Good vibrations

01 December 2009

Energy harvester in action
Energy harvester in action

Vibrations from the environments we live and work in could be harnessed as a clean source of electricity, due to cutting-edge UK research.

Known as ‘energy harvesting’, the concept has been around for over a decade, but researchers from the University of Bristol aim to make it possible to make use of a much wider range of vibrations than is currently possible.

It’s hoped that within five years ‘energy harvesting’ could be powering many more of our devices, from heart monitors to mobile phones.

The work is funded by the Engineering and Physical Sciences Research Council (EPSRC). The team are exploring how vibrations caused by machines such as helicopters and trains could be used to produce power. It is thought that vibrations from household appliances and the movement of the human body could also be harnessed in this way.

Commercial energy-harvesting devices already exist that use vibrations from industrial pumps to power sensors monitoring the pumps’ condition.

“Vibration energy-harvesting devices use a spring with a mass on the end,” says Dr. Stephen Burrow, who is leading the project. “The mass and spring exploit a phenomenon called resonance to amplify small vibrations, enabling useful energy to be extracted. Even just a few milliwatts can power small electronic devices like a heart rate monitor or an engine temperature sensor, but it can also be used to recharge power-hungry devices like MP3 players or mobile phones.”

Resonance involves the production of a large vibration in one object as a direct result of a relatively small vibration in another object. It occurs when the vibration produced by the second object has a frequency similar to the natural resonant frequency of the first object. The natural resonant frequency of an object is the frequency at which it naturally vibrates when excited by an external stimulus.

Existing devices can only exploit vibrations that have a narrow range of frequencies (the frequency is the number of vibrations occurring per second). If the vibrations don’t occur at the right frequency, very little power can be produced and it will be too low to be useable. This is a big problem in applications like transport or human movement where the frequency of vibrations change all the time.

However, the Bristol team are developing a new type of device where the mass and spring resonate over a wider range of frequencies. This would enable a much wider range of vibrations to be exploited and so increase the overall contribution that energy harvesting could make to energy supplies. The team believes it can achieve this by exploiting the properties of non-linear springs which allow the energy harvester to respond to a wider range of vibration frequencies than conventional springs.

With a conventional spring, the force needed to compress it is proportional to the distance it is compressed; known as Hooke’s law. With a non-linear spring, there is a more complicated relationship between force and distance compressed, and springs can be designed that become harder or softer as they are compressed. When these non-linear springs are used in an energy harvester, the mass and spring no longer resonate at just one frequency, but will respond to a wide range of vibration frequencies.

Energy harvesters generate low-level power on a similar scale to batteries but without the need for battery replacement or disposal of potentially dangerous and polluting chemicals. They are also suited to applications where hard wiring would be impracticable, vulnerable to damage or difficult to access for maintenance purposes.

Energy harvesters could therefore be used to provide power for wireless monitoring and diagnostic sensors that generate data on:
• Heart rate, body temperature or blood pressure
• Stresses experienced by engine components, or structural elements in buildings
• Brake temperatures in railway rolling stock.

“There’s a huge amount of free, clean energy out there in the form of vibrations that just can’t be tapped at the moment,” says Dr. Burrow. “Wider-frequency energy harvesters could make a valuable contribution to meeting energy needs more efficiently and sustainably.”

If the research at Bristol succeeds in achieving its objectives, wider-frequency energy harvesting devices could be available for real-world use within five years.

The three-year project ‘Energy Harvesting: Vibration-Powered Generators with Non-Linear Compliance’ is due to end in January 2011 and is receiving EPSRC funding of just over £197,000.

The EPSRC is the UK’s main agency for funding research in engineering and the physical sciences. It invests around £850 million a year in research and postgraduate training to help the nation handle the next generation of technological change. The areas covered range from information technology to structural engineering, and mathematics to materials science.

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