Pulling Power Out Of Thin Air

Energy harvesting over the airwaves draws increasing attention and research dollars; cell phones that don’t need to be plugged in?

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By Cheryl Ajluni

It wasn’t all that long ago that voice communication via a traditional landline was the norm. At the time, consumers would have been hard pressed to imagine a world in which anytime, anywhere communication (voice and data) with a device no bigger than the human hand was possible.

Many of those same consumers might today find it hard to conceive of a world in which their mobile phones are powered by the jacket they are wearing, but that too—like the mobile phone—may one day become a commercial reality. Given the ongoing research and developments in this area, that reality may be closer than many people think.

One technology hoping to give life to this vision is energy harvesting—a process by which energy is derived from an external ambient source (e.g., kinetic energy, RF, solar power, thermal energy, or vibration and wind energy), captured and then stored. Energy harvesting devices convert the ambient energy into electrical energy. In a wearable electronics application, for example, power captured from an ambient energy source is converted to electrical energy and stored in a device like a battery or a capacitor. The stored power can then travel through a microprocessor and be subsequently transmitted, usually wirelessly. Energy harvesters generally provide only small amounts of power (e.g., just a few milliwatts), dependent in part on their design and size, and are therefore considered suitable for powering low-energy electronics or small, autonomous devices like those used in wearable electronics and wireless sensor networks.

Energy harvesting technology has a number of critical benefits, namely cost and extended battery life. Ambient energy, the fuel that drives energy harvesters, is present in large quantities in nature and is, for all practical purposes, free. By harvesting or “scavenging” small amounts of power from these sources, the battery life of existing devices can be extended. But energy harvesting also has the potential to enable a new class of battery-free devices that can be powered indefinitely and deployed with minimal to no maintenance.

The problem with energy harvesting is that it requires ready availability of a potentially unreliable power source. In very simplistic terms: what happens if the energy harvester relies on wind and the wind dies down? Or what if it relies on sun and it’s a cloudy day? One possible solution to this dilemma is RF energy harvesting, which is now being eyed by the wireless communications industry as a viable power source for wireless electronic devices. RF energy is, after all, available everywhere—particularly in metropolitan areas. Moreover, RF energy harvesting can be coupled with a dedicated radio transmitter to provide remote power that is controllable through continuous, scheduled or on-demand power transmissions.

Energy harvesting on the cusp
RF energy harvesting devices have the potential to one day be used to power or recharge mobile phones, mobile computers, and even radio communications equipment using ambient radio waves emitted from WiFi, phone towers, television signals, and other sources. But is the technology really ready for prime time? In truth, the commercialization of this technology is likely a number of years away, but there is definitive progress being made. One company actively exploring that very possibility is the Finnish mobile phone maker Nokia. At the Nokia Research Center (NRC), researchers are investigating the concept of an energy harvesting handset—one that uses energy harvesting technology to recharge itself using only ambient radio waves. Energy harvesting would need to account for roughly 20 mW+ of power to keep a handset in standby mode indefinitely. Recharging the handset’s battery would require approximately 50 mW of power.

Intel also is actively researching the harvesting of free energy sources like the sun, kinetic energy and RF energy. In fact, earlier this year it conducted an experiment in which it harvested ambient RF energy using a television antenna pointed at a local television station tower. It harvested enough energy to actually power a wall-mounted, household weather station with an LCD screen, effectively proving that wireless power over a distance and battery-free operation is possible.

For the experiment, Intel employed an ambient RF harvesting technique similar to technology typically employed with off-the-shelf RFID tags. Here, unpowered ID tags are powered wirelessly from a tag reader that supplies just enough power to the ID tag so that it can read the information it contains. With RFID tags, the ID tag and tag reader must be in close proximity to one another. With Intel’s RF harvesting technique, the weather station was powered by a television station antenna located some 4 km away. The television antenna was connected to a 4-stage charge pump power harvesting circuit featuring the same design as that found in an RFID tag. Across an 8-KOhm load, researchers measured 0.7 V, corresponding to 60 microwatts of power harvested. That was enough to drive a thermometer/hygrometer and its LCD display, which is normally powered by a 1.5-volt AAA battery.

Key enablers
One factor playing a key role in moving RF energy harvesting forward is the advent of ultra-low power electronics for the power-conscious wireless communications industry. In the past, engineers and researchers working on energy harvesting technologies were hard-pressed to make their energy harvester designs work. They simply couldn’t harvest enough power to run a microcontroller. Today though, the power being harvested with energy harvesting devices is on the rise and the power electronic products require is decreasing. The convergence of these two trends is, for the first time, making energy harvesting technology a viable energy source in many markets, and could result in the emergence of a new class of renewable energy applications that essentially run forever—autonomously, remotely and without a battery.

Two companies that are working hard to develop low-power electronics (e.g., DSPs, microcontrollers, RF transceivers, and sensors operating on just μA’s of electrical current) for use in emerging technologies like energy harvesting are Analog Devices and Texas Instruments. ADI, for example, offers an ultra-low-power MEMS (microelectromechanical system) sensor, the ADXL345, which consumes 120 μA in full dynamic range and 25 μA in sleep mode (Figure 1). TI’s MSP430 microcontroller consumes a mere 160 μA/MHz in an active state and 1.5 μA/MHz in standby mode (Figure 2).

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Figure 1. Shown here is a functional block diagram of ADXL345—a small, thin, low power, 3-axis accelerometer with high resolution (13-bit) measurement at up to ±16 g.

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Figure 2. TI’s MSP430 MCUs are comprised of a 16-bit RISC CPU; modular, memory-mapped analog and digital peripherals; and a flexible clock system combined using a von-Neumann common memory address bus (MAB) and memory data bus (MDB). They are considered the industry’s lowest power solution for 8- to 16-bit battery-powered measurement applications.

Even University researchers have jumped on the bandwagon. Kansas State University, for example, has recently developed a single-chip microtransceiver for use in energy harvesting radio technology destined for future Mars missions as well as other earthly applications like powering radios for remote wireless sensors. The low-power RF chip uses a silicon-on-sapphire CMOS process from Peregrine Semiconductor Corporation (www.peregrine-semi.com) and operates in the 390-450 MHz band with 100mW output. It includes an integrated transmitter and superheterodyne receiver with an off-chip IF filter. To date, University researchers have already produced proof-of-concept hardware for using the single-chip radio in energy harvesting applications.

Conclusion
With the continued drive toward more energy efficient devices—especially when it comes to wireless communications applications—industry is being challenged to identify new means of powering devices. Energy harvesting, and in particular RF energy harvesting, offers one viable solution. While it may be awhile before wireless devices powered through this method make their way to market, progress is being made. Ultra-low power electronic components will play a key role in moving this technology forward. Coupling these components with energy harvesting technology is today allowing applications that were once unthinkable, like a knee brace that generates power from walking. In the years ahead, this technology will likely be leveraged with other alternate energy solutions and things like better power delivery and power management, to create electronic products that are both cheaper and more ecologically friendly.