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A tent whose flysheet charges batteries all day

Imagine wearing a jacket or rucksack that charges up your mobile phone while you take a walk. Or a tent whose flysheet charges batteries all day so campers can have light all night. Or a roll-out plastic sheet you can place on a car's rear window shelf to power a child's DVD player.

Such applications could soon become a reality thanks to a light, flexible solar panel that is a little thicker than photographic film and can easily be applied to everyday fabrics. The thin, bendy solar panels, which could be on the market within three years, are the fruit of a three-nation European Union research project called H-Alpha Solar (H-AS).

The new solar panels will be cheap, too, because they can be mass-produced in rolls that can be cut as required and wrapped around clothes, fabrics, furniture or even rooftops. "This technology will be a lot easier to handle than the old glass solar panels," claims Gerrit Kroesen, the physicist from Eindhoven University of Technology in the Netherlands who led the development team.

Kroesen's team has made its solar cells bendy simply by making them thin. But this has involved a trade-off. While the best solar cells are now working at efficiencies above 20%, the H-AS cells are only about 7% efficient. The researchers think efficiency is worth sacrificing for a cell that is going to be more generally useful, though they still hope eventually to reach 10% efficiency.

Conventional solar panels are made of pairs of sheets of semiconducting silicon, doped with phosphorus and boron atoms. Electrons in the phosphorus-doped (N-type) layer migrate across the boundary to occupy holes left in the boron-doped (P-type) material, setting up a voltage across the boundary between the two layers. When photons hit the silicon in a cell they knock electrons out of its crystal structure, generating a current that is collected by a mesh of metal contacts.

The H-AS solar panels are constructed in a similar way, but they are made just 1 micrometre thick by depositing polymorphous silicon at high pressures and temperatures. "Polymorphous silicon is as rigid as crystalline silicon. But because it is less than a micrometre thick it is flexible," Kroesen says. Today's solar panels are typically somewhere between 4 and 10 millimetres thick.

The process of producing H-AS films involves temperatures of up to 200°C, which would melt a plastic substrate. So instead of depositing the doped layers directly onto plastic they are first deposited onto aluminium foil.

After the assembly has cooled, a plastic carrier layer is added underneath it and the aluminium is removed and recycled. Contacts are then added, followed by a protective plastic layer on top, too. This sequence lends itself to continuous production on rolls of plastic film.

The Swedish and Dutch-owned company Akzo-Nobel, a partner in the H-AS research, already has a pilot plant producing rolls of silicon cells 40 centimetres wide. A projected full-scale manufacturing plant would produce panels at a cost of about 1 euro per watt. An A4-size panel sewn onto the back of a jacket and costing less than 10 euros would charge a mobile phone during a summer stroll. The company has not yet decided to go ahead with the plant.

Jeremy Leggett, chief executive of the UK solar cell supplier Solar Century, is impressed, describing the 1 euro per watt price point as "breathtaking

Here comes the sun.

 

Matt Robinson, clad in a long white lab coat and blue gloves, is tending to a shiny silver machine called a Mini-Labo. It resembles a pasta maker, but it doesn't churn out lasagna noodles. Here in the clean room at Palo Alto-based Nanosolar, Robinson feeds a thin ribbon of aluminum foil through the machine; when it emerges, it looks as though it has been coated on one side with a flat black paint.

Robinson is testing the process by which Nanosolar hopes to make a new kind of lightweight, flexible, and low-cost solar cell. The black paint is actually a solution containing millions of nanoparticles that are made to be "self-assembling" -- that is, they align themselves into the structures necessary to transform the sun's photons into electrons. This is called the "light-absorbing semiconductor layer," one of a dozen different chemical strata that will be applied to a sheet of foil to enable it to generate electricity from the sun.

Nanosolar's product is still in the test phase, and the Mini-Labo isn't designed for large-scale production. So after a few feet of foil have spooled out of the machine, Robinson leans in close and adds a bit more of the solution to a reservoir, using what looks like a small eyedropper. Nanosolar chief executive Martin Roscheisen chronicles the test with a camera.

If it seems simultaneously high tech and hand-to-mouth, well, that's life at this low-key company headquarters, adjacent to the Palo Alto dump. Roscheisen is a successful Internet entrepreneur who sold his previous company to Yahoo for $450 million. He and Nanosolar embody the new ethos of Silicon Valley innovation, postbubble: less hype and more concern about wringing the greatest possible value from every dollar. He financed the company with a combination of venture capital and government grants. And he created a frugal corporate culture that aims to get the recipe for its new solar cells right before it moves into full production.

The purse strings may be tighter these days, but the ambitions are still big. Along with industrial giants such as Sharp, Kyocera, and BP, Nanosolar is after the solar industry's golden ring: a reliable photovoltaic cell that can produce electricity from sunlight at a cost that is competitive with electricity from the existing power grid. According to Roscheisen, it costs roughly 32 cents to produce a kilowatt hour of electricity using existing solar-cell technology. (One kilowatt hour is about enough juice to run one dishwasher cycle.) That's about three times the cost of electricity purchased from the local utility, which was likely generated by a plant that burns coal, oil, or natural gas. "Our eventual goal is to generate electricity at less than the grid cost," Roscheisen says, although he concedes that the company's early products won't hit that mark everywhere in the world.

Sunlight is free, of course. The added cost comes because it's still so expensive to manufacture solar cells relative to the power they dribble out. To solve that problem, Roscheisen wanted to get a sense for how solar cells were being made and what breakthroughs academic researchers were achieving. He visited factories around the world and spoke to researchers at Cambridge University, Sandia National Laboratories, the Swiss Federal Institute of Technology, and Stanford. He discovered that one new approach to producing solar cells involved "printing" them in much the same way magazines like this one are printed: by rolling a flexible sheet through a machine, layering on liquids, and then drying them, eventually yielding a sheet that can convert photons into electricity.

Roscheisen funded the company's early research himself, and with angel investment from friends such as the founders of Google, but eventually he decided to raise $6.5 million in venture capital. The company received an additional $10 million in research grants from such agencies as the National Science Foundation, the California Energy Commission, and DARPA, the Defense Advanced Research Projects Agency.

Roscheisen decided to focus the company on creating solar cells that would be used on the roofs of buildings and by utilities that wanted to generate vast amounts of electricity. "That's the largest application area today, and we can come in at a much lower cost, with easier installation," he says. Nanosolar's product will be 100 times thinner than conventional panels that are placed on rooftops, and weigh one-twentieth as much.

Next year, Nanosolar will set up a factory for a first generation of solar cells slated to hit the market in 2006. But first, Roscheisen knows, Nanosolar has to perfect its recipe and figure out how to crank out large quantities of high-quality cells. With big rivals breathing down its neck, there's not a moment -- or a dollar -- to be wasted.

Solar fabric Iowa State

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Experimental fabric converts solar to electrical energy
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By By Teresa Krug
Daily Staff Writer
(2.7.05)

Could the day we use our sweaters to charge our cell phones be approaching fast?

Researchers at the University of Toronto have invented a flexible plastic solar cell that promises to be more efficient than any other solar cell on the market at converting energy from the sun into usable electrical energy. They hope to one day be able to weave this into clothing to charge cell phones and other electrical instruments.

According to a press release on the University of Toronto's Web site, at its very best, the new technology has a 30 percent conversion efficiency rate -- which is five times better than the average of 6 percent that current solar energy cells possess.