Energy Harvesting from PassiveHuman Power                    Faruk Yildiz            Sam Houston State University         ...
lier, not very applicable to some of the low-powered         ment, PFCB, was available to test with a small con-devices su...
age level, frequency, and corresponding time. AC                                                          signals and volt...
very-low-current outputs were still produced that             Technology based DC-DC buck-boost converter andmight be harv...
voltage drops across the capacitor when measuring                                                           the voltage wi...
Figure 7. Intermediate-voltage-sensitive switch with hysteresisodes, MOSFET switches, and resistors to transfer the       ...
voltage output of the circuit can be easily modified     battery at the nominal voltage level, which is 3.6V atby using di...
allow for the reading of output values on the oscillo-        (as reported by tests). The graph in Figure 12 com-scope dis...
Quick                                                     Charge                            Charge                        ...
ment demonstrated that if two sneakers (each with             also necessary to determine the power consumptionone PFCB at...
calculation of the total gained current from humanpower through the PFCB that is assembled in thesneaker insole during one...
need for batteries, or enabling them to be chargedwhile the energy is being discharged. These sys-              [10] “DC-D...
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  1. 1. Energy Harvesting from PassiveHuman Power Faruk Yildiz Sam Houston State University E-mail : fxy001@shsu.edu ABSTRACT dios, became available in the market. The commer-Sustaining the power resource for autonomous wire- cially available Freeplay’s [3] wind-up radios requireless and portable electronic devices is an important 60 turns in one minute of cranking, which allows forissue. Ambient power sources, such as a replace- the storage of 500 Joules of energy in a spring. Thement for batteries, can minimize the maintenance spring system drives a magnetic generator and ef-and the cost of operation by harvesting different ficiently produces enough power for about an hourforms of energy from the potential energy sources. of radio play.Researchers continue to build high-energy-density Recently researchers have performed severalbatteries, but the amount of energy available in studies in alternative energy sources that could pro-the batteries is not only finite but also low, limit- vide small amounts of electricity to low-power elec-ing the lifetime of the system. Extended lifetime of tronic devices. These studies focused on investingelectronic devices is very important and also has and obtaining power from different energy sourcesmore advantages in systems with limited accessibil- such as vibration, light, sound, airflow, heat, wasteity. This research studies one form of ambient en- mechanical energy, and temperature variations.ergy sources: passive human power generated from The piezoelectric energy-harvesting methoda shoe/sneaker insole when a person is walking or converts mechanical energy into electrical energy byrunning and its conversion and storage into usable straining a piezoelectric material [4] which causeselectrical energy. Based on source characteristics, charge separation across the device, producing anelectrical-energy-harvesting, conversion, and stor- electric field and consequent voltage drop propor-age circuits were designed, built, and tested for low- tional to the stress applied. The oscillating systempower electronic applications. is typically a cantilever beam structure with a mass at the unattached end of the lever, since it provides INDEX TERMS higher strain for a given input force [5]. The volt-Circuit Analysis, Circuit Design, Energy Conserva- age produced varies with time and strain, effectivelytion, Energy Conversion, Energy Storage, Piezoelec- producing an irregular AC signal. The piezoelectrictric Devices, Piezoelectric Materials energy conversion produces relatively higher volt- age and power-density levels than an electromag- I. INTRODUCTION netic system. Moreover, piezoelectricity has theEnergy harvesting is the conversion of ambient en- ability of elements, such as crystals, and some typesergy into usable electrical energy. When compared of ceramics to generate an electric potential from ato energy stored in common storage elements, such mechanical stress [6]. If the piezoelectric materialas batteries and capacitors, the environment repre- is not short circuited, the applied mechanical stresssents a relatively infinite source of available energy. induces a voltage across the material.Researchers have been working on many projects The problem of how to get energy from ato generate electricity from human power, such as person’s foot to other places on the body has notexploiting, cranking, shaking, squeezing, spinning, been suitably solved. For a radio frequency identi-pushing, pumping, and pulling [1]. Several types of fication (RFID) tag or other wireless device worn onflashlights were powered with wind-up generators the shoe, the piezoelectric shoe insert offers a goodin the early 20th century [2]. Later versions of these solution. However, the application space for suchdevices, such as wind-up cell phone chargers and ra- devices is extremely limited, and, as mentioned ear- 5
  2. 2. lier, not very applicable to some of the low-powered ment, PFCB, was available to test with a small con-devices such as wireless sensor networks. Active stant shaker. The shaker functioned as an ambienthuman power, which requires the user to perform a vibration source (passive human power) and wasspecific power- generating motion, is common and used to vibrate the PFCB to produce electricity formay be referred to separately as active human-pow- the energy-harvesting circuit. A photograph of theered systems [7]. PFCB with the inter-digitized electrodes to align the An example of energy harvesting using uni- field (energy-harvesting circuit) with fibers is shownmorph piezoelectric structures was conducted by in Figure 1.Thomas, Clark, and Clark [8]. This research was fo-cused on a unimorph-piezoelectricity circular plate, Device size (mm): 130 x 10 x 1which is a piezoelectric (PZT) layer assembled on Active elements: AFCBan aluminum substrate. The vibrations were driven Mode: d33from a variable-ambient- pressure source, such as a Full scale voltage range (V): ±400scuba tank or blood pressure monitor. The research- Full scale power range (mW): ±120ers showed that by creating the proper electrode pat- Figure 1. Basic specifications of the PFCBtern on the piezoelectric element, (thermal regroup-ing), the electrode was able to produce an increase in The PFCB energy source is modeled as aavailable electrical energy. In the system, as the can- steady AC power source for the circuit components oftilever beam vibrates, it experiences variable stresses the energy-harvesting circuit. Since all the electronicalong its length. Regrouping the electrodes target- components for the energy-harvesting and battery-ing specific vibration modes resulted in maximum charging circuit require DC voltage to operate, this ACcharge collection. This type of design may offer the source is then converted to a DC voltage source.possibility for miniaturization and practicality topiezoelectric energy-harvesting technology. II. SYSTEM DESIGN The piezoelectric active-fiber composites For the purpose of energy harvesting and storage,(AFCs) are made by Advanced Cerametrics Incor- shoe or sneaker insoles are good sources of me-porated (ACI) [9] from a uniquely-flexible ceramic chanical stress, deformation, and vibration when afiber that is able to capture wasted ambient energy person is walking or moving his/her feet. With thisfrom mechanical vibration sources and convert it method, waste-ambient mechanical energy was con-into electric energy. The piezoelectric composites’ verted to electrical voltage through a unique ener-fiber-spinning lines are capable of generating elec- gy-harvesting circuit. An overall energy-harvestingtricity when exposed to an electric field. In piezo- model is shown in Figure 2 to explain implemen-electric fiber composite bimorph (PFCB) architec- tation steps and potential applications. In order toture, the fibers are suspended in an epoxy matrix have the best efficiency and output power, the cir-and connected using inter-digitized electrodes to cuit was designed and developed according to thecreate an AFC. Advance Cerametrics Incorporated ambient-source, characteristics, PZT ceramic- fiberhas demonstrated through testing that thin fibers composite and load constraints. The energy- har-with a dominant dimension, a length, and a very vesting system is capable of capturing even minutesmall cross-sectional area are capable of optimizing amounts of stress and vibrations, then convertingboth the piezo and the reverse-piezo effects. them to electric power sufficient to run low-power An investigation into the improvement of electronic systems.an energy-harvesting-system performance and ef-ficiency using a PFCB is considered in this research.The PFCB characteristics and properties were inten-sively studied in order to build an efficient energyharvesting circuit for further study. The efficiencyof the PFCB was measured by building an operation-al-difference, amplifier instrumentation test circuitand following an energy-harvesting and battery-charging circuit. Only one type of piezoelectric ele-6 j o u r n a l o f a p p l i e d s c i e n c e & e n g i n e e r i n g t e c h n o l o g y 2011
  3. 3. age level, frequency, and corresponding time. AC signals and voltage outputs were similar to each other; however, the time required for the vibration to decay was observed to be longer when more mass was attached on the PFCB. All signal outputs, that occurred until the vibration decayed completely, could not be shown on the oscilloscope screen. However, it was observed that the time until vibra- tion stops is longer with more mass attached on the PFCB. The resonant frequency does not depend upon the pencil flicks. It depends upon the mass (including distributed mass and lumped-tip mass) and distrib- uted spring constant of the cantilevered beam. For a sinusoidal excitation, the most energy is transferred when excited at the resonant frequency. The decay depends upon the input resistance of the measur- ing device (electrical damping) and the mechanical damping from the material and from the air.Figure 2. Overall energy-harvesting model Power CharacteristicsA piezoelectric energy source is most often mod-eled as an AC voltage source because of its AC pow- Figure 3.The voltage decay of the open-circuit PFCBer characteristics and features. Piezoelectric fibercomposite can be connected in series with the ca- The PFCB was carefully clamped on thepacitors and resistors to reduce or smooth a high- shaker with plastic bumpers to avoid damagingvoltage input produced by PFCB. For test purposes, the part during its vibrations. The wiring betweenthe tip of a mechanical pencil was used to flick the all modules was done carefully to allow for read-tip of the PFCB product in order to provide the ini- ing the voltage outputs from the oscilloscope andtial disturbance. The test equipment used included multi-meter displays. The PFCB layer and materiala multi-meter, shaker, and oscilloscope, connected properties were not known to predict the frequencyto each other properly to obtain voltage readings rate accurately, so the value had to be determinedfrom the PFCB. The first test for voltage output de- experimentally on the test fixture.pended on time variation and was conducted with- In order to allow the calculation of the cur-out any mass placed on the tip of the PFCB. This rent output, wires from the PFCB electrodes werewas followed by a second test with variable masses connected to the oscilloscope probes through a 1that were placed on the tip of the PFCB to observe kΩ resistor. The current outputs could not be mea-the output voltage levels. As the more mass that is sured by a multi-meter and were observed from theadded on the tip of the PFCB, the more time passes oscilloscope screen when the PFCB was vibrated byuntil vibration of the PFCB stops. The voltage from the shaker at 60Hz.the PFCB increases depending on the mass and force It was not possible to plot the current andapplied to the tip of the PFCB. power outputs due to a lack of the proper data- The plot in Figure 3 is the peak-to-peak volt- acquisition system during the research. However, Energy Harvesting From Passive Human Power 7
  4. 4. very-low-current outputs were still produced that Technology based DC-DC buck-boost converter andmight be harvested with a proper energy harvesting battery-charging circuit components [15].circuit. From the vibration test results, it was de- Initially, a full-wave bridge rectifier wastermined that at the variable frequency, the power placed between the PFCB and the operational- am-generated from the PFCB is sufficient for low-pow- plifier-instrumentation circuit that converts ACer electronic devices. The obtained values would signals to DC signals. A full-wave bridge rectifier isbe enough to build an energy harvesting circuit to very efficient, converting positive and negative cy-charge a small-scale storage device such as a bat- cles from the PFCB and supplying DC voltage to thetery, capacitor, or super capacitor, albeit slowly. battery through the battery- charging part of the en- ergy-harvesting circuit. Since the current produced III. ENERGY HARVESTING CIRCUIT DESIGN from PFCB was low, an intermediate-operational-After testing the power output and the working amplifier (op-amp) circuit was used to test variouscharacteristics of the PFCB at different vibration current levels that was generated by PFCB in differ-levels and attached masses, the researcher built the ent decaying times. [16]. This instrumentation cir-energy-harvesting circuit used to charge the bat- cuit consisted of operational amplifiers, resistors,teries under low-current levels. The mechanical-to- and intermediate/storage capacitors to implementelectrical energy conversion is usually managed by the circuit at ±15V.the energy-harvesting circuits including convention- A buck-boost converter and battery- charg-al buck-boost converters, bridge rectifiers, and bat- ing circuit is the last part of the simulation inter-tery-charging circuits [10,11,12,13]. The energy har- face before the storage unit. The op-amp part of thevesting circuit was designed, developed, and built energy-harvesting circuit consisted of three singleaccording to the ambient source and piezoelectric operational amplifiers that are configured as differ-fiber composites’ low-current constraints in order ence amplifiers.to produce efficient power output. This implies that the voltage differential be- The energy-harvesting and battery charg- tween the two branches is the output of the circuit.ing circuit design was built with typical compo- This op-amp-instrumentation-circuit design (Figurenents that could decrease high-input voltages and 5) helped to observe voltage outputs from the PFCBincrease low-input currents from the PFCB to pro- and the capacity changes of the capacitors when thevide sufficient charge currents to the batteries. The PFCB was being vibrated.circuit was designed to start charging when the bat- Capacitors C1 and C11 were charged de-tery voltage drops beyond a nominal value, and it pending on the voltage generated by the PFCB,stops charging when voltage is reached at the bat- which was observed by the oscilloscope through thetery nominal voltage. operational-amplifier-instrumentation circuit. The Linear Technology SPICE simulator(LTSPICE) simulation interface that shows the over-all circuit is depicted in Figure 4; it represents thesystem circuit modules that are simulated togetherto test the output power level of the circuit [14].All the necessary simulations were conducted us-ing SwitcherCADTM Spice III, because of the LinearFigure 4. Energy-harvesting circuit simulation interface8 j o u r n a l o f a p p l i e d s c i e n c e & e n g i n e e r i n g t e c h n o l o g y 2011
  5. 5. voltage drops across the capacitor when measuring the voltage with a digital multi-meter. However, the capacity readings across the capacitor C11 would be accurate, since the operational amplifier keeps the initial voltage-level constant. DC buck-boost converter and battery charging circuit DC-DC converters efficiently step-up (boost), step- down (buck), or invert DC voltages without the neces- sity of transformers. In these structures, switching capacitors are usually utilized to reduce or increase physical size requirements. DC-DC converters assistFigure 5. Operational amplifier instrumentation in product-size reduction for portable electroniccircuit devices where increased efficiency and regulation of input power are necessary for optional require- The general operational amplifier in Figure ments. Taking the above features of the buck-boost5 was used to observe the charging phase of the converters into consideration, a linear-technologyC11 intermediate-storage capacitor. The 5V IC (ini- based LT1512 integrated circuit (DC-DC buck-boosttial voltage) was supplied across capacitors in both SEPIC constant current/voltage battery charger) wascircuits. In circuit A, the voltage across the capacitor used to regulate the high-output voltage that waswith the 10MEG impedance, representing a flux digi- produced from the PFCB to charge small-scale bat-tal multi-meter, was measured. The voltage across teries for test purposes [17].capacitor C1 dropped almost 2V when the circuit A LT1512 battery-charging circuit waswas simulated at the same input voltage. added to the energy-harvesting circuit, consideringHowever, the voltage across capacitor C11 in the its characteristics, based upon an application dataoperational-amplifier circuit stayed at constant volt- sheet. Since buck-boost converters are very sensitive,age other than negligible voltage drops. The voltage proper design, in conjunction with supporting com-level across both capacitors, C1 and C11, was simu- ponents and physical layout, is necessary to avoidlated and is plotted in Figure 6 in order to compare electrical noise generation and instability. The con-voltage drops across the capacitors. siderations, including LTSPICE modeling, converter When the PFCB was placed on the constant selection, circuitry building, debugging, and power-shaker, it started generating voltages by charging output improvements, were followed step-by-step tothe capacitors. The operational- amplifier circuit obtain adequate energy-harvesting circuitry.kept the initial voltage level constant to allow for This circuit would maximize the power flowan accurate reading of the voltage levels of the in- from the piezoelectric device and was implementedtermediate-storage capacitor. In circuit A, the volt- in coordination with a full-wave bridge rectifier, in-age readings would not be accurate because of the termediate-storage capacitor, and voltage-sensitive switching circuit. It was observed that, when using the energy-harvesting circuit, over twice the amount of energy was transferred to the battery compared to direct charging alone. However, if the power- harvesting medium produced less than 2.7V, power flow into the battery was reduced due to losses in the additional circuit components and the threshold characteristics of the LT1512. For the purpose of storing energy in the intermediate storage unit, a capacitor was placed before the voltage-sensitive circuit and buck-boostFigure 6. Voltage levels across intermediate storage converter. The voltage-sensitive circuit consists of di-capacitors Energy Harvesting From Passive Human Power 9
  6. 6. Figure 7. Intermediate-voltage-sensitive switch with hysteresisodes, MOSFET switches, and resistors to transfer the intermediate capacitor and transfers it to the batteryenergy from the intermediate capacitor to the battery through a DC-DC buck-boost converter and battery-through the DC-DC buck-boost converter [18]. charging circuit. The fourth module is the model of The MOSFET switches and zener diodes on a buck-boost converter and battery-charging circuitthe voltage-sensitive circuit sense the voltage in the representing the exact characteristics of the LT1512intermediate capacitor and transfer the energy when SEPIC-constant-current/voltage integrated circuit.the capacitor reaches specific voltage levels. The volt- The voltage level after the intermediate ca-age level in the intermediate capacitor is controlled pacitor and voltage-sensitive switch simulation isby the zener diodes until the capacitor is discharged plotted in Figure 8. VIN (the capacitor voltage level)by transferring its energy to the battery. Depending reaches only 15V, and starts discharging by trans-on the zener diode values, the stored energy in the ferring voltage to the battery. When (VIN) startscapacitor is transferred to the storage unit through decreasing, (VOUT) increases until 15V with the ca-DC-DC buck-boost converter and battery-charging pacitor voltage at 15V. Both (VIN) and (VOUT) start de-circuit (The switch should be between 5V-15V). creasing by transferring energy to the battery. (VOUT) Due to known high-discharge rates of the ca- reaches zero voltage while transferring its energypacitors, the zener-diode-voltage values of 12V and to the battery; simultaneously, the (VIN) value de-6.2V were chosen (which are small values for the pur- creases in order to reach 15V again. The charge andpose of energy harvesting from PFCB) to avoid loos- discharge steps are repeated while PFCB producesing stored energy in the intermediate capacitor. One electricity from vibrations.of the biggest benefits of the intermediate capacitorand voltage-sensitive switching circuit is to increasethe amount of transferred energy from the PFCB. Cir-cuit loss is reduced throughout the energy-harvestingcircuit caused by the electronic components. The circuit shown in Figure 7 was designedhas four phases to represent the overall energy- har-vesting circuit modules. Figure 8. Voltage input and output simulation of The first module is a mechanical-to- elec- voltage-sensitive switchtrical energy conversion module and functions thesame as PFCB producing AC power. The second The DC-DC converter and battery-chargingmodule has rectification (the conversion of AC volt- circuit design, a part of the energy harvesting circuitage to DC voltage) and an energy-storage unit (inter- simulation interface (Figure 9) is employed to han-mediate capacitor). The third module is a voltage- dle the decrease or increase of voltage levels andswitching circuit that senses the voltage level of the adjust it according to the battery specifications. The10 j o u r n a l o f a p p l i e d s c i e n c e & e n g i n e e r i n g t e c h n o l o g y 2011
  7. 7. voltage output of the circuit can be easily modified battery at the nominal voltage level, which is 3.6V atby using different resistance values if a different 60mAh for the test battery.battery is integrated to the system. The battery-charging current (IROUT) and battery-charging voltage (VOUT) simulation plots are depicted to indicate battery-charging values. Both voltage and current levels were supplied at a steady state for proper battery charging, IROUT =5mA, which is a standard charging current for the bat- tery, and VOUT =3.6V nominal charging voltage. The charging current that was generated by the PFCB was less than 1mA but was increased to 5mA by the intermediate capacitors. The intermediate capacitors were charged to the minimum charging threshold of the battery and then released to the battery terminals by discharging themselves, allow-Figure 9. Energy-harvesting and charging circuit ing the capacitors to accept charge voltages from the PFCB again. However, the current level could not Circuit Simulation increase to charge the battery at the quick-chargeThe simulation of the important circuit components phase because of the low current produced by thethrough the LT1512-SEPIC-battery- charging circuit PFCB. The specific voltage and current levels that(including input voltage, battery- charging voltage, are specified in the simulation plot can charge atand current) are depicted in Figure 10. All three im- 3.6V at 60mAh for a fully discharged battery in ap-portant aforementioned parameters of the energy- proximately 27 hours with constant vibrations.harvesting and battery- charging circuit were sim-ulated together to examine the consistency of the Building the Circuitvoltage/current levels on the circuit-design-simula- Considering the variable output voltages from thetion interface. PFCB, an energy-harvesting circuit was designed to charge small-scale NICD/NIMH batteries at the con- stant-charging phase. The printed circuit board for the energy-harvesting circuit was designed and built as small as possible to fit even the smallest places for power generation, including the battery solderedFigure 10. Battery-charging values simulation on the circuit. However, for test purposes, the in- strumentation circuit on the prototyping board and The input voltage (VIN) simulation plot was the energy-harvesting circuit were placed near thegenerated by the vibrations through the PFCB while measuring equipment with the PFCB assembled tobeing shaken. This voltage level was measured afterrectification of the AC voltage signal that came fromthe PFCB unit as a DC voltage and served as the in-put for the buck-boost converter and the voltage-regulator circuit. Since the maximum input voltageof an LT1512- integrated circuit is 30VMAX, a zenerdiode was placed between VIN and the ground ofthe LT1512 in order to avoid damaging the internalchip components of the LT1512. The input voltage(VIN) and regulated voltage (VOUT) are compared inorder to check the input and output voltage differ-ences after regulation. The input voltage levels thatwere larger or less than (3.6V) were regulated by the Figure 11. Energy-harvesting, conversion, and chargingLT1512 buck-boost converter in order to charge the circuit Energy Harvesting From Passive Human Power 11
  8. 8. allow for the reading of output values on the oscillo- (as reported by tests). The graph in Figure 12 com-scope display. The energy-harvesting circuit, which pares voltage and current levels and average poweris soldered on the printed circuit board, is shown in output in 13 seconds.Figure 11. A protective box should be designed and When a resistive load is relatively large, thebuilt to protect the circuit components and the bat- power output from the PFCB does not produce sig-tery from bending and experiencing deformations nificantly more power. The results of using a larg-from the vibration sources. er capacitor to smooth the voltage output suggest that the size of the smoothing capacitor affects the Storage Unit Tests amount of power that can be delivered to a resis-One problem often encountered when using pow- tive load (battery). This result is attributed to theer-harvesting systems is that the power produced non-ideal behavior of the capacitor, which leads toby the piezoelectric material is often not sufficient internal losses. Following construction of the ener-to power most electronics. Therefore, methods are gy-harvesting circuit, NICD- and NIMH-type batter-needed to accumulate energy in an intermediate ies were charged to determine the battery chargingstorage device so that it may be used as a power time that could be effectively observed for eachsource. A capacitor is typically used to accumulate battery, with constant frequency. After testing thethe energy. However, capacitors have characteristics voltage levels of the PFCB using the capacitors, thethat are not ideal for many practical applications PFCB was then tested with the batteries to observesuch as limited capacity and high leakage rates. For battery-charging efficiency. For this purpose, a per-the purpose of intermediate storage units, typical manent magnet shaker was used to induce vibra-capacitors were used in the energy-harvesting cir- tions; three rechargeable batteries were used forcuit without causing any critical issues. A group of the experiment (Fig. 13). A PFCB consisting of twocapacitors were connected in parallel with the re- active-fiber composites (bimorph) was clamped tosistors in order to smooth the delivered voltage, a thin piece of metal on the constant shaker for themaking the output voltage easily read by the multi- energy-harvesting experiment. The constant vibra-meter. According to the approximate displacement tions from the shaker were applied to the PFCB atand frequency levels, stored energy in the capaci- 60Hz. The voltage measurements from the batteriestors was calculated. were taken every hour, and it appeared that the in- The current levels for battery-charging pur- crease was minor. The reason for the slow chargingposes were calculated according to the input voltage was the very low current produced from the PFCBlevels. If the input voltage is increased, the output and the losses across the energy-harvesting and bat-current would automatically increase by decreas- tery-charging circuits. Because of the low-charginging the battery-charging time. All calculations were current, the test battery was not able to be chargeddone according to the energy stored in 13 seconds at the specified standard charging time. However, this charging experiment was conducted only with a single PFCB, which is not recommended for charg- ing batteries. In some applications, more than three PFCBs are connected in parallel to increase the cur- rent levels and efficiency of the energy harvesting system. The number of PFCBs would be increased to charge the batteries at a specified time frame to avoid voltage drops across the batteries while pow- ering the electronic application. The batteries used in the experiment are listed in Table I with the basic specifications that are needed as charging param- eters. In order to charge batteries, the PFCB inter- digitized electrodes were connected to the battery terminals through the energy harvesting circuit. This experimental test showed that batter- ies were being charged with constant current/volt-Figure 12. Energy stored in 13 seconds12 j o u r n a l o f a p p l i e d s c i e n c e & e n g i n e e r i n g t e c h n o l o g y 2011
  9. 9. Quick Charge Charge Nom. Ampacity Charge Type time V mAh I Tim I T h mA h mA h NIMH 1.2 80 8 15 47 NICD 3.6 60 6 14 20 7 36 NIMH 3.6 60 6 14 20 7 41 Table 1. Rechargeable Battery Specificationsage in longer time frames than the specific time son starts walking. Several pennies were attachedframe in the battery datasheets. The last column in to the tip of the PFCB to increase the vibration timeTable I shows the time required to charge the batter- while the PFCB was being shaken by the foot-steps.ies with one PFCB. If more PFCBs are used for the en- The power output of the PFCB was directly propor-ergy harvesting, charging time would be decreased tional to the force and mass applied on the PFCBconsiderably. to induce vibrations. However, the output power of the energy-harvesting circuit was a constant voltage IV. SNEAKER SOLE EXPERIMENT and current to avoid damaging the battery (becauseA sneaker-insole was considered as a possible am- of the unregulated voltages produced by the PFCB).bient energy source to generate electricity through Therefore, the amount of energy generated by hu-a PFCB that could be used to charge low-scale re- man power through the PFCB was determined bychargeable batteries. The batteries are expected to human power that was available to shake the piezo-power low-power electronic applications such as a electric material. The stronger the force and massradio, MP3 player, mobile phone, and GPS unit. A typ- applied, the more electrical power was generatedical MP3 player was chosen as an electronic device to from the PFCB while the person walked or ran. Asmake the power estimation between generated- and a result of vibrations, the fibers in the frame of theconsumed- power levels. Table 1 demonstrates that shoe sole generated electricity with high potentialthe PFCB was able to produce enough voltage with and low current. The electricity was conducted andlow current as an input power to charge a small- scale stored in a battery or capacitor in the circuit placedrechargeable battery, depending on the time a person into a special compartment of the shoe insole. Thewalks or runs. The analytical estimation of the bat- photograph of the redesigned sneaker insole withtery-charging time and walking-time relationship was the assembled PFCB is shown in Figure 13.calculated for the fully discharged batteries. Howev-er, in the case of powering the electronic application,the batteries were placed in the system fully charged.It is essential to determine if the gained and storedpower compensates for the consumption of the elec-tronic device while it is operating. If the producedpower compensates for the daily consumptions andthe leakages of the electronic device, a sneaker insoleas an ambient energy source is a feasible source forthe electronic application. For experiment purposes, the sneaker solewas cut carefully to place the PFCB in the correct po- Figure 13. Redesigned sneaker insole with PFCBsition for maximum efficiency when a person walksor runs. The base where the PFCB was placed was su- The wires coming from the PFCB were ex-per glued with a piece of thin wood in order to place tended by wires to the energy-harvesting-circuit in-the PFCB properly on a smooth surface. Following put. In order to avoid voltage drops due to long wires,the proper placement of the PFCB, the insole of the the PFCB and energy-harvesting circuit were placedsneaker was again covered with the piece taken from in close proximity. The efficient point of this experi-the sneaker sole to avoid any damages when a per- Energy Harvesting From Passive Human Power 13
  10. 10. ment demonstrated that if two sneakers (each with also necessary to determine the power consumptionone PFCB attached) are worn, the output from the vi- of the MP3 player (PMP3) separately per usage, beforebrations of both PFCBs never stops, even if a person a calculation of the overall energy consumption. Itwalks slowly. Once the first PFCB starts vibrating, it is assumed that a person walks/runs about 1 hourtakes at least one second until the vibrations stop. In in 24 hours. The power characteristics of a small,this time period, the other PFCB starts shaking when low-power MP3 player were provided by one of thethe person takes another step. In this manner, battery- MP3 manufacturer’s data sheets. The specificationscharging is decreased to almost half of the standard of the MP3 player were used to calculate the powercharging time as specified in the previous sections. consumption for one hour of use (PMP3). According Since the current level produced through to the specifications, the power usage of the MP3the energy-harvesting circuit for battery charging player in one hour is calculated as 72mW.has been calculated, the next comparison estimatedwhether the system could produce enough power IMP3 = 60mA;for a typical MP3 player. The walking time also was VMP3 = 1.2V; and PMP3 = I * V = 60mA * 1.2V =calculated to compute how much walking is neededto compensate the energy consumption of the elec- 72mWh.tronic application (Figure 14). There are certain components in the sys- tem that is always at stand-by to sense the wake-up signals. These components drain some quiescent currents from batteries while they are on standby (including the MP3 player, the energy-harvesting cir- cuit, and the battery that keeps the device up and running). The total leakages and quiescent current for the system components during the playing of the MP3 player were calculated using Equation 1. Power consumption of the MP3 player (PMP3) was found separately and included in overall current consumption in 24 hours as calculated below. I1 (LOSS/24HRS) = [(IBATTERY_LEAKAGE) + (IHARVEST_ LEAKAGE) + (IMP3_ LEAKAGE ) + (IMP3)]; soFigure 14. Block diagram of comparison estimation forenergy harvesting I1 (LOSS/24HRS) = [(270μA) + (984μA) + (13mA)] + [(60mA)] =74mAh. Overall energy used/produced relationshipestimations were conducted considering (I1), overall The value for I1 is converted to the power unit in or-current consumption, and (I2), current gained from the der to compare the power gain and the power loss.PFCB in the sneaker insole, through the human power. Then I1 (LOSS/24HRS) = [(IBATTERY_LEAKAGE) + P1 (LOSS/24HRS) = 0.074A * 1.2V = 0.088Wh (88mW). (IHARVEST_ LEAKAGE) + (IMP3_ LEAKAGE) + (IMP3)] (1) The total energy drained from the batter-where I1 (LOSS/24HRS) is the current loss per 24 hours, ies was estimated at 0.088W for one day. This valueand IMP3 is the current consumption of the MP3. is not exact energy consumption; it may change according to how often a person changes the pa- Equation 1 calculates the overall current rameters of the MP3 player while using the device.consumption of the MP3 player including the to- Since the total energy loss (P1) was estimated, thetal leakages on stand-by mode, energy- harvesting next step was to calculate the energy gain from thecircuit, and batteries. According to Equation 1, it is energy-harvesting system. This equation enables14 j o u r n a l o f a p p l i e d s c i e n c e & e n g i n e e r i n g t e c h n o l o g y 2011
  11. 11. calculation of the total gained current from humanpower through the PFCB that is assembled in thesneaker insole during one day (24 hours), the bat-tery charging time assumed.Thus: I2 (GAIN/24HRS) = IG * T; (2)where I2 (GAIN/24HRS) is the total current recovered/stored per 24 hours, IG is the current gathered whilea person walks for one hour, and T is the time per-son walked/run (in hour). For this application, the gained current fromthe PFCB (I2) should be more than or equal to the Figure15. Energy gain/loss depends on the time aoverall current loss (I1) in 24 hours (I1≤I2). Otherwise, person walks/runsthe MP3 player would be operating inconsistentlydue to insufficient current. The total energy gained Figure 15 shows that the gained power isfrom the system would depend on the time a person too low to play an MP3 player for one hour with onewalks/runs during a 24-hour period: hour of walking. To improve the energy gain, more than one PFCB should be placed into the sneaker in-I2 = (0.005A * 2hrs) = 0.01A. sole to increase the generated power to balance the (GAIN/24HRS) power consumption of the MP3 player. Another so-The value for I2 is converted to the power unit in lution to make the harvesting circuit efficient wouldorder to make comparison between the power gain be improving or redesigning the circuit to increaseand the power loss. Then the current flow to the battery to decrease the bat- tery charging time. Also, if the time a person walksP2 (GAIN/24HRS) = 0.01A * 1.2V = 0.012Wh (12mW). increases (more than one hour), the battery charging time would be decreased depending on how long aP1 and P2 were calculated and converted to the en- person walks in a day.ergy value in order to make a comparison ratio ifenergy gain is greater than energy loss in order to V. CONCLUSIONbalance the system power. The advances made from this research build the framework for further experimentation with the (3) tools necessary to use the PFCB effectively in nu- merous applications. The sensing capabilities ofwhere EG is the overall energy gain, EOUTPUT is energy the PFCB were investigated, and shown in an ener-loss through the MP3 player, and EINPUT is the energy gy-harvesting system through an experiment andgain from PFCB; battery-charging circuit. The energy-harvesting circuit can be improved to increase current levels energy loss ratio. from the PFCB while decreasing voltage levels for battery-charging purposes. The increase of current As estimated above, the energy gain is 7.3 during the vibration of PFCB would decrease thetimes smaller than the overall energy consumption battery- charging time by supplying more energy toof the MP3 player in a day. The harvested and stored the electronic device. The special device should beenergy is not sufficient to run an MP3 player for one in the proper position so that the maximum amounthour with one-hour of daily walking. The energy of vibration can be created for energy-harvestingloss/gain graph depending on time given in Figure15 purposes. As the energy yield increases and wear-to permit a visual comparison of these variables. able electronic devices become more efficient, foot- powered energy scavenging systems can drive more components of wearable computers, reducing the Energy Harvesting From Passive Human Power 15
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