Design of a mobile PEM power backup system through detailed dynamic and control analysis

In this paper we present a one dimensional dynamic model of a PEM fuel cell applied to the design of a mobile backup system for uninterruptable power units. The fuel cell is modeled using a finite difference approach where mass and energy balance equations are applied locally together with the pertinent equations of the electrochemical model yielding the profiles of any relevant thermodynamic and electrochemical cell variable. An accurate analysis of the membrane humidification is included based on state of the art models available in literature. In this system the fuel cell is fed by pure hydrogen taken from a lithium hydride hydrogen storage while ambient air is supplied to the cathode by an inverter-fed electric motor fan. A preliminary design of the main components is provided for a target operating time of 48 h. Dynamic simulations are then carried out applying to the fuel cell the actual electricity load of a computer file server that was experimentally measured by a power measurement device over a period of 0.5 h. Results of the simulations show that with an appropriate choice of the controllers parameters it is possible to maintain effective cell operation under different load variations keeping the key variables of the fuel cell within the desired set point targets.