Wireless Power Transfer....
Wireless Power Transfer is the transmission of Electrical energy from a source to Grid or consumer's load through space, without the use of conducting wires. Several researchers and world renowned scientists have been working on Wireless Power Transfer (WPT) from as early as 1900. Nikola Tesla was the first to propose WPT through the Wardenclyffe tower or Tesla tower that he designed and built at Shoreham, New York in the early twentieth century. The facility was not completed because of severe financial crisis.
Many other scientists followed in the foot steps of Nikola Tesla, but it was only in the year 2007 that a bunch of scientists from MIT (Massachusetts Institute of Technology) demonstrated wireless power transfer. They called their technology as "WiTricity". They were a able to power a 60W incandescent lamp through WiTricity at a distance of a little more than 2 metres with a transmission efficiency of 45%. Recently there has been a lot of buzz going on around WPT.
Nikola Tesla's Wardenclyffe Tower for Wireless Power Tranmission
Photo Source: Wikipedia
Many other scientists followed in the foot steps of Nikola Tesla, but it was only in the year 2007 that a bunch of scientists from MIT (Massachusetts Institute of Technology) demonstrated wireless power transfer. They called their technology as "WiTricity". They were a able to power a 60W incandescent lamp through WiTricity at a distance of a little more than 2 metres with a transmission efficiency of 45%. Recently there has been a lot of buzz going on around WPT.
Team Witricity along with their experimental set up.
Resonant Power Transfer - A 60 W lamp placed at a little more than two meters from source illuminated by resonant magnetic coupling.
There are two major categories under which the all the work related to WPT are carried out all these years. The categories are Non-Radiative or Near-field techniques and Radiative or Far-field techniques of wireless power transmission. Some of the techniques of wireless power transfer that can be brought under Non-Radiative techniques are Magnetic inductive coupling, Resonant inductive coupling, Capacitive coupling, Resonant capacitive coupling, Atmospheric plasma channel coupling and Magnetodynamic coupling. This technique is used in applications like electric tooth brush chargers, RFID tags, wireless chargers for mobiles, tablets and laptops, smartcards and chargers for implantable medical devices.
Some of radiative techniques are Microwave power transmission and Laser power transmission. Whatever may be the category, any WPT system should have a power transmitter at one end connected to an AC source, a power receiver at the other end connected to the load and air medium through which the power must be transmitted over a distance. The transmitter's antenna transfers the energy to receiver through air medium in the form of electromagnetic waves. Antenna in the power transmitter may be a coil generating magnetic field, a plate electrode that generates electric field, a laser generating light or an antenna that emits radio waves.
Block Diagram of a Wireless Power System
Image Courtesy: Wikipedia
A similar antenna available in the power receiver receives the electromagnetic wave from air medium and passes it on for conversion to appropriate Electrical signals. Accelerating charge particles (current) of antenna gives rise to electromagnetic waves. Electromagnetic wave consists of synchronized electric and magnetic fields mutually perpendicular to each other and perpendicular to the direction of propagation of the wave. Electromagnetic waves travel at the speed of light in vacuum. Electromagnetic waves carry the energy away from the source particles and deliver it to the objects with which they interact.
Propagation of Electromagnetic wave
Image Courtesy: Wikipedia
The Electromagnetic field created by accelerating charges can be divided into two regions
1. Near field or non-radiative region and
2. Far field or radiative region
The far field is that part of the Electromagnetic field that has traveled sufficient distance from the transmitter antenna producing it and has become independent of the source. The electromagnetic field exist as a transverse wave and it does not allow the transmitter to affect them. These fields contribute to Electromagnetic radiation. They propagate freely in space carrying their energy away from the source.
Another part of the electromagnetic field is that which exists with about 1 wavelength of the power transmitter antenna is called the near field. In the near field region electric and magnetic fields exist separately as their corresponding dipoles and their strength decreases rapidly with increase in distance from their source. These fields do not contribute to electromagnetic radiation. When a power receiver starts receiving energy from the near field, the electromagnetic wave starts loading the power transmitter. When there is no receiver to tap energy from near field, the energy is returned back to the transmitter. This is because of the oscillatory (does not propagate as far field) nature of the near field. The boundary between these two regions is not clearly defined and there are different techniques to transfer power in these two regions as stated above.
The biological effects Electromagnetic Radiation (which refers to the far field of electromagnetic waves) depends on the power and frequency of the electromagnetic radiation. The electromagnetic spectrum in the order of increasing frequency (decreasing wavelength) and increasing photon energy is given as radio waves, microwave waves, infra red, visible light, ultraviolet rays, x rays and gamma rays. A single photon of gamma ray carries far more power than a single photon of visible light. Electromagnetic radiation from radio wave, microwave, infrared and visible light can only affect the biological systems and other materials through heating. But the electromagnetic radiation of ultraviolet, x rays and gamma rays contain so much power that they can even damage the molecular structure of the all objects that come in their way.
All the techniques that have been worked on so far related to WPT have limited the transmitter side frequency to THz. No has ventured into frequencies beyond THz because of the radiation problems. Some of the near field techniques are discussed in the following section.
1. INDUCTIVE COUPLING:
This is the age old method and the safest method adopted for WPT. It works the basic principles of electromagnetic induction. Energy is transferred from power transmitter to the receiver by the magnetic field that couples them. Energy transfer is enabled by magnetic field. Transmission efficiency is dependant on the coefficient of coupling K between the transmitter and receiver antennas.
All the techniques that have been worked on so far related to WPT have limited the transmitter side frequency to THz. No has ventured into frequencies beyond THz because of the radiation problems. Some of the near field techniques are discussed in the following section.
Electromagnetic Wave Spectrum
Image Courtesy: Wikimedia Commons
1. INDUCTIVE COUPLING:
This is the age old method and the safest method adopted for WPT. It works the basic principles of electromagnetic induction. Energy is transferred from power transmitter to the receiver by the magnetic field that couples them. Energy transfer is enabled by magnetic field. Transmission efficiency is dependant on the coefficient of coupling K between the transmitter and receiver antennas.
Inductive (magnetic) coupling schematic
Image courtesy: Wikipedia
Mobile charging pads are the most recent development using this technique for WPT. Other applications are electric toothbrush chargers, RFID tags and Smart cards. Many tech giants have designed and marketed their products using Inductive coupling Technology following any one of the two standards viz, Power Matter Alliance (PMA standards) or Wireless Power Consortium (Qi standards). The draw backs of this scheme of WPT is that the distance of Power Transfer is limited to millimeters and positioning (alignment) of transmitter and receiver must be proper for power transfer to happen.
Mobile charging pad by Powerbyproxi
Electric toothbrush on its charging stand
Image courtesy:amazon.co.uk
2. RESONANT INDUCTIVE COUPLING:
This technique is the advancement of Inductive coupling. In this scheme, energy transfer using the near field over mid-range is possible. Here, two coupled resonant coils are used as transmitter and receiver antennas. The coils are transformed into resonant coils by just adding variable capacitors in series with them. The circuit is basically a double tuned magnetically coupled circuit. Maximum energy can be transferred between the two inductively coupled coils, if they are made resonate at same frequency and if the coupling between them is critical coupling. Additionally if the coils have high Q (quality) factors, Maximum energy is transferred between transmitter and receiver coils even if they are separated by larger distances (a few times of their diameters).
Resonant Inductive (magnetic) coupling schematic
Image courtesy: Wikipedia
The research in this area has led to concept of homes and offices without metallic wires for conducting currents to loads and appliances from the mains. This highly reduces the institutional power losses (losses occurring within the homes and offices) due to inefficient wiring and very low quality of wires used for wiring, thus making the homes and offices ideal. These ideal homes and offices are electrically smart or "E-smart". The E-smart homes and offices use E-smart appliances that are intelligent enough to stop receiving wireless power once their need is over. One such smart home is shown in the below image.
An example E-smart home
The transmitter and the receiver coils are designed to have high Q factors. High Q factors of transmitter coil implies that for each cycle, the energy stored in the magnetic field is much higher than the energy dissipated. Almost all of the electrical energy supplied to the transmitter coil is stored into the magnetic field after allowing for a small resistive loss. Similarly, high Q factors of receiver coil implies that for each cycle, the energy absorbed from the magnetic field is much higher. Almost all of the energy present in the magnetic field linking the receiver coil is absorbed and passed on to the load.
The coils are made to resonate because at resonance the impedance is minimum and current is maximum in series RLC resonant circuits. This makes it easy for the transmitter coil to store more energy in the magnetic field as the energy loss is minimum in a circuit with minimum impedance. Also, when the receiver coil is made to resonate, the receiver side impedance is minimum. Energy loss is minimum and hence almost all of the energy absorbed from the magnetic field reaches the load in the receiver circuit. So resonance is beneficial and preferred for this scheme.
High values of Q factors indicate that the value of critical coupling coefficient Kc is very small.
This scheme doesn't require the receiver to be aligned in a particular position with the transmitter for power transfer to happen. Also, this scheme enables power transfer to multiple loads at varying power levels and at distances that are a few times greater than the diameter of the transmitter and receiver antennas. Several tech giants have entered this market and the competition is intensifying. Some of companies in the forefront are Powermat, Powerbyproxi, Qi, Mojo Mobility, Media Tek, Witricity, WiPower and Mopar. Shown below are the some of the products developed by Proxibypower for wireless power transfer.
In device charger for wireless charging of battery operated accessories and toys.
3D Charging Bowl for wireless charging of wearable gadgets.
Rechargeable AA batteries for with wireless power transfer.
Wireless Power Transfer is a field which has huge potential and market. But because of the non-availability basic sciences and inhibited research woks for nearly a century, this potential and market remains untapped. In the near future, I hope to see more of this technology being researched upon and implemented to revolutionize our homes and offices. We have only discussed the basic (yet the most researched and adopted) methods of the wireless power transfer in this post. In my next post we will discuss the left out methods of wireless power transfer and the feasibility of long distance wireless power transfer in the near future.
The content is great and the flow is nice... Good work.
ReplyDeleteWireless Power Transfers offer device safety, durability of the devices in connection and also waterproof. This article provides a great insight on how it works. Great post shared!!
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