Work on the G3P3 particle receiver system at Sandia National Laboratories IMAGE © Sandia National Laboratories
In 2018, three years after the completion of the second of two utility-scale tower-type Concentrated Solar Power (CSP) plants in the US, the Department of Energy (DOE) held a competition to decide the funding direction for the next generation of Tower CSP. To transfer the heat from the solar receiver, should future projects use liquids like molten salts, solids like sand particles, or gases like air?
Liquids, solids, or gases?
Liquid heat transfer had indirectly caused problems at both US Tower projects. Water for heat transfer at Ivanpah made it impractical to incorporate energy storage (the advantage of CSP), and the leak risk of hot liquid molten salts was first revealed at Crescent Dunes.
To date, twenty-four Tower CSP projects are operating or underway in China, with several more between Morocco, the UAE, and Chile, and all these commercial Tower plants use molten salts for heat transfer.
However, at the research level, there is increasing interest in gases, such as air or carbon dioxide, and solids such as pebbles or sands for heat transfer for the next generation of Tower CSP. Both routes promise higher temperature operation, enabling more efficiency and, thus, lower costs.
For the DOE competition, the National Renewable Energy Laboratory (NREL) proposed to refine the liquids pathway, with solutions to molten salts corrosion. Brayton Energy competed for the gas pathway with a carbon-dioxide-based solar receiver system.
The Gen-3 CSP winner was Solids: particles
Sandia National Laboratories in New Mexico chose solids and won Gen-3 funding for its particle-based heat transfer system. Their team proposed using Bauxite particles; the kind refined into spheres to prop rocks apart for the fracking industry (a previous energy technology also developed with US Department of Energy funding).
The winning team at Sandia was then funded to solve all the engineering challenges in developing this new particle-based Tower CSP.
IMAGE ©Sandia National Laboratories G3-P3
Sandia’s choice of Bauxite sand for the heat transfer at $2 a kg is much cheaper than molten salts. However, it is four times the cost of untreated white desert sands, tested by colleagues in Saudi Arabia in their own particle receiver at KSU. Sandia researcher Jeremy Sment explained their decision in a call from New Mexico.
“We considered absorptivity, reflectivity, reactivity, sintering temperature, dust attrition and erosion,” he said.
“Particles bang into each other and break down quickly into dust and clouds affecting the ability of the light to penetrate the curtain. So the lifetime replacement cost and optical efficiency needs to be compared to the higher performance of bauxite. I think it will be very interesting to evaluate the difference in technologies, and see the degradation properties over time when you’re in continuous operation.”
“Is it worth the extra cost to get a harder, more durable particle? Or does it not really deliver that value, in which case lower cost sand would be more attractive. We haven’t had the opportunity to run a side-by-side comparison yet, but hopefully that will come in the future.”
The pilot plant
Sandia’s pilot plant at their test site in New Mexico is now near completion and individual components have been successfully tested.
The components are supported on a series of decks within an open metal frame structure. The sand is carried in several bucket lifts to the top, where it falls through an open solar receiver to be heated by the highly focused solar flux.
The heated sand falls into a storage bin. The heat is extracted using a heat exchanger, and the now-cooler sand is carried back up to the top to be reheated by the concentrated solar beam reflected by the heliostats.
The particles tower on the right – seen from the solar field of mirrors – was successfully tested in September 2024 IMAGE © Sandia National Laboratories
“We are preparing the system to be fully operational by the end of the summer, and expect to start sharing data in early September,”
Sment said.
“The test is planned to run for six months as continuously as possible with few interruptions or repairs, adjustments, maybe some interruptions for other test priorities at the site, but generally, we hope the world can see this plant as an example of a continuously operating site that would be used for power production or industrial heat.”
Military base
However, Sandia’s concentrated solar test site is on a US Air Force Base. This means there is heightened security regarding data sharing; online and in-person.
“If you’re interested in seeing it in person, we welcome visitors, but need to get access to the base, which can take several weeks or months, so people should call well in advance,” he cautioned.
A follow-on project to test a novel sCO2 Brayton Cycle to generate electricity is status pending.
Although initially funded with the idea that G3P3 would not include the power block, a subsequent funding opportunity currently under review may eventually provide additional funds to incorporate an sCO2 Brayton Cycle turbine.
“We initially were envisioning G3P3 as a major step toward a large 100 megawatt power plant,” he said.
”But we are also seeing a growing interest in small systems because they are easier to deploy with less risk exposure as the technology scales up and matures. We anticipate the 300 kilowatt power block project under review that would demonstrate power production would significantly boost confidence in the technology.”
But perhaps there’s a silver lining. What the world needs now is to decarbonize all the industrial processes that use heat. And these don’t need huge thermal power plants with big solar fields to generate electricity.
Solar heat for industrial processes
“We’re finding that this demo might be at the right scale for some small modular deployments, that fit in brownfield land sizes,” said Sment.
“CSP developers might have more market penetration with lower cost lower risk systems than the very large, billion-dollar systems used for electricity even if the levelized cost is a bit higher. If heat is the only thing you need, these types of smaller CST systems might be attractive. Particle technology could be used in applications needing very high temperature heat up to 1000°C.”
Sment said that the demonstration would provide near real-time operation data for potential industrial stakeholders.
“When you do a pilot, if you don’t operate it as continuously as possible, it loses its relevance in industrial settings,” he explained.
“They can’t get a sense of the true reliability if you test for a day or two and then shut down for a week. We want to answer some difficult questions: what is the mean time to failure? What is the mean time to repair? What is the actual heat variability, day to day, month to month, seasonally? We plan to publish publicly the raw data from this so that people can use it to learn about particle technologies and maybe expand and improve on what we’ve been able to do.”
Small scale is more suited to urban and retrofit industrial spaces
The tower is 53 m tall and 10 x 13 m wide. A heat-only CST system is much more compact per Watt than a typical CSP power plant because it doesn’t lose more than half of the energy in the conversion to electricity. A solar field the size of this pilot plant could produce about 5 MW of thermal power during the day or put that energy into storage.
“The small heliostat field could be a good candidate for existing industrial retrofitting because it could fit on a large parking lot like you might see at a big box store,” he said.
“The tower is low enough that it doesn’t have extra regulations for US air traffic control. So considering the reuse of land already owned and permitted for high-temperature operations, what you’re looking at is a relatively cheap way to make a small deployment. We need about 175 heliostats to aim at this tower to get the flux we want for about 2.2 megawatts of heat.”
This pilot-scale plant could be of interest for climate conscious industries – at its current size. This system generates heat at around 800°C from a relatively small solar field. The system could be versatile in the types of heat exchangers that could be plugged in at the output. The system can supply solar heat as a hot gas, such as air or CO2, or as steam or hot water.
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