# Methods and Applications of Wireless Power Transfer

March 16, 2020 by Abdulwaliy Oyekunle
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The concept of wireless power transfer (or WPT) was first developed by Nikola Tesla in the late 19th century. The near-field and the far-field are two techniques to consider when making wireless power transfer designs.

### Methods of Wireless Power Transfer

There are various techniques used to achieve wireless power transfer by electrical engineers. The methods incorporate designs for near-field and far-field wireless power transfer. These methods are discussed below:

#### Inductive Coupling

The inductive coupling method for wireless power transfer is non-radiative and near-field energy transfer. The method employs the principles of electromagnetic induction like a transformer.

Here, two coils are used in the designâ€”the primary and the secondary coil. The primary coil is used to generate a time-varying magnetic field for the secondary coil to induce a current. One advantage of using the inductive coupling for wireless power transfer is that it doesnâ€™t require contacts between the power transmitter and the power receiver.

#### Resonance Coupling

Resonance coupling is a near-field, non-radiative wireless power transfer technique developed by researchers at the Massachusetts Institute of Technology (MIT). This method comprises a source resonator and a load resonator. The source resonator emits a lossless non-radiative magnetic field oscillating at megahertz frequencies, which mediates an efficient power transfer between the source and the load resonator.

### Power Transfer by Radio Waves and Optical Waves

For far-field wireless power transfer, radio waves and microwaves can be utilised. These waves have short wavelengths and high frequencies, power transfer by this method is radiative.

A receiver circuit that comprises an antenna and a rectifier circuit is fundamentally used to construct a rectenna in electric circuits. The rectifier circuit helps convert alternating electromagnetic power to DC power. The rectenna is used to achieve wireless power transfer by radio waves.

One advantage of using radio waves or microwaves for wireless power transfer is the ability of the waves to travel with a speed as high as the speed of light. This makes it possible to transmit power in a wide range of distance.

Power transfer by optical waves can be achieved by the use of light amplification using stimulated emission of radiation (aka LASER). In this method, power transmission happens by converting electricity into a beam of laser to be directed towards a solar cell receiver.

A primary winding coil used in inductive coupling. Image Credit: Imperial College.

#### Power Transfer by Piezoelectric

This is a non-electromagnetic means for wireless power transfer. In this method for wireless power transfer, piezoelectric crystals are utilised. The crystals which include Quartz, Rochelle salt, Berlinite, Topaz and so on can convert mechanical energy to electrical energy and vice-versa.

Wireless power transfer is achieved by piezoelectric effects which are caused by vibrations and can be direct or inverse. A direct piezoelectric effect happens when mechanical stress is applied to materials generating electric potentials. An inverse piezoelectric effect occurs when there is a material deformation upon application of electric field to the materials.

The efficiency of power transfer by piezoelectric is low and can only be used in short-range transmission.

### Applications of Wireless Power Transfer

There is a wide range of areas that leverages on wireless power transfer technologies. Some of the potential applications of wireless power transfer in consumer electronics include wireless mobile chargers and floor lighting.

In biomedical, wireless power transfer finds applications in the delivery of wireless energy to biomedical sensors and implantable devices. Brain-machine interface and neural recording systems are also areas where wireless power transfer technology can be used in medicals.

Furthermore, the technology is chiefly used in automobile applications in electric vehicles for charging batteries. For industrial applications, it can be used in instrumented wheelsets.

Other applications of wireless power transfer can be seen in radio-frequency identification (aka RFID), magnetic field communication systems, aerospace and wireless sensor networks.

A smartphone being charged using a remote charging pad.

### Challenges Faced by Wireless Power Transfer Technology

Engineers face challenges when implementing the methods of wireless power transfer discussed earlier in their designs. The near field and the far-field have limitations that engineers must be wary of when dealing with wireless power transfer technology.

In inductive coupling, energy decays between the power transmitter coils and the power receiver coils when the distance between them increases. This brings about low efficiency and not suitable for far-field applications.

Also, when a non-resonant object such as the human body is very close to the transmitting and receiving element in the resonant coupling method of wireless power transfer, it reduces the efficiency of the transmitting energy and attenuates the power.

As the microwave method is radiative, there are major health concerns associated with it as it poses health risks like cancer when human is exposed to the radiation. Apart from that, designing a practicable antenna to receive power is a challenge that engineers face when dealing with this method.

The major challenge in power transfer by an optical wave is that it is expensive to carry out and solar cell receivers can be inefficient.