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Abstract Supercritical heat transfer mechanisms and heat transfer correlations are firstly reviewed in present paper. The heat transfer characteristics of S-CO 2 in circumferentially non-uniform heated vertical upward flow are then numerically studied and the fundamental mechanisms are discussed. The abnormal heat transfer for S-CO 2 is believed to be caused by the comprehensive effect of thermophysical property variations. For S-CO 2 with non-uniform heating and large mass flow rate, the enhanced heat transfer is mainly related to the large specific heat in the near wall region while the deteriorated heat transfer is dominated by the thickening viscous sublayer induced by the viscosity increase. Besides, the buoyancy effect induced by the density variation is still prominent to axial velocity distribution even in the forced convection. Finally a new correlation with higher accuracy is developed for S-CO 2 circumferentially average heat transfer by introducing the correction parameters of viscosity, specific heat, axial flow-acceleration and non-uniform heat flux. Highlights • Supercritical heat transfer mechanisms and correlations are comprehensively reviewed. • Heat transfer of S-CO 2 under non-uniform heating is numerically studied. • The relation between thermophysical property variation and abnormal heat transfer is examined. • An average heat transfer correlation for S-CO 2 under non-uniform heating is proposed and evaluated. [ABSTRACT FROM AUTHOR]

The supercritical-CO 2 (S–CO 2) cooler is one of the main components in S–CO 2 power system. This work aims at revealing the cooling mechanism of S–CO 2 under wide working conditions to assist the S–CO 2 cooler design. The pseudo-condensation is proposed with the analogy between supercritical cooling and subcritical condensation. From the pseudo-condensation and the traditional turbulent field analysis, the specific heat is found to be the dominant parameter for the enhanced heat transfer in S–CO 2 cooling. The disappearing of enhanced cooling heat transfer, in the view of pseudo-condensation, is mainly due to the thickening of pseudo liquid film, which is dominated by the gravity force, inertial force and interfacial force in low mass flux flow while the gravity force effect is negligible in high mass flux flow. However, from the turbulent field analysis, the disappearing of enhanced cooling heat transfer is mainly caused by the thickening boundary layer due to the local increase in density and viscosity in addition to the decrease in specific heat. Finally, based on the S–CO 2 cooling mechanism, two correlations for predicting heat transfer are proposed and evaluated. The present work can significantly enhance in-depth understanding on S–CO 2 cooling and promote engineering application. • The S–CO 2 cooling mechanism is analyzed with pseudo-condensation and turbulent field. • Pseudo-condensation is based on analogy between supercritical cooling and subcritical condensation. • Turbulent field analysis focuses on selection of key influencing thermophysical properties. • Two more accurate correlations are developed based on the obtained cooling mechanism. • The pseudo-condensation for S–CO 2 cooling process is proved to be with high reliability. [ABSTRACT FROM AUTHOR]

This work presents a design of 1000 MW S–CO 2 coal-fired power system, containing not only a new methodology of S–CO 2 power system modeling, but also the specific directions for engineering design. The coupling between computational fluid dynamic simulations at boiler scale and the mathematical modeling of the thermal cycle facilitates a high reliability of the comprehensive design. Our analysis helps identify key questions and then delivers specific directions for engineering design, especially for the pressure drop penalty in tail flue and thermal-hydraulic performance of cooling wall. The proposed flue split method coupled with parallel utilization method can not only effectively reduce the pressure drop penalty but also simplify the system heater configuration. Moreover, the thermal performance of S–CO 2 cooling wall is improved by using the flue gas recirculation and cooling wall arrangement optimization. The optimized position and proportion of flue gas extraction are further obtained. Comprehensively, the proposed design of S–CO 2 coal-fired power system enhances the system net efficiency up to 51.00%, with an efficient utilization of residual heat and cooling wall temperature below 700 °C. • A multiscale modelling platform is proposed for S–CO 2 coal-fired power system. • An improved system is obtained with system net efficiency up to 51.00% and cooling wall temperature all below 700 °C. • Flue split method and parallel utilization are developed for efficient recovery of tail flue heat. • Flue gas recirculation and cooling wall optimization are used to improve thermal safety of S–CO 2 cooling wall. [ABSTRACT FROM AUTHOR]

The Supercritical carbon dioxide (S–CO 2) Brayton cycle is considered as a promising alternative to traditional steam Rankine cycle due to its high efficiency and compactness. However, in coal-fired power system, the S–CO 2 temperature at cooling wall entrance is newly recognized quite high, leading to an over-temperature crisis. A coupled model of combustion and S–CO 2 heat transfer was established to predict cooling wall temperature of a 1000 MW S–CO 2 Brayton coal-fired boiler. Based on the module arrangement in S–CO 2 boiler, the "cold S–CO 2 -hot fire matching and cascaded temperature control" principle in 1D model was proposed to reduce the cooling wall temperature. Three methods were examined including the low-temperature fluid matching high heat flux burner region, the counterflow, and more cold fluid in circulation drained to match high heat flux zone. The results show that the optimal arrangement can significantly reduce the temperature in overheated region 12–44 °C and eliminate the local hot spot. In addition, the 3D model was developed to obtain the maximum wall temperature and the uneven temperature in the circumferential direction. The employment of spiral cooling wall could alleviate circumferential unevenness and reduce wall temperature. The present work can provide important guidance to design of S–CO 2 Brayton coal-fired power system. • A coupled model of combustion and S–CO 2 heat transfer was established to predict cooling wall temperature. • The "cold S–CO 2 -hot fire matching and cascaded temperature control" principle in 1D model was proposed. • The uneven temperature distribution in circumferential direction of cooling wall in 3D model was calculated. • The spiral tubes of cooling wall could effectively relieve uneven problem and reduce wall temperature. [ABSTRACT FROM AUTHOR]

Compressed carbon dioxide is a promising energy storage technology. However, renewable energy variability can lead to insufficiency during charging and discharging. The present work systematically investigates the effect of charging/discharging insufficiency on compressed carbon dioxide energy storage systems from the viewpoint of transient thermodynamic cycles. The insufficiency extent is defined based on the pressure at the high-pressure tank when disturbances occur. Then the effects of insufficiency extent are investigated in three typical scenarios. In the results, the effects of charging/discharging insufficiency on the efficiency, storage density and power output of the energy storage system during long-term operation are demonstrated. The efficiency of the system during the whole working period is 57.78%, lower than the design efficiency of 59.66%. In particular, the energy storage density is reduced dramatically to 197.60 kJ·m−3, which is 78.2% of the storage density of 252.68 kJ·m−3 at design conditions. Besides, the average output power of the turbine is reduced to 95.70% of the design value. The present work provides an improved understanding of the charging and discharging for compressed carbon dioxide energy storage, especially when renewables are integrated. • Problem on renewable energy variability effects on carbon dioxide energy storage is proposed and formulated. • Insufficient charging/discharging effects on efficiency, power and energy storage density are evaluated. • Cycle efficiency can be maintained at 57.78% but energy storage density reduced to 78.2%. • Necessity to consider renewable energy efficiency in economic analysis is suggested. [ABSTRACT FROM AUTHOR]

The heaters working with supercritical fluids normally suffer from high temperature, high pressure and non-uniform heating, which would lead to thermal fragile and stress failure in practical engineering. However, efficient and accurate methods to predict the comprehensive stress in non-uniformly heating tubes are missing. Besides, to reduce the comprehensive stress, a criterion is required to provide optimization guidance of non-uniformly heating tubes. In present study, the thermal and mechanical performances of three typical non-uniformly heating tubes are numerically investigated using the FVM-FEM method. A generalized thermal deviation factor (GTDF) is then proposed. The results showed that the GTDF can accurately predict the ratio of comprehensive stress to yield strength and GTDF > 1 indicates the plastic deformation, which is much more convenient and efficient than the conventional structural analysis. Based on the GTDF , two methods of reducing heat flux non-uniformity and enhancing tube-inside heat transfer are proposed to reduce the comprehensive stress. The results show that the unilaterally elliptic dimpled tube reduces the maximum comprehensive stress by 30.8% compared with the smooth tube and also prevents the stress concentration in the dimple zone, which can be employed to reduce the comprehensive stress in various tubes under non-uniformly heating conditions. • Thermal and comprehensive stresses in non-uniformly heating tubes were discussed. • A new criterion called GTDF was proposed to evaluate comprehensive stress. • The GTDF is more efficient and intuitive than the comprehensive stress. • Two methods are proposed to reduce the comprehensive stress based on the GTDF. • The elliptic dimple is recommended in non-uniformly heating tubes. [ABSTRACT FROM AUTHOR]

Supercritical carbon dioxide (S–CO 2) coal-fired power systems have been widely investigated due to its high efficiency and compactness. These power systems can range between 50 and 1000 MW and thus the corresponding boilers actually operate under dramatically different conditions. It is necessary to investigate the inherent relationship among different capacity S–CO 2 boilers, which not only bring new theoretical understandings but also help improve practical designs. In this work, a unified model is proposed to investigate both thermal-hydraulic and thermodynamic performance. Furthermore, it is applied to evaluate the relationship between power capacity and boiler configuration. It is shown that small capacity furnaces suffer from overheating in the furnace wall and the optimized cooling wall is suggested to tackle this issue. Moreover, it is found that high pressure drop in the cooling wall of large capacity furnaces results in significant penalty on the cycle efficiency. For the popular 1000 MW boiler, an optimized configuration is suggested by moving the second reheating into the horizontal flue of boiler, achieving a lower pressure drop and an improved cycle efficiency from 35.4% to 50.09%. Finally, three recommendations for the capacity-dependent configurations are presented. • A unified model for different capacity S–CO2 coal-fired boilers is established. • Overheating of cooling wall in boilers of small capacity is illustrated and an optimization is proposed. • Pressure drop penalty worsens in cooling wall of large capacity furnaces and results in a cycle efficiency reduction. • Three recommendations for the capacity-dependent configurations are demonstrated. [ABSTRACT FROM AUTHOR]