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Romei, Alessandro
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Romei
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Alessandro
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Alessandro Romei
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ASSESSMENT OF TRILATERAL ORGANIC RANKINE CYCLE FOR SOLAR APPLICATIONS WITH INNOVATIVE TURBOEXPANDER CONCEPTRomei, Alessandro; Giostri, Andrea; Spinelli, AndreaLow-temperature solar collectors coupled with thermal energy storage can enable stable and carbonfree energy production. The key issue is to efficiently convert solar energy into electricity without resorting to complex architecture that may hinder the technical and economic feasibility of the entire system. In this work, we propose a fully integrated organic Rankine cycle (ORC) with the solar field and energy storage, targeting 200 kW. The system consists of a single circuit of the selected organic fluid that passes through the solar collectors, the thermocline thermal energy storage, and the ORC unit. The organic fluid remains liquid inside the solar field and the thermal energy storage, leading to a trilateral thermodynamic cycle. Leveraging the thermodynamic behavior of molecularly complex fluids, the radial-inflow turbine expands from saturated liquid to superheated vapor. Following the idea of White (App. Therm. Eng., 192 (2021), 116852), the nozzle cascade expands the two-phase flow mixtures and delivers superheated vapor to the rotating cascade. In this way, the rotor processes dry organic vapors without incurring mechanical damage or suffering from additional losses due to twophase interactions. Three maximum temperatures are investigated, and each of them entails a different fluid selection to have the liquid-to-vapor expansion through the stator. Preliminary designs of the radial-inflow turbines are carried out by employing a meanline code, validated for single-phase organicfluid flows, revealing that feasible designs can be obtained. Based on these results, the proposed technology appears feasible and promising on the technical ground.- SHAPE OPTIMIZATION OF A sCO2 CENTRIFUGAL COMPRESSOR STAGERomei, Alessandro; Gaetani, Paolo; Persico, GiacomoThe design of a centrifugal compressor for supercritical carbon dioxide (sCO2) power cycle must account for non-ideal gas effects and the possible occurrence of two-phase flows. Shape optimization techniques combined with computational fluid-dynamic (CFD) simulations can produce optimized designs while inherently coping with the peculiar flow characteristics near the thermodynamic critical point. This work presents the first shape-optimization attempt of such non-conventional compressors. The compressor stage includes the impeller and the vaneless diffuser and starts the compression close to the critical point. The impeller blade angle distributions and meridional channel are parameterized with Bezier control points, which grant a local shape control within the optimization routine. The pinch of the vaneless diffuser is optimized as well. The validated CFD solver accounts for both non-ideal effects and two-phase homogeneous flows under the assumption of thermodynamic equilibrium and barotropic fluid. The constrained optimization problem is solved with genetic algorithms. To reduce the computational cost, surrogates for the objective function and constraints are trained over a limited number of CFD results. The surrogate accuracy is improved throughout the optimization process by adding optimal stage geometries to the initial training samples. The optimized geometry shows an appreciable efficiency increase (0.7 percentage points) while delivering the design pressure ratio. Although performing better in the design condition, the operating range of the compressor is altered by optimization. This finding leaves the door open for future optimizations that include both design and off-design operating points in the definition of the objective function and constraints.
