FCS Dynamics

4 Design of Hydrogen Fuel Cell Systems for Road Vehicles 

First of all it is important to analyze the response rate to transient or dynamic requirements of all individual sub-systems, included the stack [50]. The electrochemical phenomena that occur inside the fuel cell stack are very fast, while water and thermal management systems show an enormous inertia with respect to typical vehicle requirements. In particular working temperature is a crucial parameter for designing BOP components such as heat exchangers, condensers and humidifiers devices to match dynamic requirements. Equation 4.8 could be used to demonstrate that the thermal capacity of the whole stack limits the dynamic in temperature change. 

On the other hand, fuel supply system response (when hydrogen is stored in high pressure tanks) results intrinsically faster than air supply system. The dynamics of this last sub-system appear particularly significant for the evaluation of dynamic performance of an overall FCS [48], as air compressor response is the limiting step for an adequate response of stack to load requirement changes. In particular the variations in air flow rates have to guarantee instantaneous stoichiometric ratios always not much lower than 2 during fast accelerations [49]. 

During start up, when the stack temperature and then performance (voltage) are low the passive BOP devices devoted to stream and stack humidification (enthalpy wheels membrane humidifiers) do not work as they are essentially based on the difference of temperature and then are practically excluded by the FCS management on the other hand, at low temperature the requirements for stack humidification are strongly reduced and membrane could self-hydrate if a sufficient water production (proportional to power requirement) occurs. 

The presence of water in the system makes FCS susceptible to freezing, and specific procedures are necessary to purge almost completely the stack when it stops during winter seasons in cold countries [43]. A portion of catalyst surface (especially at cathode side) could be covered with ice becoming not capable to carry 

---

The FC power system could be practically applied in transportation field if it demonstrated to own some essential qualifications, not limited to energy efficiency, but extended to operation durability, also in terms of water neutrality, and to FCS dynamic response during start-up and variable driving requirement phases. 

---