MATERIALS
report. ‘Fabrication of the surface could be integrated into standard manufacturing chains for commercial silicon solar cells.’ However, Schumann says that
if the technique is to be fully commercialised, the fabrication process itself would need to be changed – because DLW cannot cope with production on an industrial scale.
As well as overcoming the
‘shadowing’ effect, the new material can gather more diffuse light – which could help to improve the efficiency of cells used in regions of weaker sunlight such as northern Europe. ‘When you illuminate the cell from oblique angles, some of the light gets trapped in the cloaking layer, and is absorbed by the solar cell,’ says Schumann. ‘This helps to raise the efficiency even further.’ Commercialisation may also
require a more robust material that can be processed using mass-production techniques. The current material is a photo-resist called OrmoComp, which is highly transparent and requires UV curing to harden it. ‘In practice, you wouldn’t want to use UV curing for high volumes, but a technique such as injection moulding instead,’ said Schumann.
Reflected glory Meanwhile, researchers at the University of Colorado in the US have devised a metamaterial that allows passive radiative cooling of surfaces – even in direct sunlight. In passive cooling, heat is emitted
from a surface as infrared radiation. It is a zero-energy and zero water – alternative to typical air conditioning systems, which work by pumping refrigerant around a set of pipes. Ordinarily, a small amount of incident sunlight is enough
Sergey Kruk of Austral- ian National Univer- sity has developed a metamaterial based on nano-structured gold and magnesium fluoride – shown in the background – that could form the basis of highly efficient thermo-photo- voltaic (TPV) cells
to overcome the effects of passive radiation, say the researchers. However, the new metamaterial – a glass-polymer hybrid – carries out two jobs at the same time: it radiates surface heat; and it reflects incoming solar energy efficiently. The material is based on a
two-layer film just 50 microns thick. The top layer, made of poly(methylpentene) plastic, incorporates randomly distributed silicon dioxide microspheres, while the base layer is a thin film of silver. The silver layer reflects around 96% of incident sunlight. The microspheres are sized such that they cause minimal scattering of visible light – yet will radiate infrared radiation effectively. When tested, the material had a cooling power of 93W/ m2
in direct sunshine (Science, doi:
An ‘invisibility cloak’ developed at Karlsruhe Institute of Technology bends light away from electrical contacts and onto the active surface of a solar cell, to boost efficiency
10.1126/science.aai7899). It could be mounted on the roof of various structures – from houses to nuclear power plants – to remove excess heat from the interior. Another possibility, say the researchers, is to boost the efficiency of rooftop solar arrays – which are prone to overheating in direct sunlight. ‘By applying this material to the
surface of a solar panel, we can cool the panel and get an extra 1-2% of solar efficiency,’ says Xiaobo Yin, co-director of the research and an assistant professor at the university’s department of mechanical engineering.
He also pointed out another
advantage: it can be produced using a simple roll-to-roll process at speeds up to 5m/minute. The researchers have applied for a patent on the technology, and are currently working on a 200m2
‘cooling farm’ prototype, which will be completed in 2017.
Saving waste heat At Duke University in the US, researchers have come up with yet another energy saving idea. They have created a metamaterial using micro-electromechanical system (MEMS) technology – to make a reconfigurable device that could convert waste heat into usable energy. It converts a wide spectrum of
infrared radiation into one with a narrower wavelength band, with very high efficiency. ‘Because the infrared emission is controllable, the emitter could provide a tailored way to collect and use energy from heat,’ says Willie Padilla, professor of electrical and computer engineering at Duke University. ‘There is great interest in using waste heat, and our technology could improve this process.’
One potential area of use is in thermo-photovoltaics (TPV), in which heat is harvested from a variety of sources – be it sunlight, emissions from a furnace or a hot car engine – and converted into electricity. One potential use of TPV could be to take waste heat from an engine block and turn it into energy to charge the car battery.
In operation, the material would
be placed on a hot object such as the engine block and ‘convert’ infrared emissions – from across a wide spectrum – into a narrower radiation band. If this is tuned to the specific needs of a thermovoltaic cell, this would lead to maximum conversion efficiency. Padilla says that the material’s emissivity – its ability to emit thermal radiation – can be altered while keeping the surface temperature constant. This could make it useful
20 09 | 2017
JAMIE KIDSTON/ANU
MARTIN SCHUMANN/KIT
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