Will Fresnel be the Model T ?

A new solar thermal plant (pictured right) employing Fresnel diffraction mirrors was officially opened in Almería (Andalusia) in Spain on 9 July. The new technology employed in the 1500 square metre 1 MWt demonstration plant on the Plataforma Solar is expected to enable solar thermal power stations to be built much more cost-effectively than in the past, says MAN Ferrostaal. MAN Ferrostaal, which has responsibilities for project management, operation management and maintenance, built the Almeria demonstration power station in collaboration with Solar Power Group (in which it has a 25% stake), the German Centre for Air and Space Flight (DLR), the Fraunhofer Institute (ISE) and PSE GmbH.

Currently, power from solar thermal power stations costs up to three times that of power from power stations in terms of p/kWh, while the generation costs for photovoltaic power stations are about ten times as much. With the Fresnel technology the cost of solar-generated electricity should be comparable with that for fossil-fuel power stations by about 2020, MAN Ferrostaal estimates.

In the new plant the moving Fresnel diffraction mirrors focus sunlight onto an absorber pipe, positioned eight metres above them (see diagram above). Water in the pipe is heated to 450 degrees Celsius and the steam used to drive a turbine.

The plant is modular in design and in a full scale power station, several modules would be connected up in series.

The plant is made up of relatively cheap standardised components, manufacturable in many parts of the world, thus creating potential for establishing a local supply chain with

high value added. "Fresnel technology is comparatively simple to construct, cost-effective to procure and reliable to operate," commented Michael Pohl, head of the solar power business unit at MAN Ferrostaal. "It has the potential to become the Model T of solar thermal technology."

The Almeria pilot plant (which is 100 m long, 21m wide and 8m high) is intended to demonstrate the commercial viability of the technology, with a test period running until 2008.

Around 1000 MW of solar thermal installed capacity is planned or already built in Spain and the medium to long term prospects for this technology are very good. The price of electricity has on average doubled throughout Europe between 2003 and 2007 and a reversal of this trend is not anticipated any time soon.

The countries around the Mediterranean Sea, of course, stand to benefit from a "solar boom" because it is here that sunlight is at it most intense, while energy-hungry Europe lies close by and as Mr Pohl observes, "solar thermal power stations would only have to be built on one percent of the Earth's desert regions to meet the total global electricity demand." It has been estimated that "by 2050 up to 25% of Europe's electricity demand could come from North Africa – if the political will exists," says Pohl.

MAN Ferrostaal announced in July that it was taking a 25% stake in Fresnel specialist Solar Power Group, following approval of a joint venture with Solar Millennium AG.

Solar Power Group has already built two Fresnel pilot plants and has been working on the technology for several years. It was responsible for the engineering of the plant. DLR is responsible for the measurement programme and will also have a technical supervisory and support role. The Fraunhofer Institute for Solar Energy Systems has made a significant contribution to the development of the coating for the absorber and will play a support role in the analysis and evaluation of the test results.

The r2.6 million demonstration plant is financially supported by the German Federal Ministry for the Environment, but most of the investment costs are being borne by MAN Ferrostaal.

Among the tasks to be performed with the Almeria demo plant will be:

• determination of investment and operational and maintenance costs for extrapolation to full scale stations;

• estimation of the efficiency of the system and measurement of the contamination and ageing of materials and the contour accuracy of the collectors; and

• learning about the practicalities of operation and maintenance, with identification of problems and implementation of solutions, including development of cleaning systems which can be automated for full scale plants.

The project partners say they are already planning power stations of up to 50 MW and more.

Solar thermal technologies compared

Solar thermal technologies compared:
According to MAN Ferrostaal Fresnel technology is the most cost-effective of the four main solar thermal technologies currently available (see diagram). Unlike a parabolic mirror, which requires multiple curved mirrors, the Fresnel facility only needs flat mirrors, which substantially reduces costs.

A "power tower", in which a large array of two-axis tracking mirrors direct sunlight towards a tower fitted with absorbers, requires a vast number of separate components and is therefore a very expensive solar thermal power option.

Parabolic trough power stations are the most technologically advanced in terms of engineering and have already proved themselves at large scale. All the components are already manufactured industrially and have therefore reached an acceptable level in terms of production costs.

However Fresnel power stations are even more cost-effective than parabolic trough power stations, their key components being very simple. The mirrors, for example, are flat and readily available flat. The structure to which they are attached has one continuous axis and is required to carry very little weight.

A Fresnel facility is relatively immune to problems arising from high winds and thus does not need the very solid foundations and robust support structures needed for parabolic trough power stations. As only the mirrors, and not the absorber tube, move in the Fresnel plant, there is also no need for flexible high-pressure compensators, which are required to enable the parabolic trough mirrors to pivot towards the position of the sun.