FOOTWEAR ENERGY HARVESTING APPARATUS AND METHOD

Abstract

An energy harvesting system for footwear is described having a sole, heel and an upper and comprising a first gaseous bellows pump formed at the sole of the footwear with the bellows hinged forward of the sole; a second gaseous bellows pump formed at the heel of the footwear with the bellows hinged forward of the heel. A gas reservoir is mounted to the upper of the footwear in fluid communication with first and second gaseous pumps downstream of the pumps and adapted to receive the gaseous medium exiting under pressure from the pumps. An axial turbine has an output shaft mounted on the footwear upper downstream of the reservoir and adapted to receive pressurized gas from the reservoir when a predetermined pressure threshold is attained in the reservoir so as to activate the axial turbine; and an electrical generator mounted on the upper, downstream of the axial turbine and engaged with the turbine output shaft which can be engaged only when a predetermined shaft velocity threshold has been attained.

Description

FIELD OF THE INVENTION

The present invention relates to an apparatus set in footwear for the purpose of generating electricity for the purposes of powering electrical accessories carried by the wearer, and the method therefore.

BACKGROUND ART

There has been a history of endeavors to harvest energy from footwear in order to produce electrical power for the purpose of energizing various accessories worn by a person, such as to operate “ . . . an electric lamp, a heating coil, a small wireless outfit, a therapeutic appliance . . . ” as described in U.S. Pat. No. 1,506,282 Barbieri in 1924. Since then many attempts have been made as illustrated by Lakic in U.S. Pat. No. 4,674,199 January 1987; U.S. Pat. No. 4,736,530 April 1988; U.S. Pat. No. 4,782,602, November 1988; U.S. Pat. No. 4,845,338 July 1989 and U.S. Pat. No. 4,941,271, July 1990; Chen in U.S. Pat. No. 5,167,082 December 1992; U.S. Pat. No. 5,367,788 November 1994 and 5,495,682 March 1996. More recently there has been Landry U.S. Pat. No. 6,201,314 March 2001; Le et al U.S. Pat. No. 6,255,799 July 2001; Sarich U.S. Pat. No. 6,281,594 August 2001 and Yang U.S. Pat. No. 7,956,476 June 2011. A paper entitled Parasitic Power Harvesting in Shoes by John Kymissis et al, from the MIT Media Laboratory, was presented at the Second IEEE International Conference on Wearable Computing in August, 1998.

With increased use of power-consuming portable electronics, the need for compact and lightweight power sources to replace batteries is becoming more urgent. Harvesting energy from walking such as from the force developed in compressing footwear soles and heels has been shown to generate anywhere from 1 to 7 W cap (continuous average power). However the challenge remains in converting this mechanical energy into useful electricity with miniaturize components.

Accordingly, improvements are desirable.

SUMMARY

It is therefore an aim of the present invention to provide an improved footwear energy harvesting apparatus and related method.

Therefore, in accordance with the present invention, there is provided an energy harvesting system for footwear comprising a first gaseous pump formed at the sole of the footwear and a second gaseous pump formed at the heel of the footwear. A reservoir is mounted to the upper of footwear in fluid communication with and downstream of the first and second pumps and adapted to receive pressurized gas exiting from the pumps. A turbine having an output shaft, is mounted on the footwear upper, in fluid communication with and downstream of the reservoir. The turbine includes an inlet port section for receiving the pressurized gas from the reservoir, when a predetermined pressure threshold is attained in the reservoir, so as to activate the turbine; and an electrical generator mounted on the upper, downstream of the turbine and disengageably connected with the turbine output shaft so that the generator is engaged by the shaft when a predetermined shaft velocity threshold has been attained whereby electricity may be generated in order to energize or be stored by a device worn by the bearer of the footwear.

In another aspect, there is provided a multistage, axial turbine for converting energy from a pressurized gas to mechanical energy which may be used within a footwear energy harvesting system. The turbine includes a casing having an inlet and an outlet at axially aligned opposite ends of the casing, and the casing houses a cylindrical hollow stator and an elongated rotor concentric with the stator. The rotor includes a plurality of stages of radially extending rotor blades spaced circumferentially in each stage, while the stator is provided with rows of radially extending stator vanes circumferentially spaced apart in each row and the rows are located inter-stage of the rotor blade stages. The casing includes a diffuser at the inlet provided to receive the pressurized gas and to direct it to the rotor and stator. The casing is provided with bearings at the inlet and the outlet and the rotor has an upstream shaft and an output shaft coaxial with the upstream shaft and the shafts rotating freely while being supported in the respective bearings.

In yet another aspect there is a method of harvesting energy from footwear comprising the steps of compressing a gas in a chamber at the sole of the footwear transferring the compressed gas to a second chamber at the heel of the footwear; further compressing the gas in the second chamber; transferring the compressed gas from the second chamber to a reservoir; repeating the compression steps until the pressure in the reservoir has reached a threshold level; once the pressure level in the reservoir has reached the threshold level, passing the pressurized gas through a turbine to convert the energy from the pressurized gas to mechanical energy by rotating the turbine rotor and dependent shaft to reach a speed threshold; once the speed threshold of the shaft has been reached, engaging the shaft with an electric generator; storing the electricity and/or driving a device carried by the bearer of the footwear.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings, showing by way of illustration a particular embodiment of the present invention and in which:

FIG. 1 is a schematic side elevation of a boot showing the location of the miniaturized components of one embodiment;

FIG. 2 is a block diagram illustrating the components shown in FIG. 1;

FIG. 3 a is an exploded view of a turbine in accordance with the embodiment;

FIG. 3 b is a perspective view, showing various details of the turbine shown in FIG. 3a;

FIG. 4 is a schematic view of the storage tank and turbine of the embodiment;

FIG. 5 a is a schematic view of the turbine in accordance with the embodiment;

FIG. 5 b is a schematic view of a crossection of the stator vanes and rotor blades of the embodiment of the turbine;

FIG. 6 a is a longitudinal cross section of the the turbine in accordance with the embodiment;

FIG. 6 b is a longitudinal crosssection of the stator shown in FIG. 6a;

FIG. 6 c is a fragmentary perspective view of the rotor shown in FIG. 6a; and

FIG. 7 is a diagram showing the circuit of the control box.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

Referring now to FIGS. 1 and 2 there is shown a work boot 10 having a heel 12 and a sole 14 along with an upper 16. As shown in the drawings, the heel 12 has a heel plate 13 that is hinged, at its forward portion, at 18 to the boot. The heel plate 12 can pivot in a vertical plane about an axis normal to the vertical plane at the hinge 18. A flexible, impermeable bellows wall 20 defines a bellows chamber 21 between the boot and the pivoting heel 12. A oneway air inlet valve 20a is defined in the flexible wall 20.

Likewise the outsole 14 a pivots about a hinge 22 in the toe region of the boot 10. The outsole 14 can pivot in a vertical plane about an axis at the hinge 22 that is normal to the vertical plane. A flexible, impermeable bellows wall 24 is provided defining a bellows chamber 25 between the boot and the pivoting outsole 14a. A oneway air inlet valve 25a is provided in the bellows wall 24, which otherwise is airtight.

An air conduit 32 communicates the bellows chamber 25 with the bellows chamber 21. A one-way check valve 31 interrupts the conduit 32 to prevent the air from returning into the chamber 25. When a person weighing 180 lbs places weight on the outsole 14a, the air in the relatively large bellows chamber 25 is compressed to 10-l3 psi. The outsole 14a has an area of approximately 10 in² (65 cm2). The air then passes through conduit 32 to the relatively smaller bellows chamber 21.

When the weight of the user is transferred to the heel 12, the pre-compressed air is now compressed to between 25-30 psi (172 kPa-207 kPa); partly because of the smaller heel area providing a smaller chamber 21.

The compressed air from the bellows chamber 21 passes through the conduit 28 to the reservoir 26, interrupted by a one-way check valve 30. The reservoir 26, mounted on the side of the upper 16 of the boot 10, typically has a capacity of 12 in³ (197 cm³), in order to provide storage capacity for the compressed air before it is released to the turbine 34.

An air conduit 36 communicates the reservoir 26 to the turbine 34. The air conduit 36 is interrupted by a pressure control valve 38. It was determined that the ideal pressure for delivering the air to the turbine 34 is between 30 and 40 psi, but the latter is preferred.

A control panel 40 is mounted to the side of the upper 16 and preferably between inner-layers forming the upper 16. An air-line 42 extends between the valve 38 and a pressure regulator 43 on the control panel 40. The valve 38 is opened by the pressure regulator, when the pressure threshold e.g. 40 psi is attained.

As shown in FIGS. 3a and 3b, the turbine 34 may be approximately 51 mm long and less than 10 mm in diameter. The turbine is considered to be minituarised and other similar dimentions are considered to be equivalent. The criteria is that it must be able to be mounted to the upper 16 of boot 10. The turbine 34 is made up of a tubular casing 60 enclosing a cylindrical stator 70, in the form of a sleeve, with a concentric, elongated rotor 68. The rotor 68 includes a coaxial shaft 66 extending upstream and downstream thereof. An inlet cap 62 is threaded onto the external threads 61 of the tubular casing 60. At the other end of the casing 60 is an outlet cap 64 with it external threads 82 adapted to engage the internal threads 63 of the tubular casing 60. The downstream or output portion of shaft 66 is supported by the inner race 80 of a bearing 81 mounted within the outlet cap 64. At the other end of the casing 60, an inlet diffuser 72, in the form of a cylindrical housing, has an inner race 78, part of an integrated bearing 79, supporting the upstream end of shaft 66. The rotor 68 mounts several stages of rotor blades 84. Preferably eight stages are provided. The blades 84 extend radially outwardly from the core surface of the rotor 68 and are spaced apart circumferentially, equally in each stage.

The stator 70 is fabricated in semi-cylindrical segments 70a and 70b, forming a sleeve which is mounted within the casing 60 and is concentric with the rotor 68. In certain conditions the stator may be in three segments. As shown in FIGS. 6a and 6b, the stator 70 has a plurality of rows of vanes 86, each exending radially inwardly. The vanes 86 are spaced apart equally in each row and the rows are inter stage with the rotor stages. Back to FIGS. 3a and 3b we see a spring 76, within the inlet cap 62, that ensures the diffuser 72 which includes bearing 79, is precisely located, once the inlet cap 62 is threaded onto the external thread 61 of the casing 60. The diffuser 72 includes an annular array of bores 74 arranged to pass the compressed air into the turbine 34. The rotor 68 and stator 70 will be described in more detail herein below.

Returning now to FIG. 2, air entering the turbine 34 will cause the shaft 66 to rotate at an increasingly higher speed, when the shaft 66 is disengaged and rotating freely. The rotational speed of the shaft 66 is measured by an optical angular velocity sensor 50 located in the control panel 40. In the present embodiment, an optical fiber 52 is directed to a marker on the shaft 66. The marker may be a small bore through shaft 66. As will be described, the shaft 66 is allowed to rotate freely until it reaches a threshold sufficient to allow it to be coupled to the generator shaft 46. The velocity will preferably attain 90,000 rpm when the air pressure is 40 psi. Once the turbine shaft 66 is engaged with the generator shaft 46 the velocity thereof will decelerate. Generator 44 which is meant to convert the mechanical energy into electricity may also be used to start the rotation of the rotor. The generator would then be driven by battery 90.

Reference will now be made to FIGS. 4, 5a and 5b. A multi-stage axial turbine 34, used to extract the available energy stored in the pressurized storage tank 26, is designed to transform the energy stored in terms of pressure and temperature into electrical energy.

For a standard axial turbine with a rotor designed in such a way that the exit velocity at all stages is oriented in the axial direction, the ideal specific work per stage (work per unit mass) is given by w_t=U_t V_θ1, U_t being the tangential velocity of the rotor at the mid-radius (Rt) and V_θ1 being the tangential velocity of the airflow at the blade leading edge. The energy extracted by a turbine equipped with V_θ1 stages is then (assuming, in this simplified case, that each stage produces the same amount of work):

E _T =φ=Δm w _t N _sη_t=Δm U_tV_θ1N_sη_t

The tangential velocity of the airflow:

$V_{\theta 1}=\frac{\varphi }{\Delta {\mathrm{mU}}_tN_s{\eta }_t}=\frac{c_p\left(T_T-T_{\mathrm{atm}}\right)}{U_tN_s}$

The tangential velocity of the airflow is a high value but is limited by the speed of sound at standard ambient temperature. It is also limited by the need to keep frictional losses as low as possible.

The mass flow rate is assumed to be constant as the available mass of air stored in the tank 26 discharges very quickly through the turbine 34. The duration of the constant velocity period is very short and what is observed is rather a regime of acceleration followed by a deceleration time.

FIGS. 5 a and 5b show a schematic view of the preferred turbine geometry. The turbine geometry is characterized by the following elements: the number of stages, the blade section at each stage, the mid-radius, the angle of attack and the trailing-edge angle of the rotor blades and the stator vanes. Moreover, one must consider the rotational speed N of the turbine as another important parameter. This parameter is related to the tangential velocity of the blade at the mid-radius by:

$U_t=N\frac{2\pi }{60}R_t$

The radius of the rotor hub is R1 while the rotor tip is R3. The radius of the inner rim of the stator is R4 while the stator vane tip is R2. It has been shown that clearances, defined by the distance between the stator vane tips and the rotor hub R2-R1 and the distance between the rotor blade tips and the stator rim R4-R3, should be kept as small as possible. Thus, the air leakage from one stage to another is minimized. The optimum design was manufactured with clearance R2-R1 of 0.120 mm and clearance R4-R3 of 0.100 mm.

The preferred rotor blade design and stator blade design is shown in FIGS. 5b, 6a, 6b and 6c. In FIG. 5b the angle of the stator vane is set to 0° at the leading edge and the angle of its trailing edge is defined by:

${\alpha }_1={\tan }^{-1}\left(\frac{V_{\theta 1}}{U_a}\right)$

The angle of attack of the rotor blade at the leading and the trailing edge are respectively defined by:

${\beta }_1={\tan }^{-1}\left(\frac{V_{\theta 1}-U_t}{U_a}\right)\mathrm{and}{\beta }_2={\tan }^{-1}\left(\frac{U_t}{U_a}\right)$

The turbine 36 has been manufactured with α1=86° and β2=35°. The blade height of the rotor varies from 0.692 mm at stage 1 to 1.004 mm at stage 8.

The number of stages should be as low as possible to limit the manufacturing difficulties, but high enough to limit tangential velocity of the air flow. The 8-stage turbine assembly 34 is shown in FIGS. 6a, 6b and 6c. The rotor blades 84 are shown extending radially from the rotor 68 while the stator vanes 86 are shown extending radially inwardly from the inner periphery of the stator 70.

Other factors affecting the turbine performamce is the temperature inside the storage tank 26 as well as the pressure and density. There is a pressure drop across each stage as a result of a temperature drop across the stages. To evaluate the pressure drop at each stage, a polytropic expansion is considered. For an ideal gas, the exit pressure at a given stage i is determined by:

$p_{e_i}={\left(\frac{T_{e_i}}{T_{i_i}}\right)}^{\frac{n}{n-1}}p_{i_i}$

The inlet temperature T_ii and exit temperature T_ei at stage i exhibits a temperature drop, such that there is, according to the above equation, a pressure drop and an air density drop across the stages of the turbine. This results in an increase of the turbine exit airflow.

For a constant axial velocity U_a of the airflow, this results in an increase of the turbine exit flow area A_ei from one stake to the other. The flow area is given by:

$A_{e_i}=\frac{\stackrel{.}{m}{\mathrm{RT}}_{e_i}}{p_{e_i}U_a}$

The exit flow area of a given stage can be defined as:

A _{e i} =π(R_outi²−R_ini²),

with the outer and inner radii of stage i respectively noted R_outi and R_ini. Given the mid-radius R_t, the outer and and inner radii may be defined as follows:

R _{out i} =R _t +ΔR _i and R_ini=R_t−ΔR_i

The following expression for ΔR_i is:

$\Delta R_i=\frac{A_{e_i}}{4\pi R_t}$

This determines the small flow area as a result of the small rotor radius parameters added to the high tangential velocity of the air flow and the small available mass of air.

The turbine 34 has been manufactured by rapid prototyping using Multi Jet Modeling technique (MJM 3D printer from 3D Systems). CNC can also be used.

Referring now to FIGS. 2 and 7 there is shown a diagram of the circuit. The air valve 38 is open and closed by an electronic switch 38a controlled by a CPU 88 in response to the pressure sensor 43. As previously described, the pressurized air from the storage tank 26 is passed to the turbine 34 only when the pressure threshold has been met, as determined by the CPU 88. In one example, the threshold is determined to be 40 psi. The pressurized air enters the turbine 34 to rotate the rotor 68 to a high velocity, in the range of 100,000 rpm. The shaft velocity is measured by the optical angular velocity sensor 50 and the information is sent to the CPU 88.

The shaft 66 of the turbine 34 is coupled to the generator 44 only when a shaft speed threshold has been attained e.g. 90,000 rpm. The CPU 88 sends a signal to ON switch 54 in order to engage the shaft 66 to the generator shaft 46. The generator 44 will generate electrical energy which can be stored in battery 90. As shown in FIG. 7, the rotor 68 may be initially rotated by electrical current supplied from the battery 90. The inertia of the rotating rotor 68 facilitates the acceleration of the rotor to its threshold velocity by the compressed air.

The nature of the pumping process and the need to constantly accelerate and decelerate, the generator shaft 46 causes a pulsing of the electrical current produced by the generator 44. As shown in FIG. 7, a regulator 92 may be provided for averaging the current flow to the battery 90 by way of a charger.

NOMENCLATURE

• c_v=specific heat at constant volume [J/kg K]
• c_p=specific heat at constant pressure [J/kg K]
• E_f=energy losses due to friction [J]
• E_T=energy extracted by the turbine [J]
• I_a=moment of inertia [kg m²]
• m_T=storage tank air mass [kg]
• N=rotational speed [rpm]
• P_T=storage tank air pressure [Pa]
• p_atm=atmospheric air pressure [Pa]
• R=ideal-gas constant for air (R=c_p−c_v) [J/kg K]
• T_atm=atmospheric air temperature [K]
• T_T=storage tank air temperature [K]
• V_T=storage tank volume [m³]
• W=work [J]
• {dot over (W)}=power [W]
• φ=exergy (useful energy) [J]
• ρ_T=storage tank air density [kg/m^3]
• ρ_atm=atmospheric air density [kg/m³]
• ω=angular velocity [rad/s]

Claims

1. A footwear energy harvesting system comprising a first gaseous pump formed at the sole of the footwear; a second gaseous pump formed at the heel of the footwear; a reservoir mounted to the upper of footwear in fluid communication with and downstream of the first and second pumps and adapted to receive pressurized gas exiting from the pumps; a turbine having an output shaft, mounted on the footwear upper, in fluid communication with and downstream of the reservoir; the turbine including an inlet port section for receiving the pressurized gas from the reservoir, when a predetermined pressure threshold is attained, so as to activate the turbine; and an electrical generator mounted on the upper, downstream of the turbine and disengageably connected with the turbine output shaft so that the generator is engaged by the shaft when a predetermined shaft velocity threshold has been attained whereby electricity may be generated in order to energize or be stored by a device worn by the bearer of the footwear.

2. The footwear energy harvesting system as defined in claim 1 wherein the pressurized gas from the first pump is communicated to the second pump to pre-pressurize the gas to the second pump and the second pump communicates the pressurized gas to the reservoir.

3. The footwear energy harvesting system as defined in claim 3 wherein the first gaseous pump formed at the sole of the footwear includes an outersole hinged to the forward part of the sole and includes an impermeable bellows wall defining a first pump chamber, and an inlet one-way valve for allowing air into the first pump chamber; and, the second gaseous pump includes a heel potion hinged to the forward part of the heel and includes an impermeable bellows wall defining a second pump chamber, and an inlet one-way valve for allowing air into the second pump chamber.

4. The footwear energy harvesting system as defined in claim 1 wherein a one way valve is provided in a gas conduit providing fluid communication between the reservoir and the turbine inlet port section, the valve is controlled by a pressure regulator to open and close the valve to control the debit of pressurized gas to the turbine.

5. The footwear energy harvesting system as defined in claim 4 wherein the predetermined pressurized gas threshold is 30 psi (207 kPa).

6. The footwear energy harvesting system as defined in claim 5 wherein the predetermined pressurized gas threshold is 40 psi (276) kPa).

7. The footwear energy harvesting system as defined in claim 1 wherein a rotary speed detector is provided to determine the speed of the turbine output shaft; the speed detector is in communication with an on/off switch means to engage or disengage the generator from the output shaft whereby the generator is engaged only when the output shaft speed is above a predetermined speed threshold.

8. The footwear energy harvesting system as defined in claim 7 wherein the output shaft speed threshold is 90,000 rpm.

9. The footwear energy harvesting system as defined in claim 1 wherein the turbine is a minituarised, multistage axial turbine with concentric rotor and stator.

10. The footwear energy harvesting system as defined in claim 9 wherein the turbine inlet port section includes a diffuser and the rotor includes an input shaft coaxial with the output shaft.

11. The footwear energy harvesting system as defined in claim 10 wherein the turbine includes eight stages.

12. The footwear energy harvesting system as defined in claim 11 wherein the turbine includes a casing with dimensions compatible with being mounted on the upper of a boot.

13. The footwear energy harvesting system as defined in claim 12 wherein the dimensions of the turbine casing that includes an axial length of 51 mm and a diameter of 10 mm.

14. The footwear energy harvesting system as defined in claim 4 wherein the gas is air and first and second bellows pumps have respective air inlets with one way valves.

15. The footwear energy harvesting system as defined in claim 14 wherein the first bellows pump has an outsole that is hinged to the toe portion of the sole and the outsole has an area of approximately 10 in2 (65 cm2) and a person of 180 lbs (82 kg) can compress the air to an exit pressure of 10-13 psi (69 kPa-90 kPa).

16. The footwear energy harvesting system as defined in claim 15 wherein the pre-pressurized air from the first bellows pump enters the second bellows pump and is compressed to 25-30 psi (172 kPa-207 kPa) upon a person shifting its weight to the heel.

17. The footwear energy harvesting system as defined in claim 1 wherein the reservoir has a volume capacity of 12 in3 (197 cm3).

18. A multistage, axial turbine for converting energy from a pressurized gas to mechanical energy; the turbine including a casing having an inlet and an outlet at axially aligned opposite ends of the casing, the casing housing a cylindrical hollow stator and an elongated rotor concentric with the stator; the rotor including a plurality of stages of radially extending rotor blades spaced circumferentially in each stage; the stator provided with rows of radially extending stator vanes circumferentially spaced apart in each row and the rows provided inter-stage of the rotor blade stages; the casing including a diffuser at the inlet provided to receive the pressurized gas and to direct it to the rotor and stator; the casing provided with bearings at the inlet and the outlet and the rotor having an upstream shaft and an output shaft coaxial with the upstream shaft and the shafts rotating freely while supported in the respective bearings.

19. The multistage, axial turbine as defined in claim 18 wherein the turbine is miniaturized for use within a footwear energy harvesting system.

20. The miniaturized, multistage, axial turbine as defined in claim 19 wherein the rotor has 8 stages.

21. The axial turbine as defined in claim 19 where the angle of the leading edge of each stator vane is set at 0° to the axis and the trailing edge is defined by:

22. The axial turbine as defined in claim 21 wherein the angle of attack of the rotor blade at the leading edge and the trailing edge are respectively defined by:

23. The axial turbine as defined in claim 22 wherein α1=86° and β2=35°.

24. The axial turbine as defined in claim 22 wherein the turbine has eight stages and the height of the rotor blades increase from 0.692 mm at the upstream stage and 1.004 mm at the downstream 8th stage.

25. A method of harvesting energy from footwear comprising the steps of compressing a gas in a chamber at the sole of the footwear; transferring the compressed gas to a second chamber at the heel of the footwear; further compressing the gas in the second chamber; transferring the compressed gas from the second chamber to a reservoir; repeating the compression steps until the pressure in the reservoir has reached a threshold level; once the pressure level in the reservoir has reached the threshold level, passing the pressurized gas through a turbine to convert the energy from the pressurized gas to mechanical energy by rotating the turbine rotor and dependent shaft to reach a speed threshold; once the speed threshold of the shaft has been reached, engaging the shaft with an electric generator; storing the electricity and/or driving a device carried by the bearer of the footwear.

26. The method as defined in claim 25 wherein the pressure threshold is 40 psi.

27. The method as defined in claim 25 wherein the speed threshold is 90,000 psi.

28. The method as defined in claim 25 wherein the turbine is an axial turbine and the pressurized gas is fed through the inlet of the axial turbine to rotate an elongated rotor that is concentric with a cylindrical stator causing the rotor to rotate at speeds exceeding 100,000 rpm.

29. The method as defined in claim 28 wherein the gas is air.