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A MECHATRONIC THERMAL MANAGEMENT SYSTEM FOR MOBILE FUEL CELLS
A MECHATRONIC THERMAL MANAGEMENT SYSTEM FOR MOBILE FUEL CELLS David R. Noble†, Dr. John R. Wagner‡, P.E., and Dr. Egidio E. Marotta§ †School of Aerospace Engineering, Georgia Institute of Technology ‡Department of Mechanical Engineering, Clemson University §College of Engineering at Texas A&M University Abstract: The evolution of ground transportation power sources from internal combustion engines to a hybrid architecture featuring a fuel cell stack and electric drives necessitates the re-design of the traditional cooling system. The common thermostat and mechanical pump are replaced by smart valves, variable speed pumps, and adjustable radiator fans. In the fuel cell, multiple cooling zones are proposed using servo-motor components to better regulate the temperature and moisture levels as prescribed by the operating conditions. In this paper, multiple thermal loops and dynamic models for a fuel cell cooling system will be presented. A scale experiment, emulating the main fuel cell cooling loop, validated the models and demonstrated temperature tracking capabilities. The coolant temperature was maintained in a ±1.5ºC neighborhood of the set point. Keywords: automotive, fuel cell, mechatronics, sensors, actuators, control, test 1. INTRODUCTION Passenger automobiles, light duty trucks, and commercial vehicles with internal combustion engines remain a prime transportation mode within society but at a significant environmental cost. The consumption of petroleum resources and contribution to global air pollution has prompted the investigation of alternative propulsion sources. An attractive technology is the fuel cell which may be coupled with electric drive motors to propel vehicles (Pukrushpan et al., 2004). Although a variety of fuel cell designs exist as noted by their electrolyte material, the proton exchange membrane (PEM) fuel cell will be considered. The PEM fuel cell converts hydrogen and oxygen to produce electrical power and thermal energy; the latter must be managed to maintain a proper membrane environment. Mosdale and Srinivasan (1995) report that roughly 40% of the fuel cell energy produced is heat. Hence, the fuel cell thermal management system must be a primary design issue. Further, the incoming hydrogen and air reactants must be humidified such that the membrane operates with the appropriate moisture level. Advanced thermal management systems have been proposed for internal combustion engines (Luptowski
et al., 2005). The traditional spark ignition internal engine cooling components (e.g., wax-based thermostat, belt driven water pump, and radiator with fan) have been upgraded with a smart valve, electric coolant pump, and variable speed electric (or hydraulic) driven radiator fan (Wagner et al., 2002). Servo-motor components allow temperature regulation based on the engine control unit's commands rather than the crankshaft speed. For a heavy-duty diesel engine, four thermal loops have been proposed which allow introduction of different thermal zones that may be independently controlled based on the operating conditions. The main loop regulates the engine block and passenger compartment temperatures. The second loop cools the liquid-to-air intercooler and exhaust gas recirculation. The two remaining loops cool the oil and transmission fluid. The philosophy of multiple loop cooling systems will be proposed for fuel cell systems since individual temperature zones can be identified for the various components. It is theorized that multiple cooling loops, with the inclusion of pre-heaters at certain locations, will allow fuel cell systems to reach operating temperature quickly and maintain that temperature within a tight neighborhood. To design
734
and demonstrate the fuel cell thermal management concept, dynamic models, numerical simulations, and experimental testing will be required. The analytical and empirical mathematical models allow the creation of simulation tools. Testing will be performed on a scale bench that features immersion heaters, smart valve, servo-motor pump, and electric radiator fan to emulate a fuel cell primary loop thermal system. A brief review of mobile fuel cells and integrated thermal management systems is now presented. Cacciola et al. (2001) discussed the application of fuel cells in transportation systems as auxiliary power units or power generators in hybrid vehicles. Rajashekara (2000) discussed fuel cell vehicle configurations, power electronics, and control strategies. Cownden et al. (2001) developed a comprehensive fuel cell system model that simulates the operation of the stack, air compressor, turbocharger, air-to-air heat exchange hydrogen supply, and thermal cooling. The cooling subsystem consisted of a radiator, water pump, radiator fan, and surge tank. Badrinarayanan et al. (2001) developed a transport model to analyze the effect of various water and thermal management parameters during steadystate and transient operations. Finally, Sugano et al. (1994) discussed the dynamic characteristics of the fuel cell stack cooling system and proposed a model for designing the temperature control systems. The paper is organized as follows. In Section 2, the concept of multiple thermal loops within the fuel cell system architecture is presented. Section 3 contains dynamic models to describe the fuel cell thermal management system. The experimental test platform is reviewed in Section 4. Experimental and numerical results are presented in Section 5. Lastly, Section 6 contains the summary.
2. FUEL CELL THERMAL MANAGEMENT SYSTEM In fuel cells, the thermal management system can be designed, integrated and controlled using distributed sensors, smart cooling actuators, and real time control algorithms to facilitate multiple temperature zones. The PEM fuel cell uses humidified hydrogen and air to generate electric power. The basic fuel cell architecture contains an air compressor, hydrogen tank, stack, condenser, turbine, and cooling system (Figure 1). A series of zones will be designed to customize the fuel cell system’s cooling for better temperature control. Three interconnected loops have been identified. The first thermal loop consists of the stack, variable speed coolant pump, adjustable position valve, radiator with electric fan, and auxiliary immersion heater for "cold start" scenarios. Loop two addresses the intercooler and condenser components using a servo-valve, coolant pump, and radiator. The intercooler reduces the air temperature after the compressor while the condenser recovers the water from the two phase mixture exiting the fuel stack. Finally, loop three humidifies the hydrogen and air using two valves, an auxiliary heater, and pump. 2.1 Loop One: Main Thermal Circuit The fuel cell stack functionality is similar to the common engine block in that the combustion, or chemical, process generates energy that may be harnessed directly or indirectly for propulsion plus heat. In fuel cells, the heat is generated by an exothermic reaction occurring between the humidified hydrogen and air across the polymer electrolytic membrane. This heat must be removed, via the circulating coolant, to maintain the set point temperature. It will be assumed that single phase heat transfer occurs in the stack’s coolant flow. Note that the insertion of a pre-heater allows the fuel cell to
Fig. 1: Fuel cell system with three separate thermal loops
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warm up quicker. As shown in Figure 2, the fuel cell system’s main thermal loop contains a radiator assembly with electric fan, electro-mechanical valve, a fuel cell stack heat source, a variable speed pump, and an auxiliary pre-heater. The electro-mechanical valve regulates the coolant flow between the bypass and radiator circuits. The variable speed pump also controls the fluid flow; for instance, the pump speed can be minimized (maximized) during heating (cooling) scenarios. Finally, the radiator rejects heat to the surrounding environment through convective heat transfer which is dependent on the fan and vehicle speed. In this configuration, all components are servo-motor driven since the mobile fuel cell system may not feature a mechanical energy source (e.g., crankshaft) to power the pump and fan. The fluid within the loop is ethylene glycol
be assumed to have the fuel stack temperature from loop one. Similarly, the condensate temperature will impact loop three. The fluid is ethylene glycol. 2.3 Loop Three: Hydrogen/Air Humidifier Circuit The final thermal loop consists of the hydrogen and air humidifiers, electric pump, pre-heater, dual solenoid valves, and insulated water tank as shown in Figure 4. Loop three is the simplest circuit, based on number of components, but important in ensuring that the inlet gas streams have the proper moisture content for the polymer membrane. Note that this paper is only concerned with providing a mechanism to impact the moisture level and not humidity measurements. The tank will contain condensate from loop two. The humidifiers are nozzles that spray high pressure water into the hydrogen and air streams as they flow into the stack. In some regard, the sprayers are comparable to gasoline/diesel fuel injectors with a variable duty cycle. Thus, the hydrogen humidifier can be controlled at a different operating condition than the air humidifier which allows both inlet reactants to have unique humidity levels for optimal power generation. The excess water is recovered and returned to the insulated tank. The fluid flowing within the circuit is water.
Fig. 2: Fuel cell stack temperature control loop 2.2 Loop Two: Intercooler/Condenser Circuit The second fuel cell thermal loop consists of the intercooler and condenser that remove heat from the compressed inlet air stream and two phase (air and water) mixture exiting the fuel cell stack. In Figure 3, the architecture of loop two consists of a radiator assembly, valve, coolant pump, and two radiators for the inlet air and two phase mixture. It should be noted that this loop is similar to a diesel engine's exhaust gas re-circulation and intercooler loop. The inlet air stream is compressed by the compressor/turbine assembly to raise the pressure, and consequentially the temperature, prior to introduction into the stack. To lower the inlet air temperature, which creates a denser stack charge, the intercooler is an air-to-water heat exchanger which transfers heat from the air to the coolant. The condenser unit's functionality is to recover water from the exhaust stream which may be recycled within the fuel cell system through the humidification process. The control set point temperature will be assigned to the inlet air stream for the fuel cell stack to ensure proper operating conditions. In loop two, the two phase mixture exiting the fuel cell stack will
Fig. 3: Intercooler/condenser thermal control loop 3. MATHEMATICAL MODELING Two different mathematical modeling approaches are presented to describe the fuel cell thermal management system. The AMESim package is used to model and integrate the various components. The analytical and empirical equations for the thermal, electro-mechanical, and fluid domains are derived and coupled together in Matlab/Simulink. 3.1 AMESim Fuel Cell Thermal Model AMESim is a modeling tool for multi-domain systems. This software was used to model the fuel cell’s thermal components and then integrated to describe the overall system behavior. For brevity, only the fuel cell stack’s primary cooling loop will be
736
discussed which consists of the stack, pre-heater, smart valve, electric pump, and electric radiator fan assembly. The fuel cell and pre-heater were represented as convective elements. Flow resistance elements were used to model the pipe restrictions. The valve, pump, fan, and radiator were located in the thermal-hydraulic library (Eberth et al., 2004).
where
V0
and
R f ( P, Q )
represent the fluid volume per
radian of shaft rotation and the nonlinear fluid resistance. The coolant mass flow rate is m& c = 2πρ c r12 b1ω p tan β1 . Radiator Fan: The radiator fan motor’s armature current and rotational speed may be written as
3.2 Analytical Fuel Cell Thermal Model
diaf
The fuel cell system’s thermal behavior will be modeled using a lumped parameter approach with thermal, electrical, mechanical, and fluid concepts. First, the smart valve, electric pump, and radiator fan actuators will be modeled. Next, the primary (e.g., fuel cell stack, valve, pump, radiator, and auxiliary heater), secondary (e.g., intercooler, condenser, valve, pump, radiator, and auxiliary heater), and humidification (e.g., pump, reservoir, valves, and nozzles) thermal circuit loops will be considered
dω f
dt
dt
(
)
=
1 Vsf − Raf iaf − K bf ω f Laf
=
1 − Beqrω f + K mf iaf − ρ a A f R f Vaf 2 J eqr
(5)
(
)
(6)
The fan's air speed and mass air flow rate become
(
)
Vaf = ⎛⎜ K mf η f ρa A f iaf ω f ⎞⎟ ⎠ ⎝
0.3
and
m& af = β r ρ a A f Vaf
.
Thermal Loop One: Two thermal nodes, fuel cell stack and radiator, will be considered. The differential equations for the stack, T fcs , and radiator, Tr
, temperatures are C fcs T& fcs = −c pc m& c (T fcs − T j ) + Qin (t ) + QH (t )
(7)
Cr T&r = −c pc Hm& c (Tr − T fcs ) − Qout (t )
(8) Tj ,
The junction coolant temperature,
is related to the
fuel cell stack and radiator temperature as T j = (1 − H )T fcs + HTr
Fig. 4: Loop three for hydrogen and air humidifiers Fluid Valve: A bi-directional motor, interfaced to a worm gear, drives the multiple position valve assembly. The motor’s armature current and angular velocity equations are di av dθ m ⎞ 1 ⎛ = ⎜ V sv − R av i av − K bv ⎟ dt L av ⎝ dt ⎠
dθ ⎛ ⎞ ⎜ − beq m + K mviav + ⎟ dt ⎟ 1 ⎜ = ⎜ ⎛ nw ⎞⎛ dh ⎞ ⎞ ⎟⎟ ⎛ J eq ⎜ dt 2 ⎟⎜ A p ΔP + cSGN ⎜ ⎟ ⎟ 0.5d ⎜⎜ ⎟ ⎟⎜ ⎜ ⎝ dt ⎠ ⎠ ⎟⎠ ⎝ nm ⎠⎝ ⎝
d 2θ m
(1)
(2)
The valve displacement is h = (d )(n w n m )θ m where
(n w
n m ) and d denote the worm to motor gear tooth
ratio and gear pitch. The normalized valve displacement is 0 ≤ H = (h − hmin ) (hmax − hmin ) ≤ 1 with H = 0(1) corresponding to fully closed (open) position. Coolant Pump: The servo-motor driven centrifugal water pump model considers the motor’s armature current and angular velocity as follows diap dt dω p dt
(
)
=
1 Vsp − Rap iap − K bpω p Lap
=
1 − b p + R f V02 ω p + K mp iap Jp
((
)
(3)
)
(4)
(9)
To simplify the analysis, the fuel cell stack will be assumed to be a heat source. The fuel cell stack heat entering the system is Qin and the auxiliary heater input is Q H . For the radiator, it shall be assumed that the air's heat capacity rate is less than the coolant (i.e., m& af c pa < m& coc pc ) so that the heat rejected to the ambient surroundings,
Qout
, is
Qout = Qο + ε c pa m& af (Tr − T∞ )
(10)
where Qο is heat lost due to uncontrollable air flow. Thermal Loop Two: The conservation of energy principle applied to the intercooler interface in Figure 5 becomes −c h m& h ΔTh = c c m& c ΔTc (11) where the temperature differences are ΔTh = Thin − Thout and ΔTc = Tcin − Tcout with Tcin = T j 2 . The terms Thin , , Tcin , and Tcout denote the compressed air inlet, compressed air outlet, coolant inlet, and coolant outlet temperatures. Note that the coolant inlet temperature will be assigned as the junction's nodal temperature; the compressed air's outlet temperature will be specified based on the fuel cell's needs. Thout
The compressed air's inlet temperature is dependent on the compressor’s exit pressure and can be determined using the ideal gas law (assuming a reversible polytropic process) as ⎛ Pcomp _ exit Thin = T∞ ⎜ ⎜ P∞ ⎝
⎞ ⎟ ⎟ ⎠
0.29
(12)
737
where T∞ , P∞ , and
denote the ambient
Pcomp _ exit
temperature, ambient pressure, and compressor exit pressure. The coolant exit temperature can be realized by substituting (12) into (11) and simplifying so that ⎛ c m& Tcout = Tcin − ⎜⎜ h h ⎝ cc m& c
⎡ ⎞ ⎢ ⎛ Pcomp _ exit ⎟ T∞ ⎜ ⎟⎢ ⎜ P∞ ⎠⎢ ⎝ ⎣
⎞ ⎟ ⎟ ⎠
0.29
⎤ − Thout ⎥ ⎥ ⎥⎦
(13)
Note that the compressor air outlet temperature, Thout , and exit air pressure, Pcomp _ exit , are required. The condenser’s thermodynamics must consider a two phase mixture for the inlet air and water stream which exits the fuel cell. An energy balance may be performed on the air/water and coolant streams in the condenser so that − cm m& m ΔTm = cc m& c ΔTc* (14) The temperature differences are ΔTm = Tmin − Tmout * * and ΔTc* = Tcin − Tcout with
* Tcin < T fcs .
If no energy loss
is assumed across the fluid loop between the * = Tcout . It may intercooler and condenser, then Tcin also be assumed that the mass flow rates in the second loop are conserved so that m& c is uniform. * This allows (14) to be rewritten to solve for Tcout as
⎛ c m& * * = Tcin − ⎜⎜ m m Tcout ⎝ cc m& c
⎞ ⎟(Tmin − Tmout ) ⎟ ⎠
(15)
Attention must now be focused on determining the inlet and exit temperatures, Tmin and Tmout , of the two phase mixture which leaves the fuel stack, enters the condenser for water removal to reuse within the system, and then exhausting the single phase air to the surroundings. Hence, the mixture's inlet temperature will be assumed to be the stack's lumped temperature, Tmin = T fcs , which couples these two thermal loops. The exit temperature of the single phase air from the condenser may be determined as ⎛ 1 * * Tcout = Tcin − ⎜⎜ ⎝ cc m& c
(
⎞ ⎟ h fg m& m ⎟ ⎠
)
(16)
with a similar flow rate as the intercooler. In the spirit of loop one, the radiator and junction nodal equations become * C r2 T&r2 = c pc H 2 m& c Tcout − Tr2 − Qout 2 (17)
(
)
* T j 2 = (1 − H 2 )Tcout + H 2Tr2
(18)
Qout 2 = Q0 2 + ε 2 c pa m& af 2 (Tr2 − T∞ )
(19)
enter the fuel cell. It is assumed that the tank temperature is 100ºC due to the saturation temperature of steam at one atmosphere in the condenser. Further, a fluid pressure of 303kPa will be initially requested to ensure atomization through the nozzles. In this paper, the humidification process thermal details are not considered. For this analysis, Bernoulli's equation, ΔP / ρ + ΔKE + ΔPE = 0 , may be applied where ΔP , ΔKE , and ΔPE denote the change in pressure, kinetic energy, and potential energy. If the humidification system is configured such that the fluid does not operate against gravity and the inlet fluid velocity at the liquid tank is zero, then the nozzle exit velocity becomes Vexit = [(2 / ρ h2 o )(Pexit − Pinlet )]0.5 (20) where ΔP = ( Pexit − Pinlet ) is the pressure across pump. 4. EXPERIMENTAL SCALE THERMAL TEST BENCH A scale cooling system experiment was created to support advanced thermal management studies. The test bench features a smart valve, radiator with electric fan, electric pump, and immersion heaters. The valve piston is linearly actuated by a worm gear assembly driven by a DC motor with optical encoder and potentiometer. A series of six immersion heater coils, maximum Qin=12.0 (kW) have been inserted into the tubing. This configuration permits a select number to be switched on/off for heating source fluctuations and auxiliary heating (i.e., Q H ). An Omega flow sensor and J-type thermocouples, with signal amplifiers, measure the coolant flow rate and temperatures. The system sensors and actuators were interfaced to a LINUX based PC workstation with Servo-to-Go I/O card for real time data acquisition and control. A custom algorithm was created in Matlab/Simulink and compiled using Mathworks Real Time Workshop. In this paper, the bench emulates the fuel cell primary loop. 5. EXPERIMENTAL AND NUMERICAL RESULTS Experimental tests and numerical simulations were performed to validate the models, develop an analysis tool, and demonstrate the underlying fuel cell thermal management concept. A series of tests were executed to investigate thermal transients in the cooling system and then explore the smart valve’s performance in tracking a set point temperature. Five heaters were operated for the temperature control studies; the sixth represented the auxiliary heat source for quick heating scenarios. The coolant pump and radiator fan were operated at maximum flows. An ambient temperature of T∞ = 25° C was maintained.
Fig. 5: Fuel cell system intercooler and condenser
A high gain controller has been implemented to regulate the coolant temperature at Tsp = 60 ° C . The
Thermal Loop Three: A fine midst of water may be sprayed into the hydrogen and air streams before they
set point and experimentally measured coolant temperatures are presented in Figure 6. As shown,
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Fig. 8: Analytical fuel cell primary loop thermal model temperature tracking for Tsp = 60 ° C Fig. 6: Experimental coolant temperature tracking for Tsp = 60 ° C in emulated fuel cell primary thermal loop the temperature is maintained within a ±1.5°C range of the target value during steady-state operation. The AMESim simulation was executed for the same test scenario and initial temperatures; the results are shown in Figure 7. The smart actuators successfully controlled the fuel cell temperature to within 2.0% of the set point at steady-state with minimal oscillations. Finally, the analytical model was executed for similar conditions. The system demonstrated tracking within 1.5% of the set point as shown in Figure 8 with few oscillations. However, this dynamic description does not consider the passive heat losses within the system. The results from the AMESim and analytical models agree favorably with the tests. The reader is referred to Setlur et al. (2005) for controller designs
REFERENCES Badrinarayanan, P., S. Ramaswamy, A. Eggert and R. Moore (2001). “Fuel Cell Stack Water and Thermal Management: Impact of Variable System Power Operation,” SAE paper 2001-01-0537. Cacciola, G., V. Antonucci and S. Ferni (2001). “Technology Update and New Strategies on Fuel Cells,” Journal of Power Sources, 100, 67-79. Cownden, R., M. Nahon and M. Rosen (2001). “Modeling and Analysis of a Solid Polymer Fuel Cell System for Transportation Applications,” Int. Journal of Hydrogen Energy, 26, 615-623. Eberth, J., J. Wagner, B. Afshar and R. Foster (2004). “Modeling and Validation of Automotive “Smart” Thermal Management System Architectures,” SAE paper 2004-01-0048. Luptowski, B., O. Arici, J. Johnson and G. Parker (2005). “Development of Enhanced Vehicle and Engine Cooling System Simulation and Application to Active Cooling,” SAE paper 2005-01-0697. Mosdale, R. and S. Srinivasan (1995). “Analysis of Performance of Water and Thermal Management in Proton Exchange Membrane Fuel Cells,” Electochemica Acta, 40, 413-421.
Fig. 7: Coolant temperature tracking for
Tsp = 60 ° C
in
Pukrushpan, J., H. Peng and A. Stefanopoulou (2004). “Modeling of Fuel Cell Reactant Flow for Automotive Applications,” Journal Dynamic Systems, Measurement and Control, 126, 14-25.
the AMESim thermal model fuel cell primary loop
Rajashekara, K. (2000). “Propulsion Strategies for Fuel Cell Vehicles,” SAE paper 2000-01-0369.
6. SUMMARY
Setlur, P., J. Wagner, D. Dawson and E. Marotta (2005). “An Advanced Engine Thermal Management System: Nonlinear Control and Test”, IEEE/ASME Trans on Mechatronics, 10, 210-220.
The integration of fuel cells into passenger, light duty, and commercial vehicles presents exciting design challenges including the thermal management system. In this paper, the concept of multiple thermal loops has been proposed to facilitate temperature regulation within specific system zones. AMESim and analytical models were presented to describe the thermal behavior of the individual components and integrated system. Lastly, experimental tests were performed to validate the models and demonstrate temperature tracking in the main thermal loop.
Sugano, N., T. Ishiwata, S. Kawai, K. Oshima and T. Uekusa (1994). “Investigation Dynamic Characteristics of Fuel Cell Stack Cooling System,” Trans Japan Soc Mech Engr, Part C, 60, 1597-1601. Wagner, J., I. Paradis, E. Marotta and D. Dawson (2002). “Enhanced Automotive Engine Cooling Systems - A Mechatronics Approach,” Int. Journal of Vehicle Design, 28, 214-240.
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et al., 2005). The traditional spark ignition internal engine cooling components (e.g., wax-based thermostat, belt driven water pump, and radiator with fan) have been upgraded with a smart valve, electric coolant pump, and variable speed electric (or hydraulic) driven radiator fan (Wagner et al., 2002). Servo-motor components allow temperature regulation based on the engine control unit's commands rather than the crankshaft speed. For a heavy-duty diesel engine, four thermal loops have been proposed which allow introduction of different thermal zones that may be independently controlled based on the operating conditions. The main loop regulates the engine block and passenger compartment temperatures. The second loop cools the liquid-to-air intercooler and exhaust gas recirculation. The two remaining loops cool the oil and transmission fluid. The philosophy of multiple loop cooling systems will be proposed for fuel cell systems since individual temperature zones can be identified for the various components. It is theorized that multiple cooling loops, with the inclusion of pre-heaters at certain locations, will allow fuel cell systems to reach operating temperature quickly and maintain that temperature within a tight neighborhood. To design
734
and demonstrate the fuel cell thermal management concept, dynamic models, numerical simulations, and experimental testing will be required. The analytical and empirical mathematical models allow the creation of simulation tools. Testing will be performed on a scale bench that features immersion heaters, smart valve, servo-motor pump, and electric radiator fan to emulate a fuel cell primary loop thermal system. A brief review of mobile fuel cells and integrated thermal management systems is now presented. Cacciola et al. (2001) discussed the application of fuel cells in transportation systems as auxiliary power units or power generators in hybrid vehicles. Rajashekara (2000) discussed fuel cell vehicle configurations, power electronics, and control strategies. Cownden et al. (2001) developed a comprehensive fuel cell system model that simulates the operation of the stack, air compressor, turbocharger, air-to-air heat exchange hydrogen supply, and thermal cooling. The cooling subsystem consisted of a radiator, water pump, radiator fan, and surge tank. Badrinarayanan et al. (2001) developed a transport model to analyze the effect of various water and thermal management parameters during steadystate and transient operations. Finally, Sugano et al. (1994) discussed the dynamic characteristics of the fuel cell stack cooling system and proposed a model for designing the temperature control systems. The paper is organized as follows. In Section 2, the concept of multiple thermal loops within the fuel cell system architecture is presented. Section 3 contains dynamic models to describe the fuel cell thermal management system. The experimental test platform is reviewed in Section 4. Experimental and numerical results are presented in Section 5. Lastly, Section 6 contains the summary.
2. FUEL CELL THERMAL MANAGEMENT SYSTEM In fuel cells, the thermal management system can be designed, integrated and controlled using distributed sensors, smart cooling actuators, and real time control algorithms to facilitate multiple temperature zones. The PEM fuel cell uses humidified hydrogen and air to generate electric power. The basic fuel cell architecture contains an air compressor, hydrogen tank, stack, condenser, turbine, and cooling system (Figure 1). A series of zones will be designed to customize the fuel cell system’s cooling for better temperature control. Three interconnected loops have been identified. The first thermal loop consists of the stack, variable speed coolant pump, adjustable position valve, radiator with electric fan, and auxiliary immersion heater for "cold start" scenarios. Loop two addresses the intercooler and condenser components using a servo-valve, coolant pump, and radiator. The intercooler reduces the air temperature after the compressor while the condenser recovers the water from the two phase mixture exiting the fuel stack. Finally, loop three humidifies the hydrogen and air using two valves, an auxiliary heater, and pump. 2.1 Loop One: Main Thermal Circuit The fuel cell stack functionality is similar to the common engine block in that the combustion, or chemical, process generates energy that may be harnessed directly or indirectly for propulsion plus heat. In fuel cells, the heat is generated by an exothermic reaction occurring between the humidified hydrogen and air across the polymer electrolytic membrane. This heat must be removed, via the circulating coolant, to maintain the set point temperature. It will be assumed that single phase heat transfer occurs in the stack’s coolant flow. Note that the insertion of a pre-heater allows the fuel cell to
Fig. 1: Fuel cell system with three separate thermal loops
735
warm up quicker. As shown in Figure 2, the fuel cell system’s main thermal loop contains a radiator assembly with electric fan, electro-mechanical valve, a fuel cell stack heat source, a variable speed pump, and an auxiliary pre-heater. The electro-mechanical valve regulates the coolant flow between the bypass and radiator circuits. The variable speed pump also controls the fluid flow; for instance, the pump speed can be minimized (maximized) during heating (cooling) scenarios. Finally, the radiator rejects heat to the surrounding environment through convective heat transfer which is dependent on the fan and vehicle speed. In this configuration, all components are servo-motor driven since the mobile fuel cell system may not feature a mechanical energy source (e.g., crankshaft) to power the pump and fan. The fluid within the loop is ethylene glycol
be assumed to have the fuel stack temperature from loop one. Similarly, the condensate temperature will impact loop three. The fluid is ethylene glycol. 2.3 Loop Three: Hydrogen/Air Humidifier Circuit The final thermal loop consists of the hydrogen and air humidifiers, electric pump, pre-heater, dual solenoid valves, and insulated water tank as shown in Figure 4. Loop three is the simplest circuit, based on number of components, but important in ensuring that the inlet gas streams have the proper moisture content for the polymer membrane. Note that this paper is only concerned with providing a mechanism to impact the moisture level and not humidity measurements. The tank will contain condensate from loop two. The humidifiers are nozzles that spray high pressure water into the hydrogen and air streams as they flow into the stack. In some regard, the sprayers are comparable to gasoline/diesel fuel injectors with a variable duty cycle. Thus, the hydrogen humidifier can be controlled at a different operating condition than the air humidifier which allows both inlet reactants to have unique humidity levels for optimal power generation. The excess water is recovered and returned to the insulated tank. The fluid flowing within the circuit is water.
Fig. 2: Fuel cell stack temperature control loop 2.2 Loop Two: Intercooler/Condenser Circuit The second fuel cell thermal loop consists of the intercooler and condenser that remove heat from the compressed inlet air stream and two phase (air and water) mixture exiting the fuel cell stack. In Figure 3, the architecture of loop two consists of a radiator assembly, valve, coolant pump, and two radiators for the inlet air and two phase mixture. It should be noted that this loop is similar to a diesel engine's exhaust gas re-circulation and intercooler loop. The inlet air stream is compressed by the compressor/turbine assembly to raise the pressure, and consequentially the temperature, prior to introduction into the stack. To lower the inlet air temperature, which creates a denser stack charge, the intercooler is an air-to-water heat exchanger which transfers heat from the air to the coolant. The condenser unit's functionality is to recover water from the exhaust stream which may be recycled within the fuel cell system through the humidification process. The control set point temperature will be assigned to the inlet air stream for the fuel cell stack to ensure proper operating conditions. In loop two, the two phase mixture exiting the fuel cell stack will
Fig. 3: Intercooler/condenser thermal control loop 3. MATHEMATICAL MODELING Two different mathematical modeling approaches are presented to describe the fuel cell thermal management system. The AMESim package is used to model and integrate the various components. The analytical and empirical equations for the thermal, electro-mechanical, and fluid domains are derived and coupled together in Matlab/Simulink. 3.1 AMESim Fuel Cell Thermal Model AMESim is a modeling tool for multi-domain systems. This software was used to model the fuel cell’s thermal components and then integrated to describe the overall system behavior. For brevity, only the fuel cell stack’s primary cooling loop will be
736
discussed which consists of the stack, pre-heater, smart valve, electric pump, and electric radiator fan assembly. The fuel cell and pre-heater were represented as convective elements. Flow resistance elements were used to model the pipe restrictions. The valve, pump, fan, and radiator were located in the thermal-hydraulic library (Eberth et al., 2004).
where
V0
and
R f ( P, Q )
represent the fluid volume per
radian of shaft rotation and the nonlinear fluid resistance. The coolant mass flow rate is m& c = 2πρ c r12 b1ω p tan β1 . Radiator Fan: The radiator fan motor’s armature current and rotational speed may be written as
3.2 Analytical Fuel Cell Thermal Model
diaf
The fuel cell system’s thermal behavior will be modeled using a lumped parameter approach with thermal, electrical, mechanical, and fluid concepts. First, the smart valve, electric pump, and radiator fan actuators will be modeled. Next, the primary (e.g., fuel cell stack, valve, pump, radiator, and auxiliary heater), secondary (e.g., intercooler, condenser, valve, pump, radiator, and auxiliary heater), and humidification (e.g., pump, reservoir, valves, and nozzles) thermal circuit loops will be considered
dω f
dt
dt
(
)
=
1 Vsf − Raf iaf − K bf ω f Laf
=
1 − Beqrω f + K mf iaf − ρ a A f R f Vaf 2 J eqr
(5)
(
)
(6)
The fan's air speed and mass air flow rate become
(
)
Vaf = ⎛⎜ K mf η f ρa A f iaf ω f ⎞⎟ ⎠ ⎝
0.3
and
m& af = β r ρ a A f Vaf
.
Thermal Loop One: Two thermal nodes, fuel cell stack and radiator, will be considered. The differential equations for the stack, T fcs , and radiator, Tr
, temperatures are C fcs T& fcs = −c pc m& c (T fcs − T j ) + Qin (t ) + QH (t )
(7)
Cr T&r = −c pc Hm& c (Tr − T fcs ) − Qout (t )
(8) Tj ,
The junction coolant temperature,
is related to the
fuel cell stack and radiator temperature as T j = (1 − H )T fcs + HTr
Fig. 4: Loop three for hydrogen and air humidifiers Fluid Valve: A bi-directional motor, interfaced to a worm gear, drives the multiple position valve assembly. The motor’s armature current and angular velocity equations are di av dθ m ⎞ 1 ⎛ = ⎜ V sv − R av i av − K bv ⎟ dt L av ⎝ dt ⎠
dθ ⎛ ⎞ ⎜ − beq m + K mviav + ⎟ dt ⎟ 1 ⎜ = ⎜ ⎛ nw ⎞⎛ dh ⎞ ⎞ ⎟⎟ ⎛ J eq ⎜ dt 2 ⎟⎜ A p ΔP + cSGN ⎜ ⎟ ⎟ 0.5d ⎜⎜ ⎟ ⎟⎜ ⎜ ⎝ dt ⎠ ⎠ ⎟⎠ ⎝ nm ⎠⎝ ⎝
d 2θ m
(1)
(2)
The valve displacement is h = (d )(n w n m )θ m where
(n w
n m ) and d denote the worm to motor gear tooth
ratio and gear pitch. The normalized valve displacement is 0 ≤ H = (h − hmin ) (hmax − hmin ) ≤ 1 with H = 0(1) corresponding to fully closed (open) position. Coolant Pump: The servo-motor driven centrifugal water pump model considers the motor’s armature current and angular velocity as follows diap dt dω p dt
(
)
=
1 Vsp − Rap iap − K bpω p Lap
=
1 − b p + R f V02 ω p + K mp iap Jp
((
)
(3)
)
(4)
(9)
To simplify the analysis, the fuel cell stack will be assumed to be a heat source. The fuel cell stack heat entering the system is Qin and the auxiliary heater input is Q H . For the radiator, it shall be assumed that the air's heat capacity rate is less than the coolant (i.e., m& af c pa < m& coc pc ) so that the heat rejected to the ambient surroundings,
Qout
, is
Qout = Qο + ε c pa m& af (Tr − T∞ )
(10)
where Qο is heat lost due to uncontrollable air flow. Thermal Loop Two: The conservation of energy principle applied to the intercooler interface in Figure 5 becomes −c h m& h ΔTh = c c m& c ΔTc (11) where the temperature differences are ΔTh = Thin − Thout and ΔTc = Tcin − Tcout with Tcin = T j 2 . The terms Thin , , Tcin , and Tcout denote the compressed air inlet, compressed air outlet, coolant inlet, and coolant outlet temperatures. Note that the coolant inlet temperature will be assigned as the junction's nodal temperature; the compressed air's outlet temperature will be specified based on the fuel cell's needs. Thout
The compressed air's inlet temperature is dependent on the compressor’s exit pressure and can be determined using the ideal gas law (assuming a reversible polytropic process) as ⎛ Pcomp _ exit Thin = T∞ ⎜ ⎜ P∞ ⎝
⎞ ⎟ ⎟ ⎠
0.29
(12)
737
where T∞ , P∞ , and
denote the ambient
Pcomp _ exit
temperature, ambient pressure, and compressor exit pressure. The coolant exit temperature can be realized by substituting (12) into (11) and simplifying so that ⎛ c m& Tcout = Tcin − ⎜⎜ h h ⎝ cc m& c
⎡ ⎞ ⎢ ⎛ Pcomp _ exit ⎟ T∞ ⎜ ⎟⎢ ⎜ P∞ ⎠⎢ ⎝ ⎣
⎞ ⎟ ⎟ ⎠
0.29
⎤ − Thout ⎥ ⎥ ⎥⎦
(13)
Note that the compressor air outlet temperature, Thout , and exit air pressure, Pcomp _ exit , are required. The condenser’s thermodynamics must consider a two phase mixture for the inlet air and water stream which exits the fuel cell. An energy balance may be performed on the air/water and coolant streams in the condenser so that − cm m& m ΔTm = cc m& c ΔTc* (14) The temperature differences are ΔTm = Tmin − Tmout * * and ΔTc* = Tcin − Tcout with
* Tcin < T fcs .
If no energy loss
is assumed across the fluid loop between the * = Tcout . It may intercooler and condenser, then Tcin also be assumed that the mass flow rates in the second loop are conserved so that m& c is uniform. * This allows (14) to be rewritten to solve for Tcout as
⎛ c m& * * = Tcin − ⎜⎜ m m Tcout ⎝ cc m& c
⎞ ⎟(Tmin − Tmout ) ⎟ ⎠
(15)
Attention must now be focused on determining the inlet and exit temperatures, Tmin and Tmout , of the two phase mixture which leaves the fuel stack, enters the condenser for water removal to reuse within the system, and then exhausting the single phase air to the surroundings. Hence, the mixture's inlet temperature will be assumed to be the stack's lumped temperature, Tmin = T fcs , which couples these two thermal loops. The exit temperature of the single phase air from the condenser may be determined as ⎛ 1 * * Tcout = Tcin − ⎜⎜ ⎝ cc m& c
(
⎞ ⎟ h fg m& m ⎟ ⎠
)
(16)
with a similar flow rate as the intercooler. In the spirit of loop one, the radiator and junction nodal equations become * C r2 T&r2 = c pc H 2 m& c Tcout − Tr2 − Qout 2 (17)
(
)
* T j 2 = (1 − H 2 )Tcout + H 2Tr2
(18)
Qout 2 = Q0 2 + ε 2 c pa m& af 2 (Tr2 − T∞ )
(19)
enter the fuel cell. It is assumed that the tank temperature is 100ºC due to the saturation temperature of steam at one atmosphere in the condenser. Further, a fluid pressure of 303kPa will be initially requested to ensure atomization through the nozzles. In this paper, the humidification process thermal details are not considered. For this analysis, Bernoulli's equation, ΔP / ρ + ΔKE + ΔPE = 0 , may be applied where ΔP , ΔKE , and ΔPE denote the change in pressure, kinetic energy, and potential energy. If the humidification system is configured such that the fluid does not operate against gravity and the inlet fluid velocity at the liquid tank is zero, then the nozzle exit velocity becomes Vexit = [(2 / ρ h2 o )(Pexit − Pinlet )]0.5 (20) where ΔP = ( Pexit − Pinlet ) is the pressure across pump. 4. EXPERIMENTAL SCALE THERMAL TEST BENCH A scale cooling system experiment was created to support advanced thermal management studies. The test bench features a smart valve, radiator with electric fan, electric pump, and immersion heaters. The valve piston is linearly actuated by a worm gear assembly driven by a DC motor with optical encoder and potentiometer. A series of six immersion heater coils, maximum Qin=12.0 (kW) have been inserted into the tubing. This configuration permits a select number to be switched on/off for heating source fluctuations and auxiliary heating (i.e., Q H ). An Omega flow sensor and J-type thermocouples, with signal amplifiers, measure the coolant flow rate and temperatures. The system sensors and actuators were interfaced to a LINUX based PC workstation with Servo-to-Go I/O card for real time data acquisition and control. A custom algorithm was created in Matlab/Simulink and compiled using Mathworks Real Time Workshop. In this paper, the bench emulates the fuel cell primary loop. 5. EXPERIMENTAL AND NUMERICAL RESULTS Experimental tests and numerical simulations were performed to validate the models, develop an analysis tool, and demonstrate the underlying fuel cell thermal management concept. A series of tests were executed to investigate thermal transients in the cooling system and then explore the smart valve’s performance in tracking a set point temperature. Five heaters were operated for the temperature control studies; the sixth represented the auxiliary heat source for quick heating scenarios. The coolant pump and radiator fan were operated at maximum flows. An ambient temperature of T∞ = 25° C was maintained.
Fig. 5: Fuel cell system intercooler and condenser
A high gain controller has been implemented to regulate the coolant temperature at Tsp = 60 ° C . The
Thermal Loop Three: A fine midst of water may be sprayed into the hydrogen and air streams before they
set point and experimentally measured coolant temperatures are presented in Figure 6. As shown,
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Fig. 8: Analytical fuel cell primary loop thermal model temperature tracking for Tsp = 60 ° C Fig. 6: Experimental coolant temperature tracking for Tsp = 60 ° C in emulated fuel cell primary thermal loop the temperature is maintained within a ±1.5°C range of the target value during steady-state operation. The AMESim simulation was executed for the same test scenario and initial temperatures; the results are shown in Figure 7. The smart actuators successfully controlled the fuel cell temperature to within 2.0% of the set point at steady-state with minimal oscillations. Finally, the analytical model was executed for similar conditions. The system demonstrated tracking within 1.5% of the set point as shown in Figure 8 with few oscillations. However, this dynamic description does not consider the passive heat losses within the system. The results from the AMESim and analytical models agree favorably with the tests. The reader is referred to Setlur et al. (2005) for controller designs
REFERENCES Badrinarayanan, P., S. Ramaswamy, A. Eggert and R. Moore (2001). “Fuel Cell Stack Water and Thermal Management: Impact of Variable System Power Operation,” SAE paper 2001-01-0537. Cacciola, G., V. Antonucci and S. Ferni (2001). “Technology Update and New Strategies on Fuel Cells,” Journal of Power Sources, 100, 67-79. Cownden, R., M. Nahon and M. Rosen (2001). “Modeling and Analysis of a Solid Polymer Fuel Cell System for Transportation Applications,” Int. Journal of Hydrogen Energy, 26, 615-623. Eberth, J., J. Wagner, B. Afshar and R. Foster (2004). “Modeling and Validation of Automotive “Smart” Thermal Management System Architectures,” SAE paper 2004-01-0048. Luptowski, B., O. Arici, J. Johnson and G. Parker (2005). “Development of Enhanced Vehicle and Engine Cooling System Simulation and Application to Active Cooling,” SAE paper 2005-01-0697. Mosdale, R. and S. Srinivasan (1995). “Analysis of Performance of Water and Thermal Management in Proton Exchange Membrane Fuel Cells,” Electochemica Acta, 40, 413-421.
Fig. 7: Coolant temperature tracking for
Tsp = 60 ° C
in
Pukrushpan, J., H. Peng and A. Stefanopoulou (2004). “Modeling of Fuel Cell Reactant Flow for Automotive Applications,” Journal Dynamic Systems, Measurement and Control, 126, 14-25.
the AMESim thermal model fuel cell primary loop
Rajashekara, K. (2000). “Propulsion Strategies for Fuel Cell Vehicles,” SAE paper 2000-01-0369.
6. SUMMARY
Setlur, P., J. Wagner, D. Dawson and E. Marotta (2005). “An Advanced Engine Thermal Management System: Nonlinear Control and Test”, IEEE/ASME Trans on Mechatronics, 10, 210-220.
The integration of fuel cells into passenger, light duty, and commercial vehicles presents exciting design challenges including the thermal management system. In this paper, the concept of multiple thermal loops has been proposed to facilitate temperature regulation within specific system zones. AMESim and analytical models were presented to describe the thermal behavior of the individual components and integrated system. Lastly, experimental tests were performed to validate the models and demonstrate temperature tracking in the main thermal loop.
Sugano, N., T. Ishiwata, S. Kawai, K. Oshima and T. Uekusa (1994). “Investigation Dynamic Characteristics of Fuel Cell Stack Cooling System,” Trans Japan Soc Mech Engr, Part C, 60, 1597-1601. Wagner, J., I. Paradis, E. Marotta and D. Dawson (2002). “Enhanced Automotive Engine Cooling Systems - A Mechatronics Approach,” Int. Journal of Vehicle Design, 28, 214-240.
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