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Energies 2020, 13, 370 4 of 31 optimization obtained for a small gas turbine having an exhaust temperature of 510 ◦C showed that the basic s-CO2 cycle already enables an effective cooling of the flue gases, hence the main function of the transcritical CO2 cycle is the recovery of the heat available at the discharge of the s-CO2 cycle. These studies clearly show the thermodynamic advantage of a cascaded system composed of traditional cycles, which can improve by up to 30% the net power output of a single s-CO2 cycle in WHR applications. On the other hand, they also highlight their major criticality, namely the high number of required components, in spite of the very recent efforts to address this point, as shown by the cascaded layouts with shared equipment. 1.4. Novel s-CO2 Power Cycles for WHR The novel s-CO2 power cycle layouts devised in the last decade for WHR applications provide a high performance with a comparatively lower number of components compared to the cascaded systems described in the previous section. The most relevant ones are here referred to as the “single flow split with dual expansion”, “dual flow split with dual expansion”, and “dual recuperated” s-CO2 power cycles. They apparently resemble a traditional s-CO2 cycle but differ from the latter by including two turbines. Among the s-CO2 layouts specifically developed for WHR applications, it is the “partial heating cycle” that can still provide a high performance using a single turbine. One of the first studies published about novel s-CO2 power cycles for WHR was an EPRI (Electric Power Research Institute) report by Kimzey [18], who presented and optimized the performance of three novel s-CO2 power cycles for WHR from gas turbines, selected out of nearly 30 configurations initially conceived. The first one is the single flow split with dual expansion (called baseline Cascade Cycle 1), which was first presented during a conference by a cutting-edge company [19]. The other two cycles were proposed by the author as an advancement on the baseline cycle and include the more complex layout with dual flow split and dual expansion. Focusing on the first novel cycle, the author showed that it can provide a 1.5% to 3% gain in power output compared to a two-pressure level steam bottoming cycle, whereas it is outperformed by the triple pressure with reheat steam cycle commonly used for larger heavy duty gas turbines. The authors showed that its main drawback is the poor thermal profile match in the low temperature recuperator, which implies an incomplete recovery of the heat at the exhaust of the CO2 turbines. Another early study on advanced s-CO2 layouts was that by Walnum et al. [20], who, referring to a patent by Held et al. [21], compared the performance of the novel dual recuperated (called “dual stage”) s-CO2 power cycle against that of a single recuperated cycle for WHR from a 32 MW gas turbine installed in an offshore platform. At the design point, the authors found that the dual recuperated cycle improves the power output of the single recuperated cycle by 10.1%. This is mainly a result of the enhanced heat extraction from the heat source, as demonstrated by the lower flue gas outlet temperature (125 versus 170 ◦C), and the higher cycle maximum temperature (426 versus 365 ◦C) attained by the dual recuperated cycle. In the latter, approximately 40% of the total CO2 mass flow rate is heated by the exhaust of the high temperature turbine (HTT) in a high temperature recuperator (HTR), and directly expanded in the low temperature turbine (LTT). The partial heating (often called “preheating”) cycle completes the picture of the most relevant novel layouts and was selected by Wright et al. [22] as one of the most promising s-CO2 power cycles for WHR applications. The authors maximized the net annual revenue by searching for the optimum cycle parameters, in particular the turbine inlet temperature and split flow fraction, and the optimum heat exchanger approach temperatures. For each cycle, they evaluated the separate contributions of the cycle thermal efficiency and heat recovery effectiveness at the optimum point. They found that the 17.1% to 22.6% gain in the net power output of the novel cycles over the single recuperated cycle is accompanied by a 10.1% to 17.6% increment in the specific investment cost. Among the novel cycles, the highest rate of return (15.6%) was achieved by the dual recuperated cycle. These first promising results paved the way for further studies aimed at comparing the performance of the novel s-CO2 power cycles against that attainable by single or cascaded layouts of traditional s-CO2 cycles or multi-pressure steam cycles. In this context, one of the earliest and most comprehensivePDF Image | Novel Supercritical CO2 Power Cycles for Waste Heat Recovery
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