Regenerative Braking using Carbon Nanotubes
Graduate Category: Engineering and Technology Degree Level: MS in Mechanical Engineering Abstract ID# 387
Regenerative Braking using Carbon Nanotubes Corbin Martin, Sanwei Liu, Carol Livermore Department of Mechanical & Industrial Engineering, NEU, Boston, MA
Design
Data
• Traditional braking systems have 0% efficiency • Electric cars can increase efficiency by ~50% • There are currently no viable mechanical regenerative braking systems
Input (left) Ragone plot of typical energy densities vs. power densities of different energy storage technologies. Adapted from T. Christen, J. Power Sources, 2000. (right) Plot of failure strain vs. Young’s Modulus for various materials.
Abstract The mechanical properties of carbon nanotubes (CNTs) have been studied extensively over the past two decades, but their practical applications have been slow to follow. One potential application for CNTs is energy storage; because of their very high tensile strength and elasticity, CNTs have tremendous potential as energy storing springs. This project uses CNTs' potential to create an all-mechanical regenerative braking system based on CNT springs. The benefits of regenerative braking are clear; conventional braking systems have 0% efficiency because all of the vehicle's kinetic energy is lost through heat (friction) in the brakes. Although hybrid cars regenerate energy during braking or downhill travel, there is an inherent energy loss in their conversion of energy from the mechanical domain to the electrical domain for storage, and back to the mechanical domain to power the car. This project aims to reduce this energy loss by leveraging CNTs' exceptional mechanical properties to enable a high performance, entirely mechanical method of regenerative braking. The system couples a wide CNT yarn to the rotating wheel via a bevel gear mechanism. When the brakes are applied, the bevel gear engages, resulting in converting the system's kinetic energy into elastic energy that is stored in the torsionally-loaded yarn. To release the energy, the bevel gear is removed, allowing for rotation in the positive direction. Initial measurements suggest that the yarn can absorb over 9000 kJ/m3 when twisted by the rotating wheel, 85% of which is available to restart the system when the stored energy is released.
This mechanism was used to measure energy densities of torsionally-loaded CNTs. A motor applies torque to one end, while a torque transducer collects measurements at the fixed end.
The simple, singlewheel design (left) can measure released energy of a twisted CNT yarn.
Output
Twisted CNT Properties (1’’ of Tape) Strain Energy 5.06E-06 kJ Torque 1.50E-04 N*m
Wheel Dynamics Acceleration 3.74 rad/s2 Max. Speed 0.502 rad/s
The wheel dynamics, above, are calculated using a moment of inertia of 4.01E-5 kg*m2 for the mechanism. The calculations also assumed 50% energy loss due to other losses, such as friction.
Various lengths of CNT yarn were loaded to failure to measure maximum energy densities. As expected, the maximum torque is independent of the yarn length. The angle of rotation, however, is dependent on length. The small fluctuations in the curves are caused by misalignments in the testing device. Energy Input at Maximum Strain from Load to Failure Tests Sample Length .5'' 1.5'' 1'' Strain Energy (kJ) 6.60E-03 2.29E-02 1.84E-02 Volumetric Strain Energy Density (kJ/m^3) 10,100 11,700 14,000 Gravimetric Strain Energy Density (kJ/kg) 34.5 40.0 48.0 Max Torque (N*m) 1.73E-04 1.55E-04 1.71E-04 Rotations 11.4 48.6 45.3 .5'' Sample Cyclic Loading
-5
A more complex mechanism (above) can be used to model other features of the braking system: • Release or collect energy on demand • Disengage to prevent overloading • Accelerate the wheel in the positive direction To compensate for increased loses in the complex mechanism, the thickness and length of the yarn can be increased. This will results in changes to the maximum torque, number of rotations and strain energy, as outlined in the table Increase thickness by ‘n’ Increase length by ‘m’ Total
Torque n3/2 1 n3/2
Rotations n-1/2 m n-1/2*m
Energy n m n*m
Acknowledgments The authors would like to thank Jon Doughty for machining most of the parts used to create the braking mechanism. Also, thank you to Nanocomp Technologies Inc. for providing CNT yarn.
9
Yarns were also loaded cyclically. The .5’’ sample reached a maximum efficiency between 30-35 rotations. Higher rotations continued to yield more released energy, but with less efficiency.
Cycle 1 Cycle 2
x 10
Cycle 1 Cycle 2
8 7 Torque (N*m)
Background
6 5 4 3 2 1 0 0
50
100 150 Angle of Rotation (rad)
200
250
Cyclic Loading Efficiency Stored Energy Released Energy Efficiency Density (kJ/kg) Density (kJ/kg) 11.72 9.93 84.8% 9.15 8.26 90.3%
Future Work Physical models have been created, and testing is ongoing. The efficiency of the braking system will be determined, and adjustments will be made to the model. Based on results, the system may be expanded to 2 wheels or a model car.
Regenerative Braking using Carbon Nanotubes Corbin Martin, Sanwei Liu, Carol Livermore Department of Mechanical & Industrial Engineering, NEU, Boston, MA
Design
Data
• Traditional braking systems have 0% efficiency • Electric cars can increase efficiency by ~50% • There are currently no viable mechanical regenerative braking systems
Input (left) Ragone plot of typical energy densities vs. power densities of different energy storage technologies. Adapted from T. Christen, J. Power Sources, 2000. (right) Plot of failure strain vs. Young’s Modulus for various materials.
Abstract The mechanical properties of carbon nanotubes (CNTs) have been studied extensively over the past two decades, but their practical applications have been slow to follow. One potential application for CNTs is energy storage; because of their very high tensile strength and elasticity, CNTs have tremendous potential as energy storing springs. This project uses CNTs' potential to create an all-mechanical regenerative braking system based on CNT springs. The benefits of regenerative braking are clear; conventional braking systems have 0% efficiency because all of the vehicle's kinetic energy is lost through heat (friction) in the brakes. Although hybrid cars regenerate energy during braking or downhill travel, there is an inherent energy loss in their conversion of energy from the mechanical domain to the electrical domain for storage, and back to the mechanical domain to power the car. This project aims to reduce this energy loss by leveraging CNTs' exceptional mechanical properties to enable a high performance, entirely mechanical method of regenerative braking. The system couples a wide CNT yarn to the rotating wheel via a bevel gear mechanism. When the brakes are applied, the bevel gear engages, resulting in converting the system's kinetic energy into elastic energy that is stored in the torsionally-loaded yarn. To release the energy, the bevel gear is removed, allowing for rotation in the positive direction. Initial measurements suggest that the yarn can absorb over 9000 kJ/m3 when twisted by the rotating wheel, 85% of which is available to restart the system when the stored energy is released.
This mechanism was used to measure energy densities of torsionally-loaded CNTs. A motor applies torque to one end, while a torque transducer collects measurements at the fixed end.
The simple, singlewheel design (left) can measure released energy of a twisted CNT yarn.
Output
Twisted CNT Properties (1’’ of Tape) Strain Energy 5.06E-06 kJ Torque 1.50E-04 N*m
Wheel Dynamics Acceleration 3.74 rad/s2 Max. Speed 0.502 rad/s
The wheel dynamics, above, are calculated using a moment of inertia of 4.01E-5 kg*m2 for the mechanism. The calculations also assumed 50% energy loss due to other losses, such as friction.
Various lengths of CNT yarn were loaded to failure to measure maximum energy densities. As expected, the maximum torque is independent of the yarn length. The angle of rotation, however, is dependent on length. The small fluctuations in the curves are caused by misalignments in the testing device. Energy Input at Maximum Strain from Load to Failure Tests Sample Length .5'' 1.5'' 1'' Strain Energy (kJ) 6.60E-03 2.29E-02 1.84E-02 Volumetric Strain Energy Density (kJ/m^3) 10,100 11,700 14,000 Gravimetric Strain Energy Density (kJ/kg) 34.5 40.0 48.0 Max Torque (N*m) 1.73E-04 1.55E-04 1.71E-04 Rotations 11.4 48.6 45.3 .5'' Sample Cyclic Loading
-5
A more complex mechanism (above) can be used to model other features of the braking system: • Release or collect energy on demand • Disengage to prevent overloading • Accelerate the wheel in the positive direction To compensate for increased loses in the complex mechanism, the thickness and length of the yarn can be increased. This will results in changes to the maximum torque, number of rotations and strain energy, as outlined in the table Increase thickness by ‘n’ Increase length by ‘m’ Total
Torque n3/2 1 n3/2
Rotations n-1/2 m n-1/2*m
Energy n m n*m
Acknowledgments The authors would like to thank Jon Doughty for machining most of the parts used to create the braking mechanism. Also, thank you to Nanocomp Technologies Inc. for providing CNT yarn.
9
Yarns were also loaded cyclically. The .5’’ sample reached a maximum efficiency between 30-35 rotations. Higher rotations continued to yield more released energy, but with less efficiency.
Cycle 1 Cycle 2
x 10
Cycle 1 Cycle 2
8 7 Torque (N*m)
Background
6 5 4 3 2 1 0 0
50
100 150 Angle of Rotation (rad)
200
250
Cyclic Loading Efficiency Stored Energy Released Energy Efficiency Density (kJ/kg) Density (kJ/kg) 11.72 9.93 84.8% 9.15 8.26 90.3%
Future Work Physical models have been created, and testing is ongoing. The efficiency of the braking system will be determined, and adjustments will be made to the model. Based on results, the system may be expanded to 2 wheels or a model car.
