Keywords

1 Introduction

The wireless power transmission technology refers to a technology for transferring power from a source to a load without relying on an power transmission line. According to the transmission mode, it can be classified into electromagnetic induction, magnetic coupling resonance, microwave, laser, ultrasonic, etc. With the breakthrough of wireless energy transmission power, transmission distance and transmission efficiency, the research and application of this technology in the fields of aerospace, rail transit, electric vehicles, household appliances, human implanted devices and weapons and equipment have developed rapidly in recent years [1].

Wireless energy transmission is not a new concept recently. As early as the 17th century, American physicist Tesla first proposed and based on the idea of wireless transmission [2]. He used a Tesla coil with a ball 60 m high and 90 cm in diameter. The masts of the toroidal coils are connected together to produce a resonance of 150 kHz to supply electrical energy. Later, he proposed the grand concept of wireless energy transmission on a global scale, and built the Woden Cliff Tower that shocked the world at that time. Tesla’s wireless transmission method is to use the earth as the inner conductor and the earth’s ionosphere as the outer conductor. By amplifying the transmitter in a radial electromagnetic wave oscillation mode, a low-frequency resonance of about 8 Hz is established between the earth and the ionosphere. The surface electromagnetic waves that surround the earth are used to transmit energy. Although the feasibility of this scheme has been fully confirmed in theory, Tesla’s bold idea has not been realized due to insufficient financial resources. Figure 1 shows Tesla’s laboratory in Long Island, New York, USA.

Fig. 1.
figure 1

Tesla wireless energy transmission test

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2 Magnetically Coupled Wireless Power Transfer Technology

2.1 Fundamentals

The magnetic coupling resonant wireless energy transmission technology utilizes the principle of resonance to reasonably set the parameters of the transmitting device and the receiving device, so that the transmitting coil and the receiving coil and the whole system have the same resonant frequency, and the system is driven by the power of the resonant frequency. An “electrical resonance” state can be achieved, enabling efficient transfer of energy at the transmitting and receiving ends. When energy is applied to the transmitting coil, a strong coupling relationship is established between the two coils and a large proportion of energy exchange occurs. When the receiving coil is connected with a power load, the load absorbs part of the energy, thereby realizing wireless transmission of electric energy, and its working principle is shown in Fig. 2.

Fig. 2.
figure 2

Magnetically coupled wireless energy transfer schematic

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In November 2006, at the American Physical Society’s Industrial Physics Forum, the Marin Soljacic research team at the Massachusetts Institute of Technology first proposed the theory of magnetic resonance wireless transmission, and then published an article in the journal Science, through the principle of magnetic resonance. Achieving 60 W wireless transmission within 2 m, the whole system efficiency is 40%. In the experiment, a pair of coil antennas with 5 mm copper wire wound 5.25 turns and diameter 600 mm and distributed inductance and capacitance characteristics are used. The LC resonant frequency is 9.90 MHz.

2.2 Key Technology

  1. (1)

    High frequency power supply technology

The reason why magnetically coupled resonant wireless energy transmission can be efficiently transmitted depends mainly on whether the system can work in the resonant state. Especially in the aspect of high-power energy transmission, the power supply not only can provide sufficient driving capability but also has corresponding output frequency. The high-power power modes that can be realized under the Hertz level are mainly oscillating, inverter and power amplification.

  1. (2)

    Resonant coil design technology

The optimization of the parameters of the resonator coil is mainly based on the number of turns of the coil itself, the winding method, the pitch spacing design, the material selection, etc., and the resonator coil is optimized in combination with the output performance requirements of the system. The efficiency of the system is improved by optimizing the number of turns of the coil at a given frequency and by varying the radius of the coil.

  1. (3)

    Frequency tracking optimization

Frequency splitting is a common phenomenon of magnetically coupled resonant wireless energy transmission technology, and it is a major factor affecting the transmission efficiency of the system. At present, the optimization and control of resonant wireless energy transmission are mostly realized around the resonant frequency. The high quality factor of the resonator coil often leads to poor stability of the system during operation. In order to improve the stability of the system, the closed-loop tracking control of the phase-locked loop can be used to solve the problem of poor stability of the resonant wireless energy transmission system.

2.3 Application Status

Domestically, it started late in magnetic coupling wireless energy transmission. It has developed rapidly in recent years, and research hot spots are mainly concentrated in the field of electric vehicle charging. China Electric Power Research Institute built a 150 m electric vehicle wireless charging test section in December 2017. It is the longest and highest power mobile wireless charging test in China.

Magnetically coupled wireless energy transmission technology can achieve better transmission efficiency within a distance of less than 2 m, and can pass through obstacles such as wood, plastic, and walls. It is the development direction of medium and short-range wireless energy transmission technology, but as the transmission distance increases, Its efficiency drops rapidly, and energy loss is more serious at longer distance.

3 Laser Wireless Energy Transfer Technology

3.1 Fundamentals

Laser wireless energy transmission is a high-energy laser beam as an energy carrier, which is transmitted by a collimating optical system, and uses a laser battery array to convert light energy into electrical energy at a remote end to realize long-distance wireless energy transmission. The working principle is shown in Fig. 3.

Fig. 3.
figure 3

Laser wireless energy transmission schematic

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3.2 Key Technology

The laser wireless energy transmission converts the electric energy into a laser beam of a certain wavelength and certain technical requirements through the laser, and is collimated by the optical system and transmitted through the spatial link; the receiving end receives the laser, and the input optical energy is passed through the high efficiency photoelectric converter. Converted to electrical energy, photoelectric energy conversion, power supply equipment. Its key technologies mainly include:

  1. (1)

    High efficiency and high beam quality laser emitter

The transmitting end of the laser wireless energy transmission system mainly converts the electric energy into a laser beam of a certain wavelength and a certain technical requirement through a laser, and is collimated by the optical system and transmitted through the space link. It is necessary to analyze the far-field distribution characteristics according to the characteristics of the laser, and study the beam conversion technology of the high-power laser array and the microlens structure of the laser array collimation.

  1. (2)

    High-efficiency laser battery photoelectric conversion technology

Ordinary solar cells are designed for the solar spectrum structure, and they have high laser reflection in practical applications, and cannot be directly applied to the laser-electric energy conversion module. The development of laser energy conversion devices with specific surface light-limiting structures is the key technology for realizing laser wireless energy transmission, including spectral matching laser cell material technology, laser cell process loss control technology, and laser cell structure radiation-resistant reinforcement. Technology, laser battery composite cooling technology, etc.

  1. (3)

    Laser energy control management technology

The output characteristics of the laser battery have nonlinear characteristics. The output power is maximum only when the voltage value is output at a certain point. The maximum power point tracking technology, energy storage technology and energy management technology of the laser battery are studied to improve the effective utilization of laser wireless energy transmission.

3.3 Application Status

Japan has been conducting ground-based experiments on laser wireless energy transmission technology since 1997. Typical experiments, such as the laser power supply experiment of kite aircraft completed by Gyeonggi University in Japan in 2006. The two fiber-coupled semiconductor lasers are excited by a power supply of about 600 W to generate about 200 W of laser energy, and the GaAs battery array is irradiated to generate 46 W of electric energy, and the kite aircraft of about 50 m in height is continuously and stably flying for more than one hour. Among them, the laser conversion efficiency is 34.2%, the conversion efficiency of GaAs photovoltaic cells is 21%, and the overall photoelectric conversion efficiency is 7.2%.

At present, China began to pay attention to laser wireless energy transmission technology from 2012. Aerospace Science and Technology Group 811 carried out various types of laser cells such as single crystal silicon, indium gallium phosphide and gallium arsenide under the conditions of 532 nm, 633 nm, 808 nm and 1064 nm laser light sources. The photoelectric test of the material system explored the law of photoelectric conversion efficiency and laser wavelength of the battery.

Laser wireless energy transmission has the advantages of high energy density, good energy convergence, small emission and receiving aperture, etc., and can maintain the beam concentration after long-distance transmission, and the beam is easy to focus, and has better directivity, and has long-distance energy transmission. Good development prospects. However, at the same time, its penetration performance is poor, and it is easily affected by the intermediate medium, and the dust mist will seriously affect the laser transmission.

4 Microwave Wireless Energy Transmission Technology

4.1 Fundamentals

Microwave wireless energy transmission refers to the use of microwave devices to convert electrical energy into electromagnetic energy [3]. The microwave electromagnetic energy of the space is transmitted wirelessly through the transmitting antenna, and the receiving antenna converts the electromagnetic energy into electrical energy, which is used for the electric load after a certain electric energy conversion. The working principle is shown in Fig. 4.

Fig. 4.
figure 4

Schematic diagram of power transmission using microwave as carrier

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In 1975, the US Jet Propulsion Laboratory established the world’s largest power microwave radio transmission test device at Goldstone, successfully transmitting 30 kW of energy through a 26 m diameter parabolic antenna at 2.5 GHz to a silicon rectifier diode antenna at 1.6 km [4].

4.2 Key Technology

Microwave long-distance wireless transmission can convert space into microwave electromagnetic energy through high-power microwave source to realize space transmission, and then adopt high-power and high-efficiency microwave rectification technology through the receiving section. The key technologies include:

  1. (1)

    High current density microwave source technology

Field emission electrons generally require the surface electric field strength of the material to reach the order of GV/m. The strong electric field on the chip will lead to chip breakdown, and the nanometer structure can effectively reduce the required electric field strength. At the same time, the output power of the radiation source is proportional to the current density of the electron source, and the on-chip electron source needs to have a low threshold and a large current density characteristic at the same time. Therefore, it is necessary to study the electron emission characteristics of different nanostructures and nanomaterials under different pulsed electromagnetic fields from a quantum angle. The mechanism of nanomaterials emission under different frequency electromagnetic fields was studied, and a low turn-on voltage, high current density, and miniaturized free electron microwave emission source were developed.

  1. (2)

    Spatial microwave non-diffraction focusing transmission technology

Due to the volatility of electromagnetic waves, microwaves exhibit diffusion transmission in space, which will result in a decrease in energy density at the target at a certain distance. A non-diffracting beam is a set of ideal solutions that satisfy the electromagnetic scalar equation, such as plane waves, Bessel beams, Cosine beams, and so on. Based on the structured non-diffracting beam design, long-distance focus transmission can be realized, and the new microwave control technology can be used to realize the non-diffracting energy transmission to the 100-m or even-kilometer target by precisely controlling the phase and timing of the radiating element. At the same time, due to the depth of focus It will directly affect the wireless energy transmission performance to unmanned devices, and achieve ultra-long-range focus depth through structured design.

  1. (3)

    Microwave rectenna design and high efficiency energy harvesting technology

The microwave receiving antenna and rectifier circuit design is the key to the electromagnetic energy-electric energy conversion efficiency of the receiving end in the wireless energy transmission architecture. Combined with the electromagnetic super-surface electromagnetic wave regulation method, the array patch microstrip antenna structure is studied to ensure the wide-band and wide-beam performance, and has a small size and a low height, which can meet the needs of the array and the device, and realize high efficiency. The principle and characteristics of electromagnetic wave energy capture and collection. Study high-efficiency microwave rectifier circuit design, RF or DC power synthesis technology, power conversion technology, including RF control, impedance matching, bandpass filtering and energy storage structure.

4.3 Application Status

Mitsubishi researchers in Japan converted 10 kW of electricity into microwaves and then used wireless energy transmission. Some of the power successfully illuminates the LEDs on the receiving device 500 m away. This is by far the longest and most powerful microwave in Japan [5].

Domestic research on microwave wireless energy transmission technology began in the 1990s. Lin Wei Qian first introduced microwave wireless energy transmission technology in China. University of Electronic Science and Technology, Shanghai University and other universities have carried out research on microwave energy wireless transmission, mainly involving medium-range and low-power microwave wireless energy transmission systems. The overall conversion efficiency is low, and most of the research is still in theoretical research or initial stage [6].

Microwave wireless energy transmission technology has high conversion and transmission efficiency, and the atmospheric and cloud penetration in a specific frequency band is very good, the beam power density is low, and high-precision pointing control can be performed through the beam, which has high security. However, due to the beam width, the size of the transmitting and receiving antennas is very large, so the miniaturization of the antenna becomes a problem that restricts the engineering application of microwave wireless energy transmission [7].

5 Summary

Compared with traditional wired transmission methods, wireless energy transmission technology has a very broad application prospect in the fields of transportation, medical electronics, consumer electronics and space solar energy, and is a revolutionary advancement of energy transmission technology. With the development of electronic devices, power conversion and measurement and control technologies, wireless energy transmission technology has gradually made breakthroughs in power, distance and efficiency, and will provide a safer, more reliable and flexible transmission method for global energy interconnection.