Method for calculating thermal performance of large opening molten salt tank type heat collection system

Abstract

The application discloses a kind of big opening molten salt tank type heat collection system heat performance calculation method, first, combining the deformation of heat collector self-weight and wind load, high-temperature thermal eccentricity of heat absorption pipe and solar position parameter, the dynamic comprehensive optical efficiency of mechanical-optical deformation coupling correction is calculated;Second, the multilayer steady heat transfer model of heat absorption pipe, glass cover and environment is established, the rheology correction of heat absorption pipe is carried out to the convection heat transfer coefficient in pipe, the heat absorption pipe-glass cover heat exchange, glass cover external heat dissipation and heavy load support thermal bridge effect are coupled, and the unit length comprehensive heat loss of heat collection pipe is solved;Finally, based on two-dimensional axial subsection steady energy balance equation, coupling nonlinear molten salt property parameter iterative calculation outlet temperature, thermal efficiency and pressure drop, and reversible solving molten salt freeze-proof critical recirculation flow.The application can accurately reflect the real heat transfer characteristics of big opening structure and high-temperature molten salt, improve the precision of heat performance calculation, and provide safety boundary for system freeze-proof operation.

Description

Technical Field

[0001] This invention relates to the field of large-aperture trough solar thermal power plant collector field technology, specifically to a method for calculating the thermal performance of a large-aperture molten salt trough collector system. Background Technology

[0002] Parabolic trough solar collectors are one of the mainstream technologies in current solar thermal power generation systems. With industry development, large-aperture parabolic trough collectors (typically with an aperture width of over 8 meters) combined with high-temperature molten salt heat transfer media (operating temperatures reaching 290°C~565°C) have become an important trend for improving the economic efficiency of power plants. However, current collector thermal performance calculations often employ empirical formulas or highly simplified steady-state models for traditional heat transfer oils and conventional aperture widths. When applied to large-aperture molten salt systems, these calculations reveal serious biases. The main technical defects include:

[0003] First, the mechanical-optical spatial coupling deformation unique to large openings is not fully considered: Due to the huge opening chord length and cantilever, the large opening collector has significant self-weight deformation, nonlinear spatial torsional deformation caused by wind load, and axial and vertical eccentricity of the collector tube under high temperature thermal expansion. Traditional fixed geometric errors and end loss constants cannot accurately characterize this dynamic spot overflow effect.

[0004] Second, the variable viscosity rheological characteristics of high-temperature molten salt are not effectively decoupled from strong heat loss: at ultra-high temperatures of 565℃, strong radiative heat dissipation between the heat absorber and the glass cover, as well as the thermal bridging effect of the heavy-duty reinforced support, become dominant. Meanwhile, the binary nitrate working fluid has high density and its viscosity changes exponentially with temperature. Traditional heat transfer formulas based on constant physical properties neglect the non-uniform viscosity boundary layer effect between the tube wall and the fluid center, leading to distortions in the calculation of convective heat transfer coefficients and pressure drop along the flow path.

[0005] Third, there is a lack of a reverse solution mechanism for the low-flow recirculation antifreeze safety boundary, which is specific to the high freezing point characteristics of molten salt: the freezing point of molten salt working fluid is as high as about 220°C, and power plants often need to implement low-flow recirculation antifreeze strategies at night. Existing calculation methods can only solve the normal performance parameters in a one-way sequential manner, and lack the ability to perform reverse evaluation by combining the characteristics of large openings and strong heat loss with the critical safety control flow rate under extreme conditions. Summary of the Invention

[0006] The purpose of this invention is to propose a method for calculating the thermal performance of a large-opening molten salt trough solar collector system. Considering the structural characteristics and high-temperature molten salt working fluid properties of such systems, this method integrates optical correction, multi-layer, multi-mechanism heat transfer calculations, and thermal bridge effect analysis of heavy-duty supports. Combined with molten salt rheological property correction and axial segmented energy balance, it achieves accurate prediction of the thermal performance of the large-opening molten salt trough solar collector system. This effectively solves the problems of large deviations and poor applicability of traditional calculation models under large-opening conditions, and can also reversely determine the critical flow rate for freeze protection, significantly improving calculation accuracy and system operational safety.

[0007] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for calculating the thermal performance of a large-opening molten salt trough solar collector system, the method comprising:

[0009] S1. Based on the opening width of the large-opening slot solar collector, the nonlinear spatial torsional deformation function caused by its own weight and wind load, and the high-temperature axial thermal eccentricity of the absorber tube, combined with the solar position parameters, the dynamic comprehensive optical efficiency of the solar collector with mechanical-optical deformation coupling correction is calculated.

[0010] S2. Establish a multi-layer steady-state heat transfer model between the heat absorber tube, the glass cover and the environment. Use the temperature-varying viscosity gradient of the molten salt between the inner wall of the heat absorber tube and the center of the fluid to perform rheological correction on the convective heat transfer coefficient inside the tube. Combine the non-uniform heat dissipation effect of the thermal bridge of the heavy-duty reinforced support to calculate the comprehensive heat loss per unit length of the heat collector tube.

[0011] S3, based on the two-dimensional axial piecewise steady-state energy balance equation, combined with the molten salt density and specific heat capacity parameters that change nonlinearly with temperature, coupled with the optical efficiency of step S1 and the heat loss of step S2, iteratively solves the outlet temperature, thermal efficiency and system pressure drop of each segment of the heat collector tube.

[0012] Step S1 further includes:

[0013] Calculate the real-time position of the sun based on the solar altitude angle, solar azimuth angle, and hour angle, and determine the solar incidence angle. And calculate the incident angle correction factor K:

[0014] ;

[0015] The loss coefficient at the base end is obtained through geometric truncation relationships or optical simulations, and is introduced into the average outer surface temperature of the heat absorber tube. Axial thermal eccentricity function The end-point loss is dynamically corrected, and the corrected end-point loss coefficient is:

[0016]

[0017] In the formula, r is the distance from any point at the end of the condenser mirror to the focal point; The length of the trough-type solar collector; The focal length of a large aperture condenser;

[0018] The shading coefficient between adjacent concentrators is dynamically determined based on the mirror field loop arrangement and the sun's position. :

[0019]

[0020] In the formula, The spacing between trough-type solar concentrators; The chord length of the concentrator opening;

[0021] The nonlinear spatial torsional deformation induced by self-weight and wind load is constructed into a spatial deformation function. Dynamic correction of mirror geometric errors:

[0022]

[0023] In the formula, It is the geometric error constant for the basic mirror assembly; The dynamic correction coefficient is obtained based on mechanical analysis or finite element simulation, and is affected by the solar altitude angle. Real-time wind speed and the opening width of the large-opening trough collector Joint control, solar altitude angle The distribution of the solar collector's self-gravity moment and real-time wind speed determine the solar collector's self-gravity Introducing wind-induced loads, opening width Characterizing the sensitivity of structural stiffness to deformation; The dimensionless coefficients output by the function are used to characterize the dynamic weakening effect of nonlinear torsional deformation on the concentrating ability of the mirror surface of the large-opening slot solar collector under the coupled action of self-weight and wind load.

[0024] Coupled with incident angle correction, end loss correction, shading coefficient, deformation correction, and optical loss factor including reflection, transmission, and cleanliness, the effective optical efficiency at the glass shell and the effective optical efficiency of the collector are calculated.

[0025]

[0026]

[0027] In the formula, , , , , These are the HCE shading correction factor, tracking error correction factor, mirror dirt correction factor, absorber tube dirt correction factor, and other factor correction factors, respectively. The transmittance of the glass casing;

[0028] Calculate solar irradiance per unit receiver length :

[0029]

[0030] In the formula, DNI is the direct radiation intensity;

[0031] The instantaneous thermal efficiency of the solar collector is obtained by combining the solar absorption of the glass shell and the absorber. for:

[0032] ;

[0033] In the formula, This represents the effective heat transfer power from the absorber tube to the molten salt per unit absorber length.

[0034] Furthermore, the spatial deformation function The data is obtained through finite element mechanical simulation combined with ray tracing analysis, specifically including the following steps:

[0035] Establish a structural mechanics model for a large-opening trough solar collector, using the solar altitude angle. Real-time wind speed and opening width Given the operating parameters, solve the nonlinear spatial torsional deformation field of the solar collector under different solar altitude angles and wind speeds;

[0036] Based on the nonlinear spatial torsional deformation field, the actual reflection path deviation of light rays at each position is calculated, and the dynamic attenuation coefficient of the mirror focusing efficiency under different working conditions is obtained.

[0037] The dynamic attenuation coefficient is constructed as a spatial deformation function related to the operating parameters. It outputs a correction coefficient between 0 and 1, which is used to dynamically correct the geometric errors of the mirror surface.

[0038] Step S2 further includes:

[0039] A multi-layer steady-state heat transfer model was established between the heat absorber tube, the glass cover, and the environment to clarify the complete heat transfer path, including convective heat transfer on the inner wall of the heat absorber tube, heat conduction on the tube wall, radiation and convective heat transfer between the outer surface of the heat absorber tube and the glass cover, convective and radiative heat dissipation between the outer surface of the glass cover and the environment, and heat dissipation through thermal bridges of the heavy-duty reinforced support.

[0040] Based on the heat transfer boundary inside the tube in the multi-layer heat transfer model, the dynamic viscosity of the molten salt at different temperatures is calculated according to the temperature difference between the inner wall of the heat absorber tube and the average temperature of the molten salt body. The Nusselt number under normal physical property conditions is rheologically corrected using the Sieder-Tate correction formula, and the convective heat transfer coefficient inside the tube is calculated based on the corrected Nusselt number.

[0041] Using the convective heat transfer coefficient inside the tube as the boundary condition of the multilayer steady-state heat transfer model, the tube wall temperature of the absorber tube is obtained by solving. For the non-uniform heat dissipation of the heavy-duty reinforced support, a thermal bridge heat transfer model of support-absorber tube is established. The tube wall temperature of the absorber tube is used as the base temperature of the thermal bridge heat transfer model. The additional heat loss of the support part is calculated, and the additional heat loss term is coupled into the thermal bridge heat dissipation path of the support in the multilayer steady-state heat transfer model.

[0042] The convection heat transfer inside the tube, the heat dissipation at each interface, and the heat bridge heat dissipation of the support are all coupled into the multi-layer steady-state heat transfer model, and the comprehensive heat loss per unit length of the heat collector tube is obtained by solving the system simultaneously.

[0043] Furthermore, to address the non-uniform heat dissipation of the heavy-duty reinforced support, a thermal bridge heat transfer model is established by treating the support as an infinitely large fin. Using the temperature of the absorber tube wall as the base temperature, the additional heat loss per unit length of the support is calculated. :

[0044]

[0045] In the formula, The average convection coefficient of the support structure; The perimeter of the support frame; The thermal conductivity of the support; This represents the minimum cross-sectional area of ​​the support frame. This refers to the wall temperature of the heat absorber tube. The ambient temperature; This is the length of the heat absorption tube.

[0046] Step S3 further includes:

[0047] The collector tube is divided into multiple calculation segments along the axial direction, with temperature continuity maintained at the boundaries of each segment. A two-dimensional axial segmented steady-state energy balance equation is used to model the collector tube segment by segment. The dynamic comprehensive optical efficiency of the collector calculated in step S1, the comprehensive heat loss per unit length of the collector tube calculated in step S2, and the molten salt density and specific heat capacity parameters that change nonlinearly with temperature are used as input conditions for the two-dimensional axial segmented steady-state energy balance equation. Based on the energy balance relationship of each segment, considering the influence of molten salt temperature change on physical properties, the energy balance equation of each segment is iteratively solved until the outlet temperature of each segment converges, thus obtaining the outlet temperature of the molten salt in each segment, and then obtaining the outlet temperature distribution, thermal efficiency distribution, and system pressure drop of the entire collector tube.

[0048] Furthermore, the method also includes:

[0049] S4 sets the molten salt freezing point and safety margin as the lower limit of the antifreeze temperature. When the direct radiation intensity is lower than the control threshold or under nighttime conditions, the two-dimensional axial piecewise energy balance equation is solved in reverse iteration to output the critical minimum antifreeze recirculation flow rate that keeps the molten salt in the heat collection loop from condensing locally, thus realizing the collaborative design of system thermal performance evaluation and antifreeze operation strategy.

[0050] Step S4 further includes:

[0051] Set the lower limit of the allowable safe temperature of the molten salt local tube wall at any location in the heat collection circuit. for:

[0052]

[0053] In the formula, This is the theoretical freezing point of the molten salt working fluid; To ensure a safety margin against freezing;

[0054] Set the direct radiation intensity DNI=0, and set the lower safety temperature limit. As a boundary condition, it is substituted into the axial piecewise steady-state energy balance equation consisting of N control volumes of equal length.

[0055] By performing inverse convergence iteration of the piecewise steady-state energy balance equation along the axial direction, the solution at the current ambient temperature can be obtained. Below, the critical minimum antifreeze recirculation mass flow rate required to prevent localized crystallization freezing at any point in the large-open circuit. This is used as the control boundary indicator for the variable frequency operation of the mirror field antifreeze pump:

[0056]

[0057] In the formula: The heat loss per unit length of the i-th segment is calculated after considering the thermal bridging effect of the support. Let be the axial length of the i-th heat collector tube; Let be the average specific heat capacity of the molten salt in the i-th segment, which varies non-linearly with temperature. and These are the inlet and outlet temperatures of the molten salt in the i-th segment, respectively.

[0058] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0059] First, the thermal performance calculation method of the large-aperture molten salt trough solar collector system of the present invention, by establishing a comprehensive optical correction model, fully considers the reflection error caused by the sun's position, collector end loss, mirror field obstruction and structural deformation, and more accurately describes the actual optical loss of the large-aperture solar collector, overcoming the defect that the traditional fixed coefficient model cannot adapt to the large-aperture structure.

[0060] Secondly, the thermal performance calculation method of the large-opening molten salt trough solar collector system of the present invention, by constructing a multi-layer, multi-mechanism heat transfer network and introducing thermal bridge heat transfer calculation of heavy-duty reinforced support, comprehensively considers heat conduction, convection and radiation coupled heat dissipation, which can accurately reflect the true heat loss distribution of the solar collector and avoid underestimation of thermal performance due to ignoring the thermal bridge effect of the support.

[0061] Third, the thermal performance calculation method of the large-opening molten salt trough heat collection system of the present invention improves the problem of heat transfer coefficient and pressure drop calculation distortion caused by constant physical property assumptions by considering the high-temperature rheological characteristics of molten salt to correct the convective heat transfer inside the tube, making the temperature distribution of the heat collection tube, outlet temperature and thermal efficiency prediction more in line with the actual operating conditions.

[0062] Fourth, the thermal performance calculation method of the large-opening molten salt trough solar collector system of the present invention solves the critical antifreeze flow rate in reverse based on the same energy balance model, directly providing a safety boundary index for the nighttime low-flow recirculation antifreeze strategy, realizing integrated support from accurate thermal performance calculation to system safe operation control, and is more suitable for the design optimization and operation decision of large-scale molten salt solar thermal power plants. Attached Figure Description

[0063] Figure 1 This is a flowchart of the thermal performance calculation method for the large-opening molten salt tank heat collection system of the present invention;

[0064] Figure 2 This is a schematic diagram of a one-dimensional heat transfer model;

[0065] Figure 3 This is a schematic diagram of a two-dimensional heat transfer model;

[0066] Figure 4 This is a schematic diagram of the loss at the end of the mirror field;

[0067] Figure 5 It is a geometric angle related to sunlight. Detailed Implementation

[0068] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0069] See Figure 1 This invention discloses a method for calculating the thermal performance of a large-aperture molten salt tank solar collector system, the method comprising:

[0070] S1. Based on the opening width of the large-opening slot solar collector, the nonlinear spatial torsional deformation function caused by its own weight and wind load, and the high-temperature axial thermal eccentricity of the absorber tube, combined with the solar position parameters, the dynamic comprehensive optical efficiency of the solar collector with mechanical-optical deformation coupling correction is calculated.

[0071] S2. Establish a multi-layer steady-state heat transfer model between the heat absorber tube, the glass cover and the environment. Use the temperature-varying viscosity gradient of the molten salt between the inner wall of the heat absorber tube and the center of the fluid to perform rheological correction on the convective heat transfer coefficient inside the tube. Combine the non-uniform heat dissipation effect of the thermal bridge of the heavy-duty reinforced support to calculate the comprehensive heat loss per unit length of the heat collector tube.

[0072] S3, based on the two-dimensional axial piecewise steady-state energy balance equation, combined with the molten salt density and specific heat capacity parameters that change nonlinearly with temperature, coupled with the optical efficiency of step S1 and the heat loss of step S2, iteratively solves the outlet temperature, thermal efficiency and system pressure drop of each segment of the heat collector tube.

[0073] I. Dynamic Comprehensive Optical Efficiency of Solar Collector

[0074] This step includes: determining the solar incidence angle and incidence angle correction coefficient based on the opening width of the large-aperture trough collector, its nonlinear spatial torsional deformation caused by its own weight and wind load, and the axial thermal eccentricity generated by the absorber tube at high temperatures, combined with solar position parameters such as solar altitude angle, azimuth angle, and hour angle. The end loss coefficient is obtained through geometric truncation relationships or optical simulations, and a thermal eccentricity function is introduced to dynamically correct the end loss. The shading coefficient is dynamically determined based on the mirror field arrangement and the solar position. The structural torsional deformation is constructed into a spatial deformation function to correct the mirror geometric error in real time. Finally, the incidence angle correction, end loss correction, shading coefficient, deformation correction, and multiple optical loss factors such as reflection, transmission, and cleanliness are coupled to obtain the effective optical efficiency at the glass shell and the effective optical efficiency of the collector. Combined with the relationship between solar irradiance per unit length and solar absorption distribution, the instantaneous thermal efficiency of the collector is calculated, completing the dynamic optical efficiency calculation of the mechanical-optical coupling.

[0075] (1.1) Angle of solar incidence and the incident angle correction factor K

[0076] Calculate the real-time position of the sun based on the solar altitude angle, solar azimuth angle, and hour angle, and determine the solar incidence angle. And calculate the incident angle correction factor K:

[0077] .

[0078] like Figure 5 As shown, the angle of solar incidence The calculation formula is:

[0079]

[0080] In the formula, The angle between the north-south axis and the concentrator. The solar altitude angle, This is the solar azimuth angle.

[0081] (1.2) End loss coefficient

[0082] Due to the long focal length, large-aperture solar collectors are prone to three-dimensional eccentricity along the focal line caused by the thermal expansion of the absorber tubes at high temperatures. Therefore, a correction for axial thermal eccentricity is introduced. Specifically, such as... Figure 4 As shown, the loss coefficient at the base end is obtained through geometric truncation relationship or optical simulation, and is introduced into the average outer surface temperature of the heat absorber tube. The axial thermal eccentricity function caused by thermal expansion sagging or axial movement The end-point loss is dynamically corrected, and the corrected end-point loss coefficient is:

[0083]

[0084] In the formula, r is the distance from any point at the end of the condenser mirror to the focal point; The length of the trough-type solar collector; The focal length of the large aperture condenser.

[0085] (1.3) Shading coefficient

[0086] When deploying large-scale trough solar concentrator fields, the shading effect between concentrators must be fully considered. Especially for trough collector systems using a north-south arrangement and east-west tracking configuration, at sunrise, the front-row collectors significantly shade the rear-row collectors, resulting in a substantial decrease in the direct solar radiation received by the rear collectors. However, as the solar altitude angle gradually increases, the shading situation improves. When the sun reaches its highest altitude angle at noon, the shading effect disappears completely. The trend of shading from noon to sunset is similar to that from sunrise to noon. The shading coefficient between adjacent concentrators is dynamically determined based on the concentrator field loop arrangement and the sun's position. :

[0087]

[0088] In the formula, The spacing between trough-type solar concentrators; The chord length of the concentrator opening.

[0089] (1.4) Geometric error of mirror surface

[0090] For large-aperture trough solar collectors, this factor is no longer a constant, but a dynamic function incorporating the spatial torsional deformation of the large opening. Specifically, the nonlinear spatial torsional deformation caused by self-weight and wind load is constructed as a spatial deformation function. Dynamic correction of mirror geometric errors:

[0091]

[0092] In the formula, It is the geometric error constant for the basic mirror assembly; The dynamic correction coefficient is obtained based on mechanical analysis or finite element simulation, and is affected by the solar altitude angle. Real-time wind speed and the opening width of the large-opening trough collector Joint control, solar altitude angle The distribution of the solar collector's self-gravity moment and real-time wind speed determine the solar collector's self-gravity Introducing wind-induced loads, opening width Characterizing the sensitivity of structural stiffness to deformation; The dimensionless coefficients output by the function are used to characterize the dynamic weakening effect of nonlinear torsional deformation on the concentrating ability of the mirror surface of the large-opening slot solar collector under the coupled action of its own weight and wind load.

[0093] Preferred, spatial deformation function The data is obtained through finite element mechanical simulation combined with ray tracing analysis, specifically including the following steps:

[0094] Establish a structural mechanics model for a large-opening trough solar collector, using the solar altitude angle. Real-time wind speed and opening width Given the operating parameters, solve the nonlinear spatial torsional deformation field of the solar collector under different solar altitude angles and wind speeds;

[0095] Based on the nonlinear spatial torsional deformation field, the actual reflection path deviation of light rays at each position is calculated, and the dynamic attenuation coefficient of the mirror focusing efficiency under different working conditions is obtained.

[0096] The dynamic attenuation coefficient is constructed as a spatial deformation function related to the operating parameters. It outputs a correction coefficient between 0 and 1, which is used to dynamically correct the geometric errors of the mirror surface.

[0097] (1.5) Effective optical efficiency at the glass casing With the effective optical efficiency of the solar collector

[0098] Coupled with incident angle correction, end loss correction, shading coefficient, deformation correction, and optical loss factor including reflection, transmission, and cleanliness, the effective optical efficiency at the glass shell and the effective optical efficiency of the collector are calculated.

[0099]

[0100]

[0101] In the formula, , , , , These are the HCE shielding correction factor (bellows, shielding, support), tracking error correction factor, mirror fouling correction factor, heat absorber fouling correction factor, and other factor correction factors. The transmittance of the glass casing.

[0102] (1.6) Instantaneous thermal efficiency of the solar collector

[0103] Calculate solar irradiance per unit receiver length :

[0104]

[0105] In the formula, DNI is the direct radiation intensity;

[0106] The instantaneous thermal efficiency of the solar collector is obtained by combining the solar absorption of the glass shell and the absorber. for:

[0107] ;

[0108] In the formula, This represents the effective heat transfer power from the absorber tube to the molten salt per unit absorber length.

[0109] (ii) Overall heat loss per unit length of collector tube

[0110] This step specifically includes: establishing a multi-layer steady-state heat transfer model between the absorber tube, the glass cover, and the environment; clarifying the complete heat transfer paths for convective heat transfer within the absorber tube, heat conduction within the tube wall, radiation and convective heat transfer between the outer surface of the absorber tube and the glass cover, convective and radiative heat dissipation between the outer surface of the glass cover and the environment, and heat dissipation via thermal bridges in the heavy-duty reinforced support; based on the heat transfer boundary within the tube in the multi-layer heat transfer model, calculating the dynamic viscosity of the molten salt at different temperatures according to the temperature difference between the inner wall of the absorber tube and the average temperature of the molten salt body; using the Sieder-Tate correction formula to perform rheological correction on the Nusselt number under constant physical property conditions; and based on the corrected Nusselt number... The convective heat transfer coefficient inside the tube is calculated using the Ernst number. This coefficient is then used as the boundary condition for a multi-layer steady-state heat transfer model to obtain the tube wall temperature. For the non-uniform heat dissipation of the heavy-duty reinforced support, a thermal bridge heat transfer model between the support and the absorber tube is established. The absorber tube wall temperature is used as the base temperature of the thermal bridge heat transfer model to calculate the additional heat loss at the support location. This additional heat loss term is coupled into the thermal bridge heat dissipation path of the support in the multi-layer steady-state heat transfer model. All heat transfer factors—including convective heat transfer inside the tube, heat dissipation at each interface, and thermal bridge heat dissipation—are coupled into the multi-layer steady-state heat transfer model, and the combined heat loss per unit length of the collector tube is obtained by solving these simultaneous equations.

[0111] (2.1) One-dimensional radial steady-state energy conservation equation

[0112] like Figure 2 As shown. For clarity, incident solar energy and optical losses have been omitted from the drag model. Optical losses are due to defects in the collector mirrors, tracking errors, shading, and the cleanliness of the mirrors and the HCE (Heat Collector Element).

[0113] The effectively incident solar energy (solar energy minus light loss) is absorbed by the glass casing ( ) and selective absorption coatings ( The energy absorbed into the selective coating is divided into three parts: the first part is conducted through the pipe wall (…). After conduction, heat is transferred through convection within the pipe. The heat is transferred to the molten salt (HeatTransferFluid); the second part is through convection between the absorber tube and the glass cover. ) and radiation ( Heat is transferred to the glass housing via heat exchange; the third part is supported by an HCE bracket in the form of a thermal bridge. The energy is directly conducted and lost to the environment. The second portion of energy transferred to the glass cover is lost through the heat conduction of the glass outer shell. The solar energy is transferred to the outer surface of the glass, along with the solar energy absorbed by the glass cover itself. Together, through convection between the outer surface of the glass and the environment ( ) and radiation ( Heat loss to the environment. The presence of the glass outer shell forms a vacuum thermal barrier between the heat absorber tube and the environment, significantly reducing heat loss from the heat collector tube. If the glass outer shell is removed, the heat absorber tube will be directly exposed to the environment, and heat loss will increase dramatically. The heat transfer path changes from "heat absorber tube → glass shell → environment" to "heat absorber tube → environment," losing the inhibitory effect of the intermediate thermal resistance. This model assumes that the temperature, heat flux, and thermophysical parameters are uniformly distributed along the circumference of the HCE. Figure 2 All heat flux directions shown are positive. Based on the above heat transfer paths, a one-dimensional radial steady-state energy conservation equation is established, providing a foundation for subsequent two-dimensional axial piecewise energy balance calculations. The one-dimensional radial steady-state energy conservation equation is shown below:

[0114]

[0115]

[0116]

[0117]

[0118]

[0119] .

[0120] (2.1) Calculation of high-temperature molten salt viscosity-variable convective heat transfer in the heat absorber tube

[0121] When the molten salt working fluid undergoes convective heat transfer inside the absorber tube, its viscosity changes exponentially due to the large temperature difference between the center and the tube wall. To correct for the convective heat transfer from the inner wall of the large-aperture collector tube to the high-temperature molten salt working fluid, a high-viscosity boundary laminar rheological correction (Sieder-Tate correction) is introduced:

[0122]

[0123] In the formula: The fundamental Nusselt number under constant physical properties; The dynamic viscosity (N) of the molten salt at the bulk average temperature T1 s / m²); The dynamic viscosity (N) of the molten salt at temperature T2 on the inner wall of the heat absorber tube. (s / m²). Using the corrected Nusselt number. Calculate the corrected in-tube convective heat transfer coefficient for:

[0124]

[0125] In the formula, The thermal conductivity of molten salt, This is the inner diameter of the heat absorption tube.

[0126] According to Newton's law of cooling, the convective heat transfer per unit length of pipe... for:

[0127]

[0128] In the formula, and These are the inner wall temperature of the heat absorber and the average temperature of the molten salt body, respectively. This is the convective heat transfer coefficient. It represents the convective heat transfer rate inside the pipe. As an inner boundary condition of the multilayer heat transfer model, it is used to solve for the wall temperature of the heat absorber tube.

[0129] (2.2) Heat transfer calculation between the heat absorber tube and the glass cover

[0130] The heat transfer between the outer surface of the absorber tube and the inner wall of the glass cover includes radiative heat transfer and annular convective heat transfer. Under high vacuum conditions, the contribution of convective heat transfer is weak, but it is still included to ensure the integrity of the model.

[0131] (2.2.1) Convective heat transfer

[0132] When the HCE annulus is under vacuum (pressure < 1 torr), convective heat transfer between the absorber and the glass cladding occurs via free molecular convection, and the convective heat transfer per unit length is:

[0133]

[0134] In the formula: The diameter of the outer surface of the absorber; The annular air convection heat transfer coefficient; This refers to the outer surface temperature of the absorber. This refers to the surface temperature of the inner glass outer shell.

[0135] (2.2.2) Radiative heat transfer

[0136] The radiative heat transfer between the outer surface of the heat absorber tube and the inner wall of the glass cover is determined by the following formula:

[0137]

[0138] In the formula, It is the Stefan-Boltzmann constant; The diameter of the external absorber; The diameter of the inner glass sleeve; This refers to the outer surface temperature of the absorber. This refers to the surface temperature of the inner glass outer shell. Emissivity of the absorber selective coating; The emissivity of the glass cladding is denoted as .

[0139] (2.2.3) Heat exchanger between heat absorber tube and glass cover

[0140] The total heat transfer per unit length between the heat absorber and the glass cover is the sum of convection and radiation terms:

[0141] .

[0142] Under steady-state heat transfer conditions, all of this heat will be transferred to the glass cover and eventually lost to the environment through convection and radiation on the outer surface of the glass.

[0143] (2.3) Heat loss from the glass shell to the environment

[0144] The heat exchange between the glass shell and the environment is an important component of the total heat loss of the heat collection tube, mainly including convective heat loss and radiative heat loss, among which convective heat loss is particularly significant under windy conditions.

[0145] (2.3.1) Convection heat dissipation of the glass outer shell to the environment

[0146] The convective heat transfer from the glass shell to the atmosphere is calculated using Newton's law of cooling:

[0147]

[0148]

[0149] In the formula: This refers to the temperature of the outer surface of the glass casing. The ambient temperature; The convective heat transfer coefficient of air; The thermal conductivity of air; The outer diameter of the glass casing; This is the average Nusselt number based on the outer diameter of the glass casing.

[0150] (2.3.2) Radiative heat transfer

[0151] The net radiative heat transfer between the glass shell and the sky is caused by the temperature difference. Assuming the glass shell is a small convex gray object inside a large blackbody cavity (the sky), its radiative heat dissipation formula is:

[0152]

[0153] In the formula: It is the Stefan-Boltzmann constant; The outer diameter of the glass casing; The emissivity of the outer surface of the glass casing; This refers to the temperature of the outer surface of the glass casing. Effective sky temperature.

[0154] (2.3.3) Glass shell - total ambient heat loss

[0155] The total heat loss per unit length between the glass enclosure and the environment is the sum of convective and radiative heat loss:

[0156] .

[0157] (2.4) Heat loss through the HCE support

[0158] HCEs are supported at the receiver focal line by support brackets extending from the receiver structure to the receiver tubes. Each HCE has a support bracket at each end. By treating the support brackets as infinitely large fins with a base temperature less than the outer receiver surface temperature T3, the loss of the brackets at the attachment point is approximated. This estimated base temperature takes into account the heat loss over a short distance from the bracket attachment to a minimum cross-sectional area, which is considered to be the base of the fins (~5cm with ~4cm insulation).

[0159] To address the non-uniform heat dissipation of heavy-duty reinforced brackets, a thermal bridge heat transfer model is established by treating the bracket as an infinitely large fin. Using the tube wall temperature as the base temperature, the additional heat loss per unit length of the bracket is calculated. :

[0160]

[0161] In the formula, The average convection coefficient of the support structure; The perimeter of the support frame; The thermal conductivity of the support; This represents the minimum cross-sectional area of ​​the support frame. This refers to the wall temperature of the heat absorber tube. The ambient temperature; This is the length of the heat absorption tube.

[0162] (2.5) Comprehensive heat loss per unit length of collector tube

[0163] The system couples the internal convection heat transfer, the heat transfer between the absorber tube and the glass cover, the heat dissipation from the glass shell to the environment, and the thermal bridge losses through the support structure. From an energy conservation perspective, the total heat loss per unit length emitted by the collector tube is composed of the heat dissipation loss from the glass shell to the environment and the thermal bridge losses through the support structure, i.e.:

[0164] .

[0165] (III) Thermal Performance Calculation

[0166] The steps include: dividing the collector tube into multiple calculation segments along the axial direction, maintaining temperature continuity at the boundaries of each segment; using a two-dimensional axial segmented steady-state energy balance equation to model the collector tube segment by segment, taking the dynamic comprehensive optical efficiency of the collector calculated in step S1, the comprehensive heat loss per unit length of the collector tube calculated in step S2, and the molten salt density and specific heat capacity parameters that change nonlinearly with temperature as input conditions for the two-dimensional axial segmented steady-state energy balance equation; based on the energy balance relationship of each segment, considering the influence of molten salt temperature changes on physical properties, iteratively solving the energy balance equation of each segment until the outlet temperature of each segment converges, obtaining the outlet temperature of the molten salt in each segment, and thus obtaining the outlet temperature distribution, thermal efficiency distribution, and system pressure drop of the entire collector tube.

[0167] This model divides the length of the absorber into N equal segments and maintains temperature continuity at the boundary surface, such as... Figure 3 As shown. It is assumed that the radial heat flux on each segment is uniform and perpendicular to the surface, and these heat fluxes are evaluated at the average temperature on both sides of the segment. It is assumed that the longitudinal temperature is approximately linear and the thermal conductivity is constant. Therefore, the longitudinal heat conduction terms on both sides cancel each other out, and only the heat transfer medium (HTF) will transfer energy in the longitudinal direction. Based on these assumptions, the radial heat transfer term can be modeled using a one-dimensional energy balance. The steady-state energy balance of the receiver can be estimated using the following equation:

[0168]

[0169] In the formula, Net heat flux per unit area The circumferential area of ​​the control volume "i" For mass flow rate h is the enthalpy value. v is the overall flow velocity .

[0170] For receiver segment i of length , the energy balance becomes

[0171]

[0172] Net heat flow includes solar absorption and heat loss; ;

[0173] Solar absorption terms include the absorber and the glass cladding. :

[0174] ;

[0175] Heat loss includes radiative and convective heat loss from the glass cladding, as well as conductive heat loss through the support structure:

[0176]

[0177] in, .

[0178] Assuming that the density of HTF is only a function of temperature (and is incompressible with respect to pressure), the enthalpy change can be approximated by the following equation:

[0179]

[0180]

[0181] The specific heat and density in the above equations are evaluated at the average HTF temperature over the length of the receiver segment.

[0182] Based on all the above results, we can obtain

[0183]

[0184] Furthermore, the outlet temperature was obtained by solving for it.

[0185]

[0186] The inlet velocity is determined by the cross-sectional area and volumetric flow rate of the absorber, both of which are input values; the remaining velocities are calculated through mass conservation and continuity at the segment boundaries.

[0187]

[0188]

[0189] Pressure changes are estimated by calculating the equation for pressure loss in a fully developed turbulent horizontal pipe:

[0190]

[0191] Where is the Darcy friction factor, which can be estimated using the following Colebrook equation for turbulent pipe flow:

[0192]

[0193] Here, It is not the equivalent roughness, but the Reynolds number calculated at the average axial heat carrier bulk temperature in each receiver section:

[0194]

[0195] In the formula, Hydrodynamic viscosity (Ns / m) 2 ).

[0196] As a preferred embodiment, the specific calculation principle and closed-loop control process for iteratively solving the comprehensive heat loss and wall temperature of the heat collection tube using a multi-layer radial heat transfer network in step S3 are as follows:

[0197] S31, Boundary condition initialization: Obtain the input parameters of the current axial segment node, including the molten salt inlet temperature T. in 1. Molten salt volumetric flow rate V, 2. Current ambient temperature T7, 3. Ambient wind speed v wind And the effective direct radiation intensity DNI after incorporating mechanical-optical deformation coupling correction. eff ;

[0198] S32, Set initial guess value: Give the initial value of the outer wall temperature of the heat absorber tube in the current segment. and the initial value of the outer wall temperature of the glass cover ） In this embodiment, to improve the convergence speed of numerical calculation, initial values ​​are assigned during the first iteration. =T in , =T7;

[0199] S33, Internal heat flow network iterative alternation: Based on the estimated value T3 of the outer wall temperature of the heat absorber tube in the current step. （k） Substituting these values ​​into the Sieder-Tate viscosity-corrected heat transfer equation in formula (2.1), the dynamic convective heat transfer coefficient of the molten salt inside the tube is calculated. The updated inner wall temperature T1 of the absorber tube is then derived from the convective heat transfer relationship from the molten salt to the inner wall of the absorber tube. （k） Furthermore, by simultaneously establishing the one-dimensional radial heat conduction equation of the absorber tube wall, the updated absorber tube outer wall temperature T3 can be obtained. （k+1） Then, using the updated outer wall temperature of the heat absorber and the inner wall temperature of the glass cover, the combined convective and radiative heat transfer inside the sandwich between the heat absorber and the glass cover is calculated according to the aforementioned formulas (2.2) and (2.3). ;

[0200] S34, External heat dissipation network and system energy residual verification: based on the estimated value of the outer wall temperature of the glass cover in the current step. Substituting these values ​​into the aforementioned formula (2.4), the radiative and convective heat dissipation from the outer surface of the glass cover towards the environment is calculated. Considering the thermal bridge heat conduction loss of the heavy-duty support bracket at the current temperature; Construct the steady-state energy conservation equation for the entire heat collector tube with respect to the glass cover subsystem, and calculate its energy balance residual. The formula for calculating the energy balance residual is:

[0201]

[0202] In the formula, The solar radiation energy absorbed by the glass cover itself;

[0203] S35, Convergence Judgment and Axial Advancement: Setting Wall Temperature Convergence Tolerance In this embodiment, take =10 -4 ℃; Verify whether the wall temperature difference between the two iterations meets the requirements. And the energy balance residual If the convergence condition is not met, the estimated values ​​of the outer wall temperature of the absorber tube and the outer wall temperature of the glass cover are updated using the relaxation factor method, and the process returns to step S33 to iterate again; if the convergence condition is met, the radial heat transfer model of the current segment is determined to have achieved physical closure, and the final comprehensive heat loss of the current segment is output. The combined heat loss is then incorporated into the flow energy equation of the two-dimensional axial segmented model to solve for the molten salt temperature and pressure at the outlet of the segment. These conditions are used as the inlet boundary conditions for the next adjacent segment. The thermal performance calculation of the entire large-opening slot heat collection loop is then completed by recursively applying the formulas along the axial direction.

[0204] (iv) Inversion solution method for safe operation boundary of low flow recirculation antifreeze

[0205] Large-aperture molten salt solar collectors suffer severe heat loss under conditions of no solar radiation at night (DNI→0) or extremely low radiation, due to the large opening and significant thermal bridging effect of the heavy-duty support structure. Given that the molten salt working fluid has a theoretical freezing point as high as 220℃, this method, based on the aforementioned multi-layer heat transfer and two-dimensional axial segmentation model, reverse-engineers the solution steps for the nighttime "low-flow recirculation" anti-freezing safety control boundary:

[0206] 1) Set the lower limit temperature of the molten salt local tube wall at any location in the heat collection circuit as follows:

[0207] T 2,min =T freeze + T safe

[0208] In the formula, T freeze The theoretical freezing point of the molten salt working medium is 220°C (in this embodiment, it is taken as 220°C for binary nitrate). T safe The safe temperature margin for frost protection is set between 10℃ and 20℃.

[0209] 2) Set the direct radiation intensity DNI=0, and set the lower safety limit temperature T. 2,min As a boundary condition, the outer wall temperature T of the heat absorber tube of the end control body i, which experiences the most severe heat dissipation in the axial segmented model, is forcibly assigned. 2,i(i.e., the outer wall temperature T3 of the heat absorber tube mentioned above), and substitute it into the comprehensive heat loss per unit length calculation formula constructed by formulas (2.2) to (2.5) in step (III) above, to back-check the critical heat loss caused by the thermal bridge effect of the large opening and heavy-duty support. ;

[0210] 3) By performing inverse convergence iterations of the control equations along the axial control volume, the solution at the current ambient temperature is obtained. Below, the critical minimum antifreeze recirculation mass flow rate required to prevent localized crystallization freezing at any point in the large-open circuit. :

[0211]

[0212] In the formula: The heat loss per unit length of the i-th segment is calculated after considering the thermal bridging effect of the support. The specific heat capacity of molten salt varies non-linearly with temperature; Let be the axial length of the i-th heat collector tube; and These are the inlet and outlet temperatures of the molten salt in the i-th segment, respectively.

[0213] Extreme flow rates obtained using this inversion calculation It is directly used as the control boundary index for the variable frequency operation of the antifreeze pump in the mirror field.

[0214] This step directly couples the heat loss of the large opening with the rheological properties of the molten salt, and outputs extreme flow rate guidance for antifreeze operation.

[0215] Example

[0216] Using the product parameters of a third-generation trough system supplier, the mass flow rate, outlet temperature, thermal efficiency, and heat loss under three operating conditions are shown in Table 1.

[0217] Table 1 Thermal performance under three operating conditions

[0218] <![CDATA[DNI(W / m 2 )]]> 450 850 1200 Mass flow rate (kg / s) 4.644 9.746 14.1828 Inlet temperature (°C) 290 290 290 Outlet temperature (°C) 565 565 565 Thermal efficiency 0.653 0.724 0.746 Heat loss (W / m) 886.42 1157.39 1405.70

[0219] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0220] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for calculating the thermal performance of a large-opening molten salt trough solar collector system, characterized in that, The method includes: S1. Based on the opening width of the large-opening slot solar collector, the nonlinear spatial torsional deformation function caused by its own weight and wind load, and the high-temperature axial thermal eccentricity of the absorber tube, combined with the solar position parameters, the dynamic comprehensive optical efficiency of the solar collector with mechanical-optical deformation coupling correction is calculated. S2. Establish a multi-layer steady-state heat transfer model between the heat absorber tube, the glass cover and the environment. Use the temperature-varying viscosity gradient of the molten salt between the inner wall of the heat absorber tube and the center of the fluid to perform rheological correction on the convective heat transfer coefficient inside the tube. Combine the non-uniform heat dissipation effect of the thermal bridge of the heavy-duty reinforced support to calculate the comprehensive heat loss per unit length of the heat collector tube. S3, based on the two-dimensional axial piecewise steady-state energy balance equation, combined with the molten salt density and specific heat capacity parameters that change nonlinearly with temperature, coupled with the optical efficiency of step S1 and the heat loss of step S2, iteratively solves the outlet temperature, thermal efficiency and system pressure drop of each segment of the heat collector tube.

2. The method for calculating the thermal performance of a large-opening molten salt trough solar collector system according to claim 1, characterized in that, Step S1 further includes: Calculate the real-time position of the sun based on the solar altitude angle, solar azimuth angle, and hour angle, and determine the solar incidence angle. And calculate the incident angle correction factor K: ； The loss coefficient at the base end is obtained through geometric truncation relationships or optical simulations, and is introduced into the average outer surface temperature of the heat absorber tube. Axial thermal eccentricity function The end-point loss is dynamically corrected, and the corrected end-point loss coefficient is: ； In the formula, r is the distance from any point at the end of the condenser mirror to the focal point; The length of the trough-type solar collector; The focal length of a large aperture condenser; The shading coefficient between adjacent concentrators is dynamically determined based on the mirror field loop arrangement and the sun's position. : ； In the formula, The spacing between trough-type solar concentrators; The chord length of the concentrator opening; The nonlinear spatial torsional deformation induced by self-weight and wind load is constructed into a spatial deformation function. Dynamic correction of mirror geometric errors: ； In the formula, It is the geometric error constant for the basic mirror assembly; The dynamic correction coefficient is obtained based on mechanical analysis or finite element simulation, and is affected by the solar altitude angle. Real-time wind speed and the opening width of the large-opening trough collector Joint control, solar altitude angle The distribution of the solar collector's self-gravity moment and real-time wind speed determine the solar collector's self-gravity moment. Introducing wind-induced loads, opening width Characterizing the sensitivity of structural stiffness to deformation; The dimensionless coefficients output by the function are used to characterize the dynamic weakening effect of nonlinear torsional deformation on the concentrating ability of the mirror surface of the large-opening slot solar collector under the coupled action of self-weight and wind load. Coupled with incident angle correction, end loss correction, shading coefficient, deformation correction, and optical loss factor including reflection, transmission, and cleanliness, the effective optical efficiency at the glass shell and the effective optical efficiency of the collector are calculated. ； ； In the formula, , , , , These are the HCE shading correction factor, tracking error correction factor, mirror dirt correction factor, absorber tube dirt correction factor, and other factor correction factors, respectively. The transmittance of the glass casing; Calculate solar irradiance per unit receiver length : ； In the formula, DNI is the direct radiation intensity; The instantaneous thermal efficiency of the solar collector is obtained by combining the solar absorption of the glass shell and the absorber. for: ； In the formula, This represents the effective heat transfer power from the absorber tube to the molten salt per unit absorber length.

3. The method for calculating the thermal performance of a large-opening molten salt trough solar collector system according to claim 2, characterized in that, The spatial deformation function The data is obtained through finite element mechanical simulation combined with ray tracing analysis, specifically including the following steps: Establish a structural mechanics model for a large-opening trough solar collector, using the solar altitude angle. Real-time wind speed and opening width Given the operating parameters, solve the nonlinear spatial torsional deformation field of the solar collector under different solar altitude angles and wind speeds; Based on the nonlinear spatial torsional deformation field, the actual reflection path deviation of light rays at each position is calculated, and the dynamic attenuation coefficient of the mirror focusing efficiency under different working conditions is obtained. The dynamic attenuation coefficient is constructed as a spatial deformation function related to the operating parameters. It outputs a correction coefficient between 0 and 1, which is used to dynamically correct the geometric errors of the mirror surface.

4. The method for calculating the thermal performance of a large-opening molten salt trough solar collector system according to claim 1, characterized in that, Step S2 further includes: A multi-layer steady-state heat transfer model was established between the heat absorber tube, the glass cover, and the environment to clarify the complete heat transfer path, including convective heat transfer on the inner wall of the heat absorber tube, heat conduction on the tube wall, radiation and convective heat transfer between the outer surface of the heat absorber tube and the glass cover, convective and radiative heat dissipation between the outer surface of the glass cover and the environment, and heat dissipation through thermal bridges of the heavy-duty reinforced support. Based on the heat transfer boundary inside the tube in the multi-layer heat transfer model, the dynamic viscosity of the molten salt at different temperatures is calculated according to the temperature difference between the inner wall of the heat absorber tube and the average temperature of the molten salt body. The Nusselt number under normal physical property conditions is rheologically corrected using the Sieder-Tate correction formula, and the convective heat transfer coefficient inside the tube is calculated based on the corrected Nusselt number. Using the convective heat transfer coefficient inside the tube as the boundary condition of the multilayer steady-state heat transfer model, the tube wall temperature of the absorber tube is obtained by solving. For the non-uniform heat dissipation of the heavy-duty reinforced support, a thermal bridge heat transfer model of support-absorber tube is established. The tube wall temperature of the absorber tube is used as the base temperature of the thermal bridge heat transfer model. The additional heat loss of the support part is calculated, and the additional heat loss term is coupled into the thermal bridge heat dissipation path of the support in the multilayer steady-state heat transfer model. The convection heat transfer inside the tube, the heat dissipation at each interface, and the heat bridge heat dissipation of the support are all coupled into the multi-layer steady-state heat transfer model, and the comprehensive heat loss per unit length of the heat collector tube is obtained by solving the system simultaneously.

5. The method for calculating the thermal performance of a large-opening molten salt trough solar collector system according to claim 4, characterized in that, To address the non-uniform heat dissipation of heavy-duty reinforced brackets, a thermal bridge heat transfer model is established by treating the bracket as an infinitely large fin. Using the tube wall temperature as the base temperature, the additional heat loss per unit length of the bracket is calculated. : ； In the formula, The average convection coefficient of the support structure; The perimeter of the support structure; The thermal conductivity of the support; This represents the minimum cross-sectional area of ​​the support frame. This refers to the wall temperature of the heat absorber tube. The ambient temperature; This is the length of the heat absorption tube.

6. The method for calculating the thermal performance of a large-opening molten salt trough solar collector system according to claim 1, characterized in that, Step S3 further includes: The collector tube is divided into multiple calculation segments along the axial direction, with temperature continuity maintained at the boundaries of each segment. A two-dimensional axial segmented steady-state energy balance equation is used to model the collector tube segment by segment. The dynamic comprehensive optical efficiency of the collector calculated in step S1, the comprehensive heat loss per unit length of the collector tube calculated in step S2, and the molten salt density and specific heat capacity parameters that change nonlinearly with temperature are used as input conditions for the two-dimensional axial segmented steady-state energy balance equation. Based on the energy balance relationship of each segment, considering the influence of molten salt temperature change on physical properties, the energy balance equation of each segment is iteratively solved until the outlet temperature of each segment converges, thus obtaining the outlet temperature of the molten salt in each segment, and then obtaining the outlet temperature distribution, thermal efficiency distribution, and system pressure drop of the entire collector tube.

7. The method for calculating the thermal performance of a large-opening molten salt trough solar collector system according to claim 1, characterized in that, The method further includes: S4 sets the molten salt freezing point and safety margin as the lower limit of the antifreeze temperature. When the direct radiation intensity is lower than the control threshold or under nighttime conditions, the two-dimensional axial piecewise energy balance equation is solved in reverse iteration to output the critical minimum antifreeze recirculation flow rate that keeps the molten salt in the heat collection loop from condensing locally, thus realizing the collaborative design of system thermal performance evaluation and antifreeze operation strategy.

8. The method for calculating the thermal performance of a large-opening molten salt trough solar collector system according to claim 7, characterized in that, Step S4 further includes: Set the lower limit of the allowable safe temperature of the molten salt local tube wall at any location in the heat collection circuit. for: ； In the formula, This is the theoretical freezing point of the molten salt working fluid; To ensure a safety margin against freezing; Set the direct radiation intensity DNI=0, and set the lower safety temperature limit. As a boundary condition, it is substituted into the axial piecewise steady-state energy balance equation consisting of N control volumes of equal length. By performing inverse convergence iteration of the piecewise steady-state energy balance equation along the axial direction, the solution at the current ambient temperature can be obtained. Below, the critical minimum antifreeze recirculation mass flow rate required to prevent localized crystallization freezing at any point in the large-open circuit. This is used as the control boundary indicator for the variable frequency operation of the mirror field antifreeze pump: ； In the formula: The heat loss per unit length of the i-th segment is calculated after considering the thermal bridging effect of the support. Let be the axial length of the i-th heat collector tube; Let be the average specific heat capacity of the molten salt in the i-th segment, which varies non-linearly with temperature. and These are the inlet and outlet temperatures of the molten salt in the i-th segment, respectively.

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