Patent Application
20070026271
The invention relates generally to fuel cell systems and more specifically to catalytically combusting an anode exhaust of a fuel cell, for example a Proton Exchange Membrane (PEM) fuel cell, to provide the heat to release hydrogen from a storage material.
Fuel cells, for example PEM fuel cells, are touted as the future of the automotive industry. Fuel cells electrochemically react a fuel, such as hydrogen, with an oxidant, such as air, to produce electricity and water. PEM fuel cells are ideally suited for use in automobiles or for in-home applications.
In order for fuel cells to become practical for use within automobiles, a storage solution must be demonstrated that will provide the necessary quantities of hydrogen to the fuel cell. One of the most common fuel cell and storage combinations is a PEM fuel cell with a metal hydride storage tank. In this system, the metal hydride storage tank is heated and stored hydrogen is released to the PEM fuel cell for electricity generation. A metal hydride must reach a certain temperature before it can release hydrogen. A metal hydride storage system has good volumetric storage density when compared to liquefied and compressed hydrogen systems. Good volumetric storage density is especially important for on-board vehicular storage because it would allow adequate hydrogen storage without taking up valuable space on the vehicle.
Several metal hydrides are available commercially, representing a good solution for hydrogen storage where weight and volume are not a significant problem, for example on buses. For most vehicles, however, the problem with metal hydride storage is the high weight of the material compared to the amount of hydrogen that is stored. The problem of weight has still not been solved in spite of extensive research. Researchers are therefore trying to think in new directions, by trying to lighten the alloys or by improving the methods of packing the hydrogen in higher concentrations.
Work is being done to find cheaper metal alloys that have the ability to absorb large amounts of hydrogen and at the same time release the hydrogen at a relatively low temperature. The International Energy Agency's (IEA) metal hydride program has a goal of developing a material that has a reversible storage capacity of 5 weight percent absorbed hydrogen and hydrogen release at less than 100° C., within the next few years. The Department of Energy (DOE) has a goal of developing a material that has reversible storage capacity of 9 weight percent absorbed hydrogen and hydrogen release at less than 100° C. by 2015, still considered to be an extremely aggressive target. Today's modern PEM fuel cells operate at relatively low temperatures, typically at about 80° C. Typically, the excess heat from the fuel cell is used to release the hydrogen from the metal hydride storage tank. Accordingly, it is widely assumed that the most practical applications would require the metal hydride storage tank to release hydrogen at about the same temperature that the fuel cell operates at, for example with PEM fuel cells, this temperature range would be from about 60° C. to about 80° C. It is widely believed that the energy efficiency of the system will be lower, and the system will be more complex, if extra heat must be independently generated to release the hydrogen from the tank.
Accordingly, there is a need to develop an improved fuel cell system that enables utilization of metal hydride storage tanks with higher hydrogen storage capacities without requiring independent heat generation to release the hydrogen from the metal hydride storage tanks.
A fuel cell system comprises a hydrogen storage system for storing and releasing hydrogen, a fuel cell in fluid communication with the hydrogen storage system for receiving released hydrogen from the hydrogen storage system and for electrochemically reacting the hydrogen with an oxidant to produce electricity and an anode exhaust. A catalytic combustor is in fluid communication with the fuel cell for receiving the anode exhaust and for catalytically reacting the anode exhaust to produce an offgas having an elevated temperature that is greater than the temperature of the anode exhaust. The heat from the offgas is used to release the hydrogen from the hydrogen storage system.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
A conventional fuel cell system 10 comprising a fuel cell 12 and a metal hydride storage tank 14 is shown in
As discussed above, a significant challenge associated with implementing fuel cell system 10 into an automobile is the weight of the metal hydride storage tank required to provide sufficient hydrogen to the fuel cell to enable adequate travel distances, for example greater than about 250 miles. Accordingly, a significant amount of research is currently being conducted around identifying reversible metal hydride materials that have a much higher hydrogen storage capacity. One additional difficulty in dealing with these systems is the operating temperatures of the fuel cells. PEM fuel cells operate at about 80° C. There are two factors that limit PEM fuel cells from operating at higher temperatures: 1) the current PEM devices cannot withstand higher operating temperatures without system degradation; and 2) the PEM fuel cells need to be kept at a temperature below the boiling point of water to ensure the system is adequately hydrated. Accordingly, the current operating temperature limit of an ambient pressure PEM system is about 80° C. There are certain advantages to operate at higher temperatures, and for this reason, there are many efforts to develop higher temperature PEM systems. Future advancements of the PEM fuel cell might permit operating temperatures to push upwards to about 150° C.
In order to meet these dueling concerns, researchers have focused on developing high capacity storage materials that release hydrogen at a relatively low temperature, for example less than 100° C. Even if the operating temperature of PEM fuel cells rises to 150° C., it is still not high enough to release most of the hydrogen stored in high-capacity hydrides. For example, the best metal hydride storage solution that releases hydrogen at temperatures less than about 150° C. is currently NaAlH4 with about 3.5 weight percent released at about 140° C. High capacity reversible metal hydride storage solutions for release at low temperatures are many years away. In fact, DOE has a goal of about 9% reversible storage capacity system, targeted at a release temperature of less than 100° C. in the year 2015. If either the weight limitations or the temperature restrictions were lifted, the implementation of these devices would surely accelerate.
Current metal hydride storage solutions exist that have a reversible storage capacity of greater than 7.5 wt. %, for example, 2LiBH4+MgH2, with a current capacity of about 10 wt % of H2. The release temperature for this material, about 400° C., however, is significantly higher than the operating temperature of PEM fuel cells. For this reason, most of these higher capacity materials have not been researched for use in PEM operated vehicles or other fuel cell applications.
In accordance with one embodiment of the instant invention, a fuel cell system 50 is shown in
Fuel cell 52, is typically a PEM fuel cell but can include a variety of other fuel cell types including but not limited to a phosphoric acid fuel cell, a solid oxide fuel cell or an alkali fuel cell. PEM fuel cells are typically associated with onboard or automotive applications, so many discussions within this application will focus on PEM fuel cells. While certain embodiments of this invention may primarily be discussed with reference to PEM fuel cells, this is not a limitation of this invention. An oxidant 58, typically air, and a fuel 60, typically hydrogen (H2), are introduced into fuel cell 52 and electrochemically react to produce electricity 62 and an anode exhaust 64 comprising water (H2O) and small quantities of unutilized H2, for example less than about 15% by volume of the anode exhaust 64, and typically less than about 10% by volume. Typical H2 utilization efficiency in a PEM fuel cell is less than about 90%, so there is always some percentage of H2 that cannot be converted inside the PEM fuel cell that is released via the anode exhaust 64. Anode exhaust 64 is typically so dilute in H2, and contains such large quantities of steam, that homogeneous combustion cannot efficiently be utilized to recover heat from the anode exhaust 64 to take advantage of this otherwise wasted energy. Instead, the anode exhaust 64 is typically used directly, at its existing temperature, around 80° C., to heat the hydrogen storage system to release the hydrogen.
In the instant invention, however, anode exhaust 64 is directed into catalytic combustor 54. The anode exhaust 64 is catalytically reacted to produce an offgas 66 having an elevated temperature, for example greater than about 150° C. and typically greater than 300° C. In some embodiments of the invention, the temperature of the offgas 66 is between about 300° C. to about 900° C. In other embodiments of the invention, the temperature of the offgas 66 is between about 300° C. to about 500° C.
In catalytic combustor 54, air is mixed with the anode exhaust 64 at a predetermined ratio and is fed to a combustion catalyst such as Pt/Al2O3, Pt—Pd/Al2O3, Pt—Rh/Al2O3, Pt—Ru/Al2O3, or Pt—Ir/Al2O3, for example. Once the constituents begin to catalytically react, the small amount of H2 in the anode exhaust 64 will react with the O2 in the air to generate heat. Depending on the H2 concentration of the anode exhaust 64, and the ratio of air to H2 feeding into the catalytic combustor 54, the temperature of the catalyst (typically a catalyst bed), and correspondingly the temperature of the offgas 66, can be controlled over a wide temperature range, for example from about 150° C. to about 900° C.
Hydrogen storage system 56 is typically a metal hydride storage system. While certain embodiments of this invention will discuss hydrogen storage system 56 as a metal hydride storage tank, this is not a limitation of this invention. In fact any hydrogen storage system that requires temperatures greater than about 80° C. to release stored hydrogen to fuel cell system 50 is contemplated within this invention. Hydrogen storage system 56 is in heat transfer relationship with offgas 66 such that the heat from the offgas 66 can be used to release stored hydrogen within hydrogen storage system 56. As discussed above, because the temperature of offgas 66 is substantially higher than the temperature of the anode exhaust 64 exiting fuel cell 52, metal hydride materials, some existing, some yet to be developed, having a reversible storage capacity greater than, for example, 7.5 wt % H2 can be used within hydrogen storage system 56. The metal hydride can be either a reversible hydride or a nonreversible hydride. An example of a reversible metal hydride is MgH2 that has a reversible hydrogen storage capacity of 7.6 wt. %. MgH2 requires about 300° C. temperature to absorb and release hydrogen. Such a hydride cannot be used in conventional fuel cell system 10, but can be used in the fuel cell system 50 of the instant invention. Another example of a reversible metal hydride storage material is a mixture of LiBH4 and MgH2 in a two to one ratio. The material has a demonstrated reversible hydrogen storage capacity of about 10 wt. %, but requires about 400° C. to absorb and release the hydrogen. Again, such a hydrogen storage material cannot be used in conventional fuel cell system 10, but can be used in the fuel cell system 50 of the current invention. One benefit of the increased temperature is that it allows new storage materials with higher absorption and adsorption temperatures to be considered for on-board storage solutions. One additional significant advantage of the increased temperature is faster kinetics that enables fast re-charge of H2. Ideally one would like to re-charge the H2 in less than 5 minutes, preferably less than 3 minutes. Many nonreversible high-capacity hydrides also require higher temperatures to release H2. An example is LiBH4 that can decompose to LiH and B and release about 13.8 wt. % H2. The decomposition temperature is about 280° C. that is not feasible for conventional fuel cell system 10, but can be used in the fuel cell system 50 of the current invention.
In addition to the above-mentioned benefits of the instant invention, fuel cell system 50 provides the following additional advantages: the higher temperature offgas 66 can also be used to vary the pressure of the metal hydride storage tank making it unnecessary to use a blower to provide the released H2 to the fuel cell 52; and an overall reduction in H2 released to the atmosphere as the catalytic combustor 54 will reclaim most of the H2 content of the anode exhaust 64.
In accordance with another embodiment of the instant invention,
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
