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EETE SEP 2014

ENERGY HARVESTING One of the more recent emerging market segments covered under the IoT which is particularly interesting from an energy harvesting perspective is the wearable electronics category. Although still in its infancy, this segment includes such products as Samsung Galaxy Gear and Google Glass. Nevertheless, one specific form factor that has garnered high expectations is that of the wrist watch. I am not talking about the Dick Tracy 1940s 2-way wrist radio in a wrist watch form factor from the classic American comic; I am referring to today’s versions which have voice and data communication, internet browsing and streaming video capabilities afforded via a Smart phone. There are many examples on the market already, a quick search on Amazon will show over a half-a-dozen such offerings, a prominent example being Qualcomm’s Toq. Nevertheless, it, and many others, look as if they are being overshadowed by the much anticipated and much rumored iWatch from Apple. Of course, wearable technology is not just for humans, there are many applications for animals too. Recent examples include ultrasound-delivering treatment patches and electronic saddle optimization for horses to collars on other animals that variously track, identify, diagnose and so on. Regardless of the application, most of these devices require a battery as the main power source. However, for human-based applications, it looks like there will soon be wearable fabrics that can generate electricity from the sun. You can think of them as “Power” suits! One company at the forefront of such research is the European Union funded project Dephotex, which has developed methods to make photovoltaic material light and flexible enough to be worn. Naturally, the material will convert photons into electrical energy, which can then be used to power various electronic devices worn by the user, or to charge their primary batteries, or even a combination of both of these. Power conversion challenges At the low end of the power spectrum are the nanopower conversion requirements of energy harvesting systems such as those commonly found in WSNs that necessitate the use of power conversion ICs, which deal in very low levels of power and current. These can be 10s of microwatts and nanoamps of current, respectively. State-of-the-art and off-the-shelf Energy Harvesting (EH) technologies, for example in vibration energy harvesting and indoor or wearable photovoltaic cells, yield power levels in the order of milliwatts under typical operating conditions. While such power levels may appear restrictive, the operation of harvesting elements over a number of years can mean that the technologies are broadly comparable to long-life primary batteries, both in terms of energy provision and the cost per energy unit provided. Moreover, systems incorporating EH will typically be capable of recharging after depletion, something that systems powered by primary batteries cannot do. Nevertheless, most implementations will use an ambient energy source as the primary power source, but will supplement it with a primary battery that can be switched in if the ambient energy source goes away or is disrupted. Of course, the energy provided by Fig. 1. LTC3331 energy harvester & battery life extender. the energy harvesting source depends on how long the source is in operation. Therefore, the primary metric for comparison of scavenged sources is power density, not energy density. EH is generally subject to low, variable and unpredictable levels of available power so a hybrid structure that interfaces to the harvester and a secondary power source is often used. The secondary source could be a re-chargeable battery or a storage capacitor (maybe even supercapacitors). The harvester, because of its unlimited energy supply and deficiency in power, is the energy source of the system. The secondary power reservoir, either a battery or a capacitor, yields higher output power but stores less energy, supplying power when required but otherwise regularly receiving charge from the harvester. Thus, in situations when there is no ambient energy from which to harvest power, the secondary power reservoir must be used to power the down-stream electronic systems or WSN. Of course, from a system designer’s perspective, this adds a further degree of complexity since they must now take into consideration how much energy must be stored in the secondary reservoir to compensate for the lack of an ambient energy source. Energy harvesting solutions Fortunately for the designer of such systems, there exist a number of power conversion ICs which have the necessary features and performance characteristics to enable such low levels of harvested power to be used in wearable technology applications. Linear Technology recently introduced its LTC3331 to specifically address this requirement, as illustrated in figure 1. The LTC3331 is a complete regulating EH solution that delivers up to 50mA of continuous output current to extend battery life when harvestable energy is available. It requires no supply current from the battery when providing regulated power to the load from harvested energy and only 950nA operating when powered from the battery under no-load conditions. The device integrates a high voltage EH power supply, plus a synchronous buck-boost DC/DC converter powered from a rechargeable primary cell battery to create a single non-interruptible output for energy harvesting applications such as those in WSNs. Fig. 2. LTC3129 15V/200mA buck-boost converter. 26 Electronic Engineering Times Europe September 2014 www.electronics-eetimes.com


EETE SEP 2014
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