Patent ID: 7588845

Claim:
A system for controlling the temperature of a fuel cell stack, said system comprising: a coolant loop for circulating a cooling fluid through the stack; a radiator for cooling heated cooling fluid from the stack; a by-pass valve for selectively causing the cooling fluid to pass through or by-pass the radiator, said by-pass valve including a wax expansion element having a heater element that heats the wax element to cause it to expand; at least one temperature sensor for measuring the temperature of the cooling fluid and providing a measured temperature signal; and a controller for providing a heater signal to the heater element in the by-pass valve to control how much of the cooling fluid flows through the radiator and how much of the cooling fluid by-passes the radiator, said controller including a proportional derivative control unit responsive to a stack power loss signal, said proportional derivative control unit generating a proportional derivative signal based on the stack power loss signal, a comparator responsive to a predetermined temperature set-point signal and the measured temperature signal and providing a difference signal of the difference therebetween, a proportional integral control unit responsive to the difference signal and providing a proportional integral signal to minimize the difference signal, and a look-up table responsive to the measured temperature signal, said look-up table providing a preheat signal for preheating the heater element based on the measured temperature signal, wherein the controller adds the proportional derivative signal, the proportional integral signal and the preheat signal to provide the heater signal.