Harvesting Energy from Radio Waves
| Since the very beginning of industrialization and modernization the demand for energy has seen an upward trend and this trend in not linear rather exponential. Scientists, researchers of different eras have founded ways, techniques and technologies to keep up with the ever increasing demand for energy. In the beginning we only had non-renewable sources of energy like coal, oil etc., then came a lot of renewable sources of energy with solar, wind, hydro power being the prime examples. Now a days non-conventional sources are being explored for harvesting energy and some of these sources are motion, pressure, vibration, radio waves etc. DC power generated for these non-conventional sources lie in the range of micro Watts to few Watts. It may appear small amount of power but it is enough to power-up our modern day low power electronic devices. |
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There is a special term coined for harvesting small but usable energy from non-conventional sources, it is called Scavenging Energy. How we can scavenge energy from radio waves is discussed in this article.
We are surrounded by radio waves all around us that are generated by many sources like mobile base station, WLAN networks, satellite TVs etc. So the energy is abundantly available in the form of electromagnetic waves. If at the radio receiver end we have proper circuitry or technology, then excess energy from the radio signals could be harvested and stored in the form of DC power. As shown in the block diagram below input RF power is transformed into DC power with the help of a voltage multiplier/rectifier circuit. A proper L-C matching network topology is used for maximum power transfer from antenna to voltage multiplier. This is followed by a battery charging circuit that will perform voltage regulation, DC-DC conversion etc. and charge the battery that further could be used in powering-up any low power electronic device.
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Antenna is a metallic device for transmitting and receiving radio waves. So, an antenna forms an intermediate structure between free space and guided device. Guided device could be coaxial cable or waveguide. For designing an antenna following design parameters are to be considered - power gain, actual maximum power density, average power density, effective height, effective antenna aperture, path loss, polarization, antenna temperature, efficiency, directivity etc. Equipment used for harvesting energy from radio waves is known as a Rectenna or rectifying antenna which transforms RF energy to DC energy. A rectenna consists of an antenna, a matching circuit and a rectifier circuit. There are a lot of topologies exist for RF-DC rectifier design and one of the popular topology is Charge Pump Rectifier. It is also known as voltage multiplier and is extensively used in micro power circuits. Charge pump rectifier features switching devices and it operates in two cycle. In first cycle the capacitor is charged to the supply voltage and in the second cycle the supply voltage in series with capacitor voltage is delivered to the load. The efficiency of charge pump could be 90-95%. Efficiency in energy harvesting system is very important, antenna efficiency will determine the amount of RF power available for harvesting. Different kinds of antennas like monopole, dipole, microstrip patch antenna etc. could be used. Similarly RF-DC conversion efficiency is determined by rectifying circuit which could be designed using nonlinear devices like IMPATT diode, Schottky diode etc. Energy harvesting using radio waves finds applications in wireless sensors network (WSN), and IoT where low power sensor nodes for remote monitoring could be powered for lifelong or battery less operation.
By: Mr. Sagar Juneja (Asso. Prof.) & Mr. Amit Monga (Asst. Prof.), ECE, Chitkara University, H.P.
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