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IoT drives energy harvesting solutions for wearables

Posted: 23 Jul 2014  Print Version  Bookmark and Share

Keywords:wireless sensor nodes  Internet of Things  IoT  Google Glass  Energy Harvesting 

The portable power application domain is both broad and diverse. Products range from wireless sensor nodes (WSNs) that consume average power measured in microwatts to cart-based medical or data acquisition systems with multi-hundred Watt-hour battery packs. However, despite this variety, a few trends have emerged; namely, designers continue to demand more power in their products to support increased functionality and they want to charge the battery from any available power source.

The first trend would imply increasing battery capacities. Unfortunately, users are often impatient and these increased capacities must be charged in a reasonable amount of time, which leads to increased charge currents. The second trend requires tremendous flexibility from the battery charging solution since they need to handle a broad range of input sources and power. Furthermore, the proliferation of wireless sensors supporting the Internet of Things (IoT) has increased the demand for small, compact and efficient power converters tailored to untethered lower power devices.

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 todays 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 Qualcomms Toq. Nevertheless, it, and many others, look as if they are being overshadowed by the much anticipated and much rumoured 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 optimisation 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 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.

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