power converter for energy harvesting - specialister i elektronik

Power Converter for EnergyHarvesting

Special Course

March 31, 2011

Johan Henning Pedersen, s052402

Supervisors:Michael A.E. Andersen, Ole C. Thomsen, Arnold Knott

Thomas Sørensen, DELTA

Department of Electrical EngineeringTechnical University of Denmark

Contents

1 Introduction 11.1 Energy Harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Project Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.1 Delimitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2.2 Goal of project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2.3 Description of report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Energy Consumer - Sensor Node 32.1 DELTA Greenlab Mote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Power Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 EH Power Converter Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3.1 Storage: Super Capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3.2 Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Energy Harvesting Technologies 83.1 Thermal Harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1.1 Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2 Vibrational harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2.1 Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.3 Photovoltaic Harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.3.1 Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4 Comparisons 174.1 Converter Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.2 Price comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.3 Choice of EH Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5 Implementation 195.1 Solar Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.1.1 Characterization of Mekoprint Solar Cell . . . . . . . . . . . . . . . . . . . 195.1.2 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5.2 Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.2.1 LTC3105 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.2.2 MPPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.2.3 Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6 Conclusion 266.1 Further work on prototype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266.2 Problems fit for a masters project . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

References 27

Appendix 29A Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

A.1 Price comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29B Solar cell test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29C Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

C.1 Components to be implemented . . . . . . . . . . . . . . . . . . . . . . . . 30D Matlabfiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

i

Contents ii

D.1 batt vs eh.m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

1. Introduction 1

1 Introduction

1.1 Energy Harvesting

Ambient energy is all around us. There is energy in all kinds of light, there is energy in theradio signals in the air, there is energy in kinetic movements (wind, vibration), heat flows etc.All these energies can in several ways be harvested. Energy harvesting (abbreviated EH), in thisproject, is defined as harvesting ambient energy in small-scale. Harvesting from small transduc-ers not larger than a coffee cup. These transducers can be photovoltaic cells, thermal generators,vibrational generators etc. The ambient energy levels are in general small and will not be able tosupply a household with power, but there is plenty of energy to supply small electronic deviceswithin the areas of ambient intelligence, condition monitoring devices, implantable and wearableelectronics, and wireless sensor networks.

The output power from the harvester is not necessarily in the form required, as loading appli-cations often demand specific voltage and impedance characteristics. It also requires energy totransfer and condition the power, in an already energy sparse system and these are some of theconverter challenges within energy harvesting.

Energy harvesting has not yet been commercialized in large scale but now the power usage inmicrocontrollers and sensory electronics has decreased so much as it is now in the level of mW’s.Along with improved energy conversion techniques several energy harvesting solutions starts topop up. Meanwhile is a demand for long lasting sensors in small form factors which makes cablesand batteries unfit (fig. 1 illustrates why batteries are unfit - they will eventually deplete, whereenergy harvesting in theory can generate energy indefinitely).

Figure 1: Illustration of the main strength of energy harvesting in comparison to battery when poweringwireless sensor networks (WSN) [21]. Batteries deplete - energy harvesting keeps on going.

Table 1 shows a quick and rough comparison on the energy levels produced by 3 different energyharvesting sources1.

1These numbers are made from the following assumptions: Solar : photovoltaic cell has 15% efficiency, Ther-mal : the temperature difference between human skin and ambient air is 5K, Vibration: human is in walkingmotion

1. Introduction 2

Table 1: Examples of energy harvesters output [4]

EH Technology Setting Output µWcm2

Solar Outdoor 15000Indoor 10

Thermal Human 20Machine 5000

Vibration Human 4Machine 200

1.2 Project Description

Existing energy harvesting technologies are to be categorized based on price and energy availabil-ity and related to real-life applications. The basis will be a sensor node from a sensor networkas the energy consumer. The available power converter topologies are clarified as an energymanagement link between harvester and sensor node. The converter topologies and relevantcriteria are summarized and an energy harvesting technology is chosen as basis to make a designand implementation of a power converter being a modification, combination or revised versionof existing topologies.

Energy Sources Power Converter Power user, eg. sensor node

Figure 2: This project deals with categorizing and choosing an energy harvesting source and implement-ing a power converter.

1.2.1 Delimitations

The project description could describe a project of a lot larger size than intended so a set ofdelimitations are set up.

• There are many ways of harvesting energy. One can extract energy from almost all kindsof processes and force, eg. chemical, kinetic etc. In this project it is chosen only to focuson the 3 energy sources: photovoltaic energy, thermal energy and vibrational energy.

• Power management of the sensor node - The energy will be supplied but this project willnot deal with how to distribute the harvested energy, ie. when a large amount of energyis available.

• A converter in discrete components will not be build, but a converter integrated in an ICwill be used.

2. Energy Consumer - Sensor Node 3

1.2.2 Goal of project

The aim of this project is to get more familiarized with energy harvesting converter challengesand to act as a pre-project for a master project within energy harvesting power converters.Hereby aiding in choosing the scope of the master project.

1.2.3 Description of report

This report starts out by describing the key parameters within energy harvesting and stating therequirements from the energy consumer. Three energy harvesting technologies are then describedincl. power conversion techniques and applications. With basis on the energy consumer andoperating environment and energy price, the different technologies are compared and one ischosen to be implemented as an energy harvesting system consisting of harvester and powerconverter. A prototype is build and the results are presented. The report concludes with furtherwork available for a master project.

2 Energy Consumer - Sensor Node

The energy consumer in this project will be a sensor node being developed at DELTA. A sensornode is a small computing device capable of collecting data, joining a mesh network and throughthis transmitting the data to a base station. A mesh of such nodes is known as a sensor network.These sensor nodes are usually supplied with power from batteries, but these require mainte-nance. To avoid this, the nodes can be made energy self-sufficient by means of energy harvesting.

2.1 DELTA Greenlab Mote

The Greenlab motes uses the Texas Instruments MSP430f1611 microcontroller, the ChipconCC2420 radio and the Atmel AT45DB161D external flash.

The microcontroller is equipped with low-quality sensors for temperature and voltage. The motehas three on-board sensors:

• Temperature and humidity Sensirion SHT11

• Ambient Light OSRAM Opto Semiconductors SFH5711

• Infrared PIN Photodiode Fairchild QSB34

2.2 Power Use

The sensor node power consumption varies depending on what task the node performs. Table 2shows the energy consumption from different operating modes 2.A normal operation cycle of a sensor node could be that at a synced time, the node: turns ON → listens for communication on the radio → takes a sensor reading → processes themeasurements → transmits the result → turns OFF.

For a sensor node cycle scenario as described above, the total power usage can be roughly esti-mated to 1s with average power use of 20mW (see table 2). This corresponds to an energy level

2To improve power consumption of sensor node, dynamic power management and especially dynamic voltagescaling will save energy. Instead of finishing the processing task fast, the microcontroller voltage can be scaleddown so it works slower, just as slow in order to finish the processing in time for the next cycle [16]. Can also bedone by lowering the frequency of the microcontroller.


Mode I [mA] P [mW]

Sleep 0.7 2.42Radio 13.2 45.5Ext. Flash 3.0 10.4Reprogramming 8.0 27.610s wait 1.2 4.14

Table 2: Power consumption of sensor node in different operating modes at an input voltage of 3.45V.Results are based on oscilloscope measurements where the average current has been estimated.

of 20mJ and is used as reference through the project.

With the energy estimates from table 1 the required harvesting times for a 1s sensor node cycleare seen in fig. 3 for the different technologies to get a feeling of how the different technologiescompare. On a bright sunny mid-day the sensor node can operate every 13 milliseconds whensupplied by a 10cm2 solar cell or every 9 minutes when operated by a equally sized vibrationharvester on a walking human.

Sun−o Sun−i Ther−h Ther−m Vib−h Vib−m0

100

200

300

400

500

600Harvesting time to operate sensor node for one cycle

Tim

e [s

]

Figure 3: Time to harvest energy for one duty cycle for sensor node (20mJ) for different harvesters:solar outdoor and indoor, thermal on human and machine, vibrations from human and machine. Basedon energy estimates from [4]. All have equal size of 10cm2.

The energy available for harvesting is very dependent on the environment. When solar light ispresent, this energy is superior in density, but in dark or warm or moving locations other energysources takes the lead.

The energy is not directly available through the various transducers. The different energies aredelivered in very different formats. Solar cells and thermal generators deliver a DC voltage, andvibrational gives an AC output. The output current is also different in between the differentEH technologies. All the technologies have in common that the energy levels are low, so theharvesting electronics needs to be specifically designed to convert and store the energy in anapplicable manner also taking into account whether the energy consumer is operating with ahigh or low duty cycle (important for the type and size of storage).


2.3 EH Power Converter Requirements

The energy harvester can in most cases not supply power enough for constant operation. Thusthe energy needs to be stored until there is enough for operation of the application.

All in all the energy harvesting power supply, consisting of converter, storage and energy har-vester, needs to fulfill these demands from the sensor node:

• Output voltage: 2V - 3,5V DC

• Maximum current draw of 20 mA 3

• Supply 20mW for 1s (= 20mJ) each 30 minutes.

Many EH transducers provide a low output voltage. Thus there is a great need for makingconverters capable of conditioning as low voltages as possible. For DC sources, like solar cellsand thermal generators, charge pumps can be used to slowly boost the voltage to a level whereregular boost converters can take over. AC sources like vibrational harvesters needs to be tunedto their resonance frequency and the power needs to be rectified.

Tasks such as charging the gate capacitance of a MOSFET could consume a large part of theharvested energy, therefore a current-source gate charge rather and a voltage-source gate chargewill often make sense.Another technique is to use more than one power converter circuit. The first circuit could beunregulated but capable of charging a capacitor. Once sufficient energy is stored in the capacitorit can be discharged and the signal conditioned by a more sophisticated power converter circuit.[4]

2.3.1 Storage: Super Capacitors

As energy storage super capacitors are able to hold large amounts of energy and release themfast, but they can only hold the energy in a limited time due to leakage currents. See fig. 4.

In order to be able to operate the node every 30 min for 1 s (20mJ) an estimate of the requiredcapacitor size is calculated. The capacitor will never be fully discharged as the sensor node canonly operate in the area of 3, 5− 2V . Here it is assumed that the storage capacitor comes aftera converter and is maintained by a hysteresis circuit controlling the output within a thresholdof 3.5V − 2V .

Enode = 20mJ (1)Ecapacitor = 0.5 · C · V 2 (2)ECavail = 0.5 · C · (V 2

H − V 2L ) (3)

C =Enode

0.5 · (V 2H − V 2

L )=

20mJ0.5 · (3.5V 2 − 2V 2)

(4)

C = 5.8mF (5)

The value of C = 5.8mF does not include leakage and ESR. The different capacitor types havevery different leakage properties, so it is important to choose the appropriate technology. Ce-ramics has no, or very low leakage current, but does not come in sizes of mF . Electrolytic andtantalum capacitors does come in mF sizes but both have high leakage currents (Lyt 6.8mFIleak = 1.2mA, Tant 6.8mF Ileak = 0.1mA). Super capacitors have very low leakage current

3Measured during start-up of node.


(∼ 1µA ), but during the initial charge from 0V the leakage can be up to ∼ 20µA [19]. The ESRof the super capacitors vary from 100Ω − 0.01Ω, the later being expensive. The ESR is veryimportant when drawing high currents. Eg. if the ESR is 50Ω and the current draw is 50mA,it will correspond in a capacitor voltage drop of 2.5V .

If the energy in the surrounding environment is only temporarily available less frequent thanevery 30 min, and the sensor node needs to be able to operate every 30 min, the storage needsto be able to provide energy enough for more than one cycle. As seen in fig. 4, super capacitorsare not appropriate for storing energy much longer than one day.

Figure 4: Super capacitor self discharge due to leakage current [9]

2.3.2 Batteries

EH only makes sense, when it is a better option than a battery. If it is not competitive inprice the energy harvesting application needs to be needed for a longer period than the batterylifetime, or can work in conditions where batteries cannot (extreme temperatures). It can beargued that a battery can always be replaced, but the replacement of a battery by a technicianis very expensive in comparison to the price of batteries/energy harvesting transducers and isthus to be avoided.

There has been an inverse Moore’s law within power consumption of embedded electronics. Ev-ery 18 months power consumption of digital systems is minimized by 50% ([3]). In comparisonbattery capacity has been very slow and only doubled every 10 years.

Battery disadvantages:

• self discharge - 0.1-5 % a month

• not happy with peak current draw

• temperature > 40C increases self discharge

• bad for the environment

Battery advantages:

• no calibration, does not influence the placement of the sensor node

• no packaging requirements


• not dependent on environmental variable energy

• predictable

• easy to convert energy - simple electronics

Fig. 5 shows the power density of different battery technologies and energy harvesting vs. time.As it is seen batteries are not suitable for long term deployment and if a sensor node has tooperate longer than a couple of years, it cannot be done with batteries without having to replacethem.

Figure 5: Power density versus lifetime for batteries, solar cells, and vibration generators [6]. Solar andvibrational power is not depend on time. The top of the shaded solar box corresponds to outdoor sunlight and the bottom corresponds to indoor office light. Both battery drain and leakage is considered inthis graph.

Batteries can also be used as buffer storage in energy harvesting power supplies. Table 3 com-pares the properties. Again the result is that batteries are not good for long term deploymentas they have a limited amount of charging cycles, but are good for power heavy applications.

Table 3: Comparison of different properties of energy storage types [18]

Li-Ion Battery Thin Film Battery Super Cap

Recharge cycles Hundreds Thousands MillionsSelf-discharge Moderate Negligible HighCharge Time Hours Minutes Sec-minutesPhysical Size Large Small MediumCapacity 0.3− 2500mAHr 12− 1000µAHr 10− 100µAHrEnvironmental Impact High Minimal Minimal

3. Energy Harvesting Technologies 8

3 Energy Harvesting Technologies

This chapter describes thermal, vibrational and photovoltaic energy harvesting and the electricalproperties and challenges.

3.1 Thermal Harvesting

Temperature gradients are found everywhere as a fact of heat flow due to temperature variationscaused by machinery, heaters, electronics and the weather.

Temperature differences can be utilized to harvest electrical energy using a thermoelectric gen-erator (TEG). The TEG creates electrical power when it is placed between a warm and a coldtemperature source. There will be a heat flow from the warm to the cold side and this heatflow makes electrons move within the TEG, creating an electrical voltage that can be harvested.Inside the TEG is a Peltier element. A Peltier element with heat sinks/thermal connectionstogether with a power conditioning module makes a thermal energy harvester (see fig. 6).

Thermal energy harvesters can be used on radiators, heat pipes, car engines, human body etc.Every place where a temperature gradient is present. Example of applications are a wristwatch,radiotormeter, car battery powered by engine excess heat etc. [1].

Peltier

Thin alu plate - contact to object

Heat sink - contact to air

TaTo

Q

Ambient airObject TEG

Figure 6: Thermo harvester (TEG) - heat flows from heat source through base of TEG, then throughthe Peltier element and out through the TEG heat sink.

A Peltier element consists of several p- and n-junctions in series (also called thermocouple). ap-plying a temperature gradient across these results in a charge carrier diffusion from the hot sidetowards the colder side. This forces electron and hole carriers to flow and creates a current thatcreates a voltage across the terminals of the thermocouple (see fig. 7). The voltage is describedby the Seebeck effect. The Seebeck coefficient of a material tells about the flow of charge carriersper heat flow.

Fig. 8 shows the electrical equivalent circuit of a thermal generator, that is a voltage source anda resistance. The corresponding IV curve is seen in the same fig. from a thermal harvester fromMicropelt. As it is seen, the voltage is low. This rises the need for a power converter capable ofoperating at low input voltages.

An interesting note is, that the available power is dependent on the thermal resistivity of theTEG. The larger the thermal resistivity, the larger the thermal difference, but the electricalresistivity is directly proportional to the thermal resistivity. So in order to maximize the outputof a TEG, the environment temperatures should be taken into account when designing the TEGin order to get the optimum resistivity [1]. As seen in the IV curve of the TEG, the powerelectronics also have to track the maximum power point in order to optimize the power output


Figure 7: Seebeck effect [27]

as the internal resistance changes with temperature[23].

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Figure 8:

Fig. 9 shows an thermal energy harvesting device along with the inside thermal generator.

3.1.1 Converter

The TEG from Micropelt shown in fig. 9 produces ∼ 75mV/K = VOC . That means when har-vesting thermal energy from just a couple of degree’s temperature difference there is a need for alow voltage power converter. Regular topologies are not able to operate below the diode/switch


(a) Thermo electric generator from Micropelt, sit-uated inside device shown beside - capable of gen-erating energy from ∆T = 3C.

(b) Thermal energy harvesting device from Micro-pelt [28].

Figure 9:

voltage threshold level. Normal MOS transistors have a threshold voltage above 0.5V, makingthem unsuited for low voltage converters. Sub 0.2V threshold transistors are available but thesehave large leakage current when the gate-source voltage is 0V also making them unsuited forconverting low power levels[14].

A solution can be a charge pumping circuit.

Charge Pump - Capacitive

Consist of X capacitors connected in parallel and thus all getting charged to the same voltage V.All the capacitors can be switched in series which then creates a total voltage of X · VIN . Thisis an efficient way of charge pumping, but it requires control of the capacitor switches whichrequires energy [17].

This can also be cascaded by several capacitor charge pumps in series which creates a voltageof 2N · VIN .

Charge pump - Armstrong Oscillator

Also known as Meissner oscillator (mostly in german literature).

The Armstrong Oscillator technique[15] (see fig. 10) utilizes a transformer instead of a single coilto be able to charge inputs below 0.5V, where an active circuit is not feasible. The secondarywinding on the transformer makes the circuit self-oscillating.

When a small input voltage is applied, the current in the primary coil will flow through the start-up JFET which is conducting at 0V gate voltage. A charge will be build up at the secondarywinding and eventually it will reach the threshold of the JFET, activating the main circuit.The JFET has a large conduction loss since its on resistance is some tens of ohms, so thistransistor is only active in the startup period and when it has initiated the main circuit an NMOStransistor with low conduction will be active instead. [14] has designed a step-up converter ableto operate down from 20mV with this technique.


Figure 10: Schematic of boost converter by [14] with low voltage start-up circuit based on the Armstrongoscillator.

3.2 Vibrational harvesting

Vibrational harvesting can be used whenever there is a movement. It can be harvested withelectromagnetic harvesters consisting of a core and a coil, electrostatic harvester utilizing a ca-pacitance change, and piezoelectric harvester made with piezo ceramic utilizing the piezoelectriceffect when the material is flexed. Vibrations are especially present in transportation and in-dustry machineries.

Table 4 shows some examples of what vibration levels can be found in the ambient environ-ment. Vibration harvesting applications can be kinetic watches, industrial motor monitoring,structural/bridge health monitoring, kinetic powered light switches, muscle powered implantsand helicopter tracking nodes[1].

Table 4: Vibration levels [6]

Vibration Source Peak Acc. [m/s2] Freq. of Peak [Hz]

Base of 5 HP 3-axis machine tool with 36inch bed 10 70Kitchen blender casing 6,4 121Clothes dryer 3,5 121Door frame just after door closes 3 125Small microwave oven 2,25 121HVAC vents in office building 0,2 - 1,5 60Wooden deck with people walking 1,3 385Bread maker 1,03 121External windows next to a busy street 0,7 100Notebook computer while CD is being read 0,6 75Washing Machine 0,5 109Second story floor of a wood frame office building 0,2 100Refrigerator 0,1 240

The piezo produces a voltage when a strain/deformation is applied in a vibrating environment.The typical setup is a two-layer bending bimorph fixed in one end and the other end attachedto a free moving mass.


Electrostatic harvesters converts mechanical energy into electrical energy by the variation in theseparation distance/overlap area of the plates in a MEMS capacitor. Changes in the capacitanceproduces energy when keeping the charge or voltage constant, Q = C · V .

Electromagnetic harvesters transforms kinetic energy into electrical by moving a coil across themagnetic field of a stationary magnet, thereby inducing a voltage across the coil.

Fig. 11 shows the power density for the 3 different vibrational harvesters. It is seen that piezo’sperforms better at ”high” frequencies (> 100Hz) and electromagnetic are best at low frequencies(< 100Hz).

Figure 11: Power density for the 3 different vibrational harvesters vs. frequency [20]

A simplified electrical equivalent circuit of a piezo generator is shown in fig. 12. The piezo actsas a AC voltage source in series with a capacitor and resistor. This capacitance can be exploitedwhen converting the energy and when tuning the resonance frequency of operation [22].

VPE

RS

VO

+

-Cshim

Figure 12: Electrical equivalent circuit of bimorph piezoelectric bender. VPE represents both layervoltages and Cshim the separation between the layers as a coupling capacitor. RS models the seriesresistance through the device.

A rather big issue with vibrational harvesters is that they are very frequency dependent (due totheir narrowband performance - see the equivalent mechanical circuit in fig. 13). They have aspecific resonance frequency where they will exhibit maximum power and not far from this res-onance the power output will be very low. Thus several techniques within tuning the frequency


of the harvester have been made [22].

Figure 13: Piezoelectric cantilever generator equivalent circuit [11]

Another issue with vibrational harvesting is the stress that the harvesters receives as theybend/shake and thus eventually might break. Electromagnetic harvesters are more durablethan piezo harvesters.

A simple method of harvesting the vibrational energy is like fig. 14. A piezoelectric generatorhas a blocking output capacitance which is connected to a diode bridge rectifier and then storedon a capacitor. When the piezoelectric output voltage is greater than the storage capacitor +diode bridge voltage, then energy is harvested.

Figure 14: Piezoelectic vibration harvester with external components [11]

Piezoelectric generators have the advantages of simple structure and easiness to fabricate. It isalso easy to be integrated into silicon devices and further fabricated with the microelectroniccircuits on the same chip[1].

The electromagnetic generators can generate high output-current levels but the voltage is verylow (typically < 1V ). Macro-scale devices are fabricated using high- performance bulk magnetsand multi-turn coils. Both piezo and electromagnetic harvesting techniques have been shown tobe capable of delivering power to the load in the range of µW to mW .

3.2.1 Converter

To maximize the energy from the harvested AC signal a method called SSHI is commonly used(see fig. 15). As seen in the electrical equivalent circuit, the piezo has a capacitance which willcause a 90 phase-shift between the voltage and current. The SSHI method utilizes an inductor


to remove this phase shift. The inductor has to be sized to the frequency of operation.

Figure 15: Two SSHI circuits: a) classical circuit[13] and b) circuit for low voltage output[12].

In fig. 15a S1 is closed when the piezo voltage is maximum and S2 is closed when minimum.These switches are closed for a very brief time period compared to the piezoelectric voltageperiod, thus shaping an oscillating network with the inductor L. [12] has made an improvedversion of the SSHI technique, where 2 diodes are replaced by the two switches (see fig. 15b).This removes one diode voltage drop, making the circuit operational at lower voltages, decreasespower loss, and it decreases the number of components.

3.3 Photovoltaic Harvesting

Harvesting energy from the sun is a mature technology in comparison to thermal and vibrational.Solar cells are found in all kinds of products and sizes. In fig. 16 the light intensities indoorsand outdoors can be seen. Together with table 5 which shows the energy in the light and table6 which shows the common efficiencies for solar cells, it gives an idea of energy availability.

Figure 16: Indoor and outdoor levels of light.

The efficiencies in table 6 are for the outdoor solar spectrum. When harvesting photovoltaicenergy inside, the efficiency is at least halved due to the spectral difference in the light [9].

The equivalent electrical diagram of a photovoltaic cell is shown in fig. 17. A simple methodof harvesting energy from a solar cell is connecting it directly through a diode to a capacitor orbattery.

The photovoltaic is a pn-junction diode generating photons that causes electrons and holes torecombine, generating an electric current when exposed to light.


Table 5: Power available in different types of light [6]

Condition Power [mW/cm2]

Mid-day, no clouds 100Outdoors, overcast 5Incandescent bulb, 3m away 10CF bulb, 3m away 1

Table 6: Photovoltaic technologies and their reported maximum efficiencies [7]

Solar panel efficiency

Silicon 25%GaAs 26,4%Amorphous Si 10,1%Organic 5,15%Multi-junction 32%

IPH

RSRRI D

IO

Figure 17: Equivalent electrical diagram of solar cell.

The current source IPH generates a current proportional to the amount of light falling uponthe cell. With no load connected, nearly all the current generated flows through the diode,whose forward voltage determines the solar cell’s open circuit voltage, VOC . This voltage variessomewhat with the exact properties of each type of solar cell. For most silicon cells, it is in therange between 0.5V and 0.6V which is the normal forward voltage of a p-n junction diode[9].

The parallel resistor, RP , represents a small leakage current that occurs in practical cells, whileRS represents the connection losses. As the load current increases, more of the current gener-ated by the solar cell is diverted away from the diode and into the load. For most values of loadcurrent, this has only a small effect on the output voltage.

Fig. 18 shows the IV characteristic of a solar cell and as it is seen the solar cell has a maximumpower point. This point can be tracked by several MPPT techniques.

3.3.1 Converter

Which converter to use for for solar panels depends on the type of panel and how it is setup.A single cell produces only 0.6V and thus needs a low voltage boost converter to operate. A


Figure 18: Solar cell I/V characteristic and power output

regular panel that produces 4-8V and above (depending on how many you put into series) canbe managed with a buck or sepic converter for broader input ranges.

MPPT Techniques

The maximum power point where the solar cell generates most energy changes with irradianceand cell temperature. In order to track the MPP of the solar cell, an extensive amount ofmethods can be used. Many utilizes a microcontroller to do the tracking: Perturb and Observemethod and the Incremental Conductance which oscillates around the MPP in order to track it,neural network method which uses fuzzy logic etc. [25].

Ripple Correlation Control When a solar cell is connected to a switching power converter,the voltage and current drawn from the cell will have a ripple. RCC correlates the time deriva-tive of the time-varying solar cell power with the time derivative of the time-varying solar cellcurrent or voltage to drive the power gradient to zero, thus reaching the MPP [25].

Fractional open circuit methodThis method works by the assumption that the MPP always is at a constant fractional, K, ofthe open circuit voltage (usually K is around 0.6-0.8).

VMPP ∼ VFV OC = VOC ·K

The open circuit voltage in the current lighting conditions needs to be measured in order totrack. This can be done by having a small extra reference solar cell dedicated to the purpose ofgenerating the reference voltage.

Another method is to periodically disconnect the load from the solar cell, take a measurementof the VOC , and the reconnect load [8].

[24] states methods of FVOC consuming from 50µW - 2mW . It is important to evaluate theefficiency gained by the MPPT in relation to how much energy the tracking circuit consumes.Results from the FVOC method in [9] shows benefits from MPPT above 200 lux.

4. Comparisons 17

4 Comparisons

4.1 Converter Comparison

Table 7 summarizes the challenges within converters for each technology. The solar technologyhas been around for long and many MPPT methods have been developed. The most challeng-ing technology is the vibrational harvester. It suffers under frequency dependence and narrowbandwidth of operation.

Table 7: Converter challenges for different EH technologies - based on example technologies used in pricecomparison in next section.

EH Technology Voltage Current Challenges

Solar 0.6V-8V (or more) low MPPT, low lightPiezo 0V-20V low AC rectification, frequency tuning, phase shiftElectromagnetic 0V-10V low AC rectification, frequency tuningTEG 0V-5V high MPPT, charge pumping

As shown earlier in table 1 the energy level in the solar light outdoor, where the sensor node isgoing to be placed, is far superior. Vibration levels and thermal levels are magnitudes lower.

The prices of the different harvesters will be evaluated in the next section and on this basis aEH technology will be chosen.

4.2 Price comparison

Different energy harvesting companies has been interviewed in regards on how they see the pricesof their energy harvesting transducers develop the coming years as they are now on a prototypelevel where the prices does not reflect the pure cost.Companies: Perpetuum (electromagnetic vibrations harvesters), Invent (piezo electric harvesters),Micropelt (thermal generators), Powercast (RF harvesters).

They all agree on the fact that the technologies are not a commercial stage yet and they arevery hesitant by giving an exact price because that is very application specific and dependstotally on scale. So another way around the price comparisons has been chosen. 3 commercialavailable transducers have been chosen to be compared with a lithium and alkaline battery inorder to give an example of where the three technologies (thermal, vibrational and photovoltaicharvesting) are in price and what prices they need to reach in the future to be competitive.

The lithium and alkaline battery have been chosen due to their low leakage current (1-3% pr.year), which makes them able to compete with energy harvesting which has timeframes for sev-eral years (other batteries like NiCD and NiMH only last a couple of months). The price paidper joule when buying a battery depends on how long time the battery is going to operate dueto its leakage current. The longer the battery has to operate, the lower total amount of joule isavailable.

See fig. 19 for the price comparison. The figure shows how the prices changes as a function ofthe number of sensor node cycles. As previously stated, the example where the sensor node is torun 1s once every half hour is chosen. That means that the sensor node needs 20mJ every halfhour. The battery price / joule changes with time, due to its leakage current. Thus the total

4. Comparisons 18

energy available in the battery decreases, and thus the price per joule increases slightly. For theharvesters the price /joule decreases the longer time it harvests energy. This means that therewill come a point where the price / joule for the battery and the harvesters intersect. After theintersection, the cheapest solution is the energy harvesting and for shorter operation the batteryis the cheapest option.

100

101

102

103

10−4

10−3

10−2

10−1

100

Days (= 48 sensor node cycles of 20mJ))

Pric

e ($

) / j

oule

Lithium Battery − 1400mAhAlkaline Battery − 2900 mAhPiezo @ 1m/s2Electromagnetic @ 1m/s2TEG − dT=20KTEG − dT=5KSolar − outdoorSolar − indoor

Figure 19: Price comparison of energy harvesters with lithium and alkaline battery as a function of time.

Table 8 compares the harvesters intersection with the alkaline batteries, to show when each EHtechnology becomes competitive on price. It is interesting to see that the solar cell has generatedenergy enough in a few days, to have a cheaper price per joule than an alkaline battery. Theindoor solar, the low vibration piezo, and the human body thermal energy generation are allvery expensive. It is obvious that they have to become much cheaper in order to be able tocompete. (It has to be noted that it is assumed that the solar, thermal, and vibrational energyis constantly available.).

Table 8: Based on results from fig. 19. Compares the harvesters prices and shows when the becomecompetitive with an alkaline battery.

EH Technology Price Competitive after [days]

Solar outdoor 20$ 5Solar indoor 20$ 2500Piezo 79$ 3500Electromagnetic 130$ 540TEG 5K 15$ 2700TEG 20K 15$ 220

See appendix A.1 for specifications of each of the chosen harvesters and the batteries. AppendixD.1 shows the matlab script for calculating the price developments.

5. Implementation 19

4.3 Choice of EH Technology

The harvesting technology chosen to be implemented for supplying energy to a sensor nodeplaced outside a building is a solar cell as there is by far the most energy available in the lightand as the price comparison showed, solar cells situated outdoors also generates energy waycheaper than the other technologies. On top of this, DELTA just got their hands on some neworganic polymer solar cells from a company called Mekoprint. Not much is known about thesecells and it is interesting to see how they perform.

The requirements from the sensor node earlier noted are to be fulfilled. In order to be able torun the sensor node once every 30 min with solar power, it needs to be able to harvest energyfor the whole day from the available light. An approximation is made that light is present 12hours a day.It is assumed that some timing circuit is present and able to activate the power circuit andsensor node every 30 min.The average power required from the solar harvester is then during the 12 hours:

Enode−day = 20mJ/cycle · 48cycles = 0.96J (6)

Pav =0.96

12h · 602= 22µW (7)

5 Implementation

This chapter describes the implementation of the photovoltaic harvester prototype for poweringthe sensor node.

5.1 Solar Cell

An organic polymer solar cell is used as power source. It is developed by a company calledMekoprint, originally developed at DTU Risø. This solar cell is chosen due to the fact that is avery new technology for processing and manufacturing solar cells.

The advantages of the organic polymer solar cell technology are its flexibility, it is printableand potentially cheap to manufacture. The flexibility opens up for building it into fabrics or onto structures that are round, and to incorporate it into designs where the shape of the objectchanges. The manufacturing process makes it possible to print it on surfaces so one could expectit being printed on e.g. big sails for a boat. The printing process also makes it potentially verycheap to produce.

5.1.1 Characterization of Mekoprint Solar Cell

The characteristics of the solar cell are unknown and there is no datasheet made yet, as it is stillunder development. Therefore it has been characterized in a controlled light environment at theLights & Optics department at DELTA. The solar cell was tested in an environment where noother light source than a XBO Zenon lamp (see fig. 20) with an IR filter D65 corresponding toa light temperature of 6500 K, which is similar to the solar spectrum, was present. The lightintensity was carefully measured and controlled (see figure 21).It was interesting to see whether the performance of the cell was comparable to a standardsilicon cell and how the MaximumPowerPoint (MPP) changed when the light intensity changed.On silicon cells, the MPP is approximately linearly related to the open circuit voltage, VOC of


Figure 20: XBO Xenon lamp - 6500K

(a) (b)

Figure 21: Setup of Mekoprint solar cell at light testing facility at DELTA

the cell [9].

3 different light intensities were tested4:

• 6950 lux - corresponds to outdoor light, heavy overcast

• 3250 lux - corresponds to inside light by a window

• 380 lux - corresponds to low artificial light inside an office

The results from the tests can be seen in fig. 22. Here both the IV-curves and the power curvesare plotted in order to easily see the maximum power points. From the results it can be con-cluded that the voltage level is not going to be a problem when applying a converter but thecurrent is very low (below 0, 5mA).

To be able to operate the solar cell on its maximum power point the relation between the opencircuit voltage, VOC , and the MPP voltage, Vmpp, (using the method of fractional open circuitfrom section 3.3.1) has been estimated in table 9. Here it can be concluded that the best point

46950 lux was the maximum the lamp could perform and therefore where no higher values tested.


0 1 2 3 4 5 6 70

2

4x 10

−4

Voltage [V]

Cur

rent

[A]

Mekoprint characteristic

0 1 2 3 4 5 6 70

0.5

1x 10

−3

Pow

er [W

]

IV−380 LUXIV−3250 LUXIV−6950 LUXPV−380 LUXPV−3250 LUXPV−6950 LUX

Figure 22: Characteristics of Mekoprint solar cell with IV curves (left y-axis) and PV curves (right y-axis) for three different luminosities - measured at DELTA Light & Optics. The maximum power pointsare found at 6950 lux: Vmpp = 3, 4V , 3250 lux: Vmpp = 3, 2V and 380 lux: Vmpp = 2, 4V .

to operate the solar cell is ∼ 0, 52 · VOC .

Table 9: Maximum power point tracking coefficient, K, for Mekoprint solar cell at different luminosities- see fig. 22

Luminosity [lux] VOC VMPP K

6950 6,6V 3,4V 0,523250 6,3V 3,2V 0,51380 4,2V 2,4V 0,57

5.1.2 Efficiency

The energy in the light at the test site is defined as 95 lux corresponds to 1W/m2. With thesolar cell size of 0, 0068m2 (8cm x 8,5cm), the energy available for the solar cell is 72µW/lux.In table 10 the efficiency of the solar cell has been calculated. The result shows that the solarcell performs quite poor. An efficiency of 0, 15% is very low compared to silicon based solar cellwhich at least performs 8% efficiency at these light intensity levels (see appendix B for referencemeasurement of silicon solar cell at same test setup).

Table 10: Mekoprint solar cell efficiency - results from fig. 22

Luminosity [lux] Plight [W] Pmeko [W] Efficiency [%]

6950 0,497 0,00064 0,1293250 0,233 0,00038 0,163380 0,027 0,000038 0,141

The solar cell has also been tested ad hoc outdoor in sunlight in the afternoon. It performed an


open circuit voltage, VOC = 8, 5, and shorted current, ISC = 2mA, which can be estimated to apower of 8mW around the MPP when using the K factor found in table 9.

The harvester is to be situated outdoors, so it is assumed that the light intensity will be some-where in between 100000 lux and 3000 lux, corresponding power from solar cell of 11mW to0.3mW .

To take the worst case of a whole day with heavy overcast the total power harvested would be0.3mW · 12602s = 13J which is more than plenty. The energy needed for the sensor node forhalf a day, as stated earlier was 0.48J . Even with a converter efficiency of 50 % there is plentyof headroom.

Then a converter able to handle low power input from solar cells is needed.

5.2 Converter

Many converter topologies can be used for solar cells, but since the output power was quite lowand the VMPPT in lower light was below the desired output voltage a search for boost converterswas made and the LTC3105 integrated step-up circuit from Linear Technology came up.

The LTC3105 is stated to be able to operate from high impedance sources like solar cells, hasmaximum power point control, low power input (225mV − 5V ) and among other burst modewhich adjusts the peak switching current. It needs a few peripheral components and is rathersimple to implement. See datasheet [26]. With these properties seeming to meet the require-ments, the LTC3105 was chosen to be implemented as the power converter for the Mekoprintsolar cell.

The LTC3105 will charge a storage capacitor, that ought not to be directly connected to thesensor node. If the sensor node was directly connected to the storage capacitor, it would turnon before the capacitor was fully charged. Thus a hysteresis circuit with a threshold around3.3V − 2V is intended to be implemented to then make sure the capacitor is charged to 3.3Vbefore the output turns ON. See appendix C.1 for a schematic of the hysteresis circuit. Thesensor node is connected to the output of the hysteresis circuit. See fig. 23 for the block diagramof the system from solar harvester to sensor node.5

Mekoprint solar cell LTC3105 Storage

capacitorHysteresis

controlSensor node

Figure 23: Block diagram of the system to be implemented.

5.2.1 LTC3105

In fig. 24 an application diagram from the datasheet is seen. It is almost identical to the circuitimplemented in this project, apart from the component values stated in the figure text.Fig. 25 shows the internal block diagram of the LTC3105. It is hard to tell how the start-upcircuit is build and also how the burst mode is implemented. If more time were available it would

5This solution is not possible to operate during the night when no light. This would require some sort oftiming circuit able to control when the power should be ON - this is beyond the scope of this project.


Figure 24: LTC3105 application diagram from the datasheet. The converter circuit was build like this,but with and feedback of 2.2MΩ and 1MΩ resulting in Vout = 3.3V , and RMPPC = 200k − 400kΩdepending on which light intensity to set the VMPP . And no battery on the output.

be interesting to contact LinearTechnology and get more specific details on these functions.

Figure 25: LTC3105 internal block diagram

How the IC operatesThe LTC3105 starts by charging the CAUX to a level of 1.4V . In this phase the maximum powerpoint control is not yet enabled. When VAUX = 1.4V the converter starts to regulate the LDOoutput. When this is done it turns the output ON (see fig. 26 for the waveforms).


Figure 26: Waveforms in the LTC3105 from datasheet

5.2.2 MPPC

LTC3105 has something called Maximum Power Point Control, which is a quite limited methodof making sure to keep the solar cell at its maximum power output. The method is to set thedesired operating voltage via a resistor RMMPC on the MPPC pin, where there is a referencecurrent of 10µA, and then the IC will make sure to operate the input at this fixed voltagelevel. This solution cannot track the MPP when the light intensity changes and this is veryimportant when harvesting outdoors, as the light intensity is very fluctuating. As the MPPCis just a reference voltage one could connect another circuit for tracking to it (remembering totake the 10µA into account). A solution could be a small reference cell or the microcontrolleron the sensor node could be used in a low power mode. Then many MPPT methods could beincorporated but each one would have to evaluated based on the power consumption vs. energygain from the MPPT method in comparison to the un-tracked case. This could be done infurther work.

5.2.3 Components

The choice of components can be seen in appendix C. The choice of the switching inductor wasa SMD 10uF with very low DC resistance as this is directly proportional to the efficiency of thecircuit. A 0.08F super capacitor is chosen as the storage element, because that size was at hand.

5.3 Results

The implemented prototype, see fig. 27, was tested first with a lab. power supply and later withthe Mekoprint solar cell.

The start-up current for the circuit has been measured to 6mA (at Vin = 3V - it decreases withhigher voltage). This current cannot be supplied by the solar cell so it cannot start the circuit itself. This is problematic. The solar cell was able to run the circuit, as long as it got a ”kick-start”being a secondary voltage input. After this it worked fine, even in indoor lighting conditions.


Figure 27: Implemented prototype of LTC3105 and external components for converting the solar energyfrom a Mekoprint organic polymer solar cell.

A solution could be a large input capacitor, but this will also collapse the solar cell outputvoltage as the LTC3105 already start drawing current at 250mV . Thus a big input capacitor isneeded to be able to supply the current draw from only 250 mV. This would require a capacitor> 1mF , and would have to be a super capacitor due to the low leakage current. Another optionis to put a switch/voltage detector on the input capacitor, making sure it is charged to a certainlevel before turning ON the input6. Then enough start-up power could be generated. This couldbe investigated in further work.7

What sets the amount of current being drawn during start-up is unknown, as the datasheetinternal diagram only shows a start-up ”block” (fig. 25).

The implemented LTC3105 with external components has a power use of 0.04mA which cor-responds well with the added quiescent currents stated in the datasheet. Under operation theconverter has been tested with a load of 470Ω at the output voltage of 3.3V - drawing 7mA.

• Input power: 33mW

• Output power: 23mW

• Efficiency 70%

This efficiency corresponds with the stated values in the datasheet.

It can be concluded that the LTC3105 is not perfectly fit for the Mekoprint solar cell, mainlydue to the high startup current. This issue can be fixed with the mentioned input capacitorsolution and then the converter is capable of performing with ok efficiency, but there also needsto be looked into a more flexible MPP technique.A very low input voltage converter is actually not that important in this case where the lightintensities will be rather high so it might be possible to look into a buck converter with low biascurrent switches.

6The LTC2935 looks interesting.7One could also use several solar cells and then it would work fine, but that is not preferable.

6. Conclusion 26

6 Conclusion

Vibrational, thermal and solar harvesters has been introduced with emphasis on their electricalproperties. The converter challenges within each technology has been described and a choice forimplementing a prototype for powering a wireless sensor network node was made based on theprice of the transducer and the available energy. It was seen that the energy generated from asolar cell after a few days only became cheaper than the energy from the alkaline battery. Theother technologies have to become much cheaper in order to compete with batteries if powersolution is only a matter of price.

The choice to be implemented was a solar harvester. At the same time DELTA got their handson a prototype polymer solar cell which then was chosen. It needed to be characterized andit showed to perform quite poor - 0.15% efficiency (compared to silicon with > 15% and otherorganic polymer cells 5.15%). The solar cell would provide between 0.3mW and 11mW whenplaced outdoor and that was plenty for the sensor node to operate once every 30 min.

The LTC3105 integrated step-up converter was quickly chosen to be implemented as it seemedlike a good match for the solar cell and simple to implement. It though showed to have a start-upcurrent draw to large for the solar cell to provide, so the system was not able to start by it self.The circuit performed a 70% efficiency under 20mW load corresponding to the sensor node.

6.1 Further work on prototype

• Address the high start-up current draw with a super capacitor on the input or switch/-voltage detector.

• Implement hysteresis control of storage

• Test it with sensor node

• Implement more sophisticated maximum power point tracking

6.2 Problems fit for a masters project

An obvious topic to carry on studying from the results from this project is a converter for lowcurrent solar cells, which would address the need of having a low current draw and while main-taining high efficiency.

Within power management the issue of how the sensor node should be activated is also a topic.If the sensor node, powered by solar harvesting, is to be operational during the night how shallit be turned ON and OFF. And then when it has to be in sync with other sensor nodes thechallenge becomes more interesting.

Power converter for vibrational harvesting - a small broadband low frequency electromagneticharvester could be interesting as it has many potential utilizations (human movement etc.).

Power converter for thermal harvesters with emphasis on maximum power point tracking as theinternal resistance of the harvester changes a lot under different temperatures.

References 27

References

[1] Shashank Priya and Daniel J. Inman (editors), Energy Harvesting Technologies,Springer, 2009.

[2] http://micropelt.com/applications/te_power_node.php June 28,2010.

[3] Adrian Valenzuela, Texas Instruments Batteryless energy harvesting for embed-ded designs http://www.eetimes.com/design/embedded/4008326/Batteryless-energy-harvesting-for-embedded-designs August1, 2009.

[4] Murugavel Raju, Mark Grazier, ULP meets energy harvesting: A game-changing combination for design engineer http://focus.ti.com/lit/wp/slyy018a/slyy018a.pdf 2010, Texas Instrument

[5] News Analysis Rare Earth Elements Fueling Innovation http://www.globalization101.org/news1/Rare-Earth-Elements October 15,2010, Levin Institute, State Uni. of New York

[6] Shadrach J. Roundy Energy Scavenging for Wireless Sensor Nodes with a Focuson Vibration to Electricity Conversion, Berkeley 2003

[7] Solar cells efficiency tables version 36 http://onlinelibrary.wiley.com/doi/10.1002/pip.1021/pdf

[8] A. Chini and F. Soci Boost-converter-based solar harvester for low power appli-cations Electronic Letters Vol. 46 No. 4, 2010

[9] W. S. WANG, T. O’DONNELL, N. WANG, and M. HAYES Design Consider-ations of Sub-mW Indoor Light Energy Harvesting for Wireless Sensor SystemsTyndall National Institute, 2010

[10] Chulsung Park and Pai H. Chou AmbiMax: Autonomous Energy HarvestingPlatform for Multi-Supply Wireless Sensor Nodes, UC Irvine, 2006

[11] Alireza Khaligh, Peng Zeng and Cong Zheng Kinetic Energy Harvesting Us-ing Piezoelectric and Electromagnetic Technologies - State of the Art, IllinoisInstitute of Technology 2010

[12] Mickael Lallart and Daniel Guyomar An optimized self-powered switching circuitfor non-linear energy harvesting with low voltage output, INSA-Lyon, 2008

[13] Taylor G W, Burns J R, Kammann S M, Powers W B and Welsh T R Theenergy harvesting eel: a small subsurface ocean/river power generator, IEEE J.Ocean. Eng., 2001

[14] Markus Pollak, Loreto Mateu, Peter Spies STEP-UP DC-DC-CONVERTERWITH COUPLED INDUCTORS FOR LOW INPUT VOLTAGES, FraunhoferIIS, 2009

[15] J. M. Damaschke Design of a Low Input Voltage Converter for ThermoelectricGenerator, University of Victoria, 1996

[16] Jihong Kim, Tajana Simunic Rosing Power-Aware Resource Management Tech-niques for Low-Power Embedded Systems Seoul National University, 2006

References 28

[17] Jerry W. Fleming Ultra-Low Power Conversion and Management Techniquesfor Thermoelectric Energy Harvesting Applications, Luna Innovations Incorpo-rated, 2010

[18] John Blyler Energy Scavenging And Storage Must Work To-gether http://chipdesignmag.com/lpd/blog/2009/07/16/energy-scavenging-and-storage-must-work-together, LowPower Engineering Community, 2009

[19] Energy Harvesting Journal Using a supercapacitor to manage yourpower http://www.energyharvestingjournal.com/articles/using-a-supercapacitor-to-manage-your-power-00001921.asp?sessionid=1, 14 December 2009

[20] C.O. Mathuna et al. Energy scavenging for long-term deployable wireless sensornetworks, Tyndall National Institute, 2008

[21] Zhi Ang Eu, Hwee-Pink Tan, Winston K.G. Seah Design and performanceanalysis of MAC schemes for Wireless Sensor Networks Powered by AmbientEnergy Harvesting National Uni. of Singapore, 2010

[22] NA KONG, DONG SAM HA, ALPER ERTURK AND DANIEL J. INMANResistive Impedance Matching Circuit for Piezoelectric Energy Harvesting, Vir-ginia Tech, 2010

[23] I. Laird, H. Lovatt, N. Savvides, D. Lu, and V. Agelidis, Comparative study ofmaximum power point tracking algorithms for thermoelectric generators PowerEngineering Conference, 2008. AUPEC ’08

[24] Alex S. Weddell, Geoff V. Merrett, Bashir M. Al-Hashimi, Ultra Low-PowerPhotovoltaic MPPT Technique for Indoor and Outdoor Wireless Sensor Nodes,Uni. of Southampton, 2011

[25] Trishan Esram, Patrick L. Chapman, Comparison of Photovoltaic Array Maxi-mum Power Point Tracking Techniques, Uni. of Illinois, 2005

[26] Linear Technology LTC3105 Datasheet http://www.linear.com/product/LTC3105

[27] Top Bits Seebeck Effect http://www.tech-faq.com/seebeck-effect.html

[28] Micropelt Energy Harvesting Thermal Generators - TE-Power Plus http://www.micropelt.com/applications/te_power_plus.php

Appendix 29

Appendix

A Comparison

A.1 Price comparison

Lithium Battery - Panasonic CR132 - 1400mAh 3V (end voltage 1.5V), 6$, max lifetime 10yearshttp://www.farnell.com/datasheets/808605.pdf

Alkaline Battery - Energizer X91 AA 1.5V 2900mAh (end voltage 0.8V), 2$, max lifetime 7yearshttp://www.farnell.com/datasheets/96696.pdf

Piezoelectric generator - Mide V20W, 0.8mW at 50Hz 0.1G, 79$http://www.mide.com/products/volture/v20w.php

Electromagnetic generator - Perpetuum PMG FSH, 10mW at 50 Hz 0.1G, 130$ 8

http://www.perpetuum.com/fsh.asp

Thermal generator - Micropelt MPG-651, 0.2mW at ∆T = 5K, 2.88mW at ∆T = 20K, 15$9

http://www.micropelt.com/down/datasheet_mpg_d651_d751.pdf

Solar cell - Sanyo AM-8801, 196mW direct outdoor sunlight, 0.3mW indoor, 20$http://www.farnell.com/datasheets/87132.pdf

B Solar cell test

In fig. 28 the reference standard silicon solar cell is tested.

Figure 28: Test setup of reference standard silicon solar cell. Only measured at 3250 lux whereit provided 0,02W at its maximum power point - corresponding to an efficiency of 8 %.

8Expected price - Source: Roy Freeland, CEO Perpetuum9Expected price - Source: Wladimir Punt, VP Sales and Marketing, Micropelt

Appendix 30

C Component list

LTC3105 peripheral components:L1 = 10uF (Coilcraft MSS5131-103MLB - DCR = 0.083Ω )R1 = 1 M OhmR2 = 2.2M OhmRMPPC = direct sunlight 4V –> R = 400k Ohm indoor 2V R=200kOhmCOUT= 10uF + super capacitor 0,08F low ESR (1s sensor node C=0,005)CAUX= 1uF ceramicCIN = 10uF ceramicCLDO=4,7uF

C.1 Components to be implemented

Output control:Hysteresis tps3806i33 - see fig. 29

R1+R2+R3 = min. 1 MOhmSwitch FDV303N

5

5

4

4

3

3

2

2

1

1

D D

C C

B B

A A

Jakob SteensenAuthor

For AC inputs

LED1 is for demo purposes.Disable with JP1 / pins 1-2.

For connectionto TI wireless tempsensor board

Hysteresis on @ V_CHARGE = [1,207V ; 2,380V].

AC_INPUT

FASTCHARGE

EXT_CHARGE

VOUT

GND_POWER

VOUT

GND_OUTPUT

V_CHARGE

RSTSENSE\

LED_ON

LED_ON

Size

Scale

CAGE Code DWG NO Rev

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DELTAVenlighedsvej 4DK - 2970 Hørsholm

Energy Harvesting Power Conditioner for small energy collection

<Cage Code>Size

Scale


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<Cage Code>Size

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<Cage Code>

R6560kR6560k

R5390kR5390k

R110RR110R

J4

2HEADER

J4

2HEADER

12

SD1BAT46SD1BAT46

R1210kR1210k

C5

10n

C5

10n

C1

10n

C1

10n

J1

2HEADER

J1

2HEADER

12

R10 10KR10 10K

Q1FDV303NQ1FDV303N

LED1LED orange

LED1LED orange

Z13V0Z13V0

SD3

BAT46

SD3

BAT46

R9100kR9100k

C7100uF / 50V elyt

C7100uF / 50V elyt

R4120kR4120k

U3 S-8354H33MCU3 S-8354H33MC

VOUT2

CONT5

NC3

VSS

4

ON/OFF1

R310RR310R

J6

2HEADER

J6

2HEADER

12

R7560kR7560k

U1 S-882Z24U1 S-882Z24

CPOUT5

VIN4

VM3

VSS

2

OUT1

C6

10n

C6

10n

Z23V0Z23V0

C42700uF/4V elyt

C42700uF/4V elyt

R11

10R

R11

10R- +

D2

Zetex ZXSBMR16PT8

- +

D2

Zetex ZXSBMR16PT8

SD2

BAT46

SD2

BAT46

J3

MINIHEADER

J3

MINIHEADER

123456U2 TPS3806J20U2 TPS3806J20

RSTSENSE1

VDD4

RESET3

HSENSE6

LSENSE5

GND2

C20.08F (supercap)

C20.08F (supercap)

R1310RR1310R

J2

CON3

J2

CON3

123

JP1

CON12A

JP1

CON12A

1 23 45 67 89 1011 12

L1

10uH

L1

10uH

R210kR210k

Figure 29: Example of how the hysteresis circuit was intended. Though setup with thetps3806i33, which has a high voltage threshold of VH = 3.3V instead of VH = 2.4V . Theswitch is a N-FET FDV303N.

D Matlabfiles

D.1 batt vs eh.m

1 %Script by Johan Pedersen for comparing prices of energy harvesting2 %technologies and batteries3 %31-3-20114

5 %Lithium battery 1400mAh 3 V (end voltage 1.5V), 6$6 %http://www.farnell.com/datasheets/808605.pdf7 E_bat=(1400e-3*60*60*(3-1.5))8 price_bat=6;%$9 bat_price_joule_init=price_bat/E_bat; % initial price $/J

10

Appendix 31

11 %Alkaline battery Energizer X91 AA 1.5V 2900mAh (stops at 0.8V)12 %http://www.farnell.com/datasheets/96696.pdf13 E_bat2=(2900e-3*60*60*(1.5-0.8));14 price_bat2=2; %$15 bat2_price_joule=price_bat2/E_bat2; % initial price $/J16 bat2_leak=0.03; % 3% pr. year17 bat2_max_life_days=7*36518

19

20 %Sensornode energy useage21 P_node=20e-3;22 t_on=1;23 E_node=P_node*t_on; %energy for one cycle24 cycles_day=48; %operate once pr. 30 min25 E_node_day=E_node*cycles_day;26

27 total_days=10*365; %10 year runtime28

29

30

31 L1=0.05; %leakage first day of lithium battery is 5%32 L1_2=0.02/(31*12); % lithium leakage pr year is 1 %, this is leakage pr.

day33 L2=bat2_leak/(31*12) % alkaline leakage pr year is 3 %, this is leakage pr.

day34

35 E_aval_tot=E_bat; %initial value of total available energy36 E_aval2_tot=E_bat2;37 E_left=E_bat;38 E_left2=E_bat2;39

40 % Loop calculating the energy available in the batteries as the sensor node41 % cycles every 30 min42 % The first day/48 cycles where the lithium has 5% capacity loss43 E_left=[E_left,E_left-(E_node_day+E_left*L1)]; %lithium44 E_left2=[E_left2,E_left2-(E_node_day+E_left2*L2)]; %alkaline45 E_aval_tot=[E_aval_tot,E_aval_tot-E_left*L1]; %available energy left

after leak46 E_aval2_tot=[E_aval2_tot,E_aval2_tot-E_left2*L2];47

48 for i=2:total_days,49 E_left=[E_left,E_left(i)-(E_node+E_left(i)*L1_2)];50 E_left2=[E_left2,E_left2(i)-(E_node+E_left2(i)*L2)];51 E_aval_tot=[E_aval_tot,E_aval_tot(i)-E_left(i)*L1_2];52 E_aval2_tot=[E_aval2_tot,E_aval2_tot(i)-E_left2(i)*L2];53 end;54

55 %the lithium battery can last for 10 years and alkaline for 756 if length(E_aval_tot)>10*365,57 E_aval_tot(10*365:total_days)=0;58 end;59 if length(E_aval2_tot)>7*365,60 E_aval_tot(7*365:total_days)=0;61 end;62

63 price_joule_bat=price_bat./E_aval_tot; %battery price per joule as afunction of cycles

64 price_joule_bat2=price_bat2./E_aval2_tot;

Appendix 32

65

66 days=1:total_days+2;67

68 %Piezoelectric generators69 %Mide V20W70 %http://www.mide.com/products/volture/volture_catalog.php#seh71 %Size= 50 x 38 x 0,76 mm72 price = 79%$73 watt=0.8e-3; %8 mW @ 1G & 50 Hz - %assumes 0,1 g = 1m/s at HVAC vent74 price_watt=price/watt75 price_joule_piezo=price_watt./(days*24*60*60);76

77 %Micropelt MPG-65178 teg_price=15; %$79 teg_watt=2.88e-3; % dT=2080 teg_price_watt=teg_price/teg_watt;81 teg_price_joule=teg_price_watt./(days*24*60*60);82

83 teg2_watt=0.2e-3; % @ dT=584 teg2_price_watt=teg_price/teg2_watt;85 teg2_price_joule=teg2_price_watt./(days*24*60*60);86

87 %Sanyo solar cell:88 %http://dk.farnell.com/jsp/search/browse.jsp;jsessionid=BBYBMX3GMBRZMCQLCIP89 %ZNFQ?N=0&Ntk=gensearch_001&Ntt=solar+cell&Ntx=mode+matchallpartial&suggest90 %ions=false&ref=globalsearch&_requestid=64900191 sol_price=20;%$92 sol_watt=196e-3; % outdoor93 sol_ind_watt=10e-6*5.5*5.5; % indoor - assumes 10uW/cm^294 sol_price_watt=sol_price/sol_watt;95 sol_ind_price_watt=sol_price/sol_ind_watt;96 sol_price_joule=sol_price_watt./(days*24*60*60);97 sol_ind_price_joule=sol_ind_price_watt./(days*24*60*60);98

99 %Perpetuum electromagnetic harvester100 eeg_price=130; %$101 eeg_watt=10e-3; % @ 1m/s^2102 eeg_price_watt=eeg_price/eeg_watt;103 eeg_price_joule=eeg_price_watt./(days*24*60*60);104

105 figure(1)106 p=loglog(days,[price_joule_bat;price_joule_bat2;price_joule_piezo;

eeg_price_joule;teg_price_joule;teg2_price_joule;sol_price_joule;sol_ind_price_joule])

107 legend(’Lithium Battery - 1400mAh’,’Alkaline Battery - 2900 mAh’,’Piezo @ 1m/s2’,’Electromagnetic @ 1m/s2’, ’TEG - dT=20K’,’TEG - dT=5K’,’Solar -outdoor’,’Solar - indoor’)

108 xlabel(’Days (= 48 sensor node cycles of 20mJ))’)109 ylabel(’Price ($) / joule ’)110 axis([1 total_days 10^(-4) 1])111 set(p,’LineWidth’,2);112 set(gca,’FontSize’,14);113 set(gcf, ’PaperUnits’, ’centimeters’); set(gcf, ’PaperSize’, [21 10.5]);114 set(gcf,’PaperPosition’,[-0.5 -0.0 21 10]); %saveas(gcf,’thermal_noise’,’

pdf’);115

116 print(gcf, ’-dpdf’, ’-r300’, ’/Users/Johan/Dropbox/Master Projekt/LaTeX/graphics/batt_vs_eh.pdf’);

Appendix 33

117

118 %26000 cycles in one 1400 mAh lithium batt when E_node=100mJ /cycle and 48119 %cycles / day -> 2000 days. Corresponds to 2000*48*E_node = 9600 J120 %E_batt_start= 15000J121 %That means that one third goes to leakage.

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