From BBC News,
It comes as the first commercially available solar-powered headphones are now on sale. The models, by Swedish firm Urbanista and German sports giant Adidas, both have solar panels built into their headbands.
In each case the flexible panels are made by another Swedish company, Exeger, which has spent the past decade working to make them light, thin and powerful enough to do the job.
Exeger's boss Giovanni Fili says it is both a matter of convenience and, more importantly, doing the right thing environmentally.
"Charging - everyone hates it," he says. "But every time you don't charge [using mains electricity] it is a good thing for the world.
"The new generation of young adults expect to be offered tools to do good [for the environment], and that is what we are offering."
Exeger's solar panels are called Powerfoyle, and they are just 1.3mm thick. The technology is based around strips of titanium dioxide covered in a natural dye. In very simple terms, the dye absorbs photons from light, which are then converted into electrons.
While approximately only half as efficient as standard silicon-based solar panels of the same size in full sunshine, the titanium dioxide panels are, in addition to being significantly thinner, much easier and cheaper to produce.
The solar-powered headphones still have a built-in battery that can power up to 80 hours of playback time. It is this that the Powerfoyle strip charges. Mr Fili says that currently the tech can create one hour's worth of power "from just 20 minutes of English or Swedish summer sunshine".
Yet the panels can also create some power from artificial light, such as indoor lighting, so the idea is that the headphones are always charging unless it is the dead of night. And the headphones also still have a power socket if back-up power is required after heavy usage.
Mr Fili adds that solar panels are unlikely to be added to mobile phones any time soon, because so many of us keep our handset in a pocket, and therefore it doesn't have access to light. Instead he envisages the panels being fitted to people's clothing and bags, and phones to be charged from these.
Finnish company Planno is already making fabrics that have built-in solar panelling.
The firm is led by Elina Ilén, who is also a professor at the Polytechnic University of Catalonia's Department of Textile and Paper Engineering, and a leading expert on wearable textile electronics.
Rather than producing clothing that can charge your mobile phone, the firm instead focuses on washable garments with built-in, solar-powered sensors. These can monitor various aspects of the wearer's health, such as heart rate, temperature, posture, sleep quality and body fat levels.
"Although these solar cells do produce enough energy to power wearable devices, placing a solar cell behind a textile will never have the same efficiency of harvesting energy as a solar cell in direct sunlight," says Ms Ilén.
"The textile's thickness, density, structure, colour and finishing treatments will all have an impact on the amount of energy-harvesting potential."
She adds: "Textile-based wearables are one of the best solutions for continuous health monitoring because product acceptability is easier to achieve. Patients are already wearing clothing, this technology can be hidden beneath the fabric to ensure comfort and discretion."
At Loughborough University researchers are taking a different approach to power-generating fabrics. Instead of using solar cells, they are harvesting the static electricity created when a person moves, and turning that into useable power.
The technology is based around tiny power converters called triboelectric nanogenerators (tengs). These are made from fibres such as cotton, polyester and nylon, which are covered in a polymer coating that attracts static. These flexible, stretchable, washable tengs can then be woven or knitted into the fabric of items of clothing.
"Our group is investigating how we can use regular textile materials and textile fabrication techniques to produce efficient tengs for wearable applications," says project leader Ishara Dharmasena.
"The aim is to produce energy-generating textiles like T-shirts, base layers and trousers which are very similar to our regular clothing, but capable of generating electricity or acting as sensors to measure body movements."
Dr Dharmasena, a Royal Academy of Engineering research fellow and lecturer at Loughborough, adds that in the future tengs could be used in conjunction with solar panels to create hybrid power-generating clothing.
Denise Wilson, author of the book Wearable Solar Systems, hopes to see the market for solar-powered clothing expand greatly in the coming years.
"I think that we could benefit from more imagination when it comes to marketing wearable solar packs," says Prof Wilson, from the University of Washington's Department of Electrical and Computer Engineering.
"We have not yet taken advantage of what they can fully offer us in the global community."
She adds that the biggest area for growth will likely be clothing that can charge mobile phones and other portable electronic devices. "For the average consumer, it may mean being able to charge on the go."
But she adds that for such clothing to find widespread popularity, the solar panel systems will need to last at least one year without a significant loss of efficiency.
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