Thermal Design of a System for Mobile Powersupply

Ever more stringent emission regulations for vehicles encourage increasing numbers of battery electric vehicles on the roads. A drawback of storing electric energy in a battery is the comparable low energy density, low driving range and the higher propensity to deplete the energy storage before reaching the destination, especially at low ambient temperatures. When the battery is depleted, stranded vehicles can either be towed or recharged with a mobile recharging station. Several technologies of mobile recharging stations already exist, however, most of them use fossil fuels to recharge battery electric vehicles. The proposed novel zero emission solution for mobile charging is a combined high voltage battery and hydrogen fuel cell charging station. Due to the thermal characteristics of the fuel cell and high voltage battery (which allow only comparable low coolant temperatures), the thermal design for this specific application (available heat exchanger area, zero vehicle speed, air flow direction) becomes challenging and is addressed in this work. Experimental methods were used to obtain reliable thermal and electric power measurement data of a 30 kW fuel cell system, which is used in the Mobile Hydrogen Powersupply. Subsequently, simulation methods were applied for the thermal design and optimisation of the coolant circuits and heat exchangers. It is shown that an battery electric vehicle charging power of 22 kW requires a heat exchanger area of 1 m2, of which 60 % is used by the fuel cell heat exchanger and the remainder by the battery heat exchanger to achieve steady state operation at the highest possible ambient temperature of 43,6 °C. Furthermore, the simulation showed that when the charging power of 22 kW is solely provided by the high voltage battery, the highest possible ambient temperature is 42 °C. When the charging power is decreased, operation up to the maximum ambient temperatures of 45 °C can be achieved. The results of maximum charging power and limiting ambient temperature give insights for further system improvements, which are: sizing of fuel cell or battery, trailer design and heat exchanger area, operation strategy of the system (power split between high voltage battery and fuel cell) as well as possible dynamic operation scenarios.