US20220235950A1

US20220235950A1 - Energy recovery wheel assembly

Info

Publication number: US20220235950A1
Application number: US17/582,198
Authority: US
Inventors: Charles-Antoine Caron
Original assignee: Broan Nutone LLC
Current assignee: Broan Nutone LLC
Priority date: 2021-01-25
Filing date: 2022-01-24
Publication date: 2022-07-28
Also published as: CA3146595A1

Abstract

An energy recovery wheel assembly includes a support frame, a motor, and a wheel rotor. The support frame at least partially defines an air-supply section that supplies outdoor air into a building and an air-exhaust section that removes indoor air from the building. The motor is coupled to the support frame. The wheel rotor is coupled to the support frame and driven in rotation about an axis relative to the support frame by the motor.

Description

PRIORITY CLAIM

This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 63/141,085, filed Jan. 25, 2021, which is incorporated by reference herein. This application incorporates by reference the subject matter of a copending application filed by the Applicant on Jan. 24, 2022 and entitled “Energy Recovery Array Wheel Array.”

BACKGROUND

The present disclosure relates to energy recovery devices, and particularly to energy recovery devices for recovering heat and/or moisture from an airflow. More particularly, the present disclosure relates to an energy recovery wheel that recovers heat and/or moisture from an airflow.

SUMMARY

According to the present disclosure, an energy recovery wheel assembly includes a support frame, a motor, and a wheel rotor. The support frame at least partially defines an air-supply section that supplies outdoor air into a building and an air-exhaust section that removes indoor air from the building. The motor is coupled to the support frame. The wheel rotor is coupled to the support frame and driven in rotation about an axis relative to the support frame by the motor.
In illustrative embodiments, the wheel rotor includes an outer case, a wheel mount coupled to the support frame, and energy absorption media located between the outer case and the wheel mount. The energy absorption media may be made up of a plurality of sheets. The energy absorption media may have a depth within a range of about 15 inches to about 40 inches. Each sheet may have a thickness within a range of about 0.003 inches to about 0.01 inches.
In illustrative embodiments, each sheet comprises at least one of aluminum, stainless steel, and copper. In illustrative embodiments, each sheet further comprises a desiccant coating.
In illustrative embodiments, the motor drives the wheel rotor to rotate at a rate of about 8 revolutions per minute or less. In illustrative embodiments, the rate is maintained when the wheel rotor is exposed to low temperature environments.
In illustrative embodiments, the wheel rotor is a first wheel rotor and the energy recovery wheel assembly further includes a second wheel rotor spaced apart from the first wheel rotor along the axis. In illustrative embodiments, the first wheel rotor is configured to rotate about the axis in a first direction and the second wheel rotor is configured to rotate about the axis in an opposite second direction.
In illustrative embodiments, the first wheel rotor is a sensible wheel rotor without any desiccant coating and the second wheel rotor includes a desiccant coating. In illustrative embodiments, the first wheel rotor is configured to rotate about the axis at a first rate and the second wheel rotor is configured to rotate about the axis at a second rate different than the first rate.
According to another aspect of the present disclosure, an energy recovery wheel assembly includes a support frame, a first wheel rotor, and a second wheel rotor. The support frame at least partially defines an air-supply section that supplies outdoor air into a building and an air-exhaust section that removes indoor air from the building. The first wheel rotor is coupled to the support frame and is configured to rotate about an axis relative to the support frame. The second wheel rotor is also coupled to the support frame and is configured to rotate about the axis relative to the support frame.
In illustrative embodiments, the first and second wheel rotors each include energy absorption media that is configured to transfer at least one of heat and moisture between air flowing through the air-supply section and air flowing through the air-exhaust section as the first and second wheel rotors are rotated about the axis.
In illustrative embodiments, the energy absorption media is made up of a plurality of sheets, the energy absorption media having a depth within a range of about 15 inches to about 40 inches, each sheet having a thickness within a range of about 0.003 inches to about 0.01 inches.
In illustrative embodiments, each sheet comprises at least one of aluminum, stainless steel, and copper. In illustrative embodiments, the first wheel rotor is configured to rotate about the axis in a first direction and the second wheel rotor is configured to rotate about the axis in an opposite second direction. In illustrative embodiments, the first wheel rotor is a sensible wheel rotor without any desiccant coating and the second wheel rotor includes a desiccant coating.
In illustrative embodiments, the first wheel rotor is configured to rotate about the axis at a first rate and the second wheel rotor is configured to rotate about the axis at a second rate different than the first rate. In illustrative embodiments, the first wheel rotor and the second wheel rotor are configured to rotate about the axis at a rate of about 8 revolutions per minute or less and the rate is maintained when the wheel rotor is exposed to low temperature environments.
According to another aspect of the present disclosure, a method of recovering energy from air in a building includes displacing outdoor air from outside of the building through an air-supply section of an air handling unit toward an interior of a building. The method may further include displacing indoor air from the interior of the building through an air-exhaust section of the air handling unit that is separate from the air-supply section. The method may further include rotating a first wheel rotor about an axis that extends parallel to and is located between the air-supply section and the air-exhaust section. The method may further include rotating a second wheel rotor about the axis.
In illustrative embodiments, the first and second wheel rotors each include energy absorption media that is configured to transfer at least one of heat and moisture between air flowing through the air-supply section and air flowing through the air-exhaust section as the first and second wheel rotors are rotated about the axis.
In illustrative embodiments, the energy absorption media is made up of a plurality of sheets, the energy absorption media having a depth within a range of about 15 inches to about 40 inches, each sheet having a thickness within a range of about 0.003 inches to about 0.01 inches.
In illustrative embodiments, the first wheel rotor is rotated about the axis in a first direction and the second wheel rotor is rotated about the axis in an opposite second direction. In illustrative embodiments, the first wheel rotor is configured to rotate about the axis at a first rate and the second wheel rotor is configured to rotate about the axis at a second rate different than the first rate.
Additional features of the present disclosure will become apparent to those skilled in the art upon consideration of illustrative embodiments exemplifying the best mode of carrying out the disclosure as presently perceived.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The detailed description particularly refers to the accompanying figures in which:

FIG. 1 is a perspective view of an energy recovery wheel assembly including a support frame, a motor, and a wheel rotor that is driven in rotation by the motor relative to the support frame to recover heat and/or moisture as indoor air and outdoor air pass through the wheel rotor;

FIG. 2 is a perspective view of another embodiment of an energy recovery wheel assembly including two wheel rotors arranged in series along the air flow paths; and

FIG. 3 is a perspective view of another embodiment of an energy recovery wheel assembly including three wheel rotors arranged in series along the air flow paths.

DETAILED DESCRIPTION

An energy recovery wheel assembly 10 in accordance with the present disclosure, includes a support frame 12, a motor 14, and the wheel rotor 16 as shown in FIG. 1. The support frame 12 is configured to support the wheel rotor 16 within an air handling unit 100 that includes an air-supply section 102 that supplies outdoor air into a building and an air-exhaust section 104 that removes indoor air from the building at the same time. The motor 14 is configured to drive and control rotation of the wheel rotor 16 relative to the air handling unit 100. The energy recovery wheel assembly 10 is arranged to lie in both the air-supply section 102 and the air-return section 104 and is configured to move heat and/or moisture between the supply air and the exhaust air as the indoor air is exchanged with outdoor air by the air handling unit 100.
The support frame 12 may form a part of the air handling unit 100 or be attached thereto and includes an outer support frame 18, a crossbeam 20, and a seal 22 as shown in FIG. 1. The support frame 12 is sized to accommodate the wheel rotor 16 within a space 26 provided by the outer support fame 18. The outer support frame 18 defines a prism that is sized to fit within the air handling unit to position the wheel rotor 16 relative to the air-supply section and the air-exhaust section. The crossbeam 20 extends from one side of the outer support frame 18 to an opposite side of the outer support frame 18 at a boarder to the both the air-supply section and the air-exhaust section. The seal 22 is coupled to the crossbeam 20 to seal between the air-supply section and the air-exhaust section of the air handling unit while the energy recovery wheel 10 rotates relative to the support frame 12.
The wheel rotor 16 rotates about a wheel rotational axis 24 relative to the outer support frame 18 and the crossbeam 20. The wheel rotor 16 includes an outer case 30, a wheel mount 32, and energy absorption media 34 between the outer case 30 and the wheel mount 32. The outer case 30 is coupled to the motor 14 and is driven by the motor 14 to rotate the wheel rotor 16 about the wheel rotational axis 24 when the wheel rotor 16 is being used. The wheel mount 32 is coupled in rotative bearing engagement with a bearing 36 mounted on the crossbeam 20 to allow the wheel rotor 16 to rotate relative to the support frame 12. The energy absorption media 34 is configured to absorb heat and/or moisture from the air passing through one of the air-supply section and the air-exhaust section and transfer the heat and/or moisture to the other of the air-supply section and the air-exhaust section to reduce energy losses from the air in the building as outdoor air is brought into the building. The wheel rotor 16 may also include a plurality of spokes 38 that extend between the wheel mount 32 and the outer case 30.
The energy absorption media 34 defines a plurality of air channels to allow flow of air through the energy absorption media 34. The energy absorption media 34 captures and releases heat and/or moisture from the air flowing through the air channels. The energy absorption media 34 may be made from any suitable material such as aluminum, stainless steel, copper, a ceramic, or any other suitable material. In some embodiments, the energy absorption media 34 may include a desiccant coating, such as a molecular sieve desiccant, a silica gel desiccant, MS3A desiccant, or a passive desiccant to capture and release moisture from the air flowing through the air channels. Accordingly, the expression “energy recovery wheel assembly” should be interpreted to include, without limitation thereto, a rotary wheel, a thermal wheel, a sensible wheel, a heat wheel, a desiccant wheel, a dehumidification wheel, a heat and/or moisture recovery wheel, a total energy recovery wheel, a enthalpy wheel, a regeneratable rotary dehumidification wheel, a rotary enthalpy wheel, a rotating wheel exchanger and the like.
The wheel rotor 16 provides a higher effectiveness or efficiency for the assembly than other energy recovery wheels and may be used in low outdoor temperature settings where other energy recovery wheels would freeze or would require a frost system to remove frost and ice. The energy absorption media 34 is sized and/or constituted to increase the thermal inertia and/or mass of the wheel rotor 16 compared to other energy recovery wheels to achieve the higher effectiveness or efficiency without using any frost system in colder environments. Example 1 below discusses the wheel rotor 16 of the illustrative embodiment while Example 2 below discusses a comparative energy recovery wheel with a lower effectiveness and that includes an anti-frost system to operate in cold environments (i.e. below 32 degrees Fahrenheit).
Example 1 is described below. The term “about” is used to account for manufacturing tolerances and includes values within about 5 percent of the stated value.
The energy absorption media 34 has a diameter 40 and a depth 42 as shown in FIG. 1. In some embodiments, the diameter 40 is within a range of about 12 inches to about 150 inches. In some embodiments, the diameter 40 is within a range of about 48 inches to about 120 inches. In some embodiments, the diameter 40 is about 48 inches. In some embodiments, the diameter 40 is about 54 inches. In some embodiments, the diameter 40 is about 62 inches. In some embodiments, the diameter 40 is about 70 inches. In some embodiments, the diameter 40 is about 78 inches. In some embodiments, the diameter 40 is about 88 inches. In some embodiments, the diameter 40 is about 96 inches. In some embodiments, the diameter 40 is about 96 inches. In some embodiments, the diameter 40 is about 108 inches. In some embodiments, the diameter 40 is about 120 inches. In some embodiments, the diameter 40 is greater than 120 inches.
In some embodiments, the depth 42 of the energy absorption media 34 is within a range of about 15 inches to about 50 inches. In some embodiments, the depth 42 is about 15 inches. In some embodiments, the depth 42 is about 18 inches. In some embodiments, the depth 42 is about 20 inches. In some embodiments, the depth 42 is about 25 inches. In some embodiments, the depth 42 is about 30 inches. In some embodiments, the depth 42 is about 35 inches. In some embodiments, the depth 42 is about 40 inches. In some embodiments, the depth 42 is about 45 inches. In some embodiments, the depth 42 is about 50 inches. In some embodiments, the depth 42 is greater than 50 inches.
The energy absorption media 34 can be comprised of corrugated or fluted sheet material. The sheet material defines a thickness perpendicular to the wheel rotational axis 24. In some embodiments, the thickness of each sheet is within a range of about 0.002 inches to about 0.01 inches. In some embodiments, the thickness of each sheet is within a range of about 0.003 inches to about 0.008 inches. In some embodiments, the thickness of each sheet is about 0.003 inches. In some embodiments, the thickness of each sheet is greater than 0.003 inches.
In one nonexclusive example, an aluminum energy absorption media 34 can have a thickness of about 0.002 and a depth 42 of about 20-30 inches. In a different nonexclusive example, an aluminum energy absorption media 34 can have a thickness of about 0.002 and a depth 42 of about 20 inches. In another nonexclusive example, an aluminum energy absorption media 34 can have a thickness of about 0.002 and a depth 42 of about 25 inches. In yet another nonexclusive example, an aluminum energy absorption media 34 can have a thickness of about 0.002 and a depth 42 of about 30 inches. Yet another nonexclusive example includes an aluminum energy absorption media 34 having a thickness of about 0.003 and a depth 42 of about 20 inches
If this invention is used with a plurality of wheel rotors, as discussed below, with one of the wheel rotors within the parameters discussed above, then at least one of the other of the plurality of wheel rotors can comprise an aluminum energy absorption media 34 can have a depth 42 of about less than 15 inches
Example 2 is described below. The diameter of the comparative energy recovery wheel in Example 2 may be the same as the diameter 40 of the wheel rotor 16.
In some embodiments, the depth of the energy absorption media of the comparative energy recovery wheel is within a range of about 6 inches to about 14 inches. In some embodiments, the depth of the energy absorption media of the comparative energy recovery wheel is about 10 inches. Thus, the depth of the energy absorption media of the comparative energy recovery wheel is less than the depth 42 of the illustrative embodiment in Example 1.
Each sheet included in the energy absorption media of the comparative energy recovery wheel includes a thickness of about 0.002 inches. Thus, the thickness of each sheet included in the comparative energy recovery wheel is less than the thickness of each fluted sheet of the illustrative embodiment in Example 1.
Because the depth of the energy absorption media and the thickness of each sheet in Example 2 is less than the depth 42 and thickness of each sheet in Example 1, Example 2 has a lower energy recovery effectiveness. Additionally, the ability of the energy recover wheel of Example 1 to hold heat energy longer than the energy recovery wheel of Example 2 permits the energy recovery wheel of Example 1 to avoid frosting in low temperature environments whereas the energy recovery wheel of Example 2 might require an anti-frost system, such as a variable frequency drive motor that decreases the rotation speed of the energy recovery wheel to reduce frost build-up on the energy absorption media. The higher thermal inertia of the wheel rotor 16 of Example 1 may eliminate the need for an anti-frost system. The wheel rotor 16 of the illustrative embodiment may also omit spokes 38 because of the increase in reinforcement provided by the greater thickness of each sheet in the energy absorption media 34 compared to Example 2.
The motor 14 of energy recovery wheel assembly 10 is configured to rotate the wheel rotor 16 at a rate within a range of about 0.2 revolutions per minute (RPM) to about 1 RPM. In some embodiments, the motor 14 rotates the wheel rotor 16 within a range of about 0.4 RPM to about 0.8 RPM. In some embodiments, the motor 14 rotates the wheel rotor 16 within a range of about 0.5 RPM to about 0.7 RPM. In some embodiments, the motor 14 rotates the wheel rotor 16 at about 0.6 RPM. In some embodiments, the motor 14 rotates the wheel rotor 16 at about 8 RPM or lower.
The RPM of the wheel rotor 16 is configured to remain constant even in low temperature environments (i.e. below 32 degrees Fahrenheit). In contrast, the comparative energy recovery wheel is rotated at about 20 RPM but can be lowered to a lower RPM in low temperature environments so that frost does not build up on the energy absorption media. This lowers the effectiveness of the comparative energy recovery wheel whereas the wheel rotor 16 of the illustrative embodiment maintains its effectiveness without reducing it's RPM even in low temperature environments and may consume less power due to the lower RPM.
The air handling unit 100 includes a housing 106, an air-movement system 108, and the energy recovery wheel assembly 10 as shown in FIG. 2. The housing 106 includes an outer shell 110 (only a portion is shown in FIG. 2) defining an interior space 112 and a divider wall 114 located within the interior space 112 to divide the interior space 112 into the air-supply section 104 and the air-exhaust section 106. The air-movement system 108 includes a first blower 116 configured to displace air through the air-supply section 102 from an exterior of the building to an interior of the building and a second blower 118 configured to displace through the air-exhaust section 104 from the interior of the building to the exterior of the building. The energy recovery wheel assembly 10 lies in the interior space 112 in both the air-supply section 102 and the air-exhaust section 104. The divider wall 114 passes generally through a center of the energy recovery wheel assembly 10 such that about one-half of the energy recovery wheel assembly 10 lies in the air-supply section 102 and the other half lies in the air-exhaust section 104.
In some embodiments, a plurality of wheel rotors may be arranged in series with one another to further increase efficiencies of the air handling unit 100. Another embodiment of an energy recovery wheel assembly 200 including multiple wheel rotors is shown in FIG. 2. The energy recovery wheel assembly 200 is substantially similar to energy recovery wheel assembly 10. Accordingly, similar reference numbers in the 200 series are used to describe similar features between energy recovery wheel assembly 200 and energy recovery wheel assembly 10. The disclosure of energy recovery wheel assembly 10 is incorporated by reference for energy recovery wheel assembly 200.
The energy recovery wheel assembly 200 includes a support frame 212, at least one motor 214, and a plurality of wheel rotors 216, 217 arranged in series with each other along a common axis 218. The support frames 212 and the motors 214 are substantially similar to support frame 12 and motor 14 of energy recovery wheel assembly 10. The wheel rotors 216, 217 are spaced apart from one another along the axis 218 and each wheel rotor 216, 217 may be substantially similar to wheel rotor 16 shown in FIG. 1. Arranging multiple wheel rotors in series increases overall efficiencies of the energy recovery wheel assembly 200 and increases the functionality and/or capabilities of the air handling unit 100.
In the illustrative embodiment, each wheel rotor 216, 217 is driven in rotation about the common axis 218 by respective motors 214 as shown in FIG. 2. The wheel rotors 216, 217 may rotate about the axis 218 in the same direction. In some embodiments, the first wheel rotor 216 is configured to rotate about the axis 218 in a first direction and the second wheel rotor 217 is configured to rotate about the axis 218 in an opposite second direction. Such an embodiment may increase an efficiency of heat and/or moisture transfer provided by the energy recovery wheel assembly 200.
In some embodiments, the first wheel rotor 216 may rotate about the axis 218 at a first rate and the second wheel rotor 217 may rotate about the axis 218 at a second rate different than the first rate. For example, the rate of rotation of each wheel rotor 216, 217 may be increased or decreased relative to one another based on indoor and/or outdoor air conditions (i.e. temperature/humidity).
Each wheel rotor 216, 217 may include a different type of energy absorption media to provide the energy recovery wheel assembly 200 with more functionality. For example, the first wheel rotor 216 may be a sensible only wheel rotor without any desiccant coating while the second wheel rotor includes a desiccant coating for moisture recovery and transfer. Additionally, the first wheel rotor 216 may have a first sheet thickness or depth 42 while the second wheel rotor 217 has a second sheet thickness or depth 42 different than the first sheet thickness or depth 42. Thus, each wheel rotor 216, 217 can be designed for a particular purpose without various features competing with one another or affecting an efficiency provided by one another.
In illustrative embodiments, the air handling unit 100 can include an energy recovery wheel assembly 300 having any number of wheel rotors in series with one another. For example, the air handling unit can include three wheel rotors 316, 317, 318 arranged along a common axis. Each wheel rotor 316, 317, 318 may be substantially similar to wheel rotor 16 shown in FIG. 1. Each wheel rotor 316, 317, 318 may be rotated at a different rate or direction and may include different features relative to one another to maximize the total efficiency of the air handling unit 100.
Energy recovery wheel assemblies 10, 200, 300 may be controlled by a control system having a microprocessor and a memory storage device. The microprocessor may be any computing device that is capable of receiving signals and operating the energy recovery wheel assemblies 10, 200, 300 (and air handling unit 100) in response to the signals. The memory storage device includes stored instructions that, when executed by the microprocessor, cause the energy recovery wheel assemblies 10, 200, 300 to operate in response to one or more sensed conditions or inputs.

Claims

1. An energy recovery wheel assembly comprising

a support frame at least partially defining an air-supply section that directs outdoor air into a building and an air-exhaust section that directs indoor air from the building,

a motor coupled to the support frame, and

a wheel rotor coupled to the support frame and driven in rotation about an axis relative to the support frame by the motor, the wheel rotor including an outer case, a wheel mount coupled to the support frame, and energy absorption media located between the outer case and the wheel mount,

wherein the energy absorption media is made up of a plurality of sheets, the energy absorption media having a depth within a range of about 15 inches to about 40 inches, each sheet having a thickness within a range of about 0.003 inches to about 0.01 inches.

2. The energy recovery wheel assembly of claim 1, wherein each sheet comprises at least one of aluminum, stainless steel, and copper.

3. The energy recovery wheel assembly of claim 2, wherein each sheet further comprises a desiccant coating.

4. The energy recovery wheel assembly of claim 1, wherein the motor drives the wheel rotor to rotate at a rate of about 8 revolutions per minute or less.

5. The energy recovery wheel assembly of claim 4, wherein the rate is maintained when the wheel rotor is exposed to low temperature environments.

6. The energy recovery wheel assembly of claim 1, wherein the wheel rotor is a first wheel rotor and the energy recovery wheel assembly further includes a second wheel rotor spaced apart from the first wheel rotor along the axis.

7. The energy recovery wheel assembly of claim 6, wherein the first wheel rotor is configured to rotate about the axis in a first direction and the second wheel rotor is configured to rotate about the axis in an opposite second direction.

8. The energy recovery wheel assembly of claim 7, wherein the first wheel rotor is a sensible wheel rotor without any desiccant coating and the second wheel rotor includes a desiccant coating.

9. The energy recovery wheel assembly of claim 7, wherein the first wheel rotor is configured to rotate about the axis at a first rate and the second wheel rotor is configured to rotate about the axis at a second rate different than the first rate.

10. An energy recovery wheel assembly comprising

a first wheel rotor coupled to the support frame and configured to rotate about an axis relative to the support frame, and

a second wheel rotor coupled to the support frame and configured to rotate about the axis relative to the support frame,

wherein the first and second wheel rotors each include energy absorption media that is configured to transfer at least one of heat and moisture between air flowing through the air-supply section and air flowing through the air-exhaust section as the first and second wheel rotors are rotated about the axis.

11. The energy recovery wheel assembly of claim 10, wherein the energy absorption media is made up of a plurality of sheets, the energy absorption media having a depth within a range of about 15 inches to about 40 inches, each sheet having a thickness within a range of about 0.003 inches to about 0.01 inches.

12. The energy recovery wheel assembly of claim 11, wherein each sheet comprises at least one of aluminum, stainless steel, and copper.

13. The energy recovery wheel assembly of claim 10, wherein the first wheel rotor is configured to rotate about the axis in a first direction and the second wheel rotor is configured to rotate about the axis in an opposite second direction.

14. The energy recovery wheel assembly of claim 13, wherein the first wheel rotor is a sensible wheel rotor without any desiccant coating and the second wheel rotor includes a desiccant coating.

15. The energy recovery wheel assembly of claim 13, wherein the first wheel rotor is configured to rotate about the axis at a first rate and the second wheel rotor is configured to rotate about the axis at a second rate different than the first rate.

16. The energy recovery wheel assembly of claim 10, wherein the first wheel rotor and the second wheel rotor are configured to rotate about the axis at a rate of about 8 revolutions per minute or less and the rate is maintained when the wheel rotor is exposed to low temperature environments.

17. A method of recovering energy from air in a building, the method comprising

displacing outdoor air from outside of the building through an air-supply section of an air handling unit toward an interior of a building,

displacing indoor air from the interior of the building through an air-exhaust section of the air handling unit that is separate from the air-supply section,

rotating a first wheel rotor about an axis that extends parallel to and is located between the air-supply section and the air-exhaust section,

rotating a second wheel rotor about the axis,

18. The method of claim 17, wherein the energy absorption media is made up of a plurality of sheets, the energy absorption media having a depth within a range of about 15 inches to about 40 inches, each sheet having a thickness within a range of about 0.003 inches to about 0.01 inches.

19. The method of claim 17, wherein the first wheel rotor is rotated about the axis in a first direction and the second wheel rotor is rotated about the axis in an opposite second direction.

20. The method of claim 19, wherein the first wheel rotor is configured to rotate about the axis at a first rate and the second wheel rotor is configured to rotate about the axis at a second rate different than the first rate.