Management of a PEM Electrolyzer in Hybrid Renewable Energy Systems - Fuzzy Modeling and Control: Theory and Applications - page 215

Table 12.1 Rules of the FLC

If the SOC is High and the irradiance is High and PV panel
temperature is Low then Vfuzzy is High
If the SOC is High and the irradiance is High and PV panel
temperature is Medium then Vfuzzy is High
If the SOC is High and the irradiance is High and PV panel
temperature is High then Vfuzzy is Medium
If the SOC is Medium and the irradiance is High and PV panel
temperature is Low then Vfuzzy is High
If the SOC is Medium and the irradiance is High and PV panel
temperature is Medium then Vfuzzy is High
If the SOC is Medium and the irradiance is High and PV panel
temperature is High then Vfuzzy is Medium
If the SOC is Low or the irradiance is Low or the compromise
current is Low or the pressure is High then Vfuzzy is Z
If the error signal is Medium then Vfuzzy is Medium
If the error signal is High then Vfuzzy is High

Three subsets have been specified, Low , Medium and High , with Z, triangular
and S function type respectively, for the error signal. The first one corresponds to
negative values, which means that the current of the electrolyzer is higher than the
theoretically available current. The cause of this situation relies on two main reasons:
deviations of Ipmax respect to the empiric equation, and factors related with the
electrolyzer. These factors are the influence of environmental andwater temperatures,
and thermic and/or chemical inertias. The fuzzy rules related with this error signal
are formulated according to the following. When the error is high, the electrolyzer
current must be increased in order to reduce that error. In the same way, if the error
is located in the Medium zone, the control signal assumes a value of the Medium
subset. This way, the error values considered acceptable have a small effect on the
output signal and the electrolyzer operation is smoother.
Three subsets have been established for Vfuzzy (the output signal): Z , Medium and
High , with Z, triangular and S function type respectively. The first one corresponds to
low voltage applied to the electrolyzer, insufficient to drive the hydrogen generation.
The High partition leads to a high value of the output, so the higher the voltage,
the higher the electrolyzer current. The intermediate subset acts as a transient zone,
contributing to a soft variation of the output signal according to the systemconditions.
Note that the output signal never becomes zero, therefore, the electrolyzer stack is
always under voltage. This behavior aims to minimize its degradation because the
ON/OFF cycling contributes to its deterioration.
The FLC behavior is defined by nine fuzzy rules, which are given in Table 12.1 .
These rules have been formulated for the output signal to have a high value (thismeans
high voltage and current of the electrolyzer) when the systemconditions are favorable