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Solar organic Rankine cycle (SORC) is a promising approach for solar energy utilization under low and moderate temperatures due to low investment and small-to-medium scale. The stable output of the SORC system under variable solar irradiation conditions is necessary to weaken the impact on the power grid and provide a base load. A novel operating mode was designed to obtain the stable output of 1 MWe for a SORC system operated under variable solar irradiation for 1 day. The parabolic trough collector was selected for solar heat reception, and a two-tank direct thermal storage unit was used for heat storage. Given the variations in solar irradiation, the mass flow rate of the heat transfer fluid (HTF) in the solar collectors was adjusted to guarantee a constant HTF inlet temperature for the ORC unit, and the mass flow rate of the HTF from the hot tank to the ORC unit was kept constant. Thus, the SORC system could operate under stable conditions. Four organic working fluids and two cycle types were subjected to thermodynamic analysis and parametric optimization. Results show that as the HTF inlet temperature increases, the optimal efficiency of system with non-recuperative cycle and toluene increases, whereas those of systems with non-recuperative cycles and cyclohexane, hexamethyldisiloxane, and pentane as working fluids first increase and then remain constant. The increment in the superheat degree increases the efficiency of the system with recuperation. By contrast, the system with non-recuperation demonstrates the maximum system efficiency in the absence of or at a low superheat degree. The maximum system efficiency reaches 17.9%. The system efficiency of this novel operating mode is 4.2% higher than that of an unstable output operating mode when operated for 1 day.

The supercritical CO2 Brayton cycle integrated with a solar power tower system has the advantages of high efficiency, compact cycle structure, strong scalability, and great power generation potential, which can positively deal with the energy crisis and global warming. The selection and optimization of design points are very important for actual operating situations. In this paper, the thermodynamic and economic models of the 10 MWe supercritical CO2 Brayton cycle for application in solar power tower system are established. Multi-objective optimizations of the simple recuperative cycle, reheating cycle, and recompression cycle at different compressor inlet temperature are completed. The thermal efficiency and the levelized energy cost are selected as the fitness functions. The ranges of the optimal compressor inlet pressure and reheating pressure on the Pareto frontier are analyzed. Finally, multiobjective optimizations and analysis of the supercritical CO2 Brayton cycle at different ambient temperature are carried out. This paper investigates the influence of the compressor inlet temperature and ambient temperature on the thermal efficiency and economic performance of the supercritical CO2 Brayton cycle.

This study proposes a novel design optimization method for the solar power tower (SPT) system with a supercritical CO2 (S-CO2) Brayton cycle. The design conditions are determined based on annual off-design performance. The fluctuating solar irradiation and ambient temperature in different locations, and various power demand scenarios, including stable, light industry, and hybrid PV + CSP system types, are considered. The parameters of solar multiple (SM), thermal energy storage (TES) capacity, and designed compressor inlet temperature are simultaneously optimized by the multi-objective optimization algorithm. Results show that compared with the conventional design method, the novel design method can improve the maximum load cover factor by 6.38% under stable power demand and reduce the levelized cost of energy by 5.62% with a load cover factor of 0.9 under light industry power demand in Daggett. The optimal design conditions and actual operating performance are significantly affected by the weather conditions, power demand scenarios, and optimization objectives. For most cases to increase load cover factor in the most economical way, increasing SM and TES capacity coordinately is preferred to be implemented first, followed by increasing designed compressor inlet temperature. Whereas, for cases with a high frequency of simultaneous high ambient temperature and high power demand, increasing designed compressor inlet temperature will be advanced to the same priority as increasing SM and TES capacity.

In this work, the load matching and techno-economic performance of concentrated solar power (CSP) plants based on four supercritical CO2 (S–CO2) Brayton cycles, different solar multiple (SM) and thermal energy storage (TES) capacities are analyzed in a CSP-PV-wind hybrid system. The performance of CSP plant for meeting varied power demand in hybrid system and stable power demand are compared. Results show the lowest levelized cost of energy (LCOE) of CSP plant based on partial-cooling cycle under load following scenarios in hybrid system is 32.1% higher than that under stable output scenarios. The optimal SM to achieve the lowest LCOE under stable output scenarios is much larger than that under load following scenarios in hybrid system. As SM and TES capacity increase, the capacity factor of hybrid system increases and can even exceed 90%, which represents that CSP plant can cooperate well with PV and wind plants to provide stable base loads. The LCOE of CSP plant first decreases and then increases with the increase of SM and TES capacities. The CSP plant based on partial-cooling cycle has the best economic performance, and achieves the lowest LCOE of 0.2169 $/kWh with SM of 2.4 and TES capacity of 20 h.

The system that integrates solar power tower, molten salt thermal storage, and supercritical CO2 (S-CO2) Brayton cycle has emerged as a promising technology to provide a dispatchable power output following the power demand. Studying the part-load performance of the S-CO2 Brayton cycle is crucial to achieve an efficient system operation during power load adjustment. In this work, the performance of four typical S-CO2 Brayton cycles including simple recuperative cycle, reheating cycle, recompression cycle, and intercooling cycle are analyzed and compared under part-load conditions. The optimal cycle layouts under different power demand scenarios are revealed. Results show that the reheating cycle achieves higher efficiency than the simple recuperative cycle, and the recompression cycle and intercooling cycle perform better than the reheating cycle under part-load conditions. However, the intercooling cycle is more efficient than the recompression cycle only when the load exceeds 60%. Given that the relative reduction in the reheating cycle efficiency is minimal as the load decreases, this cycle shows the best response to the requirement of wide-range load adjustment. When the ratio of the actual total electricity generation to the maximum total electricity generation in a certain period is lower than 62.5%, the recompression cycle performs better. When this ratio is higher than 68.3%, the intercooling cycle performs better. In general, the efficiency of intercooling cycle is not significantly improved compared with that of the recompression cycle under the load-following scenarios.

This work focuses on the off-design performance of the system integrated a simple recuperative supercritical CO2 Brayton cycle, solar power tower, and thermal energy storage (TES). The system design parameters are obtained first; then, the off-design performance of power cycle and solar receiver are analyzed from the perspective of influencing factors; finally, the daily performance simulations of overall system are conducted. This work is aimed at analyzing the system performance under more realistic conditions, and clarifying the influence mechanism between parameters of solar field, TES and power cycle under off-design conditions. Results show that the increase in ambient temperature and input energy to the receiver, and the decrease in molten salt temperature from the cold tank are beneficial to improve the solar receiver efficiency. As the power demand deviates farther from design value, the temperature of molten salt at the heater outlet is higher, which leads to lower solar receiver efficiency. On the summer solstice, the system operates continuously for 24 h with 100% electricity powered by solar energy and 17.8% daily averaged solar-to-electricity efficiency. On the winter solstice, the system operates continuously for 17 h with 72.6% electricity powered by solar energy and 17.1% daily averaged solar-to-electricity efficiency.