Periodic Reporting for period 1 - CHT-sCO2 (Coupled heat transfer and thermodynamic optimization of supercritical CO2 heat exchangers)
Berichtszeitraum: 2021-06-01 bis 2023-05-31
The supercritical CO2 (SCO2) Brayton cycle system with the advantages of high efficiency and high compactness is very suitable for clean and renewable energy like solar energy. Compared with other energy storage technologies, transcritical or supercritical CO2 energy storage technology has the advantages of low cost, high efficiency, and fewer constraints, and has a promising application potential. Whether it is a supercritical or transcritical CO2 energy system, it faces a practical technical problem. The thermophysical properties of CO2 change drastically in the regions near the critical point or pseudo-critical point, making thermodynamic processes such as heat transfer and flow extremely complex processes, far from being fully understood or well predicted. To address these problems, this project employed a research method combining theoretical analysis, numerical simulation and experimental testing, to study the characteristics of heat transfer and flow of SCO2 to improve and develop novel power systems by harnessing the unique properties of CO2 for renewables, especially solar power. The project is conducive to the large-scale development and utilization of clean energy (like solar energy), so as to reduce carbon emissions and achieve carbon neutrality to cope with climate change.
1. Thermal-hydraulic characteristics of SCO2 flows
For vertical SCO2 flow under cooling conditions, there existed a maximum heat transfer coefficient when the fluid bulk temperature was slightly higher than the pseudo-critical temperature. The influence of heat-flux condition on the local entropy generation could be attributed to the distributed matching between heat flux and the difference between the wall temperature and the fluid bulk temperature. For vertical SCO2 flows under heating conditions, heat transfer was enhanced and the irreversibility was reduced in downward flows relative to flows without gravity, whereas the heat transfer deteriorated and the irreversibility was increased in upward flows. The heat transfer further deteriorated and the irreversibility was further increased when a linearly decreasing heat-flux distribution was applied to the wall, while the heat transfer deterioration was alleviated when a linearly increasing heat-flux distribution was used. In heated horizontal flows, buoyancy acted to inhibit heat transfer when the top half of the tube wall was heated, however, when the bottom half of the tube wall was heated, this inhibition was alleviated. when the top half of the tube wall was cooled, the buoyancy effect inhibited heat transfer, while the synergy between the temperature gradient and velocity fields was improved by the secondary flow in the near-wall region at the top of the tube, which eventually led to an increase of ~21% (on average) in the heat transfer coefficient relative to the case when the bottom half of the tube wall was cooled.
2. Experimental studies on SCO2 flows in printed circuit heat exchanger
The local heat transfer coefficient was higher when the phenomenon that the local temperature of SCO2 is equal to the pseudo-critical temperature appears in the laminar sublayer than when this phenomenon appears in the buffer layer. A new heat transfer correlation was proposed, and the maximum relative error between the Nusselt number predicted by the new heat transfer correlation and Nu obtained via experiment and simulation was within 20%, with the average relative error being 3.95%.
3. Optimisation method of heat exchange system based on coordinated distribution principle
The heat load depends not only on the values of key parameters (such as heat transfer coefficients, temperature differences, etc.), but also on their distribution coordination. Moreover, the whole coordination could be improved by suitably adjusting the flow fraction among the heat exchangers, eventually improving the overall heat load. An appropriate adjustment of the flow fraction in heat exchangers that were in series/parallel was preferable to improving the match between the hot and cold fluids, leading to a decrease in the thermodynamic irreversibility.
4. Exploitation and application of unique properties of CO2
The match of the local dense distribution of fins with the region near the pseudocritical point could obtain better overall thermal performance in the modified airfoil fins heat exchanger. The front-sparse and rear-dense distribution of fins was the optimum scheme in the three distributions of the modified airfoil fins channel because its comprehensive performance is 23 % to 29 % higher than that of the uniform distribution of fins. When the channel with the spacing distribution of short rib structures was adopted in PCHE, the effectiveness of PCHE could reach 98.4 – 98.7%, the efficiency of the SCO2 Brayton cycle system based on solar power could be increased by 15.3%, and the compactness of the system could be improved by 3.8%. A concentrated photovoltaic system was integrated with advanced technologies of PV cells and CO2 battery to maximise the uninterrupted utilisation of solar energy, especially optimising the system according to the solar spectrum properties to achieve full-spectrum utilisation. The results showed that the round-trip efficiency could achieve higher than 88%, and the power produced by the turbine in discharging phase could be higher than the input PV power in the charging phase.
For vertical SCO2 flow under cooling conditions, there existed a maximum heat transfer coefficient when the fluid bulk temperature was slightly higher than the pseudo-critical temperature. The influence of heat-flux condition on the local entropy generation could be attributed to the distributed matching between heat flux and the difference between the wall temperature and the fluid bulk temperature. For vertical SCO2 flows under heating conditions, heat transfer was enhanced and the irreversibility was reduced in downward flows relative to flows without gravity, whereas the heat transfer deteriorated and the irreversibility was increased in upward flows. The heat transfer further deteriorated and the irreversibility was further increased when a linearly decreasing heat-flux distribution was applied to the wall, while the heat transfer deterioration was alleviated when a linearly increasing heat-flux distribution was used. In heated horizontal flows, buoyancy acted to inhibit heat transfer when the top half of the tube wall was heated, however, when the bottom half of the tube wall was heated, this inhibition was alleviated. when the top half of the tube wall was cooled, the buoyancy effect inhibited heat transfer, while the synergy between the temperature gradient and velocity fields was improved by the secondary flow in the near-wall region at the top of the tube, which eventually led to an increase of ~21% (on average) in the heat transfer coefficient relative to the case when the bottom half of the tube wall was cooled.
2. Experimental studies on SCO2 flows in printed circuit heat exchanger
The local heat transfer coefficient was higher when the phenomenon that the local temperature of SCO2 is equal to the pseudo-critical temperature appears in the laminar sublayer than when this phenomenon appears in the buffer layer. A new heat transfer correlation was proposed, and the maximum relative error between the Nusselt number predicted by the new heat transfer correlation and Nu obtained via experiment and simulation was within 20%, with the average relative error being 3.95%.
3. Optimisation method of heat exchange system based on coordinated distribution principle
The heat load depends not only on the values of key parameters (such as heat transfer coefficients, temperature differences, etc.), but also on their distribution coordination. Moreover, the whole coordination could be improved by suitably adjusting the flow fraction among the heat exchangers, eventually improving the overall heat load. An appropriate adjustment of the flow fraction in heat exchangers that were in series/parallel was preferable to improving the match between the hot and cold fluids, leading to a decrease in the thermodynamic irreversibility.
4. Exploitation and application of unique properties of CO2
The match of the local dense distribution of fins with the region near the pseudocritical point could obtain better overall thermal performance in the modified airfoil fins heat exchanger. The front-sparse and rear-dense distribution of fins was the optimum scheme in the three distributions of the modified airfoil fins channel because its comprehensive performance is 23 % to 29 % higher than that of the uniform distribution of fins. When the channel with the spacing distribution of short rib structures was adopted in PCHE, the effectiveness of PCHE could reach 98.4 – 98.7%, the efficiency of the SCO2 Brayton cycle system based on solar power could be increased by 15.3%, and the compactness of the system could be improved by 3.8%. A concentrated photovoltaic system was integrated with advanced technologies of PV cells and CO2 battery to maximise the uninterrupted utilisation of solar energy, especially optimising the system according to the solar spectrum properties to achieve full-spectrum utilisation. The results showed that the round-trip efficiency could achieve higher than 88%, and the power produced by the turbine in discharging phase could be higher than the input PV power in the charging phase.
This project summarised the thermal-hydraulic characteristics of SCO2 under various boundary conditions, revealed the distribution law of heat transfer enhancement and deterioration, and thus proposed a new method of heat transfer enhancement based coordinated distribution of parameters. Based on this new method, the heat transfer enhancement phenomena of SCO2 could be explained very well, and several heat transfer enhancement structures were proposed to achieve a substantial increase in heat transfer without increasing or slightly increasing pressure drop. According to the characteristics of the solar spectrum and current photovoltaic cells, a concentrated photovoltaic system integrated with CO2 energy storage was proposed. Using CO2 as the working medium to reduce CO2 emissions to achieve carbon neutralisation was far more meaningful than simply capturing and burying it as a greenhouse gas. The research scope of this project was further expanded to photovoltaic efficiency and solar spectrum utilisation. The optimal quantitative relationship between the concentration ratio and the cooling ratio was obtained in the concentrated photovoltaic systems with radiative cooling, and a photovoltaic-thermoelectric hybrid system with radiative cooling as the cold side was also proposed to maximise the solar electricity and achieve 24-h operation.
The new heat exchange structures proposed in this project could not only effectively improve the efficiencies of the SCO2 Brayton cycle system but also effectively improve the overall compactness of the system. The combined system of photovoltaic cells and CO2 energy storage proposed in this project provided an effective solution to the large-scale and long-duration energy storage of renewables. The successful implementation of this project has important technical guidance value for vigorously promoting the development of renewables, especially solar power, and is of great significance for reducing carbon emissions and achieving carbon neutrality in response to climate change.
The new heat exchange structures proposed in this project could not only effectively improve the efficiencies of the SCO2 Brayton cycle system but also effectively improve the overall compactness of the system. The combined system of photovoltaic cells and CO2 energy storage proposed in this project provided an effective solution to the large-scale and long-duration energy storage of renewables. The successful implementation of this project has important technical guidance value for vigorously promoting the development of renewables, especially solar power, and is of great significance for reducing carbon emissions and achieving carbon neutrality in response to climate change.