Street-wise streetlights

Solid-state lighting, using LEDs rather than incandescent gases or hot filaments, is seen as the next generation of lighting technologies. It produces a brighter light while using less power, potentially leading to cost savings and reductions in carbon dioxide emissions.

However, the technical difficulties of using LEDs for lighting, especially streetlights, are not yet resolved. Researchers from Manchester University have teamed up with a UK LED specialist in a 12-month project to try and develop a practical prototype.

Most road lamps in the UK and Europe run on high-pressure sodium, which replaced the old-fashioned orange sodium lights at the end of the 1990s. They produce a bright white light, which provides strong, even illumination; for every watt of power fed in, they emit 85 lumens of light. LEDs are far more efficient, with current models emitting 150 lumens/W, and this figure is increasing as new semiconductors are developed. This could reduce energy consumption by 25-50 per cent.

‘With the environment at the top of the public and political agenda, energy saving has become a very important issue,’ said Roger Shuttleworth, director of the research at Manchester. ‘When you consider how many streetlights there are in the UK alone, it’s clear there are some big opportunities for energy and cost savings.’

But there are major problems with assembling an array of LEDs big enough to act as a streetlight. Most of these are thermal, explained Shuttleworth. ‘There’s a temperature problem,’ he said. ‘Silicon can’t be run too hot. The junction temperature can’t exceed 125-150°C, depending on the type of silicon. And the problem is, the LEDs are in an ambient temperature, which can go quite high. Even if you go above 60°C, they start to degrade in terms of efficiency — so they don’t produce as much light.’

Shuttleworth’s team is working with Yorkshire-based Dialight Lumidrives, which specialises in systems based around high-powered LEDs, to develop a low-cost array that could be used for street lighting applications. The project, he said, will not develop new LEDs but look at many different versions of the concept.

‘Part of the problem seems to be that the heat comes from having a large number of LEDs packed in together to provide enough light,’ said Shuttleworth. ‘So one of the things we’re looking at is using large LEDs, or a larger number of smaller ones that each produce less light. But larger LEDs could have weight implications for the top of the [streetlight] pole.’

The project will also study different methods of cooling the LEDs, with passive cooling provided by heat sinks or heat pipes; the latter method is now being tested in China for lighting for the 2008 Olympics.

The Dialight team is working out ‘the defining rules’ for the design, said Shuttleworth, including the electronics, and ensuring that the final prototype design complies with street lighting regulations.

These include the way that the lamp spreads light, which is a tricky matter with LEDs. ‘You tend to get a more directional light than you do with incandescents,’ he said. ‘Sodium lamps throw light to the sides and even upwards, but LEDs produce more of a spotlight effect.’

While this could have the welcome effect of reducing light pollution, it also means that the team will need some method of diffusing and spreading the light from the posts. Shuttleworth said: ‘You need to have a certain amount of light, a certain distance from the lamp, otherwise you’ll have pools of light and big stretches of darkness. We need to make sure we get a smooth light.’

The project has been funded by a £175,000 grant from the DTI’s Technology Programme, which Dialight has matched. By the end of the project, Shuttleworth aims to produce a working prototype. ‘We want to have something that we can put straight into a practical trial,’ he said.