Most industrial turbines are developed for superheated steam. Modifications are obviously made to standard turbines placed in CSP plants today, but as newer plants are built with much larger capacity demands, bigger strides in turbine customization must be made.
By Bob Moser
Fast start-up, daily start-up and quick load change capabilities top the list of what new turbines must achieve to meet the needs of tomorrows top CSP plants, according to Justin Zachary, e-technology manager for Bechtel, a global leader in engineering and project management.
Modern efficient steam turbines can be tailored to accommodate the different steam conditions of three main CSP technologies: tower, trough and linear Fresnel. The bigger challenge involves identifying which modifications to CSP plants are needed to allow superior steam conditions and higher overall plant performance, says Steven Moss, spokesman for Alstom, a world leader in transport and energy infrastructure.
“Clearly, the highest inlet steam conditions are desired for better plant efficiency, but this should be accompanied by such features as the effective use of re-heat,” Moss says. “Presently, tower and trough applications employ single re-heat turbines, but an additional re-heat stage offers further plant performance potential.”
CSP turbines of tomorrow need to have a modular design capable of accommodating variable high-temperature and low-temperature flow. Assembly must be fast and easy, and the design must be robust enough to meet the demands of rapid daily start-up, while also being able to react to changes in steam conditions.
““Like a puzzle, if you don't nudge the edge of all the pieces together, it won't fit,” Zachary says. “The boiler, the turbine - all the machinery - needs to fit well with the needs [of future high-performance CSP plants].”
Lighter, more durable materials needed
If located in a desert facing temperatures of 45 degrees Celsius, pressure on a CSP turbine will be higher than in more temperate climates, explains Zachary. A good turbine must be able to handle pressure varieties.
To meet the demands of daily start-up in particular, better quality materials are needed, such as lighter bearings. A conventional steam turbine installed in a power plant is designed for about 1,800 start-ups over a 25-year lifespan. But the daily start-up demands of a CSP plant can include 300 start-ups per year alone, which proves too taxing on today’ standard turbine materials.
To scale turbines up to meet the needs of 150 MW CSP plants, the solutions are clear to engineers. A high-pressure turbine will be selected that is one or two sizes larger than what's needed for a 50 MW plant. For low-pressure it's mainly a question of the size of rotating blades, engineers say.
“It's important to have a highly efficient steam turbine in this kind of high megawatt application,” said Siemens' CSP steam turbine expert Jari Nyqvist.
“Of the total investment for a CSP plant, steam turbine is still quite a low portion, 5-8% of investment. If you can create a turbine solution that gives higher efficiency, then the size of the plant can be reduced as well. You can save on installation costs overall with the right turbine choice.”
Siemens' Steam Turbine 700 has been the company's standard CSP turbine for the past five years. The SST 700 is a two-barrel design, one high pressure and the other low, that is customized in a handful of ways to fit CSP needs, including changes in steam path, blade angle and length. The company is now releasing an updated turbine design tailored more specifically for CSP application.
Collaboration needed on identifying the solution
The success of future CSP turbine development must be a collective effort, Zachary says. Contractors, solar technology providers and turbine suppliers must work together more closely then they have in the past.
Because steam turbines have been built for powers far in excess of the current CSP ratings, existing component designs can handle greater mass flows, Moss says. Turbines in the small to mid-range are often used in applications where frequent start-stop cycles are required. This is less common for larger steam turbines, so up-scaling will be accompanied by tailoring the equipment to accommodate the daily cycling in CSP plants, Moss adds.
In terms of turbine up-scaling's effect on operations and maintenance, one good thing about CSP and other solar collectors that have no storage is they allow time in the evening – in some cases as much as 10 hours – for shutdown maintenance, without effecting capacity at the plant.
Whether it's a new turbine design or existing models being adapted for higher output in a CSP setting, larger turbines are more efficient but may not accommodate fast start-up. Reducing that time will be accomplished most easily through lighter materials being developed, like low-mass rotors and casings, and smaller seal leakage, for example.
“The larger the turbine the larger everything is going to be, it's not rocket science,” Zachary says. “The only thing we don't have is too much practical experience with it. We'll get there.”
Justin Zachary, Technology Manager, Bechtel, will be presenting on CSP steam turbine design at the forthcoming CSP Yield Optimization Conference and Expo, in Denver on October 27-28 in Denver, USA. To register for this conference, please click here.
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Image Credit: Rotor of the Siemens SST-700 steam turbine with an output of 50 megawatts. Copyright: Siemens AG