Saving the sun's energy and storing it — with mirrors

Thousands of mirrors neatly arranged in concentric circles gaze up at an enormous concrete pillar towering 195 meters (640 feet) above the desert sand. 

Not far from Las Vegas, the Crescent Dunes solar power plant looks like something from a sci-fi flick. But it's actually a real-world billion-dollar megaproject, completed in 2015 with the goal of making electricity for 75,000 homes. 

Moreover, it was supposed to mark a breakthrough for a special type of solar technology known as concentrated solar power, or CSP for short.

The project followed in the footsteps of the first large-scale CSP plant built in California in 1981 — back when the technology was more promising than photovoltaics (PV), which were expensive and only really used in space travel.

But it didn't live up to expectations. Crescent Dunes failed to produce as much power as promised, and after repeated technical failures and prolonged outages, the plant was closed down in 2019. And far from paving the way for a wave of other CSP projects, it damaged the image of the technology.

These days, there are only about 7 gigawatts worth of CSP plants worldwide, mainly in Spain or older US sites, and a few in other places with lots of direct sunlight, like Chile, Morocco and the United Arab Emirates. Meanwhile, installed global photovoltaics capacity now exceeds 2,000 gigawatts.

How does CSP work and is it still around?

Five years on, the original Crescent Dunes plant is back to generating a small amount of electricity. And while the US hasn't built a single major plant since the Nevada venture, China has. Not just one, in fact, but 30.

Some say this means CSP is set for a comeback and that it's all down to one special feature that sets it apart from normal solar panels. 

The regular kind of solar panels installed on rooftops worldwide use the photovoltaic effect to produce electricity, meaning when sunrays hit their surface, the photons contained within knock loose electrons that start to move, creating a current.

CSP plants, on the other hand, use the sun's heat. So-called heliostats — which are essentially mirrors — reflect and focus the sun's rays onto one certain point. The bundled heat is then used to create steam, which spins a turbine that makes electricity.

"It's the same kind [of turbine] that you would have in a normal fossil power plant — but without burning any fossil fuels," Xavier Lara, a mechanical engineer who's worked on many CSP projects worldwide, told DW.

There are different designs for doing this, the most iconic of which are arguably solar power towers, like the one at Crescent Dunes.

The mirrors reflect sunlight onto a receiver at the top of the tower. Inside this receiver, a liquid gets heated — usually molten salt because it is particularly good at retaining heat. The hot salt is then pumped down to the turbine, and once it has done its job there and has cooled, it is pumped back up the tower, and the cycle begins all over again.

Why did CSP lose the technology race? 

"Semiconductor technology got really cheap, and photovoltaics got super cheap," Jenny Chase, a solar analyst at energy research firm BloombergNEF, told DW.

2011 marked the first year that photovoltaic was cheaper than CSP, and the trend has continued. Since 2010, the price for PV solar power has dropped by around 90% altogether and now costs less than half as much as the rival technology.

One reason for this is that solar panels are flexible and simple to set up, whereas CSP plants are usually custom-built feats of engineering.

Also, photovoltaic solar panels are a lot easier to maintain.  

"It's just sitting there, and you have to clean it maybe from time to time. But it's not as operationally complex as CSP where you have mirrors that you need to adjust under real environmental conditions," Richard Thonig, a researcher focusing on CSP at the German Research Centre for Geosciences, told DW. 

The entire mirror field has to be painstakingly adjusted to track the sun's movement so the light is precision reflected. This makes it possible to control the temperature of the liquid circulating through the system.

Clouds that cover the sun can make the process difficult, but it is important to get it right.

"Molten salt is a pain to work with because if something goes wrong and it drops below its melting point, then you don't have molten salt anymore, you have solid salt. And then your pipes are full of solid salt, and this is a big pain to sort out," Chase said.

A new niche for CSP

But despite all this, CSP has one big advantage over photovoltaics: it can generate power at night.

Newer plants typically have large insulated tanks where hot molten salt can be stored. Inside, it only cools by about 1 degree Celsius (1.8 degrees Fahrenheit) per day, so can be used to make steam at a later point.

For example, when the sun has gone down or when there is a surge in need for power due to many people using electricity at the same time. 

This ability to store energy and turn it into electricity around the clock might give CSP a new edge.

"The future of CSP moved to a different niche, right? So it used to be a power technology like wind and PV, and now it's really very much a storage technology," Thonig said.

This is also the reason behind its sudden popularity in China. In many provinces, there are now rules mandating that every new renewable energy park with more than 1 gigawatt capacity must include at least 10% of storage.

The Chinese government also issued a notice saying it will support the "large-scale and industrialized development of solar thermal power."

The idea is simple: to get the best of both worlds. During the day, photovoltaic solar panels supply cheap electricity, while the CSP plant heats up the molten salt. At night, when the solar panels can't produce electricity, the stored heat in the CSP plants can be tapped into to deliver power. 

Bridging this night gap has been a big challenge for renewable energy, and CSP, along with other technologies like batteries, could be a part of the solution. 

China's push could bring CSP back to a certain extent, as the country builds out dedicated supply chains that might drive down the price to build new plants.

But to make the technology really take off, other countries would have to get on board and put policies in place to support it.

"I wouldn't say that we have a really big CSP renaissance," Thonig said. "But I would say that the technology is still there and still promising. There's a case for CSP in many places and — with the right framework conditions — it could be very attractive and very cheap."

Edited by: Tamsin Walker