Direct absorption solar collector based on bottom entry and method of construction

Abstract

This invention relates to a direct absorption solar collector based on a bottom-incidence method and its construction method. The method includes: establishing a collector model; optimizing the working fluid inside the collector through numerical simulation and experimental verification; optimizing key parameters of the model fluid; selecting suitable inlet flow velocity and fluid attenuation coefficient based on numerical simulation results; obtaining optimal values ​​for the collector length and height; and constructing a multi-objective optimization system for maximizing the collector's photothermal conversion efficiency, using fluid attenuation coefficient, inlet mass flow rate, collector length, height, and nanoparticle parameters as design variables. This invention effectively improves the solar-to-thermal energy conversion efficiency.

Description

Technical Field

[0001] This invention relates to a direct absorption solar collector based on bottom incidence and its construction method, belonging to the field of renewable energy technology. Background Technology

[0002] Compared to traditional flat-plate solar collectors, existing direct absorption solar collectors exhibit superior performance due to their direct absorption mechanism achieved through suspended nanoparticles. Furthermore, because the working fluid inside the collector is in a flowing state, the heat transfer process is enhanced, enabling more efficient heat absorption and exchange compared to static collector structures, resulting in superior photothermal conversion performance.

[0003] However, existing optimization techniques for solar collector systems mainly target collectors with top-injection solar energy, or those with bottom-injection heating only for static collectors. In these cases, the temperature gradient formed within the working fluid often leads to increased heat loss, limiting the potential for improving solar energy conversion efficiency. Furthermore, existing designs often require maintaining a high nanoparticle volume fraction, which significantly increases the working fluid viscosity, inducing flow instability and particle deposition. In addition, comprehensive optimization of multiple collector parameters typically involves complex computational models and extensive simulations, resulting in a heavy computational burden, which also limits the application and widespread adoption of such optimization methods in practical engineering. Summary of the Invention

[0004] This invention provides a direct absorption solar collector based on a bottom-incidence method and its construction method, aiming to solve at least one of the technical problems existing in the prior art. To this end, this invention proposes a direct absorption solar collector based on a bottom-incidence method and its construction method, which effectively improves the efficiency of solar energy to thermal energy conversion by using sunlight incident from the bottom.

[0005] The technical solution of the present invention relates, in one aspect, to a method for constructing a direct absorption solar collector based on a bottom-incidence method. The method according to the present invention includes the following steps:

[0006] S100. Establish a model of a bottom-incidence direct absorption solar collector;

[0007] S200. The working fluid inside the solar collector is optimized through numerical simulation and experimental verification methods to enhance solar energy absorption. In the established two-dimensional numerical model, the key fluid parameters are optimized to improve the absorption effect of the working fluid. The key fluid parameters to be optimized include the fluid attenuation coefficient and the inlet flow velocity.

[0008] S300. Based on the numerical simulation results obtained above, select appropriate inlet flow velocity and fluid attenuation coefficient for the collector model, and obtain the optimal values ​​of collector length and collector height to optimize the collector structural parameters.

[0009] S400 aims to maximize the photothermal conversion efficiency of the solar collector. It uses fluid attenuation coefficient, inlet mass flow rate, collector length, height, and nanoparticle parameters as design variables to construct a multi-objective optimization system for maximizing the thermal efficiency and optimizing temperature uniformity of the solar collector.

[0010] Furthermore, step S100 includes the following steps:

[0011] S110. The collector is simplified into a slender rectangular structure filled with working fluid, and the material configuration is set as follows: the absorption layer, which serves as the incident surface, is made of glass, the fluid domain is made of nanofluid, and the insulation layer is made of foam.

[0012] S120. Set the steady-state calculation conditions for the model, enable the energy equation, and select... Flow model;

[0013] S130. Set the model boundary conditions, including setting the surface heat transfer coefficient h=10W / (m²・K), emissivity ε=0.1, heat flux density of the adiabatic surface q=0, setting the pressure outlet to standard atmospheric pressure, setting the return temperature to the adjacent mesh at the outlet, and setting the bottom incident volumetric heat source mode.

[0014] Furthermore, in step S120, the energy equation is expressed as follows:

[0015] ;

[0016] In the formula, and These represent the fluid's physical properties, density and specific heat capacity, respectively. and They represent direction and velocity in direction, For fluid temperature, It serves as an internal heat source for the fluid. It represents the superposition of the fluid thermal conductivity and the turbulent thermal conductivity. In the formula The thermal conductivity of the fluid itself, The turbulent thermal conductivity is the coefficient of thermal conductivity under turbulent conditions. ,in Indicates turbulent viscosity. The Prandtl number represents the turbulent flow rate, which is the ratio of turbulent momentum diffusivity to turbulent thermal diffusivity.

[0017] Furthermore, in step S130, the internal heat source of the fluid... It is expressed as follows:

[0018] ;

[0019] In the formula, denoted as solar irradiance, Kext as fluid attenuation coefficient, H as collector height, and y as longitudinal coordinate.

[0020] Furthermore, in step S200, the optimization operation of the fluid attenuation coefficient includes:

[0021] From the unit Furthermore, the fluid attenuation coefficient, ranging from 1 to 100, is taken from 20 equally spaced values. Each time the fluid attenuation coefficient is changed, the model automatically calculates the corresponding outlet temperature. Then use equations Calculate the collector efficiency, where For inlet quality flow, The value is 0.025 kg / s; It is the specific heat capacity of the fluid. The value is 4182 kJ / kg*K; This refers to the inlet temperature of the solar collector. The value is 293.15K; It is the length of the solar collector. The value is 2m; The initial solar irradiance for the model. Value .

[0022] Furthermore, in step S200, the optimization operation of the inlet flow rate includes:

[0023] Adjust the mass flow rate parameter at the inlet of the solar collector model. Starting from a mass flow rate parameter value of 0.01 kg / s, take a mass flow rate parameter every 0.1 kg / s and calculate the outlet temperature and photothermal conversion efficiency in sequence until the mass flow rate parameter value is 0.1 kg / s.

[0024] Each time the mass flow rate parameter is changed, the model automatically calculates the corresponding outlet temperature. Then use equations Calculate the collector efficiency, where For inlet quality flow, The value is 0.025 kg / s; It is the specific heat capacity of the fluid. The value is 4182 kJ / kg*K; This refers to the inlet temperature of the solar collector. The value is 293.15K; It is the length of the solar collector. The value is 2m; The initial solar irradiance for the model. Value .

[0025] Furthermore, step S300 includes:

[0026] S310. Modify the geometry of the ANSYS Fluent 2D model, including setting up a DASC model with a collector height H=50cm and ten different values ​​for the collector length L, and ensuring that the boundary conditions and heat source are consistent with the original VAS-bh model. Obtain the corresponding outlet temperature for each length dimension through ANSYS Fluent steady-state calculation, and use the equations... The corresponding photothermal conversion efficiency is obtained, and then the optimal length of the collector is obtained when both the photothermal conversion efficiency and the outlet temperature are high.

[0027] S320. Modify the geometry of the ANSYS Fluent 2D model, including using a DASC model with ten different values ​​for the collector length L=200cm and collector height H, and ensuring that the boundary conditions and heat source are consistent with the original VAS-bh model. Obtain the corresponding outlet temperature for each length dimension through ANSYS Fluent steady-state calculations, and use equations... The corresponding photothermal conversion efficiency is obtained, and then the optimal length of the collector is obtained when both the photothermal conversion efficiency and the outlet temperature are high.

[0028] S330. Based on the optimal dimensions confirmed by simulation, manufacture a simple collector structure to build an experimental platform for direct absorption solar collectors.

[0029] Furthermore, step S400 includes:

[0030] S410. Establish a parametric geometric model. After completing the mesh generation and physical model settings, use the Latin hypercube sampling method based on the fluid attenuation coefficient Kext and the inlet mass flow rate. For the four design variables—collector length L, collector height H—one hundred sample points were extracted to uniformly cover the design variable space. Each sample point corresponds to the output collector's photothermal conversion efficiency. outlet temperature Maximum temperature difference Key indicators such as average velocity of the flow field are used to form a simulation database;

[0031] S420. A combined surrogate model is constructed using a combination of response surface methodology and radial basis functions to balance accuracy and computational efficiency.

[0032] S430, with four design variables as inputs, and the solar collector's photothermal conversion efficiency. outlet temperature and maximum temperature difference For output, the mapping function relationship between the objective function and the design variables is obtained by fitting a simulation database. The mapping function is expressed as follows: In the formula, Y represents the objective function, which is the solar collector's photothermal conversion efficiency. outlet temperature and maximum temperature difference .

[0033] The technical solution of the present invention also relates to a direct absorption solar collector based on a bottom incidence method, and a construction method for the direct absorption solar collector based on a bottom incidence method as described in the above embodiments; the direct absorption solar collector includes:

[0034] The collector inlet serves as the starting point for the nanofluid to enter the channel; the working nanofluid flows inside the collector to capture solar radiation energy incident from the bottom and convert it into heat energy; the collector outlet serves as the outlet for the nanofluid that heats up after absorbing heat, and thermocouples are placed there to monitor the outlet temperature; the top insulation layer uses high-efficiency foam insulation material to minimize heat loss from the top; and the glass layer located at the bottom of the collector ensures high transmittance of solar radiation.

[0035] Furthermore, it also includes: thermal insulation walls serving as thermal insulation walls on both sides of the channel to effectively reduce lateral heat loss; a bottom incident light source used to simulate standard solar irradiance entering the channel from the bottom of the collector; a fixed frame used to connect the sides of the collector to ensure unobstructed bottom and thus allow the light source to enter smoothly; and a support frame providing mechanical stability and structural height support for the entire collector.

[0036] The present invention also relates to a computer-readable storage medium having program instructions stored thereon, which, when executed by a processor, implement the above-described method.

[0037] The technical solution of the present invention also relates to an reachable set estimation system for periodic time-varying mechanical systems based on set intersection, the system including a computer device containing the aforementioned computer-readable storage medium.

[0038] The beneficial effects of this invention are as follows.

[0039] The present invention discloses a direct absorption solar collector and its construction method based on a bottom-incidence solar radiation method, which enhances the utilization of solar thermal energy through bottom-incidence solar radiation. Based on a dynamic direct absorption solar collector, it uses a bottom-incidence solar radiation method, which accelerates the heat transfer of the internal fluid, makes the temperature distribution of the collector more uniform, effectively reduces heat loss, and thus significantly improves the solar energy to thermal energy conversion efficiency. Attached Figure Description

[0040] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0041] Figure 1 This is a schematic diagram of the structure of a bottom-incidence direct absorption solar collector according to the present invention.

[0042] Figure 2 This is a structural perspective view of a bottom-incident direct absorption solar collector according to the present invention.

[0043] Figure 3 This is a flowchart illustrating the construction process of a bottom-incidence direct absorption solar collector according to the present invention. Detailed Implementation

[0044] The following will provide a clear and complete description of the concept, specific structure, and technical effects of the present invention in conjunction with the embodiments and accompanying drawings, so as to fully understand the purpose, solution, and effects of the present invention. It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0045] It should be noted that, unless otherwise specified, when a feature is referred to as "fixed" or "connected" to another feature, it can be directly fixed or connected to the other feature, or indirectly fixed or connected to the other feature. Furthermore, the descriptions of "upper," "lower," "left," "right," "top," and "bottom" used in this invention are only relative to the relative positional relationships of the various components of the invention in the accompanying drawings.

[0046] Furthermore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used in this specification is for the purpose of describing particular embodiments only and not for limiting the invention. The term "and / or" as used herein includes any combination of one or more of the associated listed items.

[0047] It should be understood that although the terms first, second, third, etc., may be used in this disclosure to describe various elements, these elements should not be limited to these terms. These terms are only used to distinguish elements of the same type from one another. For example, without departing from the scope of this disclosure, a first element may also be referred to as a second element, and similarly, a second element may also be referred to as a first element.

[0048] See Figure 1 The present invention discloses a direct absorption solar collector based on bottom incidence and its construction method. The direct absorption solar collector includes a collector inlet 1, a working nanofluid 4, a collector outlet 2, a top insulation layer 5, and a glass layer 6. The collector inlet 1 serves as the starting point for the nanofluid to enter the channel. The working nanofluid 4 flows inside the collector to capture solar radiation energy incident from the bottom and convert it into heat energy. The collector outlet 2 is the location where the nanofluid, after absorbing heat and heating up, flows out of the channel, and a thermocouple is placed there to monitor the outlet temperature. The top insulation layer 5 uses high-efficiency foam insulation material to minimize heat loss at the top. The glass layer 6 is located at the bottom of the collector to ensure high transmittance of solar radiation.

[0049] Further, see Figure 2 In the direct absorption solar collector and its construction method based on bottom incidence of the present invention, the collector inlet 1 and the collector outlet 2 serve as the inflow point and outflow node of the nanofluid, respectively. The insulation wall 3 serves as the thermal insulation wall on both sides of the channel to effectively reduce lateral heat loss. The working nanofluid 4 circulates in the channel and absorbs solar radiation. The top insulation layer 5 continuously performs its efficient heat insulation function. The glass layer 6 achieves channel sealing while ensuring high light transmittance. The bottom incident light source 7 simulates AM1.5 standard solar irradiation and enters the channel from the bottom of the collector. The fixed frame 8 connects to the side of the collector to ensure that the bottom is unobstructed so that the light source can enter smoothly. The support frame 9 provides the necessary mechanical stability and structural height support for the entire collector.

[0050] See Figure 3 The construction method of the direct absorption solar collector based on bottom incidence and the construction method of the present invention includes at least the following steps:

[0051] S100. Establish a model of a bottom-incidence direct absorption solar collector;

[0052] S200. The working fluid inside the solar collector is optimized through numerical simulation and experimental verification methods to enhance solar energy absorption. In the established two-dimensional numerical model, the key fluid parameters are optimized to improve the absorption effect of the working fluid. The key fluid parameters to be optimized include the fluid attenuation coefficient and the inlet flow velocity.

[0053] S300. Based on the numerical simulation results obtained above, select appropriate inlet flow velocity and fluid attenuation coefficient for the collector model, and obtain the optimal values ​​of collector length and collector height to optimize the collector structural parameters.

[0054] S400 aims to maximize the photothermal conversion efficiency of the solar collector. It uses fluid attenuation coefficient, inlet mass flow rate, collector length, height, and nanoparticle parameters as design variables to construct a multi-objective optimization system for maximizing the thermal efficiency and optimizing temperature uniformity of the solar collector.

[0055] This invention effectively improves the thermal efficiency of a dynamic direct absorption solar collector by employing a bottom-injection heating method with sunlight. Furthermore, based on the dynamic direct absorption solar collector, the photothermal conversion efficiency is further enhanced by considering the influence of parameters such as inlet flow rate, working fluid attenuation coefficient, and collector dimensions (e.g., length and height) on photothermal performance.

[0056] Specifically, this invention employs a numerical simulation method, using ANSYS Fluent to establish a two-dimensional model of a flat-plate direct absorption solar collector. Two different models are simulated using heat source settings: a top-incidence direct absorption solar collector and a bottom-incidence direct absorption solar collector. The temperature distribution and efficiency of the models are compared and analyzed after calculation, verifying the performance improvement effect of the bottom-incidence method on the collector. Furthermore, this invention optimizes parameters such as the inlet flow velocity, working fluid attenuation coefficient, and geometric dimensions (e.g., the length and height of the collector) of the bottom-incidence direct absorption solar collector to maximize the photothermal conversion efficiency of the collector.

[0057] In some embodiments of the present invention, ANSYS Fluent is used to establish a two-dimensional numerical model of a bottom-incidence direct absorption solar collector. ANSYS Fluent is a commercial software for CFD simulation. The numerical model establishment process of its DASC (Direct Absorption Solar Collector) includes:

[0058] S110. The solar collector is simplified into a slender rectangular structure filled with working fluid, which is a nanofluid. The material configuration is specified as follows: the absorption layer, which serves as the incident surface, is made of glass, the fluid domain is made of nanofluid, and the insulation layer is made of foam to ensure that the model fits the actual operating scenario.

[0059] S120. Set the steady-state calculation conditions for the model, enable the energy equation, and select... Flow model. The energy equation is expressed as follows:

[0060] ;

[0061] In the formula, and These represent the fluid's physical properties, density and specific heat capacity, respectively. and They represent direction and velocity in direction, For fluid temperature, It serves as an internal heat source for the fluid. It represents the superposition of the fluid thermal conductivity and the turbulent thermal conductivity. In the formula The thermal conductivity of the fluid itself, The turbulent thermal conductivity is the coefficient of thermal conductivity under turbulent conditions. ,in Indicates turbulent viscosity. The Prandtl number represents the turbulent flow rate, which is the ratio of turbulent momentum diffusivity to turbulent thermal diffusivity.

[0062] S130. Set the model boundary conditions. Specifically, the absorbing surface adopts a hybrid heat transfer mode, where the surface heat transfer coefficient h = 10 W / (m²・K), emissivity ε = 0.1, heat flux density of the adiabatic surface q = 0, the pressure outlet is set to standard atmospheric pressure, i.e., P = 1 atm, and the return temperature is set to the adjacent grid at the outlet.

[0063] Furthermore, a bottom-incident volumetric heat source mode is specifically configured, wherein the heat source... It is expressed as follows:

[0064] ;

[0065] In the formula, denoted as solar irradiance, Kext as fluid attenuation coefficient, H as collector height, and y as longitudinal coordinate.

[0066] This invention achieves the core mechanism of sunlight being absorbed by the volume of nanofluid after entering from the bottom of the collector by setting the above boundary conditions, while forming a comparison benchmark with the top incident model.

[0067] Furthermore, due to the complexity of the boundary conditions and heat source settings for the outlet temperature, this invention utilizes a User-Defined Function (UDF) to achieve the desired results. It should be noted that a UDF is a programming language tool in ANSYS Fluent used to extend software functionality. Based on C language syntax, it can fulfill custom requirements that cannot be covered by the software's default functions. The specific operation process is as follows: first, write the function in C language; then, compile or interpret it in ANSYS Fluent; and finally, associate the UDF with the corresponding module in ANSYS Fluent, i.e., select the compiled or interpreted UDF function in the "Heat Source" and "Outlet Reflux Temperature" options. Thus, after starting the calculation, ANSYS Fluent will automatically call the UDF during iteration to calculate the custom parameters in real time.

[0068] In some embodiments of the present invention, the present invention optimizes the working fluid inside the solar collector through numerical simulation and experimental verification methods to enhance solar energy absorption. In addition, in the established two-dimensional numerical model, key fluid parameters, including fluid attenuation coefficient and inlet flow velocity, are optimized to achieve enhanced absorption optimization of the working fluid.

[0069] Specifically, the fluid attenuation coefficient (Kext) is a parameter describing the attenuation of light intensity caused by the absorption and scattering of incident sunlight by the working fluid, reflecting the degree of attenuation of light energy during transmission in the medium. This invention first modifies the value of Kext in the two-dimensional VAS-bh model (Volumetric Absorption Solar - bottom heating) established using ANSYS Fluent, from 1 to 100 (unit: ) Take 20 values ​​at equal intervals. Each time a value is changed, the model will automatically calculate the corresponding outlet temperature. Then use equations Calculate the photothermal conversion efficiency of the solar collector, where The inlet mass flow rate is 0.025 kg / s; It is the specific heat capacity of the fluid, with a value of 4182 kJ / kg*K; This refers to the inlet temperature of the solar collector. Set to 293.15K; This is the length of the solar collector, which is 2m. The initial solar irradiance of the model is taken as... By observing the temperature field inside the steady-state collector and comparing the outlet temperature and efficiency under various Kext parameters, the optimal fluid attenuation coefficient for the bottom incident mode is determined to ensure that light energy is fully absorbed in the fluid without local overheating.

[0070] Then, adjust the mass flow rate parameter at the inlet of the collector model. Starting from a mass flow rate parameter value of 0.01 kg / s, take a mass flow rate parameter every 0.1 kg / s and calculate the outlet temperature and photothermal conversion efficiency in sequence until the mass flow rate parameter value is 0.1 kg / s. Calculate the outlet temperature and photothermal conversion efficiency using the same method as Kext above.

[0071] It should be noted that when the fluid flow rate is low, heat exchange on the heating surface inside the collector is more efficient. As the flow rate increases, the effective heat absorption ratio increases. However, when the flow rate increases to a certain extent, the fluid's heat absorption capacity approaches saturation, and if the flow rate is too high, the contact time between the fluid and the heating surface is insufficient. Therefore, the collector outlet temperature 2 will continuously decrease as the inlet mass flow rate increases, while the efficiency will first rise rapidly and then slowly stabilize as the inlet mass flow rate increases. Thus, it is necessary to find a balance between the two, that is, the inlet mass flow rate at which both the outlet temperature and efficiency are optimal.

[0072] Using numerical simulation results as experimental reference, a mixed nanofluid with a high absorption coefficient was configured. The fluid's absorption capacity for solar radiation, thermal conductivity, convective heat transfer coefficient, and stability were adjusted by modifying parameters such as the volume fraction of nanoparticles. This was primarily achieved by adjusting the nanofluid's attenuation coefficient to the optimal Kext obtained from the ANSYS Fluent simulation. Dynamic light scattering, Zeta potential analysis, and sedimentation experiments were employed to characterize the prepared nanofluid, ensuring good dispersibility and stability during long-term operation and reducing the risk of nanoparticle aggregation and deposition. The configured nanofluid was then applied to a flat-plate solar collector, and its inlet flow rate was adjusted (using the same mass flow rate experimental group as the ANSYS Fluent numerical simulation). The outlet temperature was measured and recorded at different flow rates, and the solar collector's photothermal conversion efficiency was calculated under different concentrations and inlet flow rates to verify the numerical simulation results.

[0073] In some embodiments of the present invention, based on the numerical simulation results obtained above, a suitable inlet flow velocity and fluid attenuation coefficient are selected for the bottom-incident direct absorption solar collector model. Then, the influence of collector length and height on thermal efficiency is obtained and the optimal value is found to achieve the optimization of collector structural parameters.

[0074] Specifically, the geometry of the ANSYS Fluent 2D model was modified. This included setting the collector height H=50cm and the collector length L to 100cm, 125cm, 150cm, 175cm, 200cm, 225cm, 250cm, 275cm, 300cm, 325cm, and 350cm in a DASC model, ensuring that the boundary conditions and heat source were consistent with the original VAS-bh model. The outlet temperature for each length dimension was obtained through steady-state calculations using ANSYS Fluent, and the equations were then used to calculate the temperature. The corresponding photothermal conversion efficiency was obtained, and the length curves of the outlet temperature and photothermal conversion efficiency were plotted based on the results. The influence law of the collector length on its photothermal performance was obtained, and the optimal length value was confirmed when both the photothermal conversion efficiency and the outlet temperature of the collector are high.

[0075] Then, ten DASC models with collector length L=200cm and collector height H ranging from 1-10cm were set up, ensuring that the boundary conditions and heat source were consistent with the original VAS-bh model. The outlet temperature for each length dimension was obtained through ANSYS Fluent steady-state calculations, and the equations were used to calculate the corresponding outlet temperatures. The corresponding photothermal conversion efficiency was obtained, and the height curves of outlet temperature and photothermal conversion efficiency were plotted based on the results. The influence law of collector length on its photothermal performance was obtained, and the optimal height value when both the photothermal conversion efficiency and outlet temperature of the collector are high was confirmed, so as to balance the contradiction between the increase in outlet temperature and the decrease in efficiency, while also taking into account engineering installation and material costs.

[0076] Then, based on the optimal dimensions confirmed by simulation, a simple collector structure was manufactured to build an experimental platform for a direct absorption solar collector. During the manufacturing process, it was also necessary to consider strengthening the insulation and light transmission structure, optimizing the design of the bottom and side insulation layers, using high-efficiency foam insulation materials to reduce heat loss through conduction and convection, and employing a low-iron transparent glass cover to ensure high transmittance of solar radiation.

[0077] In some specific embodiments of the present invention, with the goal of maximizing the photothermal conversion efficiency of the solar collector, factors such as fluid attenuation coefficient, inlet mass flow rate, collector length, height, and nanoparticle parameters are used as design variables to construct the solar collector photothermal conversion efficiency (SPCE). Maximize and temperature uniformity ( )Optimized multi-objective optimization system.

[0078] Specifically, this invention employs a coupled framework of numerical simulation, surrogate models, and multi-objective optimization algorithms to achieve precise mapping between design variables and objective functions. First, a parametric geometric model is established. After mesh generation and physical model setup, the Latin hypercube sampling (LHS) method is used, based on the fluid attenuation coefficient Kext and the inlet mass flow rate. Four design variables, including collector length L and collector height H, are used to select one hundred sample points (N=100) to uniformly cover the design variable space. Each sample point corresponds to the output photothermal conversion efficiency of the collector. outlet temperature Maximum temperature difference Key indicators such as average velocity of the flow field are used to form a simulation database. Then, a combined surrogate model is constructed using a combination of response surface methodology (RSM) and radial basis function (RBF) to balance accuracy and computational efficiency. Finally, four design variables and the solar collector's photothermal conversion efficiency are used as inputs. outlet temperature and maximum temperature difference For output, the mapping relationship is obtained by fitting a simulation database, that is, the functional relationship between the objective function and the design variables, which is expressed as follows: In the formula, Y represents the objective function, which is the solar collector's photothermal conversion efficiency. outlet temperature and maximum temperature difference .

[0079] Finally, the non-dominated sorting genetic algorithm (NSGA-III) is used to solve the multi-objective optimization problem. The process is as follows: First, one hundred combinations of design variables that satisfy the constraints are randomly generated as initial solutions; then, the solar collector photothermal conversion efficiency corresponding to each solution is calculated using a surrogate model. and maximum temperature difference The system performs non-dominated sorting to distinguish Pareto optimal and non-optimal solutions. Then, it generates the next generation population through selection, crossover, and mutation operations, with the crossover probability set to 0.8 and the mutation probability set to 0.05. Further, after 50 iterations, if the Pareto front converges, the iteration stops. Finally, the collector efficiency is selected from the Pareto front. Maximum and satisfying the maximum temperature difference The solutions with a value less than 10K are used as the final combination of optimization parameters.

[0080] Experimental verification was conducted on the direct absorption solar collector and its construction method based on bottom incidence of the present invention. Specifically, based on the optimized parameters, bottom incidence (VAS-bh) and top incidence (VAS-th) models were constructed in ANSYS Fluent, and their key performance indicators were compared. The results show that the heat conversion efficiency of the bottom incidence VAS-bh model is higher. The maximum temperature difference of the VAS-bh model is ≥8% higher than that of the top-incidence VAS-th model. The VAS-th of the bottom incidence method is ≥30% lower than that of the top incidence method, thus verifying the performance advantage of the bottom incidence method over the traditional top incidence method.

[0081] Furthermore, an experimental platform for a direct absorption solar collector was constructed. An AM1.5 standard solar simulator was used to provide constant irradiance. Inlet and outlet temperatures and flow rates were monitored using high-precision thermocouples and ultrasonic flow meters. The experimental measurements were compared one-to-one with the simulation calculations. The core benchmark indicators included outlet temperature. Photothermal conversion efficiency and flow resistance And set the acceptable error range as temperature error. Efficiency error Resistance error If the error of a certain indicator exceeds the acceptable range, the theoretical calculation value in the simulation is replaced with the thermal conductivity and attenuation coefficient of the actually prepared nanofluid measured experimentally. If the flow resistance error is too large, the turbulence constants in the viscosity equation are adjusted simultaneously, and the local mesh is refined in areas with large temperature gradients, such as near the bottom incident window. Then, the calculation is re-performed. After correction, the simulation calculation is re-performed and re-calibrated with the experimental data. The above correction and calibration process is repeated until all indicator errors meet the requirements. At the same time, the stability of the collector performance is analyzed under the fluctuation range of actual operating parameters, such as changes in solar irradiance and flow rate fluctuations, to ensure that the optimized parameter combination has good anti-interference ability and is suitable for complex outdoor operating environments.

[0082] This invention effectively improves photothermal conversion efficiency and optimizes parameter efficiency, accuracy, and heat transfer mechanisms. Through the synergistic effect of bottom incidence and volumetric absorption mechanisms, combined with optimized parameter design, the collector achieves a photothermal conversion efficiency of over 70%, exceeding 10% compared to top-incidence direct absorption solar collectors. Simultaneously, the temperature distribution is more uniform, effectively reducing the significant heat loss caused by excessively high surface temperatures in traditional surface collectors. Furthermore, this invention, through a systematic parameter optimization system, clarifies the optimal ranges for key parameters such as inlet mass flow rate, fluid attenuation coefficient, and collector length and height, avoiding performance waste caused by blind design. Its optimization method can be directly integrated with numerical simulation and experimental verification, significantly reducing the computational load and trial-and-error costs of traditional optimization processes, effectively shortening the product development cycle. Simultaneously, the bottom incidence method, heating against gravity, significantly enhances natural convection, Brownian motion, and thermophoresis effects. Compared to top incidence, its heat transfer process is more complete, effectively reducing temperature gradients, avoiding localized overheating, and extending the collector's lifespan.

[0083] It should be noted that a Direct Absorption Solar Collector (DASC) refers to a solar thermal collector that utilizes a fluid containing nanoparticles to directly absorb solar energy, thereby improving heat conversion efficiency. The nanofluid used is a working fluid whose thermal conductivity and radiative heat absorption capacity are enhanced by dispersing nanoparticles. The attenuation coefficient (Kext) describes the characteristic of light intensity attenuation caused by the absorption and scattering of incident sunlight by the working fluid. It reflects the degree of attenuation of light energy during transmission in the medium; a higher value indicates that light energy is absorbed or scattered more quickly. VAS-th indicates a top-incidence DASC, which receives solar radiation from above the collector. VAS-bh indicates a bottom-incidence DASC, which receives solar radiation from the bottom of the collector.

[0084] The above description is merely a preferred embodiment of the present invention. The present invention is not limited to the above-described embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this disclosure, as long as they achieve the technical effects of the present invention by the same means, should be included within the scope of protection of this disclosure and should fall within the protection scope of the present invention. Within the protection scope of the present invention, the technical solutions and / or implementation methods can have various modifications and variations.

Claims

1. A method for constructing a direct absorption solar collector based on a bottom-incidence method, characterized in that, include: S100. Establish a model of a bottom-incidence direct absorption solar collector; S200. The working fluid inside the solar collector is optimized through numerical simulation and experimental verification methods to enhance solar energy absorption. In the established two-dimensional numerical model, the key fluid parameters are optimized to improve the absorption effect of the working fluid. The key fluid parameters to be optimized include the fluid attenuation coefficient and the inlet flow velocity. S300. Based on the numerical simulation results obtained above, select appropriate inlet flow velocity and fluid attenuation coefficient for the collector model, and obtain the optimal values ​​of collector length and collector height to optimize the collector structural parameters. S400 aims to maximize the photothermal conversion efficiency of the solar collector. It uses fluid attenuation coefficient, inlet mass flow rate, collector length, height, and nanoparticle parameters as design variables to construct a multi-objective optimization system for maximizing the thermal efficiency of the solar collector and optimizing temperature uniformity. S100 includes the following steps: S110. The collector is simplified into a slender rectangular structure filled with working fluid, and the material configuration is set as follows: the absorption layer, which serves as the incident surface, is made of glass, the fluid domain is made of nanofluid, and the insulation layer is made of foam. S120. Set the steady-state calculation conditions for the model, enable the energy equation, and select... Flow model; S130. Set the model boundary conditions, including setting the surface heat transfer coefficient. Emissivity ε=0.1, heat flux density on adiabatic surface q=0, pressure outlet set to standard atmospheric pressure, return temperature set to the adjacent grid at the outlet, and bottom incident volumetric heat source mode set.

2. The method according to claim 1, characterized in that, In S120, the energy equation is expressed as follows: = ； In the formula, and These represent the fluid's physical properties, density and specific heat capacity, respectively. and They represent direction and velocity in direction, For fluid temperature, It serves as an internal heat source for the fluid. It represents the superposition of the fluid thermal conductivity and the turbulent thermal conductivity. = In the formula The thermal conductivity of the fluid itself, The turbulent thermal conductivity is the coefficient of thermal conductivity under turbulent conditions. ,in Indicates turbulent viscosity. This represents the Prandtl number for turbulence.

3. The method according to claim 2, characterized in that, In S130, the internal heat source of the fluid It is expressed as follows: ； In the formula, I0 is the solar irradiance, Kext is the fluid attenuation coefficient, H is the collector height, and y is the longitudinal coordinate.

4. The method according to claim 3, characterized in that, In step S200, the optimization operation of the fluid attenuation coefficient includes: From the unit Furthermore, the fluid attenuation coefficient, ranging from 1 to 100, is taken from 20 equally spaced values. Each time the fluid attenuation coefficient is changed, the model automatically calculates the corresponding outlet temperature. Then use equations Calculate the collector efficiency, where For inlet quality flow, The value is 0.025 kg / s; It is the specific heat capacity of the fluid. The value is 4182 kJ / kg*K; This refers to the inlet temperature of the solar collector. The value is 293.15K; It is the length of the solar collector. The value is 2m; The initial solar irradiance for the model. Value 998W / .

5. The method according to claim 4, characterized in that, In step S200, the optimization operation of the inlet flow rate includes: Adjust the mass flow rate parameter at the inlet of the solar collector model. Starting from a mass flow rate parameter value of 0.01 kg / s, take a mass flow rate parameter every 0.1 kg / s and calculate the outlet temperature and photothermal conversion efficiency in sequence until the mass flow rate parameter value is 0.1 kg / s. Each time the mass flow rate parameter is changed, the model automatically calculates the corresponding outlet temperature. Then use equations Calculate the collector efficiency, where For inlet quality flow, The value is 0.025 kg / s; It is the specific heat capacity of the fluid. The value is 4182 kJ / kg*K; This refers to the inlet temperature of the solar collector. The value is 293.15K; It is the length of the solar collector. The value is 2m; The initial solar irradiance for the model. Value 998W / .

6. The method according to claim 5, characterized in that, The S300 includes: S310. Modify the geometry of the ANSYS Fluent 2D model, including setting up a DASC model with a collector height H = 50cm and ten different values ​​for the collector length L, and ensuring that the boundary conditions and heat source are consistent with the original VAS-bh model. Obtain the corresponding outlet temperature for each length dimension through ANSYS Fluent steady-state calculation, and use the equations... The corresponding photothermal conversion efficiency is obtained, and then the optimal length of the collector is obtained when both the photothermal conversion efficiency and the outlet temperature are high. S320. Modify the geometry of the ANSYS Fluent 2D model, including using a DASC model with ten different values ​​for the collector length L=200cm and collector height H, and ensuring that the boundary conditions and heat source are consistent with the original VAS-bh model. Obtain the corresponding outlet temperature for each length dimension through ANSYS Fluent steady-state calculations, and use the equations... The corresponding photothermal conversion efficiency is obtained, and then the optimal length of the collector is obtained when both the photothermal conversion efficiency and the outlet temperature are high. S330. Based on the optimal dimensions confirmed by simulation, manufacture a simple collector structure to build an experimental platform for direct absorption solar collectors.

7. The method according to claim 6, characterized in that, The S400 includes: S410. Establish a parametric geometric model. After completing the mesh generation and physical model settings, use the Latin hypercube sampling method based on the fluid attenuation coefficient Kext and the inlet mass flow rate. For the four design variables—collector length L, collector height H—one hundred sample points were extracted to uniformly cover the design variable space. Each sample point corresponds to an output including the collector's photothermal conversion efficiency. outlet temperature Maximum temperature difference Key indicators, including average velocity of the flow field, are used to form a simulation database; S420. A combined surrogate model is constructed using a combination of response surface methodology and radial basis functions to balance accuracy and computational efficiency. S430, with four design variables as inputs, and the solar collector's photothermal conversion efficiency. outlet temperature and maximum temperature difference For output, the mapping function relationship between the objective function and the design variables is obtained by fitting a simulation database. This mapping function is expressed as Y = f(Kext, ... (L, H), where Y is the objective function, the solar collector's photothermal conversion efficiency. outlet temperature and maximum temperature difference .

8. A direct absorption solar collector based on a bottom-incidence method, comprising the construction method of a direct absorption solar collector based on a bottom-incidence method as described in any one of claims 1 to 7; characterized in that, The direct absorption solar collector includes: The collector inlet serves as the starting point for the nanofluid to enter the channel; Working nanofluids are used to flow inside the collector to capture solar radiation energy incident from the bottom and convert it into heat energy. The location serves as the outlet of the collector, where a thermocouple is arranged to monitor the outlet temperature of the nanofluid that has heated up after absorbing heat. A top insulation layer using high-efficiency foam insulation material to minimize heat loss from the top; The glass layer located at the bottom of the solar collector.

9. The direct absorption solar collector according to claim 8, characterized in that, Also includes: Insulating walls that serve as thermal insulation walls on both sides of the passage to effectively reduce lateral heat loss; Bottom incident light source used to simulate standard solar irradiance entering the channel from the bottom of the collector; A fixed frame used to connect the side of the solar collector so that the bottom is unobstructed and the light source can enter smoothly. A support frame that provides mechanical stability and structural height support for the entire solar collector.

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