Fuel Cell Hybrid Power System Modeling, Analysis and Control of Fuel Cell Electric Hybrid Power Systems Transportation Distributed Power Generation Kyungwon Suh June 9 th, 2006 Stand-Alone (APU http://www.gm.com, http://www.ballard.com 2/38 Fuel Cell Hybrid System System integration in FC hybrid power Fuel cell stack Main power source Slow and complex dynamics limited by mass and heat balances Battery or ultracapacitor Secondary power source DC/DC converter - Matches voltage or current from power source to load Inverter / Traction motor - Major load Auxiliary load - Power necessary for stack operation Literature Review Fuel cell system Dynamic model - Amphlett et al. 1994/1996, Wang & Wang 2005, Pukrushpan et al. 2004, Lin et al. 2004 Air flow control Rodatz et al. 2003, Pukrushpan et al. 2004 Load (current management - Guezenec et al. 2003, Sun & Kolmanovsky 2004, Vahidi et al. 2004/2005 Control of fuel cell hybrid power Battery management - Jiang et al. 2004 FC and bus voltage objectives - Liu et al. 2002 Non-causal optimization Lin et al. 2004, Rodatz et al. 2005 Study of fuel cell hybrid power system including Performance and limitations in high-pressure PEM FCS Electric architectures w/ dynamic interactions in voltage / current 3/38 4/38
Electric Architectures for FC Hybrid System Fuel cell system with DC-DC converter DC-DC converter boosts the fuel cell stack voltage to the battery voltage Current drawn from fuel cell is controlled by DC-DC converter Kim et al. 2005 (DCX Fuel cell system that supports the load directly Low voltage battery is used with bidirectional converter Flohr 2002 (FORD, Ishikawa et al. 2004 (TOYOTA Thesis Outline Modeling and control of fuel cell electric hybrid power system System integration and control Hybrid electric configurations Overview of the dissertation Control of fuel cell stand-alone system Fundamental performance limitations in FC air flow control (ACC 2006, IEEE TCST submitted Control of FC & DC/DC converter Control issues associated with a fuel cell augmented with DC/DC converter (ISIC-MED 2005, International Journal of Energy Research 2005 FC hybrid power system Control and coordination of the FC hybrid power system (SAE 2006 Hybrid electric architectures and control Rajashekara 2000 5/38 6/38 Contents patm Air Flow Control vcm Air Hydrogen Pressure Control Hydrogen Tank Compressor Motor M Wcp Supply Manifold psm Wca,in Wca,out pca 1. Background and Introduction 2. Performance Limitations of Air Flow Control in Power-Autonomous Fuel Cell System 3. Coordination of FC and DC/DC converter 4. Control of Fuel Cell Hybrid Power 5. Conclusion Air Ist vst Fuel Cell Stack Hydrogen Air & Water patm Fuel Cell Stack System We consider a compressor driven 75kW proton exchange membrane fuel cell. The fuel cell stack and reactant flow models are based on electrochemistry, mass balances for lumped volumes in the stack and peripheral volumes, and rotational dynamics of compressor and motor. The OXYGEN EXCESS RATIO (OER λ = WO 2, in O2 WO 2, rct is a convenient lumped variable, which if regulated to a desired ref value λ O = 2 ensures adequate 2 supply of oxygen in the fuel cell stack. 7/38 8/38
patm Fuel Cell System Dynamics Air Flow Control vcm Air Hydrogen Pressure Control Hydrogen Tank Compressor Motor M Electrochemistry ni st WO rct = M O 2, 2 4F Manifold filling dpsm RTsm = ( W dt M V W Wcp ca, in Supply Manifold = k Air psm a, atm sm ca, in ( p Humidifier and Temperatire Controller Wca,in sm cp p ca W Ist vst Fuel Cell Stack pca ca, in Hydrogen Air & Water Wca,out Mass balances dpo RT 2 st = ( W dt M O V 2 ca dpn RT 2 st = ( W dt M V patm W ca, out Compressor motor dωcp Jcp = τcm τcp dt k τ = η ( v k ω cp cm W t cm cm v Rcm f ( ωcp, psm, patm Stack voltage v = n( E v v v conc st cp N2 ca CD AT = RT = st act O2, in N2, in f ( p ca, p ohm W W atm O2, out N2, out W O2, rct Air Flow Control in Power-Autonomous FC FC air supply controller adjusts the air flow through a compressor motor command to minimize oxygen starvation periods. The volume of supply manifold including humidifier and heat exchanger causes significant lag or delay in regulating oxygen excess ratio inside the stack (Motozono et al. 2003, Rodatz et al. 2003, Pukrushpan et al. 2004 High compressor control efforts can cause instabilities due to the directly-coupled compressor power loss (Mufford and Strasky, 1999 9/38 10/38 Control Problem Formulation General control configuration Performance Measures and Constraints The performance in rejecting the disturbance Disturbance response ratio Linear transfer functions Integral constraints due to NMP zero dictate the performance Poisson integral (Freudenberg et al. 2003 where If over any frequency ranges, then necessarily at other frequencies since Integral output square error 11/38 12/38
Dynamic & Static Feedforward Control Feedforward cancellation control for RHP pole-zero cancellation Dynamic feedforward control Minimizes if Gzw is static (Goodwin et al. 2003 (cf. Static feedforward control where The disturbance is completely rejected at DC Magnitude Dynamic & Stack Feedforward Control Tradeoffs due to the Poisson Integral in both static & dynamic feedforward controllers Larger excursion in with dynamic feedforward controller Similar and with static & dynamic feedforward controllers 10 1 Disturbance response ratio R zw 10 0 10-1 static K uw real K uw open-loop 10-2 10-1 10 0 10 1 10 2 10 3 Frequency (rad/sec δ λ O2 0.05 0-0.05-0.1-0.15-0.2-0.25-0.3-0.35 I net 20A step static K uw real K -0.4 uw open-loop -0.45 0 0.2 0.4 0.6 0.8 1 Time (sec 13/38 14/38 Static Feedforward with Filter To reduce initial excursion in Feedback Control Design Feedback cancellation control Tradeoff between initial excursion and recovery time in Multiplicity bound (Freudenberg et al. 2003 Magnitude 10 1 Disturbance response ratio R zw 10 0 10-1 static K uw τ f ilter =0.01 τ f ilter =0.1 τ f ilter =0.4 open-loop 10-2 10-1 10 0 10 1 10 2 10 3 Frequency (rad/sec δ λ O2 0.05 0-0.05-0.1-0.15-0.2-0.25-0.3 I net 20A step static K uw τ filter =0.01 τ filter =0.1-0.35 τ filter =0.4 open-loop -0.4 0 0.2 0.4 0.6 0.8 1 Time (sec does not meet due to the NMP zero Combined feedback/feedforward control Static feedforward controller (map + PI feedback controller 15/38 16/38
1` Experimental Setup Nexa FC stack system with safety and measurement devices 1.2kW net power Air supply blower powered by the stack Air-cooled configuration with fan Experimental Comparisons Net current input Measurement Currents net FC current, auxiliary current Voltage stack voltage Electric load input current & voltage Static feedforward with filter =0.4 sec in simulation 17/38 18/38 Experimental Comparisons Stack voltage input Contents 1. Background and Introduction 2. Performance Limitations of Air Flow Control in Power-Autonomous Fuel Cell System 3. Coordination of FC and DC/DC converter 4. Control of Fuel Cell Hybrid Power Fuel Cell System Same air dynamics and control in FC system 5. Conclusion Lin Ist FB DC-DC Converter Iin, vout Ist, Wcp H2 Tank vst Icm CM Load d1 vout Load 1` 19/38 FF + FB vcm M Air Compressor OER Cout 20/38
Decentralized vs. Coordinated control Control objectives Oxygen excess ratio DC bus voltage 1` DC/DC Converter Model Here we consider a DC/DC converter boosting stack voltage to the 400 V DC-bus. An average nonlinear dynamic model is used to approximate the boost converter switching dynamics with DUTY CYCLE COMMAND d, and input/output voltage/currents. 1` REACTANT FLOW & CURRENT are on the same domain 21/38 22/38 Control of DC/DC Converter DC/DC converter controller is designed to transform the unregulated FC voltage to regulated bus voltage. Simulation results Step resistive load changes subject to 7.5 kw power increase Tradeoff between and with different DC/DC control tuning (PI (P 2.2 420 2.1 410 2 400 Disturbance rejection with LQR + integral control λ O 2 1.9 1.8 v out (V 390 380 The C v calibration and the closed loop time constant allow us to modify the FC current request, and thus oxygen starvation. 1.7 1.6 14.6 14.8 15 15.2 15.4 15.6 15.8 Time (sec 370 Decentralized tuning 360 14.6 14.8 15 15.2 15.4 15.6 15.8 Time (sec 23/38 24/38
Contents FC electric hybrid configuration Electric configuration Coordinates power split between fuel cell stack and battery 1. Background and Introduction 2. Performance Limitations of Air Flow Control in Power-Autonomous Fuel Cell System 3. Coordination of FC and DC/DC converter Provides regulated voltage and current to the inverter that drives the propulsion motor Protects the fuel cell stack from sudden load changes before the stack is ready Maintains battery state of charge for proper working condition 4. Control of Fuel Cell Hybrid Power 5. Conclusion Hybridization with the fuel cell stack, DC/DC converter and battery offers flexibility in managing the power demand of the vehicle. 25/38 26/38 FC Hybrid Vehicle Model A complete FORWARD-FACING, CAUSAL model for a fuel cell hybrid vehicle is desirable for detailed control simulation. Hybrid Control Strategy We use decentralized control to show the effect of controller design on the performance of FC oxygen excess ratio (OER Battery state of charge (SOC Power split and fuel economy Control calibrations of low-level controller spans the range from load-following FC to load-leveling FC 27/38 28/38
FC Usage - US06 Highway Overall fuel cell usages (histogram bar are similar in load-leveling and load-following FC. FC & Battery usage - US06 Highway The operating characteristics of the FC depends on the control calibration. Load-following FC Load-leveling FC Load-following FC Load-Leveling FC 29/38 30/38 Impact on Fuel Economy Control calibrations do NOT affect FUEL ECONOMY, but determine the HYBRIDIZATION LEVEL (FC/battery usage. Load-following Load-leveling Fuel Cell Hybrid Electric Architecture Fuel cell system that supports the load directly Small-sized, low voltage battery is used with a bidirectional converter 31/38 32/38
FC Hybrid Power Model Complete CAUSAL model with voltage-input FC & bidirectional DC/DC converter Control of bidirectional DC/DC Converter The Bidirectional DC/DC converter controller is designed to prevent abrupt changes in the stack voltage and to minimize the battery usage. (PI (P Minimum number of batteries is defined by the control of the bidirectional DC/DC converter and battery constraints using simulation. 33/38 34/38 Conclusion and Contribution Control of fuel cell stack system It is shown that the inherent design limitations due to the NMP zeros dictate the performance of the power-autonomous FC. Combined feedforward/feedback controller achieves adequate performance and complexity. Coordination of FC and DC/DC converter Tradeoff between the fuel cell performance (oxygen excess ratio and the DC/DC converter objectives (bus voltage is presented. Model-based controllers are designed to achieve the two performance objectives using decentralized control scheme. Conclusion and Contribution Control of FC hybrid power Realistic models for the FC hybrid electric architectures are presented. The model facilitates control design because it allows manipulation of physical actuators it captures dynamic behavior during transient loading Control design is performed by tuning low-level controllers without hierarchical (supervisory controller design. The control calibration is shown to have minimal effect on fuel economy primarily because typical driving cycles can be accomplished by a well controlled FC with efficiencies up to 45%. The control calibration is shown to have major impact to OER, compressor constraints, SOC and thus battery sizing. 35/38 36/38
Future Work FC hybrid vehicle study Combined control and optimization in FC air supply (cf. Jeongwoo Han s Acknowledgements Prof. Anna Stefanopoulou Committee members Prof. Huei Peng Coordination and communication in FC hybrid power Communication in centralized control Fault detection and monitoring Minimal communication in hybrid power National Science Foundation Automotive Research Center Ford Motor Company Hyundai Motor Co. & Kia Motors Corp. In-vehicle implementation 37/38 38/38