Five years ago, when the World Trade Centre fell, the shockwave rolled around the world. It ruined many lives, and sank a few companies. But who could have known that it would hit Sir Trevor Baylis's business so hard? Baylis, the British inventor best known for the wind-up radio, was developing a shoe that would charge your mobile phone battery as you walked. The shoe, complete with a slot for the battery, captured some of the power generated by the average human step, roughly eight watts, and used it to charge a phone over thousands of strides. Baylis had even walked 100 miles across the Namibian desert to raise money for the idea. But his idea collapsed along with the twin towers.

"After 9/11, anyone wearing electric shoes would look like a bomber. That's what you have to watch with any electric kit that you carry nowadays," muses Bailey. Richard Reid, who tried to blow up a plane by carrying explosives in his heels - which subsequently made customs officials particularly nervous about footwear- has a lot to answer for.

The idea of harvesting otherwise wasted energy isn't new, but it's beginning to gain traction. Not only are researchers hoping to reap the energy from people, but they're also planning to use the vibrations from motors and even passing trains to generate power. In some cases, converting vibrations into a tiny charge may be enough to power a wireless sensor. In others, thousands of footsteps could power lights and audio systems.

Historically, piezoelectricity has been the biggest hope for energy harvesters. Discovered in 1880, piezoelectric crystals, emit a charge when subjected to sudden mechanical stress. When you press the button on an electric lighter or the ignition switch on your gas oven, a piezocrystal is probably causing the spark.

Spring in your step

In 1996, MIT Media Lab doctoral student Thad Starner wrote a paper on the potential for "energy harvesting". Using a piezoelectric shoe insert that flexed with each step, or a flywheel system connected to a small spring in the back of the shoe, five to eight watts could be recovered from each footfall, Starner's paper said. Other options for harvesting human energy included finger motion (a 90 words per minute typist could generate 19 milliwatts). Starner believed that by using footfall energy alone, he could power a small wearable computer.

Building a five-watt wearable computer in the lab is one thing, but commercialising wearable computing and wearable power would involve a huge design and user testing effort. Besides, such a computing technology hasn't captured the public imagination yet. People don't want to walk around looking like one of the Borg from Star Trek.

Moreover, a key problem in wearable computing is getting power from one place to another. Cables tangle, rub and break. Dr Stan Swallow, director of Intelligent Textiles, hopes to transfer energy around the human body more easily. His company develops material that transfers power using electrical circuitry woven into "smart fabric".

And while wearable computers might be a niche market, iPods aren't. "The iPod single-handedly created a common format for everyone to work with," he says. "Suddenly your market is composed of everyone who's an iPod owner." We've already seen some jackets with built-in circuitry designed to control MP3 players. How long before clothes emerge that carry energy around the body to power that iPod while you walk or run?

Or how about clothes that harvest power directly? Dr Markys Cain, who runs the Sensor Knowledge Transfer Network at the National Physics Laboratory, hopes to see fabric that generates its own power using piezoelectric fibres woven into frequently moving joints such as elbows and knees.

"You can incorporate ceramic fibres, around which technology has increased dramatically in the last few years. You put them into polymer mats. They are flexible, and you can get millimetres of actuation," he says. Swallow puts it simply: "Your iPod will run on so little power, and your trousers will contain so much."

Finger power

While improvements in energy harvesting technology generate more energy, power consumption in computing devices is falling. That could make consumer-friendly computers running on harvested energy workable.

New generations of displays from firms like e-Ink are appearing, which only draw power when the image changes. Solid state memory removes the need to spin hard drives at high speeds, points out Simon Powell, managing director of piezoelectric lock company Servocell. "If you have Flash RAM and you only use energy to write and read from it, it all just about becomes viable," he says.

But wearable computing may not be the killer application for harvested energy. Piezoelectricity can power wireless sensors, for example, that can use the energy to communicate with other devices in a certain range. With finger motion, Starner believed he could give a wireless keyboard enough power to transmit keystroke information to another device. Today, Ennocean sells wireless light switches that use the energy generated by flipping the switch to communicate with a bulb and turn it on - no cabling necessary.

That makes Claire Price's eyes light up. She is a director of the Facility Architects, a London-based group working on energy harvesting in buildings. The company is involved in the Pacesetters project, an initiative to design energy-harvesting floors and stairs for large public spaces. Price's team will put devices under such surfaces to collect the energy from footfalls, and use it to power local systems.

"You can convert about 3-6 W per step," she says. "Think of Victoria train station. In one rush hour period there are 34,000 people walking through that space. There are many kilowatts of energy that we could be harvesting and ploughing back into low-power circuits." These include audio equipment, display screens, or, by her calculation, around 6.5million LED lights.

But Price is unlikely to use piezoelectricity for the Pacesetters project, because she doesn't think it has an high enough efficiency. Working with Jim Gilbert, a researcher at Hull University - who also worked with Baylis on the abandoned electric shoe - the Facility has investigated hydraulic systems and electromagnetic rotary generators.

She believes that the opportunities for harvesting energy on a mass scale in public spaces are huge. If floors in railway stations, then why not pavements in Trafalgar Square? If pavements in Trafalgar Square, then why not bridges? "Think about Norman Foster's wobbly bridge, where they spent millions on damping equipment," she says. "You could have applied energy harvesting devices within those dampers." Servocell's Powell, a former Cambridge lecturer and patent holder in several piezoelectric applications, agrees: "If you had piezos on that, you could probably have lit it with LEDs," he says.

Price is also working with Perpetuum, a small research company that has developed electromagnetic rotary generators that harvest vibrational energy. She wants to attach the devices to sleepers on train tracks to pick up the energy from passing trains. That could be used to power wireless transmissions, says Roy Freeland, chief executive of Perpetuum. "One of the main problems on railway systems is goods wagons breaking down. No one monitors them and there's no power source to do that. We could monitor them not only for bearing failure, but could use the generators to power transmission systems that tell people where the railway wagons are."

And what of Baylis? Currently involved in Trevor Baylis Brands, a company that helps inventors get their ideas to market, he would like to take a stab at the electric shoe idea one more time and is looking for a large company to collaborate with.

"The shoe would have to be a trendy shoe with a mark on it," he says, when asked about his original security concerns. "So that if someone was wearing this, you know they had an electronic device."

He'd employ better computer aided design techniques to refine his shoe design, he adds, and would also switch from a piezoelectric device to something else like a rotary electric generator, which would provide a bigger power kick. That would be a step in the right direction.

· Compressed crystals that generate power

Piezoelectricity was discovered in 1880 by Pierre and Paul-Jacques Curie, who found that compressing certain types of crystals - including quartz, tourmaline, and Rochelle salt - along certain axes produced a voltage that could be measured on the crystal's surface.The next year, they observed that applying an electric current could make the crystals elongate very slightly.

The charge arises when crystals which have no axis of symmetry are squeezed: the centre of the positive charge in the crystals' ions is slightly separated from the centre of the negative charge.

That creates a net electric field, which can be measured on opposite faces of the crystal. Similarly, applying an electric field across the faces induces the ions to move, and so deforms the crystal.

For decades this remained a laboratory curiosity, but was used in the first world war to generate ultrasound signals (by applying a high-frequency alternating current to make a crystal vibrate) for early sonar, and in the second world war to make impact-triggered detonators for bombs dropped from aircraft: striking the ground triggered a spark to ignite the explosives inside.

Piezoelectricity generates high voltages - a pressure sufficient to distort lead zirconate titanate by 0.05mm generates 100,000V - but tiny currents. However it is enough, for example, to create an electric spark to ignite gas in an oven or grill.

The piezoelectric effect has subsequently been used in electronic equipment, clocks and watches, cigarette lighters, and many other items - including the "needles" of vinyl record players, which generate electricity as the groove moves them.

Charles Arthur

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