A method and device for actively biasing a solar heat collecting tube

Abstract

The present invention belongs to the field of solar collectors, and specifically discloses a method and device for actively offsetting a solar heat collecting tube, which includes the steps of: determining the maximum deflection z of the heat absorption tube in the heat collecting tube under the action of gravity max ; considering the movement of the sun, determining the rotation angle θ of the entire heat collecting system according to the solar azimuth angle; determining the actual offset amount of the heat collecting tube according to the maximum deflection z max and the rotation angle θ, installing the heat collecting tube according to the actual offset amount, and determining the corresponding actual offset amount according to the change of the rotation angle θ, and adjusting the position of the heat collecting tube in real time. The present invention considers the deformation of the solar collector affected by gravity and the rotation of the system caused by the change of the sun's position, determines the offset amount of the heat collecting tube, and improves the uniformity of solar energy flow distribution and the safety of the heat collecting tube.

Description

Technical Field

[0001] The present invention belongs to the field of solar collectors, and more particularly, relates to a method and device for actively offsetting a solar heat collecting tube. Background Art

[0002] In solar power generation technology, photovoltaic power generation technology has low power generation cost and large installed capacity, but is greatly affected by solar radiation and often exhibits volatility and intermittency. Photovoltaic power generation using high-cost electrochemical energy storage can alleviate volatility to a certain extent, but cannot solve the diurnal intermittency. In comparison, the advantages of solar thermal power generation technology can make up for the deficiencies of photovoltaic power generation. The trough solar thermal power generation technology is currently the most commercialized solar thermal power generation technology, accounting for more than 76% of the total global installed capacity of solar thermal power generation. The trough solar system mainly uses a parabolic reflector to align the axis of the paraboloid with the sun and places the heat collecting tube at the focus. As long as the reflector is large enough, only the concentrated sunlight is used to heat the medium in the heat collecting tube to a predetermined temperature.

[0003] Due to the structural characteristics of the trough solar collector, the incident solar rays are mainly converged by the parabolic reflector to the bottom of the heat collecting tube, resulting in uneven distribution of the solar flux density on the upper and lower surfaces of the heat collecting tube, and causing a large temperature gradient on the heat collecting tube wall. The temperature gradient causes large thermal deformation of the heat collecting tube, piercing the glass tube and causing safety problems such as vacuum failure. Summary of the Invention

[0004] In view of the above defects or improvement requirements of the prior art, the present invention provides a method and device for actively offsetting a solar heat collecting tube, aiming to determine the offset amount of the heat collecting tube, taking into account the deformation of the collector affected by gravity and the rotation of the system caused by the change of the sun's position, so as to improve the uniformity of solar flux distribution and the safety of the heat collecting tube.

[0005] To achieve the above object, according to one aspect of the present invention, a method for actively offsetting a solar heat collecting tube is proposed, including the following steps:

[0006] Determine the maximum deflection z of the heat absorption tube in the heat collecting tube under the action of gravity max ;

[0007] Considering the movement of the sun, determine the rotation angle θ of the entire heat collection system according to the solar azimuth angle;

[0008] According to the maximum deflection z max and the rotation angle θ, determine the actual offset amount of the heat collecting tube, install the heat collecting tube according to the actual offset amount, and determine the corresponding actual offset amount according to the change of the rotation angle θ, and adjust the position of the heat collecting tube in real time.

[0009] As a further preference, the actual offset amount δz #The calculation formula is as follows:

[0010]

[0011] Among them, D ab,o is the outer diameter of the heat absorption tube; |OP| is the distance between the center of the heat collection tube and one end of the parabolic reflector when the offset is 0; W is the distance between the two end points of the parabolic reflector; the method for determining the angle α is: offset the heat collection tube by a certain amount so that the edge of the reflected beam of the edge point of the parabolic reflector is tangent to the heat absorption tube, and the included angle between this tangent line and the connection line between the two end points of the parabolic reflector is α.

[0012] As a further preference, the calculation formula for the angle α is:

[0013]

[0014] Among them, β is the half solar angular diameter.

[0015] As a further preference, the maximum deflection of the heat absorption tube under the action of gravity is calculated as follows:

[0016] Set multiple equally spaced brackets in the solar mirror field to which the trough type solar vacuum heat collection tube belongs, and regard the adjacent two brackets and the heat collection tube between them as a simply supported beam with uniformly distributed load;

[0017] Multiply the uniformly distributed load by the length of the heat collection tube between the adjacent two brackets to obtain the gravity borne by the heat absorption tube in the heat collection tube between the adjacent two brackets, and thus calculate the maximum deflection of the heat absorption tube under the action of gravity.

[0018] As a further preference, the maximum deflection z max of the heat absorption tube under the action of gravity is:

[0019]

[0020] Among them, L is the length of the heat collection tube between the adjacent two brackets, G is the gravity borne by the heat absorption tube in the heat collection tube between the adjacent two brackets, E ab is the elastic modulus of the heat absorption tube, and I ab,o is the cross-sectional moment of inertia of the heat absorption tube.

[0021] As a further preference, the heat absorption tube is a hollow circular tube, and the calculation formula for the cross-sectional moment of inertia I ab,o of the heat absorption tube is:

[0022]

[0023] Among them, D ab,o and D ab,i are the outer diameter and inner diameter of the heat absorption tube respectively.

[0024] As a further preference, the solar azimuth angle β s is calculated as follows:

[0025]

[0026] where h is the solar altitude angle, ω is the solar hour angle, and δ is the solar declination.

[0027] According to another aspect of the present invention, there is provided an apparatus for implementing the above-mentioned active offset method of a solar energy collector tube, including a bracket, a collector tube support column, a lead screw-nut mechanism, and a pipe clamp, wherein:

[0028] The bracket is used for installing a parabolic reflector, and the collector tube support column is arranged on the bracket; the lead screw-nut mechanism includes a stepping motor, a lead screw, and an adjusting nut. The stepping motor is installed on the collector tube support column, and the motor is used to drive the lead screw to rotate, so that the adjusting nut moves up and down, and the adjusting nut is connected to the pipe clamp; the pipe clamp is used to fix the collector tube.

[0029] Generally speaking, compared with the prior art by the above technical solutions conceived by the present invention, the following technical advantages are mainly possessed:

[0030] 1. When the heat absorption tube deviates downward from the focal line, part of the reflected solar beam can be diffused to the upper surface of the heat absorption tube, improving the uniformity of the solar energy flow distribution on the entire surface of the heat absorption tube; as long as the deviation distance is appropriate, the optical efficiency of the trough solar energy collection system will not change significantly. Based on this, the present invention proposes to actively offset and install the collector tube, and at the same time considers the deformation of the collector under the influence of gravity and the rotation of the system caused by the change of the sun's position, and determines the actual offset amount of the collector tube, so as to improve the uniformity of the solar energy flow distribution and the safety of the heat absorption tube.

[0031] 2. The present invention considers the situation of the heat collection system moving with the sun in practical applications. Based on the solar position tracking method, the real-time rotation angle of the solar energy collection system is determined, and the specific relationship between it and the actual offset amount is given, which is convenient to guide the control mechanism to adjust the position of the collector tube, and has stronger practicability.

[0032] 3. The collector tube is composed of a glass tube and a heat absorption tube. Since the deformation of the glass tube is small and the deformation of the heat absorption tube is large, the present invention ignores the deformation of the glass tube and only considers the deformation of the heat absorption tube under the action of gravity. Regarding the adjacent two brackets and the heat absorption tube therebetween as a simply supported beam with uniformly distributed load, the maximum deflection of the heat absorption tube under the action of gravity is calculated, which can simplify the calculation process while ensuring the calculation accuracy.

[0033] 4. The present invention also designs a specific bias adjustment device. The lead screw stepper motor operates when a pulse signal is input, and its rotation angle is determined by the number of pulse signals, which can accurately control the rising height of the heat collecting tube. The accuracy can reach the micron level, sufficient to respond to the change of tiny bias amount, with high accuracy, small space and high reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of the trough solar concentrator without bias in the embodiment of the present invention;

[0035] Figure 2 Schematic diagram of the trough solar concentrator reaching the maximum critical bias amount in the embodiment of the present invention;

[0036] Figure 3 Mathematical model diagram of the trough solar concentrator reaching the maximum critical bias amount in the embodiment of the present invention;

[0037] Figure 4 Schematic diagram of the sunglasses field of the trough solar thermal power station in the embodiment of the present invention;

[0038] Figure 5 Structural diagram of the trough solar concentrator in the embodiment of the present invention;

[0039] Figure 6 Schematic diagram of the simply supported beam in the embodiment of the present invention;

[0040] Figure 7 In (a) and (b), the bias diagrams when the rotation angle of the trough solar concentrator system is 0 and not 0 under the comprehensive influence of the embodiment of the present invention;

[0041] Figure 8 Bias mathematical model of the trough solar concentrator system under the comprehensive influence of the embodiment of the present invention;

[0042] Figure 9 Sunlight angle diagram of the embodiment of the present invention;

[0043] Figure 10 Schematic diagram of the solar collector with a bias device installed in the embodiment of the present invention;

[0044] Figure 11 Cross-sectional view of the solar collector with a bias device installed in the embodiment of the present invention;

[0045] Figure 12 Technical roadmap of the active bias method for the solar heat collecting tube in the embodiment of the present invention.

[0046] In all the drawings, the same reference numerals are used to denote the same elements or structures, where: 1 - bracket, 2 - parabolic reflector, 3 - heat collection tube support, 4 - pipe clamp, 5 - heat collection tube, 6 - stepper motor, 7 - adjusting nut. Detailed implementation mode

[0047] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to the 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.

[0048] A solar heat collection tube active biasing method provided by an embodiment of the present invention, as Figure 12 shown, by calculating the maximum critical biasing amount of the collector under the guarantee of optical efficiency, considering the deformation of the collector affected by gravity, calculating the critical biasing amount under the action of gravity, tracking the sun position, establishing a complete biasing amount database, and designing an accurate transmission mechanism to ensure the accurate implementation of the biasing method.

[0049] Specifically, it includes the following steps:

[0050] I. Calculating the maximum critical biasing amount

[0051] As Figure 5 shown, the existing commercial trough solar concentrating heat collection system mainly consists of a parabolic reflector and a vacuum heat collection tube, where the vacuum heat collection tube is located at the focal line of the parabolic reflector. The vacuum heat collection tube successively includes from the center to the outside: heat transfer fluid, heat absorption tube, vacuum gap and glass tube.

[0052] As Figure 1 shown, when the heat collection tube is not biased, that is, the biasing amount δz = 0, the incident solar rays are mainly concentrated on the bottom of the heat collection tube by the parabolic reflector, and the solar rays are vertically incident on any point P(x i , y i ) of the reflector and will be reflected to the bottom of the heat absorption tube.

[0053] As Figure 2 shown, when the heat collection tube is biased, the geometric axis of the heat absorption tube deviates downward from the focal line, and the biasing amount corresponding to the case where the edge P-c' of the reflected light beam of the edge point of the reflector just touches the heat absorption tube is denoted as: the maximum critical biasing amount δz*, at this time, if the biasing amount continues to increase, that is, |δz| > |δz * | (δz < 0, δz *When δz < 0, some of the reflected light rays will escape into the environment and cannot be concentrated on the surface of the heat absorption tube, resulting in a decrease in the optical efficiency of the trough solar collector system. Therefore, to ensure that the optical efficiency of the trough system is not lost, the offset of the heat absorption tube should not be greater than |δz * |(δz * < 0).

[0054] Abstract the mathematical model at this time and calculate the maximum critical offset. As Figure 3 shown, the circle O is translated downward by δz* along the y-axis to obtain the circle O'. P and P' are the two endpoints of the parabolic mirror. The length of the line segment PP' is W. Q is the intersection point of the circle O' and the reflected light ray PQ. Establish a Cartesian coordinate system with the midpoint S of the line segment PP' as the origin. The length of OS is f. Then the coordinates of point P are P(-W / 2, 0). Define ∠QPP' as α, and we can get:

[0055]

[0056] In the formula, β is the half solar angular diameter, β = 4.65 mrad.

[0057] The equation of the straight line PQ can be obtained as:

[0058]

[0059] The coordinates of O' are:

[0060]

[0061] The circle O' is tangent to the straight line PQ at point Q, that is, the distance from O' to the straight line PQ is the radius of the circle O'. According to the point-to-line distance formula, we can get:

[0062]

[0063] In the formula: D ab,o is the outer diameter of the heat absorption tube (O').

[0064] To sum up, the maximum critical offset δz* can be obtained as:

[0065]

[0066] II. Calculate the deformation of the heat collection tube caused by gravity

[0067] Considering the self-weight of the heat collection tube and the gravity of the fluid inside the tube, the heat absorption tube will inevitably deform. As Figure 4 shown, assume that the total designed length of the solar mirror field of a commercial trough solar power generation station is L tot, according to the design rules of existing trough systems, if n equally spaced supports are set in this solar mirror field and the heat collecting pipe is supported at the focal line of the trough-shaped mirror (the parabolic mirror is shaped like a trough, so the parabolic mirror is also called a trough-shaped mirror), then this solar mirror field consists of n - 1 Figure 5 trough solar collectors with a length of L as shown. The length L of a single trough solar collector and the total length L tot of the solar mirror field have the following relationship:

[0068] The length of the heat collecting pipe between two adjacent supports is also L. The heat collecting pipe consists of a glass tube and a heat absorbing pipe. In this invention, the deformation of the glass tube is ignored, and only the deformation of the heat absorbing pipe under the action of gravity is considered. In actual engineering, the gravity G acting on the heat absorbing pipe between two adjacent supports includes the self-weight G1 of the heat absorbing pipe and the self-weight G2 of the heat transfer fluid inside the pipe. Then:

[0069]

[0070] In the formula: D ab,o and D ab,i are the inner and outer diameters of the heat absorbing pipe; ρ1 is the density of the heat absorbing pipe; ρ2 is the density of the heat transfer fluid inside the pipe; g is the acceleration due to gravity; L is the length of the heat collecting pipe between two adjacent supports.

[0071] Two adjacent supports and the heat absorbing pipe between them can be regarded as a simply supported beam with uniformly distributed load. As Figure 6 shown, according to the theory of structural mechanics, it is easy to obtain that the maximum deflection z max of the heat absorbing pipe under the action of gravity is:

[0072]

[0073] In the formula: G is the gravity acting on the heat absorbing pipe between two adjacent supports, q is the uniformly distributed load, and qL = G; E ab.o is the elastic modulus of the heat absorbing pipe, and I ab,o is the moment of inertia of the cross-section of the heat absorbing pipe. Since the heat absorbing pipe is a hollow circular pipe, its moment of inertia I ab,o can be obtained from the following formula:

[0074]

[0075] III. Calculate the critical offset under the influence of gravity

[0076] When affected by gravity, the heat collecting pipe produces a vertical downward deformation, and the absolute value of the actual critical offset should be less than the absolute value of the maximum critical offset. When the trough solar concentrating and heat collecting system rotates as a whole with the movement of the sun, its actual critical offset δz # is as Figure 7 shown. In the figure, O1 is the position of the center of the heat collecting pipe caused by the action of gravity, and O2 is the position of the center of the circle after offset under the action of gravity (θ is the rotation angle of the heat collecting system).

[0077] The mathematical model of the collector system after abstract rotation is rotated by θ in the original direction, as Figure 8 shown. In the figure, the direction of arrow K is the original vertical ground direction. The method of establishing the coordinate system and the equation of line PQ after rotation are the same as those in the previous text, so they will not be elaborated.

[0078] The coordinates of point O1 are:

[0079] The coordinates of point O2 are:

[0080]

[0081] According to the formula for the distance from a point to a line, we can get:

[0082]

[0083] To sum up, the actual offset δz of the trough solar laser heat collecting tube # is:

[0084]

[0085] IV. Solar position tracking method

[0086] As Figure 9 shown, calculate the solar altitude angle and azimuth angle according to factors such as geographical and astronomical conditions, determine the accurate position of the sun, and determine the rotation angle of the trough solar collector system, providing a data source for the offset database.

[0087] 1. Measurement of solar altitude angle

[0088] The calculation of the solar altitude angle is as follows:

[0089]

[0090] In the formula: the geographical latitude is represented by Φ; the solar declination is represented by δ; the solar hour angle is represented by ω out. It is necessary to use a GPS receiver to locate the latitude. The calculation methods of the solar declination and the solar hour angle are given below.

[0091] The solar declination changes with seasons, and its change follows the coper equation as follows:

[0092]

[0093] In the formula: n represents the number of days of the season in a year.

[0094] The calculation method of the solar hour angle is as follows:

[0095]

[0096] Where ω is the solar hour angle in radians (rad); T is the true solar time in hours (h).

[0097] The mean solar time represents the time concept we usually understand. The true solar time represents the interval time when the sun passes through the upper culmination at a certain point continuously for two times. People imagine a mean solar time to help calculate time. Now assume that the earth rotates around the sun at a constant speed. At this time, the real time of a day is represented by 24 hours, that is, the mean solar time. The relationship between the true solar time and the mean solar time can be expressed by the following formula:

[0098] T = T p + E of = T g + E of

[0099] Where T P is the mean solar time, E of is the time difference, and T g is the Greenwich Mean Time.

[0100] The time difference E of has the following calculation formula:

[0101]

[0102] Where Ф is the daily light duration.

[0103]

[0104] Where k is the kth day of the year.

[0105] 2. Measurement of solar azimuth angle:

[0106] The azimuth angle is the angle between the projection line of the sunlight perpendicular to the ground plane and the southward ray on the ground plane. Its calculation formula is:

[0107]

[0108] Where β s is the solar azimuth angle and h is the solar altitude angle.

[0109] V. Offset amount database

[0110] Based on the solar position tracking method, determine the rotation angle of the trough solar collector system located at different geographical locations. According to the actual offset amount calculation method, determine the critical offset amount of the collector tubes of solar power plants of different models, and guide the control mechanism to adjust the position of the collector tubes.

[0111] To implement the above-mentioned active offset method of solar collector tubes, the present invention also correspondingly designs a solar collector offset mechanism device, such as Figure 10As shown, it includes a bracket 1, a heat collecting tube support 3, a screw nut mechanism and a tube clamp 4, wherein:

[0112] The screw-nut mechanism includes a stepper motor 6, a screw and an adjusting nut 7. The heat collecting tube 5 is fixed by a tube clamp 4, and the tube clamp 4 is connected to the adjusting nut 7. The adjusting nut 7 cooperates with the stepper motor 6. When the stepper motor 6 works, the adjusting nut 7 moves up and down through the screw to control the heat collecting tube 5 to make an offset movement. The stepper motor 6 is installed on the heat collecting tube support 3 and connected to the bracket 1. The parabolic reflector 2 is installed on the bracket 1.

[0113] The screw stepper motor is the core component for controlling the offset of the heat collecting tube. It works when a pulse signal is input, and its rotation angle is determined by the number of pulse signals. It can accurately control the rising height of the heat collecting tube with an accuracy of up to micron level, which is sufficient to respond to tiny changes in the offset and is suitable for the high precision, small space and high reliability requirements of the present invention.

[0114] like Figure 11 As shown, the analysis processor retrieves the bias data from the bias database, analyzes the data, converts it into a pulse signal number, generates a pulse signal through a pulse signal generator, and sends a phase pulse signal to the lead screw stepper motor. Finally, the lead screw stepper motor drives the adjusting nut to control the bias movement of the collector tube.

[0115] It should be noted that the stepper motor of this device can adopt open-loop or closed-loop control. The input of the motor control pulse of the open-loop control does not depend on the position of the rotor, but sends out the control pulse according to a fixed rule. The stepper motor only relies on this series of predetermined pulses to work. At this time, intermittent tracking is adopted, that is, after completing a bias adjustment, the device resets to the initial position. After a delay, the device re-tracks the sun and repeats the above steps. The closed-loop control adds a bias sensor to monitor the position of the collector tube in real time. Each bias adjustment is based on the previous one, with higher accuracy and stable startup, but more complex structure and reduced reliability.

[0116] In addition, the driving mechanism of the present invention is not limited to the cooperation of a stepping motor and a screw rod, but also includes a push rod drive, a hydraulic cylinder drive and other devices that can drive the offset of the heat collecting tube.

[0117] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for actively biasing a solar heat collecting tube, characterized in that, It includes the following steps: Determine the maximum deflection z of the heat-absorbing tube in the heat-collecting tube under the action of gravity max ; Considering the movement of the sun, determine the rotation angle θ of the entire heat collection system according to the solar azimuth angle; According to the maximum deflection z max and the rotation angle θ, determine the actual offset of the heat collecting tube, install the heat collecting tube according to the actual offset, and determine the corresponding actual offset according to the change of the rotation angle θ, and adjust the position of the heat collecting tube in real time.

2. The active biasing method of a solar heat collecting tube according to claim 1, wherein, The actual offset δz # is calculated as follows: Among them, D ab,o is the outer diameter of the heat absorption tube; |OP| is the distance between the center of the heat collection tube and one end of the parabolic reflector when the offset is 0; W is the distance between the two end points of the parabolic reflector; the method for determining the angle α is as follows: offset the heat collection tube by a certain amount so that the edge of the reflected beam of the edge point of the parabolic reflector is tangent to the heat absorption tube, and the included angle between this tangent line and the connection line between the two end points of the parabolic reflector is α.

3. The active biasing method for a solar heat collecting tube according to claim 1, wherein, The calculation formula for the angle α is: where β is the half solar angular diameter.

4. The active biasing method for a solar heat collecting tube according to claim 1, characterized in that The maximum deflection of the heat absorption tube under the action of gravity is calculated in the following way: A plurality of equally spaced brackets are arranged in the solar mirror field to which the trough solar vacuum heat collection tube belongs, and the adjacent two brackets and the heat collection tube therebetween are regarded as a simply supported beam with uniformly distributed load; The uniformly distributed load is multiplied by the length of the heat collection tube between the adjacent two brackets to obtain the gravity received by the heat absorption tube in the heat collection tube between the adjacent two brackets, and thus the maximum deflection of the heat absorption tube under the action of gravity is calculated.

5. The active biasing method for a solar heat collecting tube according to claim 4, wherein The maximum deflection z of the heat absorption tube under the action of gravity max is as follows: Among them, L is the length of the heat collecting pipe between two adjacent supports, G is the gravity borne by the heat absorption pipe in the heat collecting pipe between two adjacent supports, E ab is the elastic modulus of the heat absorption pipe, and I ab,o is the sectional moment of inertia of the heat absorption pipe.

6. The active biasing method for a solar heat collecting tube according to claim 5, wherein, The heat absorption tube is a hollow circular tube, and the sectional moment of inertia I of the heat absorption tube ab,o The calculation formula is as follows: Among them, D ab,o and D ab,i are the outer diameter and inner diameter of the heat absorption tube, respectively.

7. The active biasing method of a solar heat collecting tube according to any one of claims 1-6, characterized in that, The solar azimuth angle β s is calculated as follows: where h is the solar altitude angle, ω is the solar hour angle, and δ is the solar declination.

8. An apparatus for implementing the active biasing method of a solar heat collecting tube as described in any one of claims 1-7, characterized in that, It includes a bracket, a heat collection tube support column, a screw-nut mechanism and a pipe clamp, wherein: The bracket is used for installing a parabolic reflector, and the heat collection tube support column is arranged on the bracket; the screw-nut mechanism includes a stepping motor, a screw rod and an adjusting nut, the stepping motor is installed on the heat collection tube support column, and this motor is used for driving the screw rod to rotate so that the adjusting nut moves up and down, and the adjusting nut is connected to the pipe clamp; the pipe clamp is used for fixing the heat collection tube.

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