April 14, 2016

Forty years of DLR energy research: Think tank for solar power plants

The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is celebrating its fortieth year of energy research in 2016, giving decision makers a chance to reflect. One of these decision makers is Michael Geyer, who is a physicist and engineer, and an expert on solar thermal power plants. For several years he was a DLR researcher at the Plataforma Solar de Almería research centre in Spain. He also worked for German and international industrial companies before he moved to the Spanish Energy Company Abengoa Solar in 2007. As Director of International Business Development, he was involved in the development of large solar power plant projects worldwide. In the 1990s, Geyer was a professor of energy, power plant technology, and process technology at the Regensburg University of Applied Sciences. Science journalist Tim Schröder spoke with him.

Mr Geyer, the idea of using solar thermal power plants for energy production is already at least 100 years old. Shortly before the First World War, engineers built the first plant in North Africa. But the technology only succeeded in breaking through at the start of this century. Why so late?

There are a number of reasons for this. To begin with, this is because, until now, fossil fuels have dominated. What is surprising is that, actually, the American inventor Frank Shuman built the first parabolic trough power plant in Egypt in 1912, to pump Nile water to irrigate the cotton fields. He wanted to replace coal, which the Egyptians were importing from England at that time, with solar energy. With the First World War, the era of crude oil began, and discussions about solar energy stopped for a long time. With the oil crisis of 1973, the use of fossil fuels for energy had its first setback. The oil crisis gave the USA and Europe the incentive to engage with the idea of using solar energy in power plants again. At the end of the 1970s, the building of the first solar thermal pilot plants with outputs of a few megawatts began in the USA and Europe. But, there was still a long way to go from the first pilot plant through to the commercial use of the technology, because there were a lot of technical questions that remained unanswered.

How did the era of solar thermal power plants begin?

One milestone was the development and demonstration programme, which the international energy agency – IEA – launched in 1976 together with 10 of its member countries. Two solar thermal pilot plants were constructed in the only desert in Europe – the Tabernas Desert in the Spanish province of Almería. The project was called IEA-SPSS, SPSS standing for Small Solar Power Systems. The goal was to test two separate technologies for solar power plants – parabolic trough technology and solar tower technology. Each pilot plant was meant to have an electrical output of 500 kilowatts. This is, in comparison to plants nowadays that provide 100 megawatts and more, relatively low. But first of all, these new technologies had to be tested. Companies and scientists from 10 countries – Belgium, Germany, France, Greece, Italy, Austria, Switzerland, Sweden, Spain and the USA – were involved in the project. This shows how great the level of interest in this technology was at that time. The German Test and Research Institute for Aviation and Space Flight (Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt; DFVLR) as 'Operating Agent, was responsible for project management. The building of both plants was completed in 1984.

  • Solar power plants

    Solar power plants use solar radiation by collecting it with reflectors and concentrating it, thereby producing heat. The heat is collected by a thermal transport medium and is used to produce steam. The steam drives a turbine – as in normal power stations – to produce power. Their area of application is therefore dry regions in Earth's Sun Belt, between 35 degrees north and 35 degrees south. The advantage of solar thermal power plant technologies, compared to photovoltaics, is that the heat can be stored. This means that power can be produced even when the weather is cloudy or at night. Solar power plants need direct solar illumination that has not been dispersed by clouds or humidity.

  • Parabolic trough power plants

    With parabolic trough power plants, solar radiation is directed from a curved reflector onto a long pipe that runs along the trough and transports the heat storage medium. Parabolic trough power plants can reach an operating temperature of 420 degrees Celsius.

  • Solar tower power plants

    With solar tower power plants, solar radiation is directed from many individual reflectors to a collector at the top of a tower, through which the heat transfer medium circulates. Solar energy is focussed better than with parabolic troughs, so solar tower power plants can reach an operating temperature of 600 degrees.

So was it around this time that you went to Almería for DFVLR?

Yes. After studying Physics, I started at DFVLR in 1981, in Joachim Nitsch's group. There we concerned ourselves mainly with the theoretical question of how we could change the energy supply system to use renewable energy sources. My task was to investigate the potential of solar thermal power plants. After this analysis, I wanted to test solar thermal technology in practice, and so I took up the offer in 1985 to go to Tabernas for DFVLR and to get to know the plants there. The IEA-SSPS pilot plants and also the Spanish pilot power station CESA-I had at this time already been completed. The IEA-SSPS project had just finished. The work continued to be managed from then on by a new Spanish-German cooperation contract that was financed and managed equally by Spain and Germany. The site in Almería was given the name Plataforma Solar.

What research was carried out during the early years in Almería?

At the start, we investigated, in particular, the optical design of different concentrator technologies – heliostats, parabolic troughs and parabolic reflectors, together with their supporting structures and tracking systems. It is a fact that reflectors must follow the position of the Sun during the course of the day. We at DFVLR worked closely with scientists from the Spanish research institute CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológica) and partner companies. We also researched various radiation receivers, in which the Sun's rays, arriving from the reflectors, are concentrated. The receivers hold the heat transfer media, which absorbs the heat from the highly concentrated solar rays. I myself focussed on the Plataforma Solar, in particular on the storage of solar heat.

The Plataforma Solar raised concerns in the 1980s when there was a fire. The reason was said to be the heat transport medium. What happened?

In the beginning, the IEA-SSPS project used sodium as a heat transport medium in solar tower power plants, because it can absorb, transport, and retain heat very well. Germany brought to that project the sodium technology used in the SNR 300 Fast Breeder reactor at Kalkar in North Rhine-Westphalia, a modern nuclear power station at that time. Of course, sodium reacts violently when in contact with water and is also highly flammable when there is the slightest amount of moisture in the air. During the maintenance of a valve in 1986, sodium escaped and reacted with the moisture in the air; the result was a major fire. This fire certainly contributed to the fact that sodium-cooled fast breeder nuclear reactor technology was not pursued further in Germany. In solar thermal technology, we changed from sodium to molten salt – a variation that, at the time, was being tested in the American 'Solar Two' plant and in the French 'Themis' solar research centre in the Pyrenees.

Was the Plataforma Solar at the time the most important plant in the world?

The Plataforma Solar was at the time the only international facility shared by several countries. Around the same time as the IEA-SSPS project, several countries tested their own solar thermal technology, including France, the Soviet Union, Japan and the USA. All these plants and also Plataforma Solar served only for research and demonstration purposes. Only in California was the first commercial market for solar thermal power developed. There, the oil crisis caused gas prices to rise steeply. In order to protect the supply of power during the heat of summer, when thousands of air conditioners are being used in California, the local energy suppliers looked for renewable energy sources. In Southern California, the local energy supplier, Southern California Edison, offered users contracts for power from reliable non-fossil plants, referred to as 'Standard Offers'. For the first time, there was a market-based incentive for the construction of large solar thermal facilities. The Israeli-Californian firm Luz developed and built a very large 354-megawatt commercial solar thermal power plant. Luz used parabolic trough technology for this. One has to emphasise that, prior to this, Luz engineers had already been convinced about the market readiness of this technology at the Plataforma Solar. The solar reflector technology came from a German company and was tested and optimised in Almería. Luz adopted the technology for all its power plants. From 1985 to 2005, California remained the only commercial market for solar thermal power plants.

Solar thermal power stations experienced further substantial growth, but not for long.

The important thing here was that, in the 1990s, oil and gas prices dropped again and consequently the competitiveness of renewable energy waned. At the end of the IEA-SSPS project in the middle of the 1980s, most countries participating in the financing of the Plataforma Solar departed – only Germany and Spain continued to further develop solar thermal technology. The SSPS plants were handed over by the IEA to Spain. Germany and Spain negotiated a bilateral agreement that controlled the joint financing and scientific use of Plataforma Solar.

You, in the meantime, left Plataforma Solar.

Correct. In 1989 I first went from Almería back to Germany, in order to advance the industrial side of the development of commercial solar thermal power plant projects. I worked for the Flachglas Group, the German supplier of reflectors that Luz installed in power plants in California. In 1995 I then returned to Almería as German co-director of Plataforma Solar for DLR.

What was your aim?

During my time in German industry, it had become clear to me that we in Europe were only suppliers of components for solar thermal power plants. What was missing was our own 'collector' technology. So it was my aim at Plataforma Solar to develop Europe's own parabolic trough collector in cooperation with research and industry – the 'Eurotrough' that later became the origin of the most widely used parabolic trough collector fields. With the help of research in wind tunnels, we developed a completely new supporting structure for this collector that prevents the reflector from being moved by the wind, which impairs its focus. Together with industry we also developed new absorber tubes for this collector. Subsequently, this first Plataforma Solar 'EuroTrough' has been further developed by industry, and a third generation of parabolic trough collectors is on the market.

A promising technology like EuroTrough still needs a market that wants this kind of plant.

In the last ten years it really has developed; nowadays, worldwide, nearly 5000 megawatts are in use or being built. In Europe, the EU Directive 2001/77/EU was undoubtedly a driving force. In order to promote the production of energy from renewable energy sources, member states were obliged to establish national targets for the proportion of renewable energy in their power consumption. States decided themselves which support schemes should be used to achieve these goals. In Spain this EU directive was implemented from 2002 through several 'Real Decretos', or state regulations. Moreover, by 1999, the Spaniards had already set out their first national plan to promote the use of renewable energy sources with the aim of building solar thermal power plants with a total output of 200 megawatts by 2010. And so began the development boom of solar power plants in Spain. To date, 50 solar power plants with a total output of more than 2300 megawatts have been constructed, mostly in Andalusia and some further north near Madrid.

In the year 2007 you moved to the Spanish power plant builder Abengoa. Since then you have worked on many projects, particularly abroad. On the whole, how has the international market developed?

When I went to Abengoa I actually started the worldwide development of solar thermal power plant projects. The first solar thermal projects were what are referred to as 'integrated solar, combined-cycle gas and steam power plants' in Algeria and Morocco. Gas and steam power plants are most conventionally fuelled by natural gas. In this case, the 'EuroTrough' parabolic receiver fields supplemented the gas. My next project was the 100 megawatt 'Shams' parabolic trough power plant in Abu Dhabi, built by Abengoa and operated by Total. The solar thermal technology for power plants has, however, been particularly successful in South Africa. Whereas in Europe we have a plentiful supply of energy, in South Africa, as in many other emerging countries, there is a lack of power. For two years, they have experienced regular power cuts due to what is known as 'load shedding', because the capacity of the power stations is insufficient. There is a system there where, for a certain time, power is switched off in various regions, towns, and areas within towns. Therefore, in 2011, the South African government started a very successful programme to build privately financed and operated renewable energy production plants. I secured for Abengoa the contract to build three solar thermal power plants with a total output of 250 megawatts. Two of these power plants are already online. In the USA, in recent years, Abengoa has built the two largest parabolic trough power plants in the world – 'Solana' and 'Mojave', each with an output of 280 megawatts. 'Solana' also has the biggest thermal storage system in the world, which can supply up to six hours of power at full capacity during the night. In Chile, we are building a power plant that can deliver uninterrupted baseload power throughout the year.

What is the outlook for Asia and other regions?

Many enquiries now come from the rest of Africa, China, and India. I am convinced that in this case the German energy transition will set a worldwide precedent. In many countries, renewable energy now forms a major component in the expansion plans for utility companies. Ten years ago, everything looked very different. Thanks to its constant presence at Plataforma Solar, DLR has performed outstanding research and development work throughout, thereby contributing considerably to the progress of solar thermal technology for power plants. My colleagues at DLR are in demand all over the world as consultants to governments, investors, industrial companies, and energy suppliers – thanks to 40 years of acquired expertise.

A look into the future – how will the development of solar power technology in Almería continue?

More than 95 percent of systems installed today are parabolic trough power plants. But what I find really promising in particular are solar towers with molten salt storage. They can concentrate the Sun’s rays more strongly than parabolic troughs and therefore reach higher working temperatures, so that the turbines achieve higher levels of efficiency and provide more power. Solar towers use the same molten salt storage technology as today’s commercial parabolic trough power stations, but thanks to their higher operating temperature, can store almost twice as much energy in the same storage system. For several years now, the first 17-megawatt power plant in Spain, which works with molten salt, has been in operation. In the USA, a technically similar power plant with an output of 110 megawatts has just gone into operation, and Abengoa has begun building another 110 megawatt power plant. So things are on the move. These initial projects with new molten salt technology, however, need financing by state banks or at least state financial guarantees, because commercial banks generally do not want to offer credit where there is technological risk. They only get involved when new technology has proved its marketability for several years.

Do tower power stations still require a type of start-up aid?

Yes, up to a point. This is where I sincerely hope that technology support does not end with the development and construction of small pilot plants in the one-megawatt range. Further support is important if we wish to upscale a new demonstration plant – ideally to a commercial output of 100 megawatts. Otherwise, the new technology will not find a place in the market. Practical funding instruments for me would be first, credit guarantees like those that the US government, for its part, has provided for the first generation of renewable power plants; or direct financing of a project through state banks or development banks. It is clear that solar thermal power plants, which have a global installed capacity of nearly 5000 megawatts, are still experiencing a learning curve compared to photovoltaics and wind power. The next great step towards innovation that would lead to a meaningful reduction in costs would be the global commercial introduction of molten salt solar tower power plants. And solar thermal power plants have, in my opinion, an unbeatable advantage. Thanks to thermal storage, they can produce energy when it is needed, not just when the Sun shines or the wind blows.

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Dorothee Bürkle

German Aerospace Center (DLR)
Media Relations, Energy and Transport Research