The next generation of Concentrated Solar Power (CSP) plants are expected to operate at higher temperatures than those currently in use for improved efficiency [1]. A prerequisite for this is an improved compatibility between the materials used to construct the plant with environmental conditions. CSP systems use combinations of mirrors to concentrate the sun's light energy onto a receiver, which captures the energy and converts it into heat to generate electrical power [2, 3]. The cycle starts when the sunbeam is reflected onto the central receiver tower. The Heat Transfer Fluid (HTF) absorbs the thermal energy and carries it to the heat storage system. The thermal energy is then transferred through a heat exchanger (HE) to a superheated CO₂ Brayton Cycle. The sCO₂ then flows through a turbine which generates electricity. The cycle continues by recycling the sCO₂ to the HE.

This study provides a brief review on evaluating degradation mechanisms that threaten structural alloys at elevated temperatures, Figure. 1. A comprehensive evaluation should consist of thermal fatigue and creep measurement. Corrosion as a result of the dissolution of alloying elements in the alloy by liquid sodium, as HTF material, and carburization or decarburization of the material need investigation [4, 5]. Liquid metal embrittlement (LME) caused by the penetration of sodium through grain boundaries and erosion due to the movement of liquid sodium in the system are also interesting to be studied [6, 7].

Eutectic salts or metals used as Phase Change Materials (PCM) in the storage are responsible for significant corrosion damage in the storage system. Mechanisms leading to corrosion by PCMs including oxidation, fluxing, de-alloying, impurities, thermal gradient and thermal cycling need more investigation [8-10]. Erosion due to the phase transformation from liquid to solid and vice versa is also important [11]. The susceptibility of sCO₂ Brayton cycle to corrosion damage and erosion due to the flow of compressed hot gas in the system could be also considered for evaluation [12-14].

Figure captions

Figure. 1 Diagram showing common issues threatening structural materials at high temperatures in a CSP plant.

References

[1] Australian solar thermal research initiative (ASTRI), in, http://www.astri.org.au/ (accessed 6 June 2017).

[2] K. Lovegrove, J. Pye, 2 - Fundamental principles of concentrating solar power (CSP) systems, in: K. Lovegrove, W. Stein (Eds.) Concentrating Solar Power Technology, Woodhead Publishing, 2012, pp. 16-67.

[3] K. Lovegrove, W.S. Csiro, 1 - Introduction to concentrating solar power (CSP) technology, in: K. Lovegrove, W. Stein (Eds.) Concentrating Solar Power Technology, Woodhead Publishing, 2012, pp. 3-15.

[4] J. Pacio, T. Wetzel, Assessment of liquid metal technology status and research paths for their use as efficient heat transfer fluids in solar central receiver systems, Solar Energy, 93 (2013) 11-22.

[5] T. Gnanasekaran, R.K. Dayal, B. Raj, Liquid metal corrosion in nuclear reactor and accelerator driven systems, in: Nuclear Corrosion Science and Engineering, Woodhead Publishing, 2012, pp. 301-328.

[6] C.F. Old, Liquid metal embrittlement of nuclear materials, Journal of Nuclear Materials, 92 (1980) 2-25.

[7] S. Hémery, T. Auger, J.L. Courouau, F. Balbaud-Célérier, Effect of oxygen on liquid sodium embrittlement of T91 martensitic steel, Corrosion Science, 76 (2013) 441-452.

[8] M. Sarvghad, T. Chenu, G. Will, Comparative interaction of cold-worked versus annealed inconel 601 with molten carbonate salt at 450°C, Corrosion Science, 116 (2017) 88-97.

[9] M. Sarvghad, T.A. Steinberg, G. Will, Corrosion of steel alloys in eutectic NaCl+Na 2 CO 3 at 700 °C and Li 2 CO 3 + K 2 CO 3 + Na 2 CO 3 at 450 °C for thermal energy storage, Solar Energy Materials and Solar Cells, 170 (2017) 48-59.

[10] M. Sarvghad, G. Will, T.A. Steinberg, Corrosion of Inconel 601 in molten salts for thermal energy storage, Solar Energy Materials and Solar Cells, 172 (2017) 220-229.

[11] P.D. Myers, D.Y. Goswami, Thermal energy storage using chloride salts and their eutectics, Applied Thermal Engineering, 109 (2016) 889-900.

[12] F. Rouillard, F. Charton, G. Moine, Corrosion behavior of different metallic materials in supercritical carbon dioxide at 550°C and 250 bars, Corrosion, 67 (2011) 1-7.

[13] D.D. Fleming, A.M. Kruizenga, Identified corrosion and erosion mechanisms in SCO2 Brayton cycles, in, Sandia National Laboratories, 2014.

[14] D.D. Fleming, Corrosion and Erosion behavior in Supercritical CO2 power cycles, Sandia National Laboratories (SNL-NM), Albuquerque, NM (United States), 2014.