Sandia researchers are working to lower the cost of solar energy systems and improve efficiencies in a big way, thanks to a system of small particles.

Technologists John Kelton and Daniel Ray (both in Sandia’s Concentrating Solar Technologies Dept.) perform inspection of the falling-particle receiver during a cloud delay atop the NSTTF’s Solar Tower at Sandia. (Photo by Randy Montoya)

Technologists John Kelton and Daniel Ray (both in Sandia’s Concentrating Solar Technologies Dept.) perform inspection of the falling-particle receiver during a cloud delay atop the NSTTF’s Solar Tower at Sandia. (Photo by Randy Montoya)

This month, engineers lifted Sandia’s continuously recirculating falling-particle receiver to the top of the tower at the National Solar Thermal Test Facility (NSTTF), marking the start of first-of-its-kind testing that will continue through 2015. The Sandia-developed falling-particle receiver drops sand-like ceramic particles through the NSTTF’s concentrated sunlight beam, capturing and storing the heated particles in an insulated tank. The technology can capture and store heat at high temperatures without breaking down, unlike conventional molten-salt systems. Higher temperatures mean more available energy and cheaper storage costs because less material is needed to transfer heat.

Sandia engineer Cliff Ho (in Sandia’s Concentrating Solar Technologies Dept.), the project’s principal investigator, said the goal of the testing is to develop a prototype, cost-competitive falling-particle receiver that demonstrates the potential for thermal efficiency greater than 90%, while achieving 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,” Ho said. “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.”

In the falling-particle receiver, sand-like particles fall from a bucket-elevator hopper, at the top of the receiver tower, past the focused solar energy from the heliostat array. The hot particles are kept in the top tank and released into the middle one as energy is required for power generation. In the middle tank, thermal energy is extracted for the power-generation cycle (not shown). The now cooler thermal-storage particles are released from the bottom of the middle tank into the lower tank where the bucket elevator scoops them out to return them to the top of the receiver tower. The bucket elevator’s speed and hopper size are optimized to deliver a particle density to the central receiver focal point that can capture the maximum available concentrated solar energy.

In the falling-particle receiver, sand-like particles fall from a bucket-elevator hopper, at the top of the receiver tower, past the focused solar energy from the heliostat array. The hot particles are kept in the top tank and released into the middle one as energy is required for power generation. In the middle tank, thermal energy is extracted for the power-generation cycle (not shown). The now cooler thermal-storage particles are released from the bottom of the middle tank into the lower tank where the bucket elevator scoops them out to return them to the top of the receiver tower. The bucket elevator’s speed and hopper size are optimized to deliver a particle density to the central receiver focal point that can capture the maximum available concentrated solar energy.

Conventional central receiver technologies are limited to temperatures of ~600 °C with power-cycle efficiencies ~40%. At higher temperatures, nitrate salt fluids become chemically unstable. In contrast, direct-absorption receivers using solid particles that fall through a beam of concentrated solar radiation for direct heat absorption and storage have the potential to increase the maximum temperature of the heat-transfer media to over 1,000 °C, which will increase power-cycle efficiencies.

Once heated, the particles may be stored in an insulated tank and/or used to heat a secondary working fluid (e.g., steam, supercritical CO2, air) for the power cycle. Thermal energy storage costs can be significantly reduced by directly storing heat at higher temperatures in a relatively inexpensive medium (i.e., sand-like particles). Because the solar energy is directly absorbed in the sand-like working fluid, the flux limitations associated with tubular central receivers are significantly relaxed. The falling-particle receiver appears well suited for scalability ranging from 10–100 MWe power tower systems.

The research team is pursuing technical innovations that include advances in

  • receiver design, with consideration of particle recirculation, air recirculation, and novel designs for increased particle heating;
  • particle materials to increase the solar absorptance and durability; and
  • balance of plant for falling-particle receiver systems including thermal storage, heat exchange, and particle conveyance.

The research team members and key participants in this DOE SunShot Initiative-funded project include:

  • Sandia National Laboratories: Clifford Ho (Principal Investigator), Joshua Christian, Julius Yellowhair, JJ Kelton, Daniel Ray, Steve Hinken, Doug Robb, and Bill Kolb.
  • Georgia Institute of Technology: Profs. Sheldon Jeter and Said Abdel-Khalik.
  • King Saud University: Prof. Hany Al-Ansary.
  • DLR (German Aerospace Center): Reiner Buck, Birgit Gobereit, Lars Amsbeck.
Sandia engineer Joshua Christian helped design the falling particle receiver system, which converts the sun’s energy to electricity for large-scale, concentrating solar power plants. (Photo by Randy Montoya)

Sandia engineer Joshua Christian helped design the falling particle receiver system, which converts the sun’s energy to electricity for large-scale, concentrating solar power plants. (Photo by Randy Montoya)

Sandia design engineer Josh Christian (also in Sandia’s Concentrating Solar Technologies Dept.) said the on-sun testing at the solar tower will occur in two phases. First, researchers will test an insert designed by Georgia Tech that slows falling particles inside the receiver like a Pachinko board to increase the temperatures of the particles as they fall through. Later this summer, Sandia engineers will remove the Georgia Tech insert from the receiver and evaluate free-falling curtain configurations.

The falling-particle receiver system will enable higher temperatures, which will increase power-cycle efficiencies and allow cheaper thermal storage that will lower the levelized cost of energy (LCOE) toward the SunShot goal of $0.06/kWh. The flexibility of the falling-particle receiver, combined with lower costs of thermal energy storage, could enable higher penetrations of concentrating solar power (CSP) systems and help meet SunShot Initiative goals.

Read the Sandia news release.