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Sun shines on turbine makers

Unlike photovoltaic power systems that rely directly on sunlight, solar thermal power plants generate electricity from the heat transmitted by the Sun; they are much more efficient. Dedicated steam turbine technology is crucial to the economic exploitation of this valuable and limitless resource, and the blue chip turbine makers are keen to cash in.

Chris Webb

Deep inside what is often described as the ‘inhospitable’ environment of the desert, Abu Dhabi is warming to the idea of solar power. A 125 MW solar power plant will be the biggest of its kind in the world. This rich and forward-looking Emirate, surrounded by the most prosperous oil nations on Earth, is going ‘green’.

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And ‘inhospitable’ the desert is not. Locked in a sunbelt that encircles the Earth, Abu Dhabi aims to generate 1.5 GW of its electricity from solar energy in ten years, making it one of the most progressive nations on the global renewables map.

The 125 MW ‘Shams One’ project will be the largest parabolic trough power plant in the world. Its owners, Abu Dhabi Future Energy Company (Masdar), ordered the 125 MW steam turbine from MAN Turbo for the Medinat Sayed site, some 150 km from the Emirate’s vibrant capital city.

MAN will produce the biggest steam turbine ever used for a solar heating project, after supplying smaller turbines for two parabolic trough power plants in Spain; Andasol Three in Andalusia and Ibersol in the south western region of the Iberian peninsula each have a capacity of 50 MW.

Bridging the gap between more conventional forms of power production and generating electricity from solar thermal plants is relatively easy. The principle of solar thermal power is itself quite simple: an array of mirrors focus the incident solar radiation onto receivers, typically containing synthetic oils, or in some cases liquid sodium or salt, as a heat transfer medium.

This is then routed through a heat exchanger, in which steam is raised to drive a turbine. Upstream, various mirror technologies are used for concentrating solar power (CSP).

Exploring the technology options

Currently the most common, parabolic trough power plants, use single-axis sun-tracking collectors. The receiver (or absorber tube) containing the circulating heat transfer medium, is located at the mirror’s focal point.

By contrast, the linear Fresnel concept uses a large number of flat mirrors, which concentrate the solar radiation on a central absorber tube. Thirdly, solar tower technology captures the sunlight and concentrates it using two-axis, sun-tracking mirrors – or heliostats – which are arranged in an asymmetrical arc around the tower and reflect the sunlight onto a receiver at its peak. As with all these systems, the mirrors track the sun over the course of the day from east to west.

The most commonly used technology used today for concentrating solar power is that used in parabolic trough power plants. However, solar tower power plants, which concentrate the solar radiation on a single point, generate higher temperatures in the heat transfer medium, whereas linear Fresnel technology offers cost advantages due to the use of non-curved and easier-to-produce mirror strips. Both of these technologies therefore offer great promise for the future. In each case, surplus heat can be stored in large storage tanks and used to extend the running hours of the steam turbine during times without sun radiation.

“There is a huge potential for CSP,” says Ole Hansen, head of the steam turbine business unit at MAN Turbo. “CSP power has an efficiency of more than 41 per cent compared to photovoltaic’s 15-16 per cent. The only problem at the moment is the large installation investment cost of almost €6000/kW ($8325/kW).” MAN is one of a handful of companies whose technology offers high efficiency and a flexible modular concept. “This was the main reason for the Shams order,” he told PEI.

Weathering the economic storm

Though the worldwide recession has hit development of solar power as hard as any other renewable source, Hansen believes the technology will emerge in a better state than most, and the market for adaptable and purpose-built steam turbines will be stronger than before. “The USA is committed to becoming independent of oil imports and will invest in the necessary technology; on the other hand, at present there is a shortage of risk-capital. Nevertheless, the cost of the steam turbine represents less than 5 per cent of the total cost of a CSP project.”

Last October, rival Siemens Energy announced it was to supply an industrial steam turbine for one of the world’s first commercial solar tower power plants. It said the Spanish company Sener would build the innovative solar thermal power plant with a capacity of 19 MW at a site near Seville. The ‘Solar Tres’ project would use a Siemens industrial steam turbine specially adapted to meet solar technology requirements.

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The solar tower power plant will focus the sunlight captured by sun-tracking mirrors – or heliostats – arranged over a surface area of some 320 000 square metres onto a receiver located on top of a 120 metre-high tower. A unique feature of the Sener project is the use of salt as a heat transfer medium, in place of more typically used oil.

Bundling of the sunlight produces temperatures of over 850°C at the solar receiver. The salt heated to approximately 565°C flows in a molten state through the heat exchangers, in which sufficient steam is produced to operate a steam turbine generator.

Sener had commenced planning construction of the ‘Solar Tres’ solar tower power plant seven years ago. The overall project is backed with funds provided under the terms of the Fifth European Community Framework Research Programme. For the demonstration plant Siemens will supply a two-cylinder reheat SST-600 industrial steam turbine, which was specially adapted to meet solar technology requirements. The reheat enhances overall power plant efficiency.

Siemens has secured orders for the supply of more than 40 specially adapted turbines for thermal solar power plants. “The solar power market is one of the fastest growing power plant markets,” said Markus Tacke, CEO of the Siemens Steam Turbines unit. He added that solar power is an important part of Siemens environmental portfolio, which accounted for company revenues totalling €17 billion in 2007.

Earlier this year, the company announced it was to acquire a 28 per cent stake in the Italian solar company Archimede Solar Energy. ASE is the sole producer of solar receivers operating with molten salt as the heat transfer fluid. By combining ASE’s expertize and Siemens’ steam turbine technology the latter plans to enhance the efficiency of thermal solar plants and further reduce the production costs. Siemens says the market for solar thermal power plants will show double-digit growth rates per year to reach a volume of over €10 billion by 2015.

COMPLEMENTARY, NOT COMPETING

Siegfried Fischer, head of CSP at Siemens’ renewable energy division believes photovoltaics (PV) and thermal solar power will develop in parallel: “We see these two solar technologies more as complementary solutions than in direct competition. Depending on project specific requirements, PV or CSP could be the best choice for the customer – with ample room for both technologies to prosper on a global basis. We are currently discussing numerous projects for our steam turbine in solar thermal applications. While the past has been largely focused on Spain, we now see increasing activity also in other countries.”

Siemens’ ace card is, it believes, its high efficiency SST-700 DRH steam turbine. It says its proven standardization ensures low investment and life-cycle costs. As far as daily cycling capacity of turbines is concerned, when focusing on annual power production, the short start-up times the turbine can provide will be a huge attraction to CSP plant developers. Fischer believes strongly that solar power will continue to gain in importance in the future energy mix and that his company has the range of steam turbines to meet an increasing demand. “Our range includes turbines featuring high efficiency, flexibility of bleeds for feed water heating, short start-up times, coupled with a small footprint to reduce civil engineering costs.”

SOLAR SOLUTIONS

Siemens’ SST-700 DRH is based on a dual-casing industrial reheat steam turbine. The high-pressure turbine is connected via a gearbox to the generator while the low-pressure turbine drives it directly. In this way the two turbine cylinders can operate at their dedicated optimum speed, which provides for high efficiency. In the reheat cycle the exhaust from the HP turbine is routed at constant pressure before entering the downstream LP turbine. The steam cycle therefore operates at a higher mean temperature. Reheating boosts efficiency, as the turbine generates more output in return for a corresponding heat input at the boiler.

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With its low-mass rotors and casings, the SST-700 DRH is designed to cope with daily cycling and has a low minimum load, enabling maximum running hours per day for plants without heat storage. The cycle has also been optimized for standstill at night and rapid restart in the mornings. The SST-700 DRH uses high quality materials specially chosen for long and trouble-free operation in a solar plant, bearing in mind the potential wear and tear of the special cycle conditions.

Fischer says that, despite the global economic woes there remains significant demand in Spain for solar power products, accompanied by a high rate of growth US, North Africa and the Middle East. “In addition,” he told PEI, “we also see new markets emerging in India, South Africa, Israel, Chile, and Australia.”

While the SST-700 DRH turbine is popular with CSP developers, Fischer says all Siemens’ steam turbines have the potential for solar applications. Demonstration tests are currently underway with leading institutions in Spain and Germany to test both the lower end of the industrial turbine range – around 1.5 MW – and also the mid-range – around 20 MW – in solar tower applications.

INTEGRATED SOLAR COMBINED-CYCLE SYSTEMS

Siemens believes that integrated solar combined-cycle systems (ISCCS) will feature prominently in thermal solar’s future. ISCCS plants are a combination of a combined-cycle and solar thermal power plant. The steam raised to drive the turbine can be generated using the exhaust heat from a gas turbine alongside that generated in a solar plant.

In plants such as this, the gas turbine replaces an optional heat store incorporated in a solar thermal power plant. Thus ISCCS is doubly effective.

It not only minimizes the investment associated with the solar field by sharing components with the combined-cycle, it also reduces the CO2 emissions associated with a conventional plant.

The integration maximizes operation efficiency even though solar energy intensity varies according to the weather and time of day. Peak efficiency can exceed 70 per cent compared to 50–55 per cent for a conventional gas combined-cycle plant.

Globally, some 12 GW of CSP power is expected to be installed by 2015, with 2 GW of plants in development today, says Michele Taviani, steam turbine product line manager at GE Oil & Gas. GE, he says, is ready to serve the full scope of the CSP market, with steam turbines up to 100-110 MW, and larger units coming from GE Energy’s steam turbine plant in Schenectady, in the US.

“We don’t necessarily see CSP and PV as competing technologies. The technology choice depends on the size of the project, and the level of power density required. Both technologies are needed if the target of 12 GW is to be reached. The industry does not have the potential for that level of growth without both technologies,” Taviani told PEI.

To date, some 55 steam turbines have been sold for solar projects worldwide, including 15 from GE, many of them in Spain. “We’ve focused on developing steam turbine-generators and centrifugal pumps to meet the rigorous cycle duty demands of solar thermal applications.”

Just as the intensity of solar radiation can change throughout the day or throughout various seasons, so does the steam-flow range. Taviani says GE technology enables saturated steam at the turbine inlet without the need to reheat the steam before it leaves the turbine to the condenser, which greatly simplifies the turbine train configuration and helps to reduce equipment and installation costs.

The cost of CSP per MW remains a problem, as it is difficult to find organizations that are willing to finance large-scale plants and projects, particularly in these challenging economic times when a large number of projects have been delayed or cancelled. Plants based on parabolic technology, nevertheless, take preference in the financing stakes.

“It is the only [technology] that has proven endurance and reliability,” says Taviani. “That allows us to know the costs involved in running that type of plant, and the endurance of the equipment and materials. Banks are more likely to provide financing when presented with proven technology.”

“We customize our machines to meet the specific needs of customer. The general range for cycle efficiency is 39-40 per cent. Basically, during the spring and summer when the sun is out for 8-9 hours or more, after shutdown a steam turbine can start up and reach full load in less than 30 minutes. In the winter, it’s more like an hour.”

Market trends indicate that solar power will increase up to twenty-fold in the next ten years. The benefits of solar power are compelling: environmentally benign, the prospect of economic growth, particularly in developing countries in the southern hemisphere, job creation, diversity of fuel supply and rapid deployment, technology transfer and innovation.

Solar thermal technology undoubtedly has a large global potential. Where there is sun there is heat, where there is heat, there is power; clean and renewable power. And, quite aside from PV, a technology that will continue to develop in parallel, solar thermal power will ensure that the steam turbine generators have an order book that will continue to grow.

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