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A novel combined supercritical carbon dioxide recompression Brayton/organic flash cycle is investigated by means of exergoeconomic analysis. The supercritical carbon dioxide recompression Brayton/organic flash cycle is a combination of a supercritical carbon dioxide recompression Brayton cycle and an organic flash cycle where the organic flash cycle absorbs waste heat from the supercritical carbon dioxide recompression Brayton cycle for power generation. Seven different organic flash cycle working fluids are examined, including n-Nonane, n-Octane, n-Heptane, n-Hexane, n-Pentane, R365mfc and R245fa. Parametric study is employed to investigate the effects of the some decision variables on the first and second law efficiencies and the total product unit cost of the supercritical carbon dioxide recompression Brayton/organic flash cycle and the supercritical carbon dioxide recompression Brayton cycle. The performances of the supercritical carbon dioxide recompression Brayton/organic flash cycle and the supercritical carbon dioxide recompression Brayton cycle are optimized and then compared from the perspective of thermodynamics and exergoeconomics. The results show that the second law efficiency and the total product unit cost of the supercritical carbon dioxide recompression Brayton/organic flash cycle are up to 6.57% higher and up to 3.75% lower than those of the supercritical carbon dioxide recompression Brayton cycle, respectively. Compared with the supercritical carbon dioxide recompression Brayton/organic Rankine cycle, the supercritical carbon dioxide recompression Brayton/organic flash cycle can obtain slightly higher second law efficiency, and comparable or slightly lower total product unit cost. It can also be concluded that the highest second law efficiency and the lowest total product unit cost for the supercritical carbon dioxide recompression Brayton/organic flash cycle are achieved when the n-Nonane is used as the organic flash cycle working fluid.

A novel dual-pressure Allam cycle integrated with the cold energy recovery of liquefied natural gas is proposed and analyzed in this paper. The feature of this system is that the cold energy of liquefied natural gas is successively utilized in two condensers with different condensing pressures, thereby reducing the heat transfer temperature difference in the condensation process and improving the system performance. The proposed system is discussed in detail based on the results of thermodynamic analysis and optimization. According to the simulation results, the largest exergy destruction belongs to the combustion chamber. Moreover, the exergy destruction in the condenser is greatly reduced owing to the dual-pressure condensation process. The system exergy efficiency first increases and then decreases with the increment of combustor temperature and main turbine inlet pressure. However, the system performance would deteriorate as the main turbine outlet pressure and bottom turbine outlet pressure increase. According to the results of single-objective optimization, the electricity efficiency and specific work of the dual-pressure Allam cycle are 15.76% and 68.75% higher than the classical Allam cycle, respectively. In addition, the exergy efficiency of the dual-pressure Allam cycle is up to 51.88%, which is improved by 1.57% compared with the single-pressure Allam cycle.

Exergoeconomic analysis is performed for a novel combined SCRB/ARC (supercritical CO2 recompression Brayton/absorption refrigeration cycle) in which the waste heat from the SCRBC is recovered by an ARC for producing cooling. Parametric analysis is conducted to investigate the effects of the decision variables on the performance of the SCRB/ARC cycle. The performances of the SCRB/ARC and SCRBC cycles are optimized and compared from the viewpoints of first law, second law and exergoeconomics. It is concluded that combining the SCRBC with an ARC can not only enhance the first and second law efficiencies of the SCRBC, but also improve the exergoeconomic performance. The results show that the largest exergy destruction rate occurs in the reactor, while the components in the ARC have less exergy destruction. The reactor and turbine are the first and second important components from exergoeconomic aspects. When optimization is based on the exergoeconomics, the first and second law efficiencies and the total product unit cost of SCRB/ARC are 26.12% higher, 2.73% higher and 2.03% lower than those of the SCRBC. The optimization study also reveals that an increase in the reactor outlet temperature can enhance both thermodynamic and exergoeconomic performances of the SCRB/ARC. For the basic design case, the SCRB/ARC can produce 71.76 MW of the cooling capacity and 6.57 MW of the cooling exergy at the expense of only 0.36 MW of power in comparison with the SCRBC.

In this paper, a comparative investigation on the supercritical carbon dioxide power cycle for waste heat recovery of gas turbine is carried out. Four categories and six cycles are chosen to represent typical layout of supercritical carbon dioxide power cycle based on thorough literature reviews. The selected cycles are optimized and analyzed based on the results of single- and multi-objective optimization from the perspective of system efficiency, economic cost and configuration complexity. According to the exergy destruction analysis, several guidelines about the heat exchangers are proposed to enhance the thermal efficiency of gas turbine waste heat recovery system, including the avoiding of preheating, the utilization of partial heating layout, the employment of segmentation method and modified logarithmic mean temperature difference, the increase in the number of gas heater. The results indicate that turbine dominates the investment cost of power cycle. In conclusion, dual heated cascade cycle-II should be employed to pursue the maximized system efficiency. Otherwise, partial heating cycle is recommended due to its balanced overall performance.

An integration system for 1000 MW S-CO2 connected-top-bottom power cycle with overlap utilization and coal-fired boiler is developed. The adaptive modification module is introduced to eliminate the non-convergence for S-CO2 superheater thermal calculation in the system iteration. Thermal-hydraulic performance in terms of thermal performance, cycle efficiency, heater configurations, heater areas and final parameter evenness are discussed. Firstly, the strong constraint effects of inlet and outlet parameters on superheater thermal calculation convergence are demonstrated and the adaptive modification module is established. Secondly, based on the detailed thermal-hydraulic analysis, different layouts are compared to identify the flow path way and the heat absorption of S-CO2 flow in different heating processes. The midpoint between the two peaks of heat flux is recommended as the start point for the cold fluid placement in cooling wall arrangement. Finally, a new layout is proposed to obtain the highest overall thermal-hydraulic performance, with a net power efficiency up to 47.57% and a total heater area of 316426.18 m2. The design sequences for S-CO2 coal-fired boiler heaters are recommended as well.

A novel combined cooling, heating and power system which consists of carbon dioxide power cycle, absorption chiller and heaters is proposed in this paper for waste heat recovery of gas turbine. In the proposed cogeneration system, the absorption chiller and heaters are driven by the residual energy of carbon dioxide power cycle and flue gas simultaneously. Detailed mathematic models are established to simulate and evaluate the system from the perspective of thermodynamics and economics. The results of parametric analysis indicate that there is an optimal value for the turbine inlet temperature, at which the exergy efficiency is maximized and the levelized cost of exergy is minimized. As the split ratio increases, the exergy efficiency and levelized cost of exergy of the proposed system first increase then decrease simultaneously. Single-objective optimization is carried out to maximize the exergy efficiency of the proposed system. The optimal results show that the exergy efficiency and levelized cost of exergy of the cogeneration system are 4.62% higher and 0.90 cent lower than that of standalone power cycle, respectively. Compared with the alternative cogeneration system in which only low-temperature flue gas or the waste heat of power cycle is used to drive the refrigeration subsystem and heating subsystem, the exergy efficiency of proposed system is enhanced by 2.89% and 1.3%, respectively. Finally, multi-objective optimization is carried out with exergy efficiency and levelized cost of exergy as objective functions. Pareto frontier is calculated and recommended point is given for the engineering practice.

This paper proposes a novel combined cooling and power (CCP) system based on a regenerative supercritical carbon dioxide Brayton cycle (sCO2 cycle) and an ammonia-water absorption refrigeration cycle (ARC) for engine waste heat recovery. In the proposed system, the supercritical carbon dioxide Brayton cycle absorbs the waste heat of the internal combustion engine to generate power, and the absorption refrigeration cycle utilizes the low-grade energy in the supercritical carbon dioxide turbine exhaust to provide cooling. Firstly, energy and exergy analysis is performed on the combined cooling and power system. Then, detailed parametric analysis is carried out to study the effects of key parameters, such as the compressor outlet pressure, turbine inlet temperature, generator hot-end temperature difference, and evaporator temperature on the exergy efficiency and total product unit cost. Finally, the non-dominated sorting genetic algorithm II is adopted for multi-objective optimization to obtain the maximum exergy efficiency and the minimum total product unit cost. Multi-objective optimization results reveal that better thermodynamic and economic performances can be obtained by lowering the evaporator temperature. Compared with the single regenerative supercritical carbon dioxide system, the proposed system can increase the exergy efficiency by 2.29–2.54%pt (percentage point), and the thermal efficiency by 8.16–18.93%pt, but might increase the total product unit cost. Under different evaporator temperatures (−10–10 °C), the proposed system can generate 248.19–253.90 kW of net power output, accounting for 8.48–8.67% of the rated power output of the engine, and produce 70.57–168.86 kW of cooling capacity by consuming 0.445–0.532 kW of pump work. In addition, exergy destruction analyses indicate that the components in the bottoming ARC generally have less exergy destructions.

Supercritical carbon dioxide is widely used in power plant and refrigeration system as working fluid. In this paper, a novel combined cooling and power system consists of supercritical carbon dioxide recompression Brayton cycle and transcritical carbon dioxide refrigeration cycle is proposed. In the novel system, the working fluid through the recompressor in supercritical carbon dioxide recompression Brayton cycle is partially replaced by the refrigeration compressor outlet working fluid in transcritical carbon dioxide refrigeration cycle. Mathematical models are established to simulate the combined system under steady-state conditions and the simulated results show that the reactor dominates the exergy destruction followed by the cooler and high temperature recuperator. Effects of five parameters, including turbine discharge pressure, cooler outlet temperature, evaporation temperature, cooling capacity and mass flow fraction are evaluated. The performances of the proposed combined system and the separation system are optimized and then compared from the perspective of thermodynamics. The calculated results show that the combined system’s comparative advantage over separation system will increase with increasing cooling capacity and decreasing evaporation temperature. The exergy efficiency of combined system is up to 2.45% and 5.87% higher than separation system when evaporation temperature is 273.15 K and 253.15 K, respectively. In order to balance the contradiction between the investment and system performance, the multi-objective optimization is conducted with exergy efficiency (to be maximized) and annual cost per heat consumption (to be minimized) as objective functions at different evaporation temperature. Non-dominated sorting generic algorithm-II is employed and Pareto frontier solution for the proposed system is given. Suggestions for engineering practice are provided based on the distributions of some key parameters on Pareto frontier solution.