## 2 Molten Salt Mixtures
### Molten Salt Mixtures' Thermophysical Properties  
The use of nitrate mixtures both as an HTF and as a sensible HSM, and less frequently as a Phase Change Material (PCM) [19], has been investigated since the 1980's [20; 21], with a particular focus on solar salt. This composition presents several favorable characteristics, including upper temperature limits (around 600 \({}^{\circ}\)C), lower unit costs, better safety aspects, and low environmental impacts [22]. Although this nitrate mixture does not represent a eutectic composition, its higher proportion of sodium nitrate is preferable, as this significantly reduces material costs while avoiding an increase in the liquidus temperature. However, its practical operating temperature range is limited by two key factors: an initial solidification temperature of approximately 240 \({}^{\circ}\)C and an upper operational limit of about 590-600 \({}^{\circ}\)C [23; 24]. Currently, no methods have been proposed to effectively increase the upper temperature stability limit of molten salt nitrates, aside from short-term effects [25] that are not useful for a CSP plant where the maximum temperature is to be maintained over long periods of time. Moreover, experimental results show that the use of cations different from those of first group metals leads to a decrease in nitrate chemical stability [26; 27]. For the reasons mentioned above, recent research has focused on reducing the initial solidification temperature of nitrate mixtures. Lowering this temperature could decrease the costs associated with thermal insulation and backup heating systems in CSP plant pipelines and could also simplify start-up and maintenance procedures. To address the drawback of solar salt's relatively high freezing temperature, MS mixtures with significantly lower melting temperatures (<150 \({}^{\circ}\)C) have been proposed in the scientific literature. In general, the addition of other cations, such as lithium and calcium, and different anions, such as nitrites (e.g., NaNO\({}_{2}\)), has been explored. Conversely, the use of metal cations, such as silver, cesium, and rubidium [28; 29], has been considered unrealistic, with mixtures containing AgNO\({}_{3}\), TlNO\({}_{3}\), and RbNO\({}_{3}\) studied only from an academic perspective [29; 30]. The ternary nitrate mixture Na/K/Ca//NO\({}_{3}\) (commercially known as Hitec XL(r)) offers practically the same safety advantages as solar salt, combined with favorable thermophysical properties and a reduced freezing temperature of approximately 120 \({}^{\circ}\)C. However, it exhibits lower thermal stability compared to solar salt [26; 27]. Due to its relatively low costs, comparable to solar salt, this mixture is suitable for use both as an HTF and as an HSM in active, direct storage systems. Another attractive option involves lithium-containing mixtures. However, given lithium's comparatively higher costs, these mixtures have been considered economically viable only as an HTF (with solar salt serving as the HSM) within indirect storage systems employing an intermediate heat exchanger between the two fluids [14]. Literature findings indicate that the economic advantage achieved by reducing costs related to thermal insulation and backup heating systems is completely offset by the expenses incurred for an intermediate heat exchanger. Therefore, no significant economic benefit is observed. Ternary Li/K/Ca//NO\({}_{3}\) additive systems have also been investigated; however, these mixtures exhibit no appreciable improvement over the Na/K/Ca//NO\({}_{3}\) system and are likely to be more expensive due to the substitution of sodium with lithium. Mixtures containing nitrites, such as the Hitec(r) fluids, exhibit initial solidification temperatures and chemical stability similar to ternary systems, like Hitec XL(r), although they are more expensive and exhibit higher toxicity. Additionally, quaternary mixtures, such as Ca/Li/Na/K//NO\({}_{3}\), show freezing points and anticipated costs only marginally lower than those of lithium-containing ternary mixtures.