Heat Transfer Characteristics Of Upward-facing Cylindrical Cavities And Their Applications In Solar Thermoelectric Generation Systems

Environmental pollution and energy crisis have become or will continue to be the most two severe challenges faced by human survival and social development.Due to the solar energy sharing advantages of abundant,accessible and environmentally friendly,how to utilize solar energy efficiently may become the key to cope with two challenges above.Currently,solar thermal power generation is one of the main ways of solar energy utilization.In the process of power generation,cavity-type receiver,i.e.,cavity,plays an important role,and its thermal performance can directly affect the efficiency of the whole thermal power generation system.Hence,the heat transfer characteristic of cavity is one of the prior research lines in the field of solar energy.Compared with the conventional solar thermal power generation such as tower,trough,dish and linear Fresnel,solar thermoelectric power generation,as a static thermal power generation,has become another prior research line in the field of solar energy due to its high-reliability,easy to move and less occupied area.In this dissertation,taking upward-facing cylindrical cavities as object,firstly,on the basis of electric heating method,the effects of cavity tilt angle,heat flux,surface heating condition,aperture size,wind incident angle and wind speed on the heat transfer characteristics under constant heat flux condition were studied experimentally.To explain the impacts of each factor from the perspective of physical mechanism and to obtain the results of cases with high heat flux,high wind speed and back-side wind,the three-dimentional numerical analysis were also conducted.Subsequently,the heat transfer characteristics of cavities subjected to constant temperature for all surfaces condition were investigated numerically,in which the considered factors were cavity tilt angle,wall temperature,aperture size,wind incident angle and wind speed.According to the numerical results,a series of Nusselt number empirical correlations that can reflect the effects of each factor and share high prediction accuracy were developed.It is worth mentioning that the partitioned fitting approach was put forward firstly for windy condition.Finally,a novel solar thermoelectric generation system?STGS?,i.e.,Fresnel lens and heat pipe based solar thermoelectric generation system?FH-STGS?,was proposed.According to the cooling mode,FH-STGS can be classified into air-cooling FH-STGS and liquild-cooling FH-STGS.Through establishing theoretical model,the impacts of emissivity of ceramic plate,convective heat transfer coefficient of heat exchanger,total amount of incident solar energy,wind speed,wind incident angle,cavity tilt angle,ambient temperature,leg filling factor of flat-plate thermoelectric generation module?FTGM?,leg length of FTGM,leg filling factor of annular thermoelectric generation module?ATGM?and leg length of ATGM were evaluated quantitatively.And based on these,the optimum ranges of four geometrical factors?leg filling factor of FTGM,leg length of FTGM,leg filling factor of ATGM and leg length of ATGM?compromising the performace of system and cost of leg material were obtained for two FH-STGS respectively.Besides,the parametric analysis for the performance of ATGM was carried out on the basis of the theory of thermoelectricity.The main results are as follows.?1?Under the constant heat flux condition,experimental results show that the temperatures of bottom surface are nearly even distributed,and for most of cases,the temperatures of side surface decrease gradually with the distance departed from bottom surface.In the circumstance of windlessness,the temperature difference between average of cavity surfaces and ambient increases with decreasing aperture size or increasing heat flux,and rises firstly and then drops with decreasing cavity tilt angle.However,when the wind is introduced,the effect of cavity tilt angle is affected by the wind incident angle.The higher the wind speed is,the lower the temperature difference between average of cavity surfaces and ambient is.For the temperature of surface and intensity of radiative heat transfer N_ur,the difference between simulation and experiment is small,while for the intensity of convective heat transfer N_uc,the difference is very large.Simulation results show that in the windless condition,as cavity tilt angle decreases,on the one hand,the stability of free convection heat transfer decreases gradually and the free convection heat transfer can be transformed from steadiness to unsteadiness;on the other hand,N_uc(N_ur)rises?drops?firstly and then drops?rises?.With an increase of aperture size or heat flux,N_uc increases firstly and then varies slightly,and N_ur keeps increasing.Yet,due to the disturbance of wind,the effect of cavity tilt angle will beome quite complicated and is related to wind incident angle and wind speed;N_uc appropriately does not varies with heat flux.The influence of wind incident angle can be affected by wind speed.In almost all cases,as wind speed increases,N_uc and N_ur increases and decreases respectively.Compared with other factors,wind speed may be the dominant factor affecting or controlling the heat transfer of cavity.?2?Under the constant temperature for all surfaces condition,simulation results show that the effects of cavity tilt angle and aperture size are generally similar to those under the constant heat flux condition.The higher the surface temperature is,the higher the convective heat transfer rate and the lower the N_uc are.Compared with back-side windy case,the effect of wind incident angle is more complicated and related to cavity tilt angle and wind speed for front windy case.Unlike the constant heat flux condition,for almost all cases,as wind speed increases,N_uc drops before attaining a minimum after which it rises.?3?Theoretical results show that the forms of fundamental formulas about ATGM are the same as those about FTGM.The efficiency of ATGM does not affected by the annular shaped parameter under condition of load matching,while the closer the annular shaped parameter is to 1,the larger the output power of ATGM is below that of FTGM.From the performance evaluation of FH-STGS,it can be found that the performance of liquild-cooling FH-STGS increases with increasing convective heat transfer coefficient of heat exchanger,but gets saturated quickly.The smaller the emissivity of ceramic substrate or ambient temperature is or the larger the total amount of incident solar energy is,the better performance the FH-STGS is.However,the efficiency of FH-STGS will decline because of excessive total amount of incident solar energy.As wind speed increases,the performance of air-cooling FH-STGS improves firstly and then deteriorates,while the varation of the performance of liquild-cooling FH-STGS is the same as that of convective heat transfer rate.The influences of wind incident angle,cavity tilt angle and ambient temperature?only for liquild-cooling FH-STGS?on the performance of FH-STGS are quite limited when comparing with other factors.The effects of four geometrical parameters depend on other factors such as total amount of incident solar energy,wind speed or cooling mode.The optimum ranges of leg filling factor of FTGM,leg length of FTGM,leg filling factor of ATGM and leg length of ATGM are respectively 0.15⁰.205,0.005⁰.006 m,0.15⁰.202 and 0.007⁰.008 m for air-cooling FH-STGS,and are respectively 0.05⁰.1,0.006⁰.007 m,0.15⁰.202 and 0.007⁰.008 m for liquild-cooling FH-STGS.Compared with existing STGS,whether from efficiency or from output power,both the two FH-STGS are excellent,especially for liquild-cooling FH-STGS,since it may possess the highest output power of STGS so far?up to 128W?and can export thermal energy?the thermal efficiency can up to 86%?at the same time of generating electricity.