A Passive Biasing Method and Device for a Solar Heat Collection Tube

Abstract

The present invention belongs to the field of solar collectors, and specifically discloses a passive biasing method and device for a solar heat collecting tube, which includes the steps of: when the heat collecting tube is biased, the geometric axis of the heat absorbing tube in the heat collecting tube deviates downward from the focal line, and when the edge of the reflected light beam from the end point of the parabolic reflector below the heat collecting tube just touches the heat absorbing tube, the corresponding biasing amount at this time is used as the ideal critical biasing amount; the heat collecting tube is installed according to the ideal critical biasing amount, and then the heat collecting tube is successively moved upward to reduce the biasing amount, and the temperature difference between the inlet and outlet fluids of the heat collecting tube is measured accordingly. When the temperature difference between the inlet and outlet fluids no longer rises, the biasing amount of the heat collecting tube reaches near the actual critical biasing amount, and the changing critical biasing amount is continuously tracked according to the temperature difference between the inlet and outlet fluids. The present invention first determines the ideal critical biasing amount of the solar collector, and considering the influence of other factors on the solar collector, passively adjusts the biasing amount of the heat collecting tube to improve the uniformity of the 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 specifically, relates to a passive biasing method and device for 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 shows 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 contrast, 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, aligns the axis of the parabolic surface 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, punctures the glass tube, and causes 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 passive biasing method and device for a solar heat collecting tube, the purpose of which is to determine the ideal critical biasing amount of the collector, and considering the influence of other factors on the collector, passively adjust the constantly changing biasing amount of the heat collecting tube 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 passive biasing method for a solar heat collecting tube is proposed, including the following steps:

[0006] When the heat collecting tube is biased, the geometric axis of the heat absorption tube in the heat collecting tube deviates downward from the focal line. When the edge of the reflected light beam from the end point of the parabolic reflector below the heat collecting tube just touches the heat absorption tube, the corresponding biasing amount at this time is used as the ideal critical biasing amount;

[0007] Install the heat collecting tube according to the ideal critical biasing amount, and then gradually move the heat collecting tube upward to reduce the biasing amount, and correspondingly measure the temperature difference between the inlet and outlet fluids of the heat collecting tube. When the temperature difference between the inlet and outlet fluids no longer rises, the biasing amount of the heat collecting tube reaches near the actual critical biasing amount, and the installation of the heat collecting tube and the adjustment of the biasing amount are completed.

[0008] As a further preference, after the installation of the heat collecting tube and the adjustment of the offset amount are completed, during the operation of the heat collecting tube, first move the heat collecting tube downward once, and obtain the temperature difference between the inlet and outlet fluids of the heat collecting tube in real time. If the temperature difference between the inlet and outlet fluids rises, continue to move the heat collecting tube in the upward direction of the previous movement; if the temperature difference between the inlet and outlet fluids decreases, move the heat collecting tube in the direction opposite to the previous movement.

[0009] As a further preference, the calculation formula for the ideal critical offset amount is:

[0010]

[0011] where, δz* is the ideal critical offset amount, D ab,o is the outer diameter of the heat absorption tube; |OP| is the distance between the center of the heat collecting tube and one end of the parabolic reflector when the offset amount is 0; W is the distance between the two end points of the parabolic reflector; α is the angle between the tangent line of the edge of the reflected light beam of the parabolic reflector end point and the heat absorption tube and the connection line between the two end points of the parabolic reflector when the heat collecting tube is at the ideal critical offset amount.

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

[0013]

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

[0015] As a further preference, the time interval for each movement of the heat collecting tube is 2 min to 5 min.

[0016] As a further preference, the distance for each movement of the heat collecting tube is 0.5 mm to 1 mm.

[0017] According to another aspect of the present invention, there is provided a device for implementing the above-mentioned passive offset method of the solar heat collecting tube, including a bracket, a heat collecting tube support column, a lead screw nut mechanism, a pipe clamp and a fluid thermometer, wherein:

[0018] The bracket is used for installing the parabolic reflector, and the heat collecting 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 heat collecting tube support column, and this motor is used for driving 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 for fixing the heat collecting tube; the fluid thermometer is installed at the inlet and outlet of the heat collecting tube and is used for measuring the temperature difference between the inlet and outlet fluids.

[0019] Generally speaking, compared with the prior art by the above technical solution conceived by the present invention, the following technical advantages are mainly possessed:

[0020] 1. When the heat absorption tube deviates downward from the focal line, some of the reflected solar beams 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 collector system will not change significantly. Based on this, the present invention determines the ideal critical offset of the collector under the condition of ensuring the optical efficiency. Considering the influence of gravity, complex geographical conditions, etc. on the collector, this critical offset is relatively large. Therefore, the passive offset adjustment of the heat absorption tube is further carried out based on the sensor data to improve the uniformity of the solar energy flow distribution and the safety of the heat absorption tube.

[0021] 2. Since the deformation amounts caused by gravity at different positions of the heat absorption tube are different, the rotation angle of the solar energy system tracking the sun is related to its geographical location and layout method. The actual critical offset should be less than the ideal critical offset and is constantly changing, making it difficult to accurately calculate the specific value of the actual critical offset; therefore, the present invention judges the thermal economy of the system through the temperature difference between the inlet and outlet fluids of the heat absorption tube, and adjusts the offset of the heat absorption tube in real time accordingly to adapt to the system changes, realizing accurate and real-time adjustment of the offset of the heat absorption tube.

[0022] 3. The present invention also designs a specific offset adjustment device. The lead screw stepper motor works 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 absorption tube, with an accuracy reaching the micron level, sufficient to respond to small offset changes, featuring high precision, small space and high reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the trough solar concentrator without offset in the embodiment of the present invention;

[0024] Figure 2 Schematic diagram of the trough solar concentrator reaching the ideal critical offset in the embodiment of the present invention;

[0025] Figure 3 Mathematical model diagram of the trough solar concentrator reaching the ideal critical offset in the embodiment of the present invention;

[0026] Figure 4 Schematic diagram of the heliostat field of the trough solar thermal power station in the embodiment of the present invention;

[0027] Figure 5 Structural diagram of the trough solar concentrator in the embodiment of the present invention;

[0028] Figure 6 Schematic diagram of the simply supported beam in the embodiment of the present invention;

[0029] Figure 7In Figures (a) and (b), they are offset schematic diagrams when the rotation angle of the trough solar concentrator heat collection system is 0 and not 0 under the comprehensive influence of the embodiments of the present invention;

[0030] Figure 8 It is a schematic diagram of the inlet and outlet temperature difference and thermal stress changes during the offset process of the embodiments of the present invention;

[0031] Figure 9 It is a schematic diagram of a solar collector with an offset device added in the embodiments of the present invention;

[0032] Figure 10 It is a sectional view of a solar collector with an offset device added in the embodiments of the present invention;

[0033] Figure 11 It is a technical roadmap of the passive offset method for solar collector tubes in the embodiments of the present invention.

[0034] In all the drawings, the same reference numerals are used to represent the same elements or structures, where: 1 - bracket, 2 - parabolic reflector, 3 - collector tube support, 4 - pipe clamp, 5 - collector tube, 6 - stepper motor, 7 - adjusting nut. Detailed implementation manners

[0035] In order to make the objectives, technical solutions and advantages of the present invention clearer, 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.

[0036] A passive offset method for solar collector tubes provided by an embodiment of the present invention, as Figure 11 shown, calculates the ideal critical offset amount of the collector under the condition of ensuring the optical efficiency, considers the influence of factors such as gravity and complex geographical conditions on the collector, forms a passive adjustment method for the offset amount of the collector tube based on sensor data, and designs an accurate transmission mechanism to ensure the accurate implementation of the offset method.

[0037] Specifically, it includes the following steps:

[0038] 1. Analysis of the offset requirements of the trough solar collector

[0039] As Figure 5 shown, the existing commercial trough solar concentrator heat collection system mainly consists of a parabolic reflector and a vacuum collector tube, and the vacuum collector tube is located at the focal line of the parabolic reflector. The vacuum collector tube successively includes from the center outwards: heat transfer fluid, heat absorption tube, vacuum gap and glass tube.

[0040] As Figure 1As shown in the figure, when the heat collection tube is not offset, that is, the offset 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 ) on the reflector and will be reflected to the bottom of the heat absorption tube.

[0041] As Figure 2 shown in the figure, when the heat collection tube is offset, the geometric axis of the heat absorption tube deviates downward from the focal line. The offset amount corresponding to the case where the edge P-c' of the reflected beam of the edge point of the reflector is just tangent to the heat absorption tube is denoted as: the ideal critical offset amount δz*. At this time, if the offset amount continues to increase, that is, |δz| > |δz * |(δz < 0, δz * < 0), some of the above-mentioned 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 amount of the heat absorption tube should not be greater than |δz * |(δz * < 0).

[0042] 2. Calculation of the ideal critical offset amount

[0043] Abstract Figure 2 the mathematical model in it to calculate the ideal critical offset amount. As Figure 3 shown in the figure, 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 reflector. The length of the line segment PP' is W. Q is the intersection point of the circle O' and the reflected light ray PQ. A Cartesian coordinate system is established with the midpoint S of the line segment PP' as the origin, and the length of OS is f. Then the coordinates of point P are P(-W / 2, 0). Defining QPP' as α, we can get:

[0044]

[0045] In the formula, β is the half solar angular width, β = 4.65 mrad.

[0046] The equation of the straight line PQ can be obtained as:

[0047]

[0048] The coordinates of O' are:

[0049]

[0050] 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:

[0051]

[0052] Where: D ab,o is the outer diameter of the heat absorption tube (O’).

[0053] In summary, the ideal critical offset δz* can be obtained as follows:

[0054]

[0055] 3. Analysis of the critical offset in actual situations

[0056] Considering the self-weight of the heat collection tube and the gravitational force of the fluid inside the tube, the heat absorption tube will inevitably deform. According to the design rules of existing trough systems, the solar mirror field of a commercial trough solar power generation station is as Figure 4 shown. Multiple reflectors, brackets, and heat absorption tubes are spliced into a complete mirror field.

[0057] The structure of a single unit of a trough solar concentrator is as Figure 5 shown. The heat collection tube includes a glass tube and a heat absorption tube, with heat transfer fluid inside. The deformation of the glass tube can be ignored, and only the deformation of the heat absorption tube under the action of gravity is considered. In actual engineering, the gravity acting on the heat absorption tube between two adjacent brackets includes the self-weight of the heat absorption tube and the self-weight of the heat transfer fluid inside the tube. Then: The heat absorption tube between two adjacent brackets and the brackets 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 heat absorption tube deforms under the action of gravity, and its maximum deformation is located at the middle position between the two brackets. Therefore, affected by gravity, the heat collection tube will produce 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.

[0058] At the same time, the overall trough solar heat collection system will rotate with the movement of the sun. Under the dual influence of gravity and rotation, its actual critical offset δz # is as Figure 7 shown (θ is the solar tracking angle).

[0059] Therefore, due to the different deformation amounts caused by gravity at different positions of the heat collection tube, the rotation angle of the trough solar system tracking the sun is related to its geographical location and layout method. The actual critical offset δz # should be less than the ideal critical offset δz*, and the actual critical offset will change during the operation of the system, making it difficult to accurately calculate the specific value of the actual critical offset.

[0060] 4. Passive offset analysis

[0061] The purpose of placing the heat collection tube with an offset is to improve the safety of the heat collection tube and ensure its economy at the same time. The heat collection tube moves from the initial installation position to the actual critical offset δz #, continue to bias to the ideal critical bias amount δz*, and then continue to bias the change trends of the main economic indicators (the temperature difference between the inlet and outlet fluids of the heat collection pipe) and safety indicators (thermal stress) throughout the process, as Figure 8 shown:

[0062] (1) Initial installation position (bias amount is 0) - actual critical bias position

[0063] In this stage, all the reflected light irradiates on the surface of the heat collection pipe, and the irradiation area becomes larger, the heat flow is more uniform, the temperature gradient at the pipe wall decreases, the thermal stress and strain decrease, the safety increases, and at the same time, the more uniform heat flow also makes the thermal economy of the system increase slightly. In the figure, it is reflected as a slight increase in the temperature difference between the inlet and outlet fluids.

[0064] (2) Actual critical bias position - ideal critical bias position

[0065] At this time, a small amount of reflected light begins to escape from the top of the heat collection pipe, the optical efficiency of the system decreases, and at the same time the thermal economy decreases. The loss of heat makes the overall temperature of the pipe wall decrease, the temperature gradient at the pipe wall decreases, the thermal stress and strain decrease, and the safety increases. Although the thermal economy declines in this stage, the decline is small, and the temperature difference change of the inlet and outlet working fluids is also small.

[0066] (3) Ideal critical bias position - severely over-biased position

[0067] In this stage, the heat collection pipe continues to move towards the reflector, a large amount of light escapes from the top of the heat collection pipe, the thermal economy of the system decreases significantly. At the same time, the loss of heat makes the overall temperature of the pipe wall decrease, the temperature gradient at the pipe wall decreases, the thermal stress and strain decrease, and the safety increases. In this stage, the thermal economy of the system drops severely, and the temperature difference between the inlet and outlet fluids also decreases significantly, which will cause huge economic losses.

[0068] Therefore, the actual critical bias position has the following characteristics: this position is close to the peak of the system's thermal economy, and the temperature difference between the inlet and outlet fluids of the heat collection pipe is also the largest; and around this point, the safety of the system continues to increase, and the thermal strain, stress and temperature gradient of the pipe wall continue to decrease.

[0069] Based on the above content, only one indicator of the temperature difference between the inlet and outlet fluids is required to track the actual critical bias position. The present invention proposes a passive biasing method: within the range less than the ideal critical bias amount, track the actual critical bias position through a passive method. The adjustment basis is that near the actual critical bias position, the thermal economy of the system is the highest, the working fluid flowing into the pipe can obtain more heat, and the temperature rise will be more obvious. Therefore, the temperature difference between the inlet and outlet of the heat collection pipe can be measured as an indicator to track the critical bias position.

[0070] 5. Design of passive biasing device

[0071] To implement the above-mentioned passive biasing method for solar heat collecting tubes, a passive biasing device is correspondingly designed, as Figure 9 shown, which includes a bracket 1, a heat collecting tube support column 3, a lead screw-nut mechanism, and a pipe clamp 4, where:

[0072] The lead screw-nut mechanism includes a stepper motor 6, a lead screw, and an adjusting nut 7. The heat collecting tube 5 is fixed by the pipe clamp 4, and the pipe 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 lead screw to control the biasing movement of the heat collecting tube 5. The stepper motor 6 is installed on the heat collecting tube support column 3 and connected to the bracket 1. The parabolic reflector 2 is installed on the bracket 1.

[0073] The lead screw stepper motor is the main actuator to realize the biasing movement 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 up to the micron level, which is sufficient to respond to the change of a small biasing amount and meets the requirements of high accuracy, small space, and high reliability needed for the present invention.

[0074] The solar heat collector equipped with the passive biasing device is as Figure 10 shown. The fluid thermometer is installed at the inlet and outlet of the heat collecting tube to measure the fluid temperature at the inlet and outlet. The analysis processor is used to analyze the measurement data of the sensor and judge whether the biasing value reaches the critical biasing amount. The pulse signal generator is responsible for converting the analysis result of the analysis processor into a pulse signal to control the operation of the stepper motor.

[0075] In addition, the drive mechanism of the present invention is not limited to the cooperation of a stepper motor + screw, and also includes devices such as push rod drive and oil cylinder drive that can drive the biasing of the heat collecting tube.

[0076] 6. Passive biasing process

[0077] At the beginning, the heat collecting tube is installed according to the ideal critical biasing amount. After installation, the analysis processor reads the temperature of the fluid thermometer and calculates the temperature difference between the inlet and outlet fluids. Subsequently, the pulse signal generator sends a signal to control the operation of the stepper motor to reduce the biasing amount by 1 mm (the biasing amount is generally at the millimeter level). The analysis processor reads the data of the fluid thermometer at this time and compares it with the data read last time. If the temperature difference between the inlet and outlet fluids rises, continue to control the movement of the stepper motor to reduce the biasing amount. Repeat the above process until after a certain adjustment, the temperature difference between the inlet and outlet fluids no longer rises. At this time, the heat collecting tube reaches near the current actual critical biasing amount. Then control the biasing amount of the heat collecting tube to increase by 1 mm. The analysis processor continues to read and analyze the sensor data. If the temperature difference between the inlet and outlet fluids rises, continue to adjust the biasing amount in this direction. If the temperature difference between the inlet and outlet fluids drops, adjust the biasing amount in the opposite direction.

[0078] The analysis processor reads data every 3 minutes, reads the data at all times, and changes the bias of the heat collecting tube, so that the bias of the heat collecting tube is always near the actual critical bias, achieving the highest economy, and maximizing the safety of the system on this basis.

[0079] 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. A passive biasing method for a solar heat collecting tube, characterized in that, It includes the following steps: When the heat collecting tube is offset, the geometric axis of the heat absorption tube in the heat collecting tube deviates downward from the focal line. When the edge of the reflected light beam from the end point of the parabolic reflector below the heat collecting tube just touches the heat absorption tube, the corresponding offset at this time is used as the ideal critical offset. Install the heat collecting tube according to the ideal critical offset, and then gradually move the heat collecting tube upward to reduce the offset, and correspondingly measure the temperature difference between the inlet and outlet fluids of the heat collecting tube. When the temperature difference between the inlet and outlet fluids no longer rises, the offset of the heat collecting tube reaches near the actual critical offset, and the installation of the heat collecting tube and the adjustment of the offset are completed.

2. The passive biasing method of the solar heat collecting tube according to claim 1, wherein After the installation of the heat collecting tube and the adjustment of the offset are completed, during the operation of the heat collecting tube, move the heat collecting tube downward once first, and obtain the temperature difference between the inlet and outlet fluids of the heat collecting tube in real time. If the temperature difference between the inlet and outlet fluids rises, continue to move the heat collecting tube in the same direction as the previous upward movement; if the temperature difference between the inlet and outlet fluids drops, move the heat collecting tube in the direction opposite to the previous upward movement.

3. The passive biasing method of a solar heat collecting tube according to claim 1, wherein The calculation formula for the ideal critical offset is: where δz* is the ideal critical offset, 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; α is the angle between the tangent of the edge of the reflected beam of the parabolic reflector end point and the heat absorption tube and the line connecting the two end points of the parabolic reflector when the heat collection tube is at the ideal critical offset.

4. The passive biasing method of the solar heat collecting tube according to claim 3, characterized in that, The calculation method of the angle α is: Where β is the half solar angular diameter.

5. The passive biasing method of a solar heat collecting tube according to any one of claims 1-4, characterized in that, The time interval for each movement of the heat collecting tube is 2 min to 5 min.

6. The passive biasing method of a solar heat collecting tube according to any one of claims 1-4, characterized in that, The distance for each movement of the heat collecting tube is 0.5 mm to 1 mm.

7. An apparatus for implementing the passive biasing method of a solar heat collecting tube according to any one of claims 1-6, characterized in that, It includes a bracket, a heat collecting tube support column, a lead screw nut mechanism, a pipe clamp and a fluid thermometer, where: The bracket is used to install the parabolic reflector, and the heat collecting 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 heat collecting tube support column, and this motor is used to drive the lead screw to rotate, so that the adjusting nut moves up and down. The adjusting nut is connected to the pipe clamp; the pipe clamp is used to fix the heat collecting tube; the fluid thermometer is installed at the inlet and outlet of the heat collecting tube and is used to measure the temperature difference between the inlet and outlet fluids.

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