Vehicle Fuel Cell Freeze Protection

Although many researchers, including those at the FCRL, are working on fuel cell stacks that can withstand freezing temperatures without damage, commercially available stacks like those used in UD's fuel cell buses will generally sustain serious damage if frozen. This is not a serious issue in warm climates like those where most fuel cell vehicle trials have occurred, but in Delaware, a fuel cell vehicle must either be stored above 0 °C at all times or actively protected from freezing.

On UD's fuel cell buses, a dual freeze protection system has been fitted; both parts use electric resistance heating to keep critical parts above freezing, and insulation is applied where possible to reduce the power required. The low-power system protects only the fuel cell stack, using either heating blankets (on the Phase 1 bus) or immersion heaters and a pump to heat the stack from the inside out using its coolant (on Phase 2). It is powered by the vehicle's 12 volt auxiliary electrical system. The 12 volt system is charged as needed, even when the vehicle is off, by a power converter fed by the traction batteries. Starting with full batteries, the bus can be left unplugged in freezing temperatures for at least a week without damage to the fuel cell.

Even with the stack kept above freezing, damage could still occur if the fuel cell system is started up with some parts of the cooling system below freezing: sub-freezing coolant, circulating through the stack before it begins to produce heat, could freeze it from within. The higher-powered freeze protection system is designed to combat this by keeping the entire fuel cell coolant loop above freezing, except for the radiator. Since it demands more power than available from the 12 volt system, it runs on 120VAC line power, and operates only when the bus is plugged in for charging. It uses an immersion heater and pump to heat and circulate the coolant. It is controlled by two coolant temperature switches, one located on the roof of the bus, and the second located just before the coolant inlet to the heater. When the bus is subjected to cold temperatures, the coolant on the roof is the first to drop below the temperature setpoint, tripping the switch there and turning on the heating system. Heated coolant begins to circulate, moving up to the roof, while cold coolant returns to the heater, so that the second temperature switch is turned on, keeping the heater on until warm coolant has circulated around the entire loop.

The circulation pump and heater unit of the freeze protection system on the Phase 2 bus.