American soldiers could soon be taking a new piece of kit onto the battlefield – a portable solar-powered generator smaller than a laptop. The aim is to reduce the weight of batteries a soldier has to carry and simplify logistical supply chains.

The Very High Efficiency Solar Cell (VHESC) programme, made up of a group of US universities, Government research laboratories and companies, has been tasked to produce a solar-power device with an efficiency of at least 50%. The system must also deliver 1,000 working units, each measuring up to 10cm2, and produce at least 0.5W power, assuming a standard solar effluence of one kilowatt per metre squared. The consortium has so far announced an efficiency of 42.8%.

The generators include special optic coatings that concentrate the light of the sun. The highest concentrations are delivered by following the progress of the sun across the sky, but this would require a sophisticated tracking device. The VHESC system therefore has to work at whatever angle the sun’s rays hits the cell. The device will use spectral splitting, which involves directing the short, medium and long wavelengths of light onto different solar cells to make best use of the energy range of sunlight.

Since the project is sponsored by the US Defense Advanced Research Projects Agency (DARPA), much of the detail is still unknown to the wider research community. But Professor Keith Barnham of Imperial College London, UK, a long-term researcher in solar energy, is interested in several aspects of the programme. He says, ‘A non-tracking system that could produce 100 times concentration would be of great interest. If they can do that, you might start to see something that is commercially viable’.

However, he says there is a drawback to the team’s approach of spectral splitting. ‘It’s always been known that if you are able to split the light and use a lot of cells you can reach a high efficiency, but you are not saving on cost because you are using so many cells.’

Barnham suggests some changes would be needed to produce a commercial version of the DARPA prototype. ‘You certainly wouldn’t want to use their cells, because each one is expensive in itself, and so putting a few of them together means t is going to be very expensive. What we want for a commercial application is just one of those cells, and a 100 times concentration.’