An Analysis of the Altitude Impact on Roots Compressor Operation for a Fuel Cell System

Hydrogen fuel cell vehicles are one of the most promising alternatives to achieve transport decarbonization targets, thanks to their moderately high efficiency and low refueling time, combined with their zero-exhaust-emission operation. In order to reach reasonable power density figures, fuel cell systems are generally supercharged by radial compressors, which can encounter significant limitations associated with surge and choke operation, especially at high altitudes. Alternatively, the current paper explores the altitude operation of a fuel cell system combined with a Roots compressor. First, the balance of the plant model is built in the Simscape platform, combining a physical and chemical 1D fuel cell model for the stack, calibrated against literature data at different pressure and temperature values, as well as the characteristic maps of the Roots compressor. Then, the model is used to explore the balance-of-plant operation in a working range between 10 and 200 kW and an altitude range between sea level and 5 km. The results show that the compressor is capable of operating around the highest efficiency area (between 60 and 70%) for a wide range of altitude and power conditions, limiting the negative impact of the altitude on the system efficiency to up to 3%. However, once the compressor efficiency falls below 60%, the balance-of-plant performance rapidly drops, overcoming the benefits of the working pressure on the fuel cell stack operation and limiting the peak net power produced.