A transitional proposal for working fluids in csp from GEN2 to GEN3:
Evaluating their thermophysical properties, corrosion behavior, and economic
impact
Authors:   , Castro Quijada Matias Daniel 
Published: November 2024

In the pursuit of reducing the levelized cost of electricity (LCOE) in
concentrated solar power (CSP) plants, the search for working fluids (WF)
capable of withstanding higher temperatures than current nitrate-based solar
salt has emerged. This endeavor aims to enhance generation efficiency without
increasing system costs. Despite consensus favoring chlorides for the new
system, challenges such as high melting points and extreme corrosiveness of
low-melting-point chlorides (e.g., NaCl-KCl-MgCl₂) have hindered the evolution
of this technology. This research proposes a systematic transition from the
current solar salt to an equimolar salt mixture of NaNO₃, KNO₃, NaCl, and KCl.
The thermophysical properties of these salts were assessed using standard
methods and methodology such as DSC for melting point and heat capacity, TGA for
degradation temperature, Archimedean method for mass density, and rotational
viscometry for viscosity. Results revealed improved properties with increased
chloride content up to 50 mol% Cl, showcasing lower melting temperatures with
less than 30 mol% Cl, and degradation temperatures reaching 642 °C with 50 mol%
Cl, compared to 592 °C for the base case. The addition of chloride also enhances
energy density, though concerns arise regarding viscosity at low temperatures
and high chloride content.The corrosion rate (CR) and mechanisms associated with
chloride-containing salts in 304L stainless steel were investigated at 500 °C
for up to 21 days. CR was determined using gravimetry, while the morphology,
chemical composition, and microstructure of the corrosion products were
characterized using XRD, FESEM-EDS, and GD-OES. Exposure to molten salt with 0
mol% Cl (solar salt) resulted in negligible corrosion. The salt with 14 mol% Cl
produced a stable corrosion product with a rate 30 times higher than the
chloride free salt. All quaternary salts exhibited a multilayer structure with
selective chromium (Cr) removal. Salts with more than 29 mol% Cl showed similar
structures with Cr and iron (Fe) removal, leading to more brittle layers and
higher corrosion rates (90 to 250 times). Cl diffusion into the oxide layer was
confirmed, highlighting the roles of Cl₂(g) and O₂(g) in driving corrosivity.
Using salts with 29 mol% Cl at 500 °C is discouraged, while using 304L with 14
mol% Cl in a cold tank may be viable. The salt with 14 mol% Cl content exhibited
homogeneous corrosion at 100 µm/year. In contrast, salts with 29 and 50 mol% Cl
content displayed localized corrosion, with rates of 280 and 755 µm/year,
respectively, after 21 days at 500 °C in an open atmosphere.Potentiodynamic
polarization sweep studies on 304L and 316L stainless steels, as well as on
Haynes 230 and Hastelloy C-22, in the presence of three selected salts, revealed
that superalloys Haynes 230 and Hastelloy C-22 demonstrated remarkable
insensitivity to escalating chloride content. Furthermore, Hastelloy C-22
exhibited greater resilience, attributed to its higher molybdenum (Mo) content
compared to the tungsten (W) content in Haynes 230, making it a promising
material for CSP Gen3.Performance evaluations of the proposed CSP salts,
utilizing a tailored TRNSYS library with SAM and DELSOL3 in a MATLAB framework,
revealed increased generation efficiencywhen using these proposed quaternary
salts. However, considering cost factors, incorporating chlorides and new
materials reduced the LCOE to values similar to less thermally stable or more
expensive nitrates. While the higher risk may not justify the limited benefits
of quaternary salts, this transition path aligns with the stepwise methodology
devised by NREL to encourage a gradual Gen3 transition and control investment
risk. Therefore, further corrosion and material compatibility studies are
necessary to determine the appropriate cost of replacing components and assess
its impact on the LCOE through long trial assays