Research On Solar-thermal Mechanism And Optimization Of A Heat Pipe Receiver In Solar Tower Power System

Concentrated solar power generation technology is one of the effective ways to reduce the energy crisis and environmental degradation.Solar tower power system has broad prospects for commercial applications with its advantages of high concentration ratio,large capacity and high efficiency.However,large-scale heliostat field and related equipment lead to the high cost,while non-uniformity and discontinuity of heat flux density could risk receiver’s safety and efficiency.Therefore,improving the safety and efficiency of solar tower power system is the challenge.In this paper,a heat pipe solar receiver for air Brayton cycle is designed to boost the efficiency.Based on optics,heat transfer and fluid mechanics,and MCRT method and CFD simulation tools,the coupling solar thermal performance of a small-scale solar tower concentrating system and the heat pipe receiver are studied.The details are as follows:（1）Firstly,a small-scale solar tower system is introduced,with a detailed description of heliostat field and a novel heat pipe solar receiver.Based on the optical theory and Monte Carlo Ray Tracing method,the general optical analysis model of the solar tower system is developed by using MATLAB-SOLSTICE-PYTHON hybrid programming.Good results are achieved by comparing the results with other methods.It is indicated that the new model has fast calculation speed and can be easily modified.For the heat pipe solar receiver,the independent validations of grid and photon number are carried out respectively.The effect of overall error on the optical performance is studied.The results show that the overall error of heliostat has an important effect,the greater the error,that is the lower the optical efficiency,and the lower corresponding peak flux density on the receiving surface.When overall error equals to 28)(6(9,the maximum heat flux density on the aperture could reach 2.7（2?8）^-2 with an optical efficiency of 73%.（2）Using the proposed optical model,the optical performance of heliostat field and heat pipe receiver for small-scale solar tower system are studied.Firstly,the influence of solar tower shadow on the optical performance of heliostat field is studied,and the distance between tower height and heliostat field is optimized.The results show that the influence of solar tower shadow on the optical performance of heliostat field varies with the season and time.For small-scale solar field,solar tower shadow during the winter solstice has a great effect than that during summer solstice,and the effect were greater at noon than in the morning and evening.At noon on the winter solstice,the relative error of total absorbed energy can be up to 10%when ignoring the shadow of the heat-absorbing tower.The optimization results show that the average annual optical efficiency reaches the maximum value,which is about 63.5%,when theequals to 358)and₁equals to 0.2.Then the heat flux distribution on the receiver surface is studied as a function of time.The results show that the heat flux locates between[-90°,+90°]on a single heat pipe.The circumference heat flux satisfies the Gaussian distribution.The maximum heat flux density is 9596)（2?8）^-2 with total absorbed energy 6.96)(2 for a single heat pipe at noon on the summer solstice.The maximum optical efficiency during the spring equinox and summer solstice is around 75%at local noon,whereas the value during the winter solstice is only 61%.The aiming strategy is also considered.For single-aiming strategy,the heat flux distribution could be adjusted by changing the aiming point position along the receiver depth.When the aiming point is placed at the center of aperture,the receiver absorbs the most sunlight,thus gets highest efficiency.By applying the dynamic multi-aiming points strategy,the maximum heat flux density could reduce 25%with a decrease of only0.6%on optical efficiency.（3）Receiver is the vital equipment for solar-thermal conversion in solar tower system.Based on the results of optical simulation,a numerical study on the heat transfer performance of a single high temperature sodium heat pipe in a heat pipe solar receiver is conducted.The three-dimensional numerical model of high temperature sodium heat pipe is established based on kinetic theory and computational fluid dynamic.Considering the optical simulation results,a three-dimensional heat flux density distribution function on the heat pipe surface is proposed.The accuracy of the model is verified by comparing the results from existing literatures.The effects of heat flux density,temperature of cold fluid and heat pipe curvature on heat pipe performance are analyzed.The results show that the high temperature sodium heat pipe has a pretty good isothermal characteristic.For the cases of different heat flux densities（from 1006）（2?8）^-2 to 3006)（2?8）^-2),different cold fluid temperatures（from 750to 900）,different heat flux density distribution and bending angle（from 0°to 90°）,the vapor temperature difference along the heat pipe is less than 1.On the other hand,increasing the heat flux density of the evaporator section,reducing the coolant temperature and non-uniform heat flux density can lead to an increase in capillary pressure drop.Adding the heat pipe bending angle will increase the vapor temperature difference and vapor pressure drop along the heat pipe,resulting in a lower heat transfer performance of heat pipe.However,when the heat pipe bending angle varies from 0°to 90°,the difference for equivalent thermal conductivity is only about 0.47%,which could be ignored and indicate that the heat pipe still has excellent isothermal performance.Thus,using bending heat pipes in engineering applications can better meet design needs.（4）Based on the heat transfer characteristics and energy balance of high temperature heat pipe,a three-dimensional thermal conductivity model for calculating the temperature field is proposed.By comparing with the results of CFD model and thermal resistance model,it is shown that the three-dimensional thermal conductivity model can get the heat pipe temperature field quickly and accurately,especially under the condition of non-uniform heat flux density.Based on the three-dimensional thermal conductivity model,a three-dimensional numerical model and thermal efficiency model of the heat pipe solar receiver are established.Considering the non-uniform heat flux density distribution under real-world conditions,the effects of Reynolds number,transverse pitch ratio,length ratio between condenser section and evaporator section are studied in detail.The results show that for heat pipe receivers,by adjusting the transverse pitch ratio and length of the condensation section,the flow and heat transfer conditions can be changed,so as to optimize the thermal performance of the receiver.When the inlet mass flow is decided,increasing the transverse pitch ratio would decrease the flow velocity,which reduces the Nusselt number and the fluid pressure drop at the same time.Increasing the length of condenser section also reduces the flow velocity,but the overall heat exchange performance increases due to the increase of the heat exchange area.Heat pipe receiver can operate at higher heat flux densities than conventional tubular receiver.When the total radiation input of the evaporator section is 1606)(2,the thermal efficiency of the heat pipe receiver is 17.7%higher than that of the tubular receiver.（5）The condenser sections of the heat pipe receiver are arranged in staggered array which is a common structure in industrial applications.Therefore,a modified numerical method to calculate the view factor between parallel cylinders of finite length in arbitrary arrangement is developed.The new method reduces the quadruple integration into a simple double numerical integration.Besides,a simple method to judge the blocking is also given.The novel method reduces the calculation time significantly.When compared with the direct numerical method it allows reducing99.5%of the required time with similar accuracy.The accuracy of the improved algorithm is proved by comparing it with MCRT method,cross-line method and approximate algorithm.For the infinite long cylinders in a staggered array,the analytical expressions of the view factor between the center cylinder and the surrounding 12 rows of cylinders are derived.Combined with the approximation method,the calculation formulas in the case of finite long cylinders in staggered array is given.The results of the improved algorithm and the approximation algorithm are very consistent.The effects of the spacing ratio,length-to-diameter ratio and layouts on the view factor between cylinder array is analyzed.The results show that the view factor between two cylinders changes as a result of the spacing ratio and the blocking.For the view factor_1-2,there is no blocking between them,thus the view factor decreases with the increase of the spacing ratio.For the view factor_1-3-_1-12,the curves as a function of the spacing ratio can be divided into three zones:the complete blocking zone,the partial blocking zone and the non-blocking zone.In the complete blocking zone,the view factor equals to 0.In the partial blocking zone,with the increase of the spacing ratio,the view factor increases due to the decrease of the blocking effect,and then decreases due to the increase of the distance between the two surfaces.In the non-blocking zone,the view factor monotonically decreases with the increase of the spacing ratio.The view factor increases rapidly with the increase of the length-diameter ratio when/≤100.However,the length-diameter ratio has little effect on the view factor when/>100.Therefore,for cylindrical arrays with large length-diameter ratios,they can be treated as infinite-length cylindrical arrays.The proposed method might be very useful to design complex energy systems.For instance,it can be applied to design an improved compact heat exchanger in a nonstandard arrangement.