Experiments at solar thermal test facility break new ground

By Kelly Sullivan

Thursday, August 27, 2020

Facility to test world’s first particle-to-supercritical CO2 heat-exchanger system and on-sun falling particle receiver connected to a supercritical CO2 loop

engineer stands next to particle-supercritical CO2 heat exchanger
CONCENTRATED SUNLIGHT — Mechanical engineer Kevin Albrecht stands next to a particle-supercritical CO2 heat exchanger at Sandia’s National Solar Thermal Test Facility. (Photo by Cliff Ho)

Sandia’s National Solar Thermal Test Facility will host a number of “firsts” in 2020, including testing a particle-to-supercritical carbon dioxide heat-exchanger system, and the world’s first on-sun falling particle receiver connected to a supercritical CO2 loop.

“Sandia has been building and using supercritical CO2 loops for about a decade,” said Paul Gauche, concentrating solar power program manager. “There are several supercritical CO2 loops at the Brayton Laboratory, including one of the first three ever built, which are used for customer testing and research.” These loops demonstrated the first near-critical supercritical CO2 compression, recompression Brayton cycle power generation, and trans-critical and mixed gas cycle variations, he said.

In fiscal year 2015, Sandia applied to DOE’s Solar Energy Technologies Office to fund a SunShot National Laboratory Multiyear Partnership, or SuNLaMP, project in FY2016-18. The goal of the project — a precursor to the much larger Generation 3 Particle Pilot Plant concentrating solar power project — was to build a falling particle receiver system with an integrated supercritical CO2 loop to demonstrate the ability to heat supercritical CO2 using particles heated by concentrated sunlight to temperatures above 700 degrees Celsius.

Sandia Senior Scientist Cliff Ho, the principal investigator leading these projects, said the first test of the integrated test system will include the following components:

  • Falling particle receiver
  • Particle-to-supercritical CO2 primary heat exchanger
  • Supercritical CO2 loop (to represent a closed-loop recompression supercritical CO2 Brayton cycle)
  • Indirect air-cooling system for the supercritical CO2 loop

Although Sandia houses a megawatt-scale supercritical CO2 loop, the loop that will be used for the SuNLaMP project is 150 kilowatts to fit within the envelope of the facility’s elevator and accommodate operation above 700 C. Ground-based testing of this system with electrical heating has exceeded 500 hours, surpassing any other loop of its size.

“The integrated system is unique because it heats the particles up to approximately 800 C using concentrated sunlight and then transfers that heat to a working fluid through a primary heat exchanger,” Cliff said. “The heat exchanger is the world’s first to transfer heat from particles in a moving packed bed to supercritical carbon dioxide at design temperatures and pressures greater than 700 C and 20 megapascals.”

illustrations of solar facility elements
ON-SUN TESTING — This set of images illustrates the integrated falling particle receiver, heat exchanger and supercritical CO2 flow system. Click to view larger image. (Images courtesy of Sandia National Laboratories)

Ultimately, the high-temperature supercritical CO2 can be used in next-generation, high-efficiency supercritical CO2 closed-loop Brayton cycles being pursued by DOE to decrease the cost to produce electricity.

“This is a ground-breaking experiment where we plan to use concentrated sunlight to transfer heat from falling particles in the receiver and into the supercritical CO2 loop,” Paul said.

The Sandia researchers are partnering with Vacuum Processing Engineering and Solex Thermal Science on the projects. Testing will take place at Sandia’s National Solar Thermal Testing Facility using the country’s first concentrating solar tower built in 1978. The tower’s 800,000-pound-capacity elevator allows researchers to build large experiments like the SuNLaMP test and raise them to the top of the tower, which is unique in the concentrating solar power industry.

The NSTTF also will incorporate work from its Gen 3 projects by testing falling particle receiver and supercritical CO2 loop control improvements.

Related Gen 3 activities will include three projects:

  • The Gen 3 Particle Pilot Plant, where researchers are working on ways to reduce heat loss and improve efficiencies in various components of the falling particle receiver system.
  • A Gen 3 concentrating solar power supercritical CO2 coolant loop to simulate a 1-megawatt power cycle for the Gen 3 CSP pilot plant.
  • A Gen 3 lab call, where researchers from Sandia and the University of New Mexico measure particle and heat loss from the receiver aperture using novel imaging techniques.

Activities for SuNLaMP and the Gen 3 projects will continue through 2020, with the on-sun test campaign running through the summer.

Visit the National Solar Thermal Test Facility website, the Gen 2 Particle Pilot Plant website and the Brayton Lab website to learn more about Sandia’s work in concentrating solar power.