Water injected fuel cell system compressor

Details Water injected fuel cell system compressor Water injected fuel cell system compressor

1999-04-05

2001-07-31

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S006000, C429S006000

Reexamination Certificate

active

06268074

ABSTRACT:

TECHNICAL FIELD
This invention relates to fuel cell systems using compressed air as the oxidant, and more particularly an energy efficient compressor and method for compressing the air.
BACKGROUND OF THE INVENTION
A fuel cell is an electrochemical device that continuously produces electrical energy from a fuel (e.g. hydrogen) and an oxidant (e.g. O
2
) supplied continuously from external sources. One such H
2
—O
2
fuel cell, for example, is the so-called proton exchange membrane (PEM) fuel cell which uses an ion exchange membrane as the electrolyte. Hydrogen for the anode of such fuel cells may be provided from hydrogen storage tanks, or generated from dissociated methanol, gasoline, hydrazine or the like while air is used as the oxidant on the cathode side of the cell.
In addition to the fuel cell itself, fuel cell systems require a variety of auxiliary equipment (e.g. pumps, heat exchangers, fuel processors, combustors, water separators, etc.) to support the operation of the fuel cell. One such piece of auxiliary equipment is an air compressor for supplying compressed air to the cathode side of the cell, and to other of the auxiliary equipment, as needed. Fuel cell system compressors may be of the so-called “dynamic” type such as centrifugal or turbine compressors that have rapidly rotating rotor(s) that increase the velocity and pressure of the gas moving therethrough. The fuel cell compressor may also be of the so-called “positive displacement” type that has one or more rotor(s) in close proximity to each other, or to a stator. Positive displacement compressors are well known in the art and include rotary machines such as scroll machines, vane machines and screw machines, roots blowers, among others, and are generally characterized by an arrangement of members connected and constructed so that they (1) define and fill a cavity which is formed at the inlet port, (2) trap gas in the cavity, (3) transport the gas in the cavity toward a discharge port, with or without compression enroute, and (4) expel the gas from the cavity to the outlet port by mechanical displacement.
Positive-displacement air compressors can take many forms, but generally fall into two main classes, i.e. “wet” and “dry”. “Wet” compressors, by design, have rotor(s) that engage each other, or a stator, across a film of lubricant (e.g. oil, water etc.) that is typically provided from a reservoir within the compressor. The lubricant prevents wear of the rotor(s)/stator and provides a liquid seal where the rotor(s) confront each other or a stator. The liquid seal retards backflow of the compressed gas into the compressor (i.e. reduces internal leakage). “Dry” compressors, on the other hand, by design, have rotor(s) which is/are closely spaced from each other, or from a stator, and have no sealing lubricant film therebetween. Rather, there is only a close clearance between the relatively moving parts, which clearance is typically maintained by timing gears or the like.
At low gas flow rates, wet compressors are generally more efficient than dry compressors, because of the moving liquid seal and the cooling effects provided by the lubricant. However, running a “wet” compressor dry (i.e. without the lubricant) would destroy it. Dry compressors, on the other hand, can run without lubricating the rotor(s), and are generally preferred for automobile-type fuel cell systems because they (1) require less input energy at their optimum design point than a wet compressor having the same capacity, (2) don't contaminate the oxidant gas, and (3) are not susceptible to freezing in cold weather applications. Moreover, dry compressors are suitable for high temperature operations, and for quick warm-up of a fuel cell system that has been allowed to stand idle and cool down. However, dry compressors work efficiently only at high gas flow rates, and high rotor speeds, which minimize internal leakage through the clearance spaces between the co-acting relatively moving parts (i.e. rotors and stators). At low gas flow rates (e.g. when the electrical demands on the fuel cell are low), dry compressors are quite inefficient because internal leakage becomes an increasingly larger percentage of the total flow through the compressor. An inefficient compressor, in turn, demands more fuel cell energy than an efficient compressor. In this regard, fuel cell system compressors are driven by electric motors that are energized by electric current withdrawn from the fuel cell. As such, the compressor drive motors are parasitic loads on the fuel cell system, in that the electrical current that they require must be subtracted from the current produced by the fuel cell which would otherwise be available to produce useful work (e.g. propel an automobile). Hence, the more inefficient the compressor, the greater the parasitic load that is placed on the fuel cell.
SUMMARY OF THE INVENTION
The present invention contemplates a fuel cell system employing a dry compressor to provide compressed air to the fuel cell, (and other system equipment, as may be needed), wherein the energy efficiency of the dry compressor is optimized, particularly at low compressor speeds, in order to reduce the parasitic load the compressor places on the fuel cell system. More specifically, the invention contemplates spraying controlled amounts of water directly onto the rotor(s) of the dry compressor to reduce the energy required by the compressor to compress a given amount of air. In this regard, the spray forms a thin film of water on the rotor(s) that quickly evaporates which, in turn, cools and densifies the air as it is being compressed and thereby reduces the amount of work required for compression. Moreover, in positive-displacement dry compressors, the water also forms a liquid seal in the compression zone between the relatively moving parts and/or stator that serves to reduce internal leakage within the compressor, and further improve compressor efficiency, especially at low compressor speeds.
According to one aspect of the invention, the fuel cell system comprises a fuel cell having a cathode inlet for admitting compressed air into the cathode side of the fuel cell, and a cathode outlet for exhausting water-containing cathode effluent from the fuel cell. The system includes a water collector downstream of the cathode outlet to collect water extracted (e.g. condensed) from the cathode effluent. A pump removes water from the water collector and directs it into a water injector connected to a dry compressor that provides the compressed air to the cathode inlet of the fuel cell over a range of operating pressures (e.g. about 5 psig to about 30 psig) suitable to the fuel cell. The compressor has one or more rotors that co-act either with each other, or with a stator, to compress the air passing therethrough. The injector is positioned in the compressor so that it can spray a mist of atomized water directly onto the compressor's rotor(s) where it serves to: (1) form an efficiency-enhancing liquid seal between relatively moving parts in positive displacement compressors; (2) cool/densify the air passing through the compressor; and (3) humidify the air. A mass flow meter measures the mass flow rate of the input air to the compressor. A controller communicates with the mass air flow meter and the injector, and is programmed to modulate the flow of water through the injector as a function of the mass flow rate of the input air into the compressor so as to optimize the energy efficiency of the compressor at a particular mass air flow rate.
According to one preferred embodiment of the invention, the fuel cell system also includes a valved conduit that communicates the compressor exhaust conduit (i.e. leading to the fuel cell and other auxiliary equipment) to the injector's water supply conduit (i.e. from the water pump) for purging water from the injector and its supply conduit using compressor output air when the fuel cell system is shut down and the compressor running on auxiliary power (e.g. a battery). Opening and closing of the purging line valve may be carried

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