In today’s wireless, connected world, ambient Radio Frequency (RF) energy is everywhere. Technically, this free-flowing energy can be captured, converted and stored for use in other applications. In fact, it is already in use in a number of ultra-low-power, battery-free applications, such as RFID tags, contactless smart cards, and wireless sensor networks. As a result of technological advances, harvested RF energy is just beginning to realize its wider potential, including charging batteries in smartphones and other portable devices. These enabling technologies include RF transceivers, power conversion circuits, and ultra-low-power microcontrollers, all of which are becoming ever more efficient.
This article reviews the state-of-the-art of RF energy harvesting, current and emerging applications, and considers some of the recent product introductions in this sector. These include energy harvesting development kits that support RF energy harvesting, such as the M24LR EEPROM with RF interface, and associated Discovery evaluation kit from STMicroelectronics; XLP 8-bit and 16-bit microcontroller-based energy harvesting development kits from Microchip; and a range of RFID devices and development kits from Texas Instruments, based on the MSP430 range of ultra low power microcontrollers.
Applying ambient RF
RF energy is readily available in most inhabited areas from sources such as TV, radio and cellular phone antennas as well as wireless radio (Wi-Fi) networks, base stations, routers and even portable communications devices. RF energy can be captured via a power generating circuit linked to a receiving antenna, then converted into usable DC voltage. In the same way as other energy harvesting sources, the converted power can either be stored directly in a battery, or can be accumulated in a capacitor to power circuits directly or charge a battery.
Although the power available from ambient RF energy is very low (typically measured in microwatts), it has proven to be sufficient for a number of applications, such as RFID tags, wireless sensor networks for remote monitoring, and NFC functionality in contactless smart cards and, more recently, in smartphones.
RFID tags, for example, incorporate an integrated sensor, transmitter and antenna, which harvest just enough RF energy from the RF signal sent by the reader, to send back identification information. Evolving technology is enabling tags to be made smaller and to increase read ranges. Typically requiring just microwatts of energy to transmit ID data, the read range will vary depending on the power of the transmitter. Indeed, in any application, proximity to the transmitting antenna is critical and this, combined with the power output of the source, will define the types of applications that can be supported.
Read More: Tune In, Charge Up: RF Energy Harvesting Shows its Potential