Electricity is the most versatile and widely used form of energy. The global demand for electricity is continuously growing. Of the total generation worldwide, more than 60 per cent of energy is generated using coal-fired station resulting in carbon dioxide emission threatening the global warming. To mitigate the consequence of climate change, generation systems need to undergo significant changes.
One of the major issues in power system is the losses occurring during the Transmission and Distribution (T&D) of electrical power. The percentage of loss of power during T&D is approximated as 26 per cent. The main reason for power loss during T&D is the resistance of wires used in grid. According to the World Resources Institute (WRI), India’s electricity grid has the highest T&D losses in the world – a whopping 27-40 per cent. Tesla had proposed methods of transmission of electricity using electromagnetic induction.
Tesla had always tried to introduce worldwide wireless power distribution system. But due to lack of funding and technology of that time, he was not able to complete the task. Research is being going on and recent developments have been observed in this field. Despite advances, wireless power transmission has not been adopted for commercial use.
Highlight of a wireless transmission in 1891 in Tesla’s “Experimental station” at Colorado is shown in figure 1.
In 1899 Sir Nicolas Tesla and Heinrich Hertz powered a fluorescent lamp keeping it 25 miles away from source. Wireless power transmission experiments were conducted at Warden Clyffe tower. High frequency current, of a Tesla coil, could light lamps filled with gas (like neon). In this method, a closed circuit is made using transmitter, ionized path between upper atmosphere and transmitter, second ionized path connecting receiver. The circuit back to the transmitter is completed through the earth. High potential is maintained at transmitter and receiver end as well. A high potential transmitter transmits an electromotive impulse through the ionized path to the upper atmosphere where it ionizes the air and this air between the transmitter and receiver would conduct like a neon tube.
Warden Clyffe tower was designed by Tesla for trans-Atlantic wireless telephony and also for demonstrating wireless electrical power transmission today.
Methods of Wireless Transmission of Electrical Power
Induction
The principle of mutual induction between two coils can be used for the transfer of electrical power without any physical contact in between. The simplest example of how mutual induction works is the transformer, where there is no physical contact between the primary and the secondary coils. The transfer of energy takes place due to electromagnetic coupling between the two coils.
Electromagnetic Transmission
Electromagnetic waves can also be used to transfer power without wires. By converting electricity into light, such as a laser beam, then firing this beam at a receiving target, such as a solar cell on a small aircraft, power can be beamed to a single target. This is generally known as “power beaming”.
Evanescent Wave Coupling
Researchers at MIT believe they have discovered a new way to wirelessly transfer power using non-radiative electromagnetic energy resonant tunneling. Since the electromagnetic waves would tunnel, they would not propagate through the air to be absorbed or wasted, and would not disrupt electronic devices or cause physical injury like microwave or radio transmission. Researchers anticipate up to 5 meters of range.
Electrodynamic Induction
Also, known as “resonant inductive coupling” resolves the main problem associated with non-resonant inductive coupling for wireless energy transfer; specifically, the dependence of efficiency on transmission distance. When resonant coupling is used, the transmitter and receiver inductors are tuned to a mutual frequency and the drive current is modified from a sinusoidal to a non-sinusoidal transient waveform. Pulse power transfer occurs over multiple cycles. In this way, significant power may be transmitted over a distance of up to a few times the size of the transmitter.
Radio and Microwave
Power transmission through radio waves can be made more directional, allowing longer distance power beaming with shorter wavelengths of electromagnetic radiation, typically, in the microwave range. A rectenna may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized. Power beaming using microwaves has been proposed for the transmission of energy from orbiting solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been considered.
Electrostatic Induction
Also, known as ‘capacitive coupling’ is an electric field gradient or differential capacitance between two elevated electrodes over a conducting ground plane for wireless energy transmission involving high frequency alternating current potential differences transmitted between two plates or nodes.
Current Technology in the Field Of Wireless Power Transmission
Microwave Transmitter
The most current research and proposals use microwaves as the frequency range of choice for transmission. At present an efficiency of 76 per cent is possible using current technology for microwave power transmission. For transmission efficiency, the waves must be focused so that all the energy transmitted by the source is incident on the wave collection device. Higher frequencies are also impractical because of the high cost of transmitters and the relative low efficiency of current optical and infrared devices.
The most common transmitters for microwaves are the travelling wave tube (TWT), klystron and magnetron. The TWT is far too expensive and power restrictive making it impractical for the task of power transmission. The klystron has been the DC to microwave converter of choice however it is also somewhat expensive. Many researchers are looking to use magnetrons instead because they are cheap and efficient. Magnetron frequency output is not as precisely controllable as the klystron or TWT but power transmission is more lenient to frequency fluctuations than communication systems are. One of the more common proposals would be for an array of magnetrons to be used as the transmitter. One of the main advantages to using many smaller magnetrons as opposed to a few klystrons is that 300 W to 1kW magnetrons are already mass produced for microwave ovens. The efficiency of magnetrons is inconsistently reported.
Use of Microwave Power Transmission in Solar Power Satellites (SPS)
Solar power generating satellites launched into space and transmitting power to Earth stations. This idea was first proposed in 1968 and all of the experiments have only been carried out in terrestrial laboratories. The SPS satellites would be put in high earth orbit at geosynchronous location. This would allow them to receive light 99 per cent of the year. A large rectenna array facility will be built on the Earth to collect the incoming microwaves. To maintain a good lock on the rectenna, the satellite will need to be built with a retro directive transmitter which locks on to a pilot beam emanated from the ground station.
Since most of the research is done in the 2.4 GHz to 5.8 GHz range, there is some spectrum regulatory issues to deal with. Also since the retro directive antenna system is unproven, there is the health concern that the microwave beam could veer off target and microwave some unsuspecting family. However, a Japanese Government agency is planning to send up 10 to 100 kW low earth orbit satellite to prove its feasibility.
Latest Invention & Experiments
WiTricity
The new technology called WiTricity is based on using coupled resonant objects. Two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off- resonant objects. After Nicolas Tesla, there was rebirth of this in 2007 by the team from Massachusetts Institute of Technology, who call their invention ‘WiTricity’. In the first successful trial of its kind, the team was able to illuminate a 60-watt light bulb 7ft away. They simulated a transfer of 60W across two identical loops similar in dimension. The coils had a radius of 30 cm, with a cross section of 3cm and distance between the coils was 200m. Basic principle is two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off-resonant object
The investigated design consists of two copper coils, each a self-resonant system. One of the coils, attached to the power source, is the sending unit. The resonant nature of the process ensures the strong interaction between the sending unit and the receiving unit, while the interaction with the rest of the environment is weak.
Solar Power Satellite
Future suitable and largest application of the WPT via microwave is a Space Solar Power Satellite (SPS). The SPS is a gigantic satellite designed as an electric power plant orbiting in the Geostationary Earth Orbit (GEO). It consists of three segments solar energy collector to convert the solar energy into DC (direct current) electricity, DC-to-microwave converter, and large antenna array to beam down the microwave power to the ground. The first solar collector can be either photovoltaic cells or solar thermal turbine. The second DC-to-microwave converter of the SPS can be either microwave tube system or semiconductor system. It may be their combination. The third segment is a gigantic antenna array.
An amplitude taper on the transmitting antenna is adopted in order to increase the beam collection efficiency and to decrease sidelobe level in almost all SPS design. A typical amplitude taper is called 10 dB Gaussian in which the power density in the center of the transmitting antenna is ten times larger than that on the edge of the transmitting antenna.
The SPS is expected to realise around 2030. Before the realisation of the SPS, we can consider the other application of the WPT. In recent years, mobile devices advance quickly and require decreasing power consumption. It means that we can use the diffused weak microwave power as a power source of the mobile devices with low power consumption such as RF-ID. The RF-ID is a radio IC- tug with wireless power transmission and wireless information. This is a new WPT application like broadcasting.
Antennas for Microwave Power Transmission
All antennas can be applied for both the MPT system and communication system, for example, Yagi-Uda antenna, horn antenna, parabolic antenna, microstrip antenna, phased array antenna or any other type of antenna. To fixed target of the MPT system, usually large parabolic antenna selected in MPT demonstration in 1975 at the Venus Site of JPL Goldstone Facility and inground-to-ground MPT experiment in 1994-95 in Japan. In the fuel-free airship light experiment with MPT in 1995 in Japan, they changed a direction of the parabolic antenna to chase the moving airship. However, we have to use a phased array antenna for the MPT from/to moving transmitter or receiver which include the SPS because we have to control a microwave beam direction accurately and speedy. Power distribution at the transmitting antenna is given by (1-r²), where r is the radius of antenna.
The phased array is a directive antenna which generates a beam form whose shape and direction by the relative phases and amplitudes of the waves at the individual antenna elements. It is possible to steer the direction of the microwave beam. The antenna elements might be dipoles, slot antennas, or any other type of antenna, even parabolic antennas. In some MPT experiments in Japan, the phased array antenna was adopted to steer a direction of the microwave beam. All SPS is designed with the phased array antenna. We consider the phased array antenna for all following MPT system.
Japan wants to power up three million houses with wireless energy from space. They have serious plans to send a solar-panel-equipped satellite into space that could wirelessly beam a giga watt-strong stream of power down to earth. A small test model is scheduled for launch in 2015. To iron out all the kinks and get a fully functional system set up is estimated to take three decades. A major kink, presumably, is coping with the possible dangers when 1-gigawatt microwave beam aimed at small spot on Earth misses its target. The $21 billion project just received major backing from Mitsubishi and designer IHI (in addition to research teams from 14 other countries).
Wireless Power Transmission
Components of WPT system: The Primary components of Wireless Power Transmission are Microwave Generator, transmitting antenna and Receiving antenna (Rectenna).
Transmission System
In the transmission side, the microwave power source generates microwave power and the output power is controlled by electronic control circuits. The waveguide circulator which protects the microwave source from reflected power is connected with the microwave power source through the coax- waveguide adaptor. The tuner matches the impedance between the transmitting antenna and the microwave source. The transmitting antenna radiates the power uniformly through free space to the rectenna impedance matching is the practice of designing the input impedance electrical load output impedance to maximise the power transfer or minimise reflections from the load.
Magnetron
Magnetron is a crossed field tube which forces electrons emitted from the cathode to take cyclonical path to the anode. The magnetron is self-oscillatory device in which the anode contains a resonant RF structure. The magnetron has long history from invention by A W Hull in1921. The practical and efficient magnetron tube gathered world interest only after K. Okabe Average RF output power versus frequency for various electronic devices and semiconductors.
Recent Technologies
A microwave power transmission is suitable for a power transmission from/to moving transmitters or targets. Therefore, accurate target detection and high efficient beam forming are important. Retro directive system is always used for SPS. A corner reflector is the most basic retro directive system. The corner reflectors consist of perpendicular metal sheets, which meet at an apex. Incoming signals are reflected back in the direction of arrival through multiple reflections off the wall of the reflector. Van Atta array is also a basic technique of the retro directive system. This array is made up of pairs of antennas spaced equidistant from the center of the array, and connected with equal length transmission lines. The signal received by an antenna is re-radiated by its pair, thus, the order of re-radiating elements is inverted with respect to the center of the array, achieving the proper phasing for retro directivity. Usual retro directive system has phase conjugate circuits in each receiving or transmitting antenna, which plays same role as pairs of antennas spaced equidistant from the center of the array in Van Atta array. A signal transmitted from the target is received and re-radiated through the phase conjugate circuit to the direction of target. The signal is called a pilot signal. We do not need any phase shifters for beam forming. The retro directive system is usually used for satellite communication, wireless LAN, military, etc. There are many researches of the retro directive system for these applications. They use almost same frequency for the pilot signal and returned signal with a local oscillator (LO) signal at a frequency twice as high as the pilot signal frequency in the typical retro directive systems. Accuracy depends on stability of the frequency of the pilot signal and the LO signal. The retro directive system unifies target detection with beam forming by the phase conjugate circuits. There are some methods for target detecting with pilot signal which is separated to beam forming. We call the method ‘software retro directive’.
Computer is usually used for the software recto directive with the phase data from a pilot signal and for the beam forming with calculation of the optimum phase and amplitude distribution on the array. In the software recto directive, we conform microwave beam freely, for example, multi-beams. On contrary, we need phase shifters in all antennas.
Conclusion
Wireless power transmission of electrical power can be considered as a large scope in future prospects of power generation and transfer. Solar power satellites are the future for supplying non- conventional energy. The various methods and aspects regarding wireless transmission of electrical power and the details of design of solar power satellite have been discussed. The evolution of the technology from the time of Tesla has been overviewed.
Image courtesy: Authors
