People have been fascinated by the idea of ‘free’ energy for hundreds of years, with many scientists and engineers attempting to create perpetual motion machines, even after the law of Conservation of Energy became generally accepted. Unfortunately, losses due to mechanical friction and air resistance mean that no machine can be built that will continue to move forever with no need for additional energy after an initial input.

Another view of ‘free’ energy is renewable energy sources such as solar, wind and wave energy, where the supply is unlimited and no financial payment has to be made to use the energy ­– even though considerable investment has to be made in suitable energy-harvesting equipment. If only small amounts of energy are required for a given application, several other options are available. For example, self-winding wrist watches have been manufactured for over 80 years, and kinematic powered quartz watches have been on the market for around 20years. Other electrical and electronic technologies offer additional possibilities.

Passive RFID (radio frequency identification) tags can harvest sufficient energy from the RF reader/writer that they can power an integrated circuit and transmit data to the reader and/or decode and record data on the internal memory. Furthermore, devices using micro-electro-mechanical system (MEMS) technology only require tiny amounts of energy, leading many academic groups to investigate ways to harvest sufficient energy from the environment to provide the necessary power.

One option is to harvest energy from the electromagnetic waves that surround us as a result of radio and television broadcasts, mobile telephone communications, and the electromagnetic fields created by electric currents passing through conductors. This can be made to work provided a large enough collection area can be used and the RF power is high enough – which usually implies the source and receiver have to be close together.

More promising results have come from teams investigating ways to harvest energy from vibrations. Southampton University in the UK has been at the forefront of this field of research, through its membership of the multi-million euro EU-funded Vibes (Vibration energy scavenging) project (Fig.1), and in 2004 a company called Perpetuum Limited was spun off to commercialise the technology.

Production microgenerators

Perpetuums high-performance microgenerators convert vibrations into useable electrical energy to power sensors, microgenerators and RF systems. Several application areas are being pursued, focusing on wireless sensors for the industrial, transportation, aerospace and medical sectors. The benefits for end users are that the cost of installation and maintenance are lower than for conventional wired sensors because there are no cables to route and no batteries to replace. Furthermore, sensors powered by energy harvesting microgenerators open up new possibilities in application areas where the use of sensors would previously have been unfeasible.

So far Perpetuum has completed the development of a family of three microgenerators, all of which enable data to be transmitted reliably, either as several kilobytes every few minutes or as much smaller amounts of data several times per second. By harvesting vibration energy, the microgenerators can produce several milliwatts of power (Fig.2). The PMG17 is a vibration energy-harvesting microgenerator primarily designed for condition monitoring and process instrumentation in industrial applications. Alongside this, the PMG27 microgenerator is suitable for aerospace applications and the PMG37 is for transportation applications. In addition, a MEMS-based vibration energy-harvesting device is under development for medical applications.

Perpetuum’s vibration energy-harvesting microgenerators are based on a highly optimised magnetic circuit coupled to a mechanical resonator. This arrangement transforms the kinetic energy of vibration into electrical current.

The PMG17 microgenerators are intended for use on machinery driven by ac motors and harvest ‘twice-line-frequency’ vibration. Even with as little as 25mg (RMS) vibration within a 2Hz bandwidth, these microgenerators will always produce a minimum power of 0.5mW, though up to 40mW can be generated when there is more vibration available. These devices output alternating current and would normally charge a storage capacitor via rectification.

Robust, maintenance-free and capable of operating in a multitude of industrial environments, the PMG17 microgenerator is also easy to install in any orientation, with no shut down of operations required. For hazardous-area deployment, it is also available in an intrinsically-safe, ATEX-certified version. Two versions are offered, the PMG17-100 that operates in the 98 to 100Hz range, and the PMG17-120 for the 118 to 120Hz range. At the 2007 Hannover Fair, Prüftechnik unveiled what was described as the world’s first low-cost industrial wireless condition monitoring system – with sensors powered by the PMG17-100 microgenerator (Fig.3). Similar in concept, the PMG27 model for aerospace applications harvests energy from the vibrations induced by engines or rotor blades, and the PMG37 – for the transportation market – harvests energy from the vibration induced by motion.

Still in the development phase, the silicon MEMS-based microgenerator is intended to produce a few hundred microwatts from a package measuring 5x5x1.5mm.

Alternative technologies

The piezoelectric effect, in which mechanical strain applied to a piezoelectric element generates a current or voltage (or an applied current/voltage causes mechanical strain), has also been investigated as a potential way to harvest energy. In particular, prototype devices have been embedded in shoes to collect energy as the wearer walks.

Similar in some ways are electroactive polymer (EAP) materials whose shape is modified when a voltage is applied to them (and vice versa). These have also been proposed for possible use within energy harvesting devices. Compared with piezoelectric harvesters, they could enable lighter energy harvesting systems to be developed, thanks largely to their high energy conversion efficiency.

Recent research by scientists at the Fraunhofer Institute has focused on making use of the thermal energy available in the human body. In a hospital intensive care ward numerous medical instruments are attached to the patient’s body to monitor heart rate, blood pressure, body temperature, pulse and breathing rate – all of which require cables for power and signals.

In future, however, the Fraunhofer scientists believe that medical sensors may be able to draw all the power they need from the warmth of the human body, with the data wirelessly to a central monitoring station.

In collaboration with colleagues from the Fraunhofer Institute for Physical Measurement Techniques (IPM) and the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research (IFAM), scientists at the Fraunhofer Institute for Integrated Circuits (IIS) have developed a way of harnessing natural body heat to generate electricity. It works on the principle of thermoelectric generators (TEGs) made from semiconductor elements. TEGs extract electrical energy from the temperature difference between a hot and a cold environment. Normally a difference of several tens of degrees would be required in order to generate enough power, but the differences between the body’s surface temperature and that of its environment are only a few degrees.

Peter Spies, manager of this sub-project at the IIS, comments: “Only low voltages can be produced from differences like these.” A conventional TEG delivers roughly 200mV, but electronic devices require at least one or two volts. According to Spies, the project team has come up with a solution to this problem: “We combined a number of components in a completely new way to create circuits that can operate on 200mV. This has enabled us to build entire electronic systems that do not require an internal battery, but draw their energy from body heat alone” (Fig.4).

The scientists are making further improvements to this system; circuits that are excited at 50mV already exist. Peter Spies believes that in future, when further improvements have been made to the switching systems, a temperature difference of only 0.5°C will be sufficient to generate electricity.

Aside from medical sensors, Spies says the scientists have identified a wide range of possible applications: “Electricity can be generated from heat anyplace where a temperature difference occurs. That could be on the body, on radiators to meter the heating costs, when monitoring the cooling chain during the transport of refrigerated goods, or in air conditioning systems.”

As with Perpetuum’s vibration energy-harvesting microgenerators, the Fraunhofer developments demonstrate that there are plenty of potential opportunities for eliminating batteries and power cable, with additional benefits available when applications also utilise low-power wireless technologies.