energy harvesting For now, though, adds Ostaffe, the most
feasible method of RF harvesting is to rely on a dedicated transmitter and directional antennas that focuses RF energy so a receiver can gather sufficient power to have a sensor take readings and send results over a wireless link. This concept also allows the powering of multiple sensors from one transmitter.
Fig. 3: For the TE-Power Probe, software from Micropelt calculates how much power you can expect from a given set of application parameters
mobile-phone towers. The trick, explains Harry Ostaffe, VP of marketing and business development at PowerCast, is to use broadband antennas and receivers. Today, virtually all antennas and receiving circuits are optimised to receive energy at one frequency, and we all value radios that make it possible to hear a station without interference from other nearby stations and also make it possible to squeeze more stations into a given band of frequencies. For energy harvesting, though, you want just the opposite – broadband operation to receive the energy from as many transmitters on as many frequencies as possible. The ideal situation would be to harvest
the ambient RF energy that’s all around us, but for now it’s generally too weak to produce useful systems. Even so, in mid 2009 Nokia demonstrated a prototype mobile phone developed at its Cambridge research laboratory that recharges itself from ambient RF; that prototype scavenged between three to five mW, with the goal being to get at least 20mW to keep a phone in standby mode indefinitely and ultimately harvest 50mW – enough to slowly recharge the battery for making and receiving calls. In another interesting demonstration, PowerCast has used a standard iPhone in 2G mode to generate enough RF energy to power a nearby battery-free wireless sensor node – see a video of this on the company’s blog at
38 SCIENTIFIC COMPUTING WORLD
www.rfwirelesssensors.com. Such close- proximity applications could eventually lead to being able to power multiple computer devices such as a mouse or keyboard from a transmitter attached to a USB port, so says Ostaffe.
IN MID 2009 NOKIA
DEMONSTRATED A PROTOTYPE MOBILE PHONE DEVELOPED AT ITS CAMBRIDGE RESEARCH
LABORATORY THAT RECHARGES ITSELF FROM AMBIENT RF
Here, again the underlying operating
concept is simple: convert RF energy into DC using a rectifier and some power- management circuitry. In practice it’s not that easy. PowerCast developed a patented rectifier which has high efficiency across a broad range of frequencies. This is the basis for its two key products, the P1110 Powerharvester, intended for battery charging and has a peak conversion efficiency of 70 per cent and at close range can develop power in the lower mW range. The second is the P2110, designed for further distances and that stores energy in an intermediate capacitor to drive a load or pulse-charge a battery; it provides enough energy for a remote wireless sensor to send a data packet every 90 seconds.
Practical aspects But in addition to the device manufacturers, what are the views and opinions of users? One person taking a serious look at energy harvesters is Nigel Harley, director of strategic development at ML Electronics Ltd, a design consultancy in Salisbury. He’s currently working with a client to produce a battery-free drug dispenser worn on the body. For this, he chose a vibration sensor that produces roughly 12mJ; enough to run a low-power CPU to change the reading on an E-ink display and also charge a piezo- based device that dispenses medication to the air in powder form. In general, though, Harley thinks that,
depending on the application, oftentimes the best solution is to use a combination of RF/solar/vibrations/heat because each has weaknesses in terms of its energy source. Another thing holding up the market, he believes, is the fact that batteries are still a cheap alternate solution; in addition, some devices, such as vibration harvesters, need a sizable mass and he would like to see these products scaled down. In his mind, besides making harvesters
smaller and more affordable, perhaps the biggest issue is energy storage. These devices don’t generally generate enough energy to drive a load directly, so cost-effective, efficient low-leakage energy-storage systems are needed. Finally, he notes how his firm has dabbled
in energy-aware processors and software, pointing to the company Energy Micro AS as a good example. That firm’s EFM32 CPU uses only 160µA/MHz with a 3V supply while running application code; in Deep Sleep mode with full RAM retention it uses 0.9µA, and in Shutoff mode with a power-on reset it needs just 20nA. Also interesting is an energy-profiling utility that shows the power consumption of the actual piece of code running at a given moment, and a function listing provides the total energy that each function consumes. This, says Harley, is just another
addition to his toolkit which, along with all the other advances already mentioned, is allowing firms like his to develop some groundbreaking new devices that allow for what he calls ‘set and forget’ operation.
www.scientific-computing.com
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