# Investigation of Different Storage Systems for Solar-Driven Organic Rankine Cycle
## 1 Introduction  
Solar concentrating power is an alternative way in order to produce clean electricity production at a reasonable cost and to face critical problems such as the increasing energy demand [1] and the global warming issue [2]. There are different solar concentrating systems that can be used in the solar thermal power units, such as parabolic trough collector (PTC), linear Fresnel reflector, solar dish concentrator, and solar central system (tower) [3]. Among them, PTC seems to be the most developed and mature technology for various scale applications [4]. The most common power block that is combined with PTC is the organic Rankine cycle (ORC) [5] because this cycle uses organic fluid with critical temperatures up to 300 \({}^{\circ}\)C, and so these fluids are ideal candidates for oil-based PTC, which operates up to 400 \({}^{\circ}\)C [6]. The solar-driven ORCs are common in the literature. [PERSON] et al. [7] optimized a PTC-based ORC and they put the emphasis on the working fluid investigation, as well as on the proper design of the separate components. Finally, they found that the system efficiency can be 8%. [PERSON] et al. [8] optimized a double-stage ORC driven by PTC with a sensible storage tank included in the system, and they found approximately 23% exergy efficiency. In another work, [PERSON] et al. [9] optimizeda regenerative ORC, which is fed by PTC and a sensible storage tank with thermal oil. They found that cyclohexane is the best working fluid, which leads to a yearly efficiency of 15% and a payback period of around nine years. Another dynamic approach with TRNSYS software was conducted by [PERSON] et al. [10] and also an overall 15% efficiency was found. In a comparative study between solar-ORC and photovoltaic, [PERSON] et al. [11] found that the solar-ORC leads to a lower levelized cost of electricity (LCOE) compared to photovoltaics, which is 0.19 USD/kWh. Another comparative study by [PERSON] et al. [12] showed that the combination of a concentrating thermal photovoltaic with ORC is about 18% more efficient than the use of a single concentrating thermal photovoltaic. An important advantage of the solar thermal ORC is the storage of thermal energy and not the storage of electricity as in photovoltaic systems, something that offers the possibility of not using batteries, which are associated with the high cost and environmental issues. In the literature, there are various storage techniques. The most usual is the use of a sensible storage tank that can store sensible heat by increasing the temperature of the stored thermal oil [6, 9]. An alternative choice is the use of two tanks with molten salts in order to store thermal energy in an efficient way. [PERSON] et al. [13] found that this technique enhances the yearly performance by about 19%. In another approach, [PERSON] et al. [14] found that the use of a single thermocline tank has a similar performance to a two-tank design but it has a 30% lower investment cost. The use of latent heat storage systems is also common in the literature in order to keep the system temperature at lower levels and to reduce the system's thermal losses. [PERSON] et al. [15] found that the use of phase change material (PCM) for storage in a solar-ORC system is able to make the system operate for 80% of the day. [PERSON] et al. [16] found that the discharging efficiency of a solar PCM-PTC system is about 83%. In another work, [PERSON] et al. [17] stated that the use of a direct system is more efficient than an indirect system in a solar PCM-PTC unit. Moreover, [PERSON] et al. [18] calculated that the use of latent storage in a system with the ORC for heat and power production can lead to an 18% performance enhancement compared to other conventional systems. [PERSON] et al. [19] compared the use of PCM and water storage tank for an ORC, and they found a 20% performance improvement with PCM design. Another interesting idea is the use of pumped thermal energy storage, which was studied by [PERSON] et al. [20].