With its allure of free energy and maintenance-free operation for lifetimes of 10 and even 15 years, energy harvesting (EH) is grabbing the attention of potential users in many markets. Recent EH developments have made great progress, and the pieces appear to be falling into place.
System components such as microcontrollers (MCUs), RFICs, and power-supply ICs have had to drastically reduce their power consumption and increase performance to complete a useful energy-harvesting system. In addition, software that controls the power management, data collection, and transmission processes while avoiding any unnecessary power consumption is essential.
A transition from the “introduction” phase to the “growth” phase is occurring for energy harvesting and related micro battery products, according to analysts from the Darnell Group writing in Energy Harvesting & Micro Batteries: Market Forces and Demand Characteristics, Third Edition. The authors of the IDTechEx report “Energy Harvesting in Action” are similarly enthusiastic about numerous successful applications in a variety of markets such as automotive, consumer electronics, buildings and industrial sites, and military and aerospace.
Since the driving force for many energy-harvesting systems is a wireless application that must avoid battery replacement and node maintenance, wireless sensing networks (WSNs) are a common component in all of these markets. Figure 1 shows a typical energy-harvesting wireless sensor node for a WSN.
Anxiously Awaiting Applications
Cars and bridges were among the earliest targets for energy harvesting, but buildings could be the killer app. Initially, tire pressure monitoring systems (TPMS) for cars and light-duty trucks served as the poster children for energy harvesting based on microelectromechanical systems (MEMS). A microsystem version of the node in Figure 1 was supposed to displace the battery-based systems currently in use by generating energy from tire vibration using MEMS-based piezoelectric technology.
More recently, however, market research firms and companies pursuing TPMS have admitted that this EH application is unlikely to occur anytime soon, despite the fact that other countries are now pursuing the federally mandated requirement (FMVSS No. 138) as well.
Next in line for creating the need for monitoring based on energy harvesting were bridges and other structures subject to stress and aging, with inherent vibration as the ambient energy source. The 2007 collapse of the I-35W bridge in Minnesota seemed to present a compelling and reinforcing case for structural monitoring. Existing technology has been used to show several products’ viability to detect and avoid problems in this application. However, significant sales appear to be more of a future, rather than near-term, reality.
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If sensors are a limiting factor for bridge monitoring applications, Hewlett-Packard engineers think they have a solution: a new class of low-power, digital MEMS accelerometers. Using fluidic MEMS technology developed by HP, the accelerometers are up to 1000 times more sensitive than currently available high-volume products. The devices are part of HP’s Central Nervous System for the Earth (CeNSE) information ecosystem concept.
The company envisions complete sensor networks conducting real-time data collection for a range of applications, including bridge and infrastructure health monitoring, geophysical mapping, mine exploration, and earthquake monitoring. However, this sensing technology is still in the research and development phase.
Building automation is much further along. EnOcean’s patented energy-harvesting wireless sensor technology has attracted 100 manufacturers to the EnOcean Alliance. With 450 interoperable products installed in more than 100,000 buildings worldwide, the Alliance feels confident that it has established a standard in this area. However, it is not the only organization pursuing building automation. Lack of a single standard could become a limiting factor for industrial applications.
Vibration monitoring for early detection of failures in manufacturing and processing plants is a key application for an energy-harvesting WSN. Also, wireless sensing powered by other EH techniques such as thermoelectric generators (TEGs) is important in harsh environments such as oil rigs, including offshore platforms.
In contrast to building automation, major industrial (process control) users are hesitant to choose between two options. Recently, the International Electrotechnical Commission (IEC) approved the WirelessHART specification as a full international standard (IEC 62591Ed. 1.0) for process automation. However, it competes head to head with ISA-100.11a, Wireless Systems for Industrial Automation: Process Control and Related Applications, which was recently approved. Both have the support of large suppliers, but large potential users want a single standard. The lack of a wireless standard could significantly limit the application of energy-harvesting products.
Improving The Technology
At Sensors Expo 2010, especially in the Power Management: Energy Harvesting & Storage Symposium, several companies discussed the latest advancements in energy-harvesting technologies. The table summarizes the expectations for the power-generating capabilities of the most popular EH approaches. The voltage output of all of these sources typically varies from 0.5 V to less than 10 V. But some ceramic fiber piezoelectric harvesters, such as Advanced Cerametrics’ piezoelectric fiber composites (PFCs), can generate as much 40 V pk-pk. Also, the company’s PFC biomorph (PFCB) can easily generate 400 V pk-pk and much higher levels.
In addition to enhancing performance, most companies are making their products easier to use. For example, the Perpetuum Free-Standing Harvester (PMG FSH), which was announced in May, combines electromagnetic vibration energy-harvesting technology with selectable energy charge, storage, and management options. It features a power output up to 20 mW.
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Micropelt provides an example of the advances in thermoelectric generator capability. Based on the MPG-D751 thermogenerator, which uses semiconductor thin-film technology, the device converts 1 W of thermal power into about 2 mW of electrical power at approximately 2.5 mA. The company’s newest TE-Power Probe can provide over twice the output power in the 40°C to 100°C range (over 16 mW at 100°C), compared to the earlier TE-Power Node design (about 8 mW at 100°C) when operating in a 25°C ambient.
At Sensors Expo, Powercast demonstrated a battery-free wireless sensor module powered by RF energy. Designed for extremely low power consumption, the module uses Powercast’s P2110 Powerharvester receiver to convert radio waves in the 850- to 950-MHz range into dc power.
The receiver stores the harvested energy in a capacitor and boosts the voltage to provide a regulated supply to the module’s components (Fig. 2). In addition, the P2110 enables a microcontroller to determine the signal strength of the received power and provides the ability to recover low-rate data encoded in the power broadcast from the power transmitter. The efficiency of the RF energy harvesting is greater than 40% for input power levels from 0.2 to 10 mW.
Although still in the development phase, Enertia Energy Systems, a spin-out from the University of Michigan’s Center for Wireless Integrated Microsystems (WIMS), has an interesting technique for harvesting low-frequency vibration, which is typically limited by size. The company’s parametric frequency increased generator (PFIG) (Fig. 3) employs the snapping action of a movable mass to generate power over a wide range. This approach can harvest energy from arbitrary vibration instead of requiring a periodic vibration signal.
Tellurex Corp. used its established thermoelectric generator technology to power a wireless temperature-sensing measurement demo using waste heat with a temperature difference (ΔT) of only 4°C to 9°C. The temperature difference between a duct and a finned sink that dissipates the heat into the ambient air was sufficient to provide a regulated 3.3 V dc for the rest of the application hardware, which was co-developed with Dexter Research.
While one of the goals of energy harvesting is the elimination of the need to replace the battery at a wireless node, this does not preclude using a battery in an energy-harvesting system. In fact, several companies are developing rechargeable thin-film batteries for EH systems. Two examples are Cymbet Corp. and Infinite Power Solutions.
In addition to its 12- and 50-µAh EnerChip rechargeable energy storage devices, Cymbet announced the EnerChip Energy Processor, CBC915, at Sensors Expo. The CBC915 provides the power management for photovoltaic, thermoelectric, piezoelectric, and electromagnetic EH technologies.
Infinite Power Solutions’ newest micro-energy cell (MEC), the Thinergy MEC102, is a 4.1-V solid-state, thin-film, rechargeable single-cell battery available in 1.2-, 1.7-, and 2.5-mAh capacities. Capable of providing up to 10 mWh (36 J) of energy, the units have a high discharge rate capability of 100 mA (continuous current) and 300 mW (continuous power).
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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.