Although concentrated solar thermal power plants have disappointed expectations due to costs, technology allows clean electricity without interruptions
In the middle of the desert in the US state of Nevada, thousands of mirrors neatly arranged in concentric circles gaze at a concrete pillar that rises 195 m above the sand.
The Crescent Dunes solar power plant, located near Las Vegas, looks like it came straight out of a science fiction movie. But it is a US$1 billion (R$6.18 billion) megaproject, completed in 2015 with the aim of producing electricity for 75,000 homes.
It was supposed to be a milestone for a special type of technology known as concentrated solar thermal power, or solar thermal power (CSP), but the project failed to live up to expectations. After repeated technical failures and prolonged outages, the plant was unable to produce as much energy as promised, and was eventually closed in 2019.
It followed in the footsteps of the first large-scale CSP plant built in California in 1981 – when the technology was seen as more promising than the more expensive known PV (photovoltaic panels).
But the failure of Crescent Dunes damaged the image of the technology, which has been little replicated since. Currently, plants that produce solar thermal energy have a maximum production capacity of just 7 GW. Most of them are in Spain, the United States, Chile, Morocco and the United Arab Emirates – places where there is a lot of direct sunlight. Meanwhile, the capacity of photovoltaic plants installed around the world already exceeds 2,000 GW.
How CSP works
The common types of solar panels installed on rooftops around the world use the photovoltaic effect to produce electricity. When the sun’s rays reach its surface, the photons contained within it release electrons that begin to move, creating a current.
CSP plants, on the other hand, use the Sun’s heat. So-called heliostats (mirrors) reflect and focus the rays at a certain point. The accumulated heat is used to create steam, which turns a turbine to produce electricity mechanically.
“It’s the same type [de turbina] that you would have in a fossil fuel power plant.”said Xavier Lara, a mechanical engineer who has worked on CSP projects around the world.
There are different ways to apply the mechanism, such as the solar energy towers at Crescent Dunes. In this model, mirrors reflect sunlight onto a receiver at the top of the tower. Inside this receiver, there is a fluid to be heated – generally molten salt, in its liquid form.
The hot salt is then pumped into the turbine. Once it has done its job and cooled down again, it is pumped back into the tower and the cycle begins again.
Why did CSP lose the technology race?
“Semiconductor technology and photovoltaics have become very cheap”said Jenny Chase, a solar analyst at the energy research firm.
2011 was the first year in which photovoltaic energy became cheaper than CSP, and the trend continued. Since 2010, the price of solar photovoltaic energy has fallen by around 90%. Now it costs less than half the price of rival technology.
One of the reasons for this is that solar panels are flexible and simple to assemble, while CSP plants are often custom-made. Additionally, photovoltaic solar panels are much easier to maintain.
“They just sit there and you have to clean them every now and then. But it is not operationally complex like CSP, in which there are mirrors that need to be adjusted to environmental conditions.”explained Richard Thonig, a researcher focusing on CSP at the Helmholtz Center in Potsdam.
The entire field of mirrors needs to be meticulously adjusted to follow the movement of the sun so that light is reflected accurately. This makes it possible to control the temperature of the liquid that circulates through the system. Clouds can also make the process difficult.
“It’s difficult to work with molten salt because if something goes wrong and it drops below the melting point, you no longer have molten salt, you have solid salt. And then your pipes get full of solid salt, which is a big problem to solve.”disse Chase.
A new niche for CSP
Despite losing market share, CSP has an advantage over photovoltaics: it can create energy at night.
Newer plants often have large insulated tanks in which hot molten salt can be stored. Inside, it only cools by 1 ºC per day. Therefore, it can be used to produce steam later.
This ability to retain energy and turn it into electricity 24 hours a day could give CSP a new advantage. “The future of CSP has shifted to a different niche. It used to be an energy technology like wind and photovoltaics, and now it’s really a storage technology.”said Thonig.
This is also the reason behind CSP’s sudden popularity in China, where there are 30 such plants in operation. In many provinces, there are now rules that every renewable energy park with more than 1 GW of capacity must include at least 10% storage capacity. The Chinese government issued a notice stating that it will invest in large-scale production of solar thermal energy.
The idea is to get the best of both worlds. During the day, photovoltaic solar panels provide cheap electricity, while the CSP plant heats molten salt. At night, when solar panels are unable to produce electricity, the heat stored in CSP plants can be harnessed to provide energy.
Closing this nighttime production gap has been a major challenge for renewable energy, and CSP, along with other technologies like batteries, could be part of the solution.
China’s push could bring CSP back to some extent as the country develops supply chains that could reduce the price of building new plants.
However, for the technology to really take off, other countries would have to come on board and implement policies to support it. After 5 years of closure, Crescent Dunes, for example, only produces a small amount of energy.
“I wouldn’t say we have a really big renaissance of CSP”said Thonig of Potsdam’s Helmholtz Center. “But I would say the technology still exists and is still promising. There are arguments for CSP in many places, and with the right structural conditions it can be very attractive and cheap. But there are things that need to be done.”
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