Ahead of the curve? Fresnel technology in CSP

In the deserts of Spain one of the world’s most recent concentrating solar power stations has been built, based on Fresnel technology. Daniel Reinhardt says the pilot plant is set to establish new standards for power supply and prove that an efficient and reliable solar energy power plant can be commercially realized.

Jacques de Lalaing, Managing Director of the Solar Power Group (SPG), has been making frequent journeys to Spain of late to catch some sunshine, but not for a holiday. In fact, de Lalaing has been working with MAN Ferrostaal, the industrial services arm of German engineering giant MAN, to develop a prototype for a concentrating solar power system that promises to reduce the cost of solar power generation.

Concentrating solar power (CSP) generation is gaining acknowledgement for its potential to produce large volumes of power and provide an alternative to fossil fuels. So far, three methods of solar thermal power generation have been identified with the potential to generate electricity within the 10 kW to 1000 MW range. These are dish/engine technology, solar tower technology, and parabolic trough technology.

The parabolic trough is currently an economically viable technology and has proven to be bankable. In fact, a number of parabolic trough fields are being commercially used today, mostly in the United States and southern Europe.

Head in the trough

Until very recently almost all solar power plants used traditional parabolic trough mirrors to capture the sun’s radiation. The resulting energy is used to heat thermal oil, which is pumped through an absorber pipe. The heated oil then passes through a heat exchanger that in turn creates steam to drive a steam turbine.

While the team from MAN Ferrostaal and SPG acknowledged the benefits of this mature CSP generation technology, they believed solar power could be generated more cost-effectively using a different design.

With over 3000 hours of sunshine per year, Almeria in Southern Spain has long been synonymous with almost every facet of the development of solar power. It was the ideal location for an ambitious project to test what MAN Ferrostaal and SPG believe is a second, very promising model for cost effective solar thermal power generation.

The Almeria pilot project uses Fresnel lens technology to harness the power of the sun. While there are several obvious similarities between the Fresnel technology and the parabolic system, the Almeria team believes its Fresnel plant, designed in conjunction with research scientists from the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany, will be a viable alternative to the more traditional parabolic trough technology.

According to the team, the key advantage the Fresnel system has over parabolic technology lies in its simplicity. This is no more evident than in the design of the mirrors themselves. While parabolic mirrors are produced using industrial standard methods, the components are still high precision optical elements. Fresnel technology, on the other hand, uses flat reflectors, simulating a curved mirror by varying the adjustable angle of the individual rows of mirrors in relation to the absorber pipe.

The reflectors are made using standard glass mirrors – not at all dissimilar to a bathroom mirror – making the raw materials very inexpensive and opening up the possibility to produce these key components in low cost countries. Whilst the curved shape of the parabolic mirrors makes them around 15% more efficient than the Fresnel reflectors, the team believes cost savings inherent in the Fresnel design can compensate for this.

Getting off the ground

To gain initial backing for the project, de Lalaing trialled early prototype technology and was able to demonstrate its reliability, but not that it was sufficiently economical. Nonetheless, the results were impressive enough to convince partners MAN Ferrostaal to support a full-scale evaluation of the technology.

The test now is to see if the technology can work efficiently in commercial conditions. de Lalaing hopes the new system will match up on costs and will be able to operate with the minimal amount of maintenance and upkeep. Building on the original success of early prototypes and improving on original plans, de Lalaing and MAN Ferrostaal set up the trial power station project at Almeria hoping to determine whether or not this goal could be achieved.

With so much riding on the scheme, planning on the project began several years ago and construction got underway in December 2006, with the plant opening in early July 2007.

The Plataforma Solar installation 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 100 metres long by 60 cm wide, which reflect the sun’s rays onto a 100 metre-long absorber tube hanging several metres above the primary field. Above the absorber tube is a secondary mirror, which concentrates any remaining sunlight onto the linear absorber tube.

All the mirrors in the primary field are controlled by electric motors that track the position of the sun, focusing sunlight onto the absorber tube in the most efficient manner. The smaller size of the individual mirrors also makes them less sensitive to wind than parabolic trough systems.

The project is being supported by the German Aerospace Center (DLR) and the ISE, which helped with the initial trialling of the Fresnel system. The project has also received sponsorship from the German Ministry of the Environment, keen to encourage big companies to consider investing in green technology. It is believed that with the backing of the German government and MAN Ferrostaal there is enough momentum to see the technology survive in the short-term, but it is generally accepted that additional investment is required and banks and other financial institutions are being targeted as possible parties to invest.

The next level

From the results of the Almeria trial, all the partners involved ultimately aim to create a technology which will be able to be rolled out in small to medium-sized plants. Key markets are the United States, southern Europe, north Africa and the Middle East, where daylight hours are long and sunshine is intense.

The long-term goal is for the Almeria installation to become a full-scale next generation solar thermal power station, which can then act as a blueprint for future developments. Theoretically, the project already looks very promising. On the basis of this pilot plant the group will see whether these theoretically calculated advantages actually prove themselves in practice. The team is hugely confident, hence the level of investment, but they still need to convince other parties to support the technology platform.

‘While the initial investment levels might be considerable, on the flip side, the operators won’t need any fuel,’ says de Lalaing. ‘It is comparable to a situation where you would have to pay a100,000 for a mid-sized car, but would then never have to stop at a gas station again,’ he adds. de Lalaing hopes that the initial short-term investment needed to set up the power plant will pay-off long-term in continual and renewable profits.

Other backers of the technology are also keen to point out advantages that could result from a workable solar thermal power station using Fresnel technology or the parabolic trough.

A key economic benefit is its ability to work with conventional steam turbines and using the generators of existing fossil-fuelled power plants or to operate them as hybrid stations. Cannibalizing the existing technology with updated renewable solar energy is thought to be crucial to appealing to less developed nations where funds do not exist to pay for a full refit of a country’s power network system.

Away from the financial backers of the project, academic sources, too, have pointed out the benefits of the renewable energy platform. The ISE believes solar power will be an essential element in the world’s future power supply. ‘In the long-term, solar energy will be capable of replacing power generation based on oil and natural gas through an adaptation of existing technology,’ says Volker Wittwer, deputy director of the ISE. He adds: ‘This aspect of the technology is critical to penetrating developing markets which would struggle to invest in fossil fuel plants and then have to update to solar energy systems in a relatively short space of time.’

An alternative to fossil fuels

From a political perspective, solar energy plants are increasingly being viewed as important options for future global energy needs. Within such circles there is a growing perception that more energy needs to be sourced from renewable sources, rather than relying on oil from potentially politically unstable regions of the world. ‘In political terms it is less risky than depending on individual oil nations. Experts think that by 2050 it will be possible to achieve half of the world’s energy needs from renewable sources using the technologies already in existence. Rising oil prices and increasing environmental awareness are factors changing the framework for the economic use of solar energy to a perceptible degree,’ argues Wittwer.

The combination of the rising cost of carbon emissions, increasing environmental regulations and the growing public sentiment against carbon-based fuels has led to an impetus for finding an economically sound alternative energy. With demand clearly in the market for an effective and commercial green alternative to fossil fuels, de Lalaing and MAN Ferrostaal are hoping Almeria will give them the results they need to gain further backing and eventually to gain access to this currently untapped and potentially highly lucrative market. With the stakes so high for all concerned, de Lalaing will be hoping the horizon remains bright in Almeria and the future solar energy market.

Daniel Reinhardt works for MAN Ferrostaal, Germany

You can contact Daniel Reinhardt at Daniel.Reinhardt@MANferrostaal.com

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