Concentrated power from the sun

16 May 2024

Photovoltaic modules can now be seen everywhere – on roofs, balconies and open fields, for instance. However, the photoelectric effect is not the only way to convert the sun’s power into electrical energy: Electricity can also be generated using its heat. The key term here is solar thermal energy. How do solar thermal power plants work, and what contribution can they make to the energy transition?

The dusty expanses of the Atacama desert in northern Chile stretch out barren and desolate. Then suddenly you see the tower: It stretches 250 metres into the hot air of the world’s driest desert; its tip blazes with brilliant light, and thousands upon thousands of mirrors are clustered around it in kilometre-wide concentric circles. What may at first glance look like mysterious alien technology from a visually stunning science fiction blockbuster is actually a solar thermal power plant. It goes by the name of “Cerro Dominador” and is the first of its kind in South America.

Unlike photovoltaics, the system does not convert energy from the sun directly into electricity. Instead, it uses the sun’s heat to power electricity generators. In Cerro Dominador, 10,600 computer-controlled mirrors follow the sun and concentrate their rays on the focal point of the system: a heat absorber at the top of the tower. This is heated to 600 degrees by the sun’s rays. A heat exchanger continuously taps this heat, generating water vapour which is directed to turbines that use generators to produce electricity.

With its output of 110 megawatts, the solar tower power plant is intended to supply around 380,000 households with green electricity – around the clock. This is made possible by the fact that the heat tapped in this way can be stored in a tank of molten salt for over 17 hours and used at night or on one of the region’s rare cloudy days. This should save 870,000 tons of CO₂ year after year.

Since Europe wants to secure the cooperation of Chile, a country rich in sun and wind, as an energy partner for the supply of green hydrogen, the EU and the German government have joined forces to make a financial contribution to the solar tower system. “A lighthouse for the entire region” is how the then Federal Environment Minister Svenja Schulze characterised the project at its inauguration in June 2021. The minister went on to say that she hoped more power plants of this type would be built to exploit the great potential of solar energy.

The potential for concentrated solar power (CSP) in Chile is high. After all, the Atacama Desert is one of the sunniest places on earth. This is a major reason why the solar thermal power plant won out as a more economical solution in a tendering process, besting competing gas and coal-fired power plants in the process.

The idea of using the power of the sun to drive machines is not new. Way back in the middle of the 19th century, inventors such as Augustin Mouchot tried their hand at building a solar steam engine. However, it would take more than 100 years for the world’s first commercial solar thermal power plant to be connected to the grid in the USA in 1984.

Unlike Cerro Dominador in Chile, the installation in Kramer Junction, California, does not rely on a tower: Curved mirrors in rows hundreds of metres long focus the incoming sunlight on a trough at their heart. Thermal oil, heated by the sun to up to 400 degrees, flows through the trough. Water vapour is heated via a heat exchanger and fed into a turbine to drive a generator, just like the solar tower power plant, thereby generating electricity.

These parabolic trough power plants are currently the most common type of solar power plant in the world and are in use in places including South Africa, Morocco and Israel. Europe’s first solar thermal power plant was connected to the grid in Spain’s Sierra Nevada in 2008. As of the beginning of 2020, Spain had become the world’s largest solar thermal electricity producer with 50 power plants and 2.3 gigawatts of installed capacity.

Globally, however, concentrated solar heat is very much subordinate to wind power and photovoltaics. At the end of 2019, solar thermal power plants with a total capacity of around six gigawatts were in operation worldwide. By way of comparison, PV systems with a capacity of 14 gigawatts were installed in Germany alone in 2023 – more than twice the installed capacity of the whole world’s solar thermal plants, and that in a single country!

There are reasons for this. Whereas photovoltaics, which are becoming increasingly cheap and efficient, are already profitable in sizes small enough to fit on a balcony and in Central European weather conditions, solar power plants need a minimum size and a lot of sun to pay off economically. This means that they are a safe bet only in deserts and arid regions and can therefore only be found in our world’s sun belt. But even there, low-maintenance photovoltaic technology is often a more economical solution than solar thermal energy, the technology behind which is more maintenance-intensive.

All this explains why research to further improve the efficiency of solar thermal power plants is concentrated in Germany. This is no coincidence: Many of the solar thermal power plants across the globe feature technology from Siemens and other German companies.

Since 2022, for example, experts from the German Aerospace Centre (DLR) have been working in a test facility in Évora, Portugal, to investigate whether the thermal oil in parabolic trough power plants can be replaced by liquid salts. According to the researchers’ forecast, these would be cheaper, could be heated to higher temperatures and might be able to reduce the electricity generation costs of solar power plants by 20 percent.

Solar thermal power plants have a key advantage over photovoltaics: Heat can be stored more easily and cheaply than electricity in batteries. They are therefore capable of providing base load capacity and can also secure the power supply at night or on cloudy days – especially, as the DLR experts have calculated in a study, if combined with photovoltaic systems.

If the supply of electricity exceeds demand, PV parks must now be temporarily taken offline so as not to overload the power grids. The solar power is then either lost or has to be stored in batteries, which are still expensive. In a hybrid system consisting of a solar thermal power plant and a photovoltaic system, surplus PV electricity could drive an electric heater and thus be fed as heat into the storage system of the solar thermal installation – a method that the Fraunhofer Institute is also researching. According to DLR calculations, such hybrid power plants could produce electricity more cheaply than pure PV parks with battery storage systems.

In Morocco, this idea is already close to becoming reality: A hybrid plant of this kind is being built near the city of Midelt. Noor Midelt 1 is scheduled to go into operation in 2024 and will for the first time combine the advantages of photovoltaics with the benefits of concentrated solar power. If Morocco advances its energy transition in this way, this will also benefit Germany in the medium term. After all, the kingdom is scheduled to become the country’s first supplier of green hydrogen in North Africa – and this process requires a lot of green electricity.