|Fig. 1: User downloading and reading book on her smartphone (Source: Wikimedia Commons).|
Since the 1980s mobile phone technology has grown rapidly and the demands for longer battery life have risen. [1,2] While improving the energy of batteries has been a major research field, new developments in energy harvesting have helped manufacturers in their search for better power supplies.  Manufacturers looking to harvest energy through other means have to consider the following four factors: the device's power consumption; its usage pattern; its size; and the motion and vibration to which the device is generally subjected. 
Energy harvesting or power harvesting methods may prove to be attractive alternatives for batteries in wireless devices. [1-6] Solar cells, the most mature and commercially established energy-harvesting solution, are used across a wide range of size scales and power levels.  However, direct sunlight may not be readily available for handheld users; as a result, motion and vibration have emerged as potential energy sources for handheld users. [3, 4] These methods are limited in their range of power supply, but may be used in wireless communication since a supply power < 100 μW is sufficient to operate such devices in silent mode. 
In light of the shortcomings of other energy harvesting forms, research in radio frequency energy harvesting has grown in popularity.  Radio-frequency energy is common in metropolitan centers, which are saturated by television, AM, FM, cellular, WiFi, WiMax, and other instruments that transmit RF signals.  Radio frequency emitted by such sources generate high electromagnetic fields and contain electromagnetic energy that can be converted into DC voltage using a circuit system linked to the receiving antenna.  This circuit system can convert the RF signal to DC power at distances over 100 meters . To maintain a sufficient supply of energy in midst of varying RF concentrations, a capacitor can be attached to the circuit system to output the required constant voltage. 
Energy harvesting has the potential to supplement and possibly replace batteries for handheld devices; whether through solar, motion, or RF methods, energy harvesting would help consumers avoid a) changing/recharging their batteries b) carrying batteries that contain metals toxic to organisms; and c) missing backups to primary power sources. [1-6] Though the RF method may ultimately be a more versatile and ubiquitous power source over its solar and motion counterparts, each method today has its relative advantages and disadvantages and each application should be evaluated individually to finding the best EH alternative. [5,6]
© Sarkis Agaian. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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