All done by mirrors

Using Solar power to drive turbines could become much more affordable following the development of a solar energy system with cheaper components by Germany’s Fraunhofer Institute.

Using solar power to drive turbines could become much more affordable following the development of a solar energy system with cheaper components by Germany’s Fraunhofer Institute.

The prototype system, set up for testing in Almeria, Spain, last week, will be trialled until the end of next year. Designed to be more cost-effective than traditional solar energy systems, it uses linear Fresnel mirrors to collect the sun’s radiation, and water instead of thermal oil is heated to produce the steam to power turbines.

The solar platform comprises a primary mirror field, an absorber tube and a secondary mirror. The primary mirror field has 25 rows of flat mirrors on the ground, each 100m long by 60cm wide, which reflect the sun’s rays on to a 100m-long absorber tube hanging several metres above the primary field. Above the absorber tube is a secondary mirror that concentrates any remaining reflected light on to the linear tube.

All the mirrors in the primary field are controlled by electric motors that track them according to the position of the sun, to focus sunlight on to the absorber tube in the most efficient manner.

Less expensive

Project leader and co-ordinator Gabriel Morin of the Fraunhofer Institute for Solar Energy Systems said the use of Fresnel mirrors instead of traditional parabolic mirrors would make the solar power system less expensive.

‘Fresnel mirrors are cheaper because they are conventional glass mirrors, like the ones we have in our bathrooms, and they are a very cheap raw material,’ said Morin. ‘There are a small number of adaptations made to make them more transparent to get a high level of reflectivity, but it is standard technology that can be provided by many companies.

‘Parabolic trough mirrors, on the other hand, are curved to a very specific function that is quite expensive, and currently there is only one supplier that provides them.’

A single absorber tube can be coupled with a parabolic trough mirror with a typical width of 6m. However, because the linear Fresnel mirrors are only 60cm wide, one tube can support a total mirror width of 15m, reducing the number of mirrors required.

According to Morin, the smaller size of the mirrors in the primary field also makes them less sensitive to wind.

‘Because each mirror is small, you have very low wind loads, which is a problem that is often faced by parabolic trough collectors,’ he said. ‘There was a time where they tried to overcome this problem by producing a parabolic mirror with a width of 10m, but this was not successful.’

In traditional solar energy platforms, thermal oil is pumped through an absorber tube, which is heated by reflected light and then goes through a heat exchanger that creates steam to drive an electricity turbine. In the new system, water is pumped through the tube and directly heated to create steam.

By cutting out the thermal oil and heat exchanger components, the latest system is claimed to be more efficient and economical.

The Fraunhofer team also developed a special coating for the secondary mirror to cope with high temperatures, and for the absorber tube to reduce heat loss.

‘For the secondary mirror, in initial tests we found that conventional mirrors did not stand up to the high temperatures, and were damaged by the heat,’ said Morin.

‘So we developed a new material using glass, which is highly reflective, as a substrate, and the mirrors would reflect on to the front side of the glass instead of the back.’

‘We also developed the coating for the tubes to make them absorb the greatest amount of radiation possible, and also prevent heat being emitted from the tube.’

This special coating allowed the mirrors and the absorber tube to withstand temperatures of up to 450ºC.

‘We are not trying to maximise the efficiency of the technology, we’re trying to minimise the costs of the system,’ said Morin, who has clear expectations for the project.

‘It depends strongly on the position of the sun and on the operating temperature, but we expect to achieve an average efficiency of 40 per cent, this being the fraction of the primary solar energy that shines on to the mirror surface being converted into thermal energy.

‘Due to the optimised shape of parabolic mirrors, they are 15 per cent more efficient. But our expectations are of the cost advantages to compensate for the efficiency drawbacks.’