Increasing greenhouse gas emissions and electricity demands coupled with decreasing fossil fuel reserves has dramatically increased the need for alternative energy sources. Solar energy continues to be the most promising carbon-neutral source with more energy incident on the Earth in 1 hour than is consumed in an entire year (1). Concentrated solar plants (CSPs) have been gaining interest due to their relatively high efficiencies of 37-42% (2). Molten salt CSPs have been of particular interest because these designs can achieve the highest temperatures and efficiencies (1). Solar technologies also need to account for periods without sun that can only be solved by developing a method of storing excess energy produced during peak producing time. Concentrated solar plants that use a molten salt heat transfer fluid (HTF) have the ability to store sensible heat for up to 15 hours (3).

Experimental

Experiments were conducted at operating temperatures of a typical CSP using a molten nitrate salt HTF of 300 - 500°C in a static, atmospheric environment. The molten salt working fluid studied was an off-eutectic 40/60 wt.% KNO₃/NaNO₃ mixture. The alloys studied were stainless steel 316, super-austenitic Incoloy 800H, and carbon steel. Each of these alloys was exposed to the salt for 100 hour exposures and galvanic couples at 300, 400 and 500°C. The alloys were studied electrochemically to simulate long-term exposures analogous of plant operation. The oxide film that is formed on each sample was then characterized to correlate the surface chemistry to the corrosive behavior of the salt on each alloy. The surface characterizations are performed using X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. The salt was also studied by dissolving in deionized water and analyzing the solution by inductively coupled plasma optical emission.

Results

Figure 1 shows Raman spectra for SS 316 exposed at 300°C molten KNO₃/NaNO₃salt mixture for 100 hours. Based on the Raman shift, hematite and magnetite features can be observed on the surface (4). Similarly, the surface chemistry of all alloys are correlated to their corrosion behavior.

Figure 1: Raman spectrum obtain from SS316 exposed to molten nitrate salt at 300C for 100 hours.

Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant No. IIA-1301726.

References

1.            D. Barlev, R. Vidu and P. Stroeve, Solar Energy Materials and Solar Cells, 95, 2703 (2011).

2.             M. T. Dunham and B. D. Iverson, Renewable and Sustainable Energy Reviews, 30, 758 (2014).

3.             H. L. Zhang, J. Baeyens, J. Degrève and G. Cacères, Renewable and Sustainable Energy Reviews, 22, 466 (2013).

4.             RDSS - A Raman spectra library software using peaks postion for fast and accurate identification of unknown inorganic compounds, in.