2 stoichiometry of 1.35 and an air stoichmetry of 2e3 [16]. The weight of the stack is 20.5 kg and 227 148 453 mm in dimensions where each cell has an active area of 165 cm2. As the temperature of the exhaust air from the fuel cell is up to 160 C, it would be ideal to be used to heat up the methanol/water fuel mixture in the evaporator.
Fig. 3 e S165L HTPEM stack from Serenergy A/S.
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from the inlet to the outlet. Each sensor is located throughout the reformer with about 6 cm in between. This arrangement of temperature sensors makes it possible to monitor the heat generated in the activation state and gives a more detailed temperature profile of the reformer. When the reformer is being activated the measurement of the temperature is highly important to avoid damaging the catalyst. The outer shell of the reformer is covered by a 5 cm layer of isolation and the methanol fuel is introduced from the bottom. A sketch of the reformer can be seen in Fig. 4.
Air output
Oil input
Heat exchanger
Oil output
Burner Air input
A catalytic burner is used in this system to utilize the anode waste gas from the fuel cell and heat the catalytic reforming process. This is done by directing a flow of hydrogen and air through the burner catalyst. The ratio of the air and hydrogen is highly important, because in the case of a low airflow an active flame can occur. This can both be dangerous and create an unstable system. If the ratio of air is too high, the hydrogen will not react and the burner will cool down. These two situations are to be avoided and a controller for the air blower is needed. The mass flow of hydrogen is difficult to physically measure in the system, so an estimation on the amount of hydrogen is calculated. This estimation is based on the methanol fuel flow, temperature of the reformer and the fuel cell load. A sketch of the burner used in this work can be seen in Fig. 5. The burner is designed to work with an air/h2 mix which is fed in at the bottom and is directed up through a mesh with the catalytic material. At this stage the hot air travels through a heat exchanger, thereby transferring heat to the oil. The exit air will be directed to the evaporator to help heat up the input methanol fuel. The model of these systems and their interaction is presented below.
H2-rich gas
H2-rich gas Oil output
Reformer
Oil output
Oil input
Oil input
Methanol feed
Methanol feed
Fig. 4 e Reformer. left shows the design of the circular tube reformer. right figure shows how the flow of oil and methanol is directed inside the reformer.
Catalytic burner
Hydrogen input
Fig. 5 e Catalytic burner and heat exchanger. Hydrogen and air input at the bottom.
Modeling The purpose of modeling this system is to create an efficient and fast way to evaluate different control strategies and give insight into the dimensions of the cooler, methanol pump and blowers. The model can also be used as a method to obtain better system performance or to enhance the overall system response during load changes or a change in operating conditions. These operating conditions could be ambient temperatures, humidity or changes in pressure. One of the challenges in these kind of systems is the delay of gas throughout the system when a change in the fuel cell load is introduced. Two negative consequences of this fuel delay can occur. The first situation can happen if the system load is changed faster than the hydrogen can be supplied and this can result in a starvation and thereby faster degradation of the fuel cell [17]. The second problem can occur when the load is lowered or halted in the system and the amount of hydrogen that would have been used in the fuel cell is directed into the burner, thereby increasing the temperature rapidly which can result in damage of the burner catalyst or overall system instability. The model is designed to represent the dynamics of the system and to be as simple as possible, so that it can be run on hardware with limited resources. This gives the possibility to use parts of the model for predictive control in an operating system. For each system the input and output temperature of the oil is simulated and transferred to the next system. The model is also used to simulate the temperature of the different components, the fuel cell performance on reformate gas and an estimate on the gas composition from the reformer. The model for the fuel cell voltage and the reformer gas composition is based on experimental data. Each component is modeled as a lumped thermal mass and the
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model is simulated in Matlab Simulink. The model can be run on a normal sized personal computer (1 core at 2701 MHz) and 10 h simulation is computed in 733 s. This is including various controllers and gas composition observers. The following sections will give an overview of the important model equations.
Model input A series of load changes are introduced as the model input. The system starts from an idle ambient temperature and is run through a startup sequence until it reaches the operational system temperature. After the startup sequence the system will experience a stepup and stepdown sequence running from 0 to 5 kW electric output. The whole sequence will run over 30.000 s and the load changes will happen with (at minimum) 300 s intervals. The fuel input is directly coupled to the fuel cell load to investigate the impact on the fuel cell and reformer. The model input can be seen in Fig. 6 which also shows the relation between the current density of the fuel cell and the flow of the methanol/water mix. This relationship is shown in Eq. (12).
Fuel cell model The temperature of the fuel cell is varied by both the oil temperature, heat generated by the fuel cell, heat loss from conduction and heat loss from convection in the airflow. The temperature of the fuel cell is calculated by the following energy balance Q_ stack ¼ Q_ fc þ Q_ Conduction þ Q_ Convection þ Q_ oil
(4)
The different heat transfer contributions are summed up to represent the heat transfer of the system Q_ stack and is calculated by Eq. (4). The power generated inside the fuel cell stack (Q_ fc ), the airflow convection (Q_ convection ), the conduction (Q_ conduction ) and the heat from the oil circuit (Q_ oil ) is used to calculate the total heat in the stack. The temperature of the fuel cell stack is modeled by 5. The calculation integrates over the power and changes the temperature as the time changes from 0 to a given time t.
DTfc ¼
1 , mfc ,Cpfc
Zt
Q_ stack dt
(5)
0
Where Tfc is the stack temperature, mfc is the mass of the fuel cell stack, Cpfc is the specific heat capacity, and Q_ stack is the power transferred to or from the system. The heat generated in the fuel cell, shown as Q_ fc , is calculated from the difference in the cell voltage compared to the thermoneutral cell voltage (VTN) multiplied with the current density (i) and the cell area Acell. This is shown in Eq. (6). Q_ fc ¼ i,ðVTN Vcell Þ,Acell
(6)
The heat removed from the fuel cell is estimated by calculating the difference in oil exit temperature Te and the oil input temperature. This can be seen in eq. 7 m_ oil ,Cpoil ,ðTe Ti Þ ¼ hfc;oil ,Afc;oil , Tfc Ti
(7)
This equation uses the fuel cell temperature Tfc, the input _ the specific heat capacity temperature Ti, the oil mass flow m, Cpoil of the oil, the active area Afc,oil is the inside wall area and the heat transfer coefficient hfc,oil for the oil. As the temperature of the oil changes during the length of the fuel cell a logarithmic mean temperature difference (LMTD) should be used. This would increase the computational time significantly of the model and a mean temperature is used instead though this is an approximation. As the exit oil temperature is now calculated, it is possible to estimate the amount of heat that is added or removed from the system. Q_ oil ¼ m_ oil ,Cpoil ,ðTi Te Þ
(8)
The heat removed by conduction from the fuel cell is calculated by Eq. 9 kfc;cond Q_ conduction ¼ Tambient Tfc ,Afc , Dxfc
(9)
where Tfc is the fuel cell temperature, Tambient is the ambient temperature, Afc is the outer area of the fuel cell, Dx is the isolation thickness and kfc,cond is the thermal conductivity of the isolation. The cooling effect of the air blower in the fuel cell is calculated by the change in enthalpy hinlet of the air multiplied with the airflow vair. The air is assumed to be the same temperature as the fuel cell at the exit houtlet. The density of the air is shown as rair Q_ convection ¼ ðhinlet houtlet Þ,rair ,vair
(10)
The enthalpy is calculated using EES (Engineering Equation calculator) where the data is imported into the model.
Fuel cell voltage
Fig. 6 e Fuel cell simulation input.
The fuel cell voltage is estimated with a 5-dimensional lookup table based on experimental data. The inputs for the table are FC stack temperature, current density, CO gas fraction, H2 gas fraction and the air stoichiometry. The only external inputs in the fuel cell model are the CO and H2 fractions which are estimated in the reformer model.
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Reformer The reformer temperature and the steam-to-carbon ratio of the input feed are used to estimate the gas composition from the steam reformer. The temperature in the reformer is assumed uniform, although this might not be the case, it is considered acceptable in this work. The heat required in the steam reformation process is H ¼ 49.4 [kJ/mol]. A verification on the reformer temperature is presented in the results part of this work. The temperature of the reformer is considered to be the average of the bed temperature.
Burner Similar to the reformer and fuel cell stack, the burner temperature is modeled as a the fuel cell and reformer with the exception that the anode exhaust gas is burned. The burner is heated by the combustion of the excess hydrogen from the fuel cell anode and the methanol fuel under the startup sequence. This is expressed as Q_ heat Q_ heat ¼ DH+c;CH3OH ,n_CH3OH þ DH+c;H2 ,n_H2
(11)
Where DH+c;ch3oh is the heat of combustion of methanol and DH+c;H2 is that of hydrogen. Both DH+c;CH3OH and DH+c;H2 are both the lower heating value(LHV). n_CH3OH and n_H2 are the molar flows of methanol and hydrogen which are fed into the burner. The evaporator is a 3-stream heat exchanger with both hot air from the fuel cell and burner, an oil circuit heated by the fuel cell and a methanol feed line. This is to ensure the methanol feed is evaporated before it reaches the reformer. The temperature is modeled as the fuel cell as presented in Eq. (5). If there is not sufficient heat from the fuel cell to heat up the fuel mix it needs to come from the burner, which means that a higher stoichiometry is needed, which then leads to a lower overall system efficiency.
Pump model To estimate the fuel flow such that a desired H2 stoichiometry can be achieved at the fuel cell Eq. (12) is used. Flowfuel ½ml=hr ¼
i,ncell ,Acell lH2 1000 , ,r , 2,F kgas ,XH2 meoh=h2o 3600
(12)
Where i is the current density, ncell is the number of cells in the FC, Acell is the area of the cell and F is the Faraday constant. lH2 is the desired h2 stoichiometry. This equation assumes that the molar fraction of hydrogen, XH2, does not vary much from 65% under normal operating conditions. This fraction is assumed constant as long as the reformer is within the operating temperature. The ratio of 65% is also set conservatively low to ensure that the initial pump flow is always delivering a higher stoichiometry at the fuel cell. This stoichiometry can then be lowered with a controlling system where the temperature of the reformer and burner is regulated. The ratio kgas is the ratio between the methanol gas and hydrogen and is assumed to be 1.8. This ratio is based on the
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conservation of mass of the methanol fuel mix at the reformer inlet and the hydrogen output from the reformer. The ratio is estimated from the molar flow of methanol and hydrogen going through the reformer and is dependent on the reformer temperature. rmeoh=h2 o is the density of the methanol/water fuel mix. This flow estimation can be used as a feed-forward part of the controller as it estimates the pump flow. This means that the assumptions made in Eq. (12) requires the system to be in a stable operating condition. To model the transfer of liquid and gas in the system a delay is implemented in two steps. The first delay is introduced in fuel flow in the reforming phase of the fuel system line and the second is the transport delay from the fuel cell exhaust to the burner.
Model results Based on the input sequence shown in Fig. 6 it can be seen how the component temperatures change. The constants used in the model can be seen in Table 1. The system is started at ambient temperature and when the temperature in the main parts of the system reaches a minimum operational temperature, the system will change to the normal operation. This temperature is set to 150 C at the fuel cell and 180 C for the reformer. Based on Fig. 7a it can be seen how the temperature varies during the load sequence, shown in Fig. 7c. A total efficiency can be seen from Fig. 7b which varies depending on the temperature of the components and the fuel flow. After the warmup the input steps up to about 3500[ml/hr] at around 5.000 s and it can be seen that the temperature in the
Table 1 e Constants used in model. Reformer mreformer Cpoil Lengthreformer Cpmix roil SCratio AReformer Ar,oil kr,cond Burner DH+c;ch3oh Cpair mburner Aburner Ab,oil Fuel cell mfc hfc,oil lair ncell rair Afc Evaporator mevaporator Aevaporator hevaporator ke,cond
12 [kg] 0.5 [kJ/kg/K] 0.450 [m] 3.28 [kJ/kg/K] 936 [kg/m3] 1.5[] 0.51[m2] 0.4240[m2] 0.15 [W/m/K]
hr,oil Rreformer m_ oil
Dxreformer hreformer
140 [W/m2/K] 0.180 [m] 0.023 [kg/s] 1.96 [kJ/kg/K] 293.15 [K] 10 [l/min] 0.03 [m] 0.3 [W/m2/K]
681 [kJ/mol] 1.0214 [kJ/kg/K] 10 [kg] 0.5[m2] 0.25[m2]
DHc,H2 hburner cpburner Dxburner kb,cond
242 [kJ/mol] 100 [W/m2/K] 3 [kJ/kg/K] 0.02 [m] 0.15 [W/m/K]
14[kg] 978 [W/m2/K] 2.5 [] 120 [cells] 1.28 [kg/m3] 0.41 [m2]
Cpfc Acell lh2 Afc,oil kfc,cond Dxfc
0.970 [kJ/kg/K] 162.5[cm2] 1.35 [] 0.9945 [m2] 0.15 [W/m/K] 0.01 [m]
2 [kg] 0.2 [m2] 0.15 [W/m2/K] 0.15 [W/m/K]
he,oil Dxevaporator Ae,oil
200 [W/m2/K] 0.02 [m] 0.21 [m2]
Cpmix,gas Tambient m_ oil
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Fig. 8 e Heat contributions in the FC during startup and operational configuration.
Fig. 7 e (a): Components temperature during operation (b): System efficiency during operation (c): Pump flow during operation based on the fuel cell load.
reformer and burner is still not at a stable level. At 10.000 s the input fuel flow is about 6500[ml/hr] and the temperature in all components are reaching a steady-state at about 12.000 s. At 15.000 s the load is stepped down to 0 during 2000 s and it can be seen how the temperature in the burner drops to the reformer temperature as there is no fuel to heat the burner. At 17.000 s the input flow is increased to 6500[ml/hr] again and the burner regains its operating temperature and the system is operational again. This means that this system is capable to regain the operational states if the temperature in the reformer is kept above the lowest reforming temperature. Based on this model the start-up time is estimated to be about 45 min. The heat contributions in the FC can be seen in Fig. 8 and this shows how the oil circuit is the only part of the heating during the startup sequence. After the startup heating the oil circuit will be used as a cooling of the FC. As shown in Eq. (7) the main contribution to heating is from the electrochemical reaction in the fuel cell. The conduction of heat from the fuel cell is also significantly lower compared to the convection of air. Due to limitations of the heat transfer fluid, the temperature of the oil should not exceed above 316 C. This fluid also has a film temperature of 343 C which is set to be the maximum limit of the burner temperature [18]. As previously mentioned in Section 3.1 the input to the system is directly coupled to the load of the fuel cell as an open loop controller. The fuel delay from the methanol pump through the evaporator and reformer to the fuel cell is
therefore able to be evaluated and modeled. Because of this delay the fuel cell can either be affected by a fuel starvation when the load increases or the burner will experience a fuel increase when the load decreases. These spikes are modeled and can be seen in Fig. 9. To increase the efficiency and the dynamic response it is beneficial to have a knowledge of how the reformer is performing. This can give some insight into what the actual lh2 is and this can be used as a feedback input to the pump equation Eq. (12). At steady state and constant current density it is possible to get closer to the 1.35, though increases in current needs to be controlled with an increase of flow before the current is drawn from the fuel cell. So to increase the efficiency of the system, the hydrogen stoichiometry needs to be as low as possible without starving the fuel cell. From Fig. 9 the stoichiometry is about 1.4 and this corresponds to an estimated electric efficiency as shown in Fig. 7b. The efficiency is calculated using the lower heating value and is at these operating conditions about 27e30%. There is still room for improvement on the efficiency and this
Fig. 9 e Hydrogen stoichiometry in the Fuel Cell.
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Fig. 10 e System setup.
will be investigated in later work. Other work with similar system design is reporting a power efficiency at about 23e33% which corresponds well with the presented results [19]. Nicola Zuliani et al. show systems working on natural gas which shows efficiencies at about 26% with a HTPEM system and from 25 to 27% on LTPEM systems [20]. A similar methanol reforming commercial HTPEM system is analyzed by Justesen et al. and show an efficiency up to 32% [21]. To verify the model a series of experiments are made on the reformer.
Experimental work The different parts of the system are tested individually and the results from the reformer system are presented below. A setup with two electric heaters is used in this setup. One for the evaporator and one for the reformer system. This setup can be seen in Fig. 10. The model is validated against a variety of parameters on both the input temperature of the oil and the flow rate of the
Fig. 11 e (a) Reformer oil exit temperature during load stepup (b) Temperature difference between model and experiment.
Fig. 12 e (a) Comparison of different heat flows in reformer. (b) Fuel flow into the reformer.
methanol. The temperature output of the oil can be seen in Fig. 11a and describes the relationship between the input flow rate of the methanol flow and the temperature output of the oil from the reformer. Here it can be seen that the temperature decreases as the methanol flow increases. It can be seen from Fig. 11b how the predicted temperature differs from the experimental data. The agreement is good both in steady state and transient operation. The heat flows in the reformer can be seen from Fig. 12. The relationship between the methanol flow and the gas convection in the reformer can be identified around t ¼ 8000 s. This can also be seen on the steam reforming process. The conduction is based only on the temperature of the reformer and is not related directly to the methanol flow. The temperature of the oil at the exit of the reformer can be seen from Fig. 13 where the model and experiment is plotted against each other during a ramp up in methanol flow. The
Fig. 13 e Reformer oil exit temperature compared to input flow.
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temperature of the reformer starts at ambient temperature (about 20 C) and it takes about 2500 s to heat up the reformer to the operational temperature. The duration of this startup sequence can be decreased but because the reformer is heated by an oil circuit the temperature of the oil is limited to the maximum possible temperature of the catalyst (about 300 C). The methanol feed is increased in steps to investigate the dynamics of the system. As the simulation is simplified the results can deviate up to 3 C from the experimental data though the dynamics of the system show acceptable results for this application. During these experiments it was measured how the methanol flow increase was affecting the flow in the exit gas from the reformer and a delay of about 10 s was found. More work needs to be done in investigating the dynamics of the fuel flow throughout the system as this will affect the dynamics of the system significantly. The model is configured with a constant delay and this is assumed sufficient. Based on a stoichiometry of 1.4 a change ramp of 0.3% percentage points per second is the limit to avoid hydrogen fuel starvation. If a lower stoichiometry is introduced would also result in a lower possible ramp. If the system is used as a steady-state application this will not be a problem, though if the application needs a higher degree of transients this will need to be improved.
Conclusion The model was created to investigate some of the dynamics of the system and it shows how an initial system design can be operated. This can provide a good base for the control and optimization of future reformer systems focusing on the most important components of the plant. It was shown how the temperature of the system can be controlled and it shows that an efficiency of 27e30% is possible before improvements are implemented. The experiments showed that with a methanol/water mix it was possible to startup the reformer in about 45 min. The experiments also gave some insight in the dynamics of the fuel flow through the system and a delay of about 10 s from methanol pump to fuel cell was observed. The stoichiometry on the fuel cell was observed and considerations on the delay in the fuel supply is considered a main factor when designing a controller for this system. The model was sufficient in predicting the temperature of the components and how the interaction in the oil circuit affects the components. This shows a good initial base for future work with testing of startup strategies, reformer controls and control strategies with better performance during part load operation.
Future work Fuel cell characterization and durability on reformate gas Variable reformer temperature based on constant methanol slip Control system design to increase efficiency Control system based on model predictable control
Acknowledgments We gratefully acknowledge the financial support of the Danish EUDP programme and through the COBRA-II project.
Nomenclature DH+c;CH3OH heat of combustion of methanol, J/kmol heat of combustion of hydrogen, J/kmol DH+c;H2 Dxburner thickness of isolation on burner, m Dxevaporator thickness of isolation on evaporator, m isolation thickness on fuel cell, m Dxfc Dxreformer thickness of isolation on reformer, m mass flow of oil, kg/s m_ oil molar flow of methanol, kmol/s n_CH3OH molar flow of hydrogen, kmol/s n_H2 Q_ Conduction conduction in FC, J/s Q_ Convection air convection in FC, J/s Q_ fc heat generated in FC, J/s total heat from combustion in burner, J/kmol Q_ heat Q_ oil heat or cooling from oil circuit, J/s Q_ stack total heat transfer in FC stack, J/s hydrogen stoichiometry lH2 density of air, kg/m3 rair rmeoh=h2o density of methanol and water mix, kg/m3 Ab,oil active oil area in burner, m2 Aburner outer area of burner, m2 Acell cell area in fuel cell, m2 Ae,oil active oil area in evaporator, m2 Aevaporator outer area of evaporator, m2 Afc,oil active oil area in fc, m2 Afc outer area of FC, m2 Ar,oil active oil area in reformer, m2 Areformer outer area of reformer, m2 Cpfc specific heat capacity of stack, kJ/kg/K Cpoil specific heat of oil, kJ/kg/K F Faraday constant, C mol1 Flowfuel fuel flow, ml/hr hevaporator heat transfer coefficient of evaporator isolation, W/ m2/K heat transfer coefficient of oil surface in fc, W/m2/K hfc,oil heat transfer coefficient, W/m2/K hfc,oil enthalpy at FC inlet, J hinlet enthalpy at FC outlet, J houtlet i current density, A/cm2 kb,cond thermal conductivity of burner isolation, W/m/K ke,cond thermal conductivity of evaporator isolation, W/m/K the material's conductivity, W/m/K kfc,cond molar ratio before and after reformation kgas thermal conductivity of reformer isolation, W/m/K kr,cond mass of stack, kg mfc number of cells in stack ncell Rreformer radius of reformer, m t time, s Tambient ambient temperature, K oil exit temperature, K Te stack temperature, K Tfc oil inlet temperature, K Ti vair air flow, m3/s
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Vcell VTN XH2
cell voltage, V thermoneutral voltage, V hydrogen ratio [11]
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