## 2 Molten Salt Mixtures
### Molten Salt Mixtures' Thermophysical Properties
However, the presence of calcium tends to lower their maximum operating temperature [26], resulting in reduced efficiency of the thermoelectric power conversion process. Additionally, quaternary and quinary reciprocal mixtures (such as Li/Na/K//NO\({}_{2}\)/NO\({}_{3}\) and Ca/Li/Na/K//NO\({}_{2}\)/NO\({}_{3}\), respectively) have been proposed [31], yet their increased complexity does not correspond to a meaningful improvement in thermophysical properties. Recently, novel senary mixtures have been investigated, showing considerably lower melting temperatures, reaching values as low as 56 \({}^{\circ}\)C.  
Molten chloride salts can be used as an HTF and as an HSM for both sensible [32] and latent heat storage [33]. Other metal halides, such as fluorides and bromides, are expectably more expensive, and the former present particularly severe issues regarding corrosion [34]. Fluoride salts and container materials for TES applications are in the temperature range of 973 to 1400 K [35]. On the other hand, their freezing point is generally higher than that for molten nitrates [36], and they can be extremely corrosive and require costly containment materials [37; 38; 39]. Among the several possible formulations, chlorides of the first and second group are of particular interest, given their relatively low costs and high availability. For instance, the Na/K/Mg/ /Cl eutectic (with a molar percentage of around 47/23/30 for MgCl\({}_{2}\)/KCl/NaCl, respectively) presents a quite advantageous price, below 0.35 USD/kg, and a melting point of 385 \({}^{\circ}\)C [40]. [PERSON] et al. [40] reported an acceptable volatility below 800 \({}^{\circ}\)C, density and thermal conductivity comparable (but at higher temperatures) to solar salt, and a dynamic viscosity ranging from about 2 cP (at 800 \({}^{\circ}\)C) to around 5 cP near the freezing temperature. A problem using this mixture derives from the high hygroscopicity of MgCl\({}_{2}\), which leads to corrosion issues and also affects salt stability in the presence of air [41]. Similar considerations can be made for the Na/K/Ca//Cl additive eutectic (51.7/7.5/40.8 mol% for CaCl\({}_{2}\)/KCl/NaCl, respectively), with a price estimated as half of that of solar salt and a melting point of 497 \({}^{\circ}\)C [42]. Several mixtures of the Na/K/Zn//Cl system were evaluated by NREL [43]. They exhibit a significantly high density (up to 2.4 g/mL) and acceptable price (0.8 USD/kg), but present high volatility, especially above 700 \({}^{\circ}\)C.  
Molten carbonates can be potentially employed at higher temperatures than nitrates, but costly cations are to be used in order to obtain mixtures with an acceptable freezing point. The Na/K/Li//CO3 ternary eutectic has a melting temperature of 398 \({}^{\circ}\)C and an estimated cost of 2.5 USD/kg, and it was proposed to be used as an HTF at up to 650-700 \({}^{\circ}\)C [43; 44]. This temperature increase might increase the overall solar-to-electric conversion efficiency by 18.5% more with respect to systems operating at 565 \({}^{\circ}\)C with molten nitrates [45]. In addition to molten chlorides, molten carbonates present substantial issues regarding material compatibility, also depending on the gas atmosphere [39]. Mixed anion (reciprocal) mixtures have mainly been proposed for TES as a PCM [46; 47]. However, the solubility of carbonates and chlorides in molten nitrates is limited [21]: a recent scientific paper reports a sizable increase regarding chemical stability (i.e., the maximum operating temperature), if chlorides are added to nitrate mixtures [48]. Table 1 lists the names of the main MS mixtures; the type of mixture; and thermophysical properties, such as the mean value of density (\(\overline{\rho}\)) and specific heat (\(\overline{c_{p}}\)) in the operating temperatures, the melting temperature (\(T_{melt}\)), and the decomposition temperature (\(T_{deco}\)); and the references from which these properties were derived. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Mixture [-]** &