Originally published in SolarPACES, authored by Susan Kraemer.
When SolarReserve proposed a 150 MW tower Concentrated Solar Power (CSP) power plant for Port Augusta, 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.
Why multi-tower CSP?
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.
Support of local ex-coal plant workers
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.”
Grid need for power block-based renewable generation
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.”