RF Energy Harvesting: Systems, Components, and Applications

Energy harvesting systems are designed to capture and convert ambient energy from the environment into usable electrical power. The primary function of these systems is to generate electricity from various sources to power electronic devices or recharge batteries. Here are the typical energy harvesting schemes available today:

• Solar energy
• Mechanical energy
• Thermal energy
• Radio Frequency energy
• Electromagnetic energy
• Biochemical energy
• Radioactive energy etc.

RF (Radio Frequency) energy harvesting systems are designed to capture and convert radio frequency signals from the environment into electrical power.

RF energy harvesting is particularly useful in situations where ambient radio frequency signals are available, such as in urban environments, near communication towers, or in proximity to electronic devices emitting radio frequency radiation like Wi-Fi routers.

These systems can be employed to power low-power electronic devices or recharge batteries, eliminating or reducing the need for conventional power sources. This technology uses intermediate stages of power conditioning, storage batteries, or storage capacitors to conserve harvested energy from the environment. Radio Frequency waves used in an RF harvesting system are part of the electromagnetic spectrum.

Maximum power available in RF energy harvesting technique is about 0.7 µWatt for 2.4GHz and about 1 µWatt for 900 MHz frequency theoretically. This can be achieved with a free space distance of about 40 meters.

Key Features and Considerations

Key features and considerations for radio frequency harvesting systems are as follows:

• These systems are designed to operate within specific frequency ranges.
• The efficiency depends on factors such as the antenna design, matching network, and rectifier efficiency.

Applications

RF energy harvesting has various applications, including wireless sensor networks (WSNs), IoT devices, and low-power electronics. It is particularly useful in scenarios where traditional power sources are impractical or where long-term, maintenance-free operation is desired.

RF Energy Harvesting System Block Diagram

The figure below depicts a block diagram of an RF energy harvesting system. It consists of the following modules. It converts RF energy to DC energy, hence it is also referred to as an RF to DC converter. The figure also mentions discrete circuit components used in these modules.

• Antenna
• Matching Network
• Rectifier and Voltage multipliers (one or more)
• Energy storage elements and Load

Antenna

This device picks up RF radiations from various sources available in free space. The picked-up RF energy is later guided to the rest of the modules using a transmission line or guided structures. The antenna converts electromagnetic energy picked up into electrical signals.

Matching Network

It is used to couple maximum RF energy into the circuit. The circuit is made of capacitor and inductor components.

Rectifier Circuit

The captured RF signals are then fed into a rectifier circuit. The rectifier is responsible for converting the alternating current (AC) signal induced in the antenna into direct current (DC), which can be used to power electronic devices or charge a storage device like a battery or a capacitor.

Voltage Multiplier

The figure mentions a voltage multiplier or rectifier made of diodes and capacitors. The higher the number of these stages, the higher is the voltage at the load and the lesser is the current. But a higher number of stages results in an increase in charging delay before it goes for energy storage.

Energy Storage

In many RF energy harvesting systems, there is a storage component such as a battery or a capacitor. The energy harvested from RF signals can be stored in these devices for later use, allowing for continuous operation even when the RF signal source is not present or is intermittent.

Voltage Regulator

In some cases, a voltage regulator may be included to stabilize the output voltage. This ensures that the harvested energy is provided at a consistent and usable voltage level for the connected electronics. This module is optional and may not always be required.

Load or Electronic Device

The harvested and rectified energy is then supplied to the load or electronic device, powering it directly or charging a connected energy storage device.

Advantages of Radio Frequency Energy Harvesting

Following are the benefits or advantages of RF energy harvesting:

1. Ubiquitous Power Source: This harvesting technique leverages ambient radio frequency signals, providing a continuous and ubiquitous power source for electronic devices.
2. WSNs: Enables the creation of self-sustaining wireless sensor networks (WSNs) by eliminating the need for frequent battery replacements.
3. Reduced Environmental Impact: Promotes sustainability by harnessing existing RF signals, reducing the reliance on disposable batteries and minimizing environmental impact.
4. Extended Device Lifespan: Enhances the lifespan of low-power devices by continuously harvesting energy from the surrounding RF environment.
5. Cost Savings: Reduces maintenance costs and enhances cost-effectiveness by eliminating the need for frequent battery replacements in remote or inaccessible locations.

Conclusion

RF energy harvesting systems contribute to the development of self-sustaining and energy-efficient devices, reducing the reliance on conventional power sources and enabling the deployment of wireless sensor networks in diverse environments.