Concentrated solar power revived?

Concentrated solar power (CSP) looked like a really useful addition to the renewable panoply 5 years ago. It worked by using mirrors to concentrate sunlight on a central tower. The intense concentration of sunshine heated salts to ±600 C, melting them. The molten salts could be used immediately to drive steam turbines to generate electricity, or they could be stored to be used later for the same purpose. This allowed CSP to deliver 24/7 electricity, just like baseload. Only cheaper and cleaner.

However .... the most famous CSP plant, at Crescent Dunes in Nevada, failed, primarily because the tank where the molten salts were stored kept on cracking. It looked as if CSP would never be a reality, which was a great pity, because CSP is a fantastic complement to other forms of renewables.  Solar plus 4 hours of storage is cheaper than CSP, solar plus 10 or 12 hours of storage isn't.  And CSP is cheaper than new coal and much cheaper than new nuclear, while able to deliver baseload power.

But (perhaps) CSP is going to be resuscitated. An Australian company, Vast Solar, has tweaked the basic concept of CSP.  They have a 1MW pilot plant up and running, and they're planning to open a 30MW plant within a couple of years, with plans to expand that to 150 MW if it's successful.

When SolarReserve proposed a 150 MW tower Concentrated Solar Power (CSP) power plant for Port Augusta [Australia], the firm was fresh off completing Crescent Dunes, the world’s first attempt at utility scale tower CSP with storage. The startup was unable to get funding to build the Port Augusta project, but it was fully developed: SolarReserve had secured state government approval to build 150 MW of CSP with 1100 MWh of thermal energy storage and 70 MW of PV.

But now; like SolarReserve’s other fully developed projects; Likana in Chile, and Redstone in South Africa, the Aurora project is under new ownership. Several years ago, 1414 Degrees purchased the project from SolarReserve and added a 140 MWh battery project. More recently, Australian CSP developer Vast Solar has purchased 50% of the project.

“Vast Solar’s long term plan is to build up to 150 MW of modular multi-tower CSP at the Port Augusta site, beginning with a 30 MW plant we expect to have online in 2025. What we intend to do afterwards is build a larger plant on the southern end of the site,” said CEO Craig Wood. That larger project would share site infrastructure, including the O&M team, the substation, some utility services and access roading.

The firm takes a novel (and award-winning) approach to tower technology that they believe can greatly increase the ultimate capacity of tower CSP. Instead of having a single tower with its solar heat fed by one solar field of heliostats, then running a steam turbine from heat stored in a co-located power block, Vast Solar will deploy multiple solar fields and towers that link together to make up a modular power plant.

In this technology, the solar field piping transfers heat to a shared grid-connected power block housing thermal storage and a steam turbine and generator. Though various approaches for multi-tower CSP have been researched, this will be the first commercial plant.

Wood spelled out the long term rationale; that a multi-tower approach enables the controllability and scalability of trough systems with the high temperatures and performance of central tower CSP. And allows for much larger CSP plants in the long run.

“Linking multiple solar arrays and tower receivers back to one central power block means you are able to build much larger plants,” he explained. “A CSP plant with a single central tower is ultimately limited to 100 to 150 MW.”

This is because as the size of the solar field increases, the mirrors at the outer edge which are typically a mile away from the receiver on the tower, deliver lower solar flux.

“So central tower CSP is limited in terms of the number of megawatt-hours of storage that it can have which ultimately means it is limited in terms of the cost down opportunity,” added Wood, who has both an engineering and finance background.
Nuclear inspires liquid sodium for heat transfer

Because Vast Solar intends its projects to be built in multiple units all connecting to one power block, it needs an effective heat transfer fluid that can be pumped from each tower to where it is stored and used in the power block.

The search for a fluid with excellent thermal conductivity – important in its heat transfer role but also in case something goes wrong and the fluid needs to be re-melted – led to Vast Solar pioneering an innovative heat transfer fluid for CSP, albeit one with decades of experience in the nuclear industry: liquid sodium.

“We went looking for something that would allow us to have that modular configuration in a very cost-effective way that also has high thermal conductivity,” Wood explained.

“Sodium boils at 883 C and solidifies at 97 C; so it has a wide operating range. In our system, with receiver outlet temperatures of up to 580 C, the sodium is just perfect in terms of the operating temperature range. We need a temperature range between 580 C in the receivers, and 300 C at the lowest, so this is right in the middle of what sodium can do while staying liquid.”

Another key benefit of using sodium as the heat transfer fluid from the receivers to the power block is that it, if something goes wrong and it freezes, it can readily be reheated to become liquid using heat tracing elements on the pipe.

“Once the sodium arrives back at the power block, we transfer that heat into thermal energy storage in a standard molten salt system and, when we need to, we use the heat from the salt to create steam to spin a turbine,” he said. So the heat is carried in liquid sodium, stored in molten salts, and finally used in the form of steam in a Rankine cycle turbine.

As did SolarReserve before them, Vast Solar has found that locals in this former coal plant town are very motivated, understanding that due to being a form of solar that has a thermal power block, CSP brings many of the same power station jobs back – but without the coal.

“Port Augusta is an interesting community with an industrial history, having previously been home to the two major coal fired power generators in South Australia,” Wood noted.

“So the locals understand the benefits of long-term well-paying jobs in a thermal power station like CSP. When the last of the coal-fired plants was announced for closure, the community organized a group called Repower Port Augusta to actively try to secure CSP for the town. People have figured out that PV and wind – while cheap – tend not to provide many jobs.”

Official support helps too, with the grid authorities actively trying to smooth the grid connection process. “The authorities have said to us that they’re pretty excited by the prospect of the steam turbine being installed in that location,” said Wood.

“As a thermal form of solar, CSP delivers its solar energy via a turbine. In the South Australian grid there is already a lot of intermittent renewable installed and, with more slated for installation, providing the ancillary services that turbines delivers is really attractive in that location.”

With energy delivery focused on morning and evening peaks, the CSP plant would have the high earning potential of batteries in Australia’s market-based grid, where prices can briefly shoot to a high price cap of $15,500 AUD.

“Regularly you’re seeing prices of upwards of $200 to $300 a megawatt-hour,” said Wood.

“There are definitely seasonal factors but also, particularly in South Australia, you’ve got a high volume of wind and a high degree of interconnection with the eastern states. If you get a coalition of circumstances such as not much wind and then an interconnector being constrained or down for maintenance, you do find extended periods of high prices.”

And here's an article about fixing the problem with the storage tanks:

A consortium has moved to patent a new tank design for the high temperature molten salt tanks used in thermal energy storage systems, like concentrating solar thermal power (CSP) projects.

Concentrated solar power systems use mirrors and receiving towers to gather and store the sun’s energy. The technology has had a disrupted history, from being hailed a great solution, to being wedged out by cheap solar PV combined with the finicky hindrances which detracted from CSP’s great advantage of dispatchability.

One of those finicky hindrances includes the molten storage systems’ hot tank, which had tended to leak because of thermal cycling and fatigue, resulting in substantial production losses for CSP projects.

Now, a consortium including Sydney-based Vast Solar, as well as CyD, Solar Dynamics, and Alia Energy Consulting and Critical Engineering, say they have jointly developed a tank design which addresses the issue, drawing on input from the Advanced Materials Team at the Australian Solar Thermal Research Institute (ASTRI), led by the Queensland University of Technology and Flinders University.

Dubbed the Flexitank, the consortium said it developed the new design by carefully analysing, understanding and learning from previous failures. The design increases the flexibility of the floor of the tank and mitigates the risk of failure associated with thermal cycles by absorbing the repeated expansion and contraction typically encountered in such tanks, the consortium outlined.

“Once we understood the tank failure modes and started testing the physical properties of the incumbent materials, we realised that flexibility is the key to overcoming thermal cycling and fatigue,” Vast Solar CEO, Craig Wood, said. “The economics of thermal storage are compelling, and we are delighted that our work will now deliver much needed reliability.”

“We are confident that the new design will substantially improve the operation performance of CSP systems moving forward,” Dominic Zaal, ASTRI Director, added. 

[Source of articles: Vast Solar's website] 

We'll see, won't we?  I was enthusiastic about CSP 5 years ago, and it all came to nothing.  These advances might yet bring it back from the dead, and it would be excellent for green electricity generation if Vast Solar succeeded.