Wireless power transfer allows the transfer of energy from a transmitter to a receiver without electrical connections. Compared to galvanic charging, it displays several advantages, including improved user experience, higher durability and better mobility. As a result, both consumer and industrial markets for wireless charging are growing rapidly. The main market share of wireless power is based on the principle of inductive power transfer, a technology based on coupled coils that transfer energy via varying magnetic fields. However, inductive charging has some disadvantages, such as high cost, heat dissipation, and bulky inductors. A promising alternative is capacitive wireless power transfer that utilizes a varying electric field as medium to transfer energy. Its wireless link consists of conductive plates. The purpose of this paper is to review the state of the art, link the theoretical concepts to practical cases and to indicate where further research is required to take next steps towards a marketable product. First, we describe the capacitive link via a coupling model. Next, we highlight the recent progress in plate topologies. Additionally, the most common compensation networks, necessary for achieving efficient power transfer, are reviewed. Finally, we discuss power electronic converter types to generate the electric field.

Wireless power transfer provides a most convenient solution to charge devices remotely and without contacts. R&D has advanced the capabilities, variety, and maturity of solutions greatly in recent years. This survey provides a comprehensive overview of the state of the art on different technological concepts, including electromagnetic coupled and uncoupled systems and acoustic technologies. Solutions to transfer mW to MW of power, over distances ranging from millimeters to kilometers, and exploiting wave concepts from kHz to THz, are covered. It is an attractive charging option for many existing applications and moreover opens new opportunities. Various technologies are proposed to provide wireless power to these devices. The main challenges reside in the efficiency and range of the transfer. We highlight innovation in beamforming and UV-assisted approaches. Of particular interest for designers is the discussion of implementation and operational aspects, standards, and safety relating to regulations. A high-level catalog of potential applications maps these to adequate technological options for wireless power transfer.

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Capacitive wireless power transfer applies the electric field to transport energy from a transmitter to a receiver. However, if the distance between transmitter and receivers varies, the efficiency or power transfer is reduced for a system with a fixed load. By applying the bifurcation phenomenon, the system can be operated in a coupling-independent regime, resulting in a (nearly) constant efficiency or power transfer at fluctuating coupling. However, a future need can be expected to charge multiple devices simultaneously by a single transmitter. Unfortunately, the conditions for which frequency bifurcation occurs are not yet determined for capacitive wireless power transfer with multiple receivers. In this work, these constraints are analytically determined. A quartic equation is found that expresses the bifurcation conditions for a general capacitive wireless power transfer system. If the receivers are identical, the condition reduces to a quadratic equation. Finally, the analytical results are illustrated by a numerical example.