Joint-cycle high-efficiency fuel cell system with power generating expander

Abstract

The efficiency of a combination reformer / fuel cell system is significantly improved by recapturing the energy value of heat generated in the fuel cell and producing additional power. The cooling water from the fuel cell is mixed, entirely or in part, with sufficient or excess compressed air, and at least partially evaporates in the compressed air. The air is at least sufficient to support the oxidative reactions in the fuel cell and also to serve as oxidant in a burner that provides heat to reform fuel / steam mixtures into hydrogen-containing reformate. This air / steam mixture, after leaving the fuel cell, is further heated by heat exchange with the reformats stream and reformate-producing modules, and with the exhaust stream of the burner. The steam / air mixture is injected into the burner, optionally after superheating in the burner exhaust, and is reacted with fuel in the burner. The burner exhaust may be used to provide heat to a fuel reforming reaction. The high-temperature burner exhaust may also be used to drive an expander, preferably a turbine, at a location in the system which is downstream of the burner, but in which the exhaust is at a high temperature so as to run the turbine efficiently. The turbine recovers heat energy from the fuel cell as mechanical energy, typically in excess of the energy required to run a compressor, because of the addition of steam to the compressed air. Moreover, system heat removal elements, such as radiators, as well as overall system size and cost, can be markedly reduced for a given level of output.

Description

RELATED APPLICATIONS [0001] This application is a continuation of U.S. application Ser. No. 09 / 870,412, filed May 30, 2001, which claims the benefit of Provisional Application No. 60 / 208,355, filed May 31, 2000. The entire teachings of the above applications are incorporated herein by reference.TECHNICAL FIELD OF THE INVENTION [0002] The present invention is directed to a fuel cell system having improved efficiency. Particularly, the present invention relates to methods and apparatus which use the otherwise waste heat of an operating fuel cell to do useful work. More particularly, the invention uses waste heat to create an air / steam mixture which drives an expander to produce mechanical energy, thereby improving the efficiency of the fuel cell system. BACKGROUND OF THE INVENTION [0003] Fuel cells have become useful energy sources over the last few decades, but wider use of fuel cells requires a continuous available source of hydrogen. One way to provide hydrogen for fuel cells in sm...

AI Technical Summary

Benefits of technology

This approach increases system efficiency by 5% to over 18%, typically ranging from 8% to 14%, by effectively recycling waste heat, reducing fuel consumption, and minimizing heat disposal needs, making the system more compact and cost-effective.

Problems solved by technology

The carbon monoxide (CO) is an unwanted byproduct, and, in addition, is difficult to eliminate sufficiently to avoid poisoning the fuel cell.

In a small or mobile system, this is not necessarily practical.

However, the heat generated by the latter two reactions is typically not at a high enough temperature, and may be inadequate in quantity, to run the former two reactions at reasonable rates.

Several U.S. patents teach the combination of turbines and fuel reformers, but these systems are limited to the recouping of some of the work used to pressurize reactants.

Despite improvements and refinements in such systems, the reformer / fuel cell combination is still not sufficiently efficient to be competitive with turbo-compounded internal combustion in many applications.

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