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Energies 2017, 10, 413 14 of 26 • Fluid decomposition temperature: Organic fluids suffer from chemical decomposition at high temperature [89]. For this reason, it is important that the temperature of the heat source in the evaporator does not overcome the decomposition temperature of the working fluid. • Molecular weight: Fluids with high molecular weight allow for a smaller rotational speed of the expander [90], which in turn affects positively its efficiency and typically diminishes the cost of the generator. 4. Expander Selection The choice of the appropriate expander for small power ratings represents an unresolved problem. Literature reviews on small-scale expanders propose different configurations [86]. The options available present both pros and cons that complicate the selection of a single “ideal” expander. As is clear from Table 2, ORC companies are trying to develop their expanders in house. As testified by Qiu et al. [86], the commercial market lacks appropriate expanders for small-scale applications. The expander is certainly the ORC component for which the highest drop in performance arises when downsizing ORC plants from the MW to the kW power range. Efficiencies reported in the literature [17] show values below 70%. Expanders can be categorized into two main groups: volumetric and turbo expanders. Scroll, screw and vane expanders are the most common machines among the volumetric ones [91–93]. Li et al. [94] recently proposed a piston expander with the aim of reducing the component complexity and cost. A detailed explanation of the available mathematical models goes beyond the scope of this work. For an in-depth analysis, the interested readers can refer to the work of Imran et al. [17] who reviewed the published mathematical models for the design of volumetric expanders. Radial inflow, radial outflow and axial turbines are the most common turbo-machines in the literature [95–97]. As opposed to volumetric machines, turbo-machines are convenient in the high power output range, while they become inefficient for low power production. The reason for this is mainly related to very high rotational speeds leading to bearing failures. Imran et al. [17] suggest that their high rotational speed and their cost are the reasons for which radial inflow turbines are not suitable for power ratings below 50 kW. Qiu et al. [86] state that turbine expanders are used for power outputs greater than 50 kW, because the efficiency substantially drops below this value. Models for the design of turbines are available in the literature. Rahbar et al. [98] and Fiaschi et al. [99] proposed detailed models for radial turbines. Palumbo et al. [100] published the mathematical model of a radial outflow turbine. Jubori et al. [101] and Lazzaretto and Manente [102] derived a model for the design of axial turbines in ORC applications. However, the aforementioned studies have not been validated against experimental data. Kang [103] presented a model coupled to an experiment of a radial turbine for ORC applications using R 245fa as the working fluid. The maximum efficiency achieved during the experimental campaign was 78.7%, and the maximum power output was 32.7 kW. Generally, volumetric expanders are considered when the power output is low. In fact, when the power output exceeds a certain level, the performance lowers and the size increases exponentially, increasing its cost and reducing its practicality. Leibowitz et al. [104] in their analysis on cost-effective small-scale ORC systems identify that in the range of 20–50 kW, twin screw expanders are the most promising. They also state that this kind of volumetric expander offers efficiencies of 70% during low speed operation. Such small-scale ORC systems can be installed at a cost of about 1500–2000 e/kW. Kenneth and Nichols [105] presented a graph that provides a guideline to select the proper expander based on the specific speed (Ns) and the specific diameter (Ds); see Figure 7. In Figure 7, the parameters Ns and Ds are calculated as a function of the volumetric flow rate (V3) and the adiabatic expander enthalpy drop (Had). Different types of expanders present optimal operation performance in certain ranges of the non-dimensional parameters Ns and Ds. For instance, piston expanders perform well when designed for low values of a specific speed (0.01–0.1), while the optimal Ns for radial turbines is in the range of 30–300.PDF Image | Small Scale Organic Rankine Cycle (ORC)
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