Supercapacitors also provide an effective energy-storage alternative in some EH systems. Working with Perpetuum, Powercast, Micropelt, Perpetua Power Source Technologies, and other energy harvesters, the CAP-XX G and H series supercapacitors from CAP-XX have demonstrated the performance required for low-power EH systems. These include operation over a wide temperature range, low leakage current, minimal initial charging current, and predictable self-discharge characteristics. The CAP-XX HZ1, for example, is a 2.75-V (nominal), 180-mF high-temperature (–40°C to 85°C) supercapacitor.
As MicroStrain’s vice president of wireless systems Jake Galbreath noted in his presentation, users have to work to balance the energy checkbook for a successful energy-harvesting application. Compatible with piezoelectric, electro-dynamic, solar, RF field, and thermoelectric harvesters, the company’s EH-link Energy Harvesting Wireless Node with an IEEE 802.15.4 radio consumes 12 µJ at start-up, only 105 µJ for either an accelerometer or humidity sensor measurement, and 92 µJ to transmit the data.
Another system component being continuously improved is the microcontroller. Microchip Technology’s nanoWatt XLP (eXtreme Low Power) MCUs have been recognized as the industry’s leading low-power sleep-current MCU with sleep currents below 20 nA. Additional low-power MCU capabilities for EH systems include the watch-dog timer, real-time clock, and run currents. Texas Instruments, Analog Devices, and other MCU suppliers are addressing the need for ultra-low-power operation in EH applications as well.
In recognition of the importance of power management in EH systems, Linear Technology has developed a family of energy harvesting products. The LTC3108 is an ultra-low-voltage dc-dc converter and power manager designed specifically for low-input-voltage sources like thermoelectric generators. The chip can collect and dispense surplus energy in EH systems. The LTC3588-1 provides the power supply for piezoelectric energy harvesting, and the LT3652 can be used as a battery charger in solar power applications.
System Approach Required
Perhaps the biggest challenge for designers implementing energy harvesting is the total system approach that must be taken. To simplify this task, essentially all of the companies pursuing energy harvesting applications have development kits, boards, tools of some sort, and in some cases, software to efficiently manage the harvesting, measurement, and transmission processes.
Rather than duplicating efforts, suppliers of different parts of the system have readily collaborated. The benefits of those partnerships can be easily seen by investigating the design support tools of companies mentioned in this article. With a simplified design-in process, the available energy can be tapped for many wireless measurements.