Fuel Cells: Electrical Energy Conversion...

Fuel Cells: Electrical Energy Conversion Issues

P. T. KreinGrainger Center for Electric Machinery

and ElectromechanicsDept. of Electrical and Computer Engineering

University of Illinois at Urbana-Champaign

SeminarNovember 2002

Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign

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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.


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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.


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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.


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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


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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.


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Technology Types

• Proton-exchange membrane (PEM), a low-temperature 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.


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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.


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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.


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• Representative single PEM fuel cell:

0 10 20 30 400

0.5

1

Current density (mA/cm²)

Vol

tage

(vol

ts)

Single Cell Steady State, 50%, 75%, 100% Flow


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P-Curve

0

10

20

30

40

50

60

70

0 10 20 30 40

Current (Amp)

Stac

k Po

tent

ial [

V]

0200400600800100012001400160018002000

Stac

k Po

wer

[wat

t]

parasiticload


• A PEM curve for ~72 cells in series.


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• 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.


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• This shows the various curves generated as fuel flow rate changes.


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• 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.


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• Here is the behavior of an SOFC stack with a slow current ramp from 200 A to 400 A.

• Voltagedoes notkeep upeven atthis lowrate.


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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.


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• 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.


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• 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.


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• 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.

Vin +_L N

N N

N

R load

1

1

2

2


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• 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.


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• 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.


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• 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.


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• 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.


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• 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.


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• For a high-temperature system, a separate heating system is needed to bring the stack up to temperature.

• The energy levelis such that fuelmust be burnedfor this purpose.

• It can take manyhours to reachsteady state.


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• A typical fuel cell system with auxiliaries shown above. (Notice that the electrical output is shown as an afterthought!)


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• Current-sourced electrical system, with battery energy buffer.

• Auxiliaries here would be part of the ac load.


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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.


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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


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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.

Fuel Cells: Electrical Energy Conversion...

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