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From MP3 players and mobile phones to laptops and PDAs, mobile devices promise any time, anywhere connectivity.

But what happens when the power runs out? Remembering to bring along the necessary currency converters and plugs to keep gadgets running throughout a trip can cause headaches, short circuits and tangles of wires.

Unlike local power supplies, which vary greatly from place to place, there is one source of energy that remains plentiful, reliable and – for at least several hours of every day – constant: the sun.

Solar power (or photovoltaics) has been used for decades to power small consumer devices such as pocket calculators and wristwatches. Now solar energy is being harnessed to give people on the go a constant source of eco-friendly, travel plug-free power.

Over the last year or so, a niche market for solar-powered bags has sprung up on the web and in stores across the US, Europe, and Asia. Products such as the Reware Juice Bag, the Voltaic Solar Backpack, the O’Neill Solar Backpack, and the Eclipse Solar Flare Messenger Bag convert backpacks, shoulder bags and totes into more than vessels for lugging books and laptops from place to place: the bags turn the wearer into a walking, environmentally-sound power source.

“You’re not just buying a bag, you’re buying a bag that will let you charge a device,” says Henry Gentenaar, managing partner of Reluminati, the US-based sustainable energy venture marketing company behind the Juice Bag. “Every time you use a solar-powered bag to charge a device rather than conventional ‘brown’ power, you’re keeping a bit of carbon dioxide out of the atmosphere to boot.”

The solar-powered bags employ solar technology, typically in the form of photovoltaic panels stitched into the outside fabric of the bag, to convert sunlight directly into electricity. When sunlight strikes the photovoltaic cells in the panels, electrons are dislodged, creating an electrical current.

Placing a bag under direct (and sometimes indirect) sunlight for anything between one and six hours (depending on the device and the quality of the sunlight) generates enough power to recharge small 12V electronic devices such as phones, MP3 players, cameras, portable video games and PDAs.

Some products even claim to charge laptops. Electronic devices connect to most of these bags via a standard 12V car charger adapter (CLA).

Some solar-powered bags use rigid solar panels made of substances such as poly-crystalline or mono-crystalline silicon, while others use more flexible “thin film” technologies such as amorphous silicon or copper indium gallium diselenide (CIGS).

While the bendable thin film material is generally lighter and less likely to break than some of the glass-like panels, thin film technology is typically less efficient than more rigid options.

According to Lisa Frantzis, director of renewable and distributed energy with the Chicago-headquartered firm, Navigant Consulting, currently, thin film materials are averaging about 6 to 7 per cent efficient, compared to more rigid options, which have cell efficiencies of about 15 per cent.

Companies including Voltaic and Reluminati are exploring making future models out of recycled materials such as the PET plastic in fizzy drink bottles and reclaimed billboards. In spite of their ecological credentials and their potential usefulness in charging devices on the go, the nascent sector faces a number of challenges.

The most pressing issue is power. The bags on the market today have an output of anything between two and just under 7 watts.

Vendors say their products will charge small electronic devices in just a few hours, but if the sky is overcast, or if a user is trying to operate a bag in indirect sunlight, they will be lucky to get even a trickle of power.

To mitigate the power problem, some companies are offering intermediary chargeable batteries as a means to propagate extra electricity to use at night, in low sunlit areas and for juicing up more power-hungry devices such as laptops. Solar power is typically stored in lithium ion batteries attached to the bags during the day and transferred to devices for later use.

“The key to success in this industry right now is the battery pack,” says Shayne McQuade, founder of Voltaic. “People aren’t in the sun 24 hours a day, so the batteries provide an easy way to charge devices around the clock.”

Another related problem is to do with size and weight. Most solar-powered bags available today carry laptop computers but do not generate enough power to charge them up in a reasonable amount of time. “People often ask us about charging laptops. Currently, that would require making a bag with an outsize solar panel, which would be heavy and expensive,” says Randolph Gray, owner of Eclipse Solar Gear, the maker of the Eclipse Solar Flare bag.

The Juice Bag claims to charge some laptops (testing proved the author’s Apple PowerBook G4 not to be one of them.) Accordingly, the hefty 4lb pack is equipped with a 12 inch square of photovoltaics and an output of close to 7W.

“One of the biggest challenges facing these vendors right now is increasing the efficiencies of the solar photovoltaic cells that convert the sunlight into electricity,” says Ms Frantzis. “If you can then couple this improvement in supplying electricity with making the electronic devices more efficient, in other words also improving the demand side so the product requires less power to function, the market for these solar-powered products is likely to blossom.”

According to the Solar Electric Power Association, the cost of photovoltaics has fallen by 90 per cent since the early 1970s. But, retailing at anything between $130 and $250, solar carry cases cost much more than the average book bag.

Unsurprisingly, vendors are reaching out to wealthy early adopters. “Travellers always want to be connected but they often have trouble with power while on the go,” says Brad John, owner of the boutique travel store chain, Flight 001. “As they develop more compactness and efficiency, solar-powered bags will become a really interesting concept for travellers.”

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