Nonlinear multivariable sliding mode control of a reversible PEM fuel cell integrated system

•Reversible PEM fuel cell is developed integrating with several process units.•Reduced-order model is used to formulate a state observer.•Multivariable sliding mode controller is synthesized for fuel cell.•No such controller design is made for reversible PEM fuel cell. This work proposes a reduced-order sliding mode observer (SMO) based nonlinear control to regulate a multivariable proton exchange membrane fuel cell (PEMFC) integrated system. To make this system reversible, it is integrated with an air compressor, air cooler, primary manifold, supply manifold, humidifier and return manifold. To recirculate the unreacted hydrogen fuel, an ejector and to suppress the fuel cell heat of reaction, a simple water cooled heat exchange system is modeled. Among the three measured outputs, namely output voltage (main product), compressor air flow and fuel cell body temperature, the last two are proposed to control at their desired values to avoid oxygen starvation at cathode side and adverse effect of system temperature, respectively. To increase the lifetime of proton exchange membrane fuel cell and its smooth operation, a model based sliding mode controller (SMC) is synthesized, coupling with a nonlinear sliding mode observer required to estimate the unmeasured state information. This observer is formulated based on the reduced-order process model that leads to a process/model mismatch. Further, a saturation function is included in the sliding mode control theory in order to avoid the chattering effect. The performance of the proposed nonlinear observer based sliding mode controller is finally investigated by comparing with the conventional proportional integral (PI) controller in terms of servo (output voltage and fuel cell body temperature) and regulatory (load current) responses.