The Sun as Filling Station: Turning Solar Energy into Electricity

Time: 2016-09-10

Morocco is a country undergoing profound changes. The population is growing, and prosperity is rising. Companies are adding locations here. Rural areas are getting connected to the power grid. Demand for energy is expanding, especially for electricity. But this North African country has few fossil fuel resources of its own, and until now has largely had to depend on importing fossil energy. Now that’s supposed to change.

The Moroccan government has set itself some ambitious goals: by 2020, the country plans to expand capacity in solar,wind and hydroelectric power to 2,000 MW each. By that point the share of installed renewable-based power generating capacity is supposed to reach 42 percent. By 2030, it’s even supposed to climb to 52 percent. An important milestone on this path was the construction of the first of four solar power plants in Ouarzazate Province, in the south of the country. Thanks to Siemens steam turbine and generator technology from Germany, the power plants are expected to run especially efficiently.

solarImage: The parabolic mirrors on the Noor solar power plants are cleaned regularly. Credit: Siemens.

The Main Point: Implementing the Energy Projects

Implementing the energy strategy is a personal concern of the country’s king, Mohammed VI. On Feb. 4, he opened the “Noor I” solar thermal power plant in Ouarzazate. This plant is the first of four in the Ouarzazate solar complex (Noor I – Noor IV), which is projected to reach a total capacity of up to 580 MW, making it the largest of its kind in the world. It’s expected to supply electricity to 1.1 million people. The location is a good one: the sun’s intensity in Ouarzazate is very high, in excess of 2,500 kWh per square meter per year.

Complementary Technologies: Parabolic Troughs, Solar Power Tower and Photovoltaics

The technologies are also promising. The Noor projects — the name is from the Arabic word for “light” — combine a variety of mutually complementary solar thermal technologies in a single area equivalent to more than 4,200 soccer fields. A Spanish consortium made up of Acciona, Sener and TSK built the Noor I parabolic trough power plant, with a capacity of 160 MW. Chinese engineering, procurement and construction (EPC) contractor Sepco III will build the Noor II parabolic trough plant (200 MW) and the Noor III 150-MW solar power tower, as well as the Noor IV photovoltaic plant. Masen, the country’s solar energy agency, is the ultimate customer for all the projects. Noor I through III will make use of concentrated solar thermal power (CSP), while Noor IV will convert sunlight directly into electricity. Construction on Noor II and III is expected to be completed by 2018.

Here’s How it Works: Turning Sunlight into Electricity

It’s a solarthermal power plant of superlatives. Ten kilometers northeast of the city of Ouarzazate, the roughly 540,000 parabolic mirrors of Noor I rise into the sky from the desert floor, and there will be even more of them. Noor I and II focus the sun’s rays and direct them onto pipes that contain a circulating heat-absorbent medium, in this case heat transfer oil. The oil then serves to heat a steam loop.

solarFigure: This graphic shows how a concentrated solarthermal power plant with parabolic troughs (Noor I and II) works in principle.

The steam generated in a heat exchanger drives a steam turbine, which in turn drives a generator that generates electricity. In the Noor III solar-tower power plant, an array of a very large number of flat individual mirrors reflect the sun’s rays to a receiver module at the top of a tower more than 240 meters high. Temperatures there can reach as much as 1,000 degrees Celsius. The absorber medium is a molten salt mixture that heats to 555 degrees Celsius. The absorbed thermal energy is then transmitted to the associated power plant unit by way of a steam loop.

Specially Designed for Solar Power Plants: The Steam Turbines

The solar thermal systems have a high-capacity molten salt storage system that extends the system’s operating time for four to six hours after sunset, enabling it to supply power in the evening hours as well. The storage system needs up to 140,000 metric tons of a eutectic salt. The mixture of 60 percent sodium nitrate and 40 percent potassium nitrate has a fixed, determinable melting point (the eutectic point) at which both components melt simultaneously. But even with the aid of this heat storage, the three solar thermal plants can’t operate 24 hours a day.

solarFigure: This graphic shows how a concentrated solarthermal power plant with tower (Noor III) works in principle.

Depending on the amount of sunlight, they may run at full load or at partial load, or (at night) be idle. In contrast to conventional fossil-fueled power plants, the variable operating status means that the steam turbines must handle daily starts and stops, and are thus exposed to substantial stresses. It is essential for the turbines to be especially well-suited for use in solar thermal power plants and designed to start and stop quickly.

Time is Money: Steam Turbines are Especially Efficient

“The faster the power plant can power up, the sooner it can generate power,” Jürgen Keil, general project manager for steam turbines at Siemens Power and Gas, said. “That’s a key advantage for a customer. Every additional minute of production yields additional income and makes the power plant more cost-effective.”

Siemens has more than 20 years of experience with using steam turbines in CSP plants. To date, the company has supplied and commissioned more than 70 steam turbines for CSP systems of every kind. Whether parabolic-trough power plants, linear Fresnel plants with flat reflectors, solar power towers, or hybrid plants, the turbine technology is designed for all types of CSP plants. The product range in industrial steam turbines runs from just 1 MW to over 250 MW.

solarImage: Ready to assemble: the low-pressure blades for the steam turbines at the Noor power plants. Credit: Siemens.

For the Ouarzazate solar complex as a whole, Siemens is supplying a total of three turbogenerator sets, each comprising two steam turbines and one generator. For the individual Noor I to III power plants, the company connects one high-pressure turbine and one low-pressure turbine together in series.

The Goal: Cost-effective Operation of the CSP Plant

Adel Asham, in charge of steam turbine sales at Siemens Power and Gas, said: “If the investment in a solar thermal power plant is going to pay off and the plant is going to run cost-effectively, its steam turbines have to be especially efficient and reliable.”

The company has already delivered a complete turbogenerator set for Noor I, with two steam turbines (type SST-700 and SST-900), one generator and the associated systems. A configuration with one SST-800 and one SST-500 steam turbine is planned for Noor II, and one SST-700 and one SST-900 steam turbine for Noor III, each together with a generator and associated systems. The Siemens manufacturing plant in Görlitz started production on the Noor II and Noor III turbines in early May.

All of the steam turbine systems include reheating. The SST-500 and SST-800 include a direct-driven turbine for high-pressure and medium-pressure, and a double-pass low-pressure turbine. This special two-housing solution achieves very high capacity with optimum utilization of the available steam.

More Capacity: Reheating Enhances Efficiency

As a rule, steam temperatures are limited by the absorber medium. To extract the maximum power from solar energy, the CSP systems work with reheating. For the configuration of the SST-800/500 steam turbines for Noor II, that means that the steam drives the high-pressure turbine at a temperature of nearly 400 degrees Celsius and a pressure of about 100 bar. Then the steam is reheated in a boiler and drives the low-pressure turbine. This setup enables the system to deliver an output of as much as 200 MW.

solarImage: Quality Assurance: a Siemens expert checks measurements on the low-pressure blades. Credit: Siemens.

For the Noor III solar tower power plant, the steam is heated to more than 540 degrees Celsius and reaches a pressure of about 140 bar. The steam loops for the Noor projects are cooled with water or air. Water tanks with capacities of up to 300,000 cubic meters have been specially built for the purpose.

Siemens Steam Turbines for CSP Systems

Since the solar boom in the U.S. in the 1980s, solar heating has been a well-tested technology, although it poses special requirements for components like team turbines. These have to be able to start and stop fast and frequently, yet deliver a great many hours of operation. Siemens positioned itself in this segment by supplying the steam turbines for the first commercial power plants in the U.S. and Spain. The company’s steam turbines have long service lives. At the power plant near King City, Calif., for example, an SST-700 type steam turbine has been running, with daily startups and shutdowns, since 1988.

Since the SST-700 was optimized for solar power plant, Siemens has been adapted other industrial turbines in its portfolio for use in CSP systems. Siemens employees have calculated the life cycle of all critical turbine parts. They also improved the design of the housings and turbine blades and introduced a low-mass rotor.

solarImage: Final tests: before the turbine blades are delivered to Ouarzazate, Siemens runs characteristic frequency tests on the low-pressure blades. Experts check the vibration characteristics of the blades, an essential test to ensure that the rotor, and thus the entire turbine, will be properly balanced in later operation.

The company continues its research. Working with ESC contractors and power plant developers, it develops new designs for steam turbines in CSP systems. Its researchers and developers have set their main focus here on enhancing the turbines’ efficiency. That goal can be reached, for example, with reheating or with systems designed for supercritical steam parameters.

Mature Technology: Solar Heat Has Great Potential

Experts predict that solar energy will continue to gain in importance all over the world. Its advantages are obvious: the resource is free, abundant, and usable in ways that are easy on the environment. The Ouarzazate solar complex alone is estimated to save about 700,000 metric tons of CO2 each year.

There’s no doubt that solar heat has great potential. That’s also the opinion of Bernd Beyer, head of sales southwest Europe at Siemens Power and Gas: “Solar trough power plants now have a well-matured technology, and thus their risk for investors is readily calculable, so they’ll continue to play an important role. On the other hand, in view of increasing efficiency, solar power tower technology will also become more and more important.”

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