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Energy harvesting devices generate electric energy from their surroundings. MOTIVATION The history of energy harvesting dates back to the windmill and the waterwheel. People have searched for ways to store the energy from heat and vibrations for many decades. One driving force behind the search for new energy harvesting devices is the desire to power sensor networks and mobile devices without batteries. AMBIENT-RADIATION SOURCES A possible source of energy comes from ubiqious radio transmitters. Unfortunately, to derive useful amounts of energy from this source requires either a large collection area or close proximity to the radiating source. One idea is to deliberately broadcast RF devices to power remote devices. This is now commonplace in passive radio-frequency-identification systems, but the Safety and US Federal COmmunications Commission limit available power. PIEZOELECTRIC ENERGY HARVESTING The mechanical energy required to strain a piezoelectric material is converted to electrical energy through piezoelectric energy harvesting. Charge separation resulting from the strain of a piezoelectric material results in an electric field and a voltage drop. The power produced by a piezoelectric (PZT) device is on the order of milliwatts, which is too small to directly power many devices (soldiers on the battlefield require power levels of a few watts). But there are a number of ideas for PZT devices. For example, Umeda proposes a free-falling ball to hit a piezoceramic wafer. Kimura's patent is based on the vibration of a small plate. A PZT disc has placed on the back of a Helmholtz Resonator to harvest acoustic energy. The use of piezoelectric materials on the micro-scale has also been proposed, such as in a device harvesting micro-hydraulic energy. In this device, the flow of pressurized hydraulic hluid drives a reciprating piston supported by three piezoelectric elements, which convert the pressure fluctuations into an alternating current. Another device utilized a source radiating electrons. A cantilever tip collects these electrons, and the electric charge generated deflects the tip, producing spring energy. The tip eventually makes contact with a source, and the charge discharges, which causes the cantilever to spring back. The oscillations of the tip produce an alternating current. Converting motion from the human body into electrical energy is another big idea. Some sources include leg motion, blood pressure, and shoe impact. Starner claims that 8.4 watts of useable power can be harnessed from a PZT mounted in the shoe Besides storing energy, PZT devices may power microdevices or MEMS. THERMOELECTRICS In 1821, Thomas Johann Seebeck discovered that a thermal gradient formed between two dissimilar conductors produces a voltage. At the heart of the thermoelectric effect is the fact that a temperature gradient in a conducting material results in heat flow; this results in the diffusion of charge carriers. The flow of charge carriers to the low-temperature region establishes the potential difference. In 1834, Jean Charles Peltier discovered that running an electric current through the junction of two dissimilar conductors could, depending on the direction of current flow, act as a heater or coolant. The heat absorbed or produced is proportional to the current, and the proportionality constant is known as the Peltier coefficient. Today, due to knowledge of the Seebeck and Peltier effects, thermocouples exist as both heaters and coolers. Ideal thermoelectric materials have a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. Low thermal conductivity is necessary to maintain a high thermal gradient at the junction. Standard thermoelectric modules manufactured today consist of P- and N-doped bismuth-telluride semiconductors sandwiched between two metallized ceramic plates. The ceramic plates add rigidity and electrical insulation to the system. The semiconductors are connected electrically in series and thermally in parallel. Miniature thermocouples have been developed that convert body heat into electricity and generate 40μW at 3 V with a 5 degree temperature gradient. Advantages to thermoelectrics: 1) No moving parts allow continuous operation for many years. Tellurex (a thermoelectric production company) claims that thermoelectrics are capable of over 100,000 hours of steady state operation. 2) Thermoelectrics contain no materials that must be replenished. 3) Heating and cooling can be reversed. One downside to thermoelectric energy conversion is low efficiency (less than 10%). The development of materials that are able to operate in higher temperature gradients will result in increased efficiency. Future work in thermoelectrics could be to convert wasted heat, such as in automobile engine combustion, into electricity. ELECTROSTATIC (CAPACITIVE) ENERGY HARVESTING This type of harvesting is based on the changing capacitance of vibration-dependent varactors. Vibrations separate the plates of an initially charged varactor (variable capacitor), and mechanical energy is converted into electrical energy. FUTURE DIRECTIONS Electroactive polymers (EAPs) have been proposed for harvesting energy. These polymers have a large strain, elastic energy density, and high energy conversion efficiency. The total weight of systems based on EAPs is proposed to be significantly lower than those based on piezoelectric materials. EXTERNAL LINKS General review ''Energy Scavenging for Mobile and Wireless Electronics'' {Link without Title} ''Harvesting ambient energy will make embedded devices autonomous'' {Link without Title} ''Review of Energy Harvesting Techniques and Applations for Microelectronics'' {Link without Title} ''Energy-harvesting Chips: The Quest for Everlasting Life'' {Link without Title} Piezoelectric ''Ferreting Out Power'' {Link without Title} ''Sensors Get Their Field of Dreams'' {Link without Title} ''Power Considerations for Wireless Sensor Networks'' {Link without Title} ''Efficiency of energy conversion for devices containing a piezoelectric component'' {Link without Title} ''Harvesting energy using a thin unimorphed prestressed bender:geometrical effects'' {Link without Title} ''Piezoelectric energy harvesting under high pre-stressed cyclic vibrations'' {Link without Title} ''Use of piezoelectric energy harvesting devices for charging batteries'' {Link without Title} ''Piezo-effect and physics of CdS-based thin-film photovoltaics'' {Link without Title} ''Harvesting Energy by Improving Economy of Human Walking'' {Link without Title} ''Havesting Energy from Piezoelectric Material'' {Link without Title} Photovoltaic ''Luque, Antonio. Handbook of Photovoltaic Science and Engineering c2003 John Wiley and Sons.'' {Link without Title} ''Structural and photoelectrochemical characteristics of nanocrystalline ZnO electrode with Eosin-Y'' {Link without Title} Thermocouple ''Development of Thin Film Ceramic Thermocouples For High Temperature Environments'' {Link without Title} ''An Intro to Thermoelectrics'' {Link without Title} ''Brief History of Thermoelectrics'' {Link without Title} ''Introduction to Thermoelectrics'' {Link without Title} Future directions ''Electrostrictive polymers for mechanical energy harvesting'' {Link without Title} |
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