All processes that involve energy conversion are, to some degree, inefficient. Motors get hot, as do power transistors, automobile engines, and light bulbs; in each case energy is wasted as heat. Radio stations put out megawatts of RF but their signals reach antennas as microwatts. Energy harvesting devices capture some of this wasted energy, convert it to electricity, and put it to work.
The best known energy harvesting collectors are large solar panels and wind generators, which have become major alternative energy sources for the power grid. But small embedded devices must rely on energy scavenging systems that can capture milliwatts of energy from light, vibration, thermal, or biological sources. Thanks to ultra-low-power MCUs these micropower energy harvesters can greatly extend the life of batteries in consumer, industrial, and medical applications where battery replacement may be difficult, expensive, or even impossible. With careful design, energy harvesting devices can even replace batteries altogether in some applications.
Since the output from energy harvesting devices is usually small and intermittent, a system must be carefully designed that may include a boost converter, a charge controller for a rechargeable Li-Ion or thin-film battery, a regulator for the MCU and other loads, an MCU, sensors, and a wireless connectivity module. The closer an energy harvesting device can come to supplying the overall demands of an embedded system, the closer that system can come to being battery free.
Not long ago, energy harvesting was of little concern to embedded developers, thanks to the advent of ultra-low-power MCUs. However battery technology could not keep pace with the shrinking geometries of portable devices, so energy harvesting started to get a serious second look. Wireless sensor networks, for one, could not exist without ultra-low-power MCUs supported in part by micropower energy harvesting devices.
The most widely used energy harvesting devices rely on solar, thermal, RF, and piezoelectric sources of energy.
Photovoltaic (PV) or solar cells convert light energy into electricity. Photovoltaic cells have the highest power density and highest power output of the various energy harvesting devices.
Thermoelectric energy harvesters convert heat into electricity. They consist of arrays of thermocouplers that generate voltage in response to a temperature differential across their bimetal junctions (the Seebeck effect). The reverse is also true: impressing voltage on a thermocouple junction heats one junction while cooling the other which is the basis for heat pumps (the Peltier effect).
RF energy harvesters capture ambient RF radiation, rectify it, boost it, and use it to power ultra-low-power embedded devices. RFID works on that principle, though by reacting to a strong RF field that is directed at the sensor and not by harvesting ambient RF.
Piezoelectric transducers convert pressure or stress into electricity. The vibration from motors, airfoils, or roadbeds commonly power piezoelectric energy harvesters that, in turn, power an MCU that will then report any abnormalities.
There are other energy harvesting technologies under investigation, but led by the rapid proliferation of battery-powered portable consumer, industrial, and medical devices, these four energy harvesting markets will continue expanding at fast clip for many years to come.
The energy from micropower sources tends to be intermittent and, even when it is not, the output is generally so low that a boost regulator is required before the accumulated energy can be stored.
Small rechargeable Li-Ion batteries are popular in space-limited portable applications. The Texas Instruments bq24210 Li-Ion Battery Solar Charger incorporates an input voltage regulation loop with a programmable input voltage regulation threshold, making it suitable for charging from alternative power sources such as a solar panel or an inductive charging pad. TI’s bq25505 is an ultra-low-power boost converter with battery management for energy harvesting applications.
For micropower devices, thin-film batteries are an attractive storage option. The Cymbet CBC3150 EnerChip integrates a 3.3V/50 µA thin-film battery with a charge pump, power manager, and temperature-compensated charge control. The Cymbet CBC-EVAL-10 Energy Harvesting Evaluation Kit includes a small solar panel.
Micropower energy devices have trouble handling current surges such as those arise when a sensor wakes up and bursts out data. In addition to requiring power management, energy harvesters generally utilize either a large capacitor or a supercapacitor to buffer sudden peaks in demand.
Electric double-layer capacitors (EDLCs), commonly known as supercapacitors, have an energy density hundreds of times greater than electrolytics due to the extremely close proximity of their conductive layers. Compared to batteries, supercapacitors have a lower energy density but a far higher power density. Unlike batteries, they can be discharged almost immediately, which makes them well suited to handle sharp spikes in demand. However, they do have a lower internal resistance than thin-film batteries and thus discharge over time, so the two are often used together in ultra-low-power applications. Mouser carries a wide range of supercapacitors from AVX, Cellergy, Ioxus, Kemet, Maxwell, Murata, Panasonic, Cornell Dubilier, and Nichicon.
Energy harvesting represents a spectrum of rapidly evolving technologies. As design complexity increases and design cycles continue to shrink, development tools are often all but essential. In the area of energy harvesting, development kits and boards provide designers the means to evaluate and become familiar with the latest energy harvesting technologies and products.
The often iterative process of design must include development and testing beyond mere circuit simulation. Engineers frequently find themselves resorting to the seemingly inevitable Breadboard. Development kits offer a rapid method of getting to the heart of development with little set-up time.
In addition to accelerated time-to market, development kits can further provide the benefits of directly applicable, pre-tested circuits, readily available printed circuit board (PCB) layouts, and a common platform from which to create and debug customized designs.