Patent Application
20080026274
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
In order to avoid the large temperature distribution within a cell during freeze start-up and enable fast start-up of fuel cell system, a coolant flow shifting operation system and methodology is proposed in accordance with the general teachings of the present invention.
Referring to
In this operating methodology, during start-up, subzero coolant is introduced to the fuel cell stack via a coolant outlet 16 and a coolant inlet 18 intermittently through an arrangement of a series of valves 20, 22, respectively, which can function as control valves.
In accordance with one aspect of the present invention, the auxiliary coolant system 10 is employed to provide, among other things, a coolant flow shifting capability during start-up. The auxiliary coolant system 10 primarily includes a reversible coolant pump 24, valve 20, and plumbing line 26 (e.g., by-pass line), respectively, for coolant flow shifting. A radiator 28 is shown as being in fluid communication with both the primary coolant system 14 and the auxiliary coolant system 10.
More specifically, the reversible coolant pump enables the flow direction of the coolant in the fuel cell stack system 12 to be reversed. It should be appreciated that the auxiliary coolant system 10 communicates with the primary coolant system 14 via valve 22.
By way of a non-limiting example, during start-up, when coolant flow is from right-to-left through the fuel cell stack system 12, i.e., in the direction of arrow F1, valve 20 will be used to control the temperature differential between the inlet and outlet of the stack and valve 22 will be partly open. The cold-coolant make-up from the primary coolant system 14 will be controlled by the relative opening of valve 20 and valve 22. Similarly, when flow is from left-to-right through the fuel cell stack system 12, i.e., in the direction of arrow F2, valve 20 will be used to control the temperature differential between the inlet and outlet of the stack and valve 22 will be closed. The coolant shifting cycle time should be determined by the heat-up requirement and the coolant volume of a stack plus the coolant volume of an auxiliary coolant loop at a given fuel cell system.
The shorter the coolant shifting cycle is, the faster the stack heats up and the less temperature distribution across the stack. To enable a faster coolant shifting cycle, it is preferable to minimize the coolant volume in the auxiliary coolant loop and the coolant volume in the stack headers and active area. The coolant volume in the auxiliary coolant loop can be minimized by closely coupling the auxiliary coolant loop with the stack. The stack header coolant volume can be reduced by filling the header area with low pressure drop and low thermal mass porous media, such as solid hollow spheres. It has been reported that coolant volume reduction in the stack active area can be achieved through stamped metal plate design.
Referring to
In this view, the simulation results are shown for temperature profiles in a fuel cell stack system during start-up with coolant flow shifting, as previously described. As with the system shown in
In the simulation, the coolant inlet temperature profile, as a function of time, was the same for both cases. As shown in
The present invention will provide numerous benefits for controlling temperature distribution through a fuel cell stack system, including but not limited to: (1) a reduction of the down-the-channel temperature distribution within a cell during freeze start-up, thus reducing the RH variation within the cell, prolonging MEA durability, and/or allowing for faster start-up; (2) a reduction of the temperature distribution within a cell during freeze start-up, thus reducing the impact of the temperature variation on the mechanical stress of the bipolar plate, DM and/or MEA; and (3) fast start-up of the entire fuel cell system.
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
