U.S. patent application number 10/250844 was filed with the patent office on 2004-12-09 for two stage electrically powered compressor.
Invention is credited to Muenz, Stefan, O'Hara, Steve, Pflueger, Frank, Walther, Karl.
Application Number | 20040247461 10/250844 |
Document ID | / |
Family ID | 33489147 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040247461 |
Kind Code |
A1 |
Pflueger, Frank ; et
al. |
December 9, 2004 |
Two stage electrically powered compressor
Abstract
An electrically powered compressor designed for providing high
pressure ratio at low flow rate, yet having low power consumption.
The compressor is designed for optimizing efficiency in fuel cell
systems. The inventive compressors are however not limited to fuel
cell system applications.
Inventors: |
Pflueger, Frank; (Karlsruhe,
DE) ; Muenz, Stefan; (Ludwigshafen, DE) ;
Walther, Karl; (Carmel, IN) ; O'Hara, Steve;
(Zionsville, IN) |
Correspondence
Address: |
Greg Dziegielewski
Borg Warner Inc
Powertrain Technical Center
3800 Automation Avenue Suite 100
Auburn Hills
MI
48326-1782
US
|
Family ID: |
33489147 |
Appl. No.: |
10/250844 |
Filed: |
July 8, 2003 |
PCT Filed: |
November 8, 2001 |
PCT NO: |
PCT/US01/46616 |
Current U.S.
Class: |
417/407 ;
417/423.1 |
Current CPC
Class: |
F04D 17/122 20130101;
F04D 25/06 20130101 |
Class at
Publication: |
417/407 ;
417/423.1 |
International
Class: |
F04B 017/00 |
Claims
I claim:
1. A two stage centrifugal compressor assembly comprising: a rotor
shaft (20) having first and second ends; a first compressor casing
(24) having a fluid inlet (25) and a fluid outlet (26); a first
impeller (32) rotatable within the first compressor casing (24); a
second compressor casing (24') having a fluid inlet (28) and a
fluid outlet (29); a second impeller (32') rotatable within the
second compressor casing (24'); a motor disposed between the first
and second compressor casings and comprising a stator (23, 23') and
a rotor (22, 22') rotatable within the stator; wherein the first
impeller (32), second impeller (32') and rotor (22, 22') are
mounted fixed against rotation on the rotor shaft (20) and
rotatable therewith, and wherein the fluid outlet (26) of the first
compressor casing communicates with the fluid inlet (28) of the
second compressor casing.
2. A compressor assembly as in claim 1, further comprising a pulley
(21) mounted fixed against rotation on the rotor shaft (20).
3. A compressor assembly as in claim 1, wherein said motor is a
magnetically loaded composite motor.
4. A compressor assembly as in claim 1, wherein said magnetically
loaded composite motor rotor (22, 22') and stator (23, 23') serve
as a magnetic bearing system.
5. A compressor assembly as in claim 1, further comprising a bypass
conduit (30) for bypassing said first compressor casing fluid inlet
(25), and valve means for regulating flow through said bypass
conduit.
6. A two stage centrifugal compressor assembly comprising: a rotor
shaft (20); a first compressor casing (24) having a fluid inlet
(25) and a fluid outlet (26); a first impeller (32) rotatable
within the first compressor casing (24); a second compressor casing
(24') having a fluid inlet (28) and a fluid outlet (29); a second
impeller (32') rotatable within the second compressor casing(24');
a motor disposed between the first and second compressor casings
and comprising a stator (23, 23') and a rotor (22, 22') rotatable
within the stator; wherein the first impeller (32), second impeller
(32') and rotor (22, 22') are mounted fixed against rotation on the
rotor shaft (20) and rotatable therewith, and wherein the fluid
outlet (26) of the first compressor casing communicates with the
fluid inlet (28) of the second compressor casing; and wherein said
motor is a magnetically loaded composite motor.
7. A compressor assembly as in claim 6, wherein said magnetically
loaded composite motor rotor (22, 22') and stator (23, 23') serve
as a magnetic bearing system.
8. A compressor assembly as in claim 7, wherein said assembly
further comprises a pulley (21) mounted fixed against rotation on
said rotor shaft (20), wherein said assembly comprises first and
second motors, and wherein said first and second motors are
provided on first and second sides of said pulley.
9. A compressor assembly as in claim 6, further comprising a bypass
conduit (30) for bypassing said first compressor casing fluid
inlet, and valve means for regulating flow through said bypass
conduit.
10. A power generating system comprising: a fuel cell, an electric
motor (2) driven first stage centrifugal compressor (4), said
compressor having a fluid inlet and a fluid outlet, an electric
motor (1) driven second stage centrifugal compressor (3), said
compressor having a fluid inlet and a fluid outlet, wherein said
first stage centrifugal compressor fluid inlet is in communication
with a source of oxidizing gas, wherein said first stage
centrifugal compressor fluid outlet is in communication with said
second stage centrifugal compressor fluid inlet, and wherein said
second stage compressor fluid outlet is in communication with said
fuel cell.
11. A power generating system as in claim 10, wherein said
oxidizing gas is air.
12. A power generating system as in claim 10, wherein said first
and second electric motors (1, 2) are driven by a source of
feedback selected from fuel cell electrical output, fuel cell fuel
consumption, fuel cell temperature, and operator input.
13. A power generating system as in claim 10, wherein said fuel
cell is provided in a hybrid vehicle.
14. A power generating system as in claim 10, further including a
catalytic burner for decomposing any hydrocarbons, unburned fuel,
nitric oxide, carbon monoxide and particulates in the exhaust
stream leaving the fuel cell.
15. A power generating system as in claim 10, further comprising a
bypass conduit for bypassing said first compressor casing fluid
inlet, and valve means (5) for regulating flow through said bypass
conduit.
16. A method for imparting to a gas a high pressure ratio and low
flow rate, with low power consumption, said method comprising:
precompressing gas in an electric motor (2) driven first stage
centrifugal compressor (3), said compressor having a fluid inlet
and a fluid outlet, conveying gas precompressed in said first
compressor (4) to an electric motor (1) driven second stage
centrifugal compressor (3), said compressor having a fluid inlet
and a fluid outlet, further compressing said gas in said second
stage centrifugal compressor.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention is concerned with improving the
efficiency of operation of a fuel cell system. More specifically,
the invention is concerned with an electrically powered compressor
designed for providing a high pressure ratio at a low flow rate,
and with low power consumption. The inventive compressors are
however not limited to fuel cell system applications.
[0003] 2. Description of the Related Art
[0004] Fuel cell systems are being developed to generate
electricity to power vehicle accessories or as drive systems for
propelling vehicles.
[0005] For example, German Patent DE 40 32 993 C1 teaches a fuel
cell system wherein a proton-conducting electrolyte membrane (i.e.,
proton exchange membrane or PEM) is located between two electrodes
- a cathode to which oxidizing gas is supplied, and an anode to
which fuel gas (e.g., H.sub.2 and CO.sub.2) is supplied. The PEM
acts like an electrolyte for transport of hydrogen ions obtained at
the anode of the fuel cell, towards the cathode, in the form of
protons (H.sup.+). Electricity generated by the energy conversion
reaction is collected, and excess gas which has not been consumed
is exhausted.
[0006] DE 40 21 097 A1 teaches a fuel cell system in which the
exhaust air from the fuel cell is conducted to an expansion
turbine. The expansion turbine is coupled with a fresh air
compressor for boosting the pressure of air supplied to the fuel
cell.
[0007] EP 0 629 013 B1 and DE 43 18 818 A1 teach the use of an
electric motor driven compressor to boost the fuel cell fresh air
intake pressure. Compressing intake air to the usual working
pressure of, e.g., 3 bar, consumes approximately 20% of the power
developed by the fuel cell. To recover the energy contained in the
exhaust air from the fuel cell, the electric motor driven
compressor is coupled with an expander mounted on the same shaft as
the compressor. When an expander is used for energy recovery, the
energy expended in compressing air drops to about 10 to 15%. There
remains a need for further improving the efficiency of the
system.
[0008] It is also known to provide a catalytic burner to reduce
environmental emissions. Fuel is supplied to the catalytic burner
in the form of (1) moist anode offgas from the fuel cell, and (2)
methanol. In DE 40 32 993 C1 the combustion gases generated in the
catalytic burner pass through a gas turbine connected downstream to
drive a compressor to compress oxygen containing gas (e.g., air)
supplied to the catalytic burner. Thus, the fuel cell offgas and
the catalytic burner offgas pass through separate respective
expanders.
[0009] U.S. Pat. No. 6,190,791 (Hornburg) teaches that there is a
need for higher working pressures on the cathode side (i.e., air
side) in order to build a smaller fuel cell having narrower gas
channels and to achieve a higher area-related power yield in the
fuel cell. The approach taken by Hornburg involves designing the
system in such a way as to increase air mass flow, increase
temperature, and increase pressure of the exhaust gas going into
the expander. This is accomplished by (1) initially supplying the
air at the cathode outlet of the PEM fuel cell as the air supply to
the catalytic burner before. expansion, and (2) operating the
expander with the exhaust air from the catalytic burner. As a
result of the additional energy input to the expander in the form
of heat and mass flow, its performance is increased to the point
where the compressor drive (e.g., electric motor and rectifier) can
be made much smaller. Hornburg further teaches that, with an
optimum adjustment of the pressure level and/or additional supply
of combustion gas in the catalytic burner, the compressor drive can
even be completely eliminated entirely.
[0010] Hornburg also teaches an embodiment in which the combustion
gas for the PEM fuel cells is generated by a high-pressure (15 to
30 bar) gas-generating system. This pressure is harnessed by means
of a second expander/compressor stage upstream from the cathode
input of the PEM fuel cell, with the gas pressure dropping from the
system pressure of the high-pressure gas-generating system to the
working pressure of the catalytic burner (approximately 3 bar). The
second compressor stage is in the form of a compressor coupled with
an expander, without an electric motor or a turbine.
[0011] While Hornburg does achieve higher working pressures, the
inventors considered that there must be a simpler, more reliable
and more responsive way to generate these pressures. Further, the
Hornburg systems are designed for high flow rate. The inventors
considered that, when supplying fresh air to fuel cells, there is a
problem in that in fact only small volume flows are required,
though at high pressure ratios. Conventional one-step flow
compressors require extremely high speeds in combination with high
operational energy input to achieve such high pressures at low
volume flows, a feat which cannot be accomplished with the electric
engines available today.
[0012] Accordingly, a second aspect of the present invention
concerns the development of a compressor capable of providing high
pressure at low volume flow, which is reliable, economical to
construct, highly responsive and easily regulated.
[0013] The inventors first contemplated various electric motor
driven compressor assemblies. U.S. Pat. No. 6,193,473 (Mruk, et al)
teach that a major drawback associated with the use of electric
motors to drive rotating impeller compressors has been the linkage
between the electric motor and the compressor impeller. A given
compressor will have a specific speed of rotation of the impeller
in order to achieve the compression duty required of it. At the
same time, an induction electric motor will have an optimum speed
of rotation, at which the torque output is at a maximum (and which
speed of rotation is generally far lower than the operating speed
of high speed centrifugal compressors). Heretofore, in order to
link the compressor with a suitable electric drive motor, it has
been necessary to employ an arrangement of one or more expensive
gear assemblies in the compressor drive. In this way the different
optimum speeds of rotation of the compressor and the electric motor
can be accommodated.
[0014] Mruk et al have the objective of providing an electric motor
driven compressor with no gearing and wherein the electric motor
and compressor are directly linked. Mruk et al accomplish this by
using a switched reluctance motor to drive the rotating centrifugal
impeller(s). The Mruk et al compressor assembly preferably
comprises first and second compressors housed in separate
compressor casings, mounted on opposite ends of a common drive
shaft assembly and rotatable therewith. The first and second
compressors may be driven by the same switched reluctance motor.
The fluid outlet of the first compressor casing may communicate
with the fluid inlet of the second compressor casing, forming a
two-stage compressor assembly. In such an arrangement, the switched
reluctance motor is most conveniently disposed between the first
and second compressor casings, with the rotor of the switched
reluctance motor being mounted on the drive shaft assembly between
the first and second impellers.
[0015] Considering however the task of the present invention, such
an arrangement would be expensive, difficult to regulate
particularly at lower pressures, difficult to integrate into a
vehicle propulsion system, and difficult to repair.
[0016] Further, with both compressors being driven by the same
shaft, response and output may not be optimal in the case of
rapidly changing traffic conditions.
SUMMARY OF THE INVENTION
[0017] It has now been discovered that the first task of the
invention can be accomplished by connecting two electrically
powered flow compressors in series (see FIG. 1). These compressors
can be optimally coordinated with each other corresponding to the
operating requirements of the fuel cell. Each individual flow
compressor in the system operates at clearly reduced speeds and
reduced electrical power consumption as compared to single stage
compressors.
[0018] The second task of the invention has been achieved by
providing a two stage or sequential compressor driven by a single
electric motor, and preferably also connected to an internal
combustion engine of a hybrid combustion/electric vehicle via a
belt or pulley system (see FIGS. 2 and 3). The compressor electric
motor is preferably constructed using magnetically loaded composite
(MLC) rotor technology. In a preferred embodiment, the pulley is
located centrally on the rotor shaft, first and second MLC motors
are provided on opposite sides of the pulley, and first and second
compressor wheels are provided outboard of the MLC motors, on the
first and second ends of the rotor shaft. This specially designed
two stage or sequential compressor is particularly suited for use
with fuel cell systems, but has numerous other applications.
[0019] The foregoing has outlined rather broadly the more pertinent
and important features of the present invention in order that the
detailed description of the invention that follows may be better
understood, and so that the present contribution to the art can be
more fully appreciated. Additional features of the invention will
be described hereinafter, which form the subject of the claims of
the invention. It should be appreciated by those skilled in the art
that the conception and the specific embodiments disclosed may be
readily utilized as a basis for modifying or designing other
electrically powered compressors for carrying out the same purposes
of the present invention. It should also be realized by those
skilled in. the art that such equivalent structures do not depart
from the spirit and scope of the invention as set forth in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] For a fuller understanding of the nature and objects of the
present invention reference should be made by the following
detailed description taken in with the accompanying drawings in
which:
[0021] FIG. 1 is a schematic showing two electric motor driven
compressors connected in series for providing regulated high
pressure low volume air flow to a fuel cell;
[0022] FIG. 2 shows a partial sectional view of a preferred
two-stage compressor particularly suited for use with a fuel cell
system; and
[0023] FIG. 3 shows the two stage compressor of FIG. 2 in
diagrammatic form showing a belt attached to the pulley.
DETAILED DESCRIPTION OF THE INVENTION
[0024] In the first aspect of the invention, the problem of
supplying air to the fuel cell at high pressure yet low flow rate
and low compressor power consumption is solved by connecting two
separate compressors in series, as shown in FIG. 1, wherein each
compressor is independently driven an electric motor. This
arrangement makes it possible to operate each individual flow
compressor in the system at clearly reduced speeds and reduced
electrical power consumption. Further, these compressors can be
optimally coordinated with each other corresponding to the
operating requirements of the fuel cell.
[0025] A control valve is provided between the first and second
compressor. This makes it possible to bypass the first compressor,
operating only the second compressor, as desired.
[0026] The design of a low power consumption compressor system for
provision of high pressure, low flow rate for a fuel cell according
to the present invention is different from prior art systems such
as disclosed in, e.g., U.S. Pat. No. 6,079,211, wherein a
supercharging system is provided for an internal combustion engine.
In the system disclosed therein, a first compressor is driven by an
electric motor, and a second compressor is driven by an exhaust gas
driven turbine and also optionally by an electric motor. When the
throttle is opened to accelerate the engine, both electric motors
are super-energized for a short period of time to compound boost
pressure to the engine. While supercharger arrangement disclosed in
the patent is designed for rapidly boosting both pressure and flow,
the present invention is designed for boosting pressure
continuously at low flow rate, and with minimal power
consumption.
[0027] Although not necessary, it is possible in accordance with a
preferred embodiment of the invention to recover some of the energy
contained in the exhaust air from the fuel cell and, if present,
the catalytic burner, by providing an expander in the exhaust line.
The expander could be coupled to the electric motor driven
compressor, could be in-line or in series with the electric motor
driven compressors, or could be in parallel with the electric motor
driven compressors.
[0028] In a further preferred embodiment of the invention, which is
illustrated in FIGS. 2 and 3, both compressors are provided on the
same rotor shaft, and the shaft is driven by an electric motor. The
motor is preferably located between the compressors. More
preferably, a pully is additionally provided centrally on the
shaft, with first and second electrical motors (induction motors,
preferably magnetically loaded composite (MLC) motors), provided on
either side of the pulley, and compressor wheels provided on the
first and second ends of the rotor shaft.
[0029] The two-stage compressor of the present invention represents
an improvement over the closest prior art two-stage centrifugal
compressor assemblies as disclosed for example in U.S. Pat. No.
6,193,473 (Mruk et al), since Mruk et al drive the compressors via
a switched reluctance motor disposed between the first and second
compressor casings and comprising a stator and a rotor rotatable
within the stator.
[0030] The present invention in contrast provides a centrally
located pulley so that the compressors may be driven by the main
engine, and in addition provides magnetically loaded composite
(MLC) motors on either side of the pulley. MLC is a product that
incorporates magnetic material into high strength and high
integrity fibrous composite structures. One example of such a motor
is disclosed in U.S. Pat. No. 5,477,092 (Tarrant), and one
commercial source of suitable rotors is Urenco Ltd., Marlow, UK,
MLC. As far as the present inventors are aware, literature
describes MLC as useful for motor generator rotors, high surface
speed generators, flywheels, dynamometers, self-driven rollers,
transducers (linear, rotary and acoustic), actuators (linear and
rotary), and magnetic bearings (passive and active), but MLC has
never been used for the purposes of the present invention. Benefits
of MLC include weight reduction, simplified integral design, high
speed, low inertia, greater quietness both mechanically &
electrically, potential to reduce motor air gaps, reduced high
frequency losses, i.e., no laminations, versatile magnetic patterns
and numbers of poles, no PM stray fields, elimination of the back
iron requirement, magnetically anisotropic/isotropic, high specific
strength and stiffness, and good resistance to corrosion and
chemicals. Further, in the present invention, the MLC motors
provide a magnetic bearing system.
[0031] The rotor is preferably connected via a one way bearing to a
pulley, which allows a belt to provide compressor power and
generator drive from the engine crankshaft pulley at high
compressor/generator speeds and high levels of compressor power
consumption.
[0032] The electric motor(s) could easily be switched via power
electronics to function as an electricity generator.
[0033] The fuel cell system of the present invention could be a
solid oxide fuel cell as described in U.S. Pat. No. 6,230,494
(wherein oxygen in the air ionizes to O.sup.-2, producing
electricity), a reformation fuel cell as described in U.S. Pat. No.
6,232,005, a proton exchange membrane fuel cell as described in
U.S. Pat. No. 6,190,791, or any of the various known types, so long
as the fuel cell operates under elevated air pressure.
[0034] Turning now to FIG. 1, two electric motor driven compressors
are shown connected in series for providing regulated high pressure
low volume air flow to a fuel cell. Initially, when the system is
cold and is being started up, both electromotors 1 and 2 may be
energized by an external source (e.g., a battery) to drive the
compressors 3 and 4 to provide the necessary system air.
Alternatively, valve 5 may be opened so that the first compressor 4
is bypassed. Air (or any oxidizing gas), preferably boosted to 3
bar, leaves compressor 3, is introduced into conduit tube 6 and
preferably passes through a tubular heat exchanger 7 where it is
warmed prior to being directed to the cathode side of the fuel cell
BZ. Fuel gas 8 (e.g., H.sub.2 and CO.sub.2) is supplied to the
anode. Once the reaction has established itself, electricity
generated in the fuel cell BZ is used to drive motors 1 and 2.
[0035] Preferably, a burner catalyst KatBr is provided to remove
any hydrocarbons, unburned fuel, nitric oxide, carbon monoxide and
particulates from the exhaust stream prior to exiting the system,
and also to generate heat which may be used to pre-heat system air
in heat-exchanger 7. The burner catalyst KatBr may be brought
on-line using a fuel source such as methanol prior to startup of
the fuel cell, in order to provide for pre-heating the effluent
stream to the fuel cell.
[0036] In order to maintain the fuel cell at idle (e.g., when the
internal combustion engine of a hybrid vehicle is being used for
highway driving), bypass valve 5 may be open with only electromotor
1 running. As the vehicle transitions to city driving, the internal
combustion engine may be shut down and the fuel cell may be
operated at high output. For this, both electromotors are
energized, whereby air pre-compressed in the first compressor is
further compressed or boosted in the second stage of the
compressor.
[0037] Since the fuel cell system can be made more responsive, the
need for devices such as storage batteries and inertial flywheels
is reduced.
[0038] The two electromotor driven compressors can be regulated
with almost instantaneous response. Regulating can be in response
to vehicle electrical consumption, or in response to fuel gas input
into the fuel cell, temperature, gas pedal, or any combination of
these or other inputs. The power required to power the two
compressors is comparatively low, and the responsiveness is greatly
improved as compared to compressors which are only activated in
response to, e.g., exhaust gas pressure.
[0039] Turning now to the specially designed, electromotor driven,
integrated two-stage compressor shown in FIGS. 2 and 3, all
rotating parts are mounted on a single rotor shaft 20. In the
illustrated embodiment, the integrated two-stage compressor has a
generally "barbell" shape, with the pulley 21 located centrally on
the rotor shaft, centrifugal compressors on the ends of the rotary
shaft, and one MLC motor provided between the pulley and each of
the compressors. The rotor shaft is supported on bearings inside a
housing.
[0040] A belt (see FIG. 3) can be tensioned over this pulley to
connect the rotor shaft of the compressor to the driveshaft of an
internal combustion engine of a hybrid combustion/electric
vehicle.
[0041] The electric motors can be powered by batteries, by a
generator associated with an internal combustion motor, or by a
fuel cell. The electric motors may be any type, but for the reasons
listed above are preferably constructed using magnetically loaded
composite (MLC) rotor technology, with a rotor 22, 22' coupled to
the rotor shaft 20 and stators 23, 23' connected to the housing.
The MLC rotor and a stator also serve as a magnetic bearing
system.
[0042] Since centrifugal compressors draw air in axially and expel
air radially, it is necessary to place the compressors at the first
and second ends of the rotary shaft. As shown in FIGS. 2 and 3,
first 30 and second 30' compressor wheels are provided on the first
and second ends of the rotor shaft. Both compressors draw air in
axially at opposite ends of the rotor shaft. Air is drawn into
first compressor 24 housing inlet 25 at P1=atmospheric pressure and
is discharged at outlet 26 at P2= e.g., 2 bar. Air is conveyed
along conduit 27 to second compressor 24' housing inlet 28 at P2=
e.g., 2 bar and is discharged at outlet 29 at P3= e.g., 3 bar.
[0043] As shown in FIG. 1, air may flow through the first
compressor and be precompressed prior to entering the second
compressor, or may alternatively, as determined by conditions,
bypass at least in part the first compressor by opening a valve
located in a bypass conduit 30.
[0044] The motors could easily be switched via power electronics to
function as an electricity generator, in the case that the rotor
shaft is being turned either by the pulley belt which is connected
to the drive shaft of the motor, or driven by an expander.
[0045] The two-stage compressor of. the present invention
represents an improvement over the two-stage centrifugal compressor
assembly disclosed in U.S. Pat. No. 6,193,473 (Mruk et al), since
Mruk et al drive the compressors via a switched reluctance motor
disposed between the first and second compressor casings and
comprising a stator and a rotor rotatable within the stator. The
absence of a pulley in the design of Mruk et al makes it difficult
to fully integrate the compressor into a fuel cell system to
generate electricity to power vehicle accessories or as drive
systems for propelling vehicles, and in particular, hybrid
vehicles. Further, the employment of magnetically loaded composite
(MLC) motors in the present invention, one on either side of the
pulley, provides numerous advantages discussed above.
[0046] Although a two stage compressor has been described herein
with great detail with respect to an embodiment suitable for use in
a fuel cell system, and particularly a fuel cell system as used to
generate electricity to power vehicle accessories or as drive
systems for propelling vehicles, it will be readily apparent that
the two stage compressor is suitable for use in a number of other
applications. Although this invention has been described in its
preferred form with a certain of particularity with respect to an
automotive internal combustion compressor wheel, it is understood
that the present disclosure of the preferred form has been made
only by way of example and that numerous changes in the details of
structures and the composition of the combination may be resorted
to without departing from the spirit and scope of the
invention.
[0047] Now that the invention has been described,
* * * * *