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

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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

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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

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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

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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]. The FSS rectenna system allows power harvesting from two frequency band and it is polarization independent which is not possible with simple rectenna array system. As the size of FSS increases by keeping constant input power, RF to DC conversion efficiency decreases. At3x3 FSS the conversion efficiency is 25% at -6dBm incident power level and at 5x5 FSS the conversion efficiency of 15.9% is achieved.

C. Miniaturization of Antenna and Hybrid Structure

In [11] the author has introduced various shapes of antennas for RF energy harvesting. The main focus is on miniaturization of antenna. In this paper author achieved 7.96dB gain at frequency 3.43GHz but the main requirement to achieve the maximum harvested power is high bandwidth.

Fig.4 Different Miniaturized Antenna Structures

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Abdulhadi et.al has introduced a hybrid system for RF energy harvesting. In [12] a two port RFID tag antenna is integrated with latest I2C RFID chip to enhance the RF energy harvesting for self powered wireless sensors along with thin film solar cell. The hybrid RF energy harvesting system supports the operation of MCU and sensor. For simplicity of integration, testing different sensors and programming the MCU, the entire digital circuit is assembled on breadboard. The impact of thin film solar cell on multi-port patch antenna is not discussed. The RFID reading range when solar cells are used for harvesting is 27m and without solar cell (with only RF signal harvesting) is 12.1m.

Fig.5 Proposed prototype and Interconnect layout of RFID-enabled sensor with multiport patch tag antenna and energy harvesting

Fig.6 Hybrid system solar cell with TEG

In [13] hybrid system is proposed for energy harvesting and wireless communication. Here quarter wave patch antenna is integrated with thermoelectric generator (TEG) and solar cell as shown in figure above. Here solar cell converts harvested solar energy in to heat. The heat generated by solar cell is applied on top surface of the TEG. Then temperature gradient occurs between top layer and bottom layer of TEG and due to temperature gradient voltage is generated. To maintain thermal connectivity between patch antenna and ground, shorting visa are provided on patch antenna.

In [14] polarization dependent Artificial Magnetic Conductor (AMC) surfaces are used to analyze a dual band energy harvesting with loop antennas. To calculate driving point impedance of dipole antenna with AMC, surface image theory is applied from [15], [16]. According to image theory the driving point impedance is expressed as,

= + ∗ (7) Where Z11-Self impedance of real antenna and Z12-Mutual impedance between real and image antenna - Reflection phase of surface

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Fig.7 a) Antenna over reflecting surface (c) Square loop antenna and

b) Antenna representation in image theory. dipole antenna over AMC surface

Ali Mavaddat et.al [17] introduced millimeter-wave energy harvesting using microstrip patch antenna array. Antenna and step impedance five step Low Pass Filter (LPF) is fabricated on sane substrate. The function of step impedance LPF is to suppress second order harmonics generated by rectifier diode. To match 50 Ω line impedance and input impedance of diode, a single stub was introduced .To turn on diode rectifier circuit high RF input power is required, as RF input power increases simultaneously amplitude of higher order harmonics increases. So According to [18] the RF input power should be less than .

In [19] coupled E-shape microstrip patch antenna with coaxial feed technique is taken into consideration for RF energy harvesting. In market various types of substrate materials with different dielectric constant in the range of 2.2 to 12 are available. In this reference details about selection of substrate material and variation of antenna parameter is also discussed. The gain for single E-shape patch is between 5dBi to 10dBi and the gain for coupled E-shape patch antenna is up to 20dBi. According to [19] bandwidth increases with increase in substrate thickness but at the cost of difficulty in impedance matching.

By using IBM 130mm CMOS process, T.Salter et.al developed an integrated RF scavenging circuit for smartdust sensor. A conventional Villard voltage doublers circuit is modified through parasitic resistance matching network to reduce threshold voltage of diode as shown in figure7. With the integration of on chip components and improved Villard Voltage Doublers (VVD) circuit, the author achieved output voltage 1V from UHF, cellular and ISM band emissions [20].

a) b) Fig.8 a) Modified Villard voltage doublers circuit b) Test chip with RF power harvesting circuit

In reference [21] for powering a low-power wireless sensor an integration of antenna array with multi crystalline silicon solar is presented with beam switching technique. The antenna shown in figure below offers improvement in gain and beam width compared to other solar integrated antennas.

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a) b) Fig.9 a) Inverted-F antenna with solar cell b) Integration of transmission line with solar cell

In [22] author has designed three different rectenna array systems as shown in figure 9 for narrowband, dual frequency and broadband applications with power management circuitry.These are presented for harvesting low-power density incident plane waves. The details of array and scaling parameter are also mentioned. The hardware results shown here and in [23]–[25] are based on low-cost off-the-shelf components and demonstrated record efficiencies for power levels down to approximately 100 W. Below this level, the quiescent losses of available control hardware become the limiting factor. With the significant advancements in ultra-low-power wireless sensors, a range of custom solutions have been developed for power conversion at these low power levels, e.g., [26] and [27], and can be integrated with the scalable approaches demonstrated in this reference.

a) b)

c) d) Fig.10 a) Generalized scalable RF energy harvesting circuitb) Dual-band Yagi-Uda array c) Narrowband dual-linearly polarized patch array d) Broadband dual-circularly polarized spiral array.

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In reference [28] the energy is harvested from RF and microwave spectrum. The range of RF frequency is 3 KHz to 300GHz and that of microwave 300MHz (0.3GHz) to 300GHz. The antenna design and voltage doubler circuit is also discussed. The tabulations regarding free space loss and variation in output voltage with respective distance can be found in reference. The author has demonstrated the energy harvesting concept by charging the cellular phone at the distance from 0-15cm.

2. Research in RF Energy Harvesting Field.

2.1 National Level (India): In India one latest research paper from IISC Bangalore introduced opportunistic multihop relaying in cooperative wireless network and hybrid wireless sensor network. According to ICNRP guideline of 1998, GSM900, GSM1800, and UMTS2100 bands can be utilized for wireless power harvesting with safe power density of f/200. Detecting problems in the human body with the use of standalone system equipped with wireless power harvesting is facilitated to strengthen body area network using wirelessly powered sensors. Most of the wireless devices like mobile phones consume the energy in the range of microwatts to miliwatts respectively. The attempt has been made in the cellular industry to manufacture the mobile phones with capability to power its battery wirelessly using the concept of wireless power harvesting.

The recent paper on wireless power harvesting in cognitive radio network has developed a solution to increase energy and spectrum efficiency demand. Researchers from IIT Delhi, Hyderabad in collaboration with researcher from North Eastern University and university of Rochester are working on challenges and opportunities in smart energy harvesting. They have also discussed opportunities in multihop energy transfer (MHET) and simultaneous wireless information and power transfer using cooperative relaying system.

The researchers from industry like Tata Consultancy Service (TCS) are working in the field of impact of mobile transmitter sources on radio frequency wireless energy harvesting. In the St Peter University Chennai the researcher are working on various design improvement of patch antenna for increasing the efficiency of RF power harvesting and simultaneously reducing the size of patch antenna by using certain techniques.

In India the research is still lagging because of unavailability of testing facility such as an anechoic chamber and vector network analyzer in the research laboratories. Some researchers have reported that the energy is harvested from cellular towers by taking care of laws and regulations of government bodies.

Metamaterial research in India was first proposed in vision 2020 in 2004 at BARC. Researchers from Radio Physics and Electronics of Calcutta University and Society for Applied Microwave Electronics Engineering and Research Calcutta have designed and fabricated the metamaterial crystal capable of making the things invisible and can capture minute details of the object which sophisticated optical microwave cannot capture.

Researchers from IIT Mandi are also working on metamaterial [31], [32], [33], [34, [35], [36]. They have discussed challenges in manufacturing optical Metamaterials. In one of the paper form GBPUAT Pantnagar, Uttrakhand discussed metamaterial as aabsorber, superlense, antenna, sensor and predicted that the future work in the metamaterial will bring significant changes in optical and medical field and is closely related to nanotechnology [37].

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2.2 International Level: At national institute of standards and technology in U.S. Department of Commerce has developed “Active terahertz metamaterial with organic pi-conjugate materials” [38]. Professor Heinz langhals and his colleagues synthesized “Metamaterial Based on Organic Molecules” at LMU department of chemistry [39]. The researchers from Department of Chemistry, Indiana University, Bloomington, Indiana are working on “Bio-Enabled Synthesis of Metamaterials” [40]. Taconic Advanced Dielectric Division, Petersburgh, NY is doing research on “Organic ceramic microwave substrate materials” [41].

From an article by Dr Anne Horan Programme Manager, Life Sciences Royal Society of Chemistry, Thomas Graham House, Science Park, we get a clear idea about the background and the role of organic chemists in the discoveries related to Metamaterials [42].

Since 2009, Nokia is consistently working towards wirelessly chargeable cell phone which draws the energy from Wi-Fi transmitter, cell phone antenna, TV masts in the Cambridge U.K and had developed a proto type which could harvest 50 miliwatts of power which is enough to charge the cell phones by converting electromagnetic waves in to electrical signals.

At the international level NASA is working on energy harvesting from existing sources in the spacecraft and aircraft applications. To reduce the excessive mass of power system in the space application and the impact of green energy in the power generation is studied in the NASA report of Nov.2010.

Joshua Smith at Intel and Alanson Sample at Washington University developed a self sustained temperature and humidity sensor that could harvest the power from TV signal. In the Canada, USA research has been made on hybridization of solar energy harvesting with RF energy to improve the amount of harvested power. Drayson technology from London has introduced “free-volt” revolutionary energy harvesting technology on 30 Sept. 2015. They are working specially to solve the problems in the RF energy harvesting and they have developed commercial application that is “Clean Space” tag air sensor, which is currently being manufactured in the UK. Researcher from Korea have developed a system which ensures a human protection from electromagnetic fields in wireless power harvesting and wireless power transfer using decent robot. In Spain some researcher are working with hybrid frequency selective surface rectenna to increase the efficiency of RF power harvesting. In Spain University of Perugia, Italy researchers are implemented a hybrid energy harvesting system using solar cells, thermoelectric generator along with microstrip patch antenna for uninterrupted energy harvesting from multiple sources. Researcher group from Georgia tech. created a system which self sustained by scavenging the energy in the band 470-570 MHz frequency. The researcher from Poland Lodz University have developed experimental setup for UHF band energy harvesting using different types of microstrip patch antenna.

3. CONCLUSION

In this paper different RF energy harvesting techniques with microstrip antennas is discussed. Impedance matching plays an important role in energy harvesting using microstrip antennas. The hybrid system using solar cell using rectenna techniques enhances the power harvesting capability of antenna.

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