In order to improve the operation flexibility of coal-fired power units, a thermal system of coal-fired power system coupled with supercritical carbon dioxide (S-CO₂) energy storage cycle is proposed, and the effect of operation parameters on the system irreversible loss is investigated by thermodynamic exergy analysis. The results show that, the energy storage efficiency of the system can reach 56.14%, and the S-CO₂ flow rate and the S-CO₂ compressor/turbine pressure ratio have a greater influence on the system exergy efficiency. With the increase of S-CO₂ flow rate from 50 kg/s to 70 kg/s, the exergy efficiency of the system increases from 44.0% to 61.0%. With the increase of compressor/turbine pressure ratio from 3.0 to 6.0, the exergy efficiency of the system increases from 27.5% to 52.5%. The method proposed provides a theoretical reference for improving the coal-fired power unit operation flexibility, and provides ideas for large-scale grid connection of renewable energy.

A high-parameter gas turbine and supercritical carbon dioxide (S-CO₂) combined cycle model was constructed, and thermal performance analysis was carried out. The top cycle uses a high-parameter gas turbine with a combustion chamber exhaust temperature of 1 800 ℃, and the bottom cycle adopts S-CO₂ Rankine double turbine cycle, while using three-stage flue gas heating and two-stage turbine exhaust gas recovery. The parameters and thermal performance under the optimized combined cycle operating conditions are obtained by the penalty function method. The influences of the main parameters of the high-parameter gas turbine top cycle and S-CO₂ Rankine bottom cycle on the combined cycle performance is analyzed. The results show that the combined cycle thermal efficiency can reach 68.61% at the gas turbine pressure ratio of 35.5 and the combustion chamber outlet temperature of 1 800 ℃, and the efficiency of the combined cycle of the gas turbine and S-CO₂ is 2.3 percentage points higher than that of the combined cycle of the gas turbine and steam.

In order to explore the similarities and differences of the loss characteristics of carbon dioxide and steam in turbine cascades, the flow characteristics of the two kinds of working fluids in stator cascade and stage were studied by numerical methods. And the optimal Mach number for efficient operation of turbine stage under subsonic condition was obtained. The results show that with the increase of Mach number, the flow loss first increases and then decreases. When the Mach number is lower, the diffuser has a large range and is easy to backflow, which makes the wall boundary layer thicken and separate, and increases the overall flow loss. When the Mach number is higher, the strength of the secondary vortex in passage is larger, and the shock wave will be generated near the trailing edge of blades. The reason for the larger flow loss is the secondary flow and shock wave. Compared with steam, the dynamic viscosity of carbon dioxide is slightly higher, and its density is about twice that of steam. Under the same Mach number condition, the mainstream velocity is lower, the boundary layer is thicker, and the overall loss is larger. When the Mach number is lower than 0.30, the total-total efficiency of turbine stages with carbon dioxide is lower. While the Mach number is higher than 0.50, the efficiency of carbon dioxide is slightly higher than that of steam. When the optimal outlet Mach number of balde is about 0.60, the efficiency of both is the highest. The research results will provide a reference for further improving the design level for axial flow turbines of steam and carbon dioxide, and further understanding the loss characteristics of different medium in turbine stage.

In view of the actual distribution of heat boundary conditions on the cooling wall of supercritical carbon dioxide (S-CO₂) coal-fired boiler, the heat transfer characteristics of supercritical CO₂ in a vertical circular tube under axial non-uniform heat flux were numerically studied by using SST k-ω low Reynolds number turbulence model. The influence of different heat flux distribution, mass flux on heat transfer performance and wall temperature distribution was analyzed. The results show that the axial non-uniform heat flux distribution has a significant effect on the heat transfer of S-CO₂. Compared with the uniform heat flux, the total heat transfer coefficient under the axial non-uniform heat flux increases by about 8%. The non-uniform distribution of axial heat flux can inhibit the heat transfer deterioration and effectively reduce the peak wall temperature. Under the condition of non-uniform heat flux, the heat transfer of S-CO₂ is mainly affected by the thickness of the gas-like film, the thermal conductivity of the gas-like film and the specific heat near the wall. The results provide theoretical guidance for the design of supercritical CO₂ boiler.

The abnormal heat transfer behavior of supercritical carbon dioxide (S-CO₂) with low mass fluxes in a horizontal tube was studied, the S-CO₂ heat transfer process in the horizontal tube under the condition of low mass fluxes was simulated with Fluent software, and the abnormal heat transfer behavior of heating and cooling conditions and the influence of heat flux on heat transfer were analyzed. The results show that when the thermal boundary conditions are P=8 MPa, G=200 kg/(m²·s) and q/G=0.2 kJ/kg, the temperature of top and bottom walls in the S-CO₂ tube decreases along the way during the flow cooling process. When the mainstream temperature of S-CO₂ reaches the pseudo critical temperature, the heat transfer coefficient of the top wall at 551.0 mm from the inlet has a sudden peak value, heat transfer enhancement occurs here. Under heating conditions, the temperature of the top wall first rises along the tube path, then drops to 395 K and then rises slowly. The temperature of the bottom wall drops briefly and then rises slowly. At the top wall 69.5 mm away from the inlet, the heat transfer coefficient has a valley value, and the heat transfer at this point deteriorates. The increase of heat flow density aggravates the deterioration of heat transfer under heating conditions, but has no obvious effect on cooling heat transfer. It can be seen that the thermal physical property distribution of the characteristic section is the main reason for the different heat transfer behaviors. Based on the low mass fluxes conditions, thermo-physical properties and buoyancy effects, a correlation equation for predicting supercritical heat transfer enhancement is constructed, which provides theoretical guidance for the design and operation optimization of supercritical fluid heat exchanger.

As the core equipment of supercritical carbon dioxide (S-CO₂) Brayton cycle, there is a lack of reliable evaluation and test verification of the overall performance. An in-depth simulation and performance analysis of one axial turbine are carried out, focusing on the impact of inlet and exhaust housings with experiment results for different operating conditions. The results show that the numerical calculation method and model are able to evaluate the performance at different load conditions more accurately. Compared with the test results, the maximum efficiency error is 1.77 percentage point and the flow rate error remains within 5.6%. The crown pattern can reduce leakage and mixing losses, and increase efficiency by 1.4 percentage point compared to the common top clearance pattern. Simulation results show that the efficiency of turbine unit is reduced compared to turbine stage, with a maximum reduction by 2.9 percentage point. The flow loss of inlet and exhaust housings is the main reason for the reduction. The research results can provide technical support for the design and performance simulation of S-CO₂ axial turbines.

Centrifugal compressor is one of the key components in supercritical carbon dioxide (S-CO₂) cycle system, which plays a decisive role in the efficiency and stable operation of the system. Different from the traditional air compressor, the unique physical properties of S-CO₂ working medium make the internal flow field of the compressor more complex. The loss model established based on the physical characteristics of air also needs to be modified specifically to meet the performance prediction requirements of S-CO₂ centrifugal compressor. Therefore, numerical simulation is needed to investigate the internal flow field characteristics of the compressor, so as to improve the compressor performance prediction method accordingly. Firstly, one-dimensional aerodynamic parameters of the compressor were designed, and a three-dimensional model was established based on the one-dimensional design parameters to analyze the characteristics of the internal flow field of the compressor. It was found that the shunt blade had a great influence on the internal flow field, and changes in the internal flow field of the impeller under varying working conditions would also cause changes in the outlet flow Angle. Based on this, The sliding factor and the calculated blade number of the compressor under off-design conditions were corrected, and the surface friction coefficient was improved to predict the performance of the compressor under off-design conditions. The numerical simulation results show that the prediction error of the improved model is significantly reduced, and the average efficiency error decreases from 2.03% to 0.16% under off-design conditions.

"Three-renovation" (energy-saving renovation, flexibility renovations and heating renovations) of coal-fired power is an effective measure for clean and low carbon utilization of coal. The paper analyzes current energy efficiency, flexibility and heating situation of coal-fired power, and researches the policy requirement, implementation progress and expected effect of "Three-renovation". In terms of technical difficulty, energy saving renovation is the most difficult, followed by flexibility and heat supply. It is anticipated that the coal consumption for power supply will be decreased to about 297 g/(kW·h), the additional peak regulation capacity will be over 40 GW and the heating scope will be further expanded. In view of the problems such as large coal power loss, difficult technical transformation of some types, hidden safety risks of equipment under low load operation, unanticipated input and output of renovation, and small financial support of renovation, this paper puts forward measures and suggestions to promote the "Three-renovation" from the perspectives of policy, technology, standards and market.

Under the background of auxiliary heat supply of ejector, in order to study the characteristics of ejector under variable working conditions, the calculation methods of ejector at home and abroad are investigated, the ejector calculation model is established and coupled with the high back pressure unit. Moreover, the effects of working steam, ejected steam, ejector back pressure, and ejector opening on the performance of the ejector under variable working conditions are obtained. The results show that, when the working fluid pressure of the ejector changes from 0.25 MPa to 0.45 MPa, the mass flow of the working fluid first increases and then decreases, and the ejector has the best performance at the design working steam pressure. The critical back pressure increases with the steam injection pressure. The pressure of the mixed fluid after injection will not only affect the work of the ejector but also the work of the condenser. Compared with the back pumping unit, the minimum cooling flow of the low pressure cylinder of the high back pressure coupling injector unit can be reduced by 140 t/h and the power supply range can be increased by 43 MW.

The main-auxiliary combined indirect dry cooling system has attracted attention from the industry in recent years, while few scholars at home and abroad have conducted studies on its flow and heat transfer performances. In order to investigate the transport characteristics of the main-auxiliary combined indirect dry cooling system under different configurations, this paper establishes six physical models of the combined indirect dry cooling systems with the double-layer/single-layer main and auxiliary radiators. And then the cooling performances are analyzed and compared using the commercial software FLUENT. The results show that, the construction has obviously higher impacts on the auxiliary cooling system than the main one, meanwhile the air mass flow rate presents larger difference than the heat rejection; In absence of wind, the main-auxiliary combined cooling system with double-layer heat exchanger arrangement, has better cooling performance than that with the single layer heat exchanger arrangement, and case A possesses the best cooling performance; Under crosswind effects, case C has highest cooling capability of the auxiliary cooling system, while the behavior of its main cooling system should also be considered before the engineering selection. This research may provide some theoretical guidelines for the engineering design and application of the main-auxiliary combined natural draft dry cooling system.