Wireless power transfer (WPT) systems are involved in multiple and heterogeneous applications. This diversity is reflected in several factors such as the amount of power that is transferred or the distance separating the energy source and the receiver. In the current research work, the authors find several groups of technologies that try to adapt the process to the particularities of the application. In this way, wireless power transfer can be achieved with an inductive technology, with a resonant-inductive approach, or with a strongly coupled magnetic resonant configuration. This chapter focuses on strongly coupled magnetic resonant technology, which is appropriate for home applications.
Top1. Introduction
Wireless power transfer allows the transmission of energy between two objects that are not electrically connected. This idea was pioneered by Nikola Tesla with early experimental attempts over one century ago. However, commercial and efficient solutions have not been available until recent years. This technology is now a reality in several commercial appliances such as tooth brushes, mobile phone chargers, medical devices or tags. As it avoids the hassle of connections, it is being investigated how to apply it to medium and high power scenarios. This is the case of electric vehicles and some home and industrial appliances (Bi et al., 2016). For efficiency purposes, other areas are also getting benefit from the wireless power transfer. This is the case of the wireless communication networks, which try to obtain power from the signals intended for the communication with other receivers.
All these applications rely on an electromagnetic wave, which propagates in the air from an emitter to a receiver. The receiver is equipped with the electronics necessary to extract energy from the electromagnetic wave. The wavelength of the electromagnetic wave and its relationship with the distance between the emitter and the receiver is key to decide about which wireless power transfer technology is more convenient for a given scenario. According to this relationship, these technologies can be divided into three groups: near-field, mid-range and far-field technologies. In a near-field scenario, the energy emitter and the receiver are up to one wavelength separated. In contrast, the far-field scenario takes place when the distance between these two elements is greater than two wavelengths. Finally, the mid-range wireless power transfer occurs in the intermediate configuration. It is receiving recent interest as the separation between the transmitter and the receiver is suitable for home appliances.
In each group, there are a set of technologies, which appropriateness mainly depends on the power to be transmitted and on the mobility of the emitter and/or the receiver. The next chart illustrates the classification of the technologies.
Figure 1.
Scheme of the techniques for wireless power transfer
Far-field transmission can be achieved by three methods: radio-frequency, microwave and laser. Firstly, radio-frequency is based on a scheme similar to the one employed in telecommunication systems. The signals involved in the process range from 30 MHz to 5 GHz. Theoretically, the power transmission can be omnidirectional or directional. When low powers are sent, then omnidirectional transmission is allowed but for high-power applications a directional scheme should be followed in order to comply with the international restrictions about electromagnetic emissions. The enterprise PowerCast has developed this technology in order to transfer up to 3 W in a directional scheme and up to 1 W for an omnidirectional propagation . Due to the levels of power that are managed, this kind of wireless power transfer can be useful for charging low-power devices in a wide area. This is the case of wireless sensor networks.
Microwave transmission for wireless power transfer relies on a component named rectenna, which is composed of dipoles and diodes that transform the microwave signal into a direct current signal. In the decade of 1980, the SHARP (Stationary High Altitude Relay Platform) project was able to power a small plane. The distance from the sender to the receiver was 21 km. The signal employed for this 500-kW transmission was 5.8 GHz. As can be observed, the power involve in the microwave transmission is higher than those achieved by the radio-frequency schemes. However, microwave transmission requires a direct line-of-sight without any human-being around due to the potential negative impact on health. A similar problem is present in optical transmissions based on lasers. Nevertheless, they do not cause any interference with other radio systems.