# A Novel Hybrid CSP-PV Power Plant Based on Brayton Supercritical CO\({}_{2}\) Thermal Machines
## 4 Discussion and Conclusions  
The integration of a PV plant and a CSP through a high-temperature heat pump has been analysed. This configuration introduces a novel approach by employing a heat pump as the integration mechanism instead of conventional electrical resistors found in existing CSP-PV plants. For the proposed system, a recompression Brayton supercritical CO\({}_{2}\) cycle is used as the heat engine, whereas a reverse recuperative Brayton cycle, also operating with supercritical CO\({}_{2}\), is employed for the heat pump. The thermal energy storage system incorporates a two-tank configuration utilizing solar salt as the medium for storing sensible heat. The solar field consists of parabolic trough collectors, which serve as the cold source for the heat pump. It is important to highlight that the PV plant exclusively drives the heat pump, with no direct injection of PV power into the grid. The heat pump allows for storing high-grade thermal energy in the molten salts using a parabolic trough collectors field, as if it were produced in a heliostat solar field. Moreover, the integration through the heat pump reduces exergy losses in the storage system. According to Equation (3), and considering the average entropic temperatures [20] of the PTC field (616.9 K) and the molten salts (766.3 K), the exergy loss reduction (calculated in Equation (15)) for the storage system amounts to 45.24%. \[\frac{I_{conv}\ -\ I_{nov}}{I_{conv}}=\ \frac{COP}{COP_{max}}=\frac{2.32}{5.13}=45.24\% \tag{15}\]  
The high-temperature heat pump operates within the nearly ideal gas region as depicted in Figure 4. This characteristic significantly reduces the technological risk associated with the reverse recuperated Brayton cycle. On the other hand, the heat engine, which \begin{table}
\begin{tabular}{c c c} \hline \hline
**Component** & **Base Parameter** & **Specific Cost (FCl)** \\ \hline HP [USD2020/kWt] & Heat released (225 MW) & 719.44 \\ HE [USD2020/kWe] & Power produced (50 MW) & 2340 \\ TES [USD2020/kWht] & Thermal energy stored (1351 MWh) & 35.71 \\ PV field [USD2020/kWp] & Peak power (97 MW) & 501.3 \\ PTC field [USD2020/m\({}^{2}\)] & Solar aperture area (21.7 hm\({}^{2}\)) & 190.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Specific costs of the main components. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Cycle** & \multicolumn{2}{c}{**On-Site**} \\  & **Cost [USD M2020]** \\ \hline HP & 50.3 \\ HE & 32.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Molten salt loops, PTC and PV costs. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Component** & **Energy Stored** & **Salt Inventory** & **Direct Costs** \\  & **[MWh]** & **[ton]** & **[USD M2020]** \\ \hline TES & 1351 & 17,245 & 38.6 \\ PV field & — & — & 38.9 \\ PTC Field & — & — & 32.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Molten salt loops, PTC and PV costs. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Component** & **Base Parameter** & **Specific Cost (FCl)** \\ \hline HP [USD2020/kWt] & Heat released (225 MW) & 719.44 \\ HE [USD2020/kWe] & Power produced (50 MW) & 2340 \\ TES [USD2020/kWh] & Thermal energy stored (1351 MWh) & 35.71 \\ PV field [USD2020/kWp] & Peak power (97 MW) & 501.3 \\ PTC field [USD2020/m\({}^{2}\)] & Solar aperture area (21.7 hm\({}^{2}\)) & 190.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Specific costs of the main components. utilizes the recompression Brayton supercritical CO\({}_{2}\) cycle, has limited commercial experience. In early demonstration plants, it could be replaced by a conventional Rankine cycle, although losing efficiency. The proposed S-CO2 cycle achieves an efficiency of 44.4%, surpassing the less than 40% observed in current CSP plants [19]. The performance of hybrid CSP-PV plants has been analysed under two different scenarios: design conditions (Figures 6a and 7a) and full storage (Figures 6b and 7b). In the former, the conventional scheme injects 47% more electricity into the grid during a typical day compared to the novel scheme.