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Energies 2020, 13, 370 5 of 31 study is by Cho et al. [23], who compared the performance of the novel s-CO2 power cycles proposed by Kimzey [18] against that attainable by cascaded systems composed of the sequence of recompression or pre-compression and partial heating cycles. In the WHR from a gas turbine having an exhaust temperature of 580 ◦C, the authors found that the dual flow split with a dual expansion cycle improves by 6.6% to 8.5% the power generated by the cascaded s-CO2 systems, and even improves by 3.6% the power output of a triple pressure with a reheat steam cycle. Even though the single flow split with dual expansion provided only 85% of the power output of the dual flow split cycle, it was considered a promising option due to the lower number of heat exchangers and turbomachinery. Also based on these findings, Huck et al. [24] exclusively focused on the dual flow split with a dual expansion layout and compared its performance against that achievable by state-of-the-art steam bottoming cycles for heavy-duty and aeroderivative gas turbines. The author found that this advanced s-CO2 power cycle can perform even better than a three-pressure reheat steam cycle provided that maximum CO2 pressures exceeding 30 MPa are allowed, and considering very high efficiencies (95%) for CO2 turbomachinery. Instead, a 0.5% gain in the combined cycle efficiency is attainable when this s-CO2 layout replaces the dual pressure non-reheat steam cycle also using the more accessible assumptions of a near-term design. Kim M.S. et al. [25] extended the comparison to a wider set of s-CO2 power cycle layouts for WHR from a 5.0 MW gas turbine. Besides the traditional layouts (single recuperated, recompression, and pre-compression), they considered the partial heating cycle, three novel layouts with dual expansion proposed by Kimzey [18], and three original concepts. They found that the partial heating cycle improves by 23.3% to 26.2% the power output generated by the traditional layouts and even marginally improves by 2.6% the power generated by the single flow split with a dual expansion cycle. The power output of the latter was found to be equal to 83% to 86% of that achievable by the dual flow split with a dual expansion cycle, which is consistent with Cho et al. [23]. On the other hand, the new concepts devised by the authors could not exceed the power output achieved by the partial heating cycle, which was finally considered the most promising option for a megawatt scale due to the simpler layout, smaller number of components, and simpler operational scheme. A more focused study on this layout was carried out by Kim Y.M. et al. [26], who compared the performance of the partial heating cycle (called “split flow”) against that of the single recuperated cycle for WHR from a 25 MW gas turbine. The authors evaluated the separate contributions of the cycle thermal efficiency and heat recovery effectiveness in the calculation of the total heat recovery efficiency (called the “system thermal efficiency”). They showed that the maximum total heat recovery efficiency reaches 26.0% for the partial heating cycle, which is 6% higher than the maximum attained by the single recuperated cycle. This is mainly the result of the higher heat recovery effectiveness (89.6% versus 69.2%), with the cycle thermal efficiency remaining almost unaltered. The authors showed that in both cycles, the optimum TIT (in the range 390–400 ◦C) is much lower than the exhaust gas temperature (538 ◦C), which implies high exergy losses in the heater. The performance of the partial heating cycle was found to be even better than that of a cascaded system composed of the sequence of two single recuperated cycles, in spite of the lower number of components. Marchionni et al. [27] carried out a techno-economic comparison between four traditional s-CO2 power cycle layouts against four layouts specifically developed for WHR applications. The latter included the partial heating s-CO2 cycle (called preheating) and one novel layout proposed by the authors, which differs from the partial heating cycle by including a pre-compressor. In the utilization of flue gas at 650 ◦C, the authors found that the novel layouts outperform the traditional ones. Even though the comparison between novel layouts may be partially biased by the assumption a fixed flue gas outlet temperature (150 ◦C), the maximum power output was achieved by the partial heating cycle with or without the pre-compressor plant modification. In summary, these studies agree about the unrivalled performance of the dual flow split with a dual expansion cycle but, virtually, suggest the partial heating cycle and the single flow split with a dual expansion cycle as the most promising layouts due to their relevant performance and simpler layouts. With the only exception of Wright et al. [22], the performance assessment of the dual recuperated layout is limited to a comparison against traditional s-CO2 cycles. One of the most relevant studiesPDF Image | Novel Supercritical CO2 Power Cycles for Waste Heat Recovery
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