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Energies 2020, 13, 370 3 of 31 recovery efficiency of 21.3% reached by the single recuperated cycle exceeds by 2% that achieved by the recompression and pre-compression layouts. The authors claimed that the lower CO2 temperatures at the inlet of the Heater of the single recuperated cycle imply, on the one hand, a lower thermal efficiency but, on the other hand, a more effective heat extraction from the waste gas. Martinez et al. [11] evaluated the same three traditional cycles for WHR from a gas turbine having an exhaust temperature of 600 ◦C. They found that the single recuperated cycle improves by a few percentage points the power output of the recompression cycle and by several percentage points that of the pre-compression cycle. This is the outcome of the higher heat recovery effectiveness (called the WHR efficiency), which prevails over the lower cycle thermal efficiency. The majority of these studies share the main result that the single recuperated cycle is the best choice among the traditional layouts for WHR applications. Contextually, they point out that the application of traditional layouts for WHR generates a mismatch between the heat source and power cycle at the higher temperature level, and the inability to acquire heat at the lower temperature level, which results in significant exergy destruction and exergy losses, respectively. 1.3. Cascade of Two Traditional s-CO2 Power Cycles for WHR The combination of two traditional cycles in a cascaded system is considered by many authors as the best solution to solve the two aforementioned main limitations of the single cycle. In the cascaded system, the higher temperature waste heat usually feeds the recompression cycle, whereas the residual waste heat is used to generate additional power in a single recuperated cycle, as also proposed by one of the authors [12] for the recovery of flue gas from biomass combustion. Hou et al. [13] evaluated the performance of a combined s-CO2 system composed of the sequence of a recompression cycle and a single recuperated cycle that share the same compressor for WHR from a marine gas turbine having an exhaust temperature of 534 ◦C. The optimum solution obtained from a multi-objective optimization showed that the cascaded system enables a contextually high maximum temperature of the recompression cycle and a quite low gas outlet temperature to be reached, which ultimately results in a significant increment of power output (approximately 30%) compared to a single cycle. Zhang et al. [14] optimized the performance of a similar cascaded system for WHR from the exhaust gases at 490 ◦C of an offshore gas turbine, which basically differs from the previous one by allowing two different maximum cycle pressures. The solution obtained from a multi-objective optimization using the net power output and levelized energy cost as objective functions showed that the optimum cycle pressures reach the upper bounds (15 and 20 MPa). Moroz et al. [15] used the single recuperated, recompression, and basic non-recuperated cycles as building blocks to propose new configurations of cascaded s-CO2 power cycles. In the “composite” schemes devised by the authors, the different cycles share some equipment (e.g., compressor, cooler, recuperator) to reduce the number of plant components. The authors show that the composite cycles improve by 24.7% to 31.7% the power output of the single recuperated cycle in the WHR of exhaust gases at 471 ◦C. This is due to the better capability of heat extraction from the exhaust gases, as demonstrated by the much lower stack temperature (90–130 ◦C in the novel schemes versus 210 ◦C of the single recuperated cycle). Different combinations were also explored in the literature, where the organic Rankine cycle (ORC) substitutes for the second s-CO2 cycle in the cascaded system. For instance, Hou et al. [16] proposed a novel system composed of the sequence of a single recuperated s-CO2 cycle and an ORC using zeotropic mixtures of cyclopentane (cC5) and refrigerant R365mfc for WHR from a regenerative gas turbine. The authors carried out multi-objective optimization using the exergy efficiency and cost of electricity of the entire system as objective functions. They showed that the replacement of the single recuperated cycle with a recompression or a basic non-recuperated s-CO2 cycle would worsen both the thermodynamic and economic performance. Cao et al. [17] proposed a combined system composed of the sequence of a basic non-recuperated s-CO2 cycle and a transcritical CO2 cycle for WHR from a gas turbine. The transcritical cycle receives heat from the exhaust of the s-CO2 cycle and from the residual heat of the flue gases and uses the liquefied natural gas as a heat sink. The results of the thermodynamicPDF Image | Novel Supercritical CO2 Power Cycles for Waste Heat Recovery
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