Researchers improve efficiency of energy harvesters

Researchers at the UK’s National Physical Laboratory (NPL) have found they can make energy harvesters more efficient by eliminating a key material.

The team conducted experiments on piezoelectric energy harvesters that use a cantilever to convert kinetic energy into electricity.

The cantilever utilises energy from unwanted mechanical vibrations, such as the rattling of an air-conditioning duct or the movement of a bridge with passing traffic, to generate power on the microwatt scale.

Dr Paul Weaver, project leader from the NPL, told The Engineer: ‘The output of our energy harvester was 40µW with full coverage of the piezoelectric material at 5Hz. This increases to 46µW when the coverage of the piezoelectric is reduced by one third.’

The cantilever consists of an electricity-generating piezoelectric layer on top of a flexible metal layer or composite layer. ‘We put a weight on one end of the cantilever and the other end is fixed,’ said Weaver. ‘Just like twanging a ruler, if the cantilever is subject to any vibration, then it will oscillate at a particular frequency and produce an energy output.’

When pressure is applied to piezoelectric materials, such as lead zirconate titanate (PZT), they produce an electric charge that is sufficient to power sensors and microprocessors.

‘Most of the stress is applied at the clamped end, which is where most of the power is generated,’ said Weaver. ‘As you move towards the tip, it acts as a capacitor, so it’s absorbing some of the charge that is produced.’

The team obtained its results from lab-based experiments on a 20cm cantilever that was oscillated with a vibration motor.

‘What we want to do next is investigate how these effects change with scale,’ said Weaver. ‘What we really want is nice, efficient energy-harvesting output at scales that are more appropriate for the kind of devices that people want to power with them.

‘A lot of the interest is in powering wireless sensor devices so that we can put sensors into places that may be inaccessible, such as inside a material or on a vehicle.’

The project is running from 2010–2013 and has received a total of €3m (£2.5m) from the European Metrology Research Programme and the UK National Measurement System.