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Energies 2020, 13, 370 7 of 31 the elementary thermodynamic cycles, which goes in the direction of improving the thermal efficiency of both the Brayton and Rankine cycles composing the overall system. However, the improvement of the thermal efficiency does not suffice, and should be accompanied by the enhanced capability of heat recovery from the hot streams available within the system. This point was addressed by Morandin et al. [33], who analyzed the steam injected gas turbine (STIG) cycle as the combination of two partially superimposed cycles, namely an air Brayton cycle and a steam Rankine cycle. The authors first calculated the thermal efficiency of each elementary cycle disregarding any thermal interaction between them, as if the heat input to the elementary steam cycle was entirely provided by a fictitious external heat source. In this way, they could introduce the “baseline” thermal efficiency concept, as the mass flow rate weighted average of the thermal efficiencies of the two elementary cycles. The real thermal efficiency of the STIG cycle was then calculated as the sum of the baseline thermal efficiency and an efficiency gain (mainly) related to the internal heat recovery. The authors showed that the main effect in the evolution towards more advanced STIG plant configurations featuring high and low pressure steam turbines is the increase of the baseline thermal efficiency. More recently Lazzaretto et al. [34] developed an automatic procedure for the synthesis of energy system configurations (called “Synthsep”), which consists in two steps. In the first step, all meaningful system configurations are generated by combining two or more elementary Rankine cycles by sharing (partially or totally) one or more of their thermodynamic processes using a comprehensive and rigorous set of rules. In the second step, a two-level optimization is performed to select the system configuration and the associated design parameters leading to the highest value of the objective function. The authors applied this method for the synthesis/design optimization of a dual pressure ORC system using iC4 and R245fa. They found that the optimum system layout is obtained when the two elementary Rankine cycles have shared expansion and cooling processes. While the algorithm has been developed so far by Toffolo [35] for Rankine cycle-based energy systems only, yet this general approach could be extended to Brayton cycle-based energy systems as well. These studies show that the decomposition of advanced power cycles in elementary thermodynamic cycles could be a useful tool not only to identify the building blocks of the system but also for its preliminary assessment. Indeed, the calculation of the thermal efficiency of each elementary cycle could provide a first indication about the overall performance of the system, though the latter is highly affected by the thermal interactions between elementary cycles. 1.6. Aim and Main Novelty of This Work This work deals with a systematic analysis and thermodynamic optimization of three novel s-CO2 power cycles for waste heat recovery, which are selected as the most promising due to their high performance and low number of components. They are referred to in this work as single flow split with dual expansion, partial heating, and dual recuperated cycles, even though they might be known to the reader under different names (cascade, preheating, and dual stage or split cycles, respectively). The thermodynamic performance of the power cycles is optimized using the power output as the objective function and a coherent set of assumptions to allow for a fair comparison. The heat source is assumed available at an inlet temperature of 600 ◦C, which is considered as a benchmark temperature for recovery of high temperature waste heat in the industrial sector (steel industry, etc.) as well as in the electricity sector (gas turbines, fuel cells, etc.). Each advanced layout is decomposed into two elementary thermodynamic cycles, which are partially superimposed. The two cycles join after the mixer, follow a common path, and separate after the splitter. To the authors’ knowledge, such a decomposition of advanced s-CO2 power cycle layouts into elementary Brayton cycles has never been explored in the literature. An attempt is made to identify in them the features of a topping cycle, which receives heat from the waste heat source, and a bottoming cycle, fed by the exhaust of the topping cycle. The thermal efficiency of each elementary cycle is calculated to quantify the separate contribution of each elementary cycle composing the system to the output power in the cascade utilization of the waste heat.PDF Image | Novel Supercritical CO2 Power Cycles for Waste Heat Recovery
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