RF Energy Harvesting for Wireless Power
Article By : Maurizio Di Paolo Emilio
To limit the use of batteries for safety issues, an RF energy harvesting system to provide wireless power can benefit the entire application market.
The increasing demand for wireless devices such as mobile phones and computers shows the importance of wireless applications worldwide. These devices, however, require continuous power supply or long battery life. To limit the use of batteries for safety issues, an RF energy harvesting system to provide wireless power can benefit the entire application market, which is expected to grow by 22% between 2020 and 2025.
This growth can be attributed to many factors, but its primary driver is artificial intelligence (AI). While AI algorithms are strong now, they require more and more data in order to be more effective. And that’s where wireless power comes in. Wireless power has been proven to be reliable and efficient, and it can help feed AI in order to help it make better decisions. In an interview with EE Times Europe, Charlie Goetz, CEO of Powercast, said he believes that IoT, ADAS and smart cities are a number AI-related areas that could benefit most from the implementation and adoption of wireless power.
RF is available and can be found very easily in any place and at any time, regardless of time, geographical limits or weather conditions. Typically, RF ranges from 3 kHz to 300 GHz. The idea is to harvest those RF energy sources and store them for use in certain applications. RF energy harvesting offers significant benefits for many applications, but requires careful attention to key components, including the receiver antenna and power conditioning circuits needed for this approach.
Wireless power
Any industrial setting that requires sensory information for preventive maintenance and critical shutdowns can benefit from wireless power. For example, a manufacturing facility has certain areas that are hard to get to or are dangerous to access. For that reason, certain sensors (their batteries) cannot be changed. Sensors that are charged by wireless power allow for a set-and-forget mentality (i.e., you don’t need to change the batteries), which allows for uninterrupted information flow to the AI.
Autonomous vehicles can also greatly benefit from wireless power. Today, self-driving cars integrate with their environment using sonar, radar, and pattern recognition. To become safer and more reliable, cars need to sense their environment and communicate with the road: a street lamp, the yellow lines of the road, etc., via sensors. With wirelessly powered sensors, cars can also communicate their way through the environment without bumping into connecting wires resulting in a more secure and dependable way.
Further down the road, the enablement of smart cities will not occur with batteries and wires but with AI and wireless power. For smart cities to communicate with their environment, wireless power and charging must be implemented.
Energy harvesting
Radiofrequency is an abundant source for energy harvesting, although it requires the proximity of a transmitting antenna. The concept of energy harvesting from RF is not new and the process is relatively simple. Radio waves strike an antenna and cause a potential difference that moves charge carriers along the length of the antenna in an attempt to equalize the field. The harvesting circuit basically captures this movement. The energy is stored temporarily in a capacitor and then used to create a desired potential difference on the load.
RF charging technology employs several technologies (Qi, PMA/AirFuel Alliance, WPC, etc.), each with various charging methods and maximum charging distances. Our routers as well as mobile phones create regions of space with potential energies that vary over time and/or distance. And wherever a potential difference exists, there is always a way to obtain electricity.
RF transmitters for communications are typically constrained in the amount of power they can radiate. Furthermore, for a given radio source, the power at the receiving antenna falls off with distance as predicted by the Friis transmission equation:
where
PT = transmitted power
PR = received power
GT = transmitter antenna gain
GR = receiver antenna gain
λ = wavelength
d = distance between transmitter and receiver
The design of the receiving antenna plays a critical role in maximizing the efficiency of energy harvesting. Receiving antennas can be etched into printed circuit boards or available as receiver coil components specifically designed for the task (figure 1).
Powercast technology
In early 2021, Powercast announced that it has been named a winner of the BIG Innovation Awards 2021 presented by the Business Intelligence Group for its new wirelessly powered RFID temperature scanning system, which allows companies to easily and securely monitor employee temperatures in support of Covid-19 monitoring protocols.
The temperature scanning system consists of a temperature sensor, an RFID reader and a TV monitor. By using wireless technology, the device charges quickly when held near an RFID reader at the entrance to the company using patented Powercast energy harvesting technology. Employees scan their forehead using the stick to read their temperature and are either allowed in or denied entry based on the reading, which appears automatically on the monitor.
“The design of the antenna is very important, it serves to tune us to a given frequency which in our case is 915 Mhz. When you’re dealing with RF, you’re dealing with very small amounts of power at the end, even at relatively close range. You’re dealing with 10 or maybe low hundreds of milliwatts. So your energy management behind that collection and conversion has to be extremely important. Antenna and energy management must work harmoniously together to get the right user experience and actually deliver real solutions,” Goetz said.
The receiving antenna has an impedance of 50 ohms, which must match the input impedance of the rest of the device. The voltage collected in the vicinity of the antenna must then be brought to a voltage where it can be converted to DC. This can be done using a charge pump, which increases the voltage but obviously cannot increase the total power. Powercast offers an evaluation kit to help companies explore the possibilities of this technology. The P2110-EVAL-02 includes an RF receiver and transmitter, an antenna, and a charge board to collect the emitted power (figure 2).
Together with its increasing availability, RF power harvesting not only benefits cabling requirements, but also offers a system that is protected from environmental conditions and hazardous materials. For mesh networks, engineers can combine RF energy harvesting with sophisticated device-to-device communications for a variety of applications.
This article was originally published on EE Times Europe.
Maurizio Di Paolo Emilio holds a Ph.D. in Physics and is a telecommunication engineer and journalist. He has worked on various international projects in the field of gravitational wave research. He collaborates with research institutions to design data acquisition and control systems for space applications. He is the author of several books published by Springer, as well as numerous scientific and technical publications on electronics design.
