The Navier-Stokes equation, neglecting the viscosity and gravity terms, can
be used to simply calculate the total number of moles of purge gas. As shown in
the figure above, only the pressure difference between the hydrogen supply
system and the air supply system manifold will cause the gas mixture in the
hydrogen supply system pipeline to exit the hydrogen supply system. In order to
discharge the gas mixture in the hydrogen pipeline, the pressure of the hydrogen
supply system pipeline is usually controlled to be higher than the pressure of
the outlet manifold of the air supply system. Based on the concept of mole
fraction during purge, the total moles of the mixture can be divided into the
sum of the moles of each component. The mole fraction of each component is
defined as follows (ni is the number of moles of component i, n is the total
number of moles of the hydrogen supply system). The higher the mole fraction,
the greater the discharge of component i.
yi=ni/n(4)
Initial estimate of hydrogen concentration
After the fuel cell vehicle is started, it is difficult to estimate the
initial value of hydrogen concentration because the CPU in the controller is
powered off during parking. But the controller can know the parking time of the
vehicle with the help of the timer chip. It is known that the longer the
shutdown time and the higher the operating temperature of the system during
shutdown, the lower the hydrogen concentration. Therefore, drawing a map of two
parameters (parking time and operating temperature at shutdown) is a good way to
estimate the initial hydrogen concentration value at the start of a fuel cell
vehicle.
The relationship between anode hydrogen concentration and operating
temperature
estimator calibration
Simplifying the complex hydrogen flow in the fuel supply system can lead to
an estimation error, which is defined as the difference between the hydrogen
concentration estimate and the hydrogen concentration value measured by the
hydrogen concentration analyzer. Hyundai Motor Company has designed the
following three calibration parameters, as shown in the figure above. The three
calibration parameters are: (1) Nitrogen penetration gain. Calibration parameter
of estimated hydrogen concentration decay rate at normal operating temperature;
(2) Activation energy. Calibration parameter that minimizes the estimation error
caused by operating temperature; (3) Purge gain, a calibration parameter that
increases the estimated hydrogen concentration value during purging at normal
operating temperature.
Adjust the three calibration parameters of the estimator (based on the
hydrogen concentration value measured by the concentration analyzer)
Hydrogen concentration estimator validity
After adding the hydrogen concentration estimator to the NEXO fuel cell
controller, Hyundai Motor Company conducted extensive tests to verify the
effectiveness of the hydrogen concentration estimator. Based on the measurement
data of UDDS, HWFET and US06 operating conditions (wide load range from low load
to high load), Hyundai Motor Company adjusted three calibration parameters to
reduce estimation errors. After adjusting the three calibration parameters, the
error size of the hydrogen concentration estimator changes slightly in different
driving conditions, but remains within a reasonable range, indicating that it is
sufficient to be used as an estimator, as shown in the figure below. The
variation in estimation error comes from the assumption of homogeneity of the
mixture. Under low load conditions of the hydrogen supply system, due to the low
hydrogen consumption of the stack and the small amount of hydrogen
recirculation, the gaseous mixture in the hydrogen supply system is more uneven.
As shown in the figure below, this effect shows that the error is smaller under
the UDDS operating condition, but the error is slightly larger under the HWFET
and US06 operating conditions.
Validity experiment of hydrogen concentration estimator for NEXO fuel cell
system
Feedback control based on hydrogen concentration estimator
There are two methods to control the hydrogen concentration in the hydrogen
supply system pipeline. One is to increase the pressure of hydrogen flowing from
the hydrogen storage bottle through the hydrogen supply system pipeline, and the
other is to control the nitrogen and water vapor in the hydrogen supply system
pipeline. Perform a purge operation. Using a hydrogen concentration estimator
that functions in the same way as a hydrogen concentration sensor, a feedback
control as shown in the figure below can be constructed to resist internal
disturbances (such as manufacturing process deviations) and external
disturbances (such as changes in environmental pressure). The reference signal
is the optimal hydrogen concentration at the operating point, and the reference
signal is compared with the output of the hydrogen concentration estimator to
obtain the error signal. When the fuel cell system suddenly enters the power
generation state from the shutdown state, the purge controller sends a purge
command, and the pressure controller sends a pressure reference signal to the
hydrogen supply valve. The purge controller and pressure controller are designed
to reduce signal errors independent of stack load conditions, meaning the
hydrogen concentration estimator output signal follows the reference signal.
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