**6. Capacitive coupling WPT**

Capacitive coupling is a kind of coupling that depends on the electric field coupling between two plates, so it is also named electric coupling. Capacitive coupling

**137**

*WPT, Recent Techniques for Improving System Efficiency DOI: http://dx.doi.org/10.5772/intechopen.96003*

**Reference Geometry Notes**

[36] • A thin metamaterial

[37] • An efficient wireless

[38] • Metamaterials using

[39] • A study of using

slabs

(IM).

• Study of three types of metamaterials is presented: the double negative material (DNG), the isotropic μ-negative material (MNG), and indefinite material

power transfer system integrating with negative permeability (MNG) metasurface is proposed for biological applications.

• By using metasurface structure, a coupling enhancement of 15.7 dB is

a dual-layer printed circuit board (PCB) with a high dielectric constant substrate is proposed for enhancing system efficiency and reduced emf leakage in WPT systems

• 44.2% improvement in the PTE and 3.49 dBm reduction in the electromagnetic field leakage at 6.78 MHz and separation distance of 20 cm is obtained.

metamaterial structure to compensate the degradation in the power transfer efficiency due to the misalignment issues between the Tx and Rx.

obtained.

*Wireless Power Transfer – Recent Development, Applications and New Perspectives*

outgoing magnetic fields are bent back toward the receiver coil, this increases the field strength between the two coils as revealed in **Figure 25**. Thus, the efficiency is enhanced, and the EMF leakage is reduced due to applying these metamaterial surfaces in the path between the transmitter and receiver coils. **Table 1** summarizes

*(a) Metamaterial-based WPT system. (b) equivalent circuit model of applying metamaterial structures with* 

Capacitive coupling is a kind of coupling that depends on the electric field coupling between two plates, so it is also named electric coupling. Capacitive coupling

different metamaterial structures that are used in WPT systems [36–40].

**136**

**Figure 24.**

**Figure 25.**

*WPT.*

*Metamaterials categories.*

**6. Capacitive coupling WPT**

#### **Table 1.** *Different metamaterial structures used in WPT systems.*

acts as a capacitor where its metal plates one is in the transmitter and the other in the receiver and the medium in between represents the dielectric. The power can transfer between the two plates in form of a displacement current. **Figure 26** shows the WPT system for the capacitive coupling technique. As a result of electric field interacts with many different materials as well as capacitive coupling method needs very high voltages. Hence, capacitive coupling has only a few practical applications. Capacitive coupling has some special privileges over inductive coupling. The magnetic field is largely confined between the capacitor plates, reducing interference, and higher immunity for the misalignment issues between the transmitter and receiver. Therefore, capacitive coupling can be used in charging portable devices, smartcards, and transferring power between the layers of a substrate in RF integrated circuits. **Figure 27** illustrates an experiment for capacitive coupling that is executed by Nikola Tesla in 1891 [42]. He performed this experiment before his induction WPT demonstration.

In [43], a high-frequency capacitive coupling WPT using dielectric glass layers is introduced to reduce the coupling impedance and increase the coupling

**139**

same system were proposed.

**Figure 27.**

**7. Microwave power transfer (MPT)**

*WPT, Recent Techniques for Improving System Efficiency DOI: http://dx.doi.org/10.5772/intechopen.96003*

capacitance. Thus, it transfers power easily with high efficiency. Regensburger *et al.* introduced a high-performance capacitive WPT system for electric vehicles charging by using interleaved-foil coupled inductors [44]. This system used a kilowatt-scale large air-gap to achieve high power transfer density and high transfer efficiency at the operating frequency (13.56 MHz). Interleaved-foil air-core inductors provide a better quality factor; this makes them are useful at kilowatt-scale power at high frequencies. In [45], multi-loop control that is used to regulate the power transfer in capacitive wireless systems by applying variable matching networks is discussed. An adaptive multi-loop controller combines continuous frequency tracking and matching networks tuning to regulate a current/power to the receiving side at the optimal power transfer conditions. In [46–50], hybrid structures that combine inductive coupling and capacitive coupling WPT in the

*Tesla demonstrating wireless power transmission using capacitive coupling, New York, in 1891 [41].*

Microwave power transmission refers to far-field directive powering, where the power transmission occurs in the far-field using a well-defined directional transmitter. Microwave power transmission depends on the propagation of electromagnetic radiative fields where it is preferred in long-range WPT applications. This sort of WPT is useful for space-based solar power satellites (SPS) applications or with intentional powering such as using a dedicating source with a well-known direction to power a network of wireless sensors, each sensor has its built-in rectenna. One of the first applicable trails of MPT was conducted by William Brown et al. in 1965 by powering an aircraft using a MPT at an altitude of fifty feet for ten continuous hours [51]. There are many challenges regarding RF-to-DC power conversion efficiency, matching circuit design, the dependence of the DC output voltage as well as the conversion efficiency on the input power, load impedance, and operating frequency. In order to solve these issues, many rectennas have been introduced [52, 53]. Several single frequency band rectennas were used for energy harvesting [54, 55], and dual and multiband rectennas were discussed in [56–58]. In [59, 60] we proposed a dualband rectenna using voltage doubler rectifier and four-section matching network. An enhanced-gain antenna with Defected Reflector Structure (DRS) is integrated with the rectifying circuit for increasing the rectenna capability for scavenging. A voltage doubler circuit is used for the rectification. Moreover, a four-section

*WPT, Recent Techniques for Improving System Efficiency DOI: http://dx.doi.org/10.5772/intechopen.96003*

*Wireless Power Transfer – Recent Development, Applications and New Perspectives*

**Reference Geometry Notes**

[40] • A closed-form and

analytical expressions are obtained for efficiency improvement with metasurface.

acts as a capacitor where its metal plates one is in the transmitter and the other in the receiver and the medium in between represents the dielectric. The power can transfer between the two plates in form of a displacement current. **Figure 26** shows the WPT system for the capacitive coupling technique. As a result of electric field interacts with many different materials as well as capacitive coupling method needs very high voltages. Hence, capacitive coupling has only a few practical applications. Capacitive coupling has some special privileges over inductive coupling. The magnetic field is largely confined between the capacitor plates, reducing interference, and higher immunity for the misalignment issues between the transmitter and receiver. Therefore, capacitive coupling can be used in charging portable devices, smartcards, and transferring power between the layers of a substrate in RF integrated circuits. **Figure 27** illustrates an experiment for capacitive coupling that is executed by Nikola Tesla in 1891 [42]. He

In [43], a high-frequency capacitive coupling WPT using dielectric glass layers is introduced to reduce the coupling impedance and increase the coupling

performed this experiment before his induction WPT demonstration.

**138**

**Table 1.**

**Figure 26.**

*capacitive wireless power systems.*

*Different metamaterial structures used in WPT systems.*

**Figure 27.** *Tesla demonstrating wireless power transmission using capacitive coupling, New York, in 1891 [41].*

capacitance. Thus, it transfers power easily with high efficiency. Regensburger *et al.* introduced a high-performance capacitive WPT system for electric vehicles charging by using interleaved-foil coupled inductors [44]. This system used a kilowatt-scale large air-gap to achieve high power transfer density and high transfer efficiency at the operating frequency (13.56 MHz). Interleaved-foil air-core inductors provide a better quality factor; this makes them are useful at kilowatt-scale power at high frequencies. In [45], multi-loop control that is used to regulate the power transfer in capacitive wireless systems by applying variable matching networks is discussed. An adaptive multi-loop controller combines continuous frequency tracking and matching networks tuning to regulate a current/power to the receiving side at the optimal power transfer conditions. In [46–50], hybrid structures that combine inductive coupling and capacitive coupling WPT in the same system were proposed.
