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Freeing gadgets from the grid with energy harvesting

Transparent solar panels, designed to liberate gadgets from their electric leashes, were among the energy harvesting technologies getting attention at last week’s Mobile World Congress. 
As smartphones become increasingly power hungry and we are less able to live without them, developers are looking for new sources of energy to fill the void which is opening up as battery development lags behind the public appetite for gadgetry. 
“The battery is the major issue for most of the wearable and mobile devices,” Matthieu de Broca, business developer at French owned SunPartner Technologies, told E&T. His firm makes ultra-thin invisible photovoltaic solar cells which can be embedded in a display to harvest energy from the sun.
“People are looking to get the best battery life as possible with the lightest solution so it’s a major trend to do energy harvesting in any way,” he said. 
The problem is that most of the technologies currently on offer have clear limitations. SunPartner’s cells can only provide enough power for 100 minutes on standby or a two-minute phone call after ten minutes of direct sun exposure. The hope is that, eventually, new materials will be developed that will allow increased efficiency. 
An emerging family of solar-absorbing materials that could clear the way for cheaper and more efficient solar panels and LEDs are perovskites. These are  particularly good at absorbing visible light, but, up to now, had never been studied in their purest form: as perfect single crystals.   
Recently, a collaboration between the University of Toronto and the King Abdullah University of Science and Technology (KAUST) used a combination of laser-based techniques to measure selected properties of the perovskite crystals. 
"Our work sets the bar for the ultimate solar energy-harvesting performance of perovskites," researcher Riccardo Comin told Energy Harvesting Journal. "With these materials it's been a race to try to get record efficiencies, and there are no signs of stopping or slowing down."  
In recent years, perovskite efficiency has soared to over 20 per cent, very close to the current best performance of commercial-grade silicon-based solar panels you see mounted in Spanish deserts and on Californian roofs. 
"In terms of efficiency, perovskites are perfectly comparable or better than materials that have already been commercialised," says Valerio Adinolfi, a  co-first author on the paper. "The challenge is to make solar attractive from the business side. It's not just matter of making it efficient—the point is to make it efficient and cheap.”
The US military are also keen on PV technology and are currently testing wearable solar panels as one way to reduce the weight and number of batteries soldiers must carry to power electronic devices.  
The panels, which cover a soldier's backpack and helmet, are constructed from thin gallium arsenide crystals that provide flexibility to the panel's material and allow it to conform to a soldier's gear. Under bright sunlight conditions, with the PV panel facing the sun, the backpack panel is capable of delivering 10 watts while the helmet cover panels provides seven watts of electrical power.
But the technology could also receive a boost from an unlikely quarter. Some of the world’s top fashion designers are jumping on the wearable wagon to bring smart garments to the market. Tommy Hilfiger’s wool and nylon jackets are adorned with seven to ten solar panels on the back that charge your mobile devices. So, even if their charging power leaves much to be desired, these new PV technologies could become a fashion accessory before they develop a truly practical use. 
A new high performance light energy harvester for indoor applications will be the focus of a presentation by Sharp Laboratories of Europe at Energy Harvesting 2015, which is sponsored by the EHSIG. The event, which also features demonstrations and speakers from industry and academia, takes place in London on 19 March 2015.
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