GREEN MACHINE

GYM EQUIPMENT ENERGY CONVERSION

Project Team: Matthew Bruchon Blake Gates Zachary Johnson Erik Peterson Sponsor: Dr. William Edmonson

Spring 2008

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TABLE OF CONTENTS

I. Executive Summary............................................................ 3

II. Newsletter..................….................................................... 5

III. Project Report

1. Introduction.................................................................................................. 6

2. Background.................................................................................................. 7

3. Product Requirements................................................................................. 8

4. Design Alternatives...........……………………………………......................... 9

4.1 Power Generation………………………………………………………... 10

4.2 Power Regulation………………………………………………………... 14

4.3 Power Usage or Storage…………………………………….................. 17

4.4 Monitoring System……………………………………………………….. 19

4.5 Protective Devices……………………………………………………… 21

5. System Design Descriptions

5.1 Design Option 1: Direct Application…………………………………..... 22

5.2 Design Option 2: DC Network………………………………………...… 23

5.3 Design Option 3: Grid Tie................................................................... 25

6. Construction Details …………………………………………………................ 29

7 Future Improvements………………………………….................................... 31

8. Business Considerations.............................................................................. 33

8. Conclusion……………………………………………………………………........ 34

IV. Appendices.........................................................................35

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I. Executive Summary

Project Objective

This project’s objective was to research possible designs for converting the mechanical energy

from gym equipment to electrical energy, and ways to use or store that energy. Various design

options were researched, and the costs and benefits of each option were evaluated.

In addition to this research component, a second objective of the project was to build a working

prototype of one possible design. This prototype was necessary to check assumptions that were

made in our design, learn how it could be improved, and assess its feasibility for use on a larger

scale gyms.

Research & Analysis of Design Options

Research of possible energy conversion systems was divided into five basic aspects of the

design: power generation, power regulation, usage or storage of power, monitoring of the

system’s output, and protection of the system from harmful conditions. Each of these design

aspects had several possible methods of implementation; each possible method was analyzed and

the tradeoffs involved were discussed.

After discussing the design options for each area of the design aspect, we considered how these

options could be combined into a working system. When considering the design at a system

level, we considered three possible uses of the generated energy: direct powering of a load,

application to a DC network, and transferal onto the power grid. Each of these possible uses was

considered and tradeoffs were discussed.

Implemented Design

An exercise bike was used in the prototype that was built. The figure below illustrates our

implemented design.

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DC

Bike/Generator

Source

2N2905

100 5k

0.10.10.1

200

pF1.5k

150

5k

Watt Meter/

Load

LM308

LM338 LM338 LM338

Diode

Design Day Implementation

The wheel of the bike was connected to the shaft of a generator via a rubber wheel. A diode was

used to prevent the backflow of electrical energy into the generator. The diode output was

connected to a voltage regulator. In early testing, a DC to AC converter was used to power AC

loads, but for our final demonstration, only DC loads were connected to the system output. A

watt meter, powered by the system, was used to monitor the system’s output.

Results and Conclusions

The research component of the project evaluated a wide range of possible designs, both at the

level of individual components and at a system level. All three possible system designs were

assessed to feasible; direct application of the power generated is the least costly option, but the

overall benefit depends on the intended use.

The prototype was considered successful, although there were limited resources and several

setbacks. For instance, wear on the generator shaft’s wheel forced a power drill to be used to turn

the generator for demonstrations. However, the fundamental design of the system was sound and

functional, and could provide a basis for further development of a more efficient and marketable

system in the future.

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II. News Release

N.C. State Students Research “Green” Exercise Machines April 30, 2008 - In a time when oil prices and global warming have led the nation to consider

renewable energy more seriously than before, a group of students at N.C. State University

recently researched an untapped energy source: human exercise.

For their Electrical & Computer Engineering Senior

Design Project, a group of four students—Zach

Johnson, Blake Gates, Erik Peterson and Matthew

Bruchon—researched ways to use gym equipment to

produce and store electricity. Their faculty sponsor

in the E.C.E. Department was Dr. William

Edmonson.

The team began by comparing different exercise

machines see what kind would generate the most

energy and be the most marketable; they found an

exercise bike’s popularity and simple design to be ideal.

Next, they looked at three distinct uses for the generated energy: storing it on the power grid to

reduce or eliminate power bills, storing it in a system of batteries, or directly powering a load,

such as a TV screen or a cell phone charger. After doing this, they analyzed how to convert an

exerciser’s effort into electricity most efficiently.

In addition to this research project, the students also implemented a prototype of their design.

The prototype exercise bike, dubbed the “Green Machine”, was able to directly power AC and

DC loads, such as a rotating fan, and the output power could be monitored.

The success of the design, despite limited resources, gave the team high hopes that it could be

feasible for future use at a gym. If it were further developed, it could help boost gym

membership as part of a “green” marketing program. More importantly, it could save money,

reduce energy consumption, and help make the earth a little bit cleaner.

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III. Project Report

1. Introduction

The goal of this project was to analyze the feasibility of using energy generated from exercise

equipment to creating a more self-sustaining gym. To do this, a single piece of exercise

equipment was used as a test subject. The factors considered in choosing what type of machine

to conduct our study with were: ease of connection to a generator, cost, efficiency, and

popularity of the machine type.

After determining what type of equipment was ideal, research was done to find a cost-efficient

way to generate and use the energy. This research included the analysis of methods to generate

power, regulate power, apply it to a load, monitor system output, and prevent damage to the

system. It also included the analysis of three possible system-level design options: to store the

energy converted, to use it to directly power a device of some kind, or to route the power onto a

grid.

Also, a monitoring system was implemented to measure and display the system’s power, voltage,

and current output. By measuring system output at different points, conversion efficiency could

easily be measured.

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2. Background

There are several motivations for creating a system of energy conversion that gyms can use to

generate their own power. Perhaps the most obvious reason is to reduce costs and increase

profits. For the most part, the mechanical energy generated by exercise is completely wasted, so

there is an untapped potential to make the energy useful. If a gym were able to implement such a

system on a large scale, across many or all of its machines, the power generated by the gym’s

customers would offset the gym’s electricity bill. Ideally, enough power would be generated that

it could be returned to the power grid, possibly even turning electricity costs into a net profit.

As public awareness of global warming and carbon neutrality grows, the environment is

becoming a greater reason for implementing such a system at gyms. If a gym were able to

reduce its power consumption, or even produce a net amount of power, its carbon footprint

would be reduced, improving its environmental impact.

The reasons for considering the environment are not only ethical, but also commercial. If a gym

could advertise itself as a “green” gym, its public image would improve and more customers

might be attracted. In fact, the increase in membership could make such a system profitable even

if the electricity savings alone could not offset implementation costs.

Because of these reasons for generating electricity from exercise, there have been several efforts

to research and develop such a system. One case is the California Fitness health club in Hong

Kong, which connected 13 exercise machines to a system of batteries to power some of the

gym’s lights. The system was not found to be profitable, but the environmental benefits were

significant. In the Netherlands, a similar system is being made to power dance floor lights with

energy made from dancers. Research is also being conducted into parasitic generators, which

would use everyday movement such as walking to harvest energy.

These past efforts were well beyond the scope and the budget of this project. Instead of directly

implementing and analyzing large-scale systems, this project’s focus is on optimizing the design

of an individual machine and analyzing whether it would be cost-effective if implemented on a

larger scale.

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3. Product Requirements

R1. The overall cost of the product shall not exceed $250.

R2. A bicycle or gym bike shall be used as the source of the mechanical power.

R3. The generator will output DC.

R4. The power created will have two uses:

a. Power an appliance for display purposes

b. Power the tool used to measure and display system output

R5. The characteristics that will be monitored will at least include:

a. voltage

b. current

c. power

d. kWh

R6. A DC to AC converter will be use to transform created power into a useable source for

appliances and/or power grid.

R7. The output of the generator and CD to AC converter will be monitored and displayed on an

LCD screen.

R8. The device shall be capable of being connected with other power generators to create a

network of sources.

R9. A feasibility study will be done to determine the practicality and plausibility of

implementation in a medium-sized gym.

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4. Design Alternatives

When researching and designing possible design alternatives, we divided down the design into

five major design aspects:

Power generation

Power regulation

Usage or storage of energy

Monitoring of system output

Protection against system damage

Each of these design aspects is, to some extent, are dependent on the implementations of the

other aspects. For example, if the conversion system should power a TV that requires AC

voltage, an AC generator can be used and voltage can be regulated to 120 VAC 60Hz, or we can

select a DC generator and convert our generated VDC into 120 VAC 60Hz.

A more detailed discussion of design alternatives for each aspect of the design follows.

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4.1 Power Generation

The consideration of design alternatives for the generation of power can be further divided into

the areas of exercise equipment, the equipment’s interface with a generator or alternator, and the

generator or alternator.

4.1.1 Exercise Equipment

In order to determine which type of exercise equipment to use, characteristics such as popularity

of machine, cost, and ease of connection were taken into consideration. Based on these

parameters, it was decided that a standard exercise bicycle, Figure 3, will be best qualified for

this specific project. Other choices that were considered were a treadmill, rowing machine, and

an elliptical trainer.

The reason the bicycle was chosen was mainly its simple design, which often incorporates an

open wheel. This open wheel allows for the bike to easily make a strong, high-friction

connection to any standard generator. The type of bicycle used will be one with a wheel in front

of the bike with a diameter no greater than 26”. An ideal bike for this project is pictured below:

Figure 1 - Schwinn IC Elite Indoor Cycling Bike, $1099.00

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4.1.2 Generator/Alternator

The generator is one of the most important components of the entire system. Because of this,

special consideration and calculations must be taken in order to ensure that the correct one is

used. There were several options to choose from for the generator including a DC motor, AC

motor, or an alternator. For each specific motor, there were several advantages and

disadvantages that had to be compared in order to select the best possible generator for a bicycle

generator.

An alternator could have worked for this project but there are a few design flaws that would

interfere with the rest of our system. First, an alternator would only allow a person to pedal the

bike at a one constant speed no matter how hard they tried. This is because an alternator is

designed to always spin at the same rate, so that a constant output is always produced. This

could be a desired characteristic in some cases; however for our purposes this would greatly

prohibit the goal of the project. This is because it would not allow the person who is pedaling

the bike to pedal at their own pace and would hinder a person’s workout.

The next generator that was considered was an AC motor. This has several advantages,

including that it is easier to regulate the output, simpler to produce, and relatively cheap.

However, there was one major concern with the AC motor that had to be considered: AC motors

are hard to operate and relatively useless at low RPMs. This is due to thermal consideration of

the design. Because of this, most AC motors are only used for high power systems, not a bicycle

generator. This was an issue because a person using the exercise bike might be pedaling at a

RPM that was lower than the motor was designed for. If this were to happen, then it would be

difficult to produce or use the power that was generated.

The final motor that was considered was a DC motor. The characteristics of this type of motor

proved to be ideal for our problem. A permanent magnet DC motor was chosen that would

produce 12-24V at 1800RPM and 1/6-1/4 HP. These ratings were chosen since most of our

applications would operate between 12-24V. Also, it was determined through a ratio of the

circumference of the generator shaft to the circumference of the bicycle wheel that 1800RPM

would be the expected input to the generator based on the fact that the user would pedal on

average at 15 mph. Since voltage is proportional to RPMs, if the user pedaled faster or slower

than the desired 1800 RPMs, then, they would see similar results in the voltage. These results

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would be acceptable in our system since a regulator would be used later down the line in order

keep the voltage from going to high.

One problem of choosing a DC motor is that they are a little more expensive than the other

motors to purchase. This will make part procurement slightly more difficult since it must stay

within a budget. The next problem with choosing a DC motor is that the output would need to be

converted to AC for AC loads including a smart charger which would then be used to charge a

battery. During the conversion there will be some energy loses but it was a compromise that is

needed so that a smart charger could be implemented further down the line in our system. Since

this was the only negative characteristic of the DC motor for the bicycle generator application, it

was choice.

4.1.3 Equipment Interface

One of the main issues that needed to be considered in our design was how to connect the

mechanical input produced from the exercise bicycle to the generator shaft so that it could then

be converted into electrical energy. There were several issues that needed to consider here,

including how difficult it would be connect it and the efficiency of a design.

The first idea considered was to use a belt, where one end would wrap around the circumference

of the bike wheel and the other end around the shaft of the motor. In this design, as the bike was

pedaled, the generator shaft would move at a proportional rate. One issue in this case was that

depending on the input provided by the exerciser, the belt could possibly slip. This was

important because there would be several types of user on an exercise bicycle, and the belt would

have to be able to handle different torques.

Another issue was the efficiency of using either a belt or a chain. In mechanical systems, energy

losses due to friction occur due to points of contacts in a system. In the belt system previously

described, there would be a fairly large area of contact, thus producing fairly high energy loses.

The one advantage of this system is that it would be relatively easy to implement. However, the

belt system would make it difficult to keep the system portable and allow it to transfer from one

bicycle to another.

The next design that was considered was to use a system of gears and pulleys that would rotate

the generator shaft as the bike was pedaled. The main problem with this design is that it would

take a lot of mechanical knowledge to implement and would be rather complicated. Efficiency

was a problem here as well. The only thing gears due to a system is cause it to lose energy. This

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design would also make it difficult to transport and move from bike to bike if it needed to be

done.

The design that was most practical was a one-contact-point design. In this set-up a small rubber

wheel would be placed on the shaft of the generator. The size of the wheel would be chosen so

that the generator would produce on average 1800 RPMs, the rating of the motor. This rubber

wheel would then be placed in direct contact with the wheel of the stationary exercise bike so as

the stationary bike wheel spins, the wheel on the generator spins, thus producing electricity. A

strip of rubber would be placed around the wheel of the stationary bike so that slippage would be

kept at a minimum. Like the previous designs, slippage was a major concern that had to be

addressed.

The generator shaft should be placed in a way where the weight from the bicycle itself will help

keep a constant contact between the two wheels. So for this to work it is recommended that the

generator shaft be placed under the bicycle wheel. Also, the material that the wheel that sits on

the generator is made from must be durable enough to withstand wear and tear. These issues and

findings will be discussed in a later section.

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4.2 Power Regulation/Conversion

4.2.1 Regulators

The generator that was used in this project produces a DC voltage from the range of 12V – 24V.

Unfortunately, the inverter that will convert the DC voltage into AC will only allow a range of

10V – 12V before it automatically shuts off due to an installed safety feature. A problem then

remains: how does the voltage maintain a constant output in order to properly operate the

inverter continually, rather than in bursts?

Two options were researched in solving this problem. The first was a DC/DC converter in

which a voltage within a specified range will be input, and then a constant voltage will be output.

However, this still reverts back to the original problem of how to keep the voltage within a range

while someone may be pedaling at either a slow or an extremely high rate of speed. The price of

the DC/DC converter also was found to be close to $200.00.

The second option involved using a LT1083 12V voltage regulator. The voltage output would be

typical for what the project needed; however, the allowed current could not pass 7.5A. The

current output of the generator that we had chosen was anywhere from 8A – 15A. To solve this

problem, we used a parallel connection of three of the LT1083 voltage regulators in order to

keep the voltage constant, but increase the allowable current. The schematic for the voltage

regulator circuit can be seen in the section IV of this report. One component of the regulator

circuit was the use of a variable potentiometer in order to increase or decrease the gain of the

regulator.

4.2.2 Ultracapacitor

The voltage that is supplied by the generator will drastically vary depending on the user of the

bicycle. Some users will produce high voltage for longer periods of time and others will barely

reach a particular voltage level. The voltage will also change depending on what position the

pedals of the bike are in. Because of these varying voltages it is necessary to keep a constant

voltage so that the inverter can then transform the DC voltage to AC. To meet this requirement

it is necessary that an ultra-capacitor is used after the diode and before the inverter in the system.

Ultra-capacitors have become more visible in today’s applications as they have been

implemented in many systems like electric cars and other areas where they are being used to

replace batteries. One of their advantages is that ultra-capacitors have a virtually unlimited life

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cycle unlike batteries, which must be replaced often. The other advantages are that it will reduce

the DC ripple in the input voltage as mentioned earlier, as well as allows for fast charging. Also,

one would not have to worry about over charging or a protective circuit like one that would be

need for a battery.

In the designs listed above, it is recommended that a 58 Farad capacitor be used. This will

provide enough stability to make sure that the varying input voltage will remain as constant to 12

volt DC input that is needed by the inverter. These were priced at roughly $200.

During the original design process, a simple voltage regulator circuit was considered an

implemented to perform the same task as mentioned above with the ultra-capacitor. However, it

was discovered that the input voltage varied too much for the voltage regulator to perform its

function. Further information on this issue will be discussed later in this document.

4.1.3 Inverters

From our proposed design, this was the step from which the first problem arose. The ideal

choice for a generator was DC rather than AC, as stated earlier, but the required input voltage to

the Smart Charger and other AC loads would be AC.

Figure 2 – AC Inverter

Our particular voltage inverter solves this problem by converting the DC voltage from the

generator and outputting an AC voltage for the Smart Charger and other AC loads at an

efficiency rating of up to 90%. The chosen inverter can be seen in Figure 2. In addition to

inverting the voltage, the inverter also regulates the voltage coming from the DC generator.

Although the generator is rated to 12V at 1550 rpm, a higher input from the user will produce a

higher voltage output. The inverter, while classified as a 12V Power Inverter, still will accept

any input voltage between 10V – 15V. As a safety precaution, the inverter will automatically

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shut down any inversion when the input falls out of that range. However, it is important that the

input voltage to the inverter is well regulated due to this feature. This is why an ultra-capacitor

was preferred over a simple voltage regulator.

Once the 12V power inverter receives the voltage, it will convert it to a 120V, 60Hz AC voltage

which has 400 watts of continuous power and 800 watts of start up or peak power. This will be

enough to power the loads connected the design that are discussed later.

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4.3 Power Usage/Storage

The third stage of our design involves the question of how to apply the power that is produced.

4.3.1 Direct Application

There are several choices when it comes to the loads that can be hooked up to the system. The

design makes it possible for both AC and DC loads to be powered at the same time or

individually. This mean the user could power an iPod charger or a portable fan. If direct

application is not needed then the user could output the voltage to a battery or battery bank. The

size of this bank could be customizable by adding various batteries in parallel to the output of the

system. This bank could also be stored in a separate room or closet out of sight from the user of

the system. All that would be required is for cables to be run under the floor from the closet to

the location of the bicycle. These cables would then need to be connected to the DC output

terminals of the system.

The only issue that must be taken into consideration is that there is a limit to how many loads

one can power depending on how fast they are pedaling and how much power the specific load

requires. Otherwise, there are no restrictions to what one could be able to power with the

system.

4.3.2 Battery System/Smart Charger

The DC network design showed in section III.5.2 shows the implementation of a system where a

battery is charged. However, using this idea, it was realized that with a constant charge onto the

battery of certain devices, the battery could potentially be overcharged, causing harm to not only

the battery unit, but the device itself.

To fix this problem, the use of a Smart Charger could be implemented, seen in Figure 3. This

device provides circuitry for the battery to prevent any type of overcharging and opens the

connection once the battery is completely charged and once the battery voltage drops below a

certain threshold, closes the connection and allows for current flow back to the battery.

Typically, deep cycle batteries are better suited for this type of device. The reason for this being

that our design will continually charge and discharge a battery. Batteries such as a laptop battery

degrade over time whenever they are charged and discharged, and therefore will not be sufficient

for our needs.

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Figure 3 – Smart Charger

4.3.3 Grid Tie

Design three is similar to the rest of the designs except for the addition of a grid tie. Grid ties are

commonly used in PV and wind power systems to allow the energy created from these systems

to be put back on the power grid. The grid tie will take the power produced by the AC inverter

and then regulate it and put it in phase with the rest of the power grid so it can be safely added.

The benefit of having a grid tie is that one is able to basically have an unlimited power supply to

store the energy that is created. This power would then be sold back to the energy companies so

that a particular user’s power bill would then be reduced according to how much power was

generated. However, the only downfall of this device is that it is extremely expensive and would

not necessarily be practical for a system where a huge amount of power is not generated.

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4.4 Monitoring System

The ideal system for monitoring the system’s output depends on whether it will be used simply

for test purposes, or whether it will be used as a display for an end user (e.g. at a gym). Three

display systems are considered here: a “Watt’s Up” meter, a Labjack setup, and a custom GUI.

4.4.1 “Watt’s Up” monitoring system

One option is to use a “Watt’s Up” monitoring system, ideal for simple testing and

demonstrations. This device will be able to determine several characteristics of the load input

including voltage, current, power, watt hours, amp hours, peak watts, and peak volts. The Watt’s

up meter, shown in Figure 4, will be placed on the handle bars so the user will be in easy view

of his or her performance. However, the meter is portable enough to be placed anywhere around

the bicycle as long as there is enough wire to still connect it to the rest of the system.

Figure 4 – Watt’s Up Meter, $50

The Watt’s up meter was chosen over other devices like a Labjack because of its simplicity and

the ease of use. No programming or customization involved and the parameters that are

monitored will easily be available to the user. The Watt’s Up meter is also cheaper than other

similar products on the market, with a price of roughly $50.

4.4.2 Labjack

A more robust but costly option is to use a Labjack to monitor the system’s output. The Labjack

allows the system output to be monitored on a computer; it connects to the computer using a

USB cable, which also powers the Labjack. The Labjack is pictured in Figure 5.

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Figure 5 – Labjack U12, $130

The Labjack’s main advantage is its versatility. It has a large number of input ports, and they

can be simultaneously read and displayed on a graph in real-time using Labview software.

However, the cost of this versatility is high; the Labjack costs $130. Also because of this

versatility, a high degree of customization is needed. Existing sample code for graphing inputs

over time are available, but the code would have to be heavily modified to account for power,

which would be the multiplication of the input current and input voltages.

Another disadvantage is the complexity of the circuits required to connect the Labjack to the

energy conversion system. The voltage would need to be stepped down using a voltage divider,

and the current would need to be reduced using a resistive shunt circuit. Of course, reducing

these values would mean the signals input to the PC would have to be scaled by a certain factor

in Labview, and this would introduce an element of inaccuracy.

4.4.2 Custom GUI

Virtually all existing gym machines use a custom GUI, and a similar monitoring system would

be needed on any marketable version of an energy conversion system. A microcontroller could

be used to measure input current and voltage, calculate instantaneous power and the total

generated energy, and display them in a way that would demonstrate to the user how much was

being generated. However, this was beyond the scope of this project, due to the need for

substantial configuration of the microcontroller and custom coding of the GUI.

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4.5 Protective Devices

4.5.1 Diodes

As mentioned earlier, a diode is needed in the circuit to prevent backflow of power to the

generator. Since the output of the system could be connected to batteries or other storage

devices, there is a real possibility that the power generated could then cause the pedals on the

bike to spin if no one is using the equipment, which is a safety hazard. This is certainly not an

acceptable result, so a stud-mounted diode rated at 20 Amps was inserted between the generator

and the capacitor to prevent this from occurring.

4.5.2 Fuses

Another problem that was encountered is the unexpected increase in current that is generated

whenever somebody would get on the bike and pedal extremely fast in a short time. In doing so,

the generator would produce a high current which could in turn cause the voltage regulator to

become overloaded and eventually damaged. To correct this problem, a fuse was inserted into

the circuit. The job of the fuse is to be installed either after the ultracapacitor or before the

voltage regulator. The reason for the change in location is because when an ultracapacitor is

used, it can handle the fluctuation in current, but the watt meter, which is next in the circuit

diagram cannot. The same holds true when a voltage regulator is used. The regulator cannot

handle the increase in current and therefore a fuse must be inserted in order to protect it.

Because the voltage regulator can handle up to 15A, the fuse that would be used would be a 20A

fuse. The idea follows that whenever the current reaches a level higher than this, the fuse will

blow and become an open circuit, therefore blocking any current flow to the next component in

the circuit.

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5. System Design Descriptions

All of the system designs are similar to each other in one way or another. There are only a few

component additions as you progress through each design. An overview of each design can be

found below. With each, we will discuss performance costs and analysis, and the overall

marketability of each design.

5.1 Design Option 1: Direct Application

5.1.1 Design Overview

The idea of the direct connection design is that a device can be powered directly from the design.

Any type of AC or DC load which can be powered on the 12V DC power, or the 120V, 60Hz AC

voltage under 400W can be powered from this design. The use of the ultracapacitor allows for

short time storage of energy which can be discharging whenever the user stops riding for a

period of time. Whenever the capacitor is completely discharged, there will no longer be power

to the loads and therefore they will shutoff.

5.1.2 Performance and Cost Estimates

The direct application design is the next cheapest design in which the only change is the addition

of the ultra-capacitor. As mentioned before, the ultra-capacitor is a much needed component in a

high performance design and was not included in the demonstrated design due to cost factors.

Because of its importance, all other designs do include the addition of this part.

5.1.3 Marketability

Whenever used in a gym setting, this design could be extremely ideal for people who would like

to power some sort of electronic device while they are working out. Modern gyms sometimes

have televisions in front of the equipment with headphone jacks beside of the bike itself. Using

this design, it could possibly be used as a motivational tool for people who would like to power

the television or even an iPod or other MP3 player while they are working out. As long as they

are working, then the device will be powered. However, once they stop pedaling, they will only

have a short time before the capacitor discharges before the device will shutoff.

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5.2 Design Option 3: DC Network

5.2.1 Design Overview

This design takes the basic direct application design and adds onto it a battery and smart charger

system. Unlike the direct application though, a rider is storing up energy into a deep cycle

battery so that even whenever they stop pedaling for an extended amount of time, their devices

will still be charged. A smart charger is put in series with the battery in order to ensure that the

battery is never overcharged, which could in turn cause damage to the entire design. As stated

before, a deep cycle battery would be best used. This battery does not need to be stored directly

beside the machine, but rather could be kept in a storage closet out of the way of the bicycle area

itself. The final component of the design is the ability to add multiple pieces of exercise

equipment together, as seen in Figure 3. The original design of the DC network is kept the same;

however, multiple designs are connected to a single battery or battery bank.

5.2.2 Performance and Cost Estimates

The network design scheme also shows an increase in price due to the fact that a battery system

must be implemented. This could become costly due to the need to replace batteries as well as

the additional parts that must be purchased to ensure the safe storage of the energy generated.

The results that were received as far as the prototype that was built were not overwhelmingly

impressive. It was estimated that if one gym has 10 bicycles utilizing this system for 10 hours a

day for an entire year, only $365 would be saved. This is not a huge number, but if all the gyms

used them, then a somewhat substantial amount could be considered for savings. Due to this fact,

it is estimated that gym savings and profits could be increased by marketing the idea of a “green”

gym and the many benefits that would be created from that concept.

Whenever using the single battery, multiple machine design, the cost is not affected. Because

each bike can be connected with no extra components, there is a direct proportionality to the cost

and the number of bikes a certain gym might have. In fact, the more machines a gym contains,

the more beneficial it is, because they are only having to buy one battery or battery bank

compared to a battery per bike such as the design in Figure 2 suggests.

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5.2.3 Marketability

Because it is not necessarily feasible for a gym to completely produce its own power, this could

possibly be the most cost effective option for the gym. Like the direct application design, it

allows users the ability to power their own devices such as a television or MP3 player. A simple

diode could be implemented in order for the device to cutoff whenever they stop pedaling or

keep being powered by the battery system that is implemented. Even when someone is not using

any of the power they are generating, that power is still being stored in a battery somewhere to

ensure that it is not just wasted immediately as a thermal byproduct. The power that is stored in

the batteries be used to run lights, computers, cash registers, or even other machines whenever

needed. A backup system with regular outlet power would be used to ensure that the devices do

not shut down whenever the batteries are completely discharged.

Finally, the most marketable part of the DC network is that it is the one design that utilizes all of

the pieces of exercise equipment that a gym might contain. By connecting each bike or other

piece to a single battery bank, the gym can store a significantly higher amount of energy used to

power other machines, lights, cash registers and computers.

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5.3 Design Option 3: Grid Tie

5.3.1 Design Overview

The use of a grid tie is an ideal solution for any gym who thinks that they might ever produce an

excess of needed power. Whenever power is generated, it is phase matched with the power grid

installed by the local electric utility company and then power is fed onto those lines. No power

is actually stored locally, but by selling power back to the power company, a gym could reduce

their electrical bill depending on the amount of power they are generating. The overall idea of

this is similar to the use of solar panels on someone’s house whenever they are generating more

power than their house uses.

5.3.2 Performance and Cost Estimates

The grid tie design is by far the most expensive of the four designs described in this document.

This is due to the high costs of the grid tie itself. Grid ties can range anywhere from $3,500 to

$35,000 and could be a huge financial burden for a customer’s implementation. However, this is

the only way to efficiently and safely get the power back on the power grid.

5.3.3 Marketability

As stated before, this design appeals most to a gym that can create an excess of power which can

therefore be sent back to the power company. The benefits of this are obviously that money

would be saved each month, directly on their power bill based upon how much power is actually

generated. While it may be impractical to produce enough power to self-sustain the gym with

the exercise equipment alone, with the combination of solar panels, it can easily be achieved. As

of right now, electric utilities tend to steer away from the idea of buying back power from

individual customers, but with a new wave of green energy sweeping the country, this will soon

change and become much more common than it has been in the past.

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Figure 1 – AC/DC Direct Application

Bike/

GeneratorDiode Capacitor Fuse

Watt

Meter

DC

Adapter

DC/AC

Inverter

AC LoadDC Load

AC

Adapter

Bike/

GeneratorDiode Capacitor Fuse

Watt

Meter

DC

Adapter

Junction

For

Multiple

Bikes/

Generators

Smart

Charger

Diode

Battery

System

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30 April 2008 27 / 37

Figure 2 – DC Network

Figure 3 – Network Configuration

Bike System Bike SystemBike System

DiodeDiode Diode

Battery

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30 April 2008 28 / 37

Figure 4 – Grid Tie Design

Bike/

GeneratorDiode Capacitor Fuse

Watt

Meter

DC/AC

Inverter

Grid

Tie

Power

GridAC

Adapter

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30 April 2008 29 / 37

6. Construction Details

The components for this system could easily fit inside an enclosure that is 22” long by 16” wide.

The only parts that would not fit inside the enclosure would be the battery or battery bank.

These would have to be external to they system and could be stored in a closet or another room

to save space.

Possible Prototype Assembly

The following are illustrations of what a prototype of the system could look like. The design

would allow for the open wheel of the bicycle to sit in the groove in the middle of the design.

The wheel would then be in contact with the gray wheel on the shaft of the generator. This gray

wheel can easily be seen in the top view.

Top View

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Side View

From the side view, the different power outlets are easily in view. On the right is an AC plug, in

the middle are two 12 Volt DC plugs, and on the far left are two miscellaneous DC terminals that

could be connected to a battery bank or for other uses. Not pictured in the illustrations are the

two cords that would run up to the “Watt’s Up” meter. These cords could easily exit from one

side of the device go to the meter, and then enter back into the other side of the device. The legs

of the system would be adjustable so that the system could be raised or lowered depending on the

height of the bicycle. This would also make sure that the bicycle and the generator wheel are in

tight contact with one another. Another feature not pictured in the illustrations could be a strap

or connector that attaches to the front legs of the bicycle. These braces could be used to pull the

system closer to the bicycle wheel, thus ensuring better contact and connection with the

generator shaft.

Again, these are just preliminary designs so that one could get an idea of the system enclosure.

These designs could easily be altered to meet other demands as well.

Front View

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7. Future Improvements

After designing the prototype there are several areas of the design that could be improved upon

and issues that should be considered in the future design of this system.

The first area of improvement is the voltage regulator that is used to steady the input from the

generator. As mentioned earlier, the voltage from the generator was originally regulated in the

design by a simple voltage regulator circuit (See Appendix). This circuit originally consisted of

three voltage regulators in parallel so that they could handle up to 15 Amps. The circuit would

take an input voltage of and regulate it to a voltage between 4.5V and 20V. This value was

determined by a potentiometer that was available for the user to turn. However, when the

voltage regulator was tested in the system, it was apparent that it was not working with the

bicycle. The bicycle that was being tested was extremely old and did not have a smooth spinning

motion to produce a relatively constant voltage. Although this problem was anticipated—thus

the voltage regulator—the solution proved not to be as effective as was hoped. The drastic

changes in voltage as the system transitioned from not having a load to having a load proved to

be too much for the voltage regulator to handle.

As a solution to this problem, it is recommended that an ultra-capacitor be used. This concept

was mentioned earlier in this document and is believed to be the best possible way to solve the

issue. The ultra-capacitor, although more expensive, will be able to handle the sudden change in

voltage and provide the instantaneous power that is demanded once the load is turned on.

Another issue that must be taken into consideration is the connection between the generator on

the shaft of the wheel and the bicycle wheel. It was determined that for an application where it

was necessary for the system to be portable that a single point of contact of design would be best.

However, due to the friction between the two wheels, it is possible for the contact between the

two to become weaker over time. This was experienced during the original prototype testing

when the rubber composite that was on the generator shaft wheel started to wear away over time.

This wear formed a groove in the generator shaft wheel so that when bicycle wheel turned, there

was slippage.

To offset this problem, a stronger material should be used on both the generator shaft wheel and

the bicycle wheel as well. A durable rubber could easily solve this problem and provide the

efficiency in connection that is needed. Overall, a belt system might be the best solution to this

particular problem, but given the demand to be able to easily and quickly set-up the system, a

single point of contact design is still the best.

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The last area of improvement that should be looked at is the type of bicycle that is chosen to be

used with this system. As mentioned earlier, the bicycle that was chosen to test a prototype

design was extremely old and not in the best of condition. When the bicycle was pedaled, the

rotation of the wheel was inconsistent and varying. This proved to be a great problem in

reaching the results that were desired. The bicycle machines that are used today in cycling

classes would be the best design to be used with the system. They still provide the open wheel

design and have a large flywheel that continues to spin for a long period of time once the user

stops pedaling. These bicycles would be better suited to provide a smooth and constant rotation

of the generator shaft and limit problems downstream in the system with instantaneous voltage

changes.

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8. Business Considerations

To a large extent, the ideal design depends on the desired use of the electricity that is generated.

This is a business question, and is largely outside the scope of this project, but a brief discussion

of possible business considerations is included here.

The initial finding of this project is that the power savings that would result from using this

design are not enough alone to break even on the initial investment in a reasonable amount of

time. For this reason, a marketing component would help to make the investment in a “green

machine” a profitable one. One obvious idea is to purchase energy conversion machines and

create a marketing campaign to redefine a new “green”, environmentally friendly image for the

gym. This would help to increase membership at the gym.

One possible use of the energy generated would be to directly power a load, as discussed

previously. If the load were something such as a television or a cell-phone charger, this would

provide an entertaining or at least useful form of motivation for the exerciser, which would

encourage new membership and more frequent use of the gym by members.

However, the general public’s perception of “green” technologies and helping the environment is

centered on benefiting the common good; energy converters that feed the power grid or store

energy in a battery bank would lend themselves to this more easily. In addition to creating a

sense of good stewardship and responsibility among individual users, it would help foster a sense

of community at the gym. This could be done by displaying the gym’s aggregate daily energy

savings, in dollars or in pounds of greenhouse gas offset.

Another business idea some gyms have explored is to create a credit system based on the energy

a member generates. This would be done by keeping track of the kilowatt-hours generated by

the user, who could receive a small discount on his or her monthly dues if a certain amount of

energy were generated. One way to implement this would be by swiping a keycard before the

user starts exercising.

Once this keycard system were created, it could be extended to log other things, such as the

user’s cumulative calories burned and their overall workout patterns; this information would be

of interest to the individual user, and could be given to them periodically. Of course, this would

largely be dependent on exercise machine manufacturers adding this ability to their machines,

and it would not be possible for older machines.

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9. Conclusion

Despite several setbacks and many limitations that resulted from time and budget constraints, the

research and prototyping components of this project were found to be a success.

The research findings provide a wide breadth of information related to a large number of

possible designs. Based on the benefits and disadvantages of each possibility described in this

report, both at the level of individual components and system-level designs, future development

efforts have been given a good foundation to work from. To a large extent, this project found

that the ideal design depends almost completely on the desired application. For example, if the

energy converted would be used to provide instant motivation to work out, a direct load should

be powered, which affects design decisions throughout the system.

There were several setbacks and budget limits that affected the prototyped exercise machine.

Regardless of these, the prototype allowed many of the basic principles of the system to be

analyzed, and verified many of the concepts explored in the research component of the project.

While the design is far from being robust enough for market, it is a good starting point for future

development. Based on the initial findings of this project, it is reasonable to believe this design

could be developed further and eventually become efficient enough to be profitable, especially

when mass production and the economies of scale are considered along with the more intangible

benefits of a “Green Machine” being used at gyms.

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IV. Appendix

Bicycle Used for Prototype

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30 April 2008 36 / 37

DC

Bike/Generator

Source

2N2905

100 5k

0.10.10.1

200

pF1.5k

150

5k

Watt Meter/

Load

LM308

LM338 LM338 LM338

Diode

Circuit Used for Prototype (Voltage Regulator in Dashed Lines)

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30 April 2008 37 / 37

Cost Chart

Parts

Direct

Application Grid Tie Network

Chosen

Design

Bicycle Donated Donated Donated Donated

Generator $80 $80 $80 $880

Diode $10 $10 $10 $10

Capacitor $220 $220 $220 X

Fuse $8 $8 $8 $8

DC Adapter $30 X $30 $30

AC Adapter $25 $25 X $25

Watt Meter $56 $56 $56 $56

Regulator X X X $15

Inverter $34 $34 X Donated

Diode #2 X X $10 $10

Smart Charger X X $70 X

Battery Bank X X $110 X

Grid Tie X $3,000 X X

Misc. $50 $50 $50 $50

Cost $513 $3,483 $644 $284