The idea of energy harvesting isn’t new. The water wheel, wind mill, hot water thermal, and the sail have been around for a thousand years. Newer forms of energy harvesting, like solar panels, and refinements on older technologies, like wind mills, tidal power, and geothermal plants also provide a growing source of power for large communities, but in the age of shrinking and personalized electronics systems, the need for much smaller energy sources has become readily apparent. These small devices often need only miniscule amounts of power, so harvesting energy from nontraditional sources — like RF, piezoelectric and small photovoltaic cells — can be an attractive alternative to the traditional battery technology.
Energy harvesting can be an incredible boon to small-form-factor devices, like those proliferating in the Internet of Things (IoT), but often the energy available from harvesting may not match the energy needed for the application. A wireless sensor, for example, may receive enough energy from a photocell to make required measurements during the day, but if it also needs to make measurements at night, it needs an alternative source of power. Similarly, the energy generated from a harvesting activity may not be uniform, and the storage element, like a traditional battery, may not be able to efficiently store energy from a “spiky” source. An intermediate storage element may be needed to respond quickly to energy availability (similar to the way a cache memory in a hierarchical memory system can store data more quickly that a secondary DRAM can) and over time transfer excess energy to the primary storage subsystem.
Thus it is necessary to fully understand the requirements of the system being powered by the energy-harvesting application and match it with an energy storage element that optimizes the overall performance of the system. Batteries may be appropriate when power is delivered uniformly over a long period, such as that available from solar cells in a known-sunny environment. Traditional batteries may not be a good fit, however, when energy or space requirements are overly constrained. Let’s look at some options for energy storage that might prove more appropriate for overly constrained applications.
When energy delivery is less uniform and if energy needs are small, perhaps when a very-low-power MCU is being used, a simple capacitor may be sufficient to store the energy needed to operate the MCU. If more power is required, perhaps due to external memory, peripherals, or sensor, a supercapacitor can be used to store sufficient energy when more processing and communications functions are needed. Supercapacitors have specialized depending on the requirements of the application. The Maxwell BCAP Ultracapacitors, for example, are available with up to 3.3 F, and store sufficient current to run a low power MCU for weeks at a time between recharging. The AVX BestCap Pulse Supercapacitor series is optimized to deliver very-short-duration pulses, like those associated with wireless data transmission or signal processing. Panasonic Electric Double Layer Gold Capacitor series have no limit to the charge discharge cycle so they are appropriate for energy-harvesting applications that capture frequent “bursts” of energy and also release the energy in bursts, perhaps from piezoelectric sources.
Thin film batteries, like the EnerChip family from Cymbet, are an option that combines some of the advantages of both batteries and supercapacitors. Energy can be stores for long periods if needed, but can also be made available quickly when required. Perhaps even more important, the very small size of thin-film batteries, like the EnerChip device, allows them to be used in ultraminiature applications and sensors in particular. One recent example of such an application is for a sensor to measure pressure within the eye of a glaucoma patient. Energy is harvested from light using a solar cell and stored in an ultraminiature 1-µAh EnerChip solid-state battery. A MEMS pressure sensor, low-power MCU, analog-to-digital converter and wireless transceiver are all powered by the EnerChip for up to 28 days with no energy harvesting.
Energy harvesting at the miniature level is opening up entire new applications areas and is dramatically impacting developments for the IoT. Energy storage device manufacturers are specializing their devices for different applications so understanding the various options will be of critical importance in these classes of designs.
By Warren Miller
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