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Page 1 of 26 Energy Harvesting for Self-Contained Wireless Sensor Module GROUP 4 ECE 191 FALL 2008 Jean Fan Ken Yu Hector Aranda Sponsor: Spirit AeroSystems, UCSD/CEAM Mentor: Jon Isaacs Page 2 of 26 Table of Contents I. List of Figures 3 II. Executive Summary 4 III. Introduction 5 IV. Approach 5 a. Inductive Charging 6 i. Results and Discussion 7 b. Piezoelectric Energy Harvesting 12 i. Results and Discussion 13 c. Thermal Energy Harvesting 19 i. Results and Discussion 20 V. Conclusion 25 VI. Acknowledgement 26 VII. References 26 Page 3 of 26 I. List of Figures Figure 1: Schematic of Inductive Charging Circuit Figure 2: Inductive Charging Circuit Setup Figure 3: Transmitted Power (mW) vs. Distance (cm) Figure 4: Receiving Voltage vs. Distance Figure 5: Piezoelectric Effect Figure 6: Theoretical Setup of Piezoelectric Energy Harvesting circuit Figure 7: Piezoelectric Energy Harvesting Circuit Setup Figure 8: Model of Piezoelectric Circuit Figure 9: Total Voltage measured at Capacitor C89 Output Figure 10: Energy Transfer Flow Chart Figure 11: Thermal Energy Harvester Figure 12: Performance Spec Sheet Figure 13: Experimental Setup, no thermistors Figure 14: Experimental Setup with thermistors Figure 15: Power vs. R-T Coefficient Figure 16: Temp vs. R-T Coefficient Page 4 of 26 II. Executive Summary Wireless sensor modules are often embedded in airplane wings for structural analysis. Often times, powering these modules can be a hassle. These sensor modules are more convenient and efficient when powered wirelessly or self-powered. Three methods of energy harvesting were tested: inductive charging, piezoelectric energy harvesting, and thermal energy harvesting. Inductive charging is an age old technology. The most used inductive charging is the transformers. However, who to transfer large amount of power in between two coils from far away has never been considered until recently (MIT “Wielectricity” research). In order to test the inductive charging method, two inductive loops were made with magnet wires (Gauge 16, copper wire coated with insulting Polyurethane). A Matlab model is also plotted in comparison with the experimental data. The piezoelectric energy harvesting method was tested by acquiring a piezoelectric element (PCFB W-14 obtained from AmbioSystems LLC). Since the voltage generated by the element is in AC domain, a full-wave bridge rectifier was built to convert the signal to DC power that can be transferred to the battery. The thermal energy harvesting method is a lucrative method because it is a fully self- sustainable form of DC power. The harvester was tested in the lab to determine the practicality of implementing such a unit into a working prototype. A temperature differential was applied across the energy harvester and the power output was measured. After research and experiments were conducted for these three types of energy harvesting methods, we conclude that the piezoelectric harvesting method is the most appropriate method to power the wireless sensor modules. It is completely self- contained (no manual recharge necessary) and can generate enough power to support both the sensor module and the MCU. Page 5 of 26 III. Introduction Wireless sensor module using acoustic sensors are embedded in enclosed compartments of airplanes to monitor the structural integrity. To make these modules self-contained, DC power needs to be provided to the modules by energy harvesting. The objective of this project is to investigate various methods of energy harvesting need to be explored and evaluated to determine the most appropriate to implement in the lab. In addition, a functioning energy storage circuit needs to be designed to properly store the harvested energy. The three types of energy harvesting methods that were tested are inductive charging power transfer, piezoelectric energy harvesting, and thermal energy harvesting. These three methods of energy harvesting need to be explored and evaluated to determine which is the most appropriate to implement given that the sensor modules are in enclosed compartments. IV. Approach After much deliberation, we decided to research 4 different methods of energy harvesting. The first is inductive charging where a charge is induced wirelessly through electromagnetic induction. The second method is piezoelectric energy harvesting where mechanical vibrations induced an electric charge. The third method is thermal energy harvesting where a charge is induced through an applied temperature differential. Page 6 of 26 IV.a Inductive Charging The inductive charging approach uses two air-core coils to transfer power wirelessly. The type of technology used in the inductive charging is the same as in transformers, which has existed for many decades. Although it is an old technology which has been developed and matured for a very long time, it could not transmit power efficiently over a long distance until recently. A group of researchers in MIT published a paper title “Wi-tricity” (wireless electricity). Their concept is simple: to use the resonance phenomena to wirelessly transmit power. The only trick here is to add a capacitor in parallel with the inductive coil. When the impedance of the inductor is the same as the reactance of the capacitor, the two elements will resonate when connected in parallel. If the two inductive coil circuits have the same resonant frequency, power can be transmitted from one coil to the other efficiently and in long distances. The primary coil (left) of the resonance circuit is coupled to receive power from a source oscillating at natural resonant frequency. The secondary coil (right) is coupled to the power supply so that power induced in the secondary coil charges the power supply. Below is an analysis of how magnetic induction generates power (Watts). Page 7 of 26 The power is transferred in form of EM wave, where: B r2 H xI , wherex 0 2 r2 z2 EB 2 U c0 H , whereE cB 0 H: Magnetic field intensity; B: Magnetic flux; U: Energy; E: Electric field. Experimental Setup Fig. 1 Schematic of Inductive Charging Circuit Vin Vout 1 2 V1 L1 250mV Vac C1 L2 0Vdc C2 R1 2 Distance (cm) 1 0 0 Vin is set at the resonant frequency 31.8 kHz and the supply voltage is set at 250mV. We set the resistor value to be R = 10Ω to calculate power. However, the load resistance in final design will depend on the input resistance of the receiving circuit. See Fig. 2 for the actual setup. Page 8 of 26 Fig. 2 Inductive Charging Circuit Setup Equipments: Properties of Magnet wire - Breadboard and leads -Gauge: 16 - Function Generator and Oscilloscope -Outer Diameter: 0.048in - 1 Resistor, 2 Capacitors -Insulation Thermal - 2 coils of Magnet wire Class: 105oC -Length of each wire: 1.58m -Copper wire coated with -Radius of coil: 3.1cm Polyurethane as insulator -8 loops per inductor To find the value of the inductors, let C = 10uF (for calculation simplicity). The gain of Fig. 1 can be modeled by: At f = 44 kHz, Vo/Vin = ½. The equation above gives L=75.9uH. To minimize power loss during voltage transfer, C=0.33uF was chosen. Inductive Charging Experimental Results The results of the circuit can be seen in Table 1 and Fig. 3 below. Page 9 of 26 Table 1 Amount of Power Transferred Distance Vpeak Power at (cm) (mV) Load (mW) 1 86.5 187.05 2 50.0 62.5 3 33.5 28.06 4 20.0 10.0 5 14.5 5.26 6 9.00 2.03 Fig. 3 Transmitted Power (mW) vs. Distance (cm) Transmission Power vs. Distance (supply voltage = 250mW peak) 200 data 180 approximation 160 140 Power received (mW) 120 -0.888x y = 404.86*e 100 80 60 40 20 0 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 Distance between two coils (cm) The impedance of this circuit is . At resonant frequency, which means Zs goes to infinity. As a result, the only power consumed is in the parasitic impedances, which is very little and can be negligible. An inductive power transfer model was built to compare the theoretical values with the values obtained from the experiment. See Fig 4 for the comparison. 3 The magnetic field from a loop is given by which yields H K I r 2 2 r 2 z 2 2 e z where I receivez 2rreceive H z , Page 10 of 26 and γ is the loss in the medium, where 1.112610 3.3310 j , 17 2 9 where r = radius of the loop, Z = distance away from the loop, and K = coupling ratio (assume 0.5). Shown below is the matlab code written to calculate and plot the model of the inductive charging circuit. %% Magnetic field by a loop V1 = 0.25; %I1, r1 -> loop 1 (source) a = 1.291e-3/2; %radius of 16AWG wire r1 = 0.031; %I2, r2 -> loop 2 (receiving) r2 = 0.031; N1 = 8; N2 = 8; L1 = 4*pi*1e-7*r1*(log(8*r1/a)-2)*N1; L2 = 4*pi*1e-7*r2*(log(8*r2/a)-2)*N2; w = 2*pi*31.8e3; %frequency C1 = 1/(L1*w^2); %parallel capacitor values (at resonance) C2 = 1/(L2*w^2); K = 0.5; %coupling ratio I1 = V1*w*C1; gama = sqrt(-1.1126e-17*w^2 + j*3.33e-9*w); %transmission loss ratio z=0:.0001:.06; H(:,1) = K*I1*(r1^2./(2*(r1^2+z.^2).^1.5)).*exp(-1*gama*z); %mag. field at distance z on top of the loop 1 (lossy medium) H(:,2) = K*I1*(r1^2./(2*(r1^2+z.^2).^1.5)); %mag. field at distance z on top of the loop 1 (lossless medium) I2 = 2*r2.*H; plot(z, abs(I2/(w*C2)), 'linewidth',3); %plot voltage at receiving circuit vs. distance The graph of the model and the experimental results of the inductive charging circuit are shown in Fig. 4 below. The actual data is taken with a full-wave rectifier circuit. Some of the power transmitted to the load is consumed in the diodes. Thus, it is expected to be lower than the model curve. Page 11 of 26 Fig. 4 Receiving Voltage vs. Distance Receiving Voltage vs. Distance (V1=0.25) 0.14 Model Actual 0.12 0.1 Peak Voltage (V) 0.08 0.06 0.04 0.02 0 0 0.01 0.02 0.03 0.04 0.05 0.06 distance from source loop (m) Another concern with the inductive charging method is the orientation of the coils. In order to transmit power from one coil to another, the two coils must be parallel aligned with each other. If the two are aligned in an angle, the efficiency of power transfer will be greatly reduced. When the two coils are perpendicular to each other, no power will be transmitted between the two coils, due to the orientation of the magnetic field. One of the solutions to this issue is to have multiple source coils arranged in different angles. However, this method will waste more power in the generator side. In conclusion, the transferred power in the experiment is lower than the model prediction. This is caused by parasitic resistances in the coil and capacitors. The maximum power transferred by two inductors of radius 3.1 cm with eight loops per inductor is at 187.05 mW if the distance separating the inductors is 1 cm. Page 12 of 26 IV.b Piezoelectric Energy Harvesting The second approach in building a wireless energy harvester is by using the piezoelectric effect. See Fig. 1 below. Fig 5. Piezoelectric Effect Piezoelectricity can be generated by applying stress to an element (notably crystals or ceramic), which in turn creates an electric potential. *image taken from http://upload.wikimedia.org/wikipedia/commons/c/c4/SchemaPiezo.gif Since piezoelectricity is generated by applying stress to a piezoelectric element, Young’s constant measures the ratio between stress and strain of the element. F = force (N) A = area of element (m2) L = length of element (m) ΔL = change in length of element (m) As the element vibrates, there will be an elastic potential, ue, present. Lo = length of element (before compression) Ao = area of element (m2) The energy generated from a piezoelectric element (by applying stress) is modeled by: E = (piezoelectric coefficient) ue The unit conversion analysis of how a piezoelectric element can generate power (in Watts) is shown below. E = (piezoelectric coefficient) ue => Page 13 of 26 Energy has units of Joules = Nm Power from piezoelectric element => This is exactly what is expected. Piezoelectric Energy Harvesting Experimental Setup Our experimental setup can be seen in Fig. 2 and 3 below. Fig.6 Theoretical Setup of Piezoelectric Energy Harvesting circuit. *taken from IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 17, NO. 5, SEPTEMBER 2002 Fig 7 Piezoelectric Energy Harvesting Circuit Setup Page 14 of 26 Equipments used in the experiment: 1) Massager (3V) to generate vibration 2) Power supply 3) Oscilloscope 4) Piezoelectric Element (PCFB W-14 obtained from AmbioSystems LLC) 5) Energy Storage Device - 100uF capacitor 6) AC/DC Rectifier converter 7) Weights (as load on element) - LR03 AAA Alkaline - Weight: 11.3g/pc A vibrating piezoelectric element generates an ac voltage but since batteries need a dc voltage, an AC/DC rectifier (built with four zener diodes 1N4232) is connected to the output of the piezoelectric device. The energy storage capacitor is 100uF, and the vibration frequency of the massager (measured from the oscilloscope) is approximately 109 Hz. As can be seen from Fig 3, the resonant frequency of the piezoelectric element was changed by adding a load at the end. The results of how long it took for the massager to charge up the 100uF capacitor can be seen in Table 1 below. Results for Piezoelectric Element Power Output Table 1. Time (s) for Piezoelectric Element to charge 100uF capacitor Piezoelectric Element with Different Loads No 1 weight 2 weights Time (sec) to charge Capacitor load 11.6g 22.6g From 0V to 2V 19s 10s 20s From 0V to 4V 28s 5s 11s Shown below are outputs seen on the oscilloscope. Page 15 of 26 No weight 0 to 2V in 19 sec Power output = 10.5μW 22.6 g as load 0 to 4V in 11 sec Power output = 73μW Page 16 of 26 11.3 g as load 0 to 4V in 5 sec Power output = 0.16mW The optimal piezoelectric output power from the experiment was 0.16mW (calculated using E=½CV2 and dividing the quantity by the amount of time it took to charge the capacitor). From testing the circuit, some observations were that the peak-peak voltage of piezoelectric element depends on the weight (load) on the tip of the element, and the size of the temporary capacitor CP and load capacitance Cf (see Fig 2 for reference. The piezoelectric can be modeled as an AC current source with a resistive load. THe model was tested in Pspice. Fig. 8 Model of Piezoelectric Circuit Page 17 of 26 Fig 9. Total Voltage measured at Capacitor C89 Output To compare our results, we acquired datasheets from AmbioSystems give a more exact power approximation. Shown below are the specs for two kinds of piezoelectric elements, depending on the amount of power needed. Property PFCB-W14 PFCB-W24 Dimensions [mm] 132 x 14 x 1.3 132 x 24 x 1.3 Resonance Frequency fr (Hz) 30 50 Bending Stiffness [N/m] 56 138 *acquired from AmbioSystems Page 18 of 26 The power output from the piezoelectric element PFCB-14 can reach up to 48 MW if it is at its resonant frequency (30Hz). *acquired from AmbioSystem LLC Conclusion: Power output from the piezoelectric element PFCB-14 can reach up to 48mW, depending on its resonant frequency, vibration force, and resistive load. Our experimental results were acquired with a resonant frequency of 100Hz since we cannot change the vibration of the mini-massager. Page 19 of 26 IV.c Thermoelectric Energy Harvesting Concept Consider the typical commercial airplane engine. Its engine temperature can vary anywhere from a few 100 degrees Celsius to 1-2k degrees Celsius. A lot of this energy is lost in the form of mechanical energy (i.e., combustion and thrust) but there is also a portion that is just dissipated heat. Now imagine there exists a way to convert this wasted heat into electric potential energy. A thermal energy harvester does just that; it captures ambient thermal energy and converts it into DC power. Conceptually, Fig.10 Energy Transfer Flow Chart You apply a heat source which in this case can potentially be the heat exhaust from a jet engine. The applied temperature is then related to the thermal voltage, Vt, by Boltzmann’s constant which is then used thermodynamically within the thermal energy harvester. The thermal energy harvester then outputs a DC voltage and current at its terminals which give the desired power output. Thermal Energy Harvester *Fig. 11 Thermal Energy Harvester Thermal energy harvesting is the conversion of ambient thermal energy into electrical power. Fundamentally, a thermal energy harvester consists of two different conductive materials, A and B, that when attached to a resistive load, L, causes an electric potential difference. This effect is achieved by applying a temperature differential, T2 – T1, across the two conductive materials which will then cause charge to flow, Je, through the resistive load. This thermodynamic effect is known as the Seebeck Effect. The Seebeck Effect is the underlying thermodynamic phenomenon that converts thermal heat to electric power. The main equation to take into consideration is *Fig.11 = Fig. 51-3, G.N. Hatsopoulos, J.H. Keenan, “ Principles of General Thermodynamics,” Wiley & Sons Inc, p.681, June 1965. Page 20 of 26 Єb – Єa = Єabo – JeRAB . *(51.46) This equation describes the electric potential difference, measured in units of volts (V), across a resistive load L. Je is the induced current flowing through the resistive load and is measured in units of amperes (A). RAB is the total resistance of the isothermal loop and is measured in units of ohms (Ω). RAB is a function of resistivity of materials A and B and is integrated over the entire length of the temperature gradient due to its dependence on temperature. Єabo is the open-circuit voltage and is the most important quantity of the equation as is relates applied temperature to voltage. Єab0 = πAB (1/ T) dT. *(51.47) The open-circuit voltage is a function of the Peltier coefficient, πAB, and is unique for the given materials A and B. By inspection, the unit for the Peltier coefficient is joules per coulomb-Kelvin, J/(C*K). The open-circuit voltage is integrated over the entire temperature gradient to give units of voltage (V). Experiment Based off of the design requirements discussed previously and the specification sheets of various thermal energy harvesters, we decided to purchase the Z-Max G1-1.4-219-1.14 thermal energy harvester by Tellurex Corp. Fig.12 Performance Spec Sheet *Equations 51.46 and 51.47, G.N. Hatsopoulos, J.H. Keenan, “ Principles of General Thermodynamics,” Wiley & Sons Inc, p.682, June 1965. Page 21 of 26 The experiments conducted were based off of the data sheet provided by Tellurex’s website. The experiments were designed to test the performance of the energy harvester that we purchased. The results would help gauge how much power the energy harvester can potentially output in the lab. Test #1 The purpose of this test was to see if the unit functioned properly. A temperature differential was simply applied to the unit in the form of a hair dryer for the hot side and a bag of ice for the cold side. At the time of the experiment, a proper method of measuring temperature was not developed so we were restricted to deductive reasoning. The results were purely tabular but gave us a somewhat intuitive understanding of how the energy harvester functions. Setup Fig. 13 Experimental Setup, no thermistors As specified in the documentations that came with the energy harvester, a heat sink was fabricated in the ME department workshop. The purpose of the heat sink is to dissipate direct heat along each side of the energy harvester as to not damage the delicate conductive materials contained inside the unit. Page 22 of 26 Results Voltage(V) Current(mA) Power(mW) 0.239 60 14.34 Thot 0.24 60 14.4 0.236 60 14.16 0.234 60 14.04 0.228 60 13.68 0.222 60 13.32 0.215 60 12.9 0.183 50 9.15 0.182 50 9.1 0.18 50 9 0.179 50 8.95 0.156 50 7.8 0.155 50 7.75 0.151 50 7.55 0.148 40 5.92 0.147 40 5.88 0.139 40 5.56 0.095 30 2.85 0.093 30 2.79 0.077 20 1.54 Tcold 0.02 10 0.2 The measurements were taken with a DMM and measured the voltage and current output of the thermal energy harvester as steady heat was applied by the hair dryer. This was a very crude test because there were no other forms of data collected other than voltage and current. Another parameter that would have been of relevance is temperature. The test was performed with the idea that, with time, we would eventually reach a point where the hair dryer would output a maximum temperature on the hot side of the heat sink and thus would achieve a maximum heat differential across the harvester. After about 15-20 minutes of heat application there was a noticeable cap to the power output. Based on the data collected, we were able to achieve some sort of temperature differential across the thermal energy harvester as exemplified by the maximum power output of 14.4mW. In theory, this is more than enough DC power to sustain our system of interest. Page 23 of 26 TEST #2 The next test performed was much like the first except this time temperature was recorded on both sides of the energy harvester. What was recorded was the relationship between the energy output and temperature applied to the hot side of the heat sink. Setup Fig. 14 Experimental setup with thermistors In addition to the previous setup, 2 thermistors were used to record the varying temperatures on both sides of the harvester. A thermistor’s resistance depends on temperature; the resistance of the thermistor at room temperature over the new resistance at a varying temperature gives you the R-T curve coefficient. This coefficient can be used with the data sheet provided by the manufacturer to extrapolate a rather accurate measure of temperature applied. Page 24 of 26 Results Power vs. R-T Coefficient 14 12 10 Power(mW) 8 6 4 2 0 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02 R-T Coefficient Fig. 15 Power vs. R-T Coefficient Temp vs. R-T Coefficient 31 30 29 Temp(Celsius) 28 27 26 25 24 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02 R-T Coefficient Fig. 16 Temp vs. R-T Coefficient The maximum power output recorded was roughly 13mW of power. This is consistent with the previous test which is a good indication that the thermal energy harvester is working properly. It should also be noted that the temperature recorded on the cold side was fairly constant throughout the test; Tcold was roughly 25-26.5 degrees Celsius. This means that as temperature on the hot side Page 25 of 26 increases so does the temperature differential across the energy harvester. So at the highest recorded temperature we get the largest temperature differential at around 4 degrees Celsius. 4 degrees Celsius is not a huge temperature differential which might explain the drastic discrepancy between the manufacturer’s data sheet claim of 5.7 W and the measured 13mW of power. The manufacturer claims this power output at a temperature differential of 100 degrees Celsius. This type of differential was just not feasible in the lab and slightly impractical in the field without any sort of active cooling system maintaining the cold side cold while allowing the hot side to stay hot. This type of system, unless already naturally occurring within the airplane, would require a substantial power source to function properly. Achieving and maintaining a constant temperature differential is the single biggest design flaw behind the thermal energy harvesting method. For this reason, it would be impractical to implement this method into a working prototype unless otherwise overcoming the inherent temperature issues. Perhaps the engine, itself, contains cooling compartments where temperature differentials are naturally sustained for fueling purposes. V. Conclusion A list of comparisons to all three types of energy harvesting methods is listed below: Pros Cons Inductive Fast charging, allows for large Need to be manually charged power transfer Piezoelectric Doesn’t need to be manually Each element has to be manually tuned charged. Plane provides to the plane's vibration frequency sufficient vibrational force. Thermal Continually supply sufficient Temp differential impractical in an DC power enclosed compartment The piezoelectric energy harvesting method is most practical for wireless sensor modules in an enclosed compartment. Page 26 of 26 VI. Acknowledgement The work described in this paper was sponsored by Spirit AeroSystems and University of California, San Diego. Special thanks to: Professor Das John Isaacs Rigo Marin Tejaswini Narayanan Cheryle Wills VII. References 1. Marin Soljacic at el, Efficient wireless non-radiative mid-range energy transfer. (2007) 2. M.J. Guan, W.H. Liao, On Efficiencies of Piezoelectric Energy Harvesting Circuits toward Energy Storage Devices. (2005) 3. AmbioSystems Advanced Cerametrics Incorportated, Piezoelectric Fiber Composite Bimorph (PFCB) Datasheet. (2008) 4. http://upload.wikimedia.org/wikipedia/commons/c/c4/SchemaPiezo.gif 5. G.N. Hatsopoulos, J.H. Keenan, “Principles of General Thermodynamics,” Wiley & Sons Inc, p.682, June 1965. 6. Thermistor model sheet: http://content.honeywell.com/sensing/prodinfo/thermistors/diskthermistors.asp 7. Thermistor R-T conversion table: http://content.honeywell.com/sensing/prodinfo/thermistors/resistanceTables/resistance Table7.asp 8. Thermal Energy Harvester datasheet: http://www.tellurex.com/pdf/G1-1.4-219- 1.14.pdf 9. J.A. Hagerty, T. Zhao, R. Zane, Z. Popovic, “Efficient Broadband RF Energy Harvesting for Wireless Sensors,” Department of Electrical and Computer Engineering, University of Colorado at Boulder