## 2 Molten Salt Mixtures
### Molten Salt Mixtures' Thermophysical Properties
gas, nitrogen containing oxygen and small amounts (ppms) of NO, while the presence of nitrogen instead of air led to limited effects. Figure 1 shows a comparison between the thermophysical properties of the HTFs considered within their respective operating temperature ranges, referring to operations in ambient air (except where specified differently); however, they can be extended beyond these limits by operating in a controlled atmosphere [72]. Moreover, the volumetric densities were estimated, considering sensible heat release and average values for densities and heat capacities in the respective operation ranges: 290-550 \({}^{\circ}\)C for solar salt; 200-530 \({}^{\circ}\)C for Hitec(r); 200-425 \({}^{\circ}\)C for Hitec XL(r); and 200-550 \({}^{\circ}\)C for Na/K/Li nitrate. Besides thermophysical properties, an assessment of different HTF solutions includes the assessment of corrosion issues related to the steel components of a CSP plant, such as pipes, valves, tanks, welding joints, etc. Specifically, molten nitrates show some drawbacks linked to the degradation of nitrates into nitrites, with possible oxygen generation, when the fluid is heated 295 \({}^{\circ}\)C [80]. Studies carried out so far on steel corrosion with mixtures of molten nitrates are not adequately systematic. Furthermore, these tests sometimes provide contradictory results; there are no validated/shared methods for carrying out corrosion tests. From the results available in the open literature, it can be concluded that less expensive carbon steels may be employed for mixtures operating at low temperatures (200-400 \({}^{\circ}\)C), while for higher temperatures (400-600 \({}^{\circ}\)C), special stainless steels or alloys containing nickel, which have a higher corrosion resistance, should be considered [81]. Figure 1: Comparison between the densities (**a**), the dynamic viscosities (**b**), the heat capacities (**c**), and the volumetric energy densities (**d**) of the four salts in the following respective operation ranges: 290–550 \({}^{\circ}\)C for solar salt; 200–530 \({}^{\circ}\)C for Hitec®; 200–425 \({}^{\circ}\)C for Hitec XL®; and 200–550 \({}^{\circ}\)C for Na/K/Li nitrate [72]. Toxicity and environmental compatibility are clearly major issues for large-scale utilization of heat transfer materials. MSs are safe and in general not toxic. However, nitrites and lithium can also be harmful for humans and the environment, and the thermal decomposition of nitrates and nitrites can lead to the formation of nitrogen oxides [22]. From the comparison proposed in this document, it is possible to assess the following [72]: (1) the commercial MS mixture Hitec(r) is characterized by the presence of sodium nitrite and shows a relatively low freezing point, together with good features concerning specific heat, density, and viscosity. However, this material is toxic and less thermally stable than solar salt under air [22]; (2) an interesting alternative is represented by the MS mixture composed of calcium, potassium, and sodium nitrates, Hitec XL(r) [73]. However, its viscosity is much higher than that of solar salt, especially at temperatures approaching the freezing point, while its thermal stability in the presence of air is similar to that of Hitec(r) [22]. The addition of lithium nitrate significantly decreases the freezing point of an MS mixture, while, at the same time, similar thermophysical properties and stability of solar salt can be obtained [22]. However, the costs and availability of lithium can represent an issue for its utilization in CSP plants with a large-scale TES capacity.