GEN3 CSP: why USDOE thinks falling particles are better than molten salt

15 June 2023

Construction is underway of a new solar tower at the US National Solar Thermal Test Facility, Albuquerque, NM, operated by Sandia for the US DoE. The tower is part of a DOE initiative focused on the demonstration of next-generation CSP (concentrating solar power) technology, which, instead of using molten salt in the receiver would employ a flow of ceramic, sand-like particles.

Above: New solar tower at NSTTF (source: Sandia)

The Generation 3 Particle Pilot Plant-USA (G3P3-USA) is designed to enable at least six hours of particle-based energy storage and will also employ supercritical CO2 as a working fluid operating at temperatures of 700°C or higher.

The pilot plant will serve as a centralised test facility with an adaptable modular system design for exploring the full potential of particle-based thermal technologies.

The DOE has determined that particle-based systems require fewer components and are less complex to operate compared with liquid- and gas-based systems. Additionally, particle-based systems need relatively few high-cost materials to collect and transport thermal energy. These factors could increase plant availability and reliability and enable simpler plant construction and commissioning, the DOE maintains.

And unlike the other two pathways, ceramic, sand-like particles can withstand temperatures greater than 800°C, making them useful in electricity production and other solar-thermal heat applications, including industrial process heat, thermochemical energy storage, and solar fuel production.

Conventional CSP power towers have tubular receivers with a fluid, such as molten salt, flowing through the system and absorbing thermal energy. In a falling-particle receiver, sand or manufactured particles are heated directly by a beam of concentrated sunlight as they fall through open air. The Sandia project team uses particles based on aluminium oxide, with a diameter of about 300 micrometers. The heated particles are then stored in an insulated bin before passing through a particle-to-working-fluid heat exchanger. The heat exchanger’s working fluid will simulate a high-efficiency Brayton cycle using supercritical carbon dioxide (sCO2) with an exit temperature of 720°C. Then the cooled particles are collected and moved back to the top of the receiver via a bucket elevator or skip hoist.

To accelerate deployment and commercialisation a second facility (G3P3-Saudi) is being built in Saudi Arabia to test variants of key system components, allowing the research consortium to simultaneously test different potential configurations for deployment. There is also collaboration with researchers in Australia, who have helped develop key components and are performing system-level analysis of the solid-particle design, as part of the Australian Solar Thermal Research Initiative. This large-scale, integrated CSP system will help address risks associated with receiver thermal efficiency, performance and cost of particle heat exchangers, material erosion, minimisation of heat loss, and particle attrition and conveyance.

The end goal of the project is to complete > 2000 hours of combined testing of the G3P3-USA and G3P3-Saudi integrated particle-receiver systems and demonstrate a thermal duty ≥ 1 MWt for the receiver and heat exchanger, which heats a working fluid (sCO2 and air for G3P3-USA and G3P3-Saudi, respectively) to >700°C.

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