Wireless Power Transfer

Wireless power transmission in simple terms is a power source that supplies a system with energy without interconnecting wires. This process of generating energy becomes useful when instant or continuous energy is necessary but wires are impractical.

Nicholas Callan’s Induction Coil Apparatus (1836)

• Principle of Electromagnetic Induction

Heinrich Hertz (1888)

• Wireless power transmission through Radio Waves.

Nikola Tesla (1891)

• Tesla coil transformer

COUPLING :

• Transfer of energy from one medium/circuit segment to another medium/circuit segment .

MUTUAL INDUCTANCE:

• Process by which Current in one inductor induces voltage in a nearby inductor.

ELECTRICAL RESONANCE:

• Tendency for a circuit system to oscillate at maximum amplitude for a resonant frequency.

• Likely to occur when Impedance between Input and Output is minimized.

• Increases the strength of interaction between two related devices.

• Resonant frequency for LC circuit is given by  f = 1/(2*3.14*(LC)^0.5 )

Types of Wireless Power Transmission includes

• Induction
• Radio Waves
• Evanscent Wave Coupling
• LASER

When magnetic flux flowing through a circuit changes, an electromotive force (emf) along with current are induced in the circuit. This effect is for example used in dynamos, electric motors and transformers. The central principle behind electromagnetic induction is Faraday’s law, which relates to the induced electromotive force (emf) in any closed loop including a closed circuit. Induction can be used as a means of wireless power transfer. A changing current in one coil creates an emf, which in turn induces a current in another coil.
The coils are not in contact and in this way energy can very simply be transported over short distances. This is used in for example an electric toothbrush charger. The short distance that is required for induction is the largest drawback of this way of wireless energy transfer, because it limits the applicability to very close-range situations.

The key component for wireless power transfer by radio waves is the rectenna. A rectenna is a combination of a rectifying circuit and an antenna. The antenna receives the electromagnetic power and the rectifying circuit converts it to DC electric power.
A simple rectenna can be constructed from a Schottky diode placed between the antenna dipoles. The diode rectifies the current induced in the antenna by the microwaves. Schottky diodes are used because they have the lowest voltage drop and highest speed and therefore waste the least amount of power due to conduction and switching.
The amount of power that can be transferred is limited. For safety reasons, the transmitted power is limited by regulations, for instance by the Federal Communications Commission (FCC), and the received power is attenuated, mainly due to free-space path loss. Furthermore, because portable devices have small dimensions, the rectenna should have small dimensions as well. This results in a small antenna area and, consequently, a low amount of received power. Because of these limitations, wireless power transfer using radio waves is mainly suitable for low-power applications, e.g. a low-power wireless sensor.

Evanescent wave coupling (or ―WiTricity) is a technique that has recently been investigated by researchers at MIT.
The physics behind this technique is rather complicated. At a glance, it basically extends the principle of magnetic induction to mid-range applications up to a few meters. The main difference is the use of resonance; if sender and receiver have the same magnetic resonance frequency, energy can efficiently be transported, while losses to the non-resonant environment are small. Using resonance, for the same geometry, power can be transported approximately 106 times more efficiently than without resonance.
The experimental setup used by the MIT researchers is shown. The coils can be compared to antennas; the electric and magnetic fields produced by antennas can generally be divided into the near field, which is dominant at close ranges, and the far field. The far field, responsible for electromagnetic waves, radiates energy into the environment. The near field does not radiate, so no energy is lost, except when the sender and receiver have the same resonance frequency. In that case energy is transported from the sender to the receiver. The main achievement of the MIT team is to have figured out how to fine tune the system so that the near field extends to distances of a few meters, simultaneously limiting the power radiated through the far field.
One of the benefits is that most common materials do not interact with magnetic fields, so obstructing objects do not have much influence. This also goes for human tissue and therefore health risks are low. The coils shown above are too large for applications in i.e. a cell phone, but the receiving coil can be made smaller. The researchers state that the transmitted power can be kept constant, if the size of the sending coil is increased to keep the product of the sizes of both coils equal. The efficiency of the above setup is around 40 to 50% for wireless power transfer over 2 meters.

Power delivery that starts with sunlight has many advantages such as sustainability and the fact that the sun is present every day. However solar cells have limited efficiency and sunlight is not available at night. An alternative is to generate artificial light, from a laser, transmit it through air, and then convert it into electricity.
New refinements are making this alternative more attractive. NASA has demonstrated flight of a lightweight model plane powered by laser beam, directed at a panel of infrared-sensitive photovoltaic cells mounted on the bottom of the aircraft.
A theoretical setup consists of a laser (Light Amplification by Stimulated Emission of Radiation) and a photovoltaic, or solar cell.
First electricity is converted by the laser into a laser beam, which consists of coherent radiation. Next this beam is pointed towards a photovoltaic cell receiver, which in turn converts the received light energy back into electricity. This is generally called ―power beaming‖.
Both steps are not highly efficient and also a direct line of sight between laser and the photovoltaic cells is required.

• Input of about 6-12V DC supply is provided to the Electronic Ballast using Battery or Eliminator.
• Inside Electronic Ballast, Inversion process takes place resulting in conversion of DC to AC signals.
• AC signals, thus produced are of low frequency.
• Due to the transistor switching action taking place inside Electronic Ballast, Low frequency AC signals are converted into high frequency AC signals.
• High frequency AC signals obtained from Electronic Ballast are fed as Input to the Transmitting coil.
• Due to the process of Magnetic Induction along with resonant principle, an EMF is induced in the Receiving Coil.
• Output from the Receiving Coil is fed as input to the Cell Phone Charger Circuit.
• Thus, Wireless Power Transmission enables charging of Cell Phone.

Details:

Diameter of the coil (D) = 16cm

Radius of the coil (r) = 8cm

Radius of the cross-section (a) = 0.4cm

Number of Turns (N) = 9 turns

Theoretical Calculation:

1.       Inductance of the Winding:

Inductance of a circular coil = N²µ₀ r ( ln(8r/a) – 1.75 )

                                                  = 9² x 4π x 10^-7 x 8 x ( ln( (8x8)/0.4 ) -1.75 )

                                                  = 2.707mH

2.       Resistance of the Winding:

Resistance of the Winding (R) = ρl/A

Length of the coil (l) = Circumference of coil x N

                                     = 2π x D x N

                                     = 904.8cm

A = 2πr(r+h) , where , h = width of the winding

    = 442.34cm

Ρ = Resistivity of Copper

   = 1.796x10^-8

R = 3.67x10^-8 Ω

3.       Resistance of Leakage path:

R = ρl/A

Ρ = Resistivity of Air = 10⁶ (assumed )

l = length of air gap = 6.5cm ( Distance to be transmitted )

A = Area of air gap ( Rectangular Area between two coils )

R = 1.54 MΩ

4.       Resonant Frequency :

f = 1/2π sq.rt of LC

L = 2.707mH

C = 0.0047nF ( Capacitor Used )

f = 1.4 MHz

5.       Resonance Condition:

For Resonance to occur, X_L = X_C

X_L = Inductive reactance (Reactance of Coil)

X_C = Capacitive Reactance

X_L = 2π x f x L

    = 23812 Ω

    = 23.8 KΩ

X_C = 1/2π x f x C

     = 24188 Ω

     = 24.188 KΩ

 Thus, X_L = X_C and so Resonance occurs resulting in transfer of power wirelessly.

Wireless Power Transmission Using Inductive coupling also offers several advantages over other options that are as follows:

1. Simple Design – The design is very simple in theory as well as the physical implementation. The circuits built are not complex and the component count is very low too.

2. Lower Frequency Operation – The operating frequency range is in the kilohertz range. Furthermore there is low risk of radiation in the LF band.

3. Low Cost - The entire system is designed with discrete components that are readily available. No special parts or custom order parts were necessary for the design. Thus we were able to keep the cost of the entire system very low.

4. Practical for Short Distance – The designed system is very practical for short distance as long as the coupling coefficient is optimized. The design also offers the flexibility of making the receiver much smaller for practical applications.

Wireless Power Transmission using Inductive coupling also has some disadvantage's that need to be addressed.

1. High Power Loss – Due its air core design the flux leakage is very high. This results in a high power loss and low efficiency. But when combined with the resonant principle, Power loss can be reduced with an increase in efficiency.

2. Non-directionality – The current design creates uniform flux density and isn't very directional. Apart from the power loss, it also could be dangerous where higher power transfers are necessary.