INVESTIGATING TECHNIQUES & RESEARCH TRENDS IN RF ENERGY HARVESTING
Ruchi Sharma1 , Dr. S. Balaji2 1 (Research Scholar, Department of Computer Science & Engg. VTU, Bangalore, India) 2 (Centre for Emerging Technologies Jain Global Campus, Jain University Jakkasandra Post, Kanakapura Taluk Ramanagara Dist.-562112, India)
ABSTRACT: For more than two decades, there were extensive research activities towards harnessing the energy captured from the natural resources and use it for human utility. This field of research, commonly known as energy harvesting, has various characteristics and subsidiary applications. Presently, it is seen that use of mobile phones are increasing due to the advancement of communication systems and numerous services incorporated within it make the availability of free RF signals. This paper discusses various techniques and attempts initiated by research community in harnessing this free RF signals for the purpose of energy harvesting to power up low-powered electronic devices and embedded systems. It also presents some of the most significant research procedures that use RF signals for harvesting energy and their implausible research gap. Keywords: Energy Harvesting, Energy Scavenging, RF-Signals, Wireless Charging.
INTRODUCTION: Radio Frequency (RF) energy  is currently broadcasted from billions of radio transmitters around the world, including mobile telephones, hand-held radios, mobile base stations, and television / radio broadcast stations. The ability to harvest RF energy, from ambient or dedicated sources, enables wireless charging of low-power devices and has resulting benefits to product design, usability, and reliability . Battery-based systems can be trickle charged to eliminate battery replacement or extend the operating life of systems using disposable batteries. Battery-free devices can be designed to operate upon demand or when sufficient charge is accumulated. In both the cases, these devices can be free of connectors, cables, and battery access panels, and have freedom of placement and mobility during charging and usage.
RF energy can be used to charge or operate a wide range of low-power devices. At close range to a low-power transmitter, this energy can be used to trickle charge a number of devices including Global Positioning System (GPS) or Real-Time Locating System (RTLS) tracking tags, wearable medical sensors, and consumer electronics such as e-book readers and headsets. At longer range the power can be used for battery-based or battery-free remote sensors for Heating, Ventilation, and Air Conditioning (HVAC) control and building automation, structural monitoring, and industrial control. Depending on the power requirements and system operation, power can be sent continuously, on a scheduled basis, or on-demand. In large-scale sensors deployments significant labor cost avoidance is possible by eliminating the future maintenance efforts to replace batteries. Ambient radio waves are universally present over an ever-increasing range of frequencies and power levels, especially in highly populated urban areas.
These radio waves represent a unique and widely available source of energy if it can be effectively and efficiently harvested. The growing number of wireless transmitters is naturally resulting in increased RF power density and availability. Dedicated power transmitters further enable engineered and predictable wireless power solutions. With continued decreases in the power consumption of electronic components, increased sensitivity of passive receivers for RF harvesting, and improved performance of low-leakage energy storage devices, the applications for wire-free charging by means of RF-based wireless power and energy harvesting will continue to grow. The charging of mobile devices is convenient because the user can do it easily, like for mobile phones using external charger. But for other scenarios, like mobile nodes which are located in difficult to access environments, the charging of the batteries remains a major problem. This problem increases when the number of devices is large and are distributed in a wide area or located in inaccessible places. The research on RF energy harvesting provides reasonable techniques of overcoming these problems. Reducing power consumption has become a major challenge in mobile device. As a vital factor affecting system cost and lifetime, energy consumption in mobile devices is an emerging and active research area. The energy consumption of mobile nodes is of crucial concern due to the limited availability of energy. While energy is a scarce resource in every mobile device, the problem is more severe for the following reasons: (i) compared to the complexity of the task they carry out; sensing, processing, self-managing, and communication, the nodes are very small in size to accommodate high-capacity power supplies, (ii) though the research community is investigating the contribution of renewable energy and self recharging mechanisms, the size of nodes is still a constraining factor, and (iii) ideally, a mobile node consists of a large number of sub-nodes. This makes manually changing, replacing or recharging batteries almost impossible. This paper surveys the various trends, current research direction, and excavating research gap in RF energy harvesting.
ENERGY HARVESTING: The term ‘energy harvesting’ can be defined as a method to derive an alternate source of energy from various external sources. Such external sources can be thermal energy, salinity gradient, solar energy, and kinetic energy. Such energy are reposited (or stored) in miniature electronic and electrical devices which are usually positioned in energy source points. The phenomenon of energy harvesting furnishes very less amount of energy. Usually, such energy is captured from the surroundings, which is also known as ambient energy that is completely free of cost .
The categorization of the energy harvesting can be done depending on the energy that the application is trying to use. Table 1 highlights some of the common energy harvesting mechanisms along with their energy generation capabilities.
The noted researcher Fry in his work has featured some of the portable energy suppliers. The features discussed by the author for energy harvesting capabilities are (i) higher degree of current and voltage, (ii) power density, (iii) various physical attributes (e.g. shape, size, weight), (iv) various environmental characteristics (e.g. operating temperature, water resistance capabilities, and (v) maintenance characteristics.
Different types of standard energy harvesting methods explored and reported in the literature are as follows…