Improving Efficiency in Generation 3 Solar Power Systems with
New Thermal Energy Storage Materials

By Sudhakar Neti, Amanda Allekotte

As renewable energy technologies continue to replace fossil fuel power plants, solar energy is expected to play an increasing role in the nation’s energy future. The U.S. Department of Energy has identified the need to improve performance and cost-effectiveness of Generation 3 Concentrated Solar Power Systems (Gen3 CSP). Thermal energy storage (TES) is one area where further research is expected to lead to advancements toward this goal, given the intermittent generation associated with solar energy.

Dynalene Inc., a Lehigh Valley-based heat transfer fluid and coolants company, and Lehigh University’s Energy Research Center teamed up to explore the use of chloride salts for TES to increase efficiency and lower the levelized cost of energy (LCOE) for Gen3 CSPs. Solar power generation requires good heat transfer fluids and media for efficient energy storage, and the team identified that, compared to the more commonly used nitrate-based solar salts, the low cost and high thermal stability (>700ºC) of inorganic chloride salt blends make these blends excellent candidates for Gen3 CSP.

However, chloride salt blends are known for being corrosive and having poor thermal conductivity. Through prior PITA and National Science Foundation funding, the Dynalene-Lehigh team addressed this issue of corrosiveness by developing a patent-pending inhibited molten chloride salt (phase change medium “PCM” chloride salt), which provides corrosion resistance to stainless steel. For this 2019-2020 PITA project, the team sought to solve the issue of poor thermal conductivity by embedding PCM chloride salts into the pores of thermally conductive graphite foams to create a graphite foam-chloride salt composite that is ideal for high temperature TES applications.

Graphite foam is a highly porous, thermally stable carbon-based material with high thermal conductivity and tolerance to salts. By embedding a chloride salt into the porous structure of the foam, the graphite provides additional heat transfer surface area for the salt, which reduces the overall system-level costs by increasing the effective thermal conductivity of the foam and salt composite. Infiltration of the graphite foams was performed with high temperature binary- and ternary-inhibited chloride salt mixtures above the melting temperature of the salt, under pressurized and high purity inert nitrogen gas conditions, into evacuated graphite foams using an apparatus designed by the team (see Figure 1a-1c). Before and after infiltration, Dynalene performed characterization of the carbon/graphite foams using Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy. Figure 2a-2c shows representative images of successful infiltration.

The team includes Dr. Carlos Romero, Dr. Alp Oztekin, Dr. Nasser Vahedi, Dr. Sudhakar Neti, Mr. Hao Lan, and Mr. Huazhi Chen from Lehigh University and Dr. Sreya Dutta, Dr. Satish Mohapatra, and Mr. Michael Nappa from Dynalene. Dr. Neti, one of the project’s principal investigators, says that the team is grateful for the financial support provided by PITA and Dynalene for this project and hopes to continue research in this area. “Additional work is underway to fully characterize the graphite foam-chloride salt composite,” Neti says. “The project stands to impact the rapidly expanding TES field, where improvements in thermal properties of materials can have significant economic advantages.”

In the future, commercialization of the graphite foam-chloride salt composite is expected to help position Pennsylvania as a leader in
the new energy landscape.