Combing the excellent performance of carbon dioxide thermodynamic cycle and the reutilization demands for the captured carbon dioxide, the energy storagetechnologies based on carbon dioxide thermodynamic cycle have the potential to play an important role in the future energy system which mainly consists of renewable energies. This paper presents the definition of the energy storage technology based on carbon dioxide thermodynamic cycle, and then classifies this energy storage technology into six types, which are electrothermal energy storage, compressing carbon dioxide energy storage with low, mid and highpressure gas storage, compressing carbon dioxide energy storage with low-temperature and near atmospheric temperature liquid storage, and the constant pressure gas storage energy storage. The state-of-the-art research status, advantages and disadvantages of these technologies are discussed. In general, the compressing carbon dioxide energy storage with low pressure gas storage is most mature, and there are demonstration units constructed. The compressing carbon dioxide energy storage with high pressure gas storageand the electrothermal energy storage have a good overall performance, but their costs are high. The compressing carbon dioxide energy storage with constant pressure gas storage has the highest cycle efficiency, reaching 74%~75%, and the energy storage density is up to 2 (kW·h)/m³, making it one of the most promising compressing gas energy storage technologies.

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.

Based on the first and second laws of thermodynamics, parameters of the supercritical carbon dioxide (S-CO₂) recompression cycle, recompression reheat cycle, partial cooling cycle, partial cooling reheat cycle coal-fired power generation system were calculated and analyzed by using MATLAB software. Then, the impact of shunt coefficient, outlet and inlet pressure of the main compressor on the system circulation efficiency, equipment and exergy efficiency of the system were discussed respectively, and the four types of circulation systems were compared and analyzed. The results show that the cycle efficiency varies with the same parameters under different cycle layout or the same cycle layout and different operating parameters. There is a shunt coefficient for exergy efficiency and exergy efficiency to reach an optimal value. There is a coupling relationship between the influence of outlet and inlet pressure of the main compressor and the shunt coefficient on the circulation efficiency. For different parameter changes, exergy efficiency of system is mainly affected by exergy efficiency of different equipment. Reheat can increase circulation efficiency and exergy efficiency of the system, while some cooling cycles are relatively less sensitive to parameter changes.

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.

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.

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.

To ensure the safe and reliable operation of supercritical carbon dioxide (S-CO₂) power plant over a long period of time, referred to the material commonly used in traditional steam boiler, the material of S-CO₂ boiler is selected fromhigh temperature strength and corrosion characteristics. When CO₂ flows through the boiler, the temperature of CO₂ is high, the temperature difference between CO₂ and tube wall temperature is large, over the 30~50 ℃ of steam boiler. Then the grade of the heat-resistant steel has to be upgraded. However, requirements and methods for the material selection for steam boiler cannot be directly used for S-CO₂ boiler. The feasibility of heat-resistant steels such as T23, T91,T92, TP347HFG and Super304H used in S-CO₂ boilers is explored from using maturity and economic performance. The results show that for cooling wall, T91 could be selected at certain tube structure, but 12Cr1MoVG, T21 and T23 et al used in steam boiler could not be selected. For superheater, only austenitic steel TP347HFG and Super304H meet the material selection requirements at certain tube structure.

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.

The supercritical carbon dioxide cycle has many advantages, such as high cycle efficiency, small equipment size, convenient transportation and installation, and easy to reach the critical point. Considering the huge cold energy of LNG, it can not only be used as coolant in the combined cycle system, but also the natural gas after heat transfer can be used as fuel input in the combined cycle, and the rest can be supplied to urban users. A gas turbine/supercritical carbon dioxide combined cycle system based on the utilization of LNG cold energy is proposed in this paper. Select the appropriate cost formula to calculate and analyze the investment cost, operating income and recovery cycle of the circulating power generation system in detail. The influence of some key parameters (such as maximum temperature, maximum pressure, minimum temperature, minimum pressure and shunt ratio) on the power generation characteristics and economy of the supercritical carbon dioxide cycle in the combined cycle system was studied. The results show that with the increase of each single parameter, the cost of equipment investment will first increase and then decrease, but the effect of power generation on income is dominant. Taking the yield as the measurement standard, the higher the maximum temperature, the better, the lower the minimum temperature, the better. Under other parameters, there are optimal values to maximize the yield.The key parameters were optimized by genetic algorithm to maximize the cumulative income. After optimization, the recovery cycle was 5.86 years, and the cumulative income (20 years) was 2.287 billion yuan.

A cooling system was designed for a supercritical carbon dioxide (S-CO₂) axial turbine. The dry gas seal, shaft and casing were cooled by extracting the low temperature S-CO₂ in the pipeline behind the compressor to ensure that the dry gas seal operating temperature was below 200 ℃. The flow and heat transfer characteristics of the cooling system were analyzed using the coupled heat transfer method, and the temperature distributions of solid domains such dry gas seal, shaft and casing of different cooling schemes were compared. The research shows that the temperature drop of the shaft reaches 220.3 ℃ when the_shaft cooling scheme is adopted, and the maximum temperature of the dry gas seal is 229.1 ℃. Further introduction of S-CO₂ with lower temperature and larger flow rate to cool the casing can inhibit the heating effect of the high temperature mainstream at the turbine inlet. The temperature drop of the shaft increases to 244.1 ℃, and the maximum temperature of the dry gas seal decreases to 181.2 ℃. Meanwhile, the reasonable temperature gradient of the cooled domains such as the dry gas seal, shaft and casing is achieved. The cooling system designed in this paper provides a solution for the safe and reliable operation of S-CO₂ axial turbines.