Wireless power transfer (WPT) or wireless energy transmission is the transmission of electrical energy from a power source to a consuming device, without the use of discrete man-made conductors. WPT use wireless transmitter that uses any of time-varying electric, magnetic, or electromagnetic fields to convey energy to one or wore receivers, where it is converted back to an electrical current and then used. Wireless transmission of power has numerous advantages. For example, it makes fault-prone plug contacts redundant. Devices can be built into housings that are protected against moisture ingress. Users also don’t have to go to the trouble of plugging in cables. The conventional power transmission using transmission lines to carry power from one place to another is costlier in terms of cable costs with a huge transmission loss.
Wireless power techniques fall into two categories, non-radiative and radiative. In non-radiative techniques, power is typically transferred by magnetic fields using inductive coupling between coils of wire. The inductive coupling method is the most essential methods that help the experts to transfer energy wirelessly via inductive coupling. Basically, it is used for near field power transmission. However, the power transmission takes place between the two conductive materials through mutual inductance. For instance, it includes a transformer.
In radiative far-field techniques, also called power beaming, power is transferred by beams of electromagnetic radiation, like microwaves or laser beams. Microwave Power Transmission consists of two sections. It includes the transmitting section and receiving section. In the transmission section, the microwave power source generates microwave power controlled by the electronic control circuits. The waveguide circulator protects the microwave sourced from the reflecting power which connects through the co-ax waveguide adaptor.
Laser Power Transmission: Laser technology used to transfer power in the form of light energy, and the power converts to electric energy at the end of the receiver. In addition, it receives power using different sources like sun, electricity generator or high-intensity-focused light. However, the size and shape of the beam decide by a set of optics. The transmitted LASER light receives by the photo-voltaic cells. It converts the light into electrical signals. Usually, it uses optical-fiber cables for transmission.
The largest application of the WPT is the production of power by placing satellites with giant solar arrays in Geosynchronous Earth Orbit. However, it transmits the power as microwaves to the earth known as Solar Power Satellites (SPS).
Wireless power, however, has not been as successful as the technology currently faces some limitations. The transmission range of wireless power transmission through electromagnetic induction and or by magnetic resonance technique is limited. This limitation of the range poses a serious challenge for the manufacturers. The efficiency of the power is inversely proportional to the distance between the transmitter and receiver, however it is predicted to improve over time. Safety issue is also the main concern for the wireless transmission market as strong electromagnetic fields may harm the biological environment.
The first and most important classification is based on how far power transfer is possible. In the experimented methods, some are capable of delivering power wirelessly to loads at large distance away while others could only deliver power to loads only a few centimeters away from the source. So the first division is based on whether the method is of Near Field or Far Field. The difference in distance capability comes based on the type of phenomenon used by various methods to achieve wireless power transfer. For example, if the medium used by the method to deliver power is Electro-Magnetic Induction then the maximum distance can be no higher than 5cm. This is because the loss of magnetic flux increases exponentially with an increase in distance between source and load which leads to unacceptable power losses. On the other hand, if the medium used by the method to deliver power is Electro Magnetic Radiation then the maximum distance can go as high a few meters. This is because EMR can be concentrated to a focal point which is at meters away from the source. Also, methods that use EMR as a medium to deliver power have higher efficiency when compared with others.
The performance of WPT systems has been steadily increasing. In 2016, Oak Ridge National Laboratory (ORNL) developed and demonstrated a 20 kW WPT system for a Toyota RAV4 EV. INL, in collaboration with ORNL, quantified the performance and electromagnetic (EM) field safety of the WPT system. Two years later in 2018, ORNL advanced their WPT design to 120 kW, which is now the world’s highest power level WPT system for LDEV.
The development of silicon carbide (SiC) technologies makes it possible to operate at a higher power level and in a higher frequency (up to 100 kHz theoretically) as compared to conventional MOSFET because of the low switching loss and good thermal behavior. The current commercial SiC power modules are mainly available from Wolfspeed, ROHM, Infineon, SEMIKRON, and STMicroelectronics.
When the switching frequency increases, high frequency electromagnetic interference (EMI) or frequency electromagnetic compatibility (EMC) becomes a challenge. For high power SiC applications (1200 V or above) at the switching frequency of 20 kHz and 50 kHz, the EMI can be reduced to the allowed range in accordance with the standard of DO-160(Environmental Conditions and Test Procedures for Airborne Equipment) .
Electrified transportation technology is unique because it is one of the few technologies that is mobile, publicly accessible, and can be integrated into the electric grid. These unique aspects result in potential cybersecurity risks as well. With advancements in EV charging infrastructure towards higher power levels and increased sophistication, such as wireless power transfer, potential negative impacts from cybersecurity vulnerabilities are also increasing, especially for those of high power WPT and wired extreme fast charging (XFC) installed in public places. Cybersecurity vulnerabilities in physical systems may result in even greater impacts to public safety and electric grid security, in addition to denial of service, hardware damage, or theft/alteration of data. Cybersecurity should be considered early on during the design phase in order to incorporate solutions to reduce the risk of nefarious access, safeguard data and information, and enable a safe minimum state of operation during a cyber-event.
As for commercially available WPT systems, WiTricity develops a variety of WPT ranging from 3 to 11 kW with an efficiency of 90–93% operating at 85 kHz. Qualcomm presented 20 kW dynamic WPT with an efficiency of 90%. Efacec Electric Mobility in Portugal has developed their WPT system with a maximum power of 22 kW, although no frequency or efficiency information is presented. On February 11, 2019, WiTricity acquired Qualcomm Halo, which will bring the number of patents and patent applications related to wireless charging to WiTricity to over 1500.
Electromagnetic safety or cybersecurity risks
Wireless power transfer (WPT) or inductive power transfer (IPT) promises convenient, autonomous, and highly efficient charging of electric vehicles (EVs). Compared to conductive charging systems, which require heavy gauge cables with potential electrical and ergonomic hazards, wireless charging is convenient, flexible, and capable of fully automated charging, despite potential electromagnetic safety or cybersecurity risks. With power transfer levels increasing beyond 100 kW, many technical and risk management challenges emerge. The high power wireless charging for light-duty electric vehicles, which are aiming at 200 kW or higher wireless power transfer, also face future challenges and risks in the area of electromagnetic safety, resonant frequency determination, and cybersecurity risks.
Electromagnetic shielding by using an aluminum or ferrite plane is a typical solution to limit the electromagnetic emission level and ensure the safety of WPT. The SAE J2954  recommended practices document defines power classes and a recommended electromagnetic shielding design for three classes of low power WPT: WPT1(3.7 kW), WPT2(7.7 kW), and WPT3(11.1 kW). DOE has a stated goal to reduce the charging time for EVs to 15 min or less, which requires the charging system to deliver 350–400 kW. If WPT power levels continue to increase, electromagnetic safety for WPT with the constraints of LDEV space limitation becomes a critical challenge.
Misalignment is a issue specific to high power WPT. To mitigate the impact of misalignment on power transferring performance of WPT, SAE J2954 also has definitions for allowed maximum misalignment, which are no more than 0.075 m along the direction of travel and 0.1 m in the transverse direction of the vehicle. However, considering the specific electromagnetic safety risk for high power WPT, misalignment leads to additional negative impact on magnetic field emission, which makes electromagnetic safety challenge more critical.
Hence, in order to reach the power level higher than 100 kW, increasing angular resonant frequency (ω) will be the main challenge (currently 22 kHz for 120 kW WPT). Given the need for a higher resonant frequency, higher power converters operated at a higher switching frequency are required. Insulated-Gate Bipolar Transistors (IGBTs) are widely used for high power applications, such as the ones for the integration of renewable energy in the power grid or driving high power motors. However, owing to the physical limitations of IGBT, it is normally difficult to operate at a frequency higher than 20 kHz (e.g., the typical operation frequency of IGBT converters in the power grid is 10 kHz). On the other hand, conventional Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET) can work at a high frequency, but the power level is typically low.
To reach its maximum potential and meet the demands of tomorrow’s wireless war fighter, next-generation components, systems, and devices must also be designed and developed with WPT in mind to optimize form, fit, and function and also to ensure that the systems are efficient, safe, and accurate.
Power over Wi-Fi or PoWi-Fi
Power over Wi-Fi utilizes a ubiquitous part of wireless infrastructure, the Wi-Fi router, to provide far-field wireless power without significantly compromising network performance. PoWiFi combines two elements: (1) a Wi-Fi transmission strategy that delivers power on multiple Wi-Fi channels and (2) energy-harvesting hardware that can efficiently harvest from multiple Wi-Fi channels simultaneously This is attractive for three key reasons:
• In contrast to TV and cellular transmissions, Wi-Fi is ubiquitous in indoor environments and operates in the unlicensed ISM band where transmissions can be legally optimized for power delivery. Repurposing Wi-Fi networks for power delivery can ease the deployment of RFpowered devices without additional power infrastructure.
• Wi-Fi uses OFDM, an efficient waveform for power delivery because of its high peak-to-average ratio. Given Wi-Fi’s economies of scale, Wi-Fi chipsets provide a cheap platform for sending these power-optimized waveforms, enabling efficient power delivery.
• Sensors and mobile devices are increasingly equipped with 2.4 GHz antennas for communication via Wi-Fi, Bluetooth or ZigBee. We can, in principle, use the same antenna for both communication and Wi-Fi power harvesting with a negligible footprint on the device size.
The key challenge for power delivery over Wi-Fi is the fundamental mismatch between the requirements for power delivery and the Wi-Fi protocol. While the harvester can gather energy during WiFi transmissions, the energy leaks during silent periods. In this case, the Wi-Fi transmissions cannot meet the platform’s minimum voltage requirement. Unfortunately for power delivery, silent periods are inherent to a distributed medium access protocol such as Wi-Fi, in which multiple devices share the same wireless medium. Continuous transmission from the router, while optimal for power delivery, would significantly deteriorate the performance of Wi-Fi clients and other nearby Wi-Fi networks.
Quasistatic cavity resonance (QSCR)
A new method developed by Disney Research for wirelessly transmitting power throughout a room enables users to charge electronic devices as seamlessly as they now connect to WiFi hotspots, eliminating the need for electrical cords or charging cradles.
The researchers demonstrated their method, called quasistatic cavity resonance (QSCR), inside a specially built 16-by-16-foot room at their lab. They safely generated near-field standing magnetic waves that filled the interior of the room, making it possible to power several cellphones, fans and lights simultaneously.
“This new innovative method will make it possible for electrical power to become as ubiquitous as WiFi,” said Alanson Sample, associate lab director & principal research scientist at Disney Research. “This in turn could enable new applications for robots and other small mobile devices by eliminating the need to replace batteries and wires for charging.”
The QSCR method involves inducing electrical currents in the metalized walls, floor and ceiling of a room, which in turn generate uniform magnetic fields that permeate the room’s interior. This enables power to be transmitted efficiently to receiving coils that operate at the same resonant frequency as the magnetic fields. The induced currents in the structure are channeled through discrete capacitors, which isolate potentially harmful electrical fields. “Our simulations show we can transmit 1.9 kilowatts of power while meeting federal safety guidelines,” Chabalko said. “This is equivalent to simultaneously charging 320 smart phones.”
In the demonstration, the researchers constructed a 16-by-16-foot room with aluminum walls, ceiling and floor bolted to an aluminum frame. A copper pole was placed in the center of the room; a small gap was created in the pole, into which discrete capacitors were inserted.
“It is those capacitors that set the electromagnetic frequency of the structure and confine the electric fields,” Chabalko explained. Devices operating at that low megahertz frequency can receive power almost anywhere in the room. At the same time, the magnetic waves at that frequency don’t interact with everyday materials, so other objects in the room are unaffected.
Wireless power transfer enhanced by magnetic resonant field enhancers (MR-FE)
Researchers from North Carolina State University and Carnegie Mellon University, by placing a magnetic resonance field enhancer (MRFE)—a loop of copper wire resonating at the same frequency as the AC current feeding the transmitter coil—between the transmitter and receiver coil, they could boost the transmission efficiency by at least 100 percent. “Our experimental results show double the efficiency using the MRFE in comparison to air alone,” David Ricketts of NC State, said in a press release.
The researchers conducted an experiment that transmitted power through air alone, through a metamaterial, and through an MRFE made of the same quality material as the metamaterial. The MRFE significantly outperformed both of the others. In addition, the MRFE is less than one-tenth the volume of metamaterial enhancers.
“We performed a comprehensive analysis using computer models of wireless power systems and found that MRFE could ultimately be five times as efficient as using metamaterials and offer 50 times the efficiency of transmitting through air alone,” Ricketts says.
A fully integrated wireless power receiver has been demonstrated in CMOS (Complementary Metal-Oxide-Semiconductor) process. The GaN chips are also predicted to be key enablers of wireless charging, the devices help systems stay tuned to the resonance needed for wireless charging.
Wireless power transfer enhanced by Metamaterials
Metamaterial (MM) due to their unique electromagnetic properties like negative permeability can act as a near field super-lens to focus or concentrate the magnetoquasistatic field generated by the source at the receiver coil. By enhancing the evanescent waves of the near-field, inductive link can be strengthened with potential mutual coupling improvement of up to 50 times has been reported.
In a new study published in EPL, scientists at Tongji University in Shanghai, China, have experimentally demonstrated a way to improve the efficiency of wireless power transfer by using magnetic metamaterials. The new method improves the efficiency of the design from a few percent to nearly 20% at a distance of 4 cm, which could pave the way toward new applications, including wireless charging of implanted pacemakers and electric vehicles.
The performance of MM for WPT system has been proved effective in various environments. However, early reported MMs may be too thick and large in size to increase the PTE and efficient transfer distance, which may limit their practical applications. Therefore, a thin and compact MM for WPT working in ISM band is needed.
Junfeng Chen and others have proposed a MM structure is very compact and ultra-thin in size. They designed an ultra-thin and assembled planar MM structure for 13.56 MHz WPT system numerically and experimentally, which consists of a singlesided periodic array of the capacitive loaded spiral resonators (CLSRs) by FR-4 substrate.
Wireless charging through solar or ambient radio frequency sources,
Scientists at the Australian National University (ANU) have made a wireless charging breakthrough that allow wireless sensors to be charged with energy harvested from “solar or ambient radio frequency sources,” such as communication towers or other mobile phone base stations. In a first, we have accurately modelled how much energy it takes to sense and transfer information by wireless sensors and are working on further ways to analyse the problem,” says lead researcher for the study Dr Salman Durrani.
“A major problem hindering the widespread deployment of wireless sensor networks is the need to periodically replace batteries. If we can use energy harvesting to solve the battery replacement problem for wireless sensors, we can implement long-lasting monitoring devices for health, agriculture, mining, wildlife and critical national infrastructure, which will improve the quality of life,” says lead researcher for the study Dr Salman Durrani.
Team of Professor Sang-Young Lee and Professor Kwanyoung Seo of Energy and Chemical Engineering at UNIST presented a new class of monolithically integrated, portable PV-battery systems (denoted as ‘SiPV-LIBs’) based on miniaturized crystalline Si photovoltaics (c-Si PVs) and printed solid-state lithium-ion batteries (LIBs). The device uses a thin-film printing technique, in which the solid-state LIB is directly printed on the high-efficiency c-Si PV module.
“This device provides a solution to fix both the energy density problem of batteries and the energy storage concerns of solar cells,” says Professor Lee. “More importantly, batteries have relatively high power and energy densities under direct sunlight, which demonstrates its potential application as a solar-driven infinite energy conversion/storage system for use in electric vehicles and portable electronics.” The SiPV-LIB device was capable of fully charging under sunlight illumination after only 2 min. It also showed decent photo-rechargeable electric energy storage behaviour even at a high temperature of 60°C and even at an extremely low light intensity of 8 mWcm-2, which corresponds to the intensity in a dimly-lit living room. “The SiPV-LIB device presented herein shows great potential as a photo-rechargeable mobile power source that will play a pivotal role in the future era of ubiquitous electronics,” says Professor Lee.
Wireless power transfer achieved at 5-meter distance
Chun T. Rim, a professor of Nuclear & Quantum Engineering at KAIST, and his team developed the “Dipole Coil Resonant System (DCRS)” for an extended range of inductive power transfer, up to 5 meters between transmitter and receiver coils. “With DCRS,” Professor Rim said, “a large LED TV as well as three 40 W-fans can be powered from a 5-meter distance.”
The researchers improved upon MIT’s (Massachusetts Institute of Technology) Coupled Magnetic Resonance System (CMRS) proposed in 2007 by replacing its complicated coil structure with only two compact ferrite core rods with windings at their centers, compacted the bulky structure, reduced the coil’s resonant frequency from 10 MHz to 100 KHz, reduced the high Q factor of 2,000 to 100, making the resonant coils less sensitive to surroundings such as temperature, humidity, and human proximity.
The team conducted several experiments and achieved promising results: for instance, under the operation of 20 kHz, the maximum output power was 1,403 W at a 3-meter distance, 471 W at 4-meter, and 209 W at 5-meter. For 100 W of electric power transfer, the overall system power efficiency was 36.9% at 3 meters, 18.7% at 4 meters, and 9.2% at 5 meters.
Professor Rim’s team completed a research project with the Korea Hydro & Nuclear Power Co., Ltd in March this year to remotely supply electric power to essential instrumentation and control equipment at a nuclear power plant in order to properly respond to an emergency like the one happened at the Fukushima Daiichi nuclear plant. They succeeded to transfer 10 W of electricity to the plant that was located 7 meters away from the power base.
To further improve the power transfer efficiency over distance, previous works have proposed the addition of an intermediate material between the source and receiver of a WPT system. These intermediate material are placed between the sources and receive coil assemblies and are independent of the source/receiver realization.
References and Resources also include: