Indirect Measurement Method of Concentrated Moonlight for the Distribution of Concentrating Ratio of Heliostat Field in Tower-Type Power Station

Abstract

The present invention provides an indirect measurement method for the concentration ratio distribution of a heliostat field in a tower power station by using moonlight, and the steps are as follows: On the night of the concentration experiment, modify the time and date of the heliostat field control system so that the running trajectory of the sun is close to that of the moon on that night, and the heliostat field tracks the moon according to the solar position program to achieve the concentration of the heliostat field on the moon; Conduct a moonlight concentration measurement experiment in the tower heliostat field. During the measurement process, move the white board of the linear array illuminometer up and down to sweep across the receiving surface of the heat absorber, and the linear array illuminometer measures the moonlight concentration illumination distribution; The moonlight normal direct illumination measurement station continuously measures the moonlight normal direct illumination; Calculate the moonlight concentration ratio distribution of the heliostat field on the receiving surface of the heat absorber; Determine the number of times N of repeated mean smoothing filtering according to the ratio of the lunar phase on that night to the full moon; After the moonlight concentration ratio distribution is subjected to N times of repeated mean smoothing filtering, the concentration ratio distribution of the heliostat field for sunlight is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of measuring the concentrated solar energy flux density distribution in solar thermal power generation, and particularly relates to an indirect measurement method for the concentration ratio distribution of a heliostat field in a tower power station by using moonlight. Background Technique

[0002] Solar tower thermal power generation technology is a mainstream solar thermal power generation technology. Due to its high concentration ratio, high operating parameters, and high solar thermal power generation efficiency, it is a technology that is being developed worldwide. The heliostat field of a tower solar thermal power station is the main energy source, mainly composed of numerous heliostats surrounding the solar tower on the ground. Each heliostat rotates automatically with two axes to track the sun and reflects the incident direct sunlight to the absorber on the top of the tower. The light-receiving surface of the absorber is the natural interface of the energy flow from the heliostat field to the absorber. The concentrated solar energy flux density distribution of the heliostat field on the light-receiving surface of the absorber is an important basis for optimizing the layout of the heliostat field and formulating the scheduling strategy of the heliostats, and is also an important guarantee for designing and operating the absorber safely and efficiently.

[0003] The existing measurement methods for the concentrated energy flux density distribution of solar thermal power plants can be divided into three categories: direct measurement method, indirect measurement method, simulation calculation method supported by experimental data, and other special measurement methods. Among them, the optical simulation calculation method supported by experimental data relies on the existing concentrated energy flux density measurement data, and there is also a significant deviation between the simulated concentrated energy flux density distribution and the actual energy flux density distribution, which cannot be used as an independent test method for the concentrated energy flux density distribution. The direct measurement method is to statically use multiple heat flux density sensors in an array, fix them on the daylighting surface of the heat absorber to collect the heat flux density value, and directly measure. The heat flux density sensor has to face the test of high temperature environment and high heat flux density, and it is easy to be damaged. In addition, the spatial resolution of the measurement result is not high due to the limitation of the number of measurement points. The direct measurement method can also be implemented in another dynamic way, such as arranging a row of heat flux density sensors on a metal rod. During the measurement process, the metal rod quickly scans the receiving surface of the heat absorber. The heat flux density measurement values at different times correspond to different positions of the daylighting surface of the heat absorber. After data post-processing, the concentrated energy flux density distribution is obtained. Since the absorber of the tower power station is large in size and has a large light-collecting surface, it is difficult to realize large-size moving parts in a high-temperature environment. Therefore, the dynamic direct measurement method is not applicable to commercial power stations, and the spatial resolution of the measurement results is also low. The indirect measurement method generally uses the "CCD camera + experimental white target + reference heat flux density sensor" method for indirect measurement. One or several heliostats focus light on the experimental white target, and the CCD camera collects the focused light spot on the white target. The reference heat flux density sensor on the white target collects the heat flux density value at a fixed position to calibrate the grayscale image of the light spot into the focused energy flux density distribution. The indirect measurement method has a high spatial resolution of the measurement results, but the resolution of the energy flux density value is low, after all, there is too little data on the reference heat flux density value. Similarly, the absorber of the tower power station is large in size and has a high operating temperature, so the indirect measurement method of "CCD camera + white target" is not practical.

[0004] Due to the special characteristics of solar tower power stations, i.e., large light-collecting surface of the absorber, high temperature, high heat flux density, uneven distribution of concentrated energy flux density, and changing distribution of concentrated energy flux density, the measurement of the concentrated energy flux density distribution of the entire heliostat field has always been a difficult task. Therefore, research institutions and commercial companies around the world are actively developing new measurement ideas and methods.

[0005] The solar concentrator energy flux density distribution measurement method jointly developed by DLR in Germany and CSP Services company. The indirect measurement method of concentrator energy flux density distribution does not move the white target. Instead, it directly uses the outer surface of the external receiver as the concentrator target surface, uses a ground-based CCD camera to capture the concentrator image of the outer surface of the receiver, and then combines the matrix of the reflectivity of different positions on the outer surface of the receiver for incident light in different directions tested in advance to convert the original spot image into a smoother test spot image. Then, through the heat flux density value obtained by a reference heat flux density sensor on the heat absorption surface, the spot image is calibrated to the concentrator energy flux density distribution map. For related patents, see "Offergeld, M., M., Stadler. H., Verfahren zur Bestimmung von relativen Reflexionsgraden einer eines Receivers einer Solarturmanlage, Patent DE102016226033B4 (granted 08.11.2018)” (“Method for determining the relative reflectivity of the heat absorption surface of a receiver of a solar tower system, Patent DE102016226033B4 (granted on November 8, 2018)”). However, calibrating the reflection coefficients of the complex and non-uniform receiver surface at various positions and in various directions requires an artificial light source on the ground. The tower height of an actual commercial power station is more than 200 meters, which is difficult to use an artificial light source; in addition, the reflection characteristics of the receiver surface will change after being heated, so there are difficulties in practical applications. This indirect measurement method is only applicable to external receivers and is not suitable for cavity receivers with internal heat absorption surfaces.

[0006] Wang Nan et al. from the Institute of Electrical Engineering, Chinese Academy of Sciences proposed a method for measuring the concentrator energy flux density distribution of a heliostat field in a tower power station (Patent No. 201711414690.5, authorization date: April 28, 2020). On a clear full moon night, use an array of illuminometers densely arranged to measure the illuminance distribution of the moonlight concentrator spot, and compare it with the normal direct illuminance of the moonlight to obtain the relative energy flux density distribution of the moonlight concentrator spot (concentration ratio distribution, CR(x, y) moon =I(x, y) / DNI moon ), which is directly used as the concentration ratio distribution of the heliostat field for sunlight (CR(x, y) sun＝ CR(x, y) moon ). The equivalent sunlight concentrator energy flux density is: F(x, y)=CR(x, y) sun ·DNI sun =CR(x, y) moon ·DNI sun , where DNI sunis the normal direct illuminance of sunlight. The problem with the method of this patent is that it requires the condition of a full moon, which restricts its practical application. Summary of the Invention

[0007] The object of the present invention is to overcome the deficiencies of the existing method for measuring the concentrated solar energy flux density distribution in the heliostat field of a tower power station, and to propose a new indirect method for measuring the concentration ratio distribution of moonlight in the heliostat field of a tower power station.

[0008] The technical solution adopted by the present invention is as follows:

[0009] An indirect method for measuring the concentration ratio distribution of moonlight in the heliostat field of a tower power station, which uses the moonlight at night with various lunar phases to conduct a concentrated light measurement experiment; during the measurement process, a moving whiteboard driving a linear array illuminometer moves up and down, sweeping across the receiving surface of the receiver, and the linear array illuminometer measures the illuminance distribution of moonlight and the normal direct illuminance of moonlight, so as to obtain the concentration ratio distribution of moonlight with different lunar phases; the concentration ratio distribution of moonlight with different lunar phases is subjected to repeated mean smoothing filtering for different numbers of times to obtain the concentration ratio distribution of sunlight by the heliostat field.

[0010] Further, the specific steps of the indirect measurement method are as follows:

[0011] (1) On the night of the concentrated light measurement experiment, modify the time and date of the control system of the heliostat field so that the running track of the sun is close to the running track of the moon that night, and the heliostat field tracks the moon according to the solar position program to achieve the concentration of moonlight by the heliostat field;

[0012] (2) Conduct a concentrated light measurement experiment on moonlight in the heliostat field. During the measurement process, a moving whiteboard driving a linear array illuminometer moves up and down, sweeping across the receiving surface of the receiver, and the linear array illuminometer measures the illuminance distribution of moonlight;

[0013] (3) At the same time, a moonlight normal direct illuminance measurement station continuously measures the normal direct illuminance of moonlight;

[0014] (4) Calculate the concentration ratio distribution of moonlight on the receiving surface of the receiver by the heliostat field;

[0015] (5) Determine the number of times N of repeated mean smoothing filtering according to the ratio of the lunar phase that night to the full moon;

[0016] (6) After the concentration ratio distribution of moonlight is subjected to N times of repeated mean smoothing filtering, the concentration ratio distribution of sunlight by the heliostat field is obtained.

[0017] Further, in the step (2), the illuminance distribution of the moonlight spot measured by scanning with the linear array illuminometer is divided by the normal direct illuminance of moonlight measured in the step (3) to obtain the concentration ratio distribution of moonlight:

[0018] CR(x, y) moon = I(x, y) / DNI moon .

[0019] Further, in step (5), the number of times N of repeated mean smoothing filtering is determined according to the ratio of the lunar phase of the current night to the full moon.

[0020] Further, in step (6), the concentration ratio distribution of the heliostat field for sunlight is:

[0021]

[0022] where is a 3×3 mean smoothing filter.

[0023] Further, the concentration ratio distribution measurement system of the heliostat field includes a moonlight normal direct illuminance measurement station and an illuminance distribution measurement system; the moonlight normal direct illuminance measurement station includes an astronomical observation hemispherical dome house with a skylight and a lunar biaxial tracker established inside it, and a long-focus CCD camera and two micro-illuminometers with light-shielding tubes are installed on the lunar tracker.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The indirect measurement method described in the present invention uses moonlight at night to carry out the concentration experiment. Moonlight of various lunar phases can be utilized, not limited to clear full moon nights. It neither affects the normal operation of the heliostat field and the receiver during the day in the tower power plant, nor increases the number of experimental days, making the measurement method more feasible.

[0026] 2. The indirect measurement method described in the present invention does not modify the control program code of the heliostat field, but only modifies the date and time of the control system, and will not cause faults in the heliostat field control system, which makes the measurement method more independent and more general.

[0027] 3. The present invention finds that for light sources with different brightness distribution shapes in the heliostat field, the concentrated light spots (concentration ratio distribution) are similar; the brightness distribution of the circularly symmetric solar light source is more uniform and smooth; the concentration ratio distributions of moonlight of different lunar phases approach the sunlight concentration ratio distribution after different numbers of mean smoothing filterings; the similarity between the brightness distribution of the moonlight light source and that of the solar light source is high, and the number of smoothing filterings required for the moonlight concentration ratio distribution to approach the sunlight concentration ratio distribution is small. Therefore, the number of smoothing filterings should be a monotonic function of the lunar phase ratio. Based on this, the present invention uses a smoothing filter for blurring the distribution image and reducing noise, and the mean filter is a commonly used smoothing filter. Therefore, it is the most natural choice to select a mean filter with a 3x3 template, which has the effects of simplicity, practicality, and high efficiency. Description of the Drawings

[0028] Figure 1 is a schematic diagram of a tower heliostat field equipped with an illuminance distribution measurement system according to the present invention;

[0029] In the figure: 1 heliostat field, 2 solar tower, 3 moving whiteboard, 4 linear array illuminometer, 5 right slide rail, 6 receiver surface of the heat absorber, 7 left slide rail, 8 ground CCD camera;

[0030] Figure 2 is a schematic diagram of a moonlight normal direct illuminance measurement station according to the present invention;

[0031] In the figure: 9 astronomical observation hemispherical dome house, 10 skylight, 11 control box, 12 first micro-illuminometer, 13 first light-shielding tube, 14 long-focus CCD camera, 15 second micro-illuminometer, 16 second light-shielding tube, 17 moon, 18 moon two-axis tracker. Specific embodiments

[0032] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0033] As Figure 1 and 2 shown, the indirect measurement method of the present invention uses moonlight of various lunar phases at night for a concentrating experiment to indirectly measure the concentration ratio distribution of the heliostat field 1 on the receiver surface 6 of the heat absorber, and is not limited to a full moon night. The concentration ratio distribution measurement system of the heliostat field 1 mainly consists of 2 major parts, namely a moonlight normal direct illuminance measurement station and an illuminance distribution measurement system. The moonlight normal direct illuminance is represented by DNI moon and the illuminance distribution is represented by I(x, y).

[0034] As Figure 1 shown is a schematic diagram of the solar tower 2 and the heliostat field 1 equipped with an illuminance distribution measurement system according to the present invention. The illuminance distribution measurement system mainly consists of a moving whiteboard 3 arranged with 2 rows of staggered linear array illuminometers 4, a ground CCD camera 8, and left and right slide rails 7 and 5 on both sides. During the measurement, the moving whiteboard 3 moves up and down along the left slide rail 7 and the right slide rail 5, sweeping across the receiver surface 6 of the heat absorber, and the linear array illuminometer 4 measures the illuminance distribution I(x, y) of the moonlight. The ground CCD camera 8 takes pictures of the moving whiteboard 3 to determine the height position of the probe of each linear array illuminometer 4 during the scanning measurement process.

[0035] As Figure 2As shown in the figure, the moonlight normal direct illuminance measurement station includes an astronomical observation hemispherical dome house 9 with a skylight 10 and a moon biaxial tracker 18 established inside it. A long-focus CCD camera 14, a first light-shielding cylinder 13 with a first micro-illuminance meter 12 arranged thereon, a second light-shielding cylinder 16 and a second micro-illuminance meter 15 arranged thereon are installed on the moon biaxial tracker 18. The micro-illuminance meters 12 and 15 have higher precision than the weak light illuminance meter. The long-focus CCD camera 14 monitors the moon image to ensure that the moon biaxial tracker 18 accurately tracks the moon. The micro-illuminance meters 12 and 15 measure the moonlight normal direct illuminance. The probes of the micro-illuminance meters 12 and 15 are configured with the first light-shielding cylinder 13 for eliminating stray light to prevent the influence of environmental stray light and light on the measurement results. The first micro-illuminance meter 12 with the first light-shielding cylinder 13 and the second micro-illuminance meter 15 with the second light-shielding cylinder 16 are backup designs and are mutually verified to prevent the measurement failure of one of the micro-illuminance meters from affecting the test precision.

[0036] The moonlight normal direct illuminance measurement station is used to measure the moonlight normal direct illuminance, where the moon biaxial tracker 18 tracks the moon. The long-focus CCD camera 14 monitors the moon 17 and takes images to ensure that the moon biaxial tracker 18 accurately tracks the moon 17. The micro-illuminance meters 12 and 15 measure the moonlight normal direct illuminance. The control box 11 of the moon biaxial tracker 18 is also responsible for controlling the data acquisition of the two micro-illuminance meters 12 and 15.

[0037] Scan and measure the illuminance distribution of the moonlight spot, and divide it by the moonlight normal direct illuminance to obtain the moonlight concentration ratio distribution CR(x, y) moon = I(x, y) / DNI moon , after N times of repeated mean smoothing filtering, the concentration ratio distribution of the heliostat field 1 for sunlight is obtained, that is Among them, is a 3×3 mean smoothing filter.

[0038] The specific steps of the indirect measurement method of the present invention are as follows:

[0039] 1. On the night of the concentration measurement experiment, modify the time and date of the control system of the heliostat field 1 so that the running trajectory of the sun is close to the running trajectory of the moon that night. The heliostat field 1 tracks the moon according to the solar position program to achieve the concentration of the heliostat field 1 on the moon.

[0040] 2. Conduct a concentration measurement experiment on the moon for the heliostat field 1. During the measurement process, with the cooperation of the ground CCD camera 8, drive the moving whiteboard 3 of the linear array illuminometer 4 to move up and down, sweeping across the absorber receiving surface 6. The linear array illuminometer 4 measures the illuminance distribution I(x, y) of the moonlight; at the same time, the moonlight normal direct illuminance measurement station continuously measures the moonlight normal direct illuminance DNI moon .

[0041] 3. Calculate the moonlight concentration ratio distribution of the heliostat field 1 on the receiver surface 6 of the solar receiver:

[0042] CR(x, y) moon = I(x, y) / DNI moon .

[0043] 4. Determine the number of times N of repeated mean smoothing filtering according to the ratio of the lunar phase on that night to the full moon. The determination method of N is shown in Table 1 below.

[0044] Table 1 Corresponding relationship between the number of times of smoothing filtering and the lunar phase ratio

[0045] Proportion range of lunar phase (%) Number of smoothing filtering times N (97.5,100] 1 (92.5,97.5] 2 (87.5,92.5] 3 (82.5,87.5] 4 (77.5,82.5] 5 (72.5,77.5] 6 (67.5,72.5] 7 (62.5,67.5] 8 (57.5,62.5] 9 (52.5,57.5] 10 (47.5,52.5] 11 (42.5,47.5] 12

[0046] 5. After the moonlight concentration ratio distribution CR(x, y) moon is subjected to N times of repeated mean smoothing filtering, the concentration ratio distribution of the heliostat field 1 for sunlight is obtained, that is

[0047] The following uses specific embodiments to illustrate the specific implementation manner of the indirect measurement method of the present invention.

[0048] 1. Modify the time and date of the control system of the heliostat field 1 so that the running trajectory of the sun is close to the running trajectory of the moon on that night. The heliostat field 1 tracks the moon according to the solar position program to achieve the concentration of the heliostat field 1 on the moon. For example, the solar trajectory on December 16, 2021 in Yanqing, Beijing almost coincides with the lunar trajectory on March 6, 2021.

[0049] 2. As Figure 1 shown, conduct a concentration measurement experiment on the moon in the heliostat field 1. After the moonlight is reflected by the heliostats in the heliostat field 1 that track the moon, it is projected onto the receiver surface 6 of the solar tower 2. Move the line array illuminometer 4 fixed on the whiteboard 3 in two staggered upper and lower rows, 7 in the upper row and 7 in the lower row, for a total of 14. During the measurement, the shooting range of the ground CCD camera 8 in the heliostat field 1 includes the moving range of the whiteboard 3. The whiteboard 3 moves up and down along the left slide rail 7 and the right slide rail 5, sweeping across the receiver surface 6, and the line array illuminometer 4 measures the illuminance distribution I(x, y) of the moonlight. The discrete pixel size of the illuminance distribution image is 1mm×1mm.

[0050] 3. The ratio of the measured illuminance distribution to the moonlight normal direct illuminance DNI moon measured by the moonlight normal direct illuminance measurement station is the moonlight concentration ratio distribution CR(x, y) moon = I(x, y) / DNI moon .

[0051] 4. Query the lunar phase analysis software or search online to obtain the lunar phase ratio on that night. For example, for a lunar phase of 76%, determine the number of times N of mean smoothing filtering as N = 6 from Table 1.

[0052] 5. Moonlight concentration ratio distribution CR(x, y) moon After N = 6 times of repeated mean smoothing filtering, the concentration ratio distribution of the heliostat field 1 for sunlight is obtained, that is Concentration ratio distribution CR(x, y) of the sun for the heliostat field 1 sun The pixel size of the image is 1mm × 1mm.

[0053] Those skilled in the art can easily understand that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An indirect measurement method of moonlight concentration for the concentration ratio distribution of a heliostat field in a tower power station, characterized in that: The described indirect measurement method uses the moonlight at night of multiple lunar phases to conduct a condensation measurement experiment; during the measurement process, the moving whiteboard of the linear array illuminometer is driven to move up and down, sweeping across the receiving surface of the heat absorber, and the linear array illuminometer measures the light intensity distribution of the moonlight and the normal direct illuminance of the moonlight, thereby obtaining the condensation ratio distribution of the moonlight of different lunar phases; after the condensation ratio distribution of the moonlight of different lunar phases is subjected to repeated mean smoothing filtering for different times, the condensation ratio distribution of the heliostat field for sunlight is obtained; without modifying the control program code of the heliostat field, only modifying the date and time of the control system will not cause a failure in the control system of the heliostat field. The specific steps of the described indirect measurement method are as follows: (1) On the night of the condensation measurement experiment, modify the time and date of the control system of the heliostat field so that the running trajectory of the sun is close to the running trajectory of the moon that night, and the heliostat field tracks the moon with the solar position program to achieve the condensation of the heliostat field on the moon. (2) Conduct a condensation measurement experiment on the moonlight in the heliostat field. During the measurement process, drive the moving whiteboard of the linear array illuminometer to move up and down, sweeping across the receiving surface of the heat absorber, and the linear array illuminometer measures the light intensity distribution I(x, y) of the moonlight. (3) Meanwhile, the moonlight normal direct illuminance measurement station continuously measures the moonlight normal direct illuminance DNI moon ; (4) Calculate the condensation ratio distribution of the moonlight on the receiving surface of the heat absorber in the heliostat field. (5) Determine the number of times N of repeated mean smoothing filtering according to the ratio of the lunar phase that night to the full moon; use a mean filter to blur the distribution image and reduce noise. (6) After the condensation ratio distribution of the moonlight is subjected to N times of repeated mean smoothing filtering, the condensation ratio distribution of the heliostat field for sunlight is obtained. The condensation ratio distribution measurement system of the heliostat field includes a normal direct illuminance measurement station for moonlight and a light intensity distribution measurement system; the normal direct illuminance measurement station for moonlight includes an astronomical observation hemispherical dome house with a skylight and a lunar biaxial tracker established inside it. A long-focus CCD camera and two micro-illuminometers with light-shielding tubes are installed on the lunar tracker.

2. The indirect measurement method of moonlight concentration ratio distribution of a heliostat field in a tower power station according to claim 1, characterized in that: In the step (2), the illuminance distribution I(x, y) of the moonlight spot measured by the line array illuminometer is divided by the direct normal illuminance DNI of the moonlight measured in the step (3). moon to obtain the concentration ratio distribution of the moonlight: CR(x, y) moon = I(x, y) / DNI moon 。 3. The indirect measurement method of moonlight concentration ratio distribution of a heliostat field in a tower power station according to claim 2, characterized in that: In the step (6), the condensation ratio distribution of the heliostat field for sunlight is: ， Among them, , M is a 3×3 mean smoothing filter.

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