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Fuel Cells: Electrical Energy Conversion Issues Seminar November 2002 P. T. Krein Grainger Center for Electric Machinery and Electromechanics Dept. of Electrical and Computer Engineering University of Illinois at Urbana-Champaign Outline • • • • • How do fuel cells work? Some technology types. Electrical characteristics. Implications for power conversion. Key components for low-cost fuel cell conversion applications. • Conclusion. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 2 How Do Fuel Cells Work? • Two reactive components are separated by an “ionic conductor” (electrolyte). • Most widely used example: hydrogen and oxygen. • Protons flow through the electrolyte. • Electrons flow through an external circuit. • The electrochemical potential is defined by the specific reaction. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 3 How Do Fuel Cells Work? • The system is like a “proton diode.” • The action is like a battery, except that the fuel and oxidizer are allowed to “flow through,” leading to continuous operation. • Energy conversion efficiency is high. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 4 Technology Types • Fuel cell technologies are determined by the selected fuel and by the electrolyte. • Fuel: – Hydrogen – Zinc metal – Other hydrocarbons • Electrolyte: – Phosphoric acid – Proton exchange membrane (PEM) – Solid-oxide (SOFC) – Molten carbonate Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 5 Technology Types • Most types are intended for hydrogen fuel. A few can work directly with other hydrocarbons. • The proton is the simplest ion to send through the electrolyte. • Other fuels must be “re-formed” to extract hydrogen. • Example: natural gas, re-formed with steam at high temperature to yield hydrogen and CO. • Challenge: impurities, such as CO, can “poison” catalysts or electrolytes. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 6 Technology Types • Proton-exchange membrane (PEM), a lowtemperature type that uses an organic polymer that conducts protons as its electrolyte. • Molten carbonate (MCFC) types that use liquid carbonate salts as the electrolyte, at ~650°C. • Solid oxide (SOFC) types use a solid ceramic electrolyte, at ~1000°C. • Phosphoric acid (PAFC) types use phosphoric acid at ~175°C as the electrolyte. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 7 Electrical Characteristics • The ideal open-circuit voltage for a single cell is well-defined, but is a function of temperature and pressure. • It is about 1.15 V at 80°C and 1 atm pressure for hydrogen and oxygen. • In use, the voltage is closer to 0.7 V. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 8 Electrical Characteristics • Many cells are “stacked” in series to produce a sufficient voltage for application. • Considerations: – More cells facilitate power conversion. – Fewer cells make the stack simpler and make the cells easier to balance. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 9 Electrical Characteristics • Representative single PEM fuel cell: 1 Voltage (volts) Single Cell Steady State, 50%, 75%, 100% Flow 0.5 0 0 10 20 Current density (mA/cm²) Grainger Center for Electric Machines and Electromechanics 30 40 University of Illinois at Urbana-Champaign 10 Electrical Characteristics • A PEM curve for ~72 cells in series. P-Curve 70 2000 60 1800 1600 50 1400 40 1200 30 1000 800 20 600 10 400 200 0 Stack Power [watt] Stack Potential [V] parasitic load 0 0 10 20 30 40 Current (Am p) Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 11 Electrical Characteristics • The most unusual aspect of fuel cells is the dynamic behavior – tied to fuel flow. • The fuel should be supplied at just the right rate such that nearly all of it is consumed. • This fuel utilization level should be 85% or better. • What if the electrical load changes? The fuel flow must change. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 12 Electrical Characteristics • This shows the various curves generated as fuel flow rate changes. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 13 Electrical Characteristics • The curve drops quickly as load increases, then has a wide resistive region. • The cell experiences a current limit. • The current limit position depends on fuel flow. • For efficiency, we would prefer to operate at about 0.8 V, with a fuel flow close to maximum fuel utilization. • For good power and material use, a voltage of 0.6 V to 0.7 V is more suitable. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 14 Electrical Characteristics • Here is the behavior of an SOFC stack with a slow current ramp from 200 A to 400 A. • Voltage does not keep up even at this low rate. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 15 Implications for Power Conversion • In a fuel following system, the electrical response times can be several tens of seconds. • An energy buffer will be required to follow the load change. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 16 Implications for Power Conversion • How can a fuel cell be used to supply a real, unpredictable load? • Example: place a battery in parallel. • This enforces a particular voltage or voltage range. • The curves are not a good match, and control is difficult. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 17 Implications for Power Conversion • The fuel cell appears like a rather weak voltage source. • Example: use a fuel cell as the input to a push-pull converter. • This will work, but the voltage source experiences a high ripple current. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 18 Implications for Power Conversion • A current-sourced converter is a better choice. • It has the advantage of allowing battery connection at the output. • The dc output feeds to an inverter to complete the standalone system. _ V in + L N1 N2 N1 N2 Grainger Center for Electric Machines and Electromechanics R load University of Illinois at Urbana-Champaign 19 Implications for Power Conversion • Cells in the series stack must remain electrically isolated. • High voltages, while possible, can cause trouble because of the need for isolation and balance. • Modest voltages (12 V, 24 V, 48 V) are preferred because of simplicity. • Direct (0.5 V) conversion would be nice. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 20 Implications for Power Conversion • Direct conversion from 0.5 V: very hard to do this efficiently. • Cell stacks at ~400 V: hard to prepare a safe, reliable stack. • Lower voltage is best for fuel cell, higher voltage best for power conversion. • So far, voltages of about 48 V seem good for small applications. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 21 Implications for Power Conversion • Challenges: – Overloads cause rapid heating and possible failure. – The curves follow the dynamics of fuel flow, and change slowly in most cases. – Some types require very high running temperature. – Reliability must be high, and costs must be reduced. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 22 Implications for Power Conversion • An important challenge are the auxiliary and support elements for the fuel cell: – – – – Pumps for the fuel Reformer, if needed Fuel recovery for fuel utilization < 100% Diagnostics and controls • In a typical case, as much as 20% of the rated fuel cell output electrical power will be needed to support auxiliaries. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 23 Implications for Power Conversion • The challenge in a standalone system is that the auxiliaries must be up and running before any power can be delivered. • A battery set becomes even more important, as it must provide energy back to the auxiliaries for starting. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 24 Implications for Power Conversion • For a high-temperature system, a separate heating system is needed to bring the stack up to temperature. • The energy level is such that fuel must be burned for this purpose. • It can take many hours to reach steady state. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 25 Implications for Power Conversion • A typical fuel cell system with auxiliaries shown above. (Notice that the electrical output is shown as an afterthought!) Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 26 Implications for Power Conversion • Current-sourced electrical system, with battery energy buffer. • Auxiliaries here would be part of the ac load. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 27 Key Components • Current-source inductors for effective filtering and very low loss. • Example: 10 kW system output, 48 V fuel cell. • The fuel cell must deliver more than 200 A. • Need a low-cost 200 A choke that provides current ripple of just a few amps at switching frequencies (such as 50 kHz). • Low-cost high-current switching devices. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 28 Key Components • High-frequency transformers. • Example (residential): deliver 10 kW at 50 kHz into the conversion stage. • Example (automotive): deliver 100 kW at 20 kHz into an inverter stage. • Energy buffer components: – Batteries – Double-layer capacitors Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 29 Conclusion • Fuel cells act like “flow-through batteries.” • They have slow dynamics and require energy buffers for efficient use. • Current-sourced interfaces are a good approach – there is a need for large low-cost inductors to support these. • High-frequency link transformers are a significant opportunity. • Push-pull and bridge inverter topologies have been considered. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 30