Pressure controller design
When the fuel cell is shut down for a long time, the hydrogen concentration
in the hydrogen system pipeline decreases due to the nitrogen penetration
effect. However, since the stack does not need to output current at this time,
the controller will not take any measures to maintain the hydrogen concentration
at within the optimal range. If there is a sudden demand for the stack current
output, it needs to be quickly restored to the optimal range of hydrogen
concentration. The speed of supplying hydrogen from the hydrogen storage bottle
to the hydrogen supply system pipeline is much faster than the speed of purging
low-concentration hydrogen into the outlet manifold of the air supply system,
where a pressure controller needs to be designed. The hydrogen supply is
calculated based on the difference between the estimated hydrogen concentration
and the optimal hydrogen concentration for the given operating conditions. After
the hydrogen gas in the hydrogen supply system pipeline is pressurized through
the pressure control operation, the purge control operation is started under
continuous load conditions.
Purge controller design
Since most driving conditions consist of continuous load conditions, the
purging operation frequency is much higher than the pressure control operation
frequency. When the output value of the hydrogen concentration estimator is less
than the optimal hydrogen concentration range, the controller only increases the
hydrogen concentration (such as a semi-active actuator). When it is higher than
the optimal hydrogen concentration range, the purge operation is not performed.
In order to perform the above control, a minimum level of air flow velocity
should be formed in the air supply system pipeline to prevent hydrogen from
diffusing to the cathode electrode and reduce the impact on the current output.
Due to the delayed response of the air compressor and the time required for
fresh air delivery, it takes time for the air supply to form when the fuel cell
is shut down, but under stable load conditions, there will be no delay in the
blowing operation.
Performance evaluation
In order to verify the effectiveness of feedback control based on the
hydrogen concentration estimator, Hyundai Motor Company conducted a large number
of experiments in the NEXO fuel cell power system and SUV. Three main
characteristics of the traditional Q-value method were evaluated: resistance to
external interference, prevention of hydrogen starvation and efficiency
improvement. For confidentiality reasons, Hyundai Motor Company only displays
relative and expected values, not absolute and measured values.
Pressure control diagram
Resistant to external interference
Environmental stresses on Earth vary depending on factors such as climate,
altitude and latitude. Since the pressure difference between the hydrogen supply
system pipeline pressure and the ambient pressure will cause gas leakage, the
purge controller is also closely related to the ambient pressure. It can be
observed that on different daysThe changes in purge count under the same driving
conditions during the period, and the estimation error is kept within a
predetermined range. Under continuous operation under three driving conditions,
the ambient pressure difference of 1.6kPa can cause 9 purge count differences.
In the three working conditions, the working pressure of the hydrogen supply
system is the same, but a 1.6kPa decrease in ambient pressure will increase the
number of moles of purge gas, which will lead to an increase in hydrogen
concentration during the purge and extend the purge time interval. Therefore, it
was concluded that this technology has achieved optimal purge counts for ambient
pressure changes such as formal operating conditions at different altitudes.
Prevent hydrogen outgassing to improve durability
In the study, the function of the pressure controller was also checked by
suddenly applying a large load to the fuel cell system after a long shutdown.
After the fuel cell system is shut down, the estimated hydrogen concentration is
lower than the optimal concentration value due to the nitrogen permeation
effect. However, before applying to large loads, it was observed that the
hydrogen supply system pipeline pressure was increased to increase the hydrogen
concentration to the optimal position according to the design requirements. The
pressure rise of the hydrogen supply system pipeline is calculated based on the
estimated hydrogen concentration value. The output value of the hydrogen
concentration estimator does not experience overshoot, undershoot, and the error
rises to the optimal range. It has been evaluated that this function is useful
for preventing hydrogen starvation (extending fuel cell life).
Efficiency improvement
Hyundai Motor Company conducted an efficiency comparison between the
conventional Q-value method and hydrogen concentration estimator-based feedback
control technology in the NEXO fuel cell SUV. When using the Q-value method, the
hydrogen concentration estimate spreads within the optimal hydrogen range, and
the changes fluctuate greatly. However, the feedback control technology based on
the estimated hydrogen concentration can maintain the estimated hydrogen
concentration within the optimal range without crossing the upper and lower
limits, and no deviation in the average value is observed in different driving
conditions. In combined driving conditions, the efficiency is increased by
0.02MPGe because the hydrogen concentration value within the optimal range will
improve hydrogen utilization.
in conclusion
Hyundai Motor Company has implemented and proven through its NEXO fuel cell
SUV and fuel cell system experiments that a feedback controller based on a
hydrogen concentration estimator is superior to the traditional Q value method.
First, a simple cubic model with the same hydrogen supply system volume was
established for embedded software development and found to have reliable
performance and can be used as a hydrogen concentration estimator for hydrogen
supply system pipelines. Secondly, the estimation error changes within a wide
range of load conditions, such as UDDS, HWFET and US06 conditions, but the three
calibration parameters can ensure that the error is controllable within a
reasonable range. Furthermore, Hyundai Motor Company has developed feedback
control technology based on a hydrogen concentration estimator, which exhibits
good resistance to environmental pressure interference and improves durability
and efficiency by preventing hydrogen shortage. Finally, compared with the
traditional equivalent Q value method, Hyundai Motor Company's hydrogen
concentration optimal operation control achieves improved efficiency under
combined driving conditions. Dr. Fuel Cell
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