[Image above] Technologists John Kelton and Daniel Ray perform inspection of the Falling Particle Receiver during a cloud delay atop the National Solar Thermal Test Facility at Sandia National Laboratories. Credit: Randy Montoya
Ceramic particles are taking the heat at Sandia National Labs.
There, scientists are testing the utility of falling particle receivers to efficiently collect and store solar energy.
Sandia’s receivers drop ceramic particles through a beam of concentrated sunlight, where the particles absorb solar energy and store that energy as heat. Because of the heat stability of ceramic particles, the Sandia team is hoping its receivers can operate more efficiently than conventional solar energy storage systems, which use molten salt.
The project—in which Sandia pairs up with partner institutions Georgia Tech, Bucknell University, King Saud University, and the German Aerospace Center (DLR)—was first funded by the Department of Energy’s SunShot Initiative in 2012.
A new press release from Sandia reports that the project has moved beyond the development stage and has started testing. Sandia engineers have now lifted the falling particle receiver on top of the National Solar Thermal Test Facility, where the receiver will continue testing through this year.
The facility—the only test facility for large-scale solar thermal power generation in the United States—consists an 8-acre heliostat field, complete with 218 automated mirrors (heliostats) that surround a testing tower. When those mirrors are coordinated on a single focal point, they intensify the sun’s energy by 2,500 times, according to a video about the facility.
While conventional molten salt receivers operate at a cool 600 °C, the Sandia team says their ceramic particle receiver can operate in excess of 1000 °C because of the heat resistance of the ceramic particles.
According to a 2013 project update for the SunShot program, the team tested the performance of several different types of particles, including varieties of sintered bauxite. The results of testing to determine the particles’ durability and solar absorptance, along with other parameters for the falling particle receiver, are published here and here.
Cliff Ho, who leads the project at Sandia, says that the team is using Carbo Accucast ceramic particles (~280 μm mean diameter) in the system—the same ceramic proppants that are used in fracking. “These particles have a high solar absorptivity (>90%), high heat capacity, good durability and stability at high temperatures, and are commercially available,” he writes in an email.
The receiver recirculates ceramic particles through the Thermal Test Facility’s concentrated beam of reflected sunlight, where they absorb more solar energy with each pass, increasing the system’s solar efficiency.
Credit: Sandia National Labs; Youtube
Once heated, the particles travel into an insulated holding tank, where their heat will be harnessed to power a turbine and generate electricity, according to a previous Sandia press release about the project.
According to most recent press release, the team is hoping to develop a prototype receiver that can exceed 90% thermal efficiency and achieve particle temperatures of at least 700 °C.
“This technology will enable higher temperatures and higher efficiency power cycles that will bring down the cost of electricity produced from concentrating solar power,” project leader Cliff Ho says in the release. “In addition, the ability to cheaply and efficiently store thermal energy directly in the heated particles will enable power production at night and on cloudy days.”
Now that the receiver is installed on the tower, the system will undergo two phases of testing. In the first phase, scientists will test an insert in the receiver that slows down the falling particles, allowing them reach higher temperatures. The insert, designed by researchers at Georgia Tech, works like a Plinko board, with pegs that disrupt downward particle flow.
In the second phase of testing, the insert will be removed so that the team can collect results for free-falling particles, too.
“New Mexico is great for this project because our state has pretty consistent solar insolation throughout the year,” engineer Josh Christian says in the release. “However the biggest thing we need to know is how much power is going into the falling particle receiver. So a cloudy or hazy day is a big challenge for us. An ideal day for testing is a clear day with no clouds and no wind.”