Find a downloadable version of this story in pdf format at the end of the story.Some manufacturers are relying on converting various forms of energy into electric voltages and currents. These forms include piezoelectric, magnetic, capacitive, RF, inductive coupling, solar, kinetic and thermal effects. Such sensing methods are getting better all the time, improving their outputs and efficiency levels, thus making them more attractive for a growing number of applications.
Energy scavenging sources can be obtained from the human body, moving and rotating machinery, solar energy, heat differentials between materials, inductive coupling and magnetic fields. They can act as both sources as well as work with existing semiconductor IC sensors, making them flexible and valuable for a number of applications including wireless sensor networks, infrastructure monitoring, medical implants, and a host of other applications.
Making the use of energy harvesters and scavengers more attractive is the fact that the ICs that work with such sensors are not only becoming smaller, but are also more sensitive to low-level signals and need less power for their operation. These qualities play well when matching energy harvesting and scavenging sensors with interface circuitry.
In addition, research has shown that energy scavenging and harvesting devices can be integrated on the same CMOS wafer that holds electronic circuitry. That's what researchers at the Netherland's Twente University and Utrecht University, working with researchers at China's Nankai University have shown, by growing a self-powered copper-indium gallium-selenide (CIGS) photovoltaic solar cell onto CMOS chips that requires no batteries for power (Fig. 1).
Solar energy is a very omnipresent form of energy harvesting. Obtained from photovoltaic sensors, it is one of the biggest energy harvesting applications around whose use continues to grow. There are far too many developments in solar energy technology to be described aptly in this article, but suffice it to say that impressive gains are being made in this technology's development.
The most common form of energy harvesting is manifested in the piezoelectric effect. Materials like certain ceramics and crystals can generate electricity in response to an applied mechanical strain. This behavior is also reversible. An electric field applied to the material produces a mechanical movement. It should be noted that not all ceramic and crystal materials are piezoelectric. This property comes when they're formulated with other materials.
Popular piezoelectric materials are commercially available in large-area formats as piezoelectric ceramic fibers and fiber composites from companies like Advanced Ceramics Corp., Measurement Specialties Inc., and Advanced Cerametrics Inc.
An example of a piezoelectric energy harvester is the MFC macro fiber composite produced by Smart Materials Corp. MFCs are essentially encapsulated piezo ceramics to which metal is added. They're available in elongator (d33) and contractor (d31) modes, which act as powerful actuator/sensor and low-impedance sensor/energy devices, respectively (Fig. 2).
As an actuator, an MFC device operates over a range of 1 Hz to 10 kHz. As a sensor, it has a range of 0.5 Hz to 500 kHz. MFCs are flexible and robust and are available in ready to use packages that overcome the disadvantages of lead zirconate titanate (PZT) plates or patches based on solid wafers. They're rated for more than 109 cycles of actuation and more than 1011 cycles for energy harvesting. They can also be integrated with electronic components.
AdaptivEnergy is yet another company that capitalizes on the piezoelectric effect. Its Joule-Thief dc power devices harness energy from vibrations, impact and force, and convert that into usable electrical power.
One of the more interesting applications of the piezoelectric effect has been shown by France's Arveni. It has demonstrated a proof-of-concept battery-less infra-red (IR) TV remote control. The device uses a powerful piezoelectric microgenerator, which delivers up to 90 mW (for the first IR transmission). Arveni also integrated a low-power radio based on the IEEE 802.15.4 standard. This radio features bidirectional communication, ideal for point to point applications where the user wants to know if his command has been received correctly. In this case, Arveni added an LED onto the remote control, also powered by the microgenerator, which lights when the acknowledgement of receipt of the receiver is received by the remote control (Fig. 3).
POWER FROM VIBRATIONS
Vibrations are a rich source of energy that can be harvested into electrical power. Midé, for example, sells a line of Volture energy harvesters that convert vibrations into electric power using a piezeoelectric sensor. However, vibrations tend to vary in terms of frequency, amplitude and bandwidth, depending on the vibration source, so “there's no silver bullet to satisfy every application”, explains Chris Ludlow, director of mechanical engineering for Midé. “Every application is different. Vibration energy harvesters used on San Francisco's Golden Gate Bridge are different from those used on Boston's Zakim Bridge, because they have different vibration characteristics,” he adds.
For this reason, Midé came up with the SlapStick, a rechargeable USB-connected data logger that measures acceleration in three axes. Configuration software lets users tailor an energy harvester to their specific needs.
A major developer and manufacturer of energy scavenging motion sensors is the U.K.'s Perpetuum Ltd. Its FSH free-standing harvester combines electromagnetic vibration energy harvesting with a selectable set of energy charging, storage and management options. They're optimized for industrial machine-condition monitoring and work with wireless sensor networks.
Perpetuum recently successfully demonstrated a series of trials on trains in the U.K. that showed that their products can provide energy to power sensors to monitor the condition of wheel bearings. The trial showed that wireless vibration spectra can be correlated with faulty bearings, and that this data can be transmitted wirelessly, thanks to tens of milliwatts of electrical energy generated for such a task.
MicroStrain Inc. is one of the more notable companies making use of piezoelectric technology to convert vibrations to electrical energy. It is the leading provider of inertial measurement systems, displacement transducers and wireless sensing networks used in health monitoring of civil structures and military/aerospace applications. Its patented temperature-compensated differential variable-reluctance transducers provide extremely small size and high-accuracy attributes and the ability to withstand high temperature gradients, saline solutions, and pressurized environments.
A useful source of vibration energy harvesting comes from AC motors, which vibrate between roughly 50 Hz to 240 Hz. A piezoelectric sensor mounted on an ac motor to produce electrical energy and report on the motor's condition using a wireless sensor network. Helicopters and rotary wing aircraft are another source of vibration energy harvesting, vibrating between 10 Hz and 40 Hz.
The output power of a vibrating energy harvester is directly proportional to the amplitude and frequency of the vibration and to the size (seismic mass weight) of the harvester. And the output power is inversely proportional to the vibrating source's frequency bandwidth. As a result, it is much harder to efficiently harvest power from a low-frequency source with a large frequency band response and a very small size than from a stabilized high-frequency vibration source. Many of the vibrations found in natural and man-made environments are relatively low in frequency (under 120 Hz) and often depend on energy sources with varying activity levels such as engine vibrations, vehicle speed, wind level, etc.
To address this problem, researchers at the Laboratory for Electronic Information Technologies for the French Atomic and Alternative Energies Commission, CEA-LETI, are focusing on a solution. They have developed a self-adaptive and electrostatic microelectromechanical system (MEMS) vibration energy harvester that efficiently converts both low- and high-amplitude vibrations into electrical energy, thanks to a unique and patented electrode structure.
The MEMS-based electrostatic structure translates the input vibration into multiple capacitance variations, which are used to convert the input vibration energy into electrical energy. When a constant charge is placed in the variable capacitor, the voltage varies in inverse proportion to the capacitance variation and the associated energy varies proportionally to this voltage.
A silicon oxide-based dielectric electret was developed that keeps its electric charge over many years, even when built into very small electrodes less than 20 µm. The electret behaves very much like a permanent magnet in close proximity to a coil. When an electret changes position relative to two electrodes, it induces a new charge distribution on the electrodes. If an electrical load is connected between these electrodes, then the electret movement generates electrical energy. Because the structure is electrostatic, there are no resistive losses, unlike small electromagnetic systems with low operating frequencies where the losses generated by the coil grow drastically as the size and frequency decrease. Tests performed on this structure produced a desirable output voltage as a function of vibration amplitude and frequency for a 100-g seismic mass (Fig. 4).
HARNESSING TEMPERATURE GRADIENTS
Taking advantage of temperature gradients is one of the oldest sources of energy harvesting. Thermocouples in use for generations are the most common example of devices that use the temperature gradient technique.
For instance. the Dolphin platform of modules from Germany's EnOcean Inc. is based on creating electricity from ambient thermals, solar and motion environments. It allows OEMs to develop battery-less and wireless communications. It can be used for sensors and switches in new and retrofit building automation heating, ventilation and air-conditioning (HVAC) applications. EnOcean says its modules, like the STM300 868-MHz wireless sensor transceiver, feature the industry's lowest-power sleep-mode current of just 200 nA and consume one-tenth the power of conventional low-power modules (Fig. 5).
EnOcean's technology is used by Illumra Controls Inc. in key card switches to automate hotel-room energy consumption. It does this by disabling HVAC, lighting and electrical loads in unoccupied rooms. The technology is also used in battery-less active automotive tire-pressure monitoring systems (TPMSs) developed by Austria's SensoDynamics.
Germany's Micropelt GmbH employs an aluminum-oxide substrate on which multiple thermocouples are laid, thermally in parallel and electrically in series. Up to 128 thermocouples can be laid on a 1600-mm2 substrate to produce approximately 50 mV/K. A Micropelt MPG-D751 can convert 1 W of thermal energy into about 2.5 mA, an energy output that is comparable to that from a button cell-sized battery.
Nextreme Thermal is yet another company getting into the thermoelectric power generation market. The company's eTEG HV37, based on thin-film thermoelectric strips, can produce 1 mW of output power and an open-circuit voltage of 170 mV, at a temperature differential of 10K in a footprint measuring 6 mm2. A 50K temperature differential produces 24 mW at an open-circuit voltage of 850 mV.
MUCH ELECTRONIC SUPPORT
Major electronic power supply and microcontroller unit (MCU) manufacturers are working closely with energy scavenging and harvesting companies to promote the greater use of such systems by offering development kits. Linear Technology, for example, offers its LT3588-1 energy harvesting power supply that works with piezoelectric sensors (Fig. 6).
Companies such as Powercast provide the P1110 and P 2110 receivers that operate between 850 MHz to 950 MHz by harvesting RF power. The wireless harvesting approach provides controllable and reliable power that does not rely on intermittent sources. “You can turn on a Powercaster transmitter and provide power for as long as needed,” explains Harry Ostaffe, vice president of marketing and business development at Powercast. “Once charged, the receiver can send a message to have the power turned off,” he adds. The transmitter produces a 3-W effective output isotropic radiated power (EIRP) at 915 MHz, and the companion receivers can pick up that energy from 40 to 50 feet away.
Powercast has teamed up with Microchip Technology to offer a development kit that includes a Microchip Technology PIC24F XLP microcontroller and a wireless transceiver (Fig. 7).
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