# Dynamic Investigation of a Solar-Driven Brayton Cycle with Supercritical CO2
## 1 Introduction  
The energy problem is one of the key issues facing humanity. The rapid development of technology in combination with the increase in world population has led to an increase in the demand for energy production [1, 2]. In the previous decades, energy production was based on fossil fuels, a strategy that caused many environmental problems such as climate change, global warming and air pollution [3, 4]. Since energy is a fundamental factor in humanity's growth, alternative energy sources are needed [5]. Renewable energy sources (RES) can perform a significant role in tackling the energy issue. Among all RES, solar energy is one of the most promising to encounter environmental and sustainability issues [6]. Solar energy is already being utilized for household needs [7]. However, solar energy is also a very promising source for power plants, especially concentrated solar power (CSP), with many CSP plants already operating or being under construction or investigation [8]. CSP plants' heat source is solar irradiance due to sunlight. Thus, this kind of power plant can be mostly found in areas with high direct normal irradiance (DNI) [9, 10]. The concentrated irradiance is then converted to high thermal energy given to a heat transfer fluid (HTF). This energy can be utilized either directly or indirectly, where a CSP plant is integrated with a thermodynamic cycle for electricity production [11]. Among the CSP technologies, the most well-known and mature ones are the parabolic trough collector (PTC), the linear Fresnel reflector (LFR), the parabolic dish collector (PDC) and the solarpower tower (STP) [8]. The first two categories are considered linear focus applications, whereas the other two are considered point focus [9]. The most widely used technology is PTC, and the second is the STP technology [8]. These two are the most economically sustainable among the CSP categories [12]. Since STP is a single point focusing CSP, higher temperatures can be achieved in comparison to PTC [13]. STP technology can achieve temperatures greater than 1000 \({}^{\circ}\)C [14]. The main components of an STP are the heliostat field, the central receiver of the solar tower, the thermal energy storage (TES) and the power production configuration [13]. The heliostats reflect a percentage of DNI towards the central receiver. Thus, the temperature on the surface of the central receiver is increased, and, as a result, the temperature of the HTF is increased as well. Afterward, the HTF transfers its heat to a working fluid via a heat exchanger, which is used to produce power in a thermodynamic cycle [15; 16]. For this application, as HTF, molten salt is preferred since it can operate at higher temperatures [17]. In the literature, there are many studies on CSP mainly focusing on the technology of the solar collector, the temperature, the HTF and the electricity production [10]. Some studies focus on calculating the DNI based on the geometry of the solar field. [PERSON] et al. [9] have used solar angle calculations, whereas [PERSON] et al. [18] have used the ray tracing process of sunlight. A new method for designing the heliostat field proposed by [PERSON] et al. [19] saves time on the design and optimization process of the field. The annual mean optical efficiency was improved by 0.09%. Many studies focus on the calculations of central's receiver heat losses. For example, [PERSON] et al. [20] have presented a numerical simulation using computational fluid dynamics (CFD) to calculate the total thermal losses for the central receiver, and the proposed model predicted the convection and total heat losses of the central receiver with an average absolute deviation of 11.4% and 5.9%, respectively. In addition, [PERSON] et al. [21] have proposed a mathematical model based on algebraic equations. Furthermore, [PERSON] et al. [22] have proposed a numerical simulation for the prediction of the characteristics of the central receiver.