Taking the heat: steam turbines and solar

The largest solar-thermal plant in the world is utilizing special-purpose steam turbines, writes Nina Terp

Morocco has a lot to offer: a rich culture, rugged mountains, traditional crafts – but almost no fossil fuel resources.

The country’s ruler, King Mohammed VI, has now made it his personal concern to advance his country’s energy transformation. The goal is to expand the energy generated from sun, wind and water to a total of 6 GW – 2000 MW each – by 2020, and in turn become less dependent on global price fluctuations.

The showcase project is the Ouarzazate solar complex in the south of the country. On February 4, 2016, King Mohammed VI opened the Noor I solar thermal power plant, the first of four plants in the complex designed to reach a total capacity of up to 580 MW, making it the largest of its kind in the world.

It is expected to supply electricity to 1.1 million people. The sun’s intensity in Ouarzazate is extremely high – in excess of 2500 kilowatt hours per square metre per year.

Catching the sun: Thousands of parabolic mirrors focus the sun's rays in the desert near
Catching the sun: Thousands of parabolic mirrors focus the sun’s rays in the desert near Ouarzazate and direct it to pipes containing heat transfer oil

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 4200 soccer fields.

A Spanish consortium comprising Acciona, Sener and TSK, built the Noor I parabolic trough plant, with a capacity of 160 MW. Chinese engineering, procurement and construction contractor Sepco III will build the 200 MW Noor II parabolic trough plant 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 end customer for all the projects. Noor I, II and 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.

How it works

It is a solarthermal power plant of superlatives. Ten kilometres northeast of the city of Ouarzazate, the 540,000 or so parabolic mirrors of Noor I rise into the sky from the desert floor. Noor I and II focus the sun’s rays and direct them onto pipes containing a circulating heat-absorbent medium, in this case heat transfer oil.

The oil then serves to heat a steam loop. 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 reflects the sun’s rays to a receiver module at the top of a tower more than 240 metres high.

Temperatures there can reach as much as 1000à‚º Celsius. The absorber medium here is a molten salt mixture that heats to 555 à‚°C. The absorbed thermal energy is then transmitted to the associated power plant unit by way of a steam loop.

The steam turbines

The solar-thermal systems feature a high-capacity molten salt storage system that extends the system’s operating time for four to six hours after sunset. But even with the aid of this heat storage, the three plants cannot operate 24 hours a day.

Depending on the amount of sunlight, they may run at full or partial load, or at night be idle. This means that the steam turbines supplied by Siemens must handle daily starts and stops and are thus exposed to substantial stresses. So it is essential that the turbines are especially well suited for use in solar-thermal power plants and that they are designed to start and stop quickly.

“The faster the plant can power up, the sooner it can generate power,” explains Jàƒ¼rgen Keil, general project manager for steam turbines at Siemens Power and Gas. “That is a key advantage for a customer. Every additional minute of production yields additional income and makes the power plant more cost-effective,”.

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.

Cost-effective operation

Adel Asham is in charge of steam turbine sales at Siemens Power & Gas. He explains: “If the investment in a solar-thermal 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 supplied a complete turbogenerator set for Noor I, with two steam turbines (type SST-700 and SST-900), one generator and the associated systems. For Noor II it’s a configuration with one SST-800 and one SST-500 steam turbine; and for Noor III, one SST-700 and one SST-900 steam turbine, altogether driving a generator and associated systems.

At the end of 2016, all the equipment left the factory halls in the German towns of Gàƒ¶rlitz and Erfurt and the enormous turbines and generators are now on their way to Morocco.

Parabolic mirrors on the Noor solar power plants
Parabolic mirrors on the Noor solar power plants

From the Moroccan harbour of Nador, they need to be transported around 800 kilometres to the construction site.

Delivery and the beginning of installation started in January for Noor III and in March for Noor II, with all large equipment expected to arrive at project site during the next few weeks.

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 highly-special solution achieves very high capacity with optimum utilisation of the available steam.

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.

With the configuration of the SST-800/500 steam turbines for Noor II, main steam drives the high-pressure turbine at a temperature of nearly 400 à‚°C and a pressure of about 100 bar.

The steam is then reheated in a boiler and drives the low-pressure turbine. This enables the system to deliver an output of as much as 200 MW.

All the same for the Noor III solar tower power plant, the steam is heated to more than 540 à‚°C and reaches a pressure of about 140 bar with a power output of 150 MW.

Experts predict that solar energy will continue to gain in importance not only in Africa but all over the world. Siemens recently received its first steam turbine (SST-800) order for a solar-thermal power plant in China. The plant is designed as a solar tower system and is scheduled to begin commercial operation at the end of 2018.

There is no doubt that solar heat is a hot topic for the energy sector.

Nina Terp is a freelance journalist who specializes in energy technology

No posts to display