DESIGN OF A WIRELESS POWER TRANSFER SYSTEM UTILIZING MICROWAVE FREQUENCIES

Transcription

1 DESIGN OF A WIRELESS POWER TRANSFER SYSTEM UTILIZING MICROWAVE FREQUENCIES Steven Shane Ewers Department of Electrical Engineering University of Hawai i at Mānoa Honolulu, HI ABSTRACT This report describes a system, loosely based on a model originally proposed by Dr. William C. Brown in 1961, which theoretically would be able to transmit significant wireless power, with precise directivity, over a distance using a frequency of 2.45 GHz. A large-scale model of this system could be very valuable to NASA in the form of a solar powered satellite which could wirelessly transmit this power to a base station on earth. The system outlined herein is composed of a transmitter and a receiver. A helical (Kraus) antenna is the targeted radiator, capable of transmitting 10W, or 40 dbm of RF power 4 meters to a receiver. Microstrip matching networks are utilized for impedance matching on both ends. This theoretical design is mainly conceptual, including idealistic calculations for transmission efficiency, power, and radiation patterns. INTRODUCTION Nikola Tesla described the ability to transmit power to a device wirelessly as an allsurpassing importance to man. In our world of portable technology, wireless communication, and so called smart devices, it seems slightly out of place to have such a dense, wired power grid. Why, if we have the ability to send dense packets of information around the globe, has mankind not been able to free the power grid from the burdensome stitches of wires running thousands of miles around the planet? It is towards this purpose that the following research has been performed. This report will demonstrate a design whose concept has been demonstrated over 50 years ago. In 1961, William C Brown published a paper that described a system which could send energy through the air, in the form of microwaves. In 1964 he demonstrated a design could power a model helicopter completely from a microwave beam. There are several papers outlining his block diagram of the system [1]. The research done herein closely follows this model. A large-scale version could be very valuable to NASA. Solar-powered satellites (SPS) already orbit our planet. The concept is that a large array of solar panels, at high-earth orbit, would receive sunlight 99% of the year. This satellite would collect energy from the sun s rays and beam this energy to a location on the earth s surface. The transmission would occur only when a pilot beam from the receiving station is aligned with the SPS. Of course many safety concerns arise with this type of system, not the least of which is maintaining direct alignment to prevent reaction of the beam with organisms on the planet. The merits of such a system can easily be seen in the form of continuous power from space, consuming comparatively no space on the Earth s surface [2]. It is worthwhile to elaborate on the concept of wireless power transfer (WPT), in order to have a comprehensive viewpoint, prior to explaining the system governing it. There are two 17

2 general methods to transmit power that have been demonstrated; namely, near field and far field techniques. In order to understand the difference, one must look at Maxwell s equations for EM wave propagation for a plane wave detaching from an oscillating source. Basically expressed, the propagation of the electric field from a short current radiator can be described by equation 1. E θ = Idl jωε 0 4π sinθ β 2 r + jβ r r 3 e jβr Eq. 1 This equation shows the field has a dependency on r, which is the distance from the source to the observation point. As r gets closer to 0, the r 3 term dominates, thus defining the near field as a region less than one wavelength. The far field is dominated by the r-term, as r 0 - approximately two wavelengths from the source. The near field is reactive, dominated by capacitive or inductive characteristics, and is non-radiative [3]. Near field (non-radiative) methods include inductive coupling, resonance inductive coupling (RIC), and air ionization. The first can be seen in electric toothbrush chargers, and generally has a very short distance of effectiveness. RIC is similar to classic induction, with the difference being the two circuits are coupled via strong magnetic fields, at a particular frequency. This method has been demonstrated with most success, and has the lowest potential for negative effects, as only objects within the specific coupling regime are affected. Air ionization is the breakdown of air, causing a conduction current, similar to lightning. As can be imagined, this is the least safe. Far field (radiative) techniques involve microwave power transmission (MPT) and laser transmission. The former is accomplished by converting energy to microwave power, beaming this energy through a directive antenna to a rectenna, and converting it to a conventional power form. The latter method requires direct line of sight, and is considered dangerous due to the high power laser [1]. Although RIC would be cheap, efficient, and fairly simple, the distance of transfer would be limited. Therefore, far field radiative techniques were more attractive for this particular design. For legal reasons, the ISM range of frequencies, which are reserved by the FCC for scientific and medical purposes, was considered. Because of high attenuation above 6GHz, and excessively large physical structures required at frequencies less than 2GHz, the operating frequency was chosen to be 2.45 GHz, which has a wavelength of cm, as given by c λ =. f METHODOLOGY Whereas many designs follow a simple A-B-C progression, this was a non-linear process. Following fundamental research, the nominal frequency was chosen, as well as the method of transmission (radiative far-field). It was decided that a received power of 10 Watts across a distance of 4 meters was significant. This led to the application of the Friis transmission formula to aid in the design of the antenna parameters. After calculating the path loss and transmit/ receive powers, the remainder of the circuit could be designed. The transmitting circuitry consisted of a microwave power source, coaxial-waveguide adapter, input matching network, and a transmitting antenna. The receiving network, known as a rectenna (rectifier + antenna), provided a means of converting RF directly into DC [5]. It is composed of a receiving antenna on 18

3 the front end, an output-matching network, rectifying circuitry, and the load. Figure 1 shows a block diagram of the overall system. Fig 1: Block diagram showing transmitting system and rectenna components. CIRCUIT MODEL The power source would be a magnetron, commonly found in any household microwave oven. This source was chosen because it is cheap, easy to obtain, and emits microwaves, typically at a center frequency of 2.45 GHz. It can easily output 1000 kw of power and more if wired together [2]. Because the magnetron emits microwaves into free space, it would need to be coupled to the system. To solve this problem, a COTS waveguide adapter was chosen as the means of doing so. Such a device would allow for free-space waves to be collected inside a geometrically-suited waveguide, whereby the energy would be converted at the probe to electrical energy, and allowed to propagate through the system, via a coaxial coupler. The next challenge was impedance matching this coaxial line to the transmitting antenna for optimal transmission efficiency. It is well known in RF design that an impedance mismatch will cause EM power to be reflected, resulting in overall transmission power loss. This can be prevented using a microstrip technique known as quarter-wave matching. This method simply uses a 2-layer board with a microstrip trace length equal to a quarter of the transmission frequency s wavelength. To practically apply this design, a network analyzer would be needed to determine the impedance of the source, the real resistance of the load (antenna in this case), and the propagation constant must be known. If connecting directly to the antenna (load), the use of a balun could be eliminated. For the transmitting antenna, a Kraus, or helical geometry, was chosen. After simulating the directivity achievable from dipole and patch models, the Kraus antenna seemed to provide a higher directivity. Transmitting in axial (T1) mode, it can produce a highly directive beam 19

4 allowing for higher gain and lower path loss. This antenna style produces a circularly polarized wave; therefore the receiving antenna must be wound in the same direction. The receiving antenna design was identical to the transmitting antenna. Again, impedance matching played a key role at the interface with the rectifying circuitry. For the matching network, a matched-stub filter was the design of choice. This consisted of a microstrip design, using two stubs of equal length, to act as filters, as well as preventing unwanted return loss. When properly designed for the center receiving frequency, the circuit acts as a notch filter for the desired frequency, rejecting parasitic frequencies. Although this is more crucial for signal reception, the rectifying circuit could act unpredictably if they were allowed to pass. Once the wave is received, the RF power must be converted back to electrical power. This was done using a classical full-wave rectifier circuit. The rectifying part of the circuit converts a sinusoidal AC wave to a DC signal. Schottkey barrier diodes seemed to work best due to their low turn-on voltage and low junction capacitance [4]. The rectifying circuit in figure 2 shows a classic full wave rectifier, outputting a stable DC voltage. Fig 2: Full-wave rectifier circuit using Schottkey diodes The value for C would be determined from the current at the load, which would be determined after knowing the load s resistance. Once rectified, the power could either be used directly, or be stored in the form of fuel cells for transportation. The method of use would characterize the load of the device, which should be, ideally, a real resistance. PROCEDURE The first step in designing the system was to model each element of the circuit. This was a non-linear process, as many elements had to be tuned as others change value. The crucial part of the system centered on the antennas. Qualifications for the transmitting (TX) antenna required that it must be highly directive, have high power-handling capacity, and be physically realizable. Directivity describes the power density as it is related to an isotropic source (spherical, unit of 1), and is based on the physical geometry of the radiator itself. It was found from research that the Kraus antenna design would provide reasonable directivity and size. It also provided better power-handling capabilities than the two previously mentioned layouts, due to self-resonance 20

5 and lower potential for dielectric breakdown. By writing a MATLAB script, a basic model for the helical design was quantified, based on equations written by Kraus for his antennas [6]. This included solving the Friis Transmission Formula (eq.2), which showed how much power would need to be radiated through the TX antenna, in order to overcome the path loss through an atmospheric medium and be received at the other end. P R P T = G T G R 4π R λ 2 Eq. 2 Here, Pt & Pr ate the transmitted and received powers, respectively, and similarly are Gt & Gr the respective gains of each antenna. The squared term is the path loss, where R is the distance between the two antennas. For these calculations, 100% efficiency was assumed, allowing directivity to equal the gain. This simplified calculations, as no data was available for efficiency of this antenna. Also, because the TX and RX antennas are identical Gt=Gr. Using simple calculations for a helical radiator, the starting point for the antenna system is defined by the variables in table 1. Table 1: Helical Antenna Parameters (here λ is wavelength) Antenna Variables Calculated Antenna Parameters Power Transfer Values Wavelength cm Gain 16.4 dbi Pr 40 dbm Circumference 1.1 λ Characteristic Impedance Ω Path Loss dbm # of Turns 12 Input Resistance 44 Ω Gt 16.4 dbi Spacing ¼ λ HPBW 34.1 Gr 16.4 dbi Between Coils Diameter 3.9 cm BWFN 60.4 Pt dbm For the complex calculations involved, a MATLAB script was written and the design software HFSS aided in 3D representations. Figures 3 & 4 shows the model for a Kraus antenna, based on the above parameters, and details of the feed point. 21

6 Fig 3: Kraus simulation model Fig 4: Detail of feed point A radiation pattern was produced, from these calculations, showing the high directivity of the design (fig. 5). Fig 5: Radiation field pattern for the Kraus antenna design When dealing with high frequencies, impedance matching is an all-important consideration. In order to deal with this, impedance-matching input and output networks are mandatory. These would be entirely based on the final impedances of the desired circuits to be interfaced, and would be in the implementation stage of the design. I intend to use microstrip design, with quarter-wave and stub-matching geometries. These are compact, double-sided substrate circuits whose dimensions are determined by frequency, dielectric material, and can be designed simply using a Smith chart. This tool allows one to take S-parameters, discovered from using a network analyzer, and convert them to impedance values. A schematic example of a quarter-wavelength matching network was simulated in Agilent s ADS software package for verification of this technique. For the matched-stub network on the rectenna side, a similar example was simulated and can be seen in figure 6. 22

7 Fig 6: ADS schematic for a matched-stub filter, center frequency 7GHz RESULTS From this research, a system design for a WPT network has been designed. Based on calculations and theoretical models, the transmission of 10W of power (40dBm) would require about 111 dbm (125 GW) of RF power at the transmitting antenna. The path loss amounted to 104 dbm, giving a transmission efficiency of ~35%. This is very low for a small scale system. This is a theoretical model, based on 100% efficiency assumption. A realistic model would involve obtaining measurements for the real system s impedance, or equivalently, modeling each individual component with its parameters and material compositions. This cannot be done at this level. It should be noted, however, that physical dimensions of the proposed design are physically realizable. DISCUSSION As this is research on a design, many aspects are left as variables, depending heavily upon the final physical parameters. More work is necessary to complete the theoretical design/ simulations, and carry it on to the testing phase. No small portion of this is quantifying the safety hazards present. RF power at this frequency is known to be dangerous. Such a high transmission power is extremely hazardous and expert-level certification would be necessary before this could be realizable. A concern is dielectric breakdown of the transmitter, due to the high power. This could be ameliorated by a different design. The low efficiency is common with wireless systems; however in a large-scale, SPS scenario, with unlimited solar power, this would be a small concern. These systems typically decay at an exponential rate relative to the distance from the source. Future work on the project is needed. Halfway through the design, the computer doing the simulations for HFSS crashed, so a 3D representation could not be produced. This should be 23

8 done for tuning to optimum directivity, which would increase overall efficiency. Another effort could also be made for a more suitable antenna, such as a corner reflector. CONCLUSION This research has allowed me to expand upon my basic knowledge of wireless energy. Though no breakthrough has come to light, it is hoped that the concepts expanded upon herein can be taken to the next level in future work and study. If such a design for SPS could someday be a reality, it could alleviate almost all dependency on fuel sources on our planet. ACKNOWLEDGEMENTS I would like to thank the HSGC for supporting my research, the University of Hawai i for providing me with an outstanding education, Drs. Z.Q. Yun, Magdy Iskander, Wayne Shiroma, and David Garmire for their talented illustrations of EM techniques and all they have taught me. REFERENCES [1] Rajen Biswa (2012), Feasibility of Wireless Power Transmission, Retrieved from Academia.edu website: [2] Sagolsem Kripachariya Singh, T.S. Hasaramani, R.M. Holmukhe, Wireless Transmission of Electrical Power Overview of Recent Research & Development, International Journal of Computer and Electrical Engineering, vol. 4, no. 2, April 2012 [3] Umenei, A. E. (2011), Understanding Low Frequency Non-Radiative Power Transfer, Fulton Innovation [4] Christopher R. Valenta, and Gregory D. Durgin, Harvesting Wireless Power, IEEE Microwave Magazine, vol. 15, no. 4, pp , June 2014 [5] M. Venkateswara Reddy, K. Sai Hemanth, CH. Venkat Mohan, Microwave Power Transmission - A Next Generation Power Transmission System, IOSR-JEEE, vol. 4, no. 5, pp , January 2013 [6] Helical Antenna Design Calculator, Internet: Dec. 24,