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Storing sunshine

Solar thermal power plants convert the sun’s energy into electricity. The great bonus is that heat storage systems enable them to supply power even when the sun is not shining. Experts foresee a great future for this environment-friendly technology.

Hundreds of thousands of mirrors glisten in the desert sun. They are arranged in 800 lines, each of them hundreds of meters long. In Ouarzazate in southern Morocco, an electricity factory of superlatives is being created. This solar thermal power plant is intended to become the biggest in the world, covering the same area as about 4,000 soccer pitches. The first phase, known as Noor I, the Arabic for “light,” has been producing electricity with an installed capacity of 160 megawatts (MW) since its commissioning in February 2016. That is enough for 350,000 Moroccan households. Noor I is only the beginning: this flag-ship project, that is intended to comprise four power plants in all, is due to deliver a capacity of 500 MW by the end of 2017.

Electricity from the desert

“Solar thermal power plants concentrate sunlight to produce high temperatures. In sunny regions, they are thus able to deliver large quantities of environment-friendly electricity from a renewable source,” explains Professor Robert Pitz-Paal, Director of the Institute of Solar Research at the German Aerospace Center (DLR). Concentrating solar power (CSP) is best suited to countries with high sun intensity. The Moroccan solar park is in a perfect location: in the area around Ouarzazate, solar radiation reaches an intensity of more than 2,500 kilowatt-hours (kWh) per square meter per year, one of the highest levels in the world.

Interview with Professor Pitz-Paal
Director of the Institute of Solar Research at the German Aerospace Center (DLR)

What is Concentrating solar power (CSP), and what fascinates you about it?

Nearly everybody knows about photovoltaics or wind power, but only a few have heard of concentrating solar power, or CSP. What is the reason for this?

What is the main focus of research?

Location of Noor: the Moroccan electricity factory of superlatives is in one of the sunniest regions in the world.

The Noor I facility: the first phase of the world’s largest solar plant was inaugurated in 2016 in Morocco, close to the Sahara.

Concentrated solar power

At the Noor I facility, 537,000 computer-controlled parabolic mirrors turn constantly to face the sun, just like sunflowers. They concentrate the sun’s rays and convert them into thermal power. The slightly curved mirrors extend to a height of more than 10 meters. In the middle run pipes, in which a thermal oil circulates. This synthetic fluid, that can be heated to 393 degrees Celsius, is fed through to the power plant at the center of the huge area of mirrors, where steam is produced to drive the enormous power plant turbine. So that the electricity is also available after sunset, large heat storage units have been installed. As a result, solar thermal power plants can offer a decisive advantage in the renewable energy mix: electricity can be used around the clock.

More than one million households will be supplied with electricity: after the whole facility is complete, the lines of parabolic mirrors will extend across an area of 30 square kilometers.

The sunflower principle: the computer-controlled parabolic mirrors turn constantly to face the sun.

Full capacity even at night

The heat storage units at Noor I consist of two giant steel tanks. “They contain a special salt mixture of potassium and sodium nitrate, that becomes liquid at temperatures of around 240 degrees Celsius,” explains Matthias Hinrichs, PhD, Manager Solar Business and New Business Development Inorganics at BASF. In the field of storage salts, the chemical company concentrates on producing high-purity, synthetic sodium salts – and has been doing so for more than 90 years. The release of heat from the stored salt enables the full capacity of CSP plants to be maintained even at night. The salt mixtures in the storage tanks require huge quantities: for the facility in Morocco alone, BASF has produced around 27,000 metric tons of sodium nitrate in Ludwigshafen.

Solar power even at night: heat is stored in the two giant steel tanks at Noor I by a special salt mixture of potassium and sodium nitrate.

Highest-purity sodium nitrate: in solar thermal power plants, the molten salt from BASF stores solar power that can be converted into electricity as required. Microscopic image, magnification 320:1 (12 cm in width).

New investment and a jobs boom?

According to the study “Solar Thermal Electricity – Global Outlook 2016,” countries such as Morocco are thus making an important contribution to reducing emissions of green-house gases. By 2020 the countries in the “sunbelt” – those located between 40° north and south of the equator – could avoid 32 million metric tons of CO₂ by using CSP. At present, Spain and the United States are top of the table for CSP capacity, but other countries are catching up. China aims to commission solar thermal power plants with a total capacity of 10 gigawatts (GW) by 2020, and Saudi Arabia is planning a CSP capacity of 25 GW by 2032. The study, which was commissioned by SolarPACES, a technology program of the International Energy Agency (IEA), Greenpeace and the European Solar Thermal Electricity Association (ESTELA), also found that the use of this climate-neutral energy source in these countries would create investments worth €16 billion by 2020 and 70,000 new jobs in the future.

The principle of solar thermal power

Thermal energy from solar power

What is billions of years old and still full of power? The sun. Photovoltaic and solar thermal power technologies use the Earth’s closest star as a literally inexhaustible source of energy. While photovoltaic uses solar cells to convert sunlight directly into electricity, solar thermal power technology produces thermal energy. In private homes, solar thermal power is used mainly to provide hot water and well-heated rooms. Meanwhile, concentrating solar power – CSP for short – can produce electricity on a large scale: a solar thermal power plant such as Noor I in Morocco has a capacity of 160 megawatts (MW) and supplies electricity for 350,000 households. Thousands of mirrors concentrate the sunlight and warm up a liquid. This source of heat then turns water into steam, which subsequently produces electricity through a generator. Four different types of collectors are used to concentrate the sun’s rays:

In parabolic trough power plants, solar radiation is concentrated on a pipe at the mirror’s focus. The thermal power in the pipe is channeled further.

Parabolic trough collectors

Large mirrors in the shape of a trough reflect solar radiation onto a heat exchanger (receiver).

Just like parabolic troughs, Fresnel collectors too focus direct solar radiation onto a focal line to heat up a receiver pipe. However, they are laid out flat.

Fresnel collectors

Narrow, flat mirror strips are arranged so as to perform the same function.

In Dish-Stirling plants, solar radiation is concentrated onto a single point by a dish-shaped mirror. The thermal power concentrated there is converted into mechanical energy by a Stirling engine.

Dish collectors

Connected parabolic mirrors concentrate the sun’s rays on a single point.

At the top of the tower at this power plant, temperatures of up to more than 1,000 degrees Celsius are created and can be used to generate electricity.

Heliostats

At what are known as tower power plants, thousands of mirrors direct the sun’s rays onto a central receiver at the top of a tower.

The prerequisite for high yields is direct solar radiation, which occurs mainly in the Earth’s sun belt. A particularly high number of CSP plants have been and are being built in countries such as Spain and the United States, as well as the Middle East and North Africa. In these plants, the energy captured by concentrating solar power can be boosted further by large heat reservoirs containing liquid salt, that holds in the heat already obtained – for times when the sun does not shine.