Thermodynamic Analysis And Entransy Optimization Of Cascaded Latent Heat Storage For Heating

Latent heat storage(LHS)can promote heating electrification and cleaning,which is an important technology for energy conservation and carbon reduction in the heating field.At present,most LHS components/devices used for heating are filled with only one type phase change material(PCM),resulting in poor heat transfer uniformity and low thermal performance.Through the reasonable arrangement of different PCMs,the cascade latent heat storage(CLHS)can improve the heat transfer uniformity between PCM and heat transfer fluid(HTF),thereby enhancing the thermal performance of the LHS component/device at low cost.However,the existing thermodynamic and experimental research on the CLHS in heating is far from perfect.Therefore,this paper studies the entransy dissipation-based thermal resistance(EDTR)of the CLHS charge process,the thermodynamic analysis and optimization of the CLHS charge-discharge processes and cycle model,as well as the heat storage and release experimental of the CLHS device,which can provide theoretical guidance and technical support for the application of the CLHS in heating.The specific research of this paper is as follows:(1)The minimum entransy dissipation-based thermal resistance(EDTR)model of the CLHS charge process is established.Based on partial derivative solutions and example analysis,the effects of single-parameter and multi-parameter coupling on the minimum EDTR are revealed.Increasing the stage number of cascaded PCMs/HTF heat capacity rate/overall heat transfer coefficient/heat exchange area can reduce the minimum EDTR,but the effect will gradually decrease to negligible.Increasing the HTF heat capacity rate/stage number and increasing the overall heat transfer coefficient/area can synergistically strengthen the reduction effect on the minimum EDTR,while increasing the HTF heat capacity rate and increasing the stage number weaken each other.Considering the coupling effect and cost of measures,it is recommended to prioritize the coupling of increasing the stage number with increasing the overall heat transfer coefficient/area,followed by the coupling of increasing the stage number with increasing the HTF heat capacity rate.The parameter selection is guided based on the decrease of the minimum EDTR.In addition,the values of minimum EDTR under parameter infinity are derived,which can predict the limit heat storage performance of the cascaded PCMs under given constraints.(2)The research distinguishes the CLHS "charge-discharge processes" and "charge-discharge cycle" in terms of thermodynamic concepts and additional conditions.The CLHS charge-discharge cycle is a special charge-discharge processes in which the initial and final states of each PCM are consistent.Then,the relationship between energy/exergy/entransy flow between the cascaded PCMs and the HTF.The cascaded PCMs transfer the energy/exergy/entransy of the charge HTF to the discharge HTF,some PCMs with initial state smaller than final state transfer their own energy/exergy/entransy to the discharge HTF,and some PCMs with initial state greater than final state will store additional energy/exergy/entransy from the charge HTF.Moreover,through model comparison and case analysis,it is proved that the product of the charge process and the discharge process exergy efficiencies cannot be equal to the CLHS charge-discharge processes and cycle exergy efficiency models.(3)The entransy efficiency models of the CLHS charge-discharge processes and cycle are established and optimized.And the application guidance of cascaded latent heat storage device in heating is provided from three aspects: model applicability,parameter influence,and applicable scenarios for CLHS.The optimization results of the CLHS charge-discharge processes cannot guarantee that the initial and final states of each PCM are consistent,which will lead to unstable operation and performance degradation of the CLHS device.The smaller the temperature difference between the charge and discharge HTF,the greater umber of heat transfer units(NTUs)between the HTF and the PCM,and the higher the maximum cycle entransy efficiency of the CLHS.In addition,the increase in the heat transfer rate of the three-stage CLHS compared with the single-stage LHS can reach 49.7%,while the increase in the cycle entransy efficiency is only 7.4%.Considering that the improvement of the heat transfer rate is only positively correlated with the number of NTUs,the cascade LHS instead of the single-stage LHS is suitable for the large NTUs.(4)The experimental research on the CLHS device for heating is carried out,and the thermal performance index based on energy theory and entransy theory is put forward.Compared with the single-stage LHS device,the two-stage LHS device not only increases the heat transfer temperature difference between the HTF and second-stage PCM,but also prolongs the natural convection time of the second-stage PCM,thus significantly increasing the heat storage(26.5%～44.6%),the heat release(19.8%～74.5%)and the discharge HTF entransy gain(20.0%～75.7%).Besides,under the same charge HTF flow state,the heat storage increase of the three-stage phase change device group is not obvious(-1.5%～6.2%),while the heat release,energy efficiency,entransy efficiency,and the discharge HTF entransy gain decrease.Based on thermodynamic guidance alone,it cannot be guaranteed that the larger the stage number of the CLHS devices,the better the thermal performance.And the design method and control logic of the adjustable CLHS device are proposed for solving the problem of reverse heat storage during the discharge process.