energy harvesting


receiver must be within 10m or 20m, but in a free field the range extends to 300m. The principle is simple enough: the


switch has a small spring that moves in and out of a coil to build up a magnetic flux, which is converted into a voltage and then stored in a small capacitor for a few microseconds; just long enough to power the radio module. In the real world, explains Wolfgang Heller, product line manager, things aren’t quite so easy. ‘It took quite a bit of effort to reach serious production levels. We had to optimise the design for power while keeping forces low to achieve the same touch/feel of a standard wall switch.’ EnOcean’s designers employed some 3D mechanical design software along with simulation software to study the magnetic flux, in particular a package from Integrated Engineering Software (Fig. 2).


Waste heat into power EnOcean is also involved in energy harvesting from heat. It and other companies rely on Peltier elements used ‘in reverse’. Using a system to convert energy in reverse isn’t that unusual; consider that, in a pinch, you can use a small stereo speaker or even an earbud as an emergency microphone. That effect is also possible with a Peltier device, which has traditionally been used as cooling elements; when direct current runs through it, heat is moved from one side to the other at the junction of two metals. In reverse, taking advantage of the Seebeck effect, exposing the two sides to a heat differential can create a small current.


Fig. 2: The flux generated in EnOcean’s ECO 100 module when the button on a wall switch is pushed


possible to develop 600µW, sufficient to send several ‘telegrams’ to monitoring and control electronics. A company pushing the state of the art in


thermo-harvesters is Micropelt GmbH, which started as a project of Infineon Technologies in cooperation with the Fraunhofer Institute for Physical Measurement Techniques and is now a separate startup company. It is developing thin film thermoelectric components using planar technologies and silicon wafers as substrates. In this way, the devices are becoming much smaller and more sensitive. The company has created prototypes and is setting up for the first volume production towards the end of this year.


OBVIOUSLY THE CONCEPT HAS BEEN AROUND FOR A WHILE, BUT


ONLY NOW IS THE TECHNOLOGY COMING TO MAKE IT MORE COMMERCIALLY VIABLE, OFTEN USING THE HEAT DIFFERENTIAL COMMON IN INDUSTRIAL APPLICATIONS


The best condition is a constant, large


thermal gradient, so such systems have been used in deep space missions for a number of years. Obviously the concept has been around for a while, but only now is the technology coming to make it more commercially viable, often using the heat differential common in industrial applications. Note that EnOcean’s ECT 310 doesn’t


come with Peltier elements; OEMs choose the ones they want, depending on the energy demand. The module connects to these devices externally and then provides the dc/dc converter to boost the output to, say, 3V plus other electronics to connect it to a radio device. Says Heller, with a 15x15mm Peltier element and a 10K differential, it’s


www.scientific-computing.com The heart of their products is a


silicon chip. Thanks to a MEMS-like microstructuring process, MPG series thermogenerators have a density of as many as 100 thermoelectric leg pairs per mm2


. As


per Seekbeck’s Law, this translates into 1.4V at as little as 10ºC temperature differential. There are currently two standard product sizes with hot-side areas of approximately 8mm2


and 14mm2 and a chip height of


roughly 1.1mm. The thin substrate has a reaction time < 3msec, an operating range <200ºC and a voltage output of 1.7V per Watt of thermal input. This chip is placed in special units, one


of which is the TE-Power Probe (Fig. 3). A critical part of the device is its heat sink,


which must provide the largest possible difference between the temperature at the base and the ambient. To find the best heat sink design, the company works with fluid flow/heat transfer software such as CF Design from Blue Ridge Numerics (which has just announced it is being acquired by Autodesk). ‘The thermal conditions in materials and interfaces are sometimes intuitive but sometimes not,’ explains Burkhard Habbe, VP of business development. ‘We also work with customers to find the best thermal path for their application. The physical shapes, materials, interfaces, mechanical loads – these all have a significant impact on the thermal path.’ As for performance, if the hot-side temperature is at 100ºC and the ambient is 25ºC, the probe can produce enough power to replace 27 1.5V AA batteries in a year. Research is being conducted into other


approaches to waste-energy harvesting. Writing in the November 2009 issue of the Journal of Applied Physics, MIT Associate Professor Peter Hagelstein reported that theory states such energy conversion can never exceed the Carnot Limit, which defines how much heat can be turned into work. He adds that current commercial thermoelectric devices achieve only about 10 per cent of that limit. But in experiments involving a new technology called thermal diodes, he and his colleagues demonstrated efficiency to 40 per cent – and calculations show that this new kind of system could ultimately reach 90 per cent.


Capturing electrosmog One of the newer energy-harvesting mechanisms is to capture RF energy from TV signals, wireless radio networks and


APRIL/MAY 2011 37


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