Development of Fuel Cell Hybrid Vehicle Rapid Start-up from Sub-freezing Temperatures

The Fuel Cell is a highly efficient device that when integrated with hybrid technology yields even higher system-level efficiencies. This impressive efficiency is one of the key reasons fuel cell technology is one of the most promising future power sources. However, this benefit creates a significant challenge in cold climates. With so much of the energy converted directly to power, there is little waste heat compared to conventional internal combustion engine (ICE) technologies. This challenge is particularly apparent at system start up from ambient sub-freezing temperatures due to the fact that the fuel cell heats-up slower than internal combustion engines (ICEs). Clearly, the amount of heat generation can be increased if the total power produced by the system is increased proportionally, but this method can be challenging because the excess power must be consumed in some manner (such as by a cabin heater). Toyota has resolved this issue with a “rapid start-up” methodology to speed warm-up during start by limiting fuel supply to increase its concentration overvoltage, thereby reducing efficiency and maximizing waste heat generation. At this operating point, power generation can be controlled to fulfill the system requirement while waste heat generation can be maximized as much as the fuel cell polarization curve allows. This method yields 10 to 20 times the waste heat generation compared to normal idle operation without using an additional heater unit. This rapid start-up operation method was realized as a stable vehicle startup system while resolving electro-circuit topology issues and also established the fuel concentration overvoltage control methodology for operating the fuel cell stack at low efficiency.