Ambient radio frequency power harvesting: A drop in the bucket
Deloitte predicts that in 2012 ambient Radio Frequency (RF) power harvesting products will likely remain a niche market with only moderate growth potential due to fundamental limits with the technology itself. While it is expected that beamed power products56 will enjoy relatively greater success, combined global revenue for both products is likely to remain modest – probably below $100 million. While revenue may rise in the coming years, increases will likely be steady rather than dramatic. Those who imagine a future in which handheld devices and tablets are powered by the sea of ambient RF energy that surrounds us are almost certain to be disappointed.
Since 1898, when Nikola Tesla first proposed the concept of wireless power transmission57, the idea of extracting ‘free’ energy out of the air has excited public interest. The concept is particularly appealing given today’s ever-expanding constellation of mobile devices and the constant risk of experiencing a dead battery in your smartphone, tablet, GPS or other portable data device.
Several high-profile laboratory demonstrations have showcased ambient RF energy harvesting: in 2009, scientists were able to power a digital thermometer with a large antenna array pointing at a nearby TV tower58. But in other real-world applications there are a number of serious challenges – including the laws of physics – that fundamentally limit the technology’s usability59.
It can be challenging to understand the limits of harvesting ambient RF power because of the many units (volts, amps, and watts) and metric prefixes (millis, micros, picos and femtos) involved. While most people know how bright a 60W light bulb is, many are less familiar with a milliwatt or microwatt. To facilitate comparisons, all units are stated in microwatts (μW). Using this scale, the familiar 60W light bulb now equates to 60,000,000 μW!
Ambient RF harvesting faces four fundamental challenges:
There is not enough ambient RF energy available. Given the seeming ubiquity of RF transmitters, it may seem that transmissions from TV stations, cellular network towers and Wi-Fi hot spots would bathe us in a steady source of energy just waiting to be tapped. However, this is not the case: there is 25 times more ambient solar energy available than RF. Putting a solar panel on the back of a wireless device would be more practical than installing an equivalently-sized RF harvesting antenna. Existing communications networks are unlikely to generate significantly more RF energy in the future: current FCC guidelines60 limit RF exposure to the general public to less than 1000 μW/cm2. This is the highest level of RF power one would normally expect to encounter. Currently, though, there is much less power coming from cell towers, which are limited to 580 μW/ cm2 at ground level61. Compare this to an average of 25,000μW/cm2 of solar radiation that is typically available.
Distances are too great. Just like all electromagnetic radiation, RF is subject to the inverse square law. Increasing the distance between the source and receiver by four times results in a 16-fold drop in power. Directional antennas can be used on the broadcast side in other wireless power transmission schemes to reduce this effect but these techniques cannot be applied to ambient power harvesting as these are mobile targets with no fixed location or orientation. The result is that power density falls off dramatically as one moves away from an RF source, meaning the amount of power to be harvested can vary considerably depending on where you are and which way you are facing.
Converting ambient energy into usable power is too inefficient. At high power densities, such as those at or near FCC exposure limits, RF harvesting systems can be relatively efficient, with some vendors demonstrating conversion rates of 60-70 percent62. However, as power density drops, so does conversion efficiency. Simply walking a few meters further away from an RF source can drop conversion efficiency from 60 percent to less than five percent63. This is in addition to the drop in power density caused by the increased distance. Antennas and power conversion devices are tuned to operate most efficiently at specific frequencies: an 850 MHz device designed to efficiently harvest ambient 3G energy will be largely ineffective with Wi-Fi at 2.5 GHz64. Supporting multiple frequencies is possible, but requires additional conversion hardware, cost and complexity65.
Environmental factors interfere with RF power harvesting. Objects around an RF device can reflect and absorb radio waves, causing areas of increased and decreased power. This is best seen through the number> of “bars” on a wireless device and how much they can vary over time and across location. Simply placing a phone into a pocket, next to all of the RF absorbing water in the human body, can virtually eliminate the power available for harvesting66. Further, while a single device harvesting ambient RF may be able to extract some power, a bus full of people with similar devices would reduce the amount of power available to almost nothing.
The combination of low power density, distance, efficiency and interference, means that under real-world conditions ambient power harvesting systems with practical antenna sizes can recover only 10-100 μW of electrical power. Since the typical tablet battery has a capacity of 15,000,000 – 25,000,000 μWh, recharging it with a RF harvesting antenna is like filling a backyard swimming pool with a shot glass. Even under ideal conditions, charging a smartphone’s smaller battery would still take decades67.
While a smartphone powered or even recharged by ambient RF will not likely be available any time soon, there are niche areas where the technology can be useful. Small sensors, that periodically build up enough charge to report back, or that rely on separate readers to give them the RF surge needed to transmit data could be powered by RF68. An example would be a sensor to monitor ceiling temperature where there is no power supply and is too high to easily change batteries.
There is also potential for beamed or broadcast power solutions that use similar technology combined with tuned RF sources to power or charge devices at a distance of several meters. Next generation TV or game consoles could be equipped to wirelessly charge associated remotes and controllers from across the room. Also, heating, ventilation and air conditioning (HVAC), warehouse and building-telemetry solutions based on short-range (tens of meters) wireless power transmission are starting to enter the market69.
In summary, harvesting ambient RF power will likely remain a niche solution with moderate growth potential in 2012. While this technology is impressive on the lab bench, physical constraints and real world limitations will likely prevent widespread adoption. Although some of these challenges could likely be surmounted by increased research, others such, as distance and interference, are dictated by fundamental laws of physics. That being said, adjacent technologies, such as beamed power, could have greater long-term potential.
Deloitte Canada, as referenced in videos, podcasts, or online materials related to TMT Predictions 2012, refers to Deloitte & Touche LLP, the Canadian member firm of Deloitte Touche Tohmatsu Limited.
56Rather than relying only on a passive device to harvest ambient RF, beamed power products pair a receiver with an optimally tuned RF transmitter designed to transfer power over ranges of centimeters to meters.
57Tesla Biography, Tesla Website: http://www.teslasociety.com/biography.htm
58Experimental Results with two Wireless Power Transfer Systems, Intel Research Seattle http://Web.media.mit.edu/~jrs/WISP-WARP.pdf
59This should not to be confused with beamed power products designed to transmit power over short distances (centimeters to meters), or high-powered microwave and laser based wireless power transmission technologies which have been successfully used to power devices as large as aerial drones.
60Questions and Answers about Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields, Federal Communications Commission, August 1999: http://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet56/oet56e4.pdf
61Questions and Answers about Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields, Federal Communications Commission, August 1999: http://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet56/oet56e4.pdf
62Powercast Wireless Power Calculator, Powercastco: http://www.powercastco.com/wireless-power-calculator.xls
63Powercast Wireless Power Calculator, Powercastco: http://www.powercastco.com/wireless-power-calculator.xls
64Conditions vary, but outside of its optimal frequency, power harvesting antennas are able to yield less than 0.1 percent of the power that they can achieve at their optimal frequency.
65Powercast Wireless Power Calculator, Powercastco: http://www.powercastco.com/wireless-power-calculator.xls
66Microwave Absorption in Humans, Antennex: http://www.antennex.com/preview/Folder03/Jul3/mwabsor.htm
67Although in reality the self-discharge rate of the batteries used in mobile devices is greater than the re-charge rate generated by any practical ambient RF technology: the battery drains faster than you can fill it!
68Passive RFID Basics, Microchip, 1998: http://ww1.microchip.com/downloads/en/appnotes/00680b.pdf
69RF Energy Harvesting and Wireless Power for Low-Power Applications, PowerCast, 2011: http://www.powercastco.com/PDF/powercast-overview.pdf