Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.

Harvesting RF energy

Conservation of energy starts by reusing energy that has already been expended. That energy is readily available in electromagnetic (EM) form, from broadcast AM and FM radio waves to the many wireless devices that transmit signals around us, such as cellular base stations and short-distance wireless local area networks (WLANs). The key to harvesting or scavenging this “used” energy starts with a dedicated receiver capable of receiving the available wireless signals, along with some means of converting the received signal power into a supply of electrical power.

Many leading suppliers of wireless communications products are well aware of the eventual need to conserve battery power as users became more dependent upon those devices. The evolution of the cellular telephone into a personal messaging and entertainment center has meant that these portable radios can do more, but it also means that they require more power to do so. If limited to batteries as power sources, the increased current drain from the added functionality will result in less operating time per battery charge (and less billable hours for service providers).

For a while now, developers of mobile communications devices have sought to ease the load on the batteries in those devices via some form of energy recovery system. With the growing popularity of Internet of Things (IoT) and machine-to-machine (M2M) devices for automated remote control of electronic devices, IoT applications are being envisioned—for homes and factories alike—that could potentially remain powered for years awaiting a trigger. With energy harvesting capability, such devices can literally pull energy out of the air to recharge their own batteries or harvest enough energy from the environment so that a battery may not even be required for power.

Such devices are now typically referred to as “zero-power” wireless sensors for their capability of providing sensor data directly on a wireless channel or by means of the internet, using a wireless gateway with no apparent source of energy. The “batteryless” approach has been commonly used with radio-frequency-identification (RFID) tags that transmit an identifying signal based on received power from an RFID reader’s transmitted signals (as the source of power).

By harvesting power from available RF energy sources, a new generation of ultra-low-power (ULP) wireless devices, such as IoT sensors, can be developed for low-maintenance applications like remote monitoring. Energy harvesting is considered very much a “companion” technology to wireless communications, since it can enable extended battery lifetime for mobile devices and possibly battery-free operation for some electronic devices.

Creating Energy

Energy can be harvested from a number of different sources, including light, heat, vibration, motion, pressure, magnetic fields, and RF/microwave signals. Some methods of producing energy are quite creative but practical. Enocean, for example, makes wireless light-emitting-diode (LED) lamp switches that use the pressure of a user’s hand on the switch as the source of energy. Pressing on the switch generates the dc power needed to transmit wireless on/off signals to an LED lamp within line of sight of the switch.

The basic concept for harvesting energy from an RF source typically involves an extra antenna to receive the desired wireless signals via a wireless product’s primary receiver. Alternately, a secondary receiver may be dedicated to energy harvesting that covers the frequency range of interest (see figure). Received signals are applied to some form of rectifying circuitry to convert the wireless energy to dc power.

In some cases, an antenna that incorporates the rectifying circuitry, known as a rectenna, may be used to save space. The antenna portion of a rectenna can be almost any form of antenna suitable for the frequency band of interest. Options include a monopole, dipole, or microstrip patch fabricated on printed-circuit board (PCB), along with rectifying circuitry based on nonlinear rectifying devices (such as Schottky or IMPATT diodes). The antenna will be joined to the rectifying circuitry by means of impedance-matching circuitry and filters, such as lowpass filters, to block any harmonics generated by the diodes.

Conversion efficiency is critical to any energy-harvesting solution. The antenna and receiver will determine the amount of RF signal power available for rectification, while the diodes and diode rectifying circuitry will determine the RF-to-dc conversion efficiency. Energy-harvesting circuits have historically taken advantage of plentiful sources of RF energy, such as FM radio signals, which can be collected with simple stick antennas and basic diode rectifying circuits.

Voltage regulation is an important part of any energy-harvesting solution, as it ensures that a stable supply of electricity is being provided to the load. For that purpose, a number of IC suppliers offer various forms of voltage converters with built-in regulation to maintain consistent voltage and current for the intended load. Linear Technology, for example, has developed a family of energy-harvesting power supplies with capabilities that target different forms of initial energy sources.

The ICs, which integrate a full-wave bridge rectifier with a buck converter, are designed for lower current operation with lower-energy sources (like thermos-electric generators and piezoelectric sources) and higher currents with higher-power sources (e.g., solar and RF energy). In fact, many IC suppliers with energy-harvesting power supplies, including Silicon Labs and Texas Instruments, offer reference designs to show their ICs in typical circuit applications.

These ICs are not simple power-supply circuits, but sophisticated means of (in some cases) controlling trickle charges to a battery from a low-power energy supply. A device such as the LTC3588-1 from Linear Technology is a nanopower energy harvesting power supply for use with high-output-impedance energy sources such as piezoelectric, solar, or magnetic transducers. It allows charge to accumulate on an input capacitor until the IC’s buck converter can efficiently transfer a portion of the stored charge to a load (like a battery) at the output.

It provides four pin-selectable output voltages of 1.8, 2.5, 3.3, and 3.6 V with as much as 100 mA continuous output current to accommodate many different load or battery requirements. A larger output capacitor can be used with the IC when higher output current bursts are needed. This IC, with an input voltage range of 2.7 to 20.0 V dc, is part of a total energy-harvesting solution, along with an antenna, receiver, and rectifying element. Depending upon the amount of energy available from a source, the designer of an energy-harvesting solution would select the power-supply circuitry according to the expected voltage and current range to be fed to the load.

Energy sources are all around, and RF/microwave signals are just one type of those sources. Military equipment suppliers, for example, have already experimented with circuits that extract energy from motion, such as using a soldier’s walking motion to generate the power supply for recharging a portable radio system. In the medical world, where implantable devices must be powered by external power supplies, ICs are being developed with on-chip antennas and the capability to draw power from radio waves in a patient’s environment. The rapid growth of IoT devices and applications will be creating increasing demand for energy-harvesting solutions that can free many future wireless devices from their dependences on batteries.

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.