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Thermal Energy Storage for Solar Powered ORC Engines usually, the flash process is adopted only when the storage is being discharged [27]. Referring to figure 4.1, the “flashing cycle” (FC) of the working fluid in the temperature-entropy diagram is identified by the state points a, b, c, d (with qd = 1), e, f (1, 2, 3, 3vs , 4 for water). When evaluated for the exploitation of thermal energy sources whose thermal capacity can be assumed as infinite, such power cycles feature an inherently lower efficiency compared to the corresponding evaporative cycle operating between the same maximum and minimum temperature (state points a, b, c, g, h, f , and 1, 2, 5, 6 for water) [40]. However, if the working fluids is an organic compound, it can be shown that the efficiency penalty affecting the flashing cycle may be comparatively low. A detailed treatment is reported in A.1. Flashing ORC power systems for waste-heat recovery applications have been recently investigated by Ho and colleagues [53]. The flashing cycle boasts notable benefits in case of a solar ORC power system with thermal storage: i) it avoids phase transition in the SF, with major advantages [46, 54, 55]; ii) it decouples the SF and the ORC power block by means of a suitable direct thermal storage system, see figure 4.4a. A minor efficiency reduction can thus be accepted, in view of the substantial simplification it allows for, both in terms of plant layout and operation. 4.5.3 Flashing the Organic Vapor Down to Saturated Conditions A further simplification of the plant configuration derives from the possibility of reaching complete vaporization of the working fluid by flashing (see sec. 4.2, point Ia). In this way several components become redundant, namely the flashing vessel and the liquid drain circuit. More details are provided in A.2. To the authors’ knowledge, ORC power systems working according to the flashing cycle principle, whereby the working fluid is throttled down to saturated vapour conditions before entering the turbine, refer to figure 4.4b, have not been considered before, thus this concept is named here complete flashing cycle (CFC). 4.5.4 Design Analysis Results The steady state modeling of the system is performed with an in-house code implemented in a well known computer language for technical computing [56], coupled with an in-house library for the accurate estimation of the thermophysical properties of the fluids [21]. The calculated performance is reported in table 4.2, while table 4.3 shows the thermodynamic properties of the state points of the thermodynamic cycle. Notwithstanding the selected high design value for Tcond, the calculated efficiency of the ORC power system exceeds 25% which, combined with the efficiency of the SF, yields a global efficiency in design conditions close to 18%. This value can be compared to the measured values of recently-built state-of-the-art CSP plants. These steam power plants are much larger, and adopt an indirect storage system with synthetic oil as HTF, and their efficiency is of the order of 22% [57]. Even if no index of annual performance has been estimated yet, ORC power systems are char- acterized by excellent off-design performance. This characteristic can partially overcome the lower design efficiency in a highly dynamic application such as CSP [46]. The calculated values of equivalent electrical energy density (EEED) storage are lower than those characterizing traditional TES solutions, and this holds for all the considered working fluids (see fig. 8). The proposed system approaches, for the EEED, the limiting value of 6.2 [kWhE m-3 ], ST see table 4.2, assuming that the storage vessel delivers its full energy content without any variation in the discharged fluid properties (conditions corresponding to state c). Thermal losses, as well as exergy losses due to deterioration of the stratification [58] are thus neglected: such simplifications 101PDF Image | New Concepts FOR Organic Rankine Cycle Power Systems
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