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Supercritical carbon dioxide (sCO2) cycles can achieve higher efficiencies than an equivalent steam Rankine cycle at higher turbine inlet temperatures (>550 °C) with a compact footprint (tenfold). sCO2 cycles are low-pressure ratio cycles (~4–7), therefore recuperation is necessary, which reduces the heat-addition temperature range. Integration of sCO2 cycles with the boiler requires careful management of low-temperature heat to achieve higher plant efficiency. This study analyses four novel sCO2 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 sCO2 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 sCO2 cycle cost functions can diminish this to 0–3% at 95% percentile cumulative probability.

Supercritical Carbon Dioxide (sCO2) cycles can achieve higher efficiency compared to steam-Rankine or Air-Brayton cycles, therefore they are promising for concentrated solar power applications. Although sCO2 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 sCO2 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 sCO2 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.

A closed-loop, indirect, supercritical Carbon Dioxide (sCO2) 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 sCO2 cycle the whole plant performance needs to be considered. This study analyses the maximum performance and cost of electricity for five sCO2 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 sCO2 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 sCO2 cycle was higher than the equivalent steam Rankine cycle. Performance maps were developed for four high efficient cascaded sCO2 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 sCO2 cycles are presented and compared. A novel sCO2 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.

Exhaust gas recirculation (EGR) in conventional natural gas-fired oxy-combustion cycles is required to maintain the combustion temperature at an allowable level. However, EGR is not beneficial from the system performance perspective. It is difficult to achieve in oxy-fuel cycles due to the high pressure and increased pressure drop in such systems. Consequently, alternative options to control the combustion temperature need to be considered. In this study, staged oxy-fuel natural gas combined cycle (SOF-NGCC) was proposed, which does not require EGR, and its feasibility was evaluated. A process model was developed in Aspen Plus® in order to evaluate the thermodynamic performance of the proposed system and to benchmark it against the Allam cycle and conventional NGCC. The optimum net efficiency of the proposed cycle (47.63–51.32%) was shown to be lower than that for the Allam cycle (54.58%) and the conventional NGCC without post-combustion capture (PCC) (56.95%). However, the SOF-NGCC is less complex and requires smaller equipment than the Allam cycle. This is mostly because the combined volumetric flow rate into expanders in both topping and bottoming cycles is approximately 25% of that estimated for the Allam cycle. Moreover, with a backpressure of 35 bar, no further compression is required prior to the CO2 purification unit.

State-of-the-art integration scenarios of calcium looping (CaL), which is an emerging CO2 capture technology, assume that excess heat is used to raise steam for the steam cycle and result in a net efficiency penalty of 6.0–8.0% points. In this study, a concept using the supercritical CO2 cycle (s-CO2) instead of the conventional steam cycle is proposed. Retrofit of CaL with recompression s-CO2 cycle to the 580 MWel coal-fired power plant was found to result in a net efficiency penalty of 6.9%HHV points. This is 1%HHV point lower than that for the same system linked with the steam cycle having the same turbine inlet conditions (593.3 °C/242.3 bar). A further reduction of the net efficiency penalty to 5.8%HHV points was achieved through considering a pump instead of a first CO2 compression stage and increasing the turbine inlet temperature to 620 °C and pressure to 300 bar. As the s-CO2 cycle's specific capital cost is up to 27% lower than that of the equivalent steam cycle, CaL with s-CO2 cycle is a viable option for the coal-fired power plant decarbonisation. Moreover, it can be expected that this cycle can be successfully implemented in other high-temperature looping cycles, such as chemical looping combustion.