# Financial Optimization of a Solar-Driven Organic Rankine Cycle
## 1 Introduction  
Solar energy utilization is vital for facing important issues of our society such as global warming, fossil fuel depletion and increasing energy needs [1; 2; 3]. Concentrating solar power (CSP) is an important weapon for producing electricity which is the most valuable energy demand because it can cover numerous needs. Thus, a lot of research has been focused on power production with the use of solar concentrating systems [4; 5]. Parabolic trough solar collector (PTC) is the most mature solar concentrating technology [6] and thus it is usually selected in CSP. Moreover, this technology has a reasonable cost, especially in systems with great scale. One of the most usual power blocks which can be coupled to the CSP is the Rankine cycle. For PTC, the organic Rankine cycle (ORC) is usually selected due to the best compatibility between the operating temperature levels of the PTC and the ORC [7; 8]. In the literature, many studies examine the solar-driven ORC with PTC. [PERSON] et al. [9] performed a study about a system with PTC and ORC that operates with R245 fa and presented system efficiency up to 8%. [PERSON] et al. [10] examined an ORC that is driven by PTC and geothermal energy. This system presents a 9.4% system efficiency and levelized cost of electricity (LCOE) from 0.145 up to 0.280 \(\upcels\) kWh\({}^{-1}\). [PERSON] and [PERSON] [11] examined the combinations of PTC with waste heat in an ORC. They found that the best working fluid is toluene and the system efficiency can range from 11.6% up to 19.7%. [PERSON] et al. [12] investigated a system with PTC coupled to a regenerative ORC. They found that the system efficiency is close to 15%, and they studied the system parametrically. The main parameters of their work were the storage tank volume and the mass flow rate of the heat transfer fluid. [PERSON] and [PERSON] [13] compared the use of PTC and linear Fresnel reflectors (LFR) coupled to ORC. They studied different working fluids and they found the most suitable working fluids to be R113, isohexane, hexane, benzene and cyclohexane. They concluded that the system with PTC has higher efficiency, which is close to 20% and the LCOE is about 0.4 \(\upmu\)kWh\({}^{-1}\). At this point, it is important to explain that the LCOE is a critical parameter that shows how the real is the cost of the produced electricity. In the cases that the LCOE is lower than the electricity price, then the investment is viable. In cases with high LCOE, there is a need for a subsidy for creating a viable investment. [PERSON] et al. [14] studied the use of PTC in order to feed an ORC in the Greek climate. They studied four typical days and they extended the results for all the year. They found that there is a need for 25,000 m\({}^{2}\) of PTC in order to feed a system with nominal power at 1 MW. The optimum ratio of the collecting area to storage tank volume was found at 80 m\({}^{2}\)/m\({}^{3}\), and, for this case, the system efficiency is 13.46% and the payback period is at 9 years. [PERSON] et al. [15] examined a PTC with a V-shape cavity coupled to an ORC. They found that this unit is able to lead to a payback period lower than 9 years and to a system efficiency of about 25% in optimal design. [PERSON] et al. [16] investigated a system with PTC and ORC and they gave the emphasis in the storage system. They found that the toluene is the best working fluid which leads to an LCOE at 0.17 \(\upmu\)kWh\({}^{-1}\). [PERSON] et al. [17] performed a work about solar-driven ORC with different storage systems. They concluded that the use of storage is important and it has to be used in these systems. Moreover, their study case was for a system with 100 kW nominal power and 18% system efficiency. [PERSON] and [PERSON] [18] studied the idea of using a nanofluids-based PTC in a solar-ORC. They found that toluene is the best organic fluid, while the thermal oil/CuO is the best nanofluid. The nanofluid-based system presents 20.1% system efficiency, which is 1.75% higher than the system with pure thermal oil in the solar field.