# Thermoeconomic Analysis of Concentrated Solar Power Plants Based on Supercritical Power Cycles
## 1 Introduction  
The Levelised Cost of Electricity (LCOE) generated by Solar Thermal Power Plants (STPPs) has been reduced by over 50% in the last decade, primarily driven by economies of scale and improvements in the operation and maintenance (O&M) of the plants [1]. However, further research efforts are needed to make the energy provided by STPPs competitive, dispatchable, and safe. In this regard, the Gen3 program [2] proposes the use of supercritical CO\({}_{2}\) (sCO\({}_{2}\)) power cycles coupled to solar central receiver systems as a way to increase the overall efficiency; furthermore, four different pathways are identified, depending on the working fluid in the solar receiver. This study focuses on one of these research lines, proposing the use of pressurised CO\({}_{2}\) as Heat Transfer Fluid (HTF) in the solar receiver coupled to a sCO\({}_{2}\) power cycle, as shown in Figure 1.  
As shown in Figure 1, the STPP under study exhibits an indirect coupling. This is due to the significant differences in operating conditions between the HTF in the solar field (pressurised CO2) and the power cycle (supercritical CO2). The primary heat exchanger should be a Printed Circuit Heat Exchanger [3], as this type of heat exchanger performs well under high pressure difference conditions (250 bar in the supercritical cycle compared to 55 bar in the solar receiver). In STPP schemes with indirect coupling to sCO2 cycles, the working fluid in the solar receiver can be molten salts, with special attention to the new generation of advanced ternary salts [4], a fluidised bed of particles [5], pressurised gas [6], and, finally, liquid metals [7]. Direct coupling STPP schemes are less frequently mentioned in technical literature; in these layouts, the working fluid in the solar receiver is directly supercritical CO2. Although there are initial studies based on tubular external receivers [8], the large thickness required in the tubes is a challenge to the thermal performance of the receiver. Therefore, an option currently under investigation is the use of receivers based on compact structures [9], and this will be further analysed in this section. Supercritical power cycles have been extensively studied, mainly for nuclear applications [10] and more recently for solar applications [11]. Within solar applications, notable research includes the study presented by [PERSON] [12], in which different recompression sCO2 cycles are selected and compared based on various criteria, such as cycle efficiency, complexity in terms of additional elements compared to the simplest cycle, and, finally, the sCO2 temperature increase in the primary heat exchanger as it determines the volume of the working fluid to be heated in the solar receiver. A later work [13] specifically compares the conventional recompression cycle with the partial-cooling cycle, with the latter presenting very competitive advantages: higher efficiency, less complexity, and, finally, a significant increase in the sCO2 temperature in the primary heat exchanger, which leads to a smaller solar field with a reduced investment. For this reason, as shown in Figure 1, the chosen power cycle is the partial cooling configuration. Regarding the solar receiver, there are also different designs to work with pressurised gases or supercritical fluids. This study focuses on microchannel receivers, whose compact structures have been extensively studied for working with these fluids [6; 9], highlighting two prototypes. The first prototype has been developed in the Clean Energy Research Center [14] and consists of a 3 \(MW_{th}\) cavity receiver. The second has been developed by the National Renewable Energy Laboratory [15] and proposes a 10 MWe external receiver. In both cases, one of the major challenges is the thermal gradient across the thickness of the compact structure. The materials used for these panels, such as stainless steel and nickel or titanium alloys, can withstand high temperatures, but they have the disadvantage of low thermal conductivity [16].