Fields ripe for harvest

13 September 2012

Researchers from France harvest energy from stray magnetic fields using only a commercial piezoelectric diaphragm, with no expensive magnetostrictive materials involved.

Something in the air

Many of the devices that surround us every day, like laptops and their battery chargers, radiate dynamic magnetic fields as a by-product of their operation, and the energy contained in these fields is a potential source of ‘free’ energy for an increasing number of other smaller electronic devices.

Electric power can be produced from a dynamic magnetic field using either electromagnetic coupling like an electric generator or the magnetoelectric (ME) effect. The ME effect can refer to either the electrical polarisation of a material in response to an applied magnetic field or, conversely, the change in the magnetisation of a material in response to an applied electric field.

The ME effect has been used to produce electrical power previously by using laminations of piezoelectric materials with magnetostrictive materials, which change their shape in response to magnetisation. When exposed to a changing magnetic field the changing shape of the magnetostrictive material layers produces stresses in the piezoelectric layers, which transduce these stresses into a voltage.

Energy harvesters based on the ME effect can easily be made thinner than harvesters based on electromagnetic coupling and are also solid-state, easing integration on a substrate using MEMS batch fabrication techniques. However, the use of magnetostrictive materials is costly and increases the difficulty of producing devices at the microscale.

However, in the work reported in this issue, the team from the Université de Nantes generate electrical power via the ME effect using a very low cost commercial piezoelectric diaphragm with no magnetostrictive materials.

Uncommon uses

The diaphragm they use is commonly used in electronic clocks, consisting of a piezoelectric lead zirconate titanate (PZT) ceramic disc conductively bonded on one side with a brass disc and coated on the other side with a smaller silver electrode disc.

The method requires the diaphragm to be exposed to both a constant and an alternating magnetic field perpendicular to its plane. According to the Nantes team, and previous works performed at INSA Lyon, the alternating magnetic field produces eddy currents in the silver and brass electrodes of the diaphragm as expected according to the Lenz–Faraday law. The interaction of these eddy currents with the constant magnetic field then produces radial Lorentz forces which stress the PZT disc and so produce a voltage.

From their experiments so far the team expect that working at its bending mode resonant frequency and using a permanent magnet to provide the constant magnetic field, a diaphragm could produce 50 μW from the kind of magnetic field given off by a battery charger. This would be sufficient for powering RFID tags and MEMS devices like PIC microcontrollers, but increasing the magnetic field strengths would produce greater power. Team member Professor Benoit Guiffard observed that “A one order of magnitude increase in produced power could reasonably be envisaged if both magnetic field strengths are increased.”

The alternating field strength depends on the ambient source, and some devices, like hairdryers, can create magnetic fields of up to 20 Oe compared to the 1–2 Oe of a battery charger. The constant magnetic field is provided by a permanent magnet, and the main limitation on its strength is the need for compactness in an energy harvester.  Power output could also be increased by using a material with higher piezoelectric coefficient, i.e. a single crystal rather than a ceramic.

The team note that a third possibility for increasing output would be to use specialised energy harvesting circuits, citing those developed by Professor Guyomar’s group at INSA Lyon as examples.

Sensing power

Coupling of a magnetic field with Lorentz forces combined with piezoelectricity was previously reported in work from the University of Hong Kong that used similar materials to produce a magnetoelectric sensor by applying an alternating current to produce the Lorentz forces. The Nantes team only apply magnetic fields, and team member Professor Benoit Guiffard observes that “Our work shows a very simple piezoelectric compound like unimorph diaphragm or even a single ceramic disc not only has potential for magnetic energy harvesting, but also for power-free DC or AC magnetic field sensing with good linear response and sensitivity. It is even possible that the first commercial devices using this may actually be sensors.”

The Letter presenting the results on which this article is based can be found on the IET Digital Library.

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Cover of Electronics Letters, volume 50, issue 19

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