VIBRATIONS from where we live and work could be much more widely harnessed as a clean source of electricity, according to the latest research.

Called "energy harvesting", the concept has been around for more than a decade. Now, researchers at the UK's University of Bristol are examining if they can make use of a much wider range of vibrations for this purpose.

In the future, energy harvesting could be powering many more of our devices, such as heart monitors and mobile phones.

Funded by the UK's Engineering & Physical Sciences Research Council, the team is exploring how vibrations caused by large machines such as helicopters and trains could be used to produce power. Vibrations from household appliances and the movement of the human body could also be harnessed in this way.

Commercial energy-harvesting devices already exist which, for instance, use vibrations from industrial pumps to power sensors monitoring the pump's condition. Such vibration energy-harvesting devices use a spring with a mass on the end, explained Dr Stephen Burrow, who is leading the Bristol project.

"The mass and spring exploit a phenomenon called resonance to amplify small vibrations, enabling useful energy to be extracted," he added. "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."

Dr Burrow said that resonance that produces a large vibration in one object as a direct result of a relatively small vibration in another object occurs when the vibration produced by the second object has a frequency similar to the natural resonant frequency of the first.

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 number of vibrations per second). If the vibrations do not 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 such as transport or human movement where the frequency of vibrations changes all the time.

To overcome this limitation, the Bristol team is developing a new type of device in which the mass and spring resonate over a much greater range of frequencies.

This would enable a larger range of vibrations to be exploited and therefore 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.

With a conventional spring, the force needed to compress it is proportional to the distance it is compressed; this is 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 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.

Such machines could be used extensively, for example, to provide power for wireless monitoring and diagnostic sensors that generate data on a person's heart rate, body temperature or blood pressure, or on stresses experienced by engine components, structural elements in buildings, or to monitor 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," added Dr Burrow. "Wider-frequency energy harvesters could make a valuable contribution to meeting energy needs more efficiently and sustainably."

If successful, he predicted wider-frequency energy harvesting devices could be available for practical use within five years.