Popular science articles! The technical perspective takes you to understand the hidden worries of low-cost brackets
Publish Time：2024-03-20
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In photovoltaic power station projects, the use of tracking bracket than the fixed bracket to improve the power generation of about 25%, this significant advantage makes it in Europe and the United States to become the mainstream choice for photovoltaic power stations. Behind the increase in power generation, there is also a complex and professional tracking bracket technology. If the tracking bracket is designed and evaluated according to the concept of fixed bracket, the consequences will be disastrous.
In the tracking bracket program launched by different manufacturers, if anyone has a hole in reliability, there will be a large difference in cost, so as to attract attention at a low price. Wind resistance is the key point to track whether the support is reliable. This article will comprehensively analyze the differences between different manufacturers in wind load calculation, structural verification and vibration control, to help readers understand and identify the technical points of safe and reliable tracking supports.
1. Wind Load calculation risk: the "short board effect" caused by "normative school" in system design
Wind load calculation data is a very important technical parameter in the design of tracking support projects. The wind load calculation can be divided into two schools: the "normative school" based on the coefficient in the relevant load specification and the "wind tunnel school" based on the coefficient tested in the wind tunnel.
Among them, the "wind tunnel" method is the most scientific and reasonable, but its research cost is too high, and it will bring about a significant increase in the cost of some parts, and it is often not used by some "cheat" manufacturers. The design process of wind tunnel design should include the following steps (see Figure 1) : (1) Static and dynamic wind load coefficients are obtained through the rigid model pressure measurement wind tunnel test; (2) Based on the project design input, calculate the design value of wind load and its combination with other loads according to the method specified in the relevant load specifications; (3) Mechanical analysis and structural check shall be carried out according to the criteria specified in the relevant structural design specifications. In addition, the "wind tunnel school" will test the product's aeroelastic critical wind speed in the wind tunnel, compare and check with the design wind speed in the project design, and formulate scientific and reasonable wind protection control strategies, so as to prevent excessive structural vibration amplitude, which is impossible for the "standard school".
FIG. 1 Structural design flow based on wind tunnel test
The "normative school" does not calculate wind loads based on scientific wind tunnel tests, but chooses the suggestions in the "Building Structure Load Code" or the "Photovoltaic support structure Design Regulations" for the body type coefficient (similar to the static coefficient) and the wind vibration coefficient (similar to the dynamic coefficient). It should be noted that the relevant provisions in the above standards are essentially derived from the building structure or for the frame type fixed bracket, and do not fully consider the particularity of the tracking bracket structural system supported by a single row of columns. For example, the shape coefficient in the load specification does not consider enough the spatio-temporal pulsation characteristics of the wind load in the local area of the support. It is only a uniform distribution or a simple stepped distribution form, which cannot adequately and accurately describe the uneven distribution of wind pressure on the surface of the component, which will cause serious unsafe design for the torsional tracking support structural system. At the same time, the coefficient only reflects the two-dimensional characteristics, and does not consider the influence of the 360-degree wind direction and the interference/occlusion effect of the support array, so it can not include all the most unfavorable conditions, and can not make a reasonable division of the array. In addition, the wind vibration coefficient recommended by the load code is mainly based on vertical cantilever structures dominated by bending modes, such as high-rise buildings and high-rise structures. However, the tracking support structure system is complex, and the stress characteristics of different components are significantly different, such as the main beam is controlled by torsion and bending multi-mode (see Figure 2). Compared with the "wind tunnel", the "specification" will lead to the uneven safety of different components in the tracking support system, and produce a "short board effect" in the system design.
Figure 2 follows the torsion and bending modes of the main beam of the support
In contrast, the "wind tunnel" can accurately capture the uneven wind pressure by using the carefully arranged pressure measurement points, and fully simulate each incoming wind direction by using the rotatable multi-row bracket array model while considering the interference effect of the array (as shown in Figure 3), so as to obtain the static wind load coefficient that can truly reflect the structural characteristics. By combining the wind pressure time history data measured by wind tunnel test with the structural dynamic characteristics, the corresponding dynamic wind load coefficients of different structural components can be obtained by random vibration analysis, so as to obtain more accurate equivalent static wind load. The "standard" wind load model is unable to include extreme wind loads and responses of all components in a complex system, especially for wind loads considered by torsional components.
FIG. 3 Pressure measurement experiment of rigid model
FIG. 4 gives details of the comparison between wind tunnel tests and the wind load coefficients specified in load codes. In general, the current load specification is mainly based on empirical statistical values, and the consideration factors are rough and the value is radical, which will lead to greater risks in structural design. The wind tunnel test results are closer to the actual situation, taking into account comprehensive factors and accurate and reliable values, which can greatly reduce the structural design risk and avoid the "short-board effect" caused by the insufficient design of individual parts in the system.
FIG. 4 Comparison of wind load coefficient between wind tunnel test and load code
two. Structural check: Technical risk under national standard
Different national design codes have great differences on the strength and stability checking methods of different components, especially for the main beam design subject to flexural and torsional coupling. Figure 5 compares the checking criteria for main beams in the design standards for steel structures in China, the United States and Europe (since the impact of shear forces is generally small, the shear combinations are not listed in the table).
Figure 5. Differences in design specifications of different countries for the design check of the main beam of tracking support
It can be seen that our code only designs the main beam according to the bending member, mainly because the code is aimed at the steel frame building structural system based on the bending, and does not involve the torsion effect. At the same time, China's specification suggests to avoid the torsion of the component as much as possible, when it is inevitable, it is recommended to refer to foreign standards for design. In contrast, the European code can consider the individual bending and torsion design of the member, while only the United States code can consider the coupling of the bending and torsion of the member. Therefore, from the point of view of rationality, only the US standard is applicable to the design and check of the main beam of the tracking support. If the national standard is used to design the main beam, it will lead to great risks due to ignoring the torque effect that has a controlling role, as shown in Figure 6. The stress ratio (load effect/component resistance) obtained by checking the main beam based on different standards is shown in Figure 6.
FIG. 6 Comparison of the results of checking main beams in different national design codes
Third, vibration control: wind-induced vibration will seriously damage the tracking bracket
A series of wind-induced vibration phenomena such as buffeting, vortex vibration, flutter and gallop can easily occur under the action of wind, which leads to the failure of the structure due to strength failure, excessive deformation and gas bomb instability. Flutter, gallop and large eddy vibration are generally attributed to aerodynamic stability problems, which can be avoided by ensuring that the critical aeroelastic wind speed is higher than the design wind speed. Buffeting and small eddy vibration are amplitude limiting vibrations, which are generally considered based on the theory of equivalent static wind load. It is a method that treats the dynamic design problem considering structural wind vibration equivalent to the static design problem, that is, the commonly called structural wind load, which is applied to the structure as a static force, and the obtained extreme response and average response, background response and resonance response are combined and equivalent. Thus, the strength, stiffness and structural stability of the structural members and the connecting nodes are checked. On the one hand, it is necessary to prevent large wind-induced vibration of the structure, and on the other hand, it is necessary to fully consider the extra load effect caused by small vibration.
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Iv. Summary
The difference of standards used in wind load calculation, structural check and vibration control will not only cause the difference of support cost, but also affect the safety of support structure. Blindly pursuing the low price of tracking bracket will bring huge security risks to the project. This is also the main reason for tracking wind disasters in recent years. For example, in June 2017, a strong wind blew in Qinghai region, and multiple groups of flat single-axis tracking systems in a photovoltaic power station fell to the ground in response, and the pile foundation was uprooted. Another example is in November 2022, a photovoltaic project in Xinjiang, nearly 100 megawatts of photovoltaic array was blown over. According to the on-site video, a large number of photovoltaic supports collapsed, photovoltaic modules have varying degrees of damage, some severely damaged photovoltaic modules have been completely broken.
In the tracking bracket program launched by different manufacturers, if anyone has a hole in reliability, there will be a large difference in cost, so as to attract attention at a low price. Wind resistance is the key point to track whether the support is reliable. This article will comprehensively analyze the differences between different manufacturers in wind load calculation, structural verification and vibration control, to help readers understand and identify the technical points of safe and reliable tracking supports.
1. Wind Load calculation risk: the "short board effect" caused by "normative school" in system design
Wind load calculation data is a very important technical parameter in the design of tracking support projects. The wind load calculation can be divided into two schools: the "normative school" based on the coefficient in the relevant load specification and the "wind tunnel school" based on the coefficient tested in the wind tunnel.
Among them, the "wind tunnel" method is the most scientific and reasonable, but its research cost is too high, and it will bring about a significant increase in the cost of some parts, and it is often not used by some "cheat" manufacturers. The design process of wind tunnel design should include the following steps (see Figure 1) : (1) Static and dynamic wind load coefficients are obtained through the rigid model pressure measurement wind tunnel test; (2) Based on the project design input, calculate the design value of wind load and its combination with other loads according to the method specified in the relevant load specifications; (3) Mechanical analysis and structural check shall be carried out according to the criteria specified in the relevant structural design specifications. In addition, the "wind tunnel school" will test the product's aeroelastic critical wind speed in the wind tunnel, compare and check with the design wind speed in the project design, and formulate scientific and reasonable wind protection control strategies, so as to prevent excessive structural vibration amplitude, which is impossible for the "standard school".
FIG. 1 Structural design flow based on wind tunnel test
The "normative school" does not calculate wind loads based on scientific wind tunnel tests, but chooses the suggestions in the "Building Structure Load Code" or the "Photovoltaic support structure Design Regulations" for the body type coefficient (similar to the static coefficient) and the wind vibration coefficient (similar to the dynamic coefficient). It should be noted that the relevant provisions in the above standards are essentially derived from the building structure or for the frame type fixed bracket, and do not fully consider the particularity of the tracking bracket structural system supported by a single row of columns. For example, the shape coefficient in the load specification does not consider enough the spatio-temporal pulsation characteristics of the wind load in the local area of the support. It is only a uniform distribution or a simple stepped distribution form, which cannot adequately and accurately describe the uneven distribution of wind pressure on the surface of the component, which will cause serious unsafe design for the torsional tracking support structural system. At the same time, the coefficient only reflects the two-dimensional characteristics, and does not consider the influence of the 360-degree wind direction and the interference/occlusion effect of the support array, so it can not include all the most unfavorable conditions, and can not make a reasonable division of the array. In addition, the wind vibration coefficient recommended by the load code is mainly based on vertical cantilever structures dominated by bending modes, such as high-rise buildings and high-rise structures. However, the tracking support structure system is complex, and the stress characteristics of different components are significantly different, such as the main beam is controlled by torsion and bending multi-mode (see Figure 2). Compared with the "wind tunnel", the "specification" will lead to the uneven safety of different components in the tracking support system, and produce a "short board effect" in the system design.
Figure 2 follows the torsion and bending modes of the main beam of the support
In contrast, the "wind tunnel" can accurately capture the uneven wind pressure by using the carefully arranged pressure measurement points, and fully simulate each incoming wind direction by using the rotatable multi-row bracket array model while considering the interference effect of the array (as shown in Figure 3), so as to obtain the static wind load coefficient that can truly reflect the structural characteristics. By combining the wind pressure time history data measured by wind tunnel test with the structural dynamic characteristics, the corresponding dynamic wind load coefficients of different structural components can be obtained by random vibration analysis, so as to obtain more accurate equivalent static wind load. The "standard" wind load model is unable to include extreme wind loads and responses of all components in a complex system, especially for wind loads considered by torsional components.
FIG. 3 Pressure measurement experiment of rigid model
FIG. 4 gives details of the comparison between wind tunnel tests and the wind load coefficients specified in load codes. In general, the current load specification is mainly based on empirical statistical values, and the consideration factors are rough and the value is radical, which will lead to greater risks in structural design. The wind tunnel test results are closer to the actual situation, taking into account comprehensive factors and accurate and reliable values, which can greatly reduce the structural design risk and avoid the "short-board effect" caused by the insufficient design of individual parts in the system.
FIG. 4 Comparison of wind load coefficient between wind tunnel test and load code
two. Structural check: Technical risk under national standard
Different national design codes have great differences on the strength and stability checking methods of different components, especially for the main beam design subject to flexural and torsional coupling. Figure 5 compares the checking criteria for main beams in the design standards for steel structures in China, the United States and Europe (since the impact of shear forces is generally small, the shear combinations are not listed in the table).
Figure 5. Differences in design specifications of different countries for the design check of the main beam of tracking support
It can be seen that our code only designs the main beam according to the bending member, mainly because the code is aimed at the steel frame building structural system based on the bending, and does not involve the torsion effect. At the same time, China's specification suggests to avoid the torsion of the component as much as possible, when it is inevitable, it is recommended to refer to foreign standards for design. In contrast, the European code can consider the individual bending and torsion design of the member, while only the United States code can consider the coupling of the bending and torsion of the member. Therefore, from the point of view of rationality, only the US standard is applicable to the design and check of the main beam of the tracking support. If the national standard is used to design the main beam, it will lead to great risks due to ignoring the torque effect that has a controlling role, as shown in Figure 6. The stress ratio (load effect/component resistance) obtained by checking the main beam based on different standards is shown in Figure 6.
FIG. 6 Comparison of the results of checking main beams in different national design codes
Third, vibration control: wind-induced vibration will seriously damage the tracking bracket
A series of wind-induced vibration phenomena such as buffeting, vortex vibration, flutter and gallop can easily occur under the action of wind, which leads to the failure of the structure due to strength failure, excessive deformation and gas bomb instability. Flutter, gallop and large eddy vibration are generally attributed to aerodynamic stability problems, which can be avoided by ensuring that the critical aeroelastic wind speed is higher than the design wind speed. Buffeting and small eddy vibration are amplitude limiting vibrations, which are generally considered based on the theory of equivalent static wind load. It is a method that treats the dynamic design problem considering structural wind vibration equivalent to the static design problem, that is, the commonly called structural wind load, which is applied to the structure as a static force, and the obtained extreme response and average response, background response and resonance response are combined and equivalent. Thus, the strength, stiffness and structural stability of the structural members and the connecting nodes are checked. On the one hand, it is necessary to prevent large wind-induced vibration of the structure, and on the other hand, it is necessary to fully consider the extra load effect caused by small vibration.
Iv. Summary
The difference of standards used in wind load calculation, structural check and vibration control will not only cause the difference of support cost, but also affect the safety of support structure. Blindly pursuing the low price of tracking bracket will bring huge security risks to the project. This is also the main reason for tracking wind disasters in recent years. For example, in June 2017, a strong wind blew in Qinghai region, and multiple groups of flat single-axis tracking systems in a photovoltaic power station fell to the ground in response, and the pile foundation was uprooted. Another example is in November 2022, a photovoltaic project in Xinjiang, nearly 100 megawatts of photovoltaic array was blown over. According to the on-site video, a large number of photovoltaic supports collapsed, photovoltaic modules have varying degrees of damage, some severely damaged photovoltaic modules have been completely broken.
To sum up, a tracking bracket without worries must rely on more scientific tests and data analysis to determine more accurate wind load data. These requirements not only reflect the technical content of tracking bracket products, but also put forward higher professional ability and rich engineering design experience requirements for tracking bracket suppliers. Only with these conditions can we ensure the performance stability and long-term reliability of tracking bracket products, so that customers can rest assured.
