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Energy harvesting is a process of utilizing energy in the surrounding atmosphere for powering various portable devices. Energy can be harvested through different means—through wind, sun, radio frequency and so on. Amongst them, the RF energy-harvesting (RF-EH) system has received much popularity over recent years, as the method offers an alternative yet sustainable approach to supplying power to low-power electronic systems.

RF energy harvesting is the process of capturing and converting RF electromagnetic waves into usable electrical energy. It involves the use of specialized antennas and rectifiers that capture and rectify RF signals, transforming them into a direct-current (DC) power source.

This harvested energy can then be utilized to power small electronic devices or stored in energy storage systems for later use. RF energy harvesting is a potential solution for powering wireless sensor networks, IoT devices and other low-power electronics. This can be done by harnessing energy from ambient RF signals already present in the environment.

However, RF-EH systems have a limited operating range, which necessitates the device requiring power to maintain close proximity to the RF broadcaster. The efficacy of RF energy harvesting diminishes considerably as the distance between the device and the RF source increases. Another challenge faced is the requirement of a specially designed antenna to receive or transfer RF signals.

Components

RF energy harvesting consists of a core component called a rectenna (or rectifying antenna). It’s a special device that comprises an antenna, RF input filter, matching network, rectifying circuit and storage device.

RF-EH systems can function using a wide spectrum of frequency bands that provide ambient RF energy. These frequency bands include Long-Term Evolution (LTE), which operates in the 750- to 800-MHz range and is utilized for 4G mobile data services, and Digital Television (DTV), which uses the 550- to 600-MHz band for broadcasting digital TV signals, replacing analog transmissions for improved quality and multiple-channel transmission.

GSM-900, GSM-1800 and the Universal Mobile Telecommunications System (UMTS) are also featured in the ambient frequency list, of which GSM-900 works within the 850- to 910-MHz range. Similarly, GSM-1800 operates as a 2G band for mobile communication and works in the range of 1,850 to 1,900 MHz. UMTS relies on the 2,150- to 2,200-MHz band, delivering 3G mobile services, such as video calling, internet access and multimedia messaging.

The frequency spectrum also includes Wi-Fi and wireless LAN (WLAN) networks that commonly utilize the 2.4- to 2.45-GHz frequency range for wireless internet connectivity. The 900-MHz to 2-GHz spectrum is allocated for TV and radio broadcasting applications. WLAN operates within 3.1 to 4.4 GHz, offering wireless network connectivity similar to Wi-Fi but in a different frequency band.

Techniques and advantages

RF-EH comprises different techniques, including:

• RF-EH from dedicated RF sources
• RF-EH from ambient RF sources
• RF energy transfer (RFET) between mobile devices

RF-EH from dedicated sources enables high-power values compared with other methods. A circuit that harvests RF energy from a dedicated source over a short distance is expected to generate power levels in the range of 50 nW/cm². But such promising power levels are also accompanied by path loss, energy dissipation, shadowing and fading, all of which pose challenges. On the other hand, RFET shows promising advantages over non-radiative wireless energy transfer by providing more relaxed coupling and alignment specifications.

The RF-EH from ambient RF sources is categorized into two sub-parts: static and dynamic sources.

• Static sources: These sources are characterized as stable-power transmitters. However, they aren’t simplified. Signals are modulated—usually by adjusting the frequency and transmitted power—to power the sensor device. Ambient static sources include broadcast radio, mobile base stations and television.
• Dynamic sources: These are transmitters that regularly broadcast RF in such a way that isn’t monitored. To effectively harvest energy from such sources, an intelligent wireless energy-harvesting system is required to continuously monitor the channel for potential harvesting opportunities. Wi-Fi access points, microwave radio links, police radios and more are a few unnoticed examples of ambient dynamic sources.

RF energy harvesting between mobile devices enables stable power transfer among nearby devices. By utilizing power-splitting or time-switching techniques, these devices can operate sustainably without requiring any modifications to the transmitters. This approach allows for the use of a shared antenna or antenna array for both RF energy harvesting and information reception. For example, mobile devices can transfer RF energy based on information to relay nodes, preventing imbalanced energy consumption.

RF-EH systems possess distinct advantages over sources like wind, solar and vibrations. These include the following:

• RF-EH systems demonstrate the capability to regulate and facilitate consistent energy transfer over extended distances.
• The energy harvested through RF-EH systems exhibits relative stability and predictability, ensuring long-term performance within a fixed distance in an RF-EH setup.
• Notably, RF-EH system levels vary significantly across different locations of network nodes, as the overall RF energy harvesting relies on the proximity of the dedicated RF source to the ambient RF source.

Scientific principles

Microwave antennas are built based on the principles of Maxwell’s equations to transmit and receive electromagnetic waves. RF energy, which is a form of electromagnetic energy, can be transferred through electromagnetic waves. RF energy sources, such as broadcast stations, cellphone towers and Wi-Fi hotspots, provide a constant and abundant supply of RF energy in the environment.

In near-field applications, electromagnetic induction and magnetic resonance techniques are used to generate electric power within a short distance. In the far-field region, antennas can receive RF signals and convert them into power through rectifier circuits.

Far-field RF energy harvesting can be categorized into ambient and dedicated RF energy harvesting. Power transmission to a receiver within the charging range is determined by factors like distance, frequency and antenna gains. The received power is then converted to DC voltage and stored for later use. It’s worthwhile to note that the propagation properties of the environment affect the amount of received power.

In RF-EH systems, Friis transmission and the equivalent isotropically radiated power (EIRP) are the two key design boundaries to consider. The EIRP sets the upper limit for the available power at the antenna side, regardless of the range. Additionally, the power density of the Friis decreases as the distance increases, except in highly reflective 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. Tradeoffs also occur between these parameters to ensure maximum power and functionality.

Bands like DTV, GSM900, GSM1800 and 3G could be potential harvesting bands due to their presence in high power density in urban areas. Also, there’s a greater chance to capture considerable electromagnetic energies in urban areas compared with semi-urban areas.

Exploring antenna varieties

Antennas play an indispensable role in rectenna systems, serving as the receivers of RF power and converting it into a DC signal through subsequent stages of the system. They’re designed considering factors like complexity, size and overall performance. To meet the requirements of efficient energy harvesting, antennas should have a wide operating frequency range, low-profile design, omnidirectional radiation pattern, high gain and compact size.

Single-band antennas are designed to operate in a single narrow-frequency band. Amongst the non-negotiable parameters to consider while designing an antenna is circular polarization (CP), which aids in elevating the overall output power by stabilizing the output from the antenna. Furthermore, the CP antenna is a vital component to improve the overall efficiencies in rectenna systems.

Broadband and wideband antennas are designed to capture energy from various sources across a wide frequency range. The use of broadband antennas is widely studied, and the literature points toward utilizing a flower-shaped slot in a cross-dipole antenna to improve impedance matching within the 1.8-GHz to 2.5-GHz range.

Similarly, a compact slotted antenna demonstrated excellent performance with a wide bandwidth of 2 GHz to 3.1 GHz in the LTE band. These advancements in broadband and wideband antennas have expanded the possibilities for RF energy harvesting across diverse frequency ranges.

The use of multiband antennas in rectenna applications helps tap into varied advantageous frequencies. However, this method is less efficient compared with previous approaches, despite receiving more power from the surrounding environment. A major challenge in RF energy harvesting is the low conversion rate from RF to DC due to the low available RF power density. Moreover, antenna arrays provide high output power but are difficult to integrate and occupy significant chip space. Hence, the development of compact multiband circularly polarized rectennas for environmental RF energy harvesting is necessary. For example, a compact multiband rectenna covering WLAN, Wi-MAX, GSM and satellite communication bands was demonstrated, featuring fractal geometry and six radiating bands.

Rectifiers play a crucial role in RF energy harvesting by converting alternating-current (AC) signals from antennas into DC power for efficient extraction. Rectification in RF energy harvesting can be achieved using diodes, transistors and CMOS technology. There are two types of rectification topologies:

• Half-wave rectification: Only one-half of the AC waveform is allowed to pass, resulting in a unidirectional but pulsating DC.
• Full-wave rectification: The entire input waveform is converted to an output waveform with constant polarity, resulting in a higher average value output voltage.

Applications and potential

RF energy harvesting has a range of applications based on size, operating frequency, substrate and diode technology used. One of the many applications is the CP graphene field-effect transistor (GFET)-based rectenna, which was designed to detect high-frequency RF signals. A miniaturized, printed rectenna was also designed to harvest energy from ambient RF power of about 2.45 GHz.

A wearable rectenna array is designed for applications requiring mobility and comfort. Made from Cordura textile material, this rectenna is durable, light, tough and comfortable, making it suitable for wearable antenna design. An RF-EH and storage module for wearable applications can harvest 8.4 mJ of energy in less than four minutes from 915-MHz industrial and scientific medical sources.

The RF identification (RFID) augmented module for smart environmental sensing (RAMSES) was initially introduced as a fully passive device with the purpose of exploring new and unconventional applications of RFID technology.

Powering a battery through RF energy harvesting might be a promising alternative to conventional battery-based power systems. RF-EH technology enables the harvesting of ambient RF energy and converting it into usable electrical power, eliminating the need for frequent battery replacements or recharging.

RF electromagnetic waves are abundantly present in our everyday surroundings, including Wi-Fi signals, cellular networks and various communication systems. By harnessing these RF sources, wearable devices, such as smartwatches, health trackers and smart glasses, can fulfill their charging requirements seamlessly.