PEDS 2007

Energy Harvesting from Exercise Bicycle

Suchart Janjornmanit, Samart Yachiangkam, Aswin Kaewsingha.Department of Electrical Engineering, Rajamangala University of Technology Lanna, Chiangmai, Thailand 50300

E-mail: jansuchartggmail.com

Abstract-This paper presents design of energy harvestingfrom exercise bicycle. The topology is based on lead-acidbattery charge controller using dc-dc converter. The CUKconverter is selected to deliver the energy and to regulate thecharging voltage. The methodology of charging is adaptedfrom the recommendation of IEA, which is the modifiedconstant voltage charging with equalizing mode. Theexperimental exercise bicycle is constructed to verify thevalidity of the proposed design.

Index Terms-Energy Harvesting, Exercise Bicycle,Renewable Energy, Alternative Energy, CUK converter,Charge control.

I. INTRODUCTION

Nowadays, the energy crisis leads the scientist to eagerlysearching for alternative energy sources. There are manykinds of energy sources, such as fossil energy, solarenergy, kinetic energy, wind energy, e.g. Fossil basedenergy generates much of the energy but pollute theenvironment, even though there are many ways to reducethe pollutions. Furthermore the reserves of this kind ofenergy are dramatically shorten, which is expected to beused up within the next few decades.

This paper proposes energy harvesting from anotherkind of energy source, human energy. City's lifestyle,People are worrying about their weight and trying to cutthe exceeded energy stored in their body by exercise. Whydon't we collect the waste energy? The energy harvestingfrom exercise bicycle is formulated in this research. Theobjective is to deliver the energy from DC generatorthrough the converter to be kept in energy storage. We areproposing Lead-Acid battery charging control system forharvesting the energy. The battery charging system wasproposed for various kinds of energy sources such as donein photovoltaic system in [1]. The recommendation ofcharge control in [2] and the CUK converter are selected toconstruct the experimental exercise bicycle in thisresearch.

This paper is organized as follow: The constant voltagecharging is firstly detailed, followed by the designing ofcontrol in proposed system section. The section ofexperimental exercise bicycle shows structure andperformance of the proposed system. The recommendationof the design and some limitations are detailed indiscussion section. Final section gives conclusion.

II. CHARGING CONTROL

The DC generator is selected to transform mechanicalenergy to electrical energy in this research. The nature ofoutput voltage variation of solar panel due to theastronomy is similarly to the pattern of voltage changing inthe DC generator from the bicycle. Therefore the battery

charge control of the photovoltaic system is adapted to becharging system for proposed approach.

Table I and figure 1 detail some of the recommendedpractices of charge control in [2] for the modified constantvoltage charging. Table 1 gives suggested chargingsetpoints for a selection of battery types. It must be notedthat these should not be considered definitive, and thatthese may have to be adjusted depending on the type ofbattery, the type of controller, the system, the load andwhere it is install. All setpoints are given in Volts per cell(V/Cell); to find the setpoint for a normally 12 V system,for example, multiply by 6. VR setpoint is the constantvoltage used to charge the battery at the intermediate stage.Prior to the VR setpoint, the lowered VR used to boost thecharging and to limit the initial charging current. As soonas the minimum charging current is reached, the floatvoltage, which is the constant voltage lower than the VRsetpoint is used. Theses setpoints are presented assumingthat the controller is equipped to do equalization or anequalization charge is performed manually on a regularbasis.

Figure 1 show graphs of output voltage and outputcurrent of the modified constant voltage charging. Inconstant voltage charging, the amount of charge current isregulated by the controller such that the battery is held atthe voltage regulation setpoint. The nature of battery atinitial stage of charging is that the current is drawn largely,and then is gradually dropped as the battery becomescharged. Such a large amount of current drawn is thedamaging of battery and has to be avoided. Therefore atthe initial stage, the charging voltage is lowered than VRsetpoint, so that the charging current is regulated to themaximum allowance, which is recommended frommanufacturer. The second stage, constant voltage is usedwhen the charging current begin to lower than themaximum charging current at the VR setpoint voltage. TheVR setpoint is held until the charging current is droppeddown to the minimum charging current of the battery

TABLE ISuggested Setpoints for Battery Charging Voltage.

Modified Constant VoltageBattery ChargingType VR VR/Float Equalize VR

Flooded/Vented 2.35 2.40/2.25 2.50LeadAntimonyFlooded/Vented 2.40 2.45/2.30 2.50Lead CalciumFlooded/Sealed 2.35 2.45/2.30 2.50

1-4244-0645-5/07/$20.00©2007 IEEE 1138

.s1st Stage 2nd Stage 3rd Stage

CC100

VR

VFIoat

A. CUKConverterFigure 2 shows CUK converter for the proposed charge

controller. The filter capacitor (Co) in the circuit is used tolimit voltage ripple from DC generator. The reasons ofusing CUK are that the output voltage from DC generator(input of the converter) may be lower or higher thanvoltage of battery (output of the converter) because ofbiking speed, and the current drawing from DC generatorand the current feeding to the battery are continuous wherethe buck, boost and buck-boost have at least one side withpulsed current.The CUK converter uses capacitive energy transfer and

analysis is based on current balance of the capacitor. Theratio of input and output voltage of the converter is

Vo D

Vin (( - D)

Fig. 1. Modified Constant Voltage Charging.

(C/100). At the final stage, the voltage is lowered to floatvoltage, which is another constant voltage used for keepcharging current to the completely charge without harmingthe battery.Many charge controllers shift the VR setpoint up or

down depending on the condition of battery. Finding theright VR setpoint is difficult; if it is too high, the batterywill be overcharge often, and if it is too low, the batterywill never be fully charged. One approach is to normallyapply a fairly low VR setpoint, to avoid excessiveovercharge, but to occasionally raise the VR setpoint, toensure a full charge of all cells every several weeks. Thisoccasionally full charge is called an "equalization" charge.

III. PROPOSED SYSTEM

The goal of harvesting energy from exercise bicycle is toconvert the mechanical energy to electrical energy, thendeliver it to store in battery. CUK converter is used to passthe energy to battery by regulating the output voltage inaccording to charging current. For CUK converter, thecontrolling of duty cycle of PWM is the controlling ofoutput voltage of the converter. This section gives thedetails of CUK converter and the proposed PWMgeneration.

LI CI

Since the duty ratio "D" is between 0 and 1 the outputvoltage can vary between lower or higher than the inputvoltage in magnitude. The negative sign indicates areversal of sense of the output voltage.

B. Microprocessor Based Charging ControllerFigure 3 shows block diagram of PWM control. The

PWM output is generated to achieve desired output voltageby adjusting duty cycle in according to (1). PID controlleris chosen to be the regulator of the duty cycle. Themeasured output voltage is compared to referent voltagespecified in which the output current is regulated at theproper value. The relation between output voltage andcurrent is depicted from the relation shown in figure 1. Themeasured current and voltage are firstly averaged beforethe adjusting and comparison.The relation between charging voltage and current is

such a complex. This is hard to construct analog circuit tocontrol all functions, but it is much easier if this is done bythe fairly expensive microprocessor. The low cost digitalsignal processor, dsPIC is selected to control all of thesesduties. The performance of the energy harvesting by usingthis processor is acceptable and cost effective.

L2

Battery(DC

Fig. 2. CUK Converter for the charge controller.

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10/o

C/3

Vo/

(1).

Vo

Fig 3. Diagram of the PWM Control.

IV. EXPERIMENTAL EXERCISE BICYCLE

Table II details all parameters using in the experimentalmodule of the energy harvesting. Figure 4 shows theexercise bicycle and all control circuits connected. Figure5 shows process values during operation. There is a littlebit of voltage ripple around the target output voltage,especially when the empty battery is used. But at themiddle through the end of charge stage, the chargingvoltage is maintained stable.

V. DISCUSSION

The output power of the exercise bicycle is limited bythe size of DC generator and battery. For our module,1OOW DC generator is used, 7.5AH battery is tested andthe output of the bicycle is actually limited by the size ofbattery, which is about 30W. To gain more output powerthan this, the capacity of battery have to be increased andthe setpoint of charging current need to be revised. Thereis a switch to select battery capacity in our module, whichis selectable to be 7.5AH or 24AH battery. The DCgenerator may be another choice of increasing outputpower, to do this the corresponding size of battery,switching device and all set points must be considered.

TABLE IIParameters in Experimental Exercise Bicycle.

Parameters ValueDC Generator + Gear 1OOW, 24VDCSealed Lead-Acid Battery 12V, 7.5AHMaximum Charging Current C/3 (Use 2.OA)Minimum Charging Current C/100 (Use 75mA)VR Setpoint 14.7 VoltsFloat Voltage 13.8 VoltsEqualize Voltage 15.0 Volts(Manual Select)Digital Signal Processor dsPIC3OF2O 10Sampling Frequency 200kHzSwitching Frequency 2kHzSwitching Device IRFZ46Filter Capacitor 3,300uFC1 Capacitor 3,300uFC2 Capacitor 3,300uFLI Inductor l5mHL2 Inductor 15mH

Fig. 4. Experimental Exercise Bicycle.

Fig. 5. Process values.

VI. CONCLUSION

The energy harvesting from exercise bicycle is proposedin this paper. The proposed topology is based on lead-acidbattery charge controller for photovoltaic system. Themodified constant voltage charging control, which isrecommended by IEA, is adapted and implemented in thisresearch. CUK converter is used to deliver the energy,from bicycle mounted DC generator, to store in the battery.The cost effective digital signal processor, dsPIC is chosento perform PWM control in which the battery is chargedproperly by charging voltage and current. Theexperimental exercise bicycle is constructed and tested,and good performance is achieved. The designrecommendation of sizing the output power is alsodiscussed for implementation.

REFERENCES

[1] E. Koutroulis, K. Kalaitzakis. "Novel battery charging regulationsystem for photovoltaic applications". Electric Power Applications,IEE Proceedings, Volume 151, Issue 2, pp. 191 - 197, Mar. 2004.

[2] E.P. Usher, M. Ross. "Recommended practices for chargecontrollers". IEA Implementing Agreement on Photovoltaic PowerSystems. Aug. 1998.

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