This free electricity grid slide design for PowerPoint can be used to prepare presentations on transmission networks, Power Transmission Systems as the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. Alternatively, you can download other free presentation templates for PowerPoint. The technology for wireless power transmission or wireless power transfer (WPT) is in the forefront of electronic development. Applications involving microwaves, solar cells, lasers, and resonance of electromagnetic waves have had the most recent success with WPT. The main function of wireless power transfer is to allow electrical devices to be continuously charged and lose the.

Rajeev Mehrotra, ramehrotra (at) go.wustl.edu (A paper written under the guidance of Prof. Raj Jain)

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Abstract:

The transfer of power from source to receiver is a technology that has existed for over a century. Wireless power transfer (WPT) has been made feasible in recent years due to advances in technology and better implementations of transfer techniques, such as Microwave Power Transfer (MPT). The MPT system works by converting power to microwaves through a microwave generator and then transmitting that power through free space where it is received and converted back to power at a special device called a rectenna. The applications of MPT are numerous, not only to change the way existing technologies work, but also as theoretical constructs for future constructs. While the benefits are great, there are many limitations and drawbacks of MPT, necessitating the discussion of possible alternative methods for WPT. The transfer of power wirelessly has the potential to completely disrupt and revolutionize existing and future technologies.

Keywords:

Wireless Power Transfer, Microwave Power Transfer, Rectenna, Solar Power Satellites, Highly Resonant Wireless Power Transfer.

1. Introduction

The transfer of energy from a source to a receiver has traditionally necessitated the use of a physical connection. Indeed, electrical grids and power outlets span nearly the entire globe and deliver power to billions of people worldwide. Recently, there has been much interest into the area of wireless power transfer (WPT), that is, the transmission of power without the need for a physical connection [WikiWP]. Research into WPT, however, is nothing new, as experiments in the field took place as far back as Nikola Tesla in the early 20th century [Brown84].

The aim of this paper is to provide an overview of WPT, with a focus on the radiative approach to wireless power; that is, transmitting power using the electromagnetic spectrum. In particular, this paper details the transfer of power over the microwave band, called microwave power transfer or microwave power transmission (MPT). Other approaches to WPT include magnetic field resonance and capacitance, and are mentioned in this paper for completeness as alternative methods for WPT.

Section 2 of this paper provides an overview of the fundamentals of wireless power, detailing MPT and how it works. Section 3 outlines and highlights some key and interesting applications of wireless power. Section 4 poses some limitations, drawbacks, and potential safety concerns of MPT, as well as some of the alternative methods for WPT, which are discussed in this paper but not described in detail.

WPT is an extremely useful technology that has numerous applications and benefits. Cell phones, laptops and other mobile devices could function without ever having to be plugged in, cars could drive on highways burning no fossil fuels; wireless power even has the potential to solve much of the renewable energy issues we face.

2. Fundamentals of Wireless Power Transfer: Microwave Power Transfer

Transfer of power via microwaves has long been the focus of study and experimentation [Brown84]. In the early 1900s, Nikola Tesla experimented with transmission of power wirelessly, through microwaves. His work, however, was largely left unimplemented, as his experiments were vastly ahead of their time and the technology did not yet exist to make WPT via microwaves feasible [Brown84]. Advances in wireless technologies since Tesla, however, have made possible that which was not in the early 20th century [Wu13]. Described in this section are the details of those technologies behind MPT as a mechanism for WPT.

2.1 Overview of the system

As mentioned above, MPT as a mechanism for WPT is the central focus of this paper. MPT is defined as the transfer of power through space by means of microwaves. In particular, a MPT system converts direct current (DC) power to microwaves, transmits that microwave radiation to a target, and the target converts the microwave radiation back to DC power [Brown84, Reddy13]. Figure 1 below depicts a block diagram of the MPT system. First the microwaves are generated by the microwave generator. This radiation then passes through the Coax-Waveguide Adapted, which in turn passes through the waveguide circulator, a device that reduces the radiation to exposure from outside power. Finally the radiation passes through the tuner and directional coupler device, which separates the signal according to signal propagation direction. The radiation is then transmitted over the air through antennae, where it is received by the antenna at the rectenna, at which the microwave radiation passes through a low pass filter, then a matching network, then a rectifier as it is converted to DC power [Reddy13]. The details of the relevant steps are described in the sections to follow.

Figure 1: Block Diagram of MPT [Adapted from Reddy13 & Ojha13]

2.2 Transmission: Microwave Generator

Several devices, known as Microwave Vacuum Tubes, are used to transmit microwaves through space; these devices include magnetron, klystron, and the Travelling Wave Tube among others, with the magnetron being the device used the most often [Reddy13]. The magnetron device generates microwaves by passing electrons through a magnetic field [WikiMagnetron]. These microwaves are then transmitted via antenna into space.

The exact frequency at which microwave radiation is transmitted through space to achieve maximum efficiency has been an area of great deal of research. Frequencies often used within the microwave range are 2.45 GHz, 5.8 GHz, 8.5 GHz, 10 GHz and 35 GHz [Reddy13]. Experimental results have demonstrated that 2.45 GHz is the most efficient frequency to transmit microwaves, as the power capacitance achieved at that frequency is higher than the frequencies listed above [Huang13, Reddy13]. Figure 2 below shows that the lowest measured loss (solid line) occurs when transmitting at a frequency of 2.45 GHz. Table 1 shows efficiency percentages achieved using different frequencies.

Figure 2: Graph of power loss based on frequency [Harouni11].

Table 1: Table of measured and calculated efficiencies of different frequencies [Adapted from Reddy13].

Frequency (GHz)	Measured Efficiency (%)	Calculated Efficienct (%)
2.45	92.5	90.5
5.8	82	78.3
8.51	62.5	66.2

Some studies have demonstrated efficiency values, defined as the amount of power retained after being converted from microwaves, of upwards of 85 - 90% have been achieved using a 2.45 GHz frequency [Reddy13]. More details of efficiency of transfer are described in the section to follow.

The transmission of microwaves is vital to the wireless power system, however the efficiency is dictated to a large degree by the reception of those microwaves, and conversion back to DC at the receiving end of the system. The reception and conversion is described in the next section.

2.3 Reception: The Rectenna

Once DC is converted to microwaves and transmitted into space, a system that requires power needs to receive and convert microwaves back to DC. The device used for this reception and conversion is called a rectenna. The rectenna device was created by W.C. Brown in his groundbreaking work in WPT in the 1960s [Brown84, WikiRectenna]. The term is derived from the fact that a rectenna is comprised of an antenna coupled with a rectifying circuit [Huang13]. It has been determined through experimentation that the 2.45 GHz frequency, as mentioned above, achieves the greatest efficiency for the rectenna [Harouni11].

In particular, microwave radiation is received by the antenna component of the rectenna, passes through a l

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