Energy harvesting has traditionally been associated with small wireless devices, but with so many potential applications, power sourcing and conversion companies are looking further afield.
At the end of July, representatives from 30 countries will convene to discuss the latest developments in the field of monitoring buildings and structures. Among the topics at the Structural Health Monitoring of Intelligent Infrastructures (SHMII) conference will be 'intelligent' sensors and wireless sensor networks. With the EU's target of reducing energy consumption by 20% by 2020, conferences such as these will be on the increase and workable solutions will become crucial. And the term 'energy harvesting' will be on the lips of most design engineers. There are four types of energy harvesting – solar, vibration, heat and motion. While some are more established than others, the solutions often complement one and another, depending on the application. Solar technology, perhaps the most conventional form of energy harvesting, has a wide range of applications and is the basis of Freescale's range of power conversion technology. Bruno Baylac, Freescale's global director of marketing for industrial markets, explained the basics: "The challenge in energy harvesting is to convert low voltage signals into something useable and the techniques we have been using for solar energy can also be used for wider applications. The conversion process target is to meet around 3 to 5V, which most products require. If a product has no battery, the device is required to literally start from nothing from any source of energy. Once we have enough energy and the source is generating enough, then we have achieved our objective." However, Roy Freeland, chief executive at Perpetuum, warned of the limitations of solar energy. "While it can store energy overnight successfully, storing energy throughout a British winter is more difficult. There are also a number of maintenance problems – a solar panel is useless if it's covered in snow." While Freeland's company works with solar energy, it is a complementary resource to Perpetuum's primary form of energy harvesting – vibration. Perpetuum was set up in 2004 as a spin out from Southampton University, where its concept for vibration energy harvesting originated. The company developed the world's first vibration harvesting microgenerator capable of generating enough power to enable the reliable transmission of large amounts of data. "Perpetuum's vibration energy harvesting technology enables wireless and battery free sensing," said Freeland. "Vibration energy harvesting is based on the notion that mechanical vibration can be transformed into useful electrical power. Our vibration energy harvesting microgenerators feature a highly optimised magnetic circuit coupled to a mechanical resonator." This arrangement transforms the kinetic energy of vibration into an electrical current and Perpetuum's microgenerators have been designed specifically to produce high levels of power. Freeland explained: "This helps us meet the needs of wireless sensor systems created by the development of low power sensors, microprocessors and transceivers." Freeland expects massive growth in demand for wireless sensors which, he pointed out, will expose the limitations of batteries. "If you're going to go wireless, batteries aren't reliable," he said. "Batteries have a number of issues – wireless mesh networks, for example, are non deterministic, so you can't calculate how long a battery will last." The key to vibration harvesting is a mass signal vibrating resonantly and, during the development process, Perpetuum worked initially with piezoelectric materials. The piezoelectric effect converts mechanical energy by straining a piezoelectrical material into an electrical current or voltage. It can come from different sources such as human motion, low frequency seismic vibrations and acoustic noise. Except in rare instances, the piezoelectric effect operates in ac, requiring time varying inputs at mechanical resonance to be fully efficient. However, Freeland soon noticed that piezo had limitations. The voltage produced varied with time and strain and produced irregular ac signals. While the energy conversion produced higher voltage levels than electromagnetic systems, he considered the latter method to be more workable. He observed: "Although we worked with piezoelectric for a while, we found that we achieved better results with electromagnetic and that there were more benefits. It is highly reliable and is also a well known technology, so can be modelled from existing traditional materials instead of 'exotic' technology." Perpetuum's devices, Freeland stated, are designed to be compact and energy efficient. "We focus on low power – just a few hundred microwatts. It's more functional. For example, some people claim they are doing lots of research (primarily piezo based) and this usually takes place at universities. While these experiments may work in laboratories, they are not necessarily practicable outside the lab. One development team described how they managed to generate 50mW at 9g. The brain would scramble at 8g, so either these claims are untrue, or just not viable." Freeland believes Perpetuum's success is because the company's designs are based around real applications. "Look at ac induction motors," he said. "There are billions out there and our device will work with a high percentage of them." There are seemingly limitless resources for vibration, although some are more stable than others. Compressors tend to have high levels of distortion and transformers vibrate as they go through frequency changes. Trains create a lot of vibration, although these vibrations tend to be variable – nevertheless, in terms of g, they are a strong source. Perpetuum's PMG17 microgenerators are used in a number of wireless sensor nodes. The nodes enable continuous monitoring and control of plant machinery, while critical temperature and vibration information can be analysed by operations staff. Freeland observed: "The PMG17 microgenerators are intended for use on machinery driven by ac motors, harvesting the commonly found 'twice line frequency' vibration. Even with as little as 25mg vibration within a 2Hz bandwidth, they will always produce a minimum power of 0.5mW, while delivering up to 40mW when there is more vibration available." Energy harvesting using heat is not widely used, but useful within industrial environments. Baylac noted: "If process plants with heated pipes use this method, it is a viable power source. A thermocouple is a high quality source of energy for piezo or electromechanical conversion." EnOcean has been working with thermal energy for four years. Andreas Schneider, EnOcean's executive vp, explained: "We are developing a dc/dc converter that produces 20mV, which we boost to 3 to 5V. In a building environment, it is powered from temperature gauges, water and air, so it can adjust itself accordingly, even if nobody is in the building." Motion energy harvesting displays the unlimited possibilities available when creating energy from nothing. Baylac observed: "Even someone's finger pressing a switch is a method of generating energy. There is no need for wires and, in theory, this means a device could last forever." Schneider concurred: "Vibration needs dynamics, but linear harvesting doesn't and the energy output is always the same – and at a specific level. It's important to develop energy harvesting within a total system so it can be optimised accordingly. This is particularly true when focusing on energy harvesting for wireless sensors and piezo is a good way to deliver a high voltage. We can easily provide 3 to 6V for a capacitor with a simple converter." Enocean's research on rotation – using energy from body movement – has resulted in its Eco100 energy module. The kinetic energy created by physically pressing the button means it can be used to power radio modules. The energy output at every actuation of the spring is sufficient to transmit three rf sub telegrams – energy pulses sent to remote sensors which continuously monitor for signals. A typical application could be automatic stairwell lighting, where a light is switched on manually and switches off after a specified time. A common electrodynamic energy transducer is actuated by a spring, which can be pushed from outside the device. When the spring is pushed up or down, electrical energy is provided at the energy output pins. With this amount of energy it is possible to transmit an rf telegram with a connected module. "There are many applications," said Schneider. "Wireless switches for building automation, wireless position switches for industrial automation and call button transmitters." Some 100,000 buildings are equipped with Enocean's wireless light switch and the company is now working on a fully automised production line to address the high volume market. According to Schneider, the switches last as long as regular light switches, with no wiring and harvest energy every time the switch is pressed, transmitting a high power rf signal to control the lighting. Schneider noted: "We started this project based on piezo technology and developed the system from a single button press. However, there were limitations. For example, in an industrial environment, the switches will be pressed thousands of times and we were unable to optimise piezo. Therefore we moved to using a magnetic field to convert mechanical energy to electrical." The logistics of a creating a brave new energy harvesting world have not gone unnoticed. Freeland is actively seeking a resolution, as he suspects most end users will be reluctant to embrace green energy sources until standards are set in stone. "Compatibility is the real issue here," he stated. "I'm just one of many energy harvesting companies in discussions to help both systems companies and end users of wireless sensor systems. Once a standard interchangeable solution has been implemented, designers will be able to create wireless sensor systems where anything can plug in – even batteries." Such a standard would focus on performance measurement and performance under specific conditions. EnOcean has already established its EnOcean Alliance – a group of independent companies working together to establish a wireless standard for sustainable buildings. Schneider explained: "We need to educate architects and specifiers and create an ecosystem of companies all working to the same standard. They all have to deliver interoperable systems and as systems become ever more complex, there has to be a regulation and international legislation on sustainable buildings." Conferences such as the SHMII symposium are good starting points for design engineers to network with international scientists, enterprisers and researchers; and to discuss the advances in smart sensors, wireless sensor networks and signal acquisition. But the EU's energy consumption reduction directive is fast approaching – and to establish fully integrated and interchangeable systems within 11 years could be a close call. "Energy harvesting is a super, low cost way to create power," concluded Freeland. "As the volume of innovations increases, it will revolutionise our understanding and appreciation of sourcing energy. And it can't come too soon for me."