NREL Researchers Delve into Use of Heat Transfer Mediums for Concentrating Solar Power

Concentrating solar power (CSP), has been a promising technology for renewable energy. To spur CSP industry advancement and achieve an energy cost goal of 5 cents per kWh, the U.S. Department of Energy’s (DOE’s) Gen3 CSP program funds research to explore the potential of several heat transfer mediums. National Renewable Energy Laboratory (NREL) researchers are contributing to this effort, tackling several challenges related to the use of one potential medium – liquid-hot molten salt – for energy transfer and storage.

CSP uses mirrors or heliostats to harness the power and energy of the sun. They heat and store an inexpensive medium like sand, rocks or molten salt, which can be used for on-demand energy dispatch.

Three years ago, the Gen3 Program established three pathways to reach the CSP energy price goal. One was a liquid pathway that explored the use of molten sodium as a heat transport material, led NREL, and one was a particle pathway that used sand-like particles for heat transfer materials, led Sandia National Laboratories, and a third exploring the use gas as a heat exchange material (led Brayton Energy).

In March 2021, DOE selected one of the three pathways to continue research into particle-based storage. However, it also gave NREL the opportunity to develop the liquid pathway in the next two years.

Craig Turchi is the NREL’s director of thermal energy science and technologies research. He says that molten salts are a desirable option for a heat transfer and storage material – liquids are easy to work with as they can be pumped through pipes and heat exchangers to move around a CSP system. However, there are still practical issues that need to be addressed, which is the focus of current NREL research.

“Everyone initially thought that salt corrosivity would torpedo this effort,” according to Turchi. Salts are easy to transport, but they can also cause corrosion to pipes and tanks that contain them. “We actually solved that problem by and large. NREL and partners did a lot of great science on the salt chemistry – how to purify it, how to make it relatively noncorrosive if you control the chemistry, and we demonstrated that in the lab.”

Molten salts are not corrosive. The problem lies in the ability to achieve high temperatures necessary for a high efficiency power plant. The salt’s energy density requires relatively large – and therefore, expensive – storage tanks and one must keep the salts from freezing in the pipes (while thermally stable as a liquid to very high temperature, these salts freeze at a not-so-chilly 400°C).

“We had performed testing to show which materials could work but hadn’t actually built a tank to demonstrate that it did work,” adds Turchi. “Our design is a steel tank, but whereas the current tanks are insulated on the outside, our proposed tank was insulated on the inside to protect the steel.”

DOE awarded $2 million to NREL to build a prototype tank for testing its integrity in molten salt. The tank is currently being built and will be operated on the mesa above NREL’s Golden, Colo. campus.

There is more than one kind of salt, so NREL’s work developing the Gen3 CSP liquid path also involved selecting and experimenting with new salts. Commercial molten salt systems use sodium salts. However, once the system reaches a certain temperature, these salts begin to degrade. The NREL team wanted to reach higher temperatures to achieve more efficient energy conversion for higher efficiency power plants, so they explored an alternative – chloride salts.

Youyang Zhao is an NREL researcher. He has been studying the chemistry of salt for the Gen3 liquid path project for the past three year. Zhao said that he began by looking for ways to reduce the industrial salt’s impurities. Additionally, Zhao says, “We were optimizing the salt composition to lower the melting point of the salt. The lower the melting point, the more time we have to work with the material.”

Zhao’s work led to the decision to design the new prototype tank for chloride salt.

Their efforts are continuing to be a success with this new opportunity. “At a high level,” Zhao explains, “we are connecting fundamental science to future engineering. I’m not creating the component design, but trying to find out the basics, such as chemistry and material knowledge, to provide information so people can design systems better.”

Kerry Rippy is an NREL expert for inorganic chemistry. She has supported the Gen3 liquid pathway in various capacities. Her team developed electrochemical methods to remove corrosive elements from molten chloride sodium. They are now continuing this work with University of Wisconsin to demonstrate reliability of purified molecular chloride salt through a scaled up prototype that mimics an industry system.

Rippy also supports the mesa top tank project. The cost of the containment vessels are prohibitive, so the team is exploring new materials to store the salt at varying temperatures and for longer periods of time (up to 10 hours). Rippy is working with the team to develop an electrochemical sensor to monitor the purity and integrity of the salt during experiments.

Rippy says that further exploration of a molten sodium chloride pathway is warranted for the benefit CSP, and beyond. “There are multiple potential avenues for this research to be valuable. It can be beneficial for solar fuel synthesis; it could enable high-temperature fuel cells, and the nuclear industry is also really interested in this research.”

“The nuclear industry is developing a number of ‘Gen4’ reactors of its own, some of which use molten chloride salts,” agrees Turchi. The upcoming tank testing could lead to lower tank costs for many energy industries.