Part of EE Times’ RF Wireless Special Report

Energy harvesting is a method that uses energy from the environment to power different portable devices. Energy harvesting can be accomplished by various methods, including sun, wind and radio frequency, and others. Of these there is it is RF energy harvesting (RF-EH) method has gained significant attention in recent years since it provides an alternative and sustainable solution to supply energy to electronic systems that require low power.

A brief overview of the most prominent sources of energy in the environment (Source: IEEE)

RF energy harvesting refers to the process of capturing, then converting RF radio waves to electrical energy usable. It requires the use of specially designed antennas and rectifiers to are able to capture and rectify RF signals to transform them into direct-current (DC) source of power. 

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This energy harvesting can be used for small electronics or storing energy to be used later. The harvesting of energy from RF could be a viable option for supplying power to sensors wirelessly, IoT devices and other electronics with low power. It is possible to do this by utilizing energy generated by ambient radio signals that are already in the surroundings.

But, RF-EH devices have limitations in their operating range and, therefore, the device to have power in order to remain in close proximity of an RF broadcaster. The effectiveness in RF-EH decreases significantly when it gets further away from the source of RF grows. Another problem to overcome is the requirement for an antenna that is specifically designed for receiving or transmitting radio frequency signals.

Components

RF energy harvesting is the fundamental component of rectenna (or rectifying antenna). It’s a specific device that includes the following components: an antenna and an RF input filter, a matching network and rectifying circuit, as well as a storage device.

RF-EH systems operate using an array of frequency bands that supply the surrounding RF energy. These bands comprise Long-Term Evolution (LTE) which operates in the 750- 800-MHz range and is used for mobile data 4G as well as Digital Television (DTV), that utilizes the 550-600-MHz band to broadcast digital TV signals, which replaces analog transmissions to improve quality and multi-channel transmission.

GSM-900, GSM-1800 and the Universal Mobile Telecommunications System (UMTS) are also listed in the list of ambient frequencies that includes GSM-900 operates within the 850-to-910-MHz range. Similar to that, GSM-1800 operates as a 2G band for mobile communications and operates within the 1,850-to 1,900 MHz. UMTS depends on the 2,150-to 2200-MHz band to provide 3G mobile applications, including video calling Internet access, multimedia messaging.

The frequency spectrum also covers Wi-Fi as well as wireless WLAN (WLAN) network that typically use the 2.4up to 2.45-GHz frequency range to provide broadband internet wireless connectivity. The spectrum of 900-MHz up to 2-GHz is used to radio and TV broadcasting applications. WLAN operates in the range of 3.1 to 4.4 Ghz, providing wireless network connectivity that is similar to Wi-Fi, however in another frequency band.

Techniques and benefits

RF-EH comprises different techniques, including:

• RF-EH from specific RF sources
• RF-EH generated by ambient sources
• Energy transfer via radio frequency (RFET) in mobile gadgets

RF-EH generated from dedicated sources can provide the highest power levels when compared to other methods. A circuit that collects RF energies from a specific source for a small distance is likely to produce powerful levels of power in the region from 50 to 2.. However, such impressive power levels are not without the loss of path as well as energy dissipation, shadowing and blurring and all of these pose problems. However, RFET shows promising advantages over non-radiative wireless energy transfer, by offering more flexible coupling and the ability to align.

The RF-EH emitted by the ambient RF sources is classified into two parts that are dynamic and static sources.

• Static source: These sources can be classified as transmitters with stable power. But, they’re not simplified. Signals are modulated – usually by adjusting the frequency and power transmitted to enable the device that is used to sense. The sources that are static in the air include broadcast radio mobile base stations as well as television.
• Dynamic sources They are transmitters that frequently broadcast RF in an uncontrolled manner. To efficiently harvest energy from these sources, an advanced wireless energy harvesting system has to constantly monitor the channel to identify potential opportunities for harvesting. Access points for Wi-Fi as well as microwave radio links police radios, and many more are some of the lesser-known examples of dynamic sources that are in the environment.

Energy harvesting using RF between smartphones ensures steady power transfer between close devices. Through the use of power-splitting and time-switching methods they can function continuously without the need for any changes to transmitters. This allows the utilization of an antenna shared or array that can be used for both energy harvesting as well as information reception. For instance, mobile devices can transmit RF energy based on the information received into relays, thus preventing unbalanced energy consumption.

The RF-EH systems have distinct advantages over sources such as winds, solar and vibrations. They include:

• The RF-EH systems show the capability to regulate and allow for the transfer of energy over long distances.
• The energy generated by RF-EH systems is stable and predictability, which ensures longevity of performance over a set distance of an RF-EH system.
• In addition, the RF-EH system’s levels differ greatly across the different sites of network nodes, since the general RF energy harvesting depends upon the closeness of the designated RF source and the surrounding RF source.

Principles of science

Microwave antennas are designed using Maxwell’s equations that allow for the transmission or receive waves of electromagnetic energy. Energy from radio waves, a type of electromagnetic energy, is transferred via electromagnetic waves. Sources of RF energy like radio towers, cellphone towers and Wi-Fi hotspots, offer an unending supply of RF energy to the surrounding environment.

In near-field applications electromagnetic induction as well as electromagnetic resonance methods are employed to generate electricity within an extremely short distance. In the far field region antennas are able to receive RF signals and transform them into electricity using rectifier circuits.

The Far Field RF Energy Harvesting could be divided as dedicated and ambient energy harvesting RF. Transmission of power to receivers within the range of charging will be determined by several factors such as frequency, distance and gains. The power that is received is transformed into DC voltage, which is stored for future use. It’s important to consider that the propagation characteristics of the surrounding environment influence the amount of power that is received.

In RF-EH, Friis transmission as well as the equivalent isotropically radiating power (EIRP) are the two main design limits to be considered. The EIRP is the upper limit for available power on the antenna’s side regardless of the range. Furthermore the power intensity for Friis Friis decreases as distance increases, with the exception of high-reflectivity environments.

To assess the performance of a wireless-power-harvesting design, multiple parameters must be evaluated while giving due precedence to sensitivity, efficiency, output power and operation distance. There are also tradeoffs between these parameters in order to achieve the maximum amount of power and performance.

Bands such as DTV GSM900, GSM1800 and 3G may be possible harvesting bands because of their high power density regions. There’s also a higher likelihood of capturing substantial electromagnetic energy in urban areas as compared to semi-urban regions.

Exploring antenna varieties

Antennas play a crucial role in rectenna systems. They serve as receivers for RF power, transform it into the DC signal for the next stage within the structure. They are designed with consideration for factors such as size, complexity along with overall quality. To ensure efficiency in energy extraction, the antennas should be able to operate in a wide frequency range, low-profile designs with omnidirectional radiation patterns, small size and high gain.

Single-band antennas are made to function in one narrow frequency band. One of the parameters that you must think about when creating your antenna would be circular polarization (CP) which helps in boosting the power output of the entire system through stabilizing output of the antenna. Additionally CP is a crucial component to consider. CP antenna is an essential component in improving efficiency of the entire system.

The wideband and broadband antennas have been designed to absorb energy from various sources over an extensive frequency range. Broadband antennas is extensively researched, and the literature suggests the use of a flower-shaped slot on the cross-dipole antenna in order to increase impedance-to-impedance matching in that 1.8-GHz to 2.5-GHz band.

A similar slotted antenna showed excellent performance with a broad bandwidth from 2 GHz to 3.1. GHz in the LTE band. These advances in wideband and broadband antennas have increased the possibilities of RF energy harvesting across various frequency bands.

Multiband antennas in rectenna applications allows access a wide range of beneficial frequencies. But, this approach is not as efficient as prior methods, even though it draws greater power from the environment. One of the major challenges in RF Energy harvesting is limited conversion rate of RF to DC due to the small accessible RF energy density. Antenna arrays also provide large output power, however they are hard to integrate and consume a lot of space on chip. Therefore, the development of compact circularly polarized rectennas to enable environmental RF energy harvesting is essential. For instance, a compact multiband rectenna, which covers WLAN Wi-MAX GSM as well as satellite communications bands has been shown, with the fractal geometry as well as 6 radiating bands.

Rectifiers play an essential part within RF energy harvesting, by converting the alternating-current (AC) signals generated by antennas to DC energy for efficient extraction. Rectification is a crucial step in RF energy harvesting is done using transistors, diodes as well as CMOS technology. There are two kinds of topologies used for rectification:

• Half-wave rectification: Only one half of an AC waveform can pass, creating the unidirectional and constantly pulsing DC.
• Full-wave rectification: The whole input waveform is converted into an output waveform that has constant polarity. This results in a higher average output voltage.

Applications and the potential

The RF energy harvesting technology can be used for a variety of purposes dependent on the size and operating frequency, as well as the diode and substrate technology that is employed. One of the numerous possibilities is one of them being the CP graphene Field-Effect Transistor (GFET)-based rectenna, specifically designed to detect high frequency radio signals. A printed, miniaturized rectenna was designed to harness energy from an RF signal that is around 2.45 GHz.

An adjustable and wearable rectenna array has been made for use in applications that require mobility and ease of use. Constructed from Cordura fabric it is tough lightweight, robust and comfy, which makes it ideal for wearable antenna designs. A RF-EH storage and RF-EH module that can be worn by a wearer will harvest 8.4 millijoules of energy in under 4 minutes, from industrial and medical sources.

The RF identification (RFID) enhanced module for sensoring environmental smartness (RAMSES) was first designed as a completely passive device, with the intention of evaluating new and unique applications for RFID technology.

A battery powered by RF energy harvesting could be an attractive alternative to conventional battery-powered power systems. The RF-EH technology allows the collection of all-natural RF energy and converts it into electrical energy usable which eliminates the requirement for regular battery replacements or recharges.

Radio frequency electromagnetic waves are found in our daily surroundings such as Wi-Fi signals, cell networks, and other communication systems. Through harnessing these radiofrequency sources, wearable devices like smartwatches or health trackers, as well as smart glasses, are able to meet their requirements for charging seamlessly.

Suggested Reading

• TSMC’s 3-nm Push Faces Tool Struggles – EE Times
• U.S. Crawls Toward Rebuilding Frail PCB Industry – EE Times
• Processor Startup Improves Memory Function – EE Times
• Open-Source GPT Model Trained at Wafer Scale – EE Times
• Special Report: Energy Needs a Smarter Approach

RELATED TOPICS: 5G IOT, ARTIFICIAL INTELLIGENCE OF THINGS (AIOT), EV WIRELESS CHARGING, FUTURE OF IOT, IOT NETWORK, OTA IOT, REVERSE WIRELESS CHARGING