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• Integration of four novel sCO 2 cycles with coal-fired power plant. • Efficiency of sCO 2 cycle is higher than steam Rankine cycle by 3–4%pts. • sCO 2 cycles can reduce the cost of electricity by 6–8% than steam Rankine cycle. • Increasing turbine inlet temperature by 140 °C increases efficiency by 3–4%pts. • Increasing turbine inlet temperature doesn't show reduction in cost of electricity. Supercritical carbon dioxide (sCO 2) cycles can achieve higher efficiencies than an equivalent steam Rankine cycle at higher turbine inlet temperatures (>550 °C) with a compact footprint (tenfold). sCO 2 cycles are low-pressure ratio cycles (~4–7), therefore recuperation is necessary, which reduces the heat-addition temperature range. Integration of sCO 2 cycles with the boiler requires careful management of low-temperature heat to achieve higher plant efficiency. This study analyses four novel sCO 2 cycle configurations which capture the low-temperature heat in an efficient way and the performance is benchmarked against the state-of-the-art steam Rankine cycle. The process parameters (13–16 variables) of all the cycle configurations are optimised using a genetic algorithm for two different turbine inlet temperatures (620 °C and 760 °C) and their techno-economic performance are compared against the advanced ultra-supercritical steam Rankine cycle. A sCO 2 power cycle can achieve a higher efficiency than a steam Rankine cycle by about 3–4% points, which is correspond to a plant level efficiency of 2–3% points, leading to cost of electricity (COE) reduction. Although the cycle efficiency has increased when increasing turbine inlet temperature from 620 °C to 760 °C, the COE does not notably reduce owing to the increased capital cost. A detailed sensitivity study is performed for variations in compressor and turbine isentropic efficiency, pressure drop, recuperator approach temperature and capacity factor. The Monte-Carlo analysis shows that the COE can be reduced up to 6–8% compared to steam Rankine cycle, however, the uncertainty of the sCO 2 cycle cost functions can diminish this to 0–3% at 95% percentile cumulative probability. [ABSTRACT FROM AUTHOR]

• Identified the optimal GT pressure ratio requirements for four sCO 2 cascaded cycles. • Pressure ratio of a sCO 2 cycle at maximum efficiency is higher than a steam cycle. • Multi-objective optimisation is done to compare sCO 2 cycles on equivalent basis. • Performance prediction maps were produced to help selecting an optimal GT. • A new sCO 2 cycle is proposed that improves efficiency by 1.4 percentage points. A closed-loop, indirect, supercritical Carbon Dioxide (sCO 2) power cycle is attractive for fossil-fuel, solar thermal and nuclear applications owing to its ability to achieve higher efficiency, and compactness. Commercial Gas Turbines (GT's) are optimised to yield maximum performance with a conventional steam Rankine cycle. In order to explore the full potential of a sCO 2 cycle the whole plant performance needs to be considered. This study analyses the maximum performance and cost of electricity for five sCO 2 cascaded cycles. The plant performance is improved when the GT pressure ratio is considered as a design variable to a GT to optimise the whole plant performance. Results also indicate that each sCO 2 Brayton cycle considered, attained maximum plant efficiency at a different GT pressure ratio. The optimum GT pressure ratio to realise the maximum cost reduction in sCO 2 cycle was higher than the equivalent steam Rankine cycle. Performance maps were developed for four high efficient cascaded sCO 2 cycles to estimate the specific power and net efficiency as a function of GT turbine inlet temperature and pressure ratio. The result of multi-objective optimisation in the thermal and cost (c$/kWh) domains and the Pareto fronts of the different sCO 2 cycles are presented and compared. A novel sCO 2 cycle configuration is proposed that provides ideal-temperature glide at the bottoming cycle heat exchangers and the efficiency of this cycle, integrated with a commercial SGT5-4000F machine in lieu of a triple-pressure steam Rankine cycle, is higher by 1.4 percentage point. [ABSTRACT FROM AUTHOR]

• Off-design Pareto fronts of sCO 2 cycle using multi-objective optimisation approach. • Storage capacity reduces by 25% at 55 °C compressor inlet temperature (design = 42 °C). • Off-design compressor inlet pressure estimation method to maximise efficiency. • Capacity factor increases by 10.8% when operating the plant in maximum power mode. • Number of start-ups reduce by about 50% when operating in maximum efficiency mode. Supercritical Carbon Dioxide (sCO 2) cycles can achieve higher efficiency compared to steam-Rankine or Air-Brayton cycles, therefore they are promising for concentrated solar power applications. Although sCO 2 cycles show higher design efficiency, the off-design efficiency is highly sensitive to the ambient conditions, impacting the power block net-power and heat input. In the present work a recompression sCO 2 cycle is connected to a central-tower solar field with two-tank thermal storage delivering molten chloride salt at 670 °C. The temperature of the molten-salt exiting from the power block and returning to the cold storage tank increases by 46 °C with respect to the design value when the compressor inlet temperature is raised by 13 °C relative to the design condition of 42 °C, which implies that the capacity of the thermal storage reduces by 25%. The main focus of this work is to investigate the off-design performance of a sCO 2 recompression cycle under variable ambient temperature, molten-salt inlet temperature and molten-salt flow rate. Multi-objective optimisation is carried-out in off-design conditions using an in-house code to explore the optimal operational strategies and the Pareto fronts were compared. Since the power cycle can either be operated in maximum power mode or maximum efficiency mode, this study compares these two operational strategies based on their annual performance. Results indicate that the capacity factor of the concentrated solar power can be increased by 10.8% when operating in maximum power mode whilst the number of start-ups is reduced by about 50% when operating in maximum efficiency mode. [ABSTRACT FROM AUTHOR]