Experimental And Numerical Study Of Solar Phase Change Thermal Storage With Different Tube Types And Fin Arrangement

Solar thermal energy storage technology based on phase change material plays an important role in the optimisation of our energy structure by shifting the thermal energy from solar energy in time as well as in space.The principle of this technology is to use the large amount of latent heat absorbed or released by phase change materials during the energy storage or application process.This technology is of great interest for its applicability for excellent economic,stability for heat storage and flexibility for application in solar thermal applications.However,the technology suffers from constraints such as the generally low thermal conductivity of phase change materials and the slow thermal response rate.Therefore,this paper focused on optimising the design of phase-change-based heat exchangers,which improve the storage or exhaust efficiency,the overall thermal conductivity and the thermal response rate.This study started with a widely used phase-hange-based solar thermal energy accumulator,which is made up of phase-change-based thermal energy storage units with phase change materials encapsulated inside.Numerical simulations were used to reveal the flow characteristics and heat transfer mechanism of the phase-change-based thermal energy storage cavity during the thermal energy storage process.Based on the above numerical simulation results,the design was optimised in two main directions: 1.optimisation of the tube shape of the phase-change-based thermal energy storage unit;2.reinforcement of the heat transfer designed by embedding fins inside the phase-change-based thermal energy storage unit.By means of the above-mentioned optimisation solutions,this paper finally achieved an optimised construction of each phase-change-based thermal energy storage unit in the phasechange-based heat exchanger.In addition,an experimental platform for the visualisation of the phase change heat transfer cavity had been built and combined with image processing techniques,the flow of the phase change material and the heat transfer performance during the heat storage process inside the phase-change-based thermal energy storage unit were further analysed statistically and analytically.This paper provided some new ideas and references for the further development of solar phase-change-based heat storage technology and its efficient utilisation.The following elements were investigated in this paper.(1)After optimizing the tube type,the triangular T2 case was the optimal design,with an improvement in thermal storage efficiency of about 27.86% compared to the circular case.That was mainly due to the fact that different tube designs had different degrees of influence on natural convection.The phase-change-based thermal energy storage unit with higher natural convection intensity would improve the heat storage efficiency;(2)It was found that the phase change material in the phase-change-based heat storage energy unit spent longer to melt away from the heating wall.Therefore,this paper added fin optimization design,using fins to transfer heat to the phase change material away from the wall.When the effect of fin length on heat storage was investigated,it was found that a fin design of0.75r＿SF was optimal,achieving a 40% improvement in thermal performance at the expense of a 5% reduction in phase change material filling.The reason for the above effect was that the fins increased the heat transfer area of the heat transfer wall and also promoted the natural convection intensity in the phase change heat transfer cavity,thereby increasing the heat transfer efficiency of the phase-change-based thermal energy storage unit;(3)After selecting an optimisation scheme for the better fin angle,this paper focused on the effect of fin angle on heat storage.It was proposed that fins with an angle of 0° had the greatest impact on the thermal performance of the phase change thermal energy cavity in the middle and early stages of melting process,but fins with an angle slightly less than 0° had a more significant impact on the global process of melting,such as the-15°＿SF fin proposed in this study.The main reason for the influence of the above fin angles on heat storage was that different fin angle designs would change the natural convection inside the phase change heat transfer cavity,thus acting as a facilitator of convective heat transfer;(4)The DF model was proposed in this subject in order to achieve a more efficient thermal response rate.In the optimised design of the DF model,0.75r＿DF was the better design,and when liquid fraction was 0.6,its volume fraction of metal fins increased by 1.19% compared to the-15°＿SF model,which improved the heat storage rate by nearly 15%.The increase in thermal response rate was mainly due to the additional heat transfer area of the metal fins;(5)Tree-shaped fins had the ability to increase the surface area of the metal heat transfer surface without taking up more space for filling with phase change material,thus improving the storage or exhaust efficiency of the phase-change-based heat accumulator.This study compared tree-shaped fins with the plate fins described above.It was found that the tree-shaped fin design had the highest enhancement rate for the early and middle stages of the phase change material melting process,with an enhancement rate of 68%.It can be used in special thermal demand scenarios with high thermal response rates and high-volume requirements,such as solar thermal energy storage in aerospace applications.