| A novel polygeneration can be composited of solar thermal power,heating and absorption cooling subsystems.In the top of the polygeneration,solar thermal power subsystem takes fully advantage of focusing action of parabolic or butterfly heliostats to collect solar energy.Thermal energy concentrated from heliostats is then pumped to a heat transfer device with the purpose of producing high temperature and pressure working fluid that is then transferred to drive the turbine in a conventional power generation technology for the sake of generating electricity.The released working fluid from the turbine is adopted as efficient heat source and introduced into a heating subsystem or an absorption subsystem in the bottom cycle.Heating and cooling capacity produced in the polygeneration can be employed to achieve the demand of household heating,living heating and refrigeration.Therefore,polygeneration exhibits the capacity of notably eliminating energy crisis and further avoiding dependence on non-renewable fossil fuels(mainly coal,oil and natural gas),due to its sufficient thermal source,green generating technology,no secondary pollution,etc In addition,polygeneration system is generally operating under the condition of relatively simple equipments with higher thermal efficiency over conventionally separated systems.However,there exists obvious mismatch among the need of household heating,cooling and supply of residual thermal energy which is primarily aroused by the time and space influence,such as geography,season and day and night,etc.And this mismatch has severely weakened the thermal efficiency of the whole polygeneration system.It is believed that thermal storage system(TES)is one of the most efficient methods to address this problem.Naturally,TES transforms energy into relatively stable thermal energy with the goal of temporarily storing energy and reutilizing it in high-load or night time.As is well known that performance and cost of a TES device is largely determined by the properties of therml energy materials.And carbonates as a molten salt are supposed to be promising high-temperature thermal energy materials.Carbonates obtain the advantages of large latent heat,low cost,superior thermal stability and chemical stability as well as preferable compatibility with metal containers.Their operating temperature is in excellent agreement with the high temperature and pressure turbine,and therefore carbonate TES enables to improve the overall thermal efficiency of the CCHP system.Nevertheless,to our best knowledge,single carbonate has high melting point and relatively low thermophysical properties(mainly specific heat and thermal conductivity),which has presented enormous limitations for applications in CCHP system.Thermodynamic parameters of the carbonate multielement system are elaborated based on thermal dynamics and eutectic theory.According to the minimum principle of Gibbs energy,carbonate binary and ternary system are conducted for the sake of solving instinct problems in single carbonate.It is found that carbonate multielement system indicates the function of effectively broadening operating temperature scope of carbonate system and provides more material choices for heat storage and transfer in high temperature applications.Melting temperature and corresponding latent heat range are between 407.13 and 898.6?,138.2 and 506.7J/g,respectively.Lithium fluoride with relatively large specific heat is introduced into carbonate ternary as specific heat enhanced medium through high temperature melting method.The primary aim is to improve specific heat of composed carbonate that is just physically mixed during the high-temperature fabrication.Results indicate that lithium fluoride shows remarkable enhancement in specific heat of composite carbonate and the maximum specific heat in solid and liquid are 1.416 and 1.912J/(g·?),respectively,when the addition of lithium fluoride is 15wt.%.Three carbon materials(expanded graphite,EG,graphene nanosheets,GNSs and multi-walled carbon nanotubes,MWCNTs)are brought into the carbonate ternary as thermal conductivity enhanced medium through mechanical mixing and results indicate that three carbon materials all have obvious improvement in thermal conductivity.EG shows the largest enhancement,orderly followed by GNSs and MWCNTs.The various improvement is contributed to their structural characteristics.EG can form effective heat transfer links in three-dimensional space,while GNSs and MWCNTs can only form effective heat transfer links in two-dimensional surface and one-dimensional axis,respectively.The two-step method is adopted to scatter three nano-Al2O3(20,50 and 80nm)and Tetra-needle like ZnO whiskers(T-ZnOw)as specific heat and thmal conductivity enhanced medium into carbonate ternary in order to prepare carbonate based nanofluids.Nanofluids exhibit outstanding enhancement in specific heat and thermal conductivity and the enhancement variation is obviously affected by the particle size and content of nanoparticles.The enhancement to specific heat of nano-Al2O3 is superior to that of T-ZnOw,while the enhancement to thermal conductivity of T-ZnOw is superior than that of nano-Al2O3.The optimized content is 1.0wt.%for 20 nm nano-Al2O3,1.0wt.%for 50 nm nano-Al2O3 and 0.8wt.%for 80nm nano-Al2O3.The corresponding enhancement of specific heat is 18.5%in solid and 33.0%in liquid for 20nm nano-Al2O3;17.9%in solid and 22.7%in liquid for 50nm nano-Al2O;13.2%in solid and 17.5%in liquid for 80nm nano-Al2O3,respectively.The maximum thermal conductivity of carbonate/T-ZnOw composites is approximately 4.483W/(m·?)with enhancement of 167.80%over pure carbonate ternary,when content of T-ZnOw is 1.4wt.%?The total heat storage and average heat storage density of composite carbonates are all larger than pure carbonate ternary during the temperature range of solid sensible heat(300-370?)and liquid sensible heat(450-600?).While during the sensible-latent heat range(300-600?),the total heat storage of composite carbonates with Al2O(20nm),Al2O(50nm),lithium fluoride,Al2O(80nm)and T-ZnOw are larger than pure carbonate ternary.Only the average heat storage density of composite carbonates with lithium fluoride and three nano-alumina are larger than pure carbonate ternary.Heat transfer performance under three conditions of forced convection turbulence,natural convection turbulence and natural convection laminar flow are compared by introduction of figure of merits(FOMs)as the criterion.Results indicate that heat transfer performance of all composite carbonates are better than pure carbonate ternary and the diversified effect is mainly caused by the various liquid specific heat.Heat exchanger area and pump consumption are also investigated according to the FOMs.Heat exchanger area of composite carbonates increase respectively in the order of T-ZnOw,lithium fluoride,nano-Al2O3(50nm),nano-Al2O3(20nm),EG,nano-Al2O3(80nm),GNSs and MWCNTs,and the variation is mainly related to thermal conductivity.Pump consumption of composite carbonates decrease respectively in the order of lithium fluoride,nano-Al2O3(20nm),nano-Al2O3(50nm),nano-Al2O3(80nm),T-ZnOw,GNSs,MWCNTs and EG,and the variation is mainly related to liquid specific heat. |
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