The unreasonable structure of energy use, low energy use efficiency and serious pollutant emissions are the current problems, which seriously restrict the development and progress of society. In the context of "carbon peaking and carbon neutrality", the development of advanced and efficient power cycles (like the supercritical carbon dioxide (S-CO2) Brayton cycle) can contribute significantly to optimising the energy use structure, improving energy use efficiency and reducing pollutant emissions.

The S-CO2 Brayton cycle offers higher efficiency and compactness compared to ordinary power cycles, but also places completely new demands on the performance of the heat exchangers in the cycle, which need to withstand higher temperatures and pressures and have larger specific surface areas. Printed circuit plate heat exchangers (PCHEs) are considered one of the most promising heat exchangers in S-CO2 power generation systems, transcritical heat pumps and refrigeration systems.

Scholars have now systematically analysed the heat transfer characteristics of S-CO2 flow in a single tube in a microchannel (a channel with a diameter of less than 3 mm) under different influencing factors, summarised the heat transfer characteristics of supercritical fluids, and explored issues such as buoyancy forces, and thermal acceleration effects. The thermal-hydraulic characteristics of S-CO2 flow in a PCHE with different flow paths have also been investigated and the correlation equations for the heat transfer parameters under the corresponding operating conditions have been summarised. However, most of the correlations proposed in the current study are only applicable to a small range of operating conditions, and there is a lack of universal heat transfer and flow correlations for a wide range of boundary conditions.

Two new wing rib structures based on the NACA0020 wing type were proposed, and the results showed that the improved wing ribs effectively reduced the (fire product) dissipation of the heat transfer process and improved the heat transfer performance. while the heat transfer was enhanced well by changing the arrangement of the fin ribs in the wing channel compared to the uniform distribution case, and the design of the heat transfer structure is optimised. There is still much room for improvement in the optimal design of heat transfer structures, and subsequent research should focus on the development of new and efficient heat transfer structures to achieve enhanced heat transfer with little or no increase in flow resistance and to further increase the power density of the PCHE.

As for the design method of S-CO2 PCHE, due to the drastic changes in the thermophysical properties of S-CO2 near the proposed critical point, the overall heat exchanger design methods based on constant physical fluids (efficiency-heat transfer unit number method, logarithmic mean temperature difference (LMTD) method) cannot be applied to the design of S-CO2 heat exchangers. A segmented design method to analyse the performance of the PCHE was proposed and this method could accurately capture the drastic changes in the heat transfer performance of S-CO2 flow within the heat exchanger and also improve the heat transfer performance of the heat exchanger. As for the optimisation methods, the heat transfer characteristics of a straight channel PCHE were analysed from the perspective of synergy in the distribution of heat transfer parameters, two methods to improve the distribution synergy were proposed, namely variable cross-section and insertion of fins. A matrix analysis method to divide the heat exchanger was proposed and the factors affecting the total heat load were analysed. The results show that when the total heat transfer is fixed and equally distributed among the sub-heat exchangers, the smaller the heat transfer density and temperature difference distribution synergy angle, the better the distribution of the corresponding local (fire product) dissipation rate within the heat exchanger, which also provides a practical approach to heat exchanger optimisation. But it also needs to be based on the unique nature of supercritical fluids to develop the new enhanced heat transfer theory to guide the development and optimization of new high-efficiency heat exchangers, some scholars use the multi-objective optimization method to get Pareto optimal solution to improve the heat exchanger integrated heat transfer performance.

Current research has focused on the heat transfer flow within the design conditions under steady-state conditions. Considering the changes of conditions in practical applications, future research needs to analyse the performance of S-CO2 heat exchangers under non-design conditions, especially the transient response performance of heat exchangers under drastic changes in conditions, in order to cope with the dynamic oscillation characteristics of supercritical fluids.

The microchannel heat exchanger is related to the overall cycle, and the performance of the heat exchanger affects the efficiency of the whole cycle. As the key equipment for thermal power conversion in the cycle, a comprehensive and systematic study and analysis of the heat exchanger will help to improve the efficiency of the whole system and truly achieve the "double carbon target".

These works have received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. [882628], the National Key Research and Development Program - China (2017YFB0601803), National Natural Science Foundation of China (51676185) and National Energy Group Major Pilot Project-China (GJNY2030XDXM-19-10).

The relevant works can be found in:

(1)       Jiangfeng Guo*, Jian Song, Suray Narayan, Konstantin Pervunin, Chrisots Markides, Numerical investigation of the thermal-hydraulic performance of horizontal supercritical CO2 flows with half-wall heat-flux conditions, Energy 264 (2023) 125845. https://doi.org/10.1016/j.energy.2022.125845

(2)       Jiangfeng Guo*, Xinying Cui, Xiulan Huai, Keyong Cheng, Haiyan Zhang, The coordination distribution analysis on the series schemes of heat exchanger system, International Journal of Heat and Mass Transfer 129 (2019) 37-46. https://doi.org/10.1016/j.ijheatmasstransfer.2018.09.068   

(3)       Zengxiao Han, Jiangfeng Guo*, Haiyan Liao, Zhongmei Zhang, Xiulan Huai, Numerical investigation on the thermal-hydraulic performance of supercritical CO2 in a modified airfoil fins heat exchanger, The Journal of Supercritical Fluids 187 (2022) 105643. https://doi.org/10.1016/j.supflu.2022.105643

(4)       Xinying Cui, Jiangfeng Guo*, Xiulan Huai, Keyong Cheng, Haiyan Zhang, Mengru Xiang, Numerical study on novel airfoil fins for printed circuit heat exchanger using supercritical CO2, International Journal of Heat and Mass Transfer 121 (2018) 354-366. https://doi.org/10.1016/j.ijheatmasstransfer.2018.01.015

(5)       Jiangfeng Guo*, Design analysis of supercritical carbon dioxide recuperator, Applied Energy 164 (2016) 21-27. https://doi.org/10.1016/j.apenergy.2015.11.049