JASC: Journal of Applied Science and Computations ISSN NO: 1076-5131 A Review on Radio Frequency[RF] Energy Harvesting Systems 1 P.S.Chindhi, 2 H. P. Rajani, 3G.B.Kalkhambkar 1Electrical Engineering, 2,3Electronics and Communication Engineering 1,3Shivaji University Kolhapur, 2Visvesvaraya Technological University, Belagavi Abstract:Presently solar and wind energy systems are considered as most prominent alternative energy systems.Looking at the energy scenario in the fast growing technological world it has become necessary to adopt new renewable energy system apart from solar and wind like Radio Frequency (RF) which is abundant and never ending source. This paper presents a review on RF Energy Harvesting (RFEH) system through rectenna. Various impedance matching technics are discussed in designing the rectenna Keywords: Energy Harvesting, Patch Antenna, Rectenna, Polarization, Solar Cells, Frequency Selective Surface (FSS), Hybrid system,PIFA 1. INTRODUCTION Batteries are an important a part of our lives. We’ve mature thus passionate about them for pretty much everything – from mobile phones, cars, remote controls, watches, laptops, and anything in between. At present majority of low power remote sensor devices and embedded equipment’s are powered through rechargeable chemical batteries, but batteries require periodic replacement and have finite life span. The battery waste processing is difficult. Battery waste pollutes the land and water. To overcome above limitations we have to adopt RF energy harvesting system.In present scenario the wireless systems such as TV and radio base stations, wireless field systems (Wi-Fi), wireless sensor network (WSNs), mobile base station and wireless routers are increasing which directly increases the level of RF signal. In the past decade 1800’s and 1900’s researchers focused on wireless power transmission (WPT) system.In WPT system the RF signal is converted into DC signal [1-2]. Recently sensor based networks are gaining more importance and the concept of self-powered sensors is revolutionizing various fields such as smart cities, Internet of Things (IOT), smart skins etc. Since the self- powered sensor nodes are continuously powered, they make the system autonomous [3-4].The RF energy source is the means of powering the sensor nodes spontaneously. It can be done through a rectenna system.Rectenna isa combination of antenna, rectifier and power conditioning circuitry. Rectenna not only power the sensor networks,the low powered electronic devices also be made auto chargeable. But there are various challenges in designing the rectenna system. The RF energy is having least energy density and hence a powerful control circuitry is required for making this concept practicableThis paper presentsa reviewon RF energy harvesting system by referring different design concept from various reputed papers. The power received for the line of sight propagation in case of RF energy harvesting system is given by ⋋ Pow = (1) r () Where Powr- Power received by received antenna Gtr- Gain of transmitting antenna Gre-Gain of receiving antenna ⋋- Wavelength of electromagnetic signal Volume 5, Issue 8, August /2018 Page No:211 JASC: Journal of Applied Science and Computations ISSN NO: 1076-5131 R-Distance between transmitting and receiving antenna L-System loss factor () () L= = = = () (2) ⋋ Where ⋋= and = ⋋ The scattering of RF signal and reflection due to obstacles in the propagation, the receiving antenna collects signal from multiple path. The received power in such case is given by = (3) Where h and hhight of transmitting and receiving antenna TABLE 1 ALTERNATIVE SOURCE, POWER DENSITY AND HARVESTING TECHNIQUE Harvesting Source Power Density tech. RF GSM: 0.1/ Antenna Wi-Fi:1mW/ Solar Indor:10 / Photovoltaic Outdoor:10 / Thermal Human:30 / Thermoelectric Industrial 1-10 Pyro electric / Vibration Human: 4 / Piezoelectric Industrial: 1-100 Electrostatic / Electromagnetic Table 2 Frequency band and Applications Sr.No. Name Frequency Application 01 ELF 3-30 Hz 02 SLF 30-300 Hz Power Grids 300-3000 03 ULF Hz 04 VLF 3-30 kHz Submarines 300-3000 AM broadcast 06 MF kHz 07 HF 3-30 MHz Short wave broadcast 30-300 FM,TV 08 VHF MHz 300-3000 TV, LAN, Cellular, 09 UHF MHz GPS 3-30 GHz Radar, GSO satellites, 10 SHF Data 30-300 Radar, Automotive, 11 EHF GHz data Volume 5, Issue 8, August /2018 Page No:212 JASC: Journal of Applied Science and Computations ISSN NO: 1076-5131 II. Impedance Matching Techniques to Improve Efficiency of Energy Harvesting [EH] A. Georgiadis et. al. [5] has proposed a rectenna in which dual polarised square aperture coupled micro- strip antenna with cross shaped slot on patch layer is placed on top of three substrate layers and is separately designed using Electromagnetic Simulator(EM) simulator. By the applying reciprocity theory the open circuit voltage sources of antenna is computed. ( , ∅ , ) = (Ѳ , ) (4) , , The rectifier circuit and circuital model of antenna is optimized using Harmonic Balance Analyzer (HBAM) for a specific power density and angle. The impedance matching is performed with series and shunt inductor as shown in figure 1.The impedance matching improves the efficiency of RF to DC conversion. The table 1and 2 shows the results in case of horizontal and vertical polarization respectively. Fig.1Theveninequivalent of antenna with rectifier circuit The limitation of this reference was the 1% fabrication errors caused due to adhesive used for joining the substrates, which resulted in the frequencyshift. The distance between reference antenna and rectenna is 3.45m and the available received power Pav=11.96. TABLE 3 SIMULATED RESULTS Resonance Gain Type of Efficiency frequency in Polarization in % in GHz dB Vertical Polarization 2.45GHz 8.25 14.1% Horizontal Polarization TABLE 4 MEASURED RESULTS AT S=0.15 Resonance Type of Gain in Efficiency frequency in Polarization dB in % GHz Vertical 2.44GHz 15.3 % Polarization 7.7 Horizontal 2.45GHz 11.3% Polarization Volume 5, Issue 8, August /2018 Page No:213 JASC: Journal of Applied Science and Computations ISSN NO: 1076-5131 In reference [6] two squareshapeelectromagnetically coupled aluminum microstrip antennas of 2mm thickness as a radiating patch are stacked for harvesting RF energy from GSM 900 band. Here details of power received from GSM 900 band with respective distance and number of operators is tabulated. To avoid impedance matching complicity between input power, input impedance and frequency, resistive impedance matching is utilized. RF to DC conversion Dickson Multiplier circuit is used to increase the output voltage level. The detail of number of number of voltage doubler stages and output voltage with respective distance from cell tower is tabulated. According to S.Agrawal et al. the efficiency of RF to DC conversion is improved by implementing resonating L-type matching circuit with low pass filter at the last stage. The resonating circuit strengthens the low power RF signal and LPF reduces harmonics and ripples from output voltage [7]. In reference [8] the author has designed two different RF energy harvesting circuits,one for Low Power Density (LPD) and other for Higher Power Density (HPD) and plotted the combine efficiency curve. The efficiency curve is a function of impedance matching network and is optimized to maximize the efficiency of EH. This covered larger region and can work separately for varying power densities while powering the sensor nodes. He has given mathematical description that optimized curve is the function of impedance matching network (L,C), number of rectifier stages (N) and he has optimized combined efficiency curve stating the constraints on effective operational range of the circuit with respective cross over point and its impacts. The optimization framework is subject to the equations (5) & (6). , , , , , , ∫ ( ` ) dx>∫ ( ` ) dx (5) , , , , , , and∫ ( ` ) dx>∫ ( ` ) dx(6) Thelimitation of design given in [8] is that the output harvested energy decreases with increases in number of antennas. Complexity of impedance matching increases with increase in number of antennas stages. Fig.3 a) Efficiency curve b) EH with multiple antennas stages Fig.2 a) Hybrid FSS/Rectenna b) Unit cell hybrid FSS Volume 5, Issue 8, August /2018 Page No:214 JASC: Journal of Applied Science and Computations ISSN NO: 1076-5131 c) PCB layout for RF-DC circuit A. Impedance Matching with Inset Feed In [7] the focus is given on inset fed technique to achieve better impedance matching and to reduce size of antenna by varying the depth of inset fed. In this paper phase distribution is achieved by changing the length of feeding lines and amplitude distribution adjustment is carried out by using three power divider networks which are located along the central vertical axis. After simulation of designed hexagonal microstrip antenna array author observed that there are no sidelobes in electric field pattern.According to [8], [9] if the sidelobes are absent then maximum RF energy harvesting is possible. Sidelobes decrease the performance of antenna array. Here the maximum transmission distance for lighting the Light Emitting Diode (LED) is 51cm. B. Frequency Selective Surface (FSS) In case of simple rectenna array configuration, polarization matching is required between transmitter and receiver to maximize the harvested power. To overcome the polarization and matching network problem, frequency selective surface (FSS) is taken in to consideration for the rectenna design [10].