## 2 Techno-Economic Assessment of CSP Based on sCO\({}_{2}\)  
### Operating Conditions  
The initial works on supercritical power cycles acknowledged the superior thermal performance of this technology, with respect to standard steam cycles, when turbine inlet temperatures are higher than 600/650 \({}^{\circ}\)C [6]. Below this value, the little gain (if any) brought about by sCO\({}_{2}\) cycles does not pay off the lower reliability and higher cost of a still less mature technology. Unfortunately, even at the lower end of the temperature range where sCO\({}_{2}\) is substantially better than steam, the operating temperatures that are needed are far from the values currently achieved by commercial solar technologies, as recently confirmed by a report issued by the SCARABEUS consortium [12]. Two major hurdles are identified. Receiver technologies able to achieve temperatures in excess of 700 \({}^{\circ}\)C are available but they have not reached the commercial stage yet [13]. This also applies to heat transfer fluids able to harvest this high-temperature thermal energy available in the receiver, in order to feed both the power block and Thermal Energy Storage system, which are currently available but not widely commercialised [14]. This availability of high temperature heat transfer fluids is assessed in Table 1 where the characteristics of different intermediate to high-temperature molten salts are listed. Amongst them, this work makes use of FLiNaK for the sake of continuity with past works by the authors. Thermodynamic-wise, this is a very interesting option even though the very high cost of this salt compromises the economic feasibility of its practical implementation. Still, it is assumed that adopting this fluid yields a safe (conservative) estimate of the cost of electricity associated to this technology. The thermodynamic potential of sCO\({}_{2}\) power cycles for different boundary conditions and applications was assessed by the authors in a previous work [8], based on an earlier, thorough \begin{table}
\begin{tabular}{c c c c c c} \hline
**Salt** & **Composition [\%]** & **Freezing Point [ \({}^{\circ}\)C]** & **Boiling Point [ \({}^{\circ}\)C]** & **Price [5/kg]** & **Price [8/4 wh\({}_{\text{d}}\)] \({}^{\ast}\)** \\ \hline NaNO\({}_{\text{3}}\)/KNO\({}_{\text{3}}\) (Solar Salt) [15] & 60–40 & 220 & 600 & 0.8 & 10 \\ LiF-NaK-FF (FLNaK) [14] & 46.5–11.5–42 & 454 & 1570 & 8.6 [16] & 54.8 \\ Li\({}_{\text{3}}\)/C\({}_{\text{3}}\)/Na\({}_{\text{3}}\)/K\({}_{\text{3}}\)/K\({}_{\text{3}}\)/K\({}_{\text{3}}\)/K\({}_{\text{3}}\) & 397 & 662 & 2.5 [15] & 26.1 \\ Na\({}_{\text{2}}\)/CO\({}_{\text{3}}\)/NaOH [17] & 19–81 & 284 & 714 & - & 2.3 \\ MgCl\({}_{\text{3}}\)/K\({}_{\text{3}}\)/K\({}_{\text{3}}\)/K\({}_{\text{3}}\) & 37.5–62.5 & 426 & 1412 & 0.35 & 5 \\ ZnCl-NaK-KCl [15] & 69–7-24 & 204 & 732 & 0.8 & 18 \\ \hline \end{tabular} \({}^{\ast}\) Values taken from [15] are obtained for \(\Delta T_{salt}=200\)\({}^{\circ}\)C. For the FLiNaK, authors calculated this price for \(\Delta T_{salt}=290\)\({}^{\circ}\)C. Values taken from [17] are estimated with market price of raw components in Q1 2015, for a maximum cycle temperature of 650 \({}^{\circ}\)C.  
\end{table} 
Table 1: Comparison between different molten salts for intermediate-high temperature Concentrated Solar Power (CSP) plants. literature review [4]. In a second step, turbine inlet temperature was set to 750 \({}^{\circ}\)C, based on several works in literature [18; 19] and on private communications with Abengoa (a SCARABEUS partner), and the assessment of thermal performance was complemented by an economic analysis with the aim to calculate the capital cost of a representative CSP plant using each cycle layout [10]. The main specifications of this reference plant are presented in Table 2.  
The thermo-economic performance of the reference power plant in Table 2 considering different cycle layouts is presented in Figure 2 for a pressure range deemed affordable in the medium term (15 to 40 MPa).