Funding for research is necessary to take concentrated solar to the next level

Kenneth Sandhage, professor at Purdue University, and his collaborators have recently made a breakthrough by redesigning and testing another ceramic–metal composite for the heat exchanger in the GEN3 CSP plant. Their material is a combination of zirconium carbide (ZrC) as the ceramic component and tungsten (W) metal. The ZrC/W cermet has an ultrahigh melting point, it is thermally conductive, thermally cyclable, very stiff and tough, and has a relatively high failure strength. The researchers can also mold it in complicated shapes via a patented process. “We start with a 50% porous tungsten-carbide plate, which we immerse in a Zr-bearing liquid. The liquid infiltrates the pores, and then it reacts to give ZrC and W. The new solid materials fill the pores of the plate, with very little change of its shape or size,” said Sandhage.

Renkun Chen, professor at the University of California, San Diego, is developing a diagnostic tool that can measure, with high precision, thermophysical properties, such as thermal conductivity and heat capacity, in several materials used in a GEN3 CSP plant. He believes that providing the other teams with the exact values of these properties can help them choose the right combination of salts, alloys, and coatings. “There is currently no commercial set up to measure with certainty these properties in situ at very high temperatures while the plant operates,” he said. The big challenge is to design their instrument in a way that it will be resilient to the thermal stress and damage that can occur in the proximity of a CSP solar tower.

Even if they end up with an inexpensive solution for the HTF, and they are able to solve all corrosion issues, other challenges await researchers. “The temperatures that we want to run at necessitate that we build the hot tank out of a temperature-resistant alloy, like Inconel, and that is too expensive,” said Turchi. They now focus a large amount of effort on designing an internal liner for the tank that will provide insulating capability, so they can build the tank from stainless steel or carbon steel instead

The melting points of the chloride salts are also quite high, meaning that there is a danger that the salt may freeze in the pipes. The lowest mp that they have been able to get is around 400°C, double what it is currently commercially used (around 220°C). “We plan to deploy a different power cycle, which will allow us to keep it at higher temperatures, but the danger remains,” said Turchi. Finally, Turchi refers to the thermal conductivity and heat capacity of the salt, which are lower than preferred. “We can tolerate that, but it will definitely impact the overall efficiency and cost of the process.”

To bring a Generation 3 concentrating solar power plant to life, researchers will have to cut the GEN3 CSP plant Gordian knot and fill knowledge and technology gaps that are tightly interconnected. When asked about what will happen at the end of the first two years of evaluation, Turchi said, “We will not have answered all the questions; there will be a certain amount of uncertainty, but our pathway is shaped to move forward with design that will address questions that need to be answered before an initial commercial plant can be built.”