Novel Supercritical CO2 Power Cycles for Waste Heat Recovery

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Novel Supercritical CO2 Power Cycles for Waste Heat Recovery ( novel-supercritical-co2-power-cycles-waste-heat-recovery )

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Energies 2020, 13, 370 2 of 31 and 54% in the transportation sector are available at temperatures higher than 300 ◦C. Focusing on the industrial waste heat in the EU, Papapetrou et al. [2] found that the waste heat potential at temperatures higher than 500 ◦C amounts to 124 TWh/year, which represents 41% of the total. Vance et al. [3] estimated the potential and barriers for waste heat recovery (WHR) at temperatures higher than 650 ◦C in five industries (steel, aluminum, glass, cement, and lime). The authors pointed out that reactive constituents, often present in the waste gases from these industries, may complicate the heat recovery process and require new technologies. They estimated a potential for WHR from these high temperature streams in the U.S. equal to 113.6 TWh/year. Firth et al. [4] forecasted the global waste heat emissions in 2030 from the power generation, industry, transport, and building sectors. Their analysis suggested an important need for effective medium and high temperature WHR technologies. All these studies point to a huge amount of wasted heat at high temperatures that could be conveniently converted into electricity with high thermal efficiencies. However, there appears to be a lack of consensus about the best technology to accomplish this goal. The supercritical CO2 (s-CO2) power cycle represents a promising option due to the high thermal efficiency, scalability to low power ranges (hundreds of kW), small footprint, and favorable economics. The s-CO2 cycle was originally proposed by Feher [5] and Angelino [6] to overcome the thermodynamic and technological limitations of the Rankine and Brayton cycles. Both authors suggested nuclear power as the foremost application, as also considered by Dostal [7] in his popular doctoral thesis, which still represents one of the most comprehensive studies on “traditional” s-CO2 power cycle layouts. In the meantime, the renewed interest in concentrating solar power and, in particular, on the central receiver layout suggested the s-CO2 power cycle as a suitable power block substitute for the Rankine cycle due to its higher thermal efficiency and compactness. Since the beginning of this last decade, many efforts have been devoted to extending its range of application to other fields, especially in the recovery of waste heat, where the maximization of net power output rather than of thermal efficiency is to be pursued. In this field, three main approaches are considered. The first one simply takes the traditional s-CO2 layouts and optimizes their cycle parameters for maximum power. The second one considers a sequence of two traditional s-CO2 layouts in a cascade utilization of the waste heat. The third one suggests novel layouts specifically developed to overcome the limitations of the traditional layouts in heat extraction from the waste heat source. In the following, the main findings and results obtained from the applications of these approaches in the literature are summarized, to ultimately focus on the third approach. 1.2. Traditional s-CO2 Power Cycles for WHR The traditional s-CO2 layouts generally considered in the literature for WHR applications are the single recuperated, recompression, and pre-compression layouts, and the objective function is the net power output or, in relative terms, the ratio between the net power output and overall heat available from the heat source from the inlet temperature to the ambient temperature, called the “total heat recovery efficiency”. One of the most enlightening studies was performed by Mohagheghi and Kapat [8], who optimized the net power output of the single recuperated and recompression s-CO2 power cycles for WHR in a wide temperature range between 230 and 830 ◦C. The authors showed that the optimum recompression cycle reduces to the single recuperated layout for waste gas temperatures lower than 450 ◦C, whereas it only marginally improves the power output at higher temperatures. The authors highlighted the high gap between the waste gas inlet temperature and optimum turbine inlet temperature (TIT), and the high outlet temperature of the waste heat source (stack temperature). These drawbacks were found to be more pronounced at high waste gas temperatures and for the recompression layout. A different finding was obtained by Khadse et al. [9] for a waste gas at 630 ◦C. The authors showed that the recompression cycle improves by 22.9% the power output of the single recuperated cycle. This result is probably ascribable to the different assumptions compared to the previous study, in particular the higher maximum pressure (33 MPa). Moroz et al. [10] extended the performance comparison to the pre-compression cycle and a wide set of gas turbines having exhaust temperatures in the range 450–650 ◦C. For a gas temperature of 550 ◦C, the maximum total heat

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