Patent Application
20080081238
The following discussion of the embodiments of the invention directed to a fuel cell system that employs a charge air cooler for reducing the temperature of the cathode inlet air sent to a fuel cell stack during certain system operating conditions so that the cathode inlet air is able to absorb more humidity in a water vapor transfer unit is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
Fuel cell systems typically include a stack cooling sub-system that drives a cooling fluid through cooling fluid flow channels in the fuel cell stack 12. The stack cooling fluid flows through a coolant loop 32 outside of the stack 12. The cooling fluid is pumped through the coolant loop 32 and the fuel cell stack 12 by a pump 34. The heated cooling fluid from the fuel cell stack 12 is cooled by a radiator 36 before it is returned to the stack 12 to gather waste heat. The radiator 36 would typically be part of a radiator sub-system that included a cooling fan (not shown). A line 38 directs some of the cooling fluid to the CAC 18, as will be discussed in more detail below.
As is well understood in the art, operation of the stack 12 produces significant water that is transferred to the cathode exhaust gas. If the stack 12 is operated below 100% humidity and the relative humidity set-point of the cathode inlet gas is not achieved, the present invention enables an increase of stack relative humidity without increasing the size of the WVT unit 20. This can occur during certain operating conditions of the system 10, such as at high loads.
According to this embodiment of the invention, the system 10 includes a gas/liquid heat exchanger 40 that receives the cooling fluid on the line 38 before it is sent to the CAC 18. The heat exchanger 40 reduces the temperature of the cooling fluid in the line 38 after the heat exchanger 40 so that the CAC 18 reduces the temperature of the cathode inlet air so that it is able to absorb more water vapor in the WVT unit 20. Therefore, even though the relative humidity of the cathode exhaust gas may be reduced, the ability of the cathode inlet air to absorb water is increased, so that the relative humidity requirements of the cathode inlet air can be maintained.
A three-way valve 42 is provided in the line 38 that is controlled to either send the cooling fluid flowing through the line 38 and the heat exchanger 40, as discussed above, or to by-pass the heat exchanger 40 on by-pass line 44 during those times where the relative humidity of the cathode inlet air is at the desired set-point, where the cathode inlet air does not need to be further cooled by the CAC 18. Particularly, if the cathode outlet gas is 100% or above, then the cathode inlet air typically only needs to be cooled to the temperature of the cooling fluid flowing through the stack 12 to meet the relative humidity requirements of the cathode inlet air. Control algorithms can be employed that use the temperature of the cathode inlet air from the CAC 18, as provided by the temperature sensor 26, and the relative humidity of the cathode inlet air, as provided by the relative humidity sensor 28, to control the valve 42.
The heat exchanger 40 can be any heat exchanger suitable for the purposes described herein. In one embodiment, the heat exchanger uses air to provide the cooling through the movement of the vehicle that receives ambient air on line 46. In one specific example, the heat exchanger 40 is the already existing wheel-house heat exchanger, known to those skilled in the art, positioned proximate to the wheels of a vehicle. In that design, the cooling fluid flow for the stack 12 would be re-plumbed in the vehicle so that it traveled through the heat exchanger 40. However, other heat exchangers can be employed in the system 10 consistent with the discussion herein, including those heat exchangers that may include a cooling fan. The amount of heat that is rejected by the heat exchanger 40 depends on the amount of airflow on line 46, i.e., the speed of the vehicle, and the ambient air temperature.
In an alternate embodiment, the valve 42 can be one or more valves that selectively allow some of the cooling fluid to by-pass the heat exchanger 40 and some of the cooling fluid to flow through the heat exchanger 40 to provide increased cathode inlet air temperature control. A suitable example for providing this type of design is to use two proportional or digital valves. Alternately, a valve can be provided in the line 38 or a valve can be provided in the by-pass line 44, and the pressure drop of the other line can be adapted to provide the flow distribution through the heat exchanger 40 or around the heat exchanger 40. Also, the heat exchanger 40 can be used to heat the cathode inlet air for reasons that will be discussed below
There may be certain operating situations where the temperature of the cathode inlet air is too low, such as during start-ups at ambient temperatures below 0° C., where it may absorb too much water vapor and possibly block the cathode flow channels in the fuel cell stack 12.
It may be desirable in certain designs to eliminate the valve 66 so that the CAC 62 is continuously maintaining the temperature of the inlet air at the temperature of the cooling fluid, and use the CAC 52 to decrease the temperature from the temperature of the cooling fluid during those times that the relative humidity of the cathode inlet air is below the RH set-point, as discussed above. Further, it may be desirable to make the valve 66 some type of proportional valve that can be selectively controlled to control the amount of the cooling fluid that travels through the CAC 62. Also, the combination of the CAC 62 and the valve 66 can be provided in the system 10 for the same purposes described herein.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
