There have been substantial advancements in the different forms of Wireless Power Transfer (WPT) technology in recent years. Emerging from the theoretical realm, WPT is now utilized in household appliances. While inductive coupling works over a distance of a few millimeters, magnetic resonant coupling systems can achieve wireless power transmission for a few meters and have increased efficiency [1][2]. The concept of inductive coupling and some common applications are discussed in Chapter One of this series, with magnetic resonance coupling discussed in Chapter Two.

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'Electromagnetic Radiation' is the most efficient method of transmitting power across long distances [3]. Research on the possibilities of transmitting power from orbiting satellites started back in the 1950s. A notable early use of WPT occurred in 1975 when NASA successfully transmitted 30 kW of power over a distance of one mile at 50% efficiency using beamed microwaves [4]. Table 1 provides a basic summary of the three methods of WPT.

WPT Type	Initial Range	Power Delivery	Function
Inductive Coupling	Short	High	Communication
Magnetic Resonant Coupling	Medium	High/Medium	Power
Electromagnetic Radiation	Long	Low	Power and Communication Sensor

^{Table 1: A summary of the different forms of WPT, with attributes.}

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Electromagnetic Radiation

Wireless power transfer by means of electromagnetic radiation (ER) is an emerging technology. Unlike the other two methods, in ER power transfer the transmitting and receiving antennas are not electromagnetically coupled. This allows for WPT to be implemented over longer distances. The increased range has proven to be useful in applications where power transmissions must cover large gaps. The most efficient forms of ER are visible light from lasers and microwaves.

Microwave Power Transmission

Microwave power transmission is capable of sending large amounts of power to a receiving station using radio waves. The wavelength of these radio waves range from 1-30 GHz. The transmitter needs to have line-of-sight with the receiver for the technology to work. A device known as a Magnetron is often used because of its efficiency and flexibility to be used for either power transmission or communication systems.

This form of WPT works as follows:

• AC current is converted into DC current using a rectifier.
• The DC current is fed into the magnetron, where microwaves are generated.
• Microwaves are sent through an RF cable to a circulator where they are transmitted.
• A rectifying antenna is used as a receiver to capture and convert microwaves back to DC current.
• An inverter converts the DC current into AC current where it can be applied to an electrical load [5].

Microwave power transmission can achieve up to 75% efficiency in ideal conditions. There is active research into using this method for transmitting energy from Solar Power Satellites (SPS).

Laser Power Transfer

Laser power transfer technology works by concentrating power into a small area with mirrors. It has the potential to transmit large amounts of power over a long distance. An efficient way of transmitting wireless power using laser beams is called ‘Direct pumping’. Recent studies on this method have revealed the advantages of using semiconducting materials in laser medium [6].

The process of direct pumping works like this:

• Solar energy from sunlight are absorbed directly with solar collectors.
• The absorbed energy is then routed to the emitter which generates directional electromagnetic radiation in form of laser beams.
• A photovoltaic panel is used to collect the beamed laser energy at receiving end and covert it to DC current.
• Finally, an inverter is used to convert the DC current to AC output.

One major drawback of this method is that the transmitted power is attenuated as it propagates through the atmosphere. During transmission, some of the energy of the laser is lost through the generation of heat [6]. Researchers are currently investigating using polymer films as a way to minimize this loss.

Laser technology is being used in remotely-controlled rovers to search for the presence of ice in lunar craters. The initial energy is captured by solar collectors and converted into laser energy before being transmitted to the rover [3].

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Applications of ER

Electromagnetic Radiation (ER) has a number of applications ranging from electric vehicles to space rover operation. Researchers are continually trying to improve the efficiency and functionality of this technology.

Wireless Health Monitoring in Spacecraft

The ability to monitor the health and safety of astronauts in space is critically important. Using wireless transmission for this purpose enables more flexibility as well as efficient remote control of the over monitoring equipment. Multi-Input Multi-Output (MIMO) based stations are set up with a sufficient number of sensor tags to capture all pertinent data. The implementation of wireless technology for health monitoring in spacecraft offers higher reliability and comparatively lower expense than wired sensors [5].

Powering Space Rovers

A group of Japanese researchers have developed an exploration rover that could operate on Mars using microwave power transmission. The rover is equipped with a 120 W, 600x340 mm² patch antenna array [5]. Powering rovers using WPT technology solves many issues related to efficiently and remotely ensuring continued operation.

Electric Vehicles

Electric Vehicles can now be charged using microwave power transmission. At Kyoto University of Japan, researchers have developed a wireless charging station with 125 kW of power to charge city distribution trucks. A rectifying microwave antenna with a power density of 100 kW/m² has been developed to transmit the power over a distance of 1-6 m [7][8].

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Biological Exposure To WPT

Wireless power transmission systems have been used for decades. With technological advancements in WPT, the impact of electromagnetic radiation on biological tissues is increasingly an issue of concern. Larger fields may be induced in human tissues due to the strength of the electromagnetic field (EMF). Researchers are working to demonstrate compliance of WPT with exposure limits to EMFs [9].

The first study that considered the effects of EMF on human health was performed in 1979. A research group containing epidemiologist N. Wertheimer and physicist E. Leeper noticed the rate of children affected with the cancer ‘leukemia’ was up to twice as high among those who lived near high tension power lines that produce strong EMFs [9].

People who experience significant exposure to EMFs are at risk of adverse health effects. The effects of EMF exposure in humans depends on factors such as:

• Radiation type

• Amount absorbed

• Rate of absorption

• Radio-sensitivity of the cell

• Distance from the source, etc.

   

The amount of EMF radiation is measured in a unit called grays (gy), sometimes referred to as the ‘Specific Absorption Rate’ (SAR). The most convenient way of calculating the SAR is with the following equation:

SAR = σ E² / ρ

Where, σ represents the conductivity of the tissue involved (Siemens/meter), E is the strength of the electric field induced in the tissue (Volts/meter), and ρ is the mass density in kg/m³. A human body absorbing an EMF of 1.5-10 gy will lead to the destruction of bone marrow and infections. The reduction of fluids and electrolytes in intercellular space can occur with exposure to EMFs ranging from 10-40 gy. Absorption of more than 400 gy can cause death within 48 hours by damaging the human cardiovascular system. The effects of radiation is also dependent on the sensitivity and health of the individual. This can lead to situations where people with the same body weight have drastically different responses when exposed to the same amount of radiation [9].

Harmful radiation originating from home appliances like wireless charging docks is not likely due to the small amount of power being transferred and the short range. However, technology like solar powered satellites will not be commercially available until researchers are able to reduce the amount of radiation to a standard that is safe for humans. A basic guideline of the maximum exposure limits for EMFs is shown in the tables below [10],

For CNS Tissue of the Head

Frequency Range	Internal Electric Field (V/m)
1-10 Hz	0.1
10-25 Hz	0.01
25-1000 Hz	4x10-4
1-3 kHz	0.4
3 KHz - 10 MHz	1.35x10-4

For all tissues of the head and body

Frequency Range	Internal Electric Field (V/m)
1 Hz - 3 kHz	0.4
3 KHz - 10 MHz	1.35x10-4

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Conclusions

The modern era of technological advancements brings endless possibilities for state-of-the-art innovations that can assist us in everyday life. WPT has the potential to revolutionize the energy industry. Shorter-range applications of WPT will continue to replace the traditional wired charging systems used in home appliances. Mid-range WPT technology of magnetic resonant coupling already has several applications including electric vehicle charging, remote sensing, and biomedical implants. For long range applications, WPT has a promising future in spacecraft and in harvesting energy from solar powered satellites. As WPT begins to overcome some of the remaining obstacles pertaining to health issues, it is expected to be one of the most useful and promising technologies in upcoming decade.

References

_{[1] A. Kurs, A. Karalis, R. Moffatt, J. Joannopoulos, P. Fisher, and M. Soljacˇic´, ‘‘Wireless power transfer via strongly coupled magnetic resonances’’, Science, vol. 317, no. 5834, pp. 83–86, Accessed on 27th April, 2018.
[2] A. Karalis, J. Joannopoulos, and M. Soljacic, ‘‘Efficient wireless non-radiative mid-range energy transfer’’, Annals of Physics, vol. 323, no. 1, pp. 34–48, Accessed on 27th April, 2018.
[3] W.C. Brown & E. E. Eves., “Beamed Microwave Power Transmission and its Application to Space.”, iEEE Xplore, Accessed on 27th April, 2018.
[4] R.M. Dickinson & W.C. Brown, “Radiated Microwave Power Transmission System Efficiency Measurements.”, NASA Technical Reports, Accessed on 27th April, 2018.
[5] S. Kawasaki, Y. Kobayashi, S. Yoshida “High-Power, High-Efficiency Microwave Circuits and Modules for Wireless Power Transfer Based on Green-Eco Technology”, iEEE Xplore, Accessed on 27th April, 2018.
[6] M. Weksler, J. Shwartz “Solar-pumped solid-state lasers”, iEEE Xplore, Accessed on 27th April, 2018.
[7] N. Shinohara, Y. Kubo, and H. Tonomura, “Mid-Distance Wireless Power Transmission for Electric Truck via Microwaves”, iEEE Xplore, Accessed on 27th April, 2018.
[8] N. Shinohara, Y. Kubo, and H. Tonomura, “Wireless Charging for Electric Vehicle with Microwaves”, iEEE Xplore, Accessed on 27th April, 2018.
[9] B. O. Anyaka, U. B. Akuru, “Electromagnetic Wave Effect on Human Health: Challenges for Developing Countries”, iEEE Xplore, Accessed on 27th April, 2018.
[10] International Commission on Non-Ionizing Radiation Protection, “Guidelines For Limiting Exposure To Time-Varying Electric And Magnetic Fields (1 Hz To 100 Khz)”, Health Physics, vol. 99, no. 6, pp. 818-836, Accessed on 27th April, 2018.}

A big thanks to @gra for the huge support in mentoring this article.

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