【作者】 杨华；

【导师】 刘朝；

【作者基本信息】 重庆大学 ， 能源动力（专业学位）， 2024， 硕士

【摘要】 随着我国经济的快速增长,能源消耗总量也在不断攀升,如何有效地利用工业生产和自然界中的低品位热能对我国能源结构转型有着重要意义。近年来,基于膜蒸馏发展起来的热渗透能量转换技术(Thermo-Osmotic Energy Conversion,TOEC)可同时实现低品位热能发电以及盐水淡化。目前研究中存在膜孔径较大导致的系统工作压力低,进而导致系统性能低下的问题。针对这一问题,本文将纳米孔径疏水碳纳米管(CNT)构成的膜应用于TOEC系统,基于分子动力学模拟的方法研究了材料特性、运行工况以及工质对TOEC系统性能的影响。主要研究内容及获得的成果如下:首先,建立了基于疏水CNT膜的TOEC系统模型,模拟了TOEC系统的传质传热过程,并探究了不同孔径、疏水性、结构以及纳米材料形状对系统性能的影响规律。研究表明,疏水CNT构成的膜材料应用于TOEC系统是可行的。(18,18)CNT构成的超疏水(接触角140°)膜材料,在冷侧温度293 K、热侧温度400 K、冷热侧压力差40 MPa运行条件下,可获得3.7 g/(cm~2·s)的质量流量以及150 W/cm~2的输出功率,热效率为1.3%。疏水CNT构成的膜材料承压能力强,在疏水性较弱(接触角95°)、孔径较大((18,18)CNT)时,依然具有50 MPa的极限工作压力。TOEC系统的质量流量和输出功率均随孔径的增大、疏水性的减弱而逐渐增大,而极限工作压力随孔径、疏水性的变化则呈相反规律。由小孔径、强疏水性CNT与大孔径、弱疏水性CNT组成的复合膜结构,其无法同时获得大孔径、弱疏水性CNT的高通量以及小孔径、强疏水性CNT的高极限工作压力。纳米材料的孔道形状会影响系统的传质以及极限工作压力。三角形、圆形、正方形三种几何形状的纳米材料孔道,在孔道面积接近相等时,传质能力最弱的为三角形孔道,其次为圆形孔道,最强为正方形孔道;极限工作压力则是三角形孔道最强,圆形孔道次之,正方形孔道最弱。其次探究了TOEC系统在不同压差、温差、盐浓度下性能变化规律。研究表明,膜两侧液压差对系统的传质影响很小,系统的输出功率、热效率随冷侧水压增大而近乎线性增大。一定条件下,当冷侧水压从10.1 MPa逐渐升高至40.1 MPa时,质量流量接近不变,输出功率从73 W/cm~2上升至281 W/cm~2,热效率从0.38%上升至1.33%。因此,为获得更高的系统输出功率和热效率,应使系统在极限工作压力附近工作。膜两侧液体的温差为系统传质过程的主要驱动力。在一定压力下,质量流量、输出功率随温差增大而呈现指数式增大。相同温差下,高温区间的传质及热功转换能力要优于低温区间。系统在利用低温区间的热能时,应采用疏水性较弱的大孔径CNT膜来保证系统正常运行。疏水CNT膜材料除了能将低温热能转换为机械(压力)能外,还可以用于除盐,且除盐效果较好。TOEC系统在冷侧温度293 K、热侧温度353 K、冷热侧压力差40 MPa、接触角95°、(18,18)CNT、盐浓度为0.5 mol/L下,能产出2 g/(cm~2·s)的淡水以及输出80 W/cm~2的功率。产水量随盐浓度的升高而逐渐减小,这主要是由于离子数目增多其对水分子的吸引力增强导致的。本文从微观角度研究了基于疏水CNT膜的热渗透热-功转换系统中热质传输以及能量转换规律,可为热渗透能量转换技术性能提升提供理论支撑。更多还原

【Abstract】 With the rapid growth of our country’s economy,the total energy consumption is continuously increasing.Effectively utilizing low grade thermal energy from industrial production and the natural environment is crucial for the transformation of our energy structure.In recent years,the development of(Thermo-Osmotic Energy Conversion TOEC)technology based on membrane distillation has emerged,enabling simultaneous generation of electricity from low-grade thermal energy and desalination of saltwater.However,a challenge in current research lies in the low system operating pressure due to the large pore size of membrane materials,leading to decreased system performance.To address this issue,this study applies membrane composed of hydrophobic carbon nanotubes(CNT)with nanometer-scale pores to TOEC systems.Utilizing molecular dynamics simulations,the study investigates the impact of material properties,operating conditions,and working fluids on the performance of TOEC systems.The main research content and findings are as follows:Firstly,a model of a TOEC system based on hydrophobic carbon nanotubes(CNT)membrane was established to simulate the mass and heat transfer processes within the TOEC system and investigate the influence of different pore sizes,hydrophobicity,structures,and nanomaterial shapes on system performance.The research demonstrates the feasibility of employing membrane composed of hydrophobic CNT in TOEC systems.A superhydrophobic membrane material composed of(18,18)CNT,with a contact angle of 140°,achieves a mass flow rate of 3.7 g/(cm~2·s)and an output power density of 150W/cm~2 under operating conditions of 293 K on the cold side,400 K on the hot side,and a pressure difference of 40 MPa between the cold and hot sides,with a thermal efficiency of 1.3%.The pressure-bearing capacity of hydrophobic CNT membrane material is robust,maintaining a limit working pressure of 50 MPa even when the hydrophobicity is weak(contact angle of 95°)and the pore size is large((18,18)CNT).Both the mass flow rate and output power of the TOEC system gradually increase with increasing pore size and decreasing hydrophobicity,while the limit working pressure exhibits an opposite trend with changes in pore size and hydrophobicity.Composite membrane structures consisting of small-pore,highly hydrophobic CNT and large-pore,weakly hydrophobic CNT cannot simultaneously achieve the high flux of large-pore,weakly hydrophobic CNT and the high limit working pressure of small-pore,highly hydrophobic CNT.The pore shape of nanomaterials affects both the mass transfer and limit working pressure of the system.Among the three geometric shapes of nanomaterial pores(triangle,circle,square),when the pore areas are approximately equal,triangular pores exhibit the weakest mass transfer capability,followed by circular pores,with square pores being the strongest;whereas,the limit working pressure is strongest in triangular pores,followed by circular pores,and weakest in square pores.Secondly,the performance variation of the TOEC system under different pressure differentials,temperature differentials,and salt concentrations was investigated.The study shows that the hydraulic pressure difference on both sides of the membrane has little influence on mass transfer in the system.The output power and thermal efficiency of the system increase nearly linearly with increasing cold side water pressure.Under certain conditions,as the cold side water pressure increases from 10.1 MPa to 40.1 MPa,the mass flux remains almost constant,while the output power increases from 73 W/cm~2to 281 W/cm~2,and the thermal efficiency increases from 0.38%to 1.33%.Therefore,to achieve higher system output power and thermal efficiency,the system should operate near the limit working pressure.The temperature difference between the liquids on both sides of the membrane is the main driving force for mass transfer in the system.Under certain pressure,the mass flux and output power increase exponentially with increasing temperature difference.The mass transfer capability in the high-temperature range is superior to that in the low-temperature range under the same temperature difference.The mass transfer capability in the low-temperature range is weaker,and when utilizing the thermal energy in the low-temperature range,CNT membrane with larger pore size and weaker hydrophobicity should be used to ensure the normal operation of the system.In addition to converting low-temperature thermal energy into mechanical(pressure)energy,membrane materials composed of hydrophobic CNT membrane can also be used for desalination,with good desalination efficiency.Under the conditions of a cold-side temperature of 293 K,a hot-side temperature of 353 K,a cold-hot side pressure difference of 40 MPa,a contact angle of 95°,(18,18)CNT,and a salt concentration of 0.5 mol/L,the system can produce 2 g/(cm~2·s)of freshwater and output 80 W/cm~2 of power.The freshwater production decreases gradually with increasing salt concentration,mainly due to the increased number of ions enhancing their attraction to water molecules.The paper investigates the heat and mass transfer,as well as energy conversion phenomena,in a thermopermeable heat-power conversion system based on hydrophobic CNT membranes from a microscopic perspective.The research findings can provide theoretical support for enhancing the performance of thermopermeable energy conversion technologies.更多还原