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, such as wireless sensor nodes (WSNs), 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.
Although primary batteries claim to be able to provide up to 10 years of life, this greatly depends on both the level of power pulled out of it and the frequency with which it occurs. Systems incorporating EH capabilities will typically be able 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. This can be thought of as a “battery life extender” capability, giving the system a long working life – approaching that of the working life of the battery which is usually about 12 years for Lithium Thionyl Chloride chemistry.
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. 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 the ambient energy is not available for some reason, the secondary power reservoir can be used to power the down-stream electronic systems or WSN.
IoT drives demand too
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 embryonic, this segment includes such products as the FitBit, Google Glass and the Apple Watch. 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.