Butterfly-inspired coatings give wings to solar glass for shipping

Anti-reflective coatings combined with recycled solar glass could boost sustainability in global shipping.

Researchers at the University of Exeter, UK, believe the smaller, lighter, solar panels could lead to reduced emissions, lower costs, savings in materials and less waste. 

The new solar panels concentrate the sun’s energy, enabling greater compactness. This counters a key problem, whereby current solar cells maximise energy capture by directly facing the sun, which isn’t always possible on ships whose routes and schedules vary.

To establish integrated solar panels on cargo ships – in various orientations – their performance needs to be boosted without solar trackers moving the solar panels to face the sun. 

Dr Katie Shanks at Exeter sought to develop coatings that mimic the energy-trapping properties of glasswing butterflies. Project partner, the University of Pittsburgh, USA, has manufactured the resulting 50-150nm-thick structures directly into the fused silica glass surface of solar panels.

The coating is made of silicon dioxide. The surface structure is etched directly into the cover glass. The nanofabrication process is performed in two steps – reactive ion etching and plasma-enhanced chemical vapour deposition and surface treatment with fluorination.

The structures are modified with flourosilane after silicon dioxide deposition to create a low surface energy by spin coating, developed by Paul Leu at Pittsburgh University.

The work also took inspiration from another butterfly – the cabbage white butterfly. Its white reflective wings position in a V-shape to funnel and concentrate sunlight to its body and flight muscles, which need warming up before flight. 

Further investigations highlighted the glasswing butterfly’s wide-angle anti-reflectance properties, which led to the use of compact solar concentrator optics – e.g. lenses and mirror-type optics, which focus and redirect light towards smaller areas of photovoltaics. 

The unique feature of the work is the wide-angle anti-reflectance properties, Shanks says. 'The wide-angle anti-reflective layer allows more energy generation annually due to the wide range of incident sunlight angles experienced for a stationary solar panel (e.g. on a building). This also works for a moving vehicle such as a car or marine cargo ship.  

'As this is a property of the cover glass only, this can also be applied to any type of solar photovoltaic (PV) panel – boosting the annual energy performance of the latest PV developments such as perovskites or high-efficiency multijunction PV, or lightweight thin-film cadmium telluride (both used in space applications)'.

Solar concentrator modules can have payback periods under a year because of the significantly reduced photovoltaic material required for equal energy generation. Shanks says though this was proven years ago, 'it has been an overlooked/underestimated factor, which I hope is beginning to be more valued now'. 

She adds, 'By using PV panels to provide onboard energy…the cargo ships can reduce their carbon emissions and save costs of fuel. The ratios of the different surfaces of the cargo ships are also fairly scalable depending on the weight of cargo it’s designed to ship – so easy to model energy gains for a range of ship sizes.' 

The specific concentrator design developed for this research is optimised for vertical installation in buildings and cargo ships.  

The Exeter team have also worked with ReSolar and Upcycled Glass Company to produce the solar concentrator optics from recycled PV glass.

Discussions with maritime transport companies are planned and scale-up of the recycling technology is being explored with glass manufacturers. Focus on development and testing aims to produce a prototype solar panel within two to three years.

Branched collaborations with ReSolar are investigating the antimony content in PV glass and its effect on secondary uses and potential environmental impact, building on previous work.