Energy harvesting embraces many techniques

One of the best-known systems that extract the energy they require from the ambient environment is the self-powered wristwatch. However, modern ICs can perform sophisticated functions on not much more energy than what a quartz-controlled electronic watch consumes, and so there is a race to develop energy harvesting techniques and standards. In many applications, the environment itself could supply the energy required through temperature difference, vibration or light.

Since energy harvesters typically capture very small amounts of energy over a long period, which then is consumed by the data transmitting system attached to the sensor, in most cases the harvester also contains an energy storage subsystem in the form of a capacitor. In application areas as diverse as aircraft construction, personal health care monitoring systems or burglary detection systems, energy harvesting, or energy scavenging, as it is sometimes called, is on the verge of developing a market with a multibillion dollar potential.

Energy harvesting is not a single technology but a broad approach that embraces many techniques that can be classified by the type of energy used: temperature differences, light radiation, electromagnetic fields, kinetic energy and so on. Industry experts such as Jrmie Bouchaud from Wicht Technology Consulting also divide the topic into macro- and micro-energy harvesting, with macro harvesting already being used on an industrial scale.

In the works
The importance of energy harvesting has motivated the German federal government to include the topic in its about Rs.2,770.71 crore ($685 million) research support program, said Marco Voigt, coordinator for the 'Autonomous Microsystems' research program at research consulting company VDI/VDE Innovation + Technik GmbH.

Research is already under way. For instance, Fraunhofer Institute for Integrated Circuits (Fraunhofer IIS) has presented a semiconductor-based thermo electric generator (TEG) that transforms temperature differences into electrical energy. Worn on the body, the device aims at using the human body heat as an energy source. An obvious application would be medical monitoring devices, said Peter Spies, group manager at Fraunhofer IIS.

The device exploits the Seebeck effect that converts temperature differences across a pair of conductors in electrical energy. Tapping the temperature differences between the human body and the ambient, the device generates very small voltages of about 200mV, too low for many electronic devices to work. "One theoretically could connect several Seebeck elements in series, but then it would become too clumsy," explained Spies. "We have developed a circuit that starts working even at 50mV, which makes it possible to turn very low temperature differences into electrical energy." Typical currents are in the low single-digit mA range. The design uses a charge pump in connection with a step-up DC-DC converter, Spies explained. The resulting energy is stored in a capacitor to have it available when needed.

Another research project recently presented by IMEC's Netherland's subsidiary site uses the piezoelectric effect. The device turns mechanical vibrations into electrical energy of about 40mW. The material of choice to be used for piezo elements today is lead zirconate titanate (PZT), explained IMEC researcher Bert Gyselinckx. The problem, however, is that this material is very difficult to handle in mass production. Gyselinckx hopes that in about five years the problems will be solved and piezo energy harvesters could enter industrial production.

Coil-magnet system
Others are one step further. Perpetuum Ltd, for instance, already has energy harvesters in industrial production. "We primarily use energy from vibrations to turn them into energy by means of a coil-magnet system," explained Roy Freeland, CEO of Perpetuum.

One problem with such systems is that they are susceptible to resonance effects, which limits the bandwidth of the vibrations that can be used to generate electricity. "The issue is to achieve a good bandwidth," acknowledged Freeland. "You need a device that picks up over the entire frequency range." How Perpetuum achieves the high bandwidth however is kept confidential by the company. The main application field for Perpetuum's energy harvesters is power sensor nodes that monitor machinery conditions in the petrochemical industry and other pump works. Perpetuum supplies 'nude' microgenerators, the customers have to add the monitoring circuitry and the data transmitting system which typically is done wirelessly.

However, Perpetuum has produced a reference design around its PMG17 electricity microgenerator that includes a CC2420 single-chip RF transceiver and an MSP430 16bit MCU, both from Texas Instruments, to make a battery-less, wireless sensor.

Freeland discriminates between vibration energy and kinetic energy. "Our definition is that kinetic energy has very low frequencies of about one hertz, the frequency of a heartbeat, for instance," he said. This indicates the application area for one kinetic energy project Perpetuum is working on: an energy harvester for a patient monitoring system While Perpetuum manufactures its energy harvesting systems in relatively low unit figures, another company has already entered mass production.

Wireless conversion
EnOcean GmbH offers electro-dynamic energy converters that are linked wirelessly to power switches: thus the switch can be placed anywhere in a room, without wires that connect it to the power grid and without a battery. The energy required to push the button is transformed in electrical energy for a wireless transmitter that triggers the circuitry to turn on the light in a room. Even though it may be considered a toy for architects, the company has already sold tens of thousands of units, claimed Zeljko Angelovski, marketing manager for EnOcean.

Besides wireless light switches, EnOcean produces thermo and solar energy harvesters. The company's thermal harvester, launched in July, starts operation at an input voltage level of 10mV, much lower than other designs, the company said.

Nevertheless, the breakthrough for energy harvesters is yet to come. The most significant market for this technology in terms of volumes is likely to be tyre pressure monitoring systems (TPMS), according to Wicht analyst Bouchaud. Implemented in MEMS technology, the TPMS systems will take off about the year 2012, he predicted. Conventional TPMS are powered by a relatively heavy and bulky battery which also makes up for the biggest part of the costs. Equipped with vibration energy harvesters, the systems could be moved from the rim of the wheel and into the tyres and acquire much more information than just the pressure: Intelligent tyres could transmit data on tyre pressure, traction, wear, temperature and much more.

- Christoph Hammerschmidt
EE Times Europe