Design And Thermal Performance Of Flexible Textile-based Solar Air Heaters And Lowtemperature Heat Storage Research

Solar energy is an inexhaustible and renewable green energy blessed by nature.How to best collect and store solar thermal energy is always one of the hot spots among the scientific community.The flat-plate solar air heater,which takes airflow as heat transfer medium,collects solar energy and converts it into thermal energy to satisfy the needs of production and life.However,flat-plate solar air heaters are mainly composed of metals and glasses,of which heavy structure and low heat transfer efficiency limits its large-scale application.Some researchers got inspiration from the light collection and heat insulation mechanism of polar bear fur and designed flexible three-dimensional(3D)textile-based composites with spacer fabric and transparent films to collect solar thermal energy,which supplies good research direction for the usage of 3D textile-based composites.At present,the research on the flexible textile-based solar air heater(FSAH)is still in the initial stage of development.The structural stability of textile-based composites,the thermal performance of FSAH and the way of its heat storage need to be further explored and optimized.This paper mainly selected materials for each part of the flexible 3D textile-based composites and evaluated its outdoor durability.The effect of the structure of FSAH and the parameter of spacer fabric on thermal efficiency of FSAHs were explored,which laid foundations for the industrialized production of such textile-based FSAHs.In addition,the energy storage materials that can be used for low-temperature heat storage were explored,which provides a new choice for the thermal energy storage of solar air heaters.The specific research contents and conclusions are as follows:(1)According to the working principle and flexibility requirements of the FSAH,the second chapter analyzed and put forward the selection basis and evaluation method of each component of the FSAH(spacer fabric,transparent layer,absorbing layer and Thermal insulator).Results showed that the upper surface of spacer fabric should be designed as hexagonal mesh structure in order to let more sunlight passing through.After accelerated aging in the artificial accelerated aging machine for 2400h,the transparent layer prepared by DC184still kept better apparent morphology,solar transmittance and mechanical properties,which indicates that the DC184 could resist outdoor aging and can be suitable for the preparation of textile-based solar air heaters.After mixing DC184 and nano-black carbon at a mass ratio of1:25,the absorptivity of the prepared absorbing layer was 94.5%and can be used as flexible absorbing layer of FSAH.Additionally,the mechanical properties of flexible textile-based composites prepared by spacer fabric and DC184 were evaluated(including tensile elastic recovery,low-speed compression elastic recovery and high-speed compression elastic recovery).The results demonstrated that the transparent coating layer enhanced the tensile strength of textile-based composites,so that they could withstand the repeated tensile elongation within 40%in the weft direction without damaging the coating surface.Furthermore,the 3D textile-based composite also could withstand the multiple vertical low-speed and high-speed cyclic compression with good resilience.(2)The third chapter mainly refers to the preparation and assessment of thermal performance of FSAHs with different structures.An outdoor experimental test platform was built to study the effects of heat absorption layer,collector length and thermal insulation layer on the thermal efficiency of FSAHs.Although the heat loss coefficient was the highest when tested in a stagnant condition,the heater FSAH-M1,which bottom layer and spacer filaments coated with black coating,still had a higher outlet temperature and thermal efficiency.Besides,the thermal efficiency increased with the increment of mass flow rate.When the mass flow rate was increased from 0.025 kg/(m~2·s)to 0.055 kg/(m~2·s),the thermal efficiency?of FSAH-M1increased from around 66.2%to around 77.8%.The net energy gain Q_eand effective efficiency?_e of the FSAH-M1 increased first and then decreased with the increase of mass flow rate.When the mass flow rate was 0.037 kg/(m~2·s),the maximum effective efficiency?_e of FSAH-M1 was 68.4%.Experimental showed that the temperature rise?T and net energy gain Q_e of FSAH-M1 increased with the increase of heater length,but its thermal efficiency?decreased with the increase of heater length.Therefore,without considering economic benefits,the length of FSAH can be increased when the outlet temperature is required to be high.In addition,when another layer of transparent textile-based composites was added to the upper surface of the FSAH-M1,the heater performed better.At the mass flow rate of 0.037 kg/(m~2·s),the thermal efficiency?and effective efficiency?_e of FSAH-M4 were 97.5%and 85.8%,respectively.(3)In order to explore the influence of spacer fabric parameters on the performance of FSAHs,the heat transfer and flow characteristics of FSAH are numerically simulated in Chapter IV.The 3D-spatial-trajectory of spacer filaments in one cycle was given by analyzing the weaving parameters,and then a reasonable simplified model of spacer fabric was proposed.The 3D flow field and temperature field of the FSAH was numerical simulated by Flo EFD(for Solid Works)method.The effects of fluid inlet direction,spacer filament density d_n,diameter of spacer filament d_fand thickness of spacer fabric H on the friction factor and heat transfer of FSAH were analyzed.Results showed that the experimental values of Nusselt number Nu and friction factor f were in good agreement with the numerical values,which means that the established fabric model and simulated method can effectively evaluate the heat transfer and flow characteristics of FSAHs.When the spacer filaments and bottom layer were coated with black absorption coating,the thermal performance parameter THPP of FSAH was much larger than 1,indicating that the improvement of the heat exchange of the model overcomes the increase of frictional resistance caused by the spacer filament and enhancing the overall thermal performance of the model.In addition,the THPP of FSAH model when air flows from rear view of fabric was slightly larger than that of the model when air flows from front view of fabric.Moreover,results demonstrated that the THPP of FSAH all increased with the increase of spacer filament density d_n,diameter of spacer filament d_fand thickness of spacer fabric H when other parameters were the same.The Nu and f of the FSAH model were fitted by using the least square regressed empirical correlation equation.It is found that the predicted Nu and f according to the equations were similar to the simulation results,and the relative errors were within 10%,signifying that the method of fitting the correlation is reasonable and effective.(4)The Chapter?investigated the low-temperature air heat storage technology of collected solar thermal energy.A new type of energy storage material Sr Cl₂@SG composite adsorbent was prepared by using silica gel SG particles and Strontium chloride hexahydrate Sr Cl₂·6H₂O.Moreover,an open sorption system was built to explore the new method for low-temperature air heat energy storage.The mainly three stages of dehydration process of Sr Cl₂·6H₂O was confirmed,and dehydration process of Sr Cl₂@SG composite adsorbent was explained by dehydrated curves.Kinetic hydration experiments showed that the equilibrium water absorption increased first and then decreased as the increment of the salt content of composite sorbents.Sr Cl₂@SG?10 had the highest water uptake among the series of composite adsorbents.The temperature had little effect on the equilibrium water absorption of composite adsorbents,while the equilibrium water absorption of composite adsorbents increased with the increase of relative humidity.When the relative humidity exceeds 70%,the equilibrium water absorption of the adsorbent reached the theoretical equilibrium water absorption.Therefore,in order to fully release the energy of the adsorbent and ensure the stability of the composite adsorbent,the environmental relative humidity should be greater than 70%.The desorption and adsorption experiments were carried out in the open sorption system filled with 800g Sr Cl₂@SG?10 adsorbent.Results showed that the energy storage density of Sr Cl₂@SG?10adsorbent was up to 100 k Wh/m~3.After 15 charging/discharging cycles,the Sr Cl₂@SG?10adsorbent can still maintain stability and high energy output,showing that the composite adsorbent and system can be applied to low-temperature air thermal energy storage.