Energy Harvesting for Remote Sensor Networks and Low Power Electronic Devices

Transcription

1 Energy Harvesting for Remote Sensor Networks and Low Power Electronic Devices Nwabueze, C. A. and Nwosu, A. W. Department of Electrical/Electronic Engineering, Faculty of Engineering, Anambra State University, Uli. A paper presented during NIEEE's International Conference and Exhibition on Power and Telecommunications (ICEPT 2015) ABSTRACT Energy harvesting also known as power harvesting or energy scavenging is the process by which energy is derived from external sources such as solar power, thermal energy, wind energy, salinity gradients, and kinetic energy, captured, and stored. This term is frequently applied to small, wireless autonomous devices, like those used in wearable electronics and wireless sensor networks. One driving force behind the search for new energy harvesting devices is the desire to power sensor networks and mobile devices without batteries. Future applications may include high power output devices or arrays of such devices, deployed at remote locations to serve as reliable power stations for large systems. This paper presents a comparative analysis of the various energy harvesting dimensions with particular emphasis on its application in wireless sensor networks, mobile computing and wearable monitoring devices. An interesting aspect of this technology is its relatively low cost, reliability and easy application which are also presented. Keywords: Energy Harvesting, Harvested Power, Wireless Sensor Networks. INTRODUCTION Energy harvesting has been around for centuries in the form of windmills, watermills and passive solar power systems. In recent decades, technologies such as wind turbines, hydro-electric generators and solar panels have turned harvesting into a small but growing contributor to the world s energy needs. This technology offers two significant advantages over battery-powered solutions: virtually inexhaustible sources and little or no adverse environmental effects. Macro-scale harvesting technologies differ in many ways but have one thing in common: they feed the grid, typically adding kilowatts or megawatts to the power distribution system. On the other hand, energy harvesting new frontier is an array of micro-scale technologies that scavenge milliwatts from solar, vibration, thermal and biological sources. Micro-harvesting provides power at the same order of magnitude that carefully designed Ultra Low Power (ULP) circuits typically consume. The three most promising technologies based on light, motion and thermal scavenging have different characteristics. Large solar panels have made photovoltaic harvesting a well characterized technology. Approximately 1 mw of average power can be harvested from each 100-mm 2 photovoltaic cell. Typical efficiency is roughly 10 percent and the capacity factor of photovoltaic sources (the ratio of average power produced to power that would be produced if the sun was always shining) is about 15 to 20 percent. Commercially available kinetic energy systems also produce power in the milliwatt range. Energy is most likely to be generated by an oscillating mass (vibration) and electrostatic energy harvested by piezoelectric cells or flexible Page 1 of 9

2 elastomers is also classified as kinetic energy. Vibration energy is available from structures such as bridges and in many industrial and automotive scenarios. Basic kinetic harvester technologies include: (1) a mass on a spring; (2) devices that convert linear to rotary motion; and (3) piezoelectric cells. An advantage of (1) and (2) is that voltage is not determined by the source itself but by the conversion design. Electrostatic conversion produces voltages as high as 1,000 V or more. Thermoelectric harvesters exploit the Seebeck effect, which states that voltage is created in the presence of a temperature difference between two different metals or semiconductors. A thermoelectric generator (TEG) consists of thermopiles connected thermally in parallel and electrically in series. The latest TEGs are characterized by an output voltage of 0.7 V at matched load, which is a familiar voltage for engineers designing ultra-low-power applications. Generated power depends on the size of the TEG, the ambient temperature, and in the case of harvesting heat energy from humans, the level of metabolic activity (Raju, M. (2010)). Characteristics of Energy Harvesting applications include: Ability to operate with lowest standby current to maximize storage of energy, Consume lowest possible power when active, Ability to turn on and turn off instantaneously, Efficient operation with lowest duty cycle of active versus standby modes, Analog capability for sensor interfacing and measurements, Ability to operate with low voltage range, Lowest leakage currents to maximize harvested energy. Various energy harvesting techniques are presented, their application and impact on low and ultra low power capabilities in various dimensions are also discussed. Figure 1: Energy Harvesting System (Raju, M. (2010)). ENERGY HARVESTERS Vibration Harvesting A vibration harvester is typically composed of a frame anchored to the vibration source and a mass m connected to the frame by a suspension having a stiffness k. The vibration induces a relative motion of the mass and the frame, a transducer transforms the energy of this relative motion into electricity. The main characteristic of a vibration energy harvester is that it operates in resonance. The resonance frequency is given by: Page 2 of 9

3 f = 1 2π k m (1) The power in resonance for a mass spring system is given by Mitcheson, P. D. et al (2004): p( f ) = 4π mf Yz 2 3 max (2) where Y is the external vibration amplitude and z max the maximum displacement of the proof mass, limited by the dimensions of the frame. It follows from the above equation that large output powers are enabled by higher vibration frequencies. There are three ways of converting vibration into energy Electrostatic (ES): A capacitor is made consisting of two opposing metal structures. One of these structures is fixed, the other one moves in the presence of an external force. The change in voltage is proportional to the capacitance change. Piezoelectric (PE): A mass is suspended by a beam, with a piezoelectric layer on top of the beam. When the mass starts to vibrate, the piezoelectric layer is deformed and a voltage is generated Electromagnetic or Inductive (EM): A mass of magnetic material moves through a magnetic field. The change in flux generates a voltage. Thermal Harvesting For thermal harvesting, two effects can be deployed. Using the Seebeck effect, one makes use of a spatial temperature difference. Another option is using a pyroelectric element, which can turn a temporal temperature difference into electricity. Since the efficiency of the latter is very low, only the first effect will be considered. Thermal energy harvesters are based on the Seebeck effect. Their core element is a thermopile, formed by a large number of thermocouples placed between a hot and a cold plate as shown in figure 2. The thermocouples are thermally connected in parallel and electrically in series. The generator may include a radiator for efficient dissipation of heat in the ambient and specific structures aimed to increase thermal isolation between the hot and cold plates. The Seebeck Effect The Seebeck effect is associated with the generation of voltage along a conductor when it is subjected to a temperature difference. Charged carriers (electrons or holes) diffuse from the hot side to the cold side, creating an internal electric field that opposes further diffusion. The Seebeck coefficient is defined as the voltage generated per degree of temperature difference between two points. In practice, the measurement of the potential difference drifting of the Seebeck electric field requires the use of a second driver to carry out the contact with the Voltmeter, which leads to the direct measurement of the difference of the Seebeck coefficients of two materials concerned: Page 3 of 9

4 V = ( α A α B )( Th Tc ) (3) where V is the observed potential difference (in Volts), and α i is the Seebeck coefficients of materials A and B (in V/K) respectively, and (T h T c ) the temperature difference between the hot source and the cold source (in Kelvin). This equation is valid if the variation in temperature is sufficiently weak to be able to neglect the dependence in temperature of the Seebeck coefficient (Catrene (2009)). Figure 2: Thermoelectric thermocouple fabricated from n and p type semiconductor (Catrene (2009)). Photovoltaic Harvesting Photovoltaic (PV) converts incoming photons into electricity. These cells have been used outdoor for many years, where power densities are available for up to 100 mw/cm 2. Efficiencies range from 5% to 30%, depending on the material. The situation is much different indoor, since the illumination levels are much lower than outdoor (100 to 1000µW/cm 2 ). Furthermore, at low illumination levels, the efficiency of solar cells will drop considerably. Working Principle of PV Cells Conversion of light directly into electric energy involves the photovoltaic effect where photon energy is used to excite an electron from its ground state to an excited state, see figure 3. A functional photovoltaic scheme should implement at least the following three steps: Light harvesting: photons are absorbed and their energy used to excite electrons. Charge separation: the excited electrons are separated spatially from the ground state to avoid recombination. Selective charge transport/extraction: electrons and holes are transported to the terminals of the device where the high energy electrons are selectively extracted at one terminal while the holes selectively are replenished from the other terminal. In an ideal case, there should be a one to one relationship between light and electric current: each photon that strikes the device delivers its energy to an electron, which in turn transports the energy to an electric load connected to the terminals of the device. Here the energy can be released in the form of work. Page 4 of 9

5 The PV scheme of figure 3 is implemented in the classical semiconductor solar cell (first generation). In these PV s all the steps necessary for conversion of light to electric energy are provided by the manipulated band structure of a single semiconductor crystal. Figure 4 shows the band structure of such a semiconductor pn-junction. An excited electron hole pair will (provided that its lifetime is sufficiently long) diffuse to the junction region where the charge carriers are separated. The activated electron is carried through the n-doped region to the negative terminal while the hole is replenished through the p-doped region from the cells positive terminal. Because impurities and grain boundaries acts as recombination centres, these devices should preferably be produced from high purity single or multi crystalline semiconductor material. In order to reduce the high cost of materials inherent in the first generation of PV technology, several thin film technologies are being pursued to enable higher volumes to be produced at lower cost. Even where the materials cost of thin film PV devices can be reduced to a very low level, it may still be difficult to bring down the overall production cost to the level where solar power becomes economically competitive to power produced from fossil fuels. This is mainly because of the vacuum processes involved in the production. Figure 3: Photovoltaic Principle showing the elemental steps: light harvesting; 2) charge separation; and 3) charge transport (Catrene (2009)). Figure 4: Principle of Photovoltaic Device based on a semiconductor junction (Catrene (2009)). RF (Radio Frequency) Harvesting RF harvesting converts electromagnetic radiation into electricity. This can be done in two ways. a) Make use of existing EM radiation (e.g. GSM, FM, WiFi). b) Broadcast an EM signal at a specific wavelength in order to power a wireless node. Page 5 of 9

6 The first solution has the advantage that radiation can be used that is already present. The disadvantage is the low energy density (typically µw/cm 2 ). Furthermore, it is not always desirable or even legal to block radiation. Therefore, a solution is to use dedicated broadcasting device, which can power sensors in the neighborhood. A point of concern is the maximum power that is allowed to be transmitted into the environment, which is typically around 100mW. A good example is of this principle used for charging mobile devices developed by a company called Powercast is shown in figure 5. Using 3W of transmitted power at an operating frequency of 900MHz and a distance between device and transmitter around 30 cm, about 100mW of power is received. Larger distances means larger radiation power, but 3W is already much higher than the allowed energy (around 100mW). The received power decreases very rapidly with distance. There is room for improvement, though, in transmission (e.g. beam steering), receiving (improved antenna design) and the conversion efficiency. For example, at a transmission power of 100mW, values of 1.5mW at 20 cm have been reported (Vullers, R. J. M. et al (2008)). Human Harvesting Figure 5: Solution by Powercast: an RF transmitter (1) is plugged into a wall socket, it radiates radio waves (2) to tiny receivers integrated into devices (3). (Vullers, R. J. M. et al (2008)) People dissipate between Watts of energy while sleeping, eating, running, etc as depicted in Table 1. This can be harnessed especially in wearable electronic devices for health monitoring. Table 1: Human Energy Expenditure Activity Kilocal/hr Watts Sleeping Lying Quietly Sitting Standing at ease Conversation Eating Meal Strolling Driving Car Playing Violin or Piano Housekeeping Carpentry Hiking, at 6 km/hr Swimming Mountain Climbing Long Distance Run Sprinting , ,048 1,630 Source: Paradiso, J. (2006). Page 6 of 9

7 TYPICAL HARVESTED POWER VALUES Table 2 shows estimated figures of energy output values per harvesting principle. It should be noted that these numbers are just first order indications with some margin of error. However, they do give some indication of the typical expected power levels. Table 2: Estimated Power Output Values per Harvesting Principle. Source Source Characteristics Physical Efficiency Harvested Power Photovoltaic Office Outdoor 0.1 mw/cm % 10 µw/cm mw/cm 2 15 mw/cm 2 Vibration/Motion Human Industry Thermal Energy Human 0.5m at 1Hz 1 m/s 2 at 50Hz 1m at 5Hz 10 m/s 2 at 1kHz 20mW/cm2 maximum power is source dependent 0.10% 4 µw/cm µw/cm 2 25 µw/cm 2 Industry 100mW/cm2 3% 1 10 mw/cm 2 RF GSM 900MHz 1800MHz µw/cm 2 50% 0.1 µw/cm µw/cm 2 Source: van Hoof, C. (2008) and PSMA Energy Harvesting Forum (2012). APPLICATION OF ENERGY HARVESTING Remote patient monitoring, efficient office energy control, surveillance and security, agricultural management, home automation, long range asset tracking, implantable sensors, structural monitoring are some of the applications of energy harvesting devices. Figures 6 and 7 shows applications of energy harvesting devices for urban interface centre monitoring and optimal irrigation application for soil moisture measurement in agriculture respectively. In figure 8, Bio-Medical: Electroencephalography (EEG), ECG Electrocardiogram (ECG), (EMG) (muscular), Blood Pressure, Glucose Sensor, Respiration, Temperature, Performance: Distance, Speed, Posture (Body Position), Sports Training Aid are shown. Electromyography Fall Detection and Sports Figure 6: Urban Interface Centre Interface Monitoring (Dallemagne, P. (2010)). Page 7 of 9

8 Figure 7: Optimal Irrigation Application through Soil Moisture Measurement (Dallemagne, P. (2010)). Figure 8: Wearable Health and Medical Application (Dallemagne, P. (2010)). CONCLUSION Much has been written about the benefits of wireless sensors and the potential of energy harvesting to provide power for the life of these devices. Disposable, long-life batteries will continue to be used in wireless sensor applications but, as the technologies mature, energy harvesting will create some shift in battery usage from primary to rechargeable batteries for applications that need higher power over the life of the device. The greatest potential, however, lies in a new class of devices that will be battery-free and thus enable applications that would have been prohibitively expensive due to the maintenance cost of eventual and repeated battery replacement. Page 8 of 9

9 The energy harvesting industry is developing technologies to take advantage of varied sources of micropower (power measured in milliwatts) including solar, vibration, thermal, human and RF energy. For any specific installation, there will likely be a clear choice of the optimal energy harvesting technology to be used, and depending on the application, all are capable of providing the micropower needed for low/ultra low power devices and wireless sensor applications. REFERENCES Catrene-Cluster for Application and Technology Research in Europe on Nanoelectronics (2009), Energy Autonomous Systems: Future Trends in Devices, Technology and Systems, Dallemagne, P., (2010), Solar Energy for Wireless Sensor Networks, Swiss Centre for Electronics and Microtechnique. Mitcheson, P. D., Green, T. C., Yeatman, E. M. and A.S Holmes, A. S. (2004); Architectures for Vibration Driven Micropower, Journal of MicroElectroMechanical Systems, vol. 13, no. 3, pp Paradiso, J. (2006), Energy Harvesting for Mobile Computing, PSMA Energy Harvesting Forum (2012), Energy Harvesting End-to-End: Technologies and Techniques for Building Zero Power Systems, Raju, M. and Grazier, M. (2010), ULP Meets Energy Harvesting: A Game-Changing Combination for Design Engineers, van Hoof, C. (2008), Micro Power Generation using Thermal and Vibrational Energy Scavengers, 12 Years of Energy Harvesting An overview, IEEE International Solid State Circle Conference (ISSCC). Vullers, R. J. M., Visser, H. J., Ophet Veld, B. and Pop, V. (2008), RF Harvesting Using Antenna Structures On Foil, Proceedings of Power MicroElectroMechanical Systems, Sendai, Japan, pp Page 9 of 9