Optimizing micro-channel structure in a collector/evaporator using multi-objective algorithm

Abstract: Solar collector can serve as an evaporator (i.e., collector/evaporator) in a direct-expansion solar-assisted heat pump (DX-SAHP) system. Among them, the refrigerant can directly absorb the heat from the solar energy and/or ambient air. Much effort has been made to optimize the traditional structure of micro-channel in the heat exchangers. However, it is still lacking on the influence of multiple structural parameters coupling on the flow and heat transfer characteristics of the collector/evaporator. In this study, the experimental platform of DX-SAHP water heater was designed and built using R290 (propane) as refrigerant in Qingdao City, Shandong Province, China. A series of experiment and simulation were applied to explore the flow and heat transfer characteristics of the micro-channel collector/evaporator. Firstly, the appropriate and accurate correlations were selected to establish the flow and heat transfer coupled mathematical model of micro-channel collector/evaporator. The accuracy of the model was then verified by a large number of experimental data. Secondly, three structural parameters were selected as the hole length, width, and spacing in the flow channel of micro-channel collector/evaporator. 25 groups of simulation was then conducted. Response surface method (RSM) was used to establish the ternary quadratic nonlinear objective functions of collector efficiency and pressure drop after simulation. Response surface graphs were obtained to clarify the influence of three structural parameters on the collector efficiency and pressure drop. Finally, the multi-objective particle swarm optimization (MOPSO) algorithm was introduced to optimize the structure parameters of micro-channel collector/evaporator flow channel. The representative solutions were obtained by k-means clustering. The optimal solutions were achieved within a certain range of working conditions. The eight kinds of structures were calculated by MOPSO. Specifically, the collector efficiencies were much greater than that of the original, while the corresponding pressure drops were lower than before. The typical conditions were selected in spring, summer, autumn, and winter to obtain the optimal channel size of micro-channel collector/evaporator in the end. The results showed that the mathematical model of the micro-channel collector/evaporator was predicted better the experimental data within an average relative error of 10%. The multivariate statistical coefficients R2 of the objective functions were 0.995 and 0.999, respectively, for the collector efficiency and pressure drop, indicating the higher accuracy and better regression. The hole length, width, and spacing of micro-channel collector/evaporator were posed the greatest influence on the collector efficiency and pressure drop. The influence degree of the parameters was ranked in the descending order of the hole width, hole length, hole spacing. The Pareto solutions were calculated for the collector efficiency and pressure drop using the MOPSO. Moreover, the different ambient conditions were considered to determine the optimum structural parameters with the best overall performance in the whole year. The better optimization was obtained under the optimal combination, where the hole length was 1.27 mm, the hole width was 1.53 mm, and the hole spacing was 0.39 mm under different seasonal conditions after experimental verification. Compared with the original, the collector efficiency increased by 8.29%, whereas, the pressure drop decreased by 11.05%, indicating the significantly improved performance of flow and heat transfer in the micro-channel collector/evaporator. The finding can provide a theoretical basis for the practical engineering design of DX-SAHP systems.