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Light years ahead

New technology - Photovoltaics: Transforming sunlight directly into electricity has long been an important goal for mankind. Alan Sparks explores how thin film photovoltaic technology could turn this old dream into reality.

Sustainable development is the buzz word of the modern engineer and renewable energy arguably its purest embodiment. In the past solar power has striven to make a real contribution to the nation's energy supply, but has too often been found wanting. Where photovoltaics have previously failed to break through, new advances in thin film technology will look to capitalise, as for the first time a competitive commercial product has emerged.

Huge growth in energy demand is predicted for the new century, creating a market strongly biased in favour of the seller. The opportunity exists for renewable energy to stake a claim for a larger slice of the energy production cake.

Government policy aims to provide 10% of the country's energy through renewable means by 2010. Such initiatives may fall short of the proposals in other European countries, such as Denmark, where they hope 50% of supply will come from renewable sources by 2035, but there appears to be a growing trend in incorporating self generated energy into new building schemes. Now that both construction and energy are under Department of Trade & Industry minister Brian Wilson, a potential exists for more sustainable new building developments and there have already been calls for solar arrays on 70,000 domestic properties and several hundred commercial buildings in the UK.

Photovoltaic (PV) solar power cells offer a proven alternative to, and are more socially acceptable than, sometimes unsightly and noisy wind turbines. Major companies are already taking an active interest as they appreciate the increasingly desirable nature of a green image. One such subscriber to the benefits of solar power is BAA, which plans to incorporate a large array of solar cells on the Pier 6 development at Gatwick Airport when it eventually goes ahead.

'Britain has some of the best solar designers in the world, ' says BP Solar international projects manager, Ray Noble. 'And together with new processes, like thin film technology, and the positive steps taken by the government to invest in solar power, UK industry has a real chance of catching up with its rivals in Germany and Japan.'

Thin film technology, (see box) though not new, is just emerging as a commercial product, opening new doors to the potential use of PV cells. With their semi transparent and maintenance-free nature, windows, facades and skylights are real possible uses for PV cells. One of the most innovative and extensive uses of thin film technology will be seen on Cambridge University's new campus and science park. A £1.35M European grant was awarded last month to the Urban Integrated Solar to Hydrogen Energy Realisation (USHER) project, which will power a hydrogen fuelled bus link into the city centre from the largest solar array in the country - covering 3500m 2. Whitby Bird project engineer for USHER, Shane Slater, enthuses about thin film panels, 'Every architect we've shown them to has gone absolutely crazy about them and the new options they offer.'

'These technologies are on steep downward cost curves, so as well as the aesthetic and environmental benefits they will become an economically viable solution too, ' says Whitby Bird project engineer Ben Madden.

'Thin film is available at £150/m 2and can produce around 50W per hour, ' Nobel adds.

Traditional solar panels currently generate more power than thin films. 'Conventional screen printed monocrystalline solar cells have efficiencies in the range of 11-15% with limited potential for improvement. We can already create thin film with efficiencies approaching this level. In the future thin film technology will drive down the costs of solar energy with improving technology, ' Noble adds.

However, the production process is significantly different.

Solar panels use silicon crystals and have to compete for this material with many other electronic manufacturing processes.

Thin film uses hundreds of times less silicon than crystalline PV cells in a paint like application that can then be etched, usually by laser, to create any desired pattern, transparency, or colour.

'Crucially, this manufacturing process lends itself to mass production in a way that traditional solar panels do not, ' explains Arup associate director Chris Twinn. This online production is where cost savings can be made and from where a truly competitive product may emerge.

'At the moment demonstration projects with European grants are the only way these technologies can be justified as a construction material, though when used as an alternative to architectural glass there's little difference in cost, ' says Whitby Bird engineer Hannah Routh.

Throughout their life traditional solar panels are expected to lose a significant amount of efficiency. Tests show that with thin film, however, this loss is minimal. Solar energy may prove a more productive tool in sunny climes, but in a north European climate scepticism has constantly questioned the effectiveness of solar panels. Thin film will improve this performance as it works more efficiently over lower levels of energy.

Twinn believes thin film is a proven technology which in the medium term can reduce energy losses through the day, especially when allied to other energy saving schemes. He sees thin film's likely influence as a cladding material to be greater than traditional solar cells as far less lifetime maintenance would be required.

A thin tale Thin film PV cells initially consist of a tin oxide coated glass layer which is then overlain with a semiconductor, usually silicon - although cadmium telluride is also suitable. Cadmium is considered to be an environmentally harmful material, yet the amount required for each square metre of PV module is less than that in a standard battery, and when disposed of would be regarded as standard waste. Once several layers of the semiconductor have been applied, a rear sheet of aluminium studded glass is then fixed. Light passing through the finished product excites electrons which pass a positive charge through the tin oxide coating.

Unlike monocrystalline modules there are no parts or mechanical connections between the cells to break or need repair. Two different types of semiconductors can be used to increase the efficiency of the cell by trapping different portions of the light spectrum. Less silicon is required compared to crystalline silicon PV cells, which means there is less embedded energy within the silicon. As a result thin films are able to work with less input than traditional cells - ie when sunlight levels are low.

Thin film silicon has an energy payback of 6 to 12 months depending on the amount of sunlight available.

For monocrystalline and polycrystalline PV cells this figure is four to seven and two to four years respectively. A conventional PV cell would expect to generate about 20 times the energy required to manufacture it. This will improve as thin film takes a greater share of the market.

Energy output of thin film modules is generally around 50W/m 2, although using cadium telluride as the semiconductor can boost this to 80W/m 2. This compares to 120W/m 2for conventional crystalline cells - sometimes rising to140W/ m 2for high efficiency monocrystalline.

INFOPLUS www. bpsolarex. com

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