1. Technical Field
The invention relates to a source of energy. In particular, the invention relates to a fuel cell configured to minimize the internal and external gas leakage of the reactant gases.
2. Background of Related Art
Due to an increasing demand on the earth's limited energy resources and to low conversion efficiencies of conventional power generation systems as well as environmental concerns, the need for a clean and reliable, alternative source of energy has greatly escalated. Fuel cells have been considered for power generation applications for years. Many innovative improvements in operational performance capability have been achieved. Efficiencies have been increased; water-management problems have been resolved; and the use of proton exchange membranes with reduction of the thin film catalyst layers has been achieved.
Fuel cell assemblies with proton exchange membrane cells, in which a hydrogen-oxygen reaction is employed for power generation, have become a popular source of energy in an automobile industry. Unfortunately, the development of suitable stacked assemblies using the proton exchange membrane fuel cell has been subject to various problems, mainly related to corrosion and leakage.
To obviate the corrosion problem, proton exchange membrane (PEM) fuel cells have been overwhelmingly manufactured with bipolar plates made from graphite, not metal. However, increased resistance to corrosion comes at a penalty of an increased porosity leading to an internal leakage through the graphite bipolar plates, and reduced structural integrity leading to an external leakage due to the inherent brittleness of graphite. In additional graphite bipolar plates have relatively low electrical conductivity leading to substantial energy losses, poor machinability and, thus are cost-inefficient.
To generate sufficient power for industrial use, a number of fuel cells have to be combined together in a series to increase the current proportionally to the electrode area. A major problem of the fuel cell stack is that the internal and external leakage of the reactant gases stem partially from the use of the brittle graphite plates that cannot withstand high pressure and partially from structural drawbacks of the known fuel cell stack structures.
In a typical structure of a fuel cell pack, forces generated during assembling the fuel cell stack are applied to peripheral regions of base plates flanking a plurality of stacked bipolar plates. Accordingly, when the assembly tie rods are tightened up, the base plates each are deformed and have an outwardly curved cross-section, resulting in the non-uniform distribution of the compressing forces creating passages that connect reactant gases together and can cause a leak or explosion. As a consequence, the deformed base plates do not apply sufficient compression forces to those regions of the bipolar plates that are traversed by gas-conveying conduits. Hence, the risk of external leakage is increased.
Furthermore, to simply apply greater forces is not practical because a) the forces would still be applied to the periphery, not to the inner regions of interest of the base plates, and b) greater forces can eventually crack graphite bipolar plates, which, as known, are brittle.
To increase the capacity of the fuel cell, it is necessary to have a great number of the stacked bipolar plates membranes and electrodes, which all have to be compressed to avoid the external and internal leakages, as discussed above. To accomplish this, opposite base plates have numerous circular holes traversed by tie rods, which, as a rule, have their heads provided with a circular cross-section. Accordingly, the application of a torque to the rod's head may be ineffective because the entire rod can simply keep rotating in response to the torque. Furthermore, guiding the tie rods through numerous holes can be an onerous job because of the substantial height of a fuel cell.
It is, therefore, desirable to optimize the design of fuel cell stack in a manner minimizing the risk of the internal and external leakage of the reactant gases and to provide a simple and reliable structure facilitating the assembly of the fuel cell stack.