## 2 Techno-Economic Assessment of CSP Based on sCO\({}_{2}\)
### Economic Assessment
This is due to the need to enable condensation of the working fluid, which is only possible if the inlet temperature to the _Transcritical CO\({}_{2}\)_ pump is set to about 15 \({}^{\circ}\)C (strictly speaking, a temperature lower than the critical temperature of CO\({}_{2}\) would suffice to enable the implementation of a _Transcritical_ cycle. Nevertheless, performance-wise, this layout only makes sense if the saturation temperature in the condenser is substantially lower than the critical temperature. This is why a value of 15 \({}^{\circ}\)C at the pump inlet is usually adopted). This translates into ambient temperatures in the order of 8 \({}^{\circ}\)C if an evaporative cooling tower is considered or less than 5 \({}^{\circ}\)C if air coolers are used, values that are only rarely found in the arid (even desertic) locations where CSP plants are typically found. The dismissal of the _Transcritical CO\({}_{2}\)_ layout for these reasons narrows the selection of cost-effective \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Cycle** & \(\eta_{th}\) & \(\Delta T_{solar}\) & **OCC** & \(C_{TES}\) & \(C_{SF}\) & \(C_{TRR}\) & \(C_{PB}\) \\  & [\%] & [ \({}^{\circ}\)C] & [\({}^{\circ}\)K\(/\)\(kW_{inst}\)] & [kS] & [kS] & [kS] & [kS] \\ \hline SR & 45.8 & 290 & 6404 & 78,184 & 85,657 & 75,123 & 50,585 \\ TC & 48.3 & 290 & 5656 & 76,648 & 80,675 & 75,307 & 21,896 \\ PrC & 50.6 & 254 & 6515 & 80,145 & 76,373 & 79,697 & 58,835 \\ RC & 50.5 & 220 & 6867 & 91,640 & 76,547 & 90,000 & 52,498 \\ PC & 51.1 & 290 & 5907 & 70,230 & 75,603 & 70,945 & 50,568 \\ Al & 45.0 & 290 & 5943 & 79,403 & 87,074 & 75,778 & 26,227 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Thermo-economic assessment of different cycle layouts. Adapted from [10]. configurations to the Allam and Partial Cooling cycles, the layouts of which are presented in Figure 4. The main features of these cycles are:  
* Allam cycle: it is an extremely high recuperative cycle, an evolution of a standard Brayton cycle incorporating a three-step compression process with two compressors and a pump separated by an intercooler and a condenser respectively. Originally proposed by [PERSON] [24] for oxy-combustion applications, this layout has been adapted considering pure sCO\({}_{2}\) as working fluid [8]. * Partial Cooling cycles: this cycle derives directly from [PERSON]'s work [6], and it is an evolution of a Recompression cycle with the addition of a cooler and a pre-compressor before the flow-split. The most interesting features of the Partial Cooling cycle are its high specific work [25] and a very low sensitivity of global efficiency to deviations of pressure ratio from the optimum value [26]. The installation costs reported in Table 3 are presented graphically in Figure 3, showing that the high temperature heat exchanger (heat adder or heater) is the key contributor to installation costs, accounting for approximately half of the total OCC of the power block. This is due to the very high operating pressure and temperature of the sCO\({}_{2}\)-FLNaK heat exchanger which, as a consequence, requires the utilisation of special alloys like _Inconel 617_[10]. Figure 4: Layouts of the Allam and Partial Cooling cycles. Figure 3: Breakdown of the Overnight Capital Cost of the power block.