Film focus

Solar concentrators can improve the effectiveness of solar generators by focusing energy into a small area, which improves the efficiency of the system and reducing the overall cost.

The Carbon Trust has awarded light management company Microsharp a £175,000 grant to develop a process of making optical film for solar concentrators cheaply using a reel-to-reel process.

'We develop somewhat unusual optical films,' said Dr Nicholas Walker, Microsharp's chief executive. 'One area in which we've been building up capabilities is surface structures or micro-optical films. We put a UV-curable coating on the surface of an ordinary film of optical-grade plastic, and mould tiny lenses or prisms on it, about 50 microns across and maybe 30 microns deep.'

These films could be used in solar concentrators in place of traditional lenses.

Microsharp is a small, privately-owned, UK-based firm founded in 1998 to develop optical films and use them in applications along with other optical elements. Its prime market to date has been flat panel displays.

The aim of economical mass production of optical film is to maintain very high accuracy of the moulds to maintain the required optical quality and ensure the cast is very faithful to the original mould.

'We carve the structures using a diamond cutting system on a precision lathe,' said Walker. 'We cut the microstructures on the surface of a casting drum, which is mounted in a production machine to make film using a reel-to-reel fabrication process. We've put a lot of effort into cutting these drums, so we're soon going to have a world-class facility to be able to do this.'

The prism-like structures act together as a Fresnel lens, which works like one in a lighthouse, but in reverse. 'In a lighthouse you have a point source and then the lens turns it into light coming out in one direction,' said Walker. 'We take the light from the sun which is all going in one direction and try and take it down to a small area.'

According to Walker, concentrator systems are well established. 'Instead of having a large piece of silicon, you have a small piece and a lens which is effectively a magnifying glass,' he said. 'The aim is that the cost of the lens plus the tracking system that needs to point it at the sun is less than you're saving by having a much smaller piece of silicon.'

Microsharp's reel-to-reel process can make large volumes of the concentrator film relatively cheaply. 'What we want to do next is build a few varieties of these kind of systems, and work with other companies to demonstrate that you do save money this way,' said Walker.

One is Whitfield Solar which has a concentrator that currently uses standard cast lenses. 'We're producing a film that concentrates the light to a thin line, so it only needs to track the sun by rotating in one direction, said Walker. 'Then we're going to make films that concentrate the light down to a point, which will require tracking in two directions.'

Microsharp is also working with Hydrogen Solar to research using the films in hydrogen generation. The process could potentially be made more efficient by directing a line of light on the appropriate parts of the system and keeping the other parts on the other side.

According to the Carbon Trust, a government-funded independent company, it selected Microsharp's project for its commercial potential. One of the ways it aims to assist the transition to a low-carbon economy is to back emerging technologies that will form its backbone.

Garry Staunton, head of low-carbon research at the Carbon Trust, said: 'With the applied research grant programme, we have an open call for proposals for projects which are innovative, can demonstrate how they would reduce carbon emissions and can show how the project will commercialise the technology. Microsharp's project was received through this process.

'The company made a very strong case for this being an innovative approach which contributed to the commercialisation of solar concentrator technologies by reducing the cost of the systems. It was very strongly focused on making the system cheaper,' said Staunton.

'At some point the people who make and sell low carbon technologies have got to make a financial return otherwise they will not be a viable business selling low carbon technologies.'