Design considerations for fuel cell powered UPS

In this paper design consideration for a fuel cell powered, passive stand-by single-phase UPS system is detailed. A commercially available Ballard Nexa (43-26 V, 1.2 kW) PEMFC fuel cell along with suitable DC/DC and DC/AC power electronic converter modules is employed. The proposed topology provides stable power to the load when the utility is interrupted. The design considerations include: obtaining an electrical equivalent circuit for fuel cell to facilitate the mathematical analysis of interactions between the fuel cell internal impedance and DC/DC converter. The analysis presented provides a design methodology to check for steady state and transient stability of fuel cell and DC/DC converter interaction in closed loop control to regulate the output DC voltage. For this example system it is shown that the fuel cell´s dominant time constant is load dependent and varies from 8.97 ms (light load) to 20.37 ms (full load) resulting in fuel cells relatively slow dynamic response. Design inequalities are reviewed to better understand the interaction between the DC/DC converter, fuel cell and potential instability conditions. Finally, a supercapacitor module is incorporated to overcome the electrical transients such as instantaneous power fluctuations, slow dynamics of the fuel processor and overload conditions. Energy stored in the supercapacitor is also utilized to handle a momentary overload for a short duration. It is shown that the supercapacitor value calculated for overload conditions are sufficient to enhance stability and improve dynamics respond of the fuel cell. Due to the absence of batteries the system satisfies the demand for an environmentally friendly clean source of energy. A complete design example illustrating the amount of hydrogen storage required for 1 hour power outage and sizing of supercapacitors for transient load demand is presented for a 1.5 kVA UPS.