|Fig. 1: A simplified circuit schematic of a resonant inductive CPT system. (Source: Wikimedia Commons)|
Contactless Power Transfer (CPT) was introduced to the world in the 1890's, when after successful experiments, Nikola Tesla demonstrated a lightbulb powered wirelessly by a tesla coil at the 1893 Chicago World fair.  The tesla coil in this demonstration produced a very high voltage that oscillated at a high frequency, but with a low current. Electricity was sent through the bulb, but the high frequency of oscillation prevented it from penetrating the skin of the onlookers, making this a harmless but impressive experience. Though this transfer was by no means power efficient, as it was using air directly as a conductor, this paved the way to the multiple types of CPT that have been developed today.
The principle of CPT is that electricity is transmitted between two circuits without the use of a typical conductive material (metal). In this way, electrical power is transmitted through air, or plastic, or other insulating materials without the direct use of wires. There are multiple types of CPT, each of which has its own advantages and disadvantages, which will be presented in the next section.
While inductive charging is the most widely used form of CPT in consumer-oriented industries, there are in fact three main ways to transfer energy through CPT: inductive CPT, capacitive CPT, and radiant CPT. 
Inductive CPT is good for power transfers up to the several-meter range. It retains efficiency above 90% under a meter. [2-4] The technology is being researched heavily and many developments are expected to increase the efficiency for longer distances and under more rigorous transmission conditions. It can handle a wide range of power loads without problems. This is the method that manufacturers of consumer goods have chosen to focus on. One big drawback is the production of electromagnetic radiation due to oscillating magnetic fields.  In addition, the range limitation may not be optimal for all applications.
|Fig. 2: A simplified circuit schematic of a capacitive CPT system. (Source: Wikimedia Commons)|
Capacitive CPT relies on the capacitive coupling of metal plates through an electric field. It has over 90% efficiency at 30cm (and exceptionally high efficiency at closer distances), but it is unable to handle high power applications. 
Radiant CPT is efficient over long distances, but relies on electromagnetic waves to transfer energy.  Since the waves are directed, it also requires a line of sight to the receiving module. Because of the use of electromagnetic waves, this method is also dangerous to anyone who comes into the area into which the transmitter is sending waves. It is, however, the method that can transmit over much greater distances than any of the other discussed methods, and therefore may be useful in certain applications.
One last method has been developed and proposed as CPT, called magneto-dynamic CPT.  This method operates via rotating magnets and gears, and has been shown to be over 90% efficient compared to contact power transfer (using wires) over a distance of 6 inches. An electric motor rotates one of the magnets, causing the coupled magnet on the receiving end to rotate as well, powering an electric generator. The great benefit that this method provides is a lack of electromagnetic radiation, but the difficulty in downscaling the system and the high quantity of moving parts make this method unpopular for consumer product makers. Due to its low popularity, this method will not be explored in greater detail.
Inductive CPT is the most widely used method of CPT, and is used from appliances to phones to electric vehicles. Inductive charging transfers energy magnetically between two coils as the transmitting coil oscillates at a high frequency (Fig. 1). [2, 7-11] An important innovation that increases efficiency and range of inductive charging makes use of coils with the same magnetic resonant frequency (which is in the MHz region). [9,10] This method causes the magnetic power transfer to act primarily between the two coils, reducing the effect of the magnetic field on the surroundings.  However, oscillating magnetic fields generate electromagnetic radiation, which can be harmful to people in high enough quantities. High power applications of inductive CPT, such as electric vehicles, will have to keep this in mind and mitigate such dangers. 
Inductive CPT has the limitation that efficiency of the power transfer relies on both the magnetic resonance frequency and physical alignment of the magnetic coils.  In motion, for example, an object must be continuously tuned in order to maintain effective power transfer.  Recent work has overcome this obstacle by adding a voltage amplifier and feedback resistor to the circuit.  This allows for automatic tuning of the magnetic frequency to the one that provides maximum efficiency, regardless of the orientation or motion of the charging circuit. 
|Fig. 3: A functional schematic of a radiant CPT system. (Source: Wikimedia Commons)|
Capacitive CPT works by using capacitive plates on both the charger and receiver such that a capacitor is created when the charging circuit comes in proximity with the charger circuit (Fig. 2). [12,13] A high frequency alternating current on the charger end therefore causes an alternating current through the charging circuit, which can then be stored in a battery or used. The higher the frequency, the less impedance there is, which means high frequencies in MHz or GHz range are desirable.  Because this charging method is based on capacitance, it has the same limits as exist with a regular capacitor - that is, smaller capacitive plate area means less capacitance as does greater distance between plates. In addition, the relative static permittivity (dielectric constant) of the material between the plates limits capacitance as well. Since the power transferred, W = 1⁄ 2CV2, where C is the capacitance and V is voltage, this charging system is not ideal for high-power applications. However, it does not emit electromagnetic radiation and therefore is a good candidate for biomedical implants and related technologies.  In addition, it can be useful in chip design to increase data transfer density.  Capacitive CPT is also extremely efficient at low-gap power transfer, making it an appealing choice for applications where separation is below a centimeter.
Radiant CPT is a method that is directly based on sending energy through electromagnetic waves. [14-17] There are two main types of radiant CPT: microwave-based and laser-based. Microwave CPT is more efficient through most mediums, notably in atmospheric conditions, but lasers are more accurate, which is vital at long distances.  In microwave CPT, a transmitting antenna sends electromagnetic waves in a tight beam, and a receiving antenna receives it, after which a circuit known as a rectenna converts the waves into DC power (Fig. 3).  The laser CPT model shines a laser at a photovoltaic cell. Many of the challenges with this technology relate to shaping the beam of electromagnetic waves to efficiently hit the receiver with minimal loss.  In general, the efficiency of radiant CPT is much lower than that of inductive and capacitive CPT, but it is able to transmit over distances limited only by the accuracy and scattering of electromagnetic waves.  The efficiency for laser CPT is based on the photovoltaic efficiency, currently around 50%.  The theoretical efficiency of microwave CPT can reach 100%, but in experimental measurements is closer to 54% DC to DC (which is posited to rise as high as 76% with improved technology).  This technology is meant to be used in applications where people will not intersect with the beam of electromagnetic radiation, as this is harmful to human health. The most prominent application of radiant CPT is aerospace technology.
© Alec Arshavsky. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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