Power Converter for Energy Harvesting

Transcription

1 Power Converter for Energy Harvesting Special Course March 31, 2011 Johan Henning Pedersen, s Supervisors: Michael A.E. Andersen, Ole C. Thomsen, Arnold Knott Thomas Sørensen, DELTA Department of Electrical Engineering Technical University of Denmark

2 Contents 1 Introduction Energy Harvesting Project Description Delimitations Goal of project Description of report Energy Consumer - Sensor Node DELTA Greenlab Mote Power Use EH Power Converter Requirements Storage: Super Capacitors Batteries Energy Harvesting Technologies Thermal Harvesting Converter Vibrational harvesting Converter Photovoltaic Harvesting Converter Comparisons Converter Comparison Price comparison Choice of EH Technology Implementation Solar Cell Characterization of Mekoprint Solar Cell Efficiency Converter LTC MPPC Components Results Conclusion Further work on prototype Problems fit for a masters project References 27 Appendix 29 A Comparison A.1 Price comparison B Solar cell test C Component list C.1 Components to be implemented D Matlabfiles i

3 Contents ii D.1 batt vs eh.m

4 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 the radio 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 this project, is defined as harvesting ambient energy in small-scale. Harvesting from small transducers 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 to supply a household with power, but there is plenty of energy to supply small electronic devices within the areas of ambient intelligence, condition monitoring devices, implantable and wearable electronics, and wireless sensor networks. The output power from the harvester is not necessarily in the form required, as loading applications often demand specific voltage and impedance characteristics. It also requires energy to transfer and condition the power, in an already energy sparse system and these are some of the converter challenges within energy harvesting. Energy harvesting has not yet been commercialized in large scale but now the power usage in microcontrollers 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 to pop up. Meanwhile is a demand for long lasting sensors in small form factors which makes cables and batteries unfit (fig. 1 illustrates why batteries are unfit - they will eventually deplete, where energy harvesting in theory can generate energy indefinitely). Figure 1: Illustration of the main strength of energy harvesting in comparison to battery when powering wireless 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 energy harvesting sources 1. 1 These numbers are made from the following assumptions: Solar: photovoltaic cell has 15% efficiency, Thermal: the temperature difference between human skin and ambient air is 5K, Vibration: human is in walking motion

5 1. Introduction 2 Table 1: Examples of energy harvesters output [4] EH Technology Setting Output µw cm 2 Solar Outdoor Indoor 10 Thermal Human 20 Machine 5000 Vibration Human 4 Machine Project Description Existing energy harvesting technologies are to be categorized based on price and energy availability and related to real-life applications. The basis will be a sensor node from a sensor network as the energy consumer. The available power converter topologies are clarified as an energy management link between harvester and sensor node. The converter topologies and relevant criteria are summarized and an energy harvesting technology is chosen as basis to make a design and implementation of a power converter being a modification, combination or revised version of 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 implementing a power converter Delimitations The project description could describe a project of a lot larger size than intended so a set of delimitations are set up. There are many ways of harvesting energy. One can extract energy from almost all kinds of processes and force, eg. chemical, kinetic etc. In this project it is chosen only to focus on 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 will not deal with how to distribute the harvested energy, ie. when a large amount of energy is available. A converter in discrete components will not be build, but a converter integrated in an IC will be used.

6 2. Energy Consumer - Sensor Node Goal of project The aim of this project is to get more familiarized with energy harvesting converter challenges and 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 Description of report This report starts out by describing the key parameters within energy harvesting and stating the requirements from the energy consumer. Three energy harvesting technologies are then described incl. power conversion techniques and applications. With basis on the energy consumer and operating environment and energy price, the different technologies are compared and one is chosen to be implemented as an energy harvesting system consisting of harvester and power converter. A prototype is build and the results are presented. The report concludes with further work 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 sensor node is a small computing device capable of collecting data, joining a mesh network and through this 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 maintenance. 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 Chipcon CC2420 radio and the Atmel AT45DB161D external flash. The microcontroller is equipped with low-quality sensors for temperature and voltage. The mote has three on-board sensors: Temperature and humidity Sensirion SHT11 Ambient Light OSRAM Opto Semiconductors SFH5711 Infrared PIN Photodiode Fairchild QSB Power Use The sensor node power consumption varies depending on what task the node performs. Table 2 shows 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 the measurements transmits the result turns OFF. For a sensor node cycle scenario as described above, the total power usage can be roughly estimated to 1s with average power use of 20mW (see table 2). This corresponds to an energy level 2 To improve power consumption of sensor node, dynamic power management and especially dynamic voltage scaling will save energy. Instead of finishing the processing task fast, the microcontroller voltage can be scaled down so it works slower, just as slow in order to finish the processing in time for the next cycle [16]. Can also be done by lowering the frequency of the microcontroller.

7 2. Energy Consumer - Sensor Node 4 Mode I [ma] P [mw] Sleep Radio Ext. Flash Reprogramming s wait 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 cycle are seen in fig. 3 for the different technologies to get a feeling of how the different technologies compare. On a bright sunny mid-day the sensor node can operate every 13 milliseconds when supplied by a 10cm 2 solar cell or every 9 minutes when operated by a equally sized vibration harvester on a walking human. 600 Harvesting time to operate sensor node for one cycle Time [s] Sun o Sun i Ther h Ther m Vib h Vib m 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. Based on energy estimates from [4]. All have equal size of 10cm 2. The energy available for harvesting is very dependent on the environment. When solar light is present, this energy is superior in density, but in dark or warm or moving locations other energy sources takes the lead. The energy is not directly available through the various transducers. The different energies are delivered in very different formats. Solar cells and thermal generators deliver a DC voltage, and vibrational gives an AC output. The output current is also different in between the different EH technologies. All the technologies have in common that the energy levels are low, so the harvesting electronics needs to be specifically designed to convert and store the energy in an applicable manner also taking into account whether the energy consumer is operating with a high or low duty cycle (important for the type and size of storage).

8 2. Energy Consumer - Sensor Node EH Power Converter Requirements The energy harvester can in most cases not supply power enough for constant operation. Thus the 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 harvester, 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 making converters capable of conditioning as low voltages as possible. For DC sources, like solar cells and thermal generators, charge pumps can be used to slowly boost the voltage to a level where regular boost converters can take over. AC sources like vibrational harvesters needs to be tuned to 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 the harvested energy, therefore a current-source gate charge rather and a voltage-source gate charge will often make sense. Another technique is to use more than one power converter circuit. The first circuit could be unregulated but capable of charging a capacitor. Once sufficient energy is stored in the capacitor it can be discharged and the signal conditioned by a more sophisticated power converter circuit.[4] Storage: Super Capacitors As energy storage super capacitors are able to hold large amounts of energy and release them fast, 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 required capacitor size is calculated. The capacitor will never be fully discharged as the sensor node can only operate in the area of 3, 5 2V. Here it is assumed that the storage capacitor comes after a converter and is maintained by a hysteresis circuit controlling the output within a threshold of 3.5V 2V. C = E node = 20mJ (1) E capacitor = 0.5 C V 2 (2) E Cavail = 0.5 C (VH 2 VL 2 ) (3) E node 20mJ 0.5 (VH 2 V L 2) = 0.5 (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 have very different leakage properties, so it is important to choose the appropriate technology. Ceramics has no, or very low leakage current, but does not come in sizes of mf. Electrolytic and tantalum capacitors does come in mf sizes but both have high leakage currents (Lyt 6.8mF I leak = 1.2mA, Tant 6.8mF I leak = 0.1mA). Super capacitors have very low leakage current 3 Measured during start-up of node.

9 2. Energy Consumer - Sensor Node 6 ( 1µA ), but during the initial charge from 0V the leakage can be up to 20µA [19]. The ESR of the super capacitors vary from 100Ω 0.01Ω, the later being expensive. The ESR is very important 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 than every 30 min, and the sensor node needs to be able to operate every 30 min, the storage needs to be able to provide energy enough for more than one cycle. As seen in fig. 4, super capacitors are not appropriate for storing energy much longer than one day. Figure 4: Super capacitor self discharge due to leakage current [9] Batteries EH only makes sense, when it is a better option than a battery. If it is not competitive in price the energy harvesting application needs to be needed for a longer period than the battery lifetime, or can work in conditions where batteries cannot (extreme temperatures). It can be argued that a battery can always be replaced, but the replacement of a battery by a technician is very expensive in comparison to the price of batteries/energy harvesting transducers and is thus to be avoided. There has been an inverse Moore s law within power consumption of embedded electronics. Every 18 months power consumption of digital systems is minimized by 50% ([3]). In comparison battery capacity has been very slow and only doubled every 10 years. Battery disadvantages: self discharge % a month not happy with peak current draw temperature > 40 C increases self discharge bad for the environment Battery advantages: no calibration, does not influence the placement of the sensor node no packaging requirements

10 2. Energy Consumer - Sensor Node 7 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 to operate longer than a couple of years, it cannot be done with batteries without having to replace them. Figure 5: Power density versus lifetime for batteries, solar cells, and vibration generators [6]. Solar and vibrational power is not depend on time. The top of the shaded solar box corresponds to outdoor sun light and the bottom corresponds to indoor office light. Both battery drain and leakage is considered in this graph. Batteries can also be used as buffer storage in energy harvesting power supplies. Table 3 compares the properties. Again the result is that batteries are not good for long term deployment as 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 Millions Self-discharge Moderate Negligible High Charge Time Hours Minutes Sec-minutes Physical Size Large Small Medium Capacity mAHr µAHr µAHr Environmental Impact High Minimal Minimal

11 3. Energy Harvesting Technologies 8 3 Energy Harvesting Technologies This chapter describes thermal, vibrational and photovoltaic energy harvesting and the electrical properties and challenges. 3.1 Thermal Harvesting Temperature gradients are found everywhere as a fact of heat flow due to temperature variations caused by machinery, heaters, electronics and the weather. Temperature differences can be utilized to harvest electrical energy using a thermoelectric generator (TEG). The TEG creates electrical power when it is placed between a warm and a cold temperature source. There will be a heat flow from the warm to the cold side and this heat flow 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 connections together 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]. Object To Thin alu plate - contact to object TEG Heat sink - contact to air Ambient air Ta Q Peltier Figure 6: Thermo harvester (TEG) - heat flows from heat source through base of TEG, then through the Peltier element and out through the TEG heat sink. A Peltier element consists of several p- and n-junctions in series (also called thermocouple). applying a temperature gradient across these results in a charge carrier diffusion from the hot side towards the colder side. This forces electron and hole carriers to flow and creates a current that creates a voltage across the terminals of the thermocouple (see fig. 7). The voltage is described by the Seebeck effect. The Seebeck coefficient of a material tells about the flow of charge carriers per heat flow. Fig. 8 shows the electrical equivalent circuit of a thermal generator, that is a voltage source and a resistance. The corresponding IV curve is seen in the same fig. from a thermal harvester from Micropelt. As it is seen, the voltage is low. This rises the need for a power converter capable of operating at low input voltages. An interesting note is, that the available power is dependent on the thermal resistivity of the TEG. The larger the thermal resistivity, the larger the thermal difference, but the electrical resistivity is directly proportional to the thermal resistivity. So in order to maximize the output of a TEG, the environment temperatures should be taken into account when designing the TEG in order to get the optimum resistivity [1]. As seen in the IV curve of the TEG, the power electronics also have to track the maximum power point in order to optimize the power output

12 voltag Energy Harvesting Technologies load resistance [ ] 9*0"$()"$:3:(+*%'($":.:&%#-"( T = 30 K T = 20 K T = 10 K T = 5 K power [mw] Figure 7: Seebeck effect [27] load resistance [ ] as the internal resistance changes with temperature[23]. E*+&%,"()"$:3:(-3$$"#&(%&('.=="$"#&(+*%'($":.:&%#-":( T = 30 K T = 20 K T = 10 K T = 5 K R S 3.5 V TH + = V O + (a) Electrical equivalent of thermal generator - voltage [V] load resistance = 440 matched load resistance = current [ma] F"%&(=+*0()"$:3:(+*%'($":.:&%#-"(%&('.=="$"#&( >( heat flow [W] (b) IV curve of TEG MPG-651 from Micropelt [28]. Figure 8: T = 30 K T = 20 K T = 10 K T = 5 K Fig. 9 shows an thermal energy harvesting device along with the inside thermal generator Converter The TEG from Micropelt shown in fig. 9 produces 75mV/K = V OC. That means when harvesting thermal energy from just a couple of degree s temperature difference there is a need for a 1.5 low voltage power converter. Regular topologies are not able to operate below the diode/switch 0.5

13 3. Energy Harvesting Technologies 10 (a) Thermo electric generator from Micropelt, situated inside device shown beside - capable of generating energy from T = 3 C. Figure 9: (b) Thermal energy harvesting device from Micropelt [28]. voltage threshold level. Normal MOS transistors have a threshold voltage above 0.5V, making them unsuited for low voltage converters. Sub 0.2V threshold transistors are available but these have large leakage current when the gate-source voltage is 0V also making them unsuited for converting 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 V IN. This is an efficient way of charge pumping, but it requires control of the capacitor switches which requires energy [17]. This can also be cascaded by several capacitor charge pumps in series which creates a voltage of 2 N V IN. 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 coil to be able to charge inputs below 0.5V, where an active circuit is not feasible. The secondary winding 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 startup JFET which is conducting at 0V gate voltage. A charge will be build up at the secondary winding 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 this transistor is only active in the startup period and when it has initiated the main circuit an NMOS transistor with low conduction will be active instead. [14] has designed a step-up converter able to operate down from 20mV with this technique.

14 3. Energy Harvesting Technologies 11 Figure 10: Schematic of boost converter by [14] with low voltage start-up circuit based on the Armstrong oscillator. 3.2 Vibrational harvesting Vibrational harvesting can be used whenever there is a movement. It can be harvested with electromagnetic harvesters consisting of a core and a coil, electrostatic harvester utilizing a capacitance change, and piezoelectric harvester made with piezo ceramic utilizing the piezoelectric effect when the material is flexed. Vibrations are especially present in transportation and industry machineries. Table 4 shows some examples of what vibration levels can be found in the ambient environment. Vibration harvesting applications can be kinetic watches, industrial motor monitoring, structural/bridge health monitoring, kinetic powered light switches, muscle powered implants and helicopter tracking nodes[1]. Table 4: Vibration levels [6] Vibration Source Peak Acc. [m/s 2 ] Freq. of Peak [Hz] Base of 5 HP 3-axis machine tool with 36inch bed Kitchen blender casing 6,4 121 Clothes dryer 3,5 121 Door frame just after door closes Small microwave oven 2, HVAC vents in office building 0,2-1,5 60 Wooden deck with people walking 1,3 385 Bread maker 1, External windows next to a busy street 0,7 100 Notebook computer while CD is being read 0,6 75 Washing Machine 0,5 109 Second story floor of a wood frame office building 0,2 100 Refrigerator 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 attached to a free moving mass.

15 3. Energy Harvesting Technologies 12 Electrostatic harvesters converts mechanical energy into electrical energy by the variation in the separation distance/overlap area of the plates in a MEMS capacitor. Changes in the capacitance produces energy when keeping the charge or voltage constant, Q = C V. Electromagnetic harvesters transforms kinetic energy into electrical by moving a coil across the magnetic 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 s performs 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 acts as a AC voltage source in series with a capacitor and resistor. This capacitance can be exploited when converting the energy and when tuning the resonance frequency of operation [22]. R S + V PE V O C shim - Figure 12: Electrical equivalent circuit of bimorph piezoelectric bender. V P E represents both layer voltages and C shim the separation between the layers as a coupling capacitor. R S models the series resistance through the device. A rather big issue with vibrational harvesters is that they are very frequency dependent (due to their narrowband performance - see the equivalent mechanical circuit in fig. 13). They have a specific resonance frequency where they will exhibit maximum power and not far from this resonance the power output will be very low. Thus several techniques within tuning the frequency

16 3. Energy Harvesting Technologies 13 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 they bend/shake and thus eventually might break. Electromagnetic harvesters are more durable than piezo harvesters. A simple method of harvesting the vibrational energy is like fig. 14. A piezoelectric generator has a blocking output capacitance which is connected to a diode bridge rectifier and then stored on 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 is also easy to be integrated into silicon devices and further fabricated with the microelectronic circuits on the same chip[1]. The electromagnetic generators can generate high output-current levels but the voltage is very low (typically < 1V ). Macro-scale devices are fabricated using high- performance bulk magnets and multi-turn coils. Both piezo and electromagnetic harvesting techniques have been shown to be capable of delivering power to the load in the range of µw to mw 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 will cause a 90 phase-shift between the voltage and current. The SSHI method utilizes an inductor

17 3. Energy Harvesting Technologies 14 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 voltage period, thus shaping an oscillating network with the inductor L. [12] has made an improved version 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, decreases power 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 indoors and outdoors can be seen. Together with table 5 which shows the energy in the light and table 6 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 photovoltaic energy 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 method of harvesting energy from a solar cell is connecting it directly through a diode to a capacitor or battery. The photovoltaic is a pn-junction diode generating photons that causes electrons and holes to recombine, generating an electric current when exposed to light.

18 3. Energy Harvesting Technologies 15 Table 5: Power available in different types of light [6] Condition Power [mw/cm 2 ] Mid-day, no clouds 100 Outdoors, overcast 5 Incandescent bulb, 3m away 10 CF 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% I O R S I PH I D R R Figure 17: Equivalent electrical diagram of solar cell. The current source I P H generates a current proportional to the amount of light falling upon the 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, V OC. This voltage varies somewhat with the exact properties of each type of solar cell. For most silicon cells, it is in the range between 0.5V and 0.6V which is the normal forward voltage of a p-n junction diode[9]. The parallel resistor, R P, represents a small leakage current that occurs in practical cells, while R S represents the connection losses. As the load current increases, more of the current generated by the solar cell is diverted away from the diode and into the load. For most values of load current, 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 maximum power point. This point can be tracked by several MPPT techniques 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

19 3. Energy Harvesting Technologies 16 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) can be 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 irradiance and cell temperature. In order to track the MPP of the solar cell, an extensive amount of methods can be used. Many utilizes a microcontroller to do the tracking: Perturb and Observe method 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 derivative of the time-varying solar cell power with the time derivative of the time-varying solar cell current or voltage to drive the power gradient to zero, thus reaching the MPP [25]. Fractional open circuit method This method works by the assumption that the MPP always is at a constant fractional, K, of the open circuit voltage (usually K is around ). V MP P V F V OC = V OC K The open circuit voltage in the current lighting conditions needs to be measured in order to track. This can be done by having a small extra reference solar cell dedicated to the purpose of generating the reference voltage. Another method is to periodically disconnect the load from the solar cell, take a measurement of the V OC, and the reconnect load [8]. [24] states methods of FVOC consuming from 50µW - 2mW. It is important to evaluate the efficiency 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.

20 4. Comparisons 17 4 Comparisons 4.1 Converter Comparison Table 7 summarizes the challenges within converters for each technology. The solar technology has been around for long and many MPPT methods have been developed. The most challenging technology is the vibrational harvester. It suffers under frequency dependence and narrow bandwidth of operation. Table 7: Converter challenges for different EH technologies - based on example technologies used in price comparison in next section. EH Technology Voltage Current Challenges Solar 0.6V-8V (or more) low MPPT, low light Piezo 0V-20V low AC rectification, frequency tuning, phase shift Electromagnetic 0V-10V low AC rectification, frequency tuning TEG 0V-5V high MPPT, charge pumping As shown earlier in table 1 the energy level in the solar light outdoor, where the sensor node is going 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 a EH technology will be chosen. 4.2 Price comparison Different energy harvesting companies has been interviewed in regards on how they see the prices of their energy harvesting transducers develop the coming years as they are now on a prototype level 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 are very hesitant by giving an exact price because that is very application specific and depends totally on scale. So another way around the price comparisons has been chosen. 3 commercial available transducers have been chosen to be compared with a lithium and alkaline battery in order to give an example of where the three technologies (thermal, vibrational and photovoltaic harvesting) 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 several years (other batteries like NiCD and NiMH only last a couple of months). The price paid per joule when buying a battery depends on how long time the battery is going to operate due to its leakage current. The longer the battery has to operate, the lower total amount of joule is available. See fig. 19 for the price comparison. The figure shows how the prices changes as a function of the number of sensor node cycles. As previously stated, the example where the sensor node is to run 1s once every half hour is chosen. That means that the sensor node needs 20mJ every half hour. The battery price / joule changes with time, due to its leakage current. Thus the total

21 4. Comparisons 18 energy available in the battery decreases, and thus the price per joule increases slightly. For the harvesters the price /joule decreases the longer time it harvests energy. This means that there will come a point where the price / joule for the battery and the harvesters intersect. After the intersection, the cheapest solution is the energy harvesting and for shorter operation the battery is the cheapest option. Price ($) / joule Lithium Battery 1400mAh Alkaline Battery 2900 mah 1m/s2 1m/s2 TEG dt=20k TEG dt=5k Solar outdoor Solar indoor Days (= 48 sensor node cycles of 20mJ)) Figure 19: Price comparison of energy harvesters with lithium and alkaline battery as a function of time. Table 8 compares the harvesters intersection with the alkaline batteries, to show when each EH technology becomes competitive on price. It is interesting to see that the solar cell has generated energy enough in a few days, to have a cheaper price per joule than an alkaline battery. The indoor solar, the low vibration piezo, and the human body thermal energy generation are all very expensive. It is obvious that they have to become much cheaper in order to be able to compete. (It has to be noted that it is assumed that the solar, thermal, and vibrational energy is constantly available.). Table 8: Based on results from fig. 19. Compares the harvesters prices and shows when the become competitive with an alkaline battery. EH Technology Price Competitive after [days] Solar outdoor 20$ 5 Solar indoor 20$ 2500 Piezo 79$ 3500 Electromagnetic 130$ 540 TEG 5K 15$ 2700 TEG 20K 15$ 220 See appendix A.1 for specifications of each of the chosen harvesters and the batteries. Appendix D.1 shows the matlab script for calculating the price developments.

22 5. Implementation Choice of EH Technology The harvesting technology chosen to be implemented for supplying energy to a sensor node placed outside a building is a solar cell as there is by far the most energy available in the light and as the price comparison showed, solar cells situated outdoors also generates energy way cheaper than the other technologies. On top of this, DELTA just got their hands on some new organic polymer solar cells from a company called Mekoprint. Not much is known about these cells 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 to run the sensor node once every 30 min with solar power, it needs to be able to harvest energy for the whole day from the available light. An approximation is made that light is present 12 hours a day. It is assumed that some timing circuit is present and able to activate the power circuit and sensor node every 30 min. The average power required from the solar harvester is then during the 12 hours: E node day = 20mJ/cycle 48cycles = 0.96J (6) P av = 0.96 = 22µW 12h 602 (7) 5 Implementation This chapter describes the implementation of the photovoltaic harvester prototype for powering the sensor node. 5.1 Solar Cell An organic polymer solar cell is used as power source. It is developed by a company called Mekoprint, originally developed at DTU Risø. This solar cell is chosen due to the fact that is a very new technology for processing and manufacturing solar cells. The advantages of the organic polymer solar cell technology are its flexibility, it is printable and potentially cheap to manufacture. The flexibility opens up for building it into fabrics or on to structures that are round, and to incorporate it into designs where the shape of the object changes. The manufacturing process makes it possible to print it on surfaces so one could expect it being printed on e.g. big sails for a boat. The printing process also makes it potentially very cheap to produce Characterization of Mekoprint Solar Cell The characteristics of the solar cell are unknown and there is no datasheet made yet, as it is still under development. Therefore it has been characterized in a controlled light environment at the Lights & Optics department at DELTA. The solar cell was tested in an environment where no other light source than a XBO Zenon lamp (see fig. 20) with an IR filter D65 corresponding to a light temperature of 6500 K, which is similar to the solar spectrum, was present. The light intensity was carefully measured and controlled (see figure 21). It was interesting to see whether the performance of the cell was comparable to a standard silicon 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, V OC of

23 5. Implementation 20 Figure 20: XBO Xenon lamp K (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 curves are plotted in order to easily see the maximum power points. From the results it can be concluded that the voltage level is not going to be a problem when applying a converter but the current is very low (below 0, 5mA). To be able to operate the solar cell on its maximum power point the relation between the open circuit voltage, VOC, and the MPP voltage, Vmpp, (using the method of fractional open circuit from section 3.3.1) has been estimated in table 9. Here it can be concluded that the best point lux was the maximum the lamp could perform and therefore where no higher values tested.

24 5. Implementation 21 Current [A] 4 x Mekoprint characteristic x IV 380 LUX IV 3250 LUX IV 6950 LUX PV 380 LUX PV 3250 LUX PV 6950 LUX 0.5 Power [W] Voltage [V] 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 points are found at 6950 lux: V mpp = 3, 4V, 3250 lux: V mpp = 3, 2V and 380 lux: V mpp = 2, 4V. to operate the solar cell is 0, 52 V OC. Table 9: Maximum power point tracking coefficient, K, for Mekoprint solar cell at different luminosities - see fig. 22 Luminosity [lux] V OC V MP P K ,6V 3,4V 0, ,3V 3,2V 0, ,2V 2,4V 0, Efficiency The energy in the light at the test site is defined as 95 lux corresponds to 1W/m 2. With the solar cell size of 0, 0068m 2 (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 solar cell performs quite poor. An efficiency of 0, 15% is very low compared to silicon based solar cell which at least performs 8% efficiency at these light intensity levels (see appendix B for reference measurement of silicon solar cell at same test setup). Table 10: Mekoprint solar cell efficiency - results from fig. 22 Luminosity [lux] P light [W] P m eko [W] Efficiency [%] ,497 0, , ,233 0, , ,027 0, ,141 The solar cell has also been tested ad hoc outdoor in sunlight in the afternoon. It performed an

25 5. Implementation 22 open circuit voltage, V OC = 8, 5, and shorted current, I SC = 2mA, which can be estimated to a power 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 somewhere in between lux and 3000 lux, corresponding power from solar cell of 11mW to 0.3mW. To take the worst case of a whole day with heavy overcast the total power harvested would be 0.3mW s = 13J which is more than plenty. The energy needed for the sensor node for half a day, as stated earlier was 0.48J. Even with a converter efficiency of 50 % there is plenty of 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 low and the V MP P T in lower light was below the desired output voltage a search for boost converters was 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, has maximum power point control, low power input (225mV 5V ) and among other burst mode which adjusts the peak switching current. It needs a few peripheral components and is rather simple to implement. See datasheet [26]. With these properties seeming to meet the requirements, the LTC3105 was chosen to be implemented as the power converter for the Mekoprint solar cell. The LTC3105 will charge a storage capacitor, that ought not to be directly connected to the sensor node. If the sensor node was directly connected to the storage capacitor, it would turn on before the capacitor was fully charged. Thus a hysteresis circuit with a threshold around 3.3V 2V is intended to be implemented to then make sure the capacitor is charged to 3.3V before the output turns ON. See appendix C.1 for a schematic of the hysteresis circuit. The sensor node is connected to the output of the hysteresis circuit. See fig. 23 for the block diagram of the system from solar harvester to sensor node. 5 Mekoprint solar cell LTC3105 Storage capacitor Hysteresis control Sensor node Figure 23: Block diagram of the system to be implemented LTC3105 In fig. 24 an application diagram from the datasheet is seen. It is almost identical to the circuit implemented 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-up circuit is build and also how the burst mode is implemented. If more time were available it would 5 This solution is not possible to operate during the night when no light. This would require some sort of timing circuit able to control when the power should be ON - this is beyond the scope of this project.

26 5. Implementation 23 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 V out = 3.3V, and R MP P C = 200k 400kΩ depending on which light intensity to set the V MP P. 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 operates The LTC3105 starts by charging the C AUX to a level of 1.4V. In this phase the maximum power point control is not yet enabled. When V AUX = 1.4V the converter starts to regulate the LDO output. When this is done it turns the output ON (see fig. 26 for the waveforms).

27 5. Implementation 24 Figure 26: Waveforms in the LTC3105 from datasheet MPPC LTC3105 has something called Maximum Power Point Control, which is a quite limited method of making sure to keep the solar cell at its maximum power output. The method is to set the desired operating voltage via a resistor R MMP C on the MPPC pin, where there is a reference current of 10µA, and then the IC will make sure to operate the input at this fixed voltage level. This solution cannot track the MPP when the light intensity changes and this is very important when harvesting outdoors, as the light intensity is very fluctuating. As the MPPC is just a reference voltage one could connect another circuit for tracking to it (remembering to take the 10µA into account). A solution could be a small reference cell or the microcontroller on the sensor node could be used in a low power mode. Then many MPPT methods could be incorporated but each one would have to evaluated based on the power consumption vs. energy gain from the MPPT method in comparison to the un-tracked case. This could be done in further work Components The choice of components can be seen in appendix C. The choice of the switching inductor was a SMD 10uF with very low DC resistance as this is directly proportional to the efficiency of the circuit. 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 with the Mekoprint solar cell. The start-up current for the circuit has been measured to 6mA (at V in = 3V - it decreases with higher voltage). This current cannot be supplied by the solar cell so it cannot start the circuit it self. 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.

28 5. Implementation 25 Figure 27: Implemented prototype of LTC3105 and external components for converting the solar energy from a Mekoprint organic polymer solar cell. A solution could be a large input capacitor, but this will also collapse the solar cell output voltage as the LTC3105 already start drawing current at 250mV. Thus a big input capacitor is needed 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 option is to put a switch/voltage detector on the input capacitor, making sure it is charged to a certain level before turning ON the input 6. Then enough start-up power could be generated. This could be investigated in further work. 7 What sets the amount of current being drawn during start-up is unknown, as the datasheet internal diagram only shows a start-up block (fig. 25). The implemented LTC3105 with external components has a power use of 0.04mA which corresponds well with the added quiescent currents stated in the datasheet. Under operation the converter 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, mainly due to the high startup current. This issue can be fixed with the mentioned input capacitor solution and then the converter is capable of performing with ok efficiency, but there also needs to be looked into a more flexible MPP technique. A very low input voltage converter is actually not that important in this case where the light intensities will be rather high so it might be possible to look into a buck converter with low bias current switches. 6 The LTC2935 looks interesting. 7 One could also use several solar cells and then it would work fine, but that is not preferable.