Transcript November 2002

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
•
•
•
•
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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 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.
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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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University of Illinois at Urbana-Champaign
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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)
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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)
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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.
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Electrical Characteristics
• This shows the various curves generated as
fuel flow rate changes.
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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.
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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.
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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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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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University of Illinois at Urbana-Champaign
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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.
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University of Illinois at Urbana-Champaign
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Implications for Power Conversion
• A typical fuel cell system with auxiliaries shown
above. (Notice that the electrical output is
shown as an afterthought!)
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University of Illinois at Urbana-Champaign
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Implications for Power Conversion
• Current-sourced electrical system, with battery
energy buffer.
• Auxiliaries here would be part of the ac load.
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University of Illinois at Urbana-Champaign
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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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University of Illinois at Urbana-Champaign
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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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University of Illinois at Urbana-Champaign
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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.
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