Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2001-10-24
2004-12-14
Kalafut, Stephen J. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000
Reexamination Certificate
active
06830842
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to fuel cell systems, and more particularly to an apparatus and method for recycling hydrogen fuel gas to a fuel cell stack.
BACKGROUND OF THE INVENTION
A fuel cell is a device that converts chemical energy directly into electrical energy and heat. In perhaps its simplest form, a fuel cell comprises two electrodes—an anode and a cathode—separated by an electrolyte. During use, the anode is supplied with fuel and the cathode is supplied with an oxidizer, which is usually oxygen in ambient air. With the aid of a catalyst, the fuel undergoes oxidation at the anode, producing protons and electrons. The protons diffuse through the electrolyte to the cathode where, in the presence of a second catalyst, they combine with oxygen and electrons to produce water and heat. Because the electrolyte acts as a barrier to electron flow, the electrons travel from the anode to the cathode via an external circuit containing an electrical load that consumes power generated by the fuel cell. A fuel cell generates an electrical potential of about one volt or less, so individual fuel cells are “stacked” in series to achieve a requisite voltage.
Because of their high efficiency, their potential for fuel flexibility, and their ability to generate electricity with zero or near zero emission of pollutants, fuel cells have been proposed as replacements for internal combustion engines in vehicles. Among fuels that have been considered for vehicle applications, hydrogen (H
2
) appears to be the most attractive. Hydrogen has excellent electrochemical reactivity, provides sufficient power density levels in an air-oxidized system, and produces only water upon oxidation.
FIG. 1
schematically shows a hydrogen-based fuel cell system
10
. The fuel cell system
10
includes a fuel cell stack
12
, which is made up of individual fuel cells
14
and includes cathode
16
and anode
18
terminals that are electrically connected via an external circuit
20
. The external circuit
20
includes a load
22
(e.g., electrical motor) which consumes power generated by the fuel cell stack
12
. Air (oxygen) and pressurized hydrogen enter the fuel cell stack
12
through cathode
24
and anode
26
gas inlets, respectively. The fuel cell stack
12
includes internal flow paths
28
,
30
, which distribute air and hydrogen to the cathode and anode of each fuel cell
14
. Oxygen-depleted air exits the fuel cell stack
12
through a cathode gas outlet
32
. Water, nitrogen, and unreacted hydrogen exit the fuel stack
12
through an anode gas outlet
34
.
As shown in
FIG. 1
, a first conduit
36
carries the anode gases (H
2
, N
2
, and H
2
O) away from the fuel cell stack
12
. A portion of the anode gas stream may vent into an exhaust line
38
through a draw-off valve
40
; a recycle line
42
returns the balance of the anode gas stream to the fuel cell stack
12
. Besides pressure losses from anode gas venting, frictional losses within the anode gas flow path
30
typically result in about a thirty kPa pressure drop across the fuel cell stack
12
. To overcome these pressure losses, the fuel cell system
10
employs a motor
44
driven blower
46
to boost the pressure of the anode gas within the recycle line
42
. For clarity, the motor
44
and blower
46
are depicted without an enclosure to show that a rigid shaft
48
transmits torque between the motor
44
and blower
46
. Furthermore, as indicated by an arrow
50
, a dynamic seal
52
reduces, but may not eliminate the flow of the anode gas from the blower
46
to the motor
44
.
Pressurized anode re-circulation gas exits the blower
46
through an outlet
54
and flows into a discharge line
56
, which directs the anode gas recycle stream into the anode gas inlet
26
of the fuel cell stack
12
. A second conduit
58
, which communicates with a hydrogen gas reservoir
60
or other source of hydrogen, introduces make-up hydrogen into the blower discharge line
56
. A control valve
62
and a mass flow meter
64
, which communicate with a flow controller (not shown), regulate the amount of hydrogen added to the anode gas re-circulation stream. During operation, a heat exchanger
66
removes excess heat generated by the blower motor
44
. The heat exchanger
66
typically comprises a fluid coolant loop
68
, which circulates the fluid coolant through the motor
44
housing.
Although the fuel cell system
10
shown in
FIG. 1
represents a useful scheme, existing motor
44
driven blowers
46
for fuel cell applications present several difficulties. Because hydrogen is a small molecule, the dynamic seal
52
may be unable to completely prevent H
2
from leaking into the blower motor
44
air space. In addition, water in the anode gas re-circulation stream may leak into the motor
44
housing, which may contaminate the motor lubricant and promote corrosion of motor parts. Finally, since the motor
44
generates a substantial amount of heat, a relatively large heat exchanger
66
must be used, which adds to the bulk and expense of the fuel cell system
10
.
The present invention overcomes, or at least helps mitigate, one or more of the problems set forth above.
SUMMARY OF THE INVENTION
The present invention provides a fuel cell system that can be used to power a vehicle. The system includes a fuel cell stack, which uses hydrogen and an oxidizer (typically oxygen in ambient air) to generate electricity. The system includes a re-circulation loop for returning unreacted hydrogen, along with water and nitrogen, to the fuel cell stack, and a hermetically sealed assembly, which comprises a blower portion for pressurizing hydrogen in the re-circulation loop and a motor portion for driving the blower.
The system also includes a source of make-up hydrogen for replenishing hydrogen in the re-circulation loop. The source introduces make-up hydrogen in the motor portion of the assembly at a pressure greater than the pressure in the blower portion of the assembly. As a result, at least some of the make-up hydrogen flows from the motor portion of the assembly into the blower portion assembly, which helps prevent components in the re-circulation loop from entering the motor portion of the assembly. Make-up hydrogen purges the motor of undesirable compounds (e.g., water and oxygen) and removes heat generated by the motor and controller (if present). Passing make-up hydrogen through the blower portion of the assembly preheats the make-up hydrogen and, in some cases, obviates the need for a separate heat exchanger.
The present invention also provides an apparatus for replenishing hydrogen in a fuel cell stack. The apparatus includes a re-circulation loop for returning unreacted hydrogen to the fuel cell stack, and a hermetically sealed assembly comprising a blower portion and a motor portion. The blower portion of the assembly, which communicates with the re-circulation loop, pressurizes hydrogen in the re-circulation loop, and the motor portion of the assembly drives the blower. The apparatus includes a source of make-up hydrogen, which is adapted to introduce hydrogen in the motor portion of the assembly at a pressure greater than the pressure in the blower portion of the assembly.
Finally, the present invention provides a method of replenishing hydrogen in a fuel cell stack. The method comprises re-circulating unreacted hydrogen from an outlet to an inlet of the fuel cell stack using a motor-driven blower. The motor, which is hermetically coupled to the blower, has a flow path that provides fluid communication between the motor and the blower. The method thus includes introducing make-up hydrogen in the motor at a pressure higher than the pressure in the blower. Make-up hydrogen flows within the motor and through the flow path into the blower where it mixes with unreacted hydrogen.
REFERENCES:
patent: 4075396 (1978-02-01), Grehier
patent: 4769297 (1988-09-01), Reiser et al.
patent: 2002/0098393 (2002-07-01), Dine et al.
patent: 2002/0142208 (2002-10-01), Keefer et al.
patent: 2004/0023084 (2004-02-01), Sterchi et al.
Dumke Ulrich
Siepierski James S.
Brooks Cary W.
General Motors Corporation
Kalafut Stephen J.
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