Kinetic Energy Recovery System in a Bicycle using a Flywheel

Table of Contents

1 Introduction

2 Design of Components

3 Fabrication of Components

4 Assembly and Working

5 Cost Estimation

6 Conclusion

7 Bibliography

CHAPTER 1 INTRODUCTION

This chapter introduces the principles and the problems and ideas that were encountered during the fabrication process. It also included the proposed actions, the expected outcomes

1.1. OVERVIEW AND BACKGROUND

Flywheel is a rotating mechanical device used to store rotational kinetic energy. Flywheels have a significant moment of inertia and thus resist changes in rotational speed. The amount of energy inside a flywheel depends on rotational speed, mass, and geometry. Flywheels are used to store and release energy as kinetic energy. Since there is no energy transformation (for example: from mechanical to electrical energy) involved during storage and release, the efficiency is high. Flywheels can act like mechanical batteries as an alternative to chemical batteries.

Brakes are devices that regulate the motion of a rotating body or a shaft. Most brakes are friction devices. These devices convert the kinetic energy to heat energy by friction. This, however, releases energy into the surrounding, which could be considered as a waste of energy. Harnessing and recovering this energy for useful work is possible by using regenerative braking.

Automotive that uses internal combustion engines (ICE) create harmful pollutant gases as their byproducts. Efforts have been made to improve the efficiency of cars and to reduce the harmful emissions. Alternative replacements to ICE vehicles are electric cars powered by chemical batteries. An electric car does not emit pollutants and does not emit so much heat into the atmosphere. However, charging the chemical battery for electric cars can take hours, compared to filling up a fuel tank of internal combustion engine vehicles.

Flywheels can store energy and if used as a mechanical battery, it can be charged to its maximum capacity in minutes or even seconds. A flywheel can be charged by increasing its rotational speed. It releases energy as it slows down. If used on a vehicle, an external power source would rotate the flywheel to a certain RPM (revolutions per minute). The flywheel will then be engaged to the wheels, thus driving the vehicle. As the vehicle run, the flywheel will slow down, emptying the energy stored.

Regenerative braking is the process of braking while recovering the kinetic energy and stored for use. The recovered energy can be stored in a chemical battery using a generator, and then retrieved using an electric motor for acceleration. Another way is using the strong moment of inertia of the flywheel for braking, and upon acceleration, uses it to drive the wheels again.

The flywheel speeds up as the bicycle brakes and slows down immediately as it accelerates. That work demonstrates the FKES but not as storage for energy from an external source but only as a temporary storage for recovered kinetic energy. Bicycle is an alternative to vehicles that requires an energy source. It has no emissions and it does not pollute. However, it has very limited power. The capability of the driver determines the performance of the vehicle. At level ground, the human power is enough to propel the vehicle forward. Going up a slope, however, usually requires a different speed ratio between the wheels and the driver. Additional torque is supplied to the wheel while sacrificing speed. Going down a slope generates plenty of kinetic energy for the bicycle but drivers tend to use brakes and regulate the speed for safety. Regenerative braking would allow the driver to store the generated kinetic energy while going down then use it for additional power while going up or speeding up.

A flywheel mechanical battery can be used as storage both for powering the vehicle and regenerative braking. This system on a vehicle would allow it to store energy using charging station that will rotate the flywheel to its maximum speed. The flywheel will slow down and speed up as the vehicle decelerates and accelerates using the regenerative braking system. In theory, the kinetic energy of the vehicle from the flywheel would not be lost upon deceleration. It would be recovered and used again. Practically, mechanical and frictional losses occur, thus, not all of the energy will be recovered. By law of conservation of energy, the vehicle should never lose kinetic energy, but in real situation, machine components tend to have losses.

1.2. FLYWHEEL KINETIC ENERGY STORAGE

A vehicle requires an energy source that has enough power to accelerate the vehicle and the occupants. It should also have enough energy density to keep continuous supply of power. Flywheel kinetic energy storage (FKES) should have enough energy density to propel its own mass and the whole vehicle and occupants. It should also have a means of transmitting the required power to the wheels at varying speeds since the flywheel will slow down as it gives up energy. This study demonstrated all these requirements on a bicycle.

Regenerative braking has a means to recover and reuse the energy upon deceleration and acceleration. The brakes should be able to reduce the speed of the vehicle or to put it into halt. The moment of inertia of the flywheel must be enough to affect and control the speed of the vehicle. The flywheel kinetic energy storage serves as the storage for regenerative braking. Both systems are integrated in a single mechanism.

The bicycle is the platform to demonstrate both FKES and regenerative braking. A mechanism would engage the driving wheel to a flywheel mounted on the bicycle. The flywheel is charged by using an external power source to spin it to a certain speed. The speed of the flywheel indicates the energy it contains since both its mass and geometry are constant. The bicycle is not powered entirely by the flywheel. In this setup, the flywheel provides the needed boost in situations, as going up a slope. The setup allows the vehicle to draw and store energy from an external source and recover the energy upon braking and use it again for boost.

1.3. OBJECTIVES OF THE PROJECT

With the problems solved and defined, the following general and specific objectives were accomplished.

1.3.1. General Objective

This project is aimed to design, fabricate, and demonstrate FKES and regenerative braking on a bicycle. The design was intended to, first allow the bicycle to store kinetic energy and use it to power the driving wheel. Second, it allows the flywheel to recover the kinetic energy when it is engaged. Both systems are installed on a normal bicycle. The prototype demonstrates both flywheel kinetic energy storage and regenerative braking.

1.3.2. Specific Objectives

The following were the specific objectives of the study:

1. Design, select, and install a flywheel on the bicycle that has enough energy density and strength to store energy for boosting and powering the bicycle and the driver. The flywheel should also have enough moment of inertia to affect the bicycle speed.
2. Design and fabricate a mechanism that enables the pedal or the rear wheels to spin the flywheel.
3. Design and fabricate a mechanism that will transfer power between the driving wheels and the flywheel during charging and boosting.

1.4. SIGNIFICANCE OF FKES

The advantages and disadvantages of FKES were demonstrated using the bicycle as a platform. The essential mechanisms of a FKES system are tested to give rise to improving the system.

The design and selection of the flywheel gave rise to factors that must be considered first in order to create proper FKES for vehicles. The design of mechanism for transfer of energy from the external source can be used as basis to design charging stations in case FKES system is used in the production vehicles. The design of the mechanism for transfer of power between the flywheel and the wheels explores the essential techniques to be considered to design a similar mechanism for other vehicles. The use of regenerative braking extends the range of the FKES system. All of the knowledge that was generated can be used to design the same system for other vehicles or applications other than transportation.

1.5. SCOPE AND LIMITATIONS

This project included the selection and testing of a flywheel that can store energy to power a bicycle. Design and fabrication were also an option. However, this study was not intended to invent a new flywheel. All information and knowledge that were used currently exist and no new knowledge was generated in the design of the flywheel.

The mechanism for spinning the flywheel using an external power source did not include the design for that external source. The external power source was the pedals, which can be cranked for pre-charging. The mechanism just allowed other means to charge the flywheel aside from braking regeneratively.

This project did not intend to reinvent the bicycle or to create something for mass production. The bicycle still functioned as a vehicle powered by the driver. The FKES just aided the bicycle to attain more power and speed and reduce the input from the driver. The FKES alone was not enough to drive the bicycle for prolonged distances. The overall performance optimization of the design was not included. The design of the transmission was not part of the thesis.

The use of regenerative braking extended the range of the FKES. The flywheel had very limited energy density. Instead of releasing the energy to the surroundings as heat during braking, it was brought back to the flywheel. Conventional friction brakes was still required for safety. It did not intend to replace conventional braking system. When the flywheel was at its maximum rotating speed, the friction brakes was used.

1.6. THEORETICAL CONSIDERATIONS

The flywheel kinetic energy storage and regenerative braking system on a bicycle have three major parts: flywheel, charging mechanism, and power transfer mechanism. This chapter discusses the important factors considered in the design.

1.6.1. Flywheel Kinetic Energy Storage

The kinetic energy of a spinning flywheel depends on three factors: mass, geometry, and rotation speed. Mass moment of inertia refers to the resistance of an object against change in angular momentum. The moment of inertia of the flywheel depends on its mass and geometry. Mass and rotation speed are directly proportional to energy content. The relationship between kinetic energy, moment of inertia, and angular speed is shown in equation (1.1).

Abbildung in dieser Leseprobe nicht enthalten

Where:

Ek = kinetic energy, in joules

I = moment of inertia, in kg-m2

ω = angular speed, in rad/sec

The relationship of moment of inertia to mass and geometry of a solid disc or cylinder with axis perpendicular to circular face is shown in equation (1.2).

Abbildung in dieser Leseprobe nicht enthalten

The relationship of moment of inertia to mass and geometry of a thinwalled cylinder with axis parallel to face is shown in equation (1.3).

Abbildung in dieser Leseprobe nicht enthalten

Where:

I = moment of inertia, in kg-m2

m = mass, in kg

r = rotation radius, in meters

Energy density refers to the kinetic energy content per mass of a flywheel of certain mass density and geometry. In practical applications, flywheels have maximum rotational speed, depending on the material, before begin to yield or shatter. The centrifugal forces can break a flywheel apart. The tensile strength of the material limits the mass, geometry, and rotation speed in designing a flywheel. The geometry of the flywheel corresponds to a geometric shape factor. The energy density is defined in equation (1.4).

Abbildung in dieser Leseprobe nicht enthalten

Where:

Ek = kinetic energy, in joules

m = moment of inertia, in kg

K = angular speed, dimensionless

σ = tensile strength, Pa

ρ = mass density, kg/m2

In designing a FKES on a vehicle, flywheels with too much mass are not ideal. The added weight on the vehicle reduces the effective driving force. Increasing the speed generates greater kinetic energy but it could induce greater stress on the flywheel and its speed ratio with the wheels will be too high. Another consideration is the geometry of the flywheel, specifically, its moment of inertia. The thin-walled cylinder (as shown in equation 1.3) has the greatest moment of inertia at a given mass. It implies that all of the flywheel’s mass must be concentrated at the rim or at a point furthest from the center of rotation.

1.6.2. Power Transfer

There should always be a good balance between torque and speed for every situation. Torque refers to the moment of force or the ability of a force to create a rotational motion to a body at an axis. As the flywheel speeds up, lesser torque is needed to keep it rotating at a certain supply of power. The relationship between torque, force and rotational radius is shown in equation (1.5).

Abbildung in dieser Leseprobe nicht enthalten

Where:

τ = torque, in N-m

F =force, in Newton

r = lever arm or radius of rotation, in meters

The relationship between torque, angular speed, and power is shown in equation (1.6)

Abbildung in dieser Leseprobe nicht enthalten

Where

P =force, in Watts

τ = torque, in N-m

ω = angular speed, in rad/sec

The mechanism for power transfer between the flywheel and the driving wheel could use a chain drive or a belt drive. Chain drive offers uniform gear ratio while belt drive could slip. Chain drive is favorable since standard bicycles use chain drives and the commonality of parts would lessen the complication in the design. A transmission is also required to keep the torque on the flywheel and the wheels uniform at different rotation speeds. The speed ratio between the flywheel and the driving wheel should be minimized upon braking to minimize the moment of force on the flywheel while putting it at higher speed. This creates a stronger braking force. The speed ratio upon acceleration or boosting would depend on the current speed of the bicycle and whether the bicycle is on level ground or going uphill where more torque is needed. When going uphill or accelerating from very low speed or from rest, very large torque is needed. This can be attained by using high speed ratio. When cruising on a level ground at relatively higher speed, a lower speed ratio is necessary.

The transmission has limited speed ratio, thus the flywheel also has maximum speed. At maximum speed, it can no longer be used for braking since there is not enough torque to be induced on the wheels. When the flywheel is at minimum speed or at rest, it cannot be used for boosting. Instead of driving the vehicle forward, it will impede the movement of the vehicle. The transmission should be as simple and compact as possible to reduce the total weight. Continuously Variable Transmission (CVT) is ideal to facilitate smooth braking and acceleration. The CVT allows infinite number of ratios between two limits, thus the optimal ratio can always be attained. A Derailleur transmission can also be used for simplicity and lesser weight in expense of having less control of braking and boosting intensity.

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