## 4 Results and Discussion  
The most important results of the analysis are presented in this section. Additional data on the simulation of the plant can be found in the Appendix A of the paper. The exergy efficiencies obtained are shown in Figure 2 and the components with the highest thermodynamic inefficiencies in Figure 3. The most important inefficiencies in the proposed plant were found in the boiler, since the combustion process constitutes one of the main sources of irreversibilities, i.e., exergy destruction. The relative irreversibilities could be reduced through the preheating of the incoming flows, which in this case were assumed to have the ambient temperature. Another source of inefficiencies within the plant was the cooling system, composed of the condenser and the cooling tower. As a dissipative component, the cooling system does not generate a useful product, but it facilitates the operation of the overall plant. Thus, the destroyed exergy here should be kept to a minimum, without preventing the efficient operation of the rest of the plant components. The solar power tower also resulted in a relatively low efficiency. However, it is expected that working fluids will be further developed in the near future and the current technology will progress, allowing for higher operating temperatures [53]. Heat exchange between fluids of significantly different temperatures also led to high exergy destruction. This is the reason that some heat exchangers resulted in relatively low exergy efficiencies. The overall exergy efficiency of the plant was found to be 35.8%, with a total exergy destruction of 917.4 MW. This efficiency was calculated considering only the coal as the fuel and not including the solar heat as a power input. The efficiency was thus higher than that of a conventional coal power  
Figure 3: Exergy destruction share of the plant components. Figure 2: Exergetic efficiencies of the plant components. plant, as the necessary fuel to achieve the same power output was lower, given that the power was obtained by burning coal and from solar energy. If we also accounted for the solar heat as a fuel, the efficiency of the plant would drop down to 30.7%. This result is comparable to previous studies of solar-coal hybrid power plants that have reported efficiencies in the order of 30% [56]. The calculated efficiency offers an improvement, when compared to solar power tower plants that operate with efficiencies of around 25% [19]. When we compared the operation of the hybrid power plant during the day with its operation during the night (fired only by coal), we obtained the data presented in Table 2. The efficiency of the plant at night was found to be 34.2%. This value is within the range of common efficiencies reported for coal power plants [41]. The reason for this lower efficiency during night hours is the lack of solar energy. To achieve the desired power output in this case, the plant was solely dependent on coal. The use of coal was reduced by 4.6% when solar energy was used, correspondingly reducing the generated CO\({}_{2}\). The total CO\({}_{2}\) emissions per day of the proposed hybrid plant were calculated to be 18,539.7 tons, while the emissions generated from an analogous coal-fired power plant were 19,430.9 tons. In other words, with the proposed hybrid power plant cam achieve an annual reduction of more than 325,273 tons of CO\({}_{2}\). Hybrid plants allow for higher capacity factors, when compared to renewable plants. For example, when solar irradiation is not available (night hours, cloudy days), the hybrid power plant is able to continue its operation by using a higher fuel mass flow (coal, in this case) and meet demand requirements in a more flexible manner. The TCI of the plant was found to be 8050.32 $/kW. This cost agrees with investment values reported for solar power tower plants, fluctuating between 6100 and 8100 $/kW [57]. The main expense of solar technology, when compared to conventional coal power plants, is the relatively high purchased equipment cost.