- Wind load
Due to different environments, the requirements for the design of the photovoltaic system support structure are getting higher and higher, and ensuring the safety and application of the support structure, cost saving, and convenient construction and maintenance have become the main factors to be considered in the design. The ground photovoltaic support has its particularity as a framework. There are two forms of wind load design value for the photovoltaic support structure abroad. One is to refer to the American Building Structure Load Code [Minimum Design Loads for Buildings and Other Structures (ASCE/SEI7-05)] The calculation formula of wind load for single-slope non-enclosed structure in the other is to refer to the calculation formula of wind pressure load of array in Japan’s “Design and Construction of Photovoltaic Power Generation System”. At present, due to the lack of research results on the wind load design value of photovoltaic support structures in China, when there is no design specification for photovoltaic support structures, the standard value of wind load in the design of photovoltaic module support structures is mainly selected from the “Code for Loads of Building Structures”. (GB50009-2012) calculation formulas and parameter values in related content. There are two main calculation formulas in China.
Method 1 is calculated according to the design method of calculating the main force-bearing structure, and the formula is as shown in Figure 1:
Where: Wk——standard value of wind load (kN/m²);
βz——wind vibration coefficient at height Z;
μs——wind load body type coefficient;
μz——wind pressure height variation coefficient;
W. ——Basic wind pressure (kN/m²).
Method 2 is calculated according to the design method of calculating the envelope structure, and the formula is as shown in Figure 2:
Where: Wk——standard value of wind load (kN/m²);
βgz – gust coefficient at height Z;
μsi——wind load local body shape coefficient.
(1) Change coefficient of wind pressure height
For photovoltaic power stations built on flat or slightly undulating terrain, the wind pressure height variation coefficient is determined according to the ground roughness category. For photovoltaic power stations on offshore sea surfaces, islands, coasts, lake shores and desert areas, μz is taken at 1.1~1.3 value; for photovoltaic power stations near fields, villages, hills, and towns with sparse houses, μz ranges from 1.0 to 1.1.
For mountain or mountain photovoltaic power stations, the change coefficient of wind pressure height can be determined according to the roughness category of flat ground according to the “Code for Loading of Building Structures” (GB50009-2012), and the correction of terrain conditions should also be considered. The terrain correction coefficient is generally 7 Indicates that, according to the height of the hillside or peak, the correction coefficient of the sunny or south-facing hillside or the top of the peak, n can take a value between 1.95 and 2.75. Set the correction factor for the bottom of the hillside or peak on the sunny or south side to 1, and the correction factor between the top and bottom of the hillside or peak is determined by linear interpolation.
For photovoltaic power stations on the far sea surface and islands, the coefficient of variation of wind pressure height can be determined by the “Code for Loading of Building Structures” (GB50009-2012) according to the roughness category of the flat ground, and the correction of the distance from the coast should also be considered. The correction factor 7 for 40km is 1.0; the correction factor n for the distance from the coast is 40~60km is 1.0~1.1; the correction factor n for the distance from the coast is 1.1~1.2.
For photovoltaic modules and support structures on high-rise buildings and high-rise structures, the value of the basic wind pressure should be appropriately increased on the basis of the above terrain conditions, and should comply with the current national standards “Technical Regulations for Concrete Structures of High-rise Buildings” (JGJ3) and “High-rise Buildings”. Structural Design Specifications (GB50135).
(2) Form factor of wind load
In the design process of photovoltaic support, the shape coefficient is the most important factor affecting the wind load value under the same basic wind pressure condition.
Referring to the “Design Specifications for Photovoltaic Power Stations” (GB50797-2012), the wind load coefficient of the ground and roof photovoltaic supports is 1.3; some scholars have proposed that the wind direction angle y=180° represents the wind pressure, and the inclination angle of the photovoltaic module is 40°. The simulated body shape coefficient of exhaust load is 0.97~1.01, and the second row is 0.25~0.35; The wind direction angle y=0° represents wind suction, the inclination angle of photovoltaic modules is 40°, the simulated body shape coefficient of the first row of wind load is 1.27~1.28, and the second row is 0.32~0.33; It is stable at 0.74~0.78. The wind direction angle y=180° represents the wind pressure, and the inclination angle of the photovoltaic modules is between 10° and 40°. The corresponding first row wind load simulation body shape coefficient is 0.47 to 1.01, and the intermediate value is calculated according to the interpolation method.
The size of the wind load is an important factor affecting the cross-sectional size of the main components of the photovoltaic support. In the construction of photovoltaic power stations, the photovoltaic support is relatively closely arranged. The design situation has declined. If this part of the wind load reduction can be taken into account at the beginning of the design through research and calculation, the steel consumption of the photovoltaic support can be effectively reduced. Under the wind pressure condition, the effect of the wind load on the foundation of the photovoltaic support is pressure; while under the wind suction condition, the effect of the wind load on the foundation of the photovoltaic support is the tensile force, which is not good for the foundation’s anti-overturning or pull-out resistance. Therefore, in the engineering design practice of photovoltaic power stations, in order to facilitate the design, the wind suction condition is generally considered, and the same wind load body shape coefficient is preferred for each row.
(3) Wind Vibration Coefficient
There are many methods of wind vibration analysis. In the book “New Progress in Random Vibration Theory and Application”, the stochastic simulation time history analysis method directly acts on the structure of the wind load time history, and then analyzes the dynamic time history response of the structure through the step-by-step integration method. The method has high precision, and can obtain the whole process information of the dynamic response of the structure as the wind vibration response of the calculation dynamic action, which is expected to be applied in the field of photovoltaic integration.
At present, the simulation methods of wind speed time history in China include the CAWS (constant 8mplitude wave superposition) method, the WAWA (waves with weighted amplitude) method, the inverse Fourier method, the wavelet method, and the linear regression filter method. Among them, the CAWS method and the WAWA method have a huge amount of calculation, and the generated wind speed process cannot consider the time correlation, and it is more effective to use the linear regression filter law. Aiming at the effect of pulsating wind in the downwind direction, the simulation technology of linear autoregressive filter is used to compile a program to simulate the wind speed time history of Davenport spectrum, and the equivalent inertial force of each node caused by pulsating wind can be obtained. Then use ANSYS software to conduct random vibration After calculation, the displacement and stress at the nodes and the wind vibration coefficient of the structure can be finally obtained.
Some scholars use the stochastic simulation time history analysis method for time domain analysis, and use two calculation models:
1) Calculate the displacement and stress of the photovoltaic module, adopt a simplified single-piece photovoltaic module model, and divide it into 10 × 10 and a total of 100 plate units;
2) To calculate the displacement and stress of the frame beams and columns, the overall frame model is used, and its blessing number is 10 × 10. Each component is divided into 4 plate elements according to 2 × 2, and each beam and column is divided into 2 beam elements.
The wind direction is the horizontal wind direction perpendicular to the short side direction. The specific steps are: according to the statistical characteristics of the wind load, artificially generate the wind speed time history (excitation sample) with specific spectral density and spatial correlation, and convert it into the wind pressure time history to act on the structure; use the Newmark step-by-step integration method in the time domain The equation of motion is solved to obtain the node response at each time step; the response samples are statistically analyzed to determine the mean value and mean square error of the wind vibration response. Through ANSYS simulation calculation and analysis, the results show that the displacement wind vibration coefficient does not change much on each node of the single photovoltaic module panel plane, and the distribution is relatively uniform, and its value changes in the range of 2.3~2.6. The displacement wind vibration coefficient of the overall model edge frame is larger than the middle position displacement wind vibration coefficient. The displacement wind vibration coefficient of the overall model does not change much, among which the displacement wind vibration coefficient of the middle span is about 2.32, while the displacement wind vibration coefficient of the two sides is 2.41. For the convenience of engineering application, the wind vibration coefficient of photovoltaic modules and their supporting frame structures can be taken as 2.3~2.6.
(4) Gust coefficient
For photovoltaic power stations built on flat or slightly undulating terrain, the gust coefficient is determined according to the category of ground roughness. For photovoltaic power stations on offshore sea surfaces, islands, coasts, lake shores and desert areas, z can be a value of 1.60~1.65; For photovoltaic power stations near fields, villages, hills, and towns with sparse houses, z can take a value between 1.66 and 1.70.
(5) Basic wind pressure
The basic wind pressure shall be the wind pressure of the 50-year recurrence period determined by the method specified in the “Code for Loads of Building Structures” (GB50009-2012). ) Appendix OK.
When the basic wind pressure value of the construction site or city is not given in the appendix of “Code for Loading of Building Structure” (GB50009-2012), the definition and regulation of basic wind pressure in “Code for Loading of Building Structure” (GB50009-2012) can be adopted. calculated by statistical methods. When there is no local wind speed data, it can be determined according to the basic wind pressure or long-term data specified in the nearby area, through the comparison of meteorological and topographic conditions, or it can be approximately determined by referring to the national basic wind pressure distribution map in the specification. as follows:
1) When determining the wind pressure, the observation site should meet the following requirements:
(a) The observation site and its surroundings should be open and flat terrain;
(b) It can reflect the meteorological characteristics in a larger range of the region, and avoid the influence of local topography and environment.
2) The wind speed observation data should meet the following requirements:
(a) The 10-min average wind speed data recorded by the self-recording anemometer should be used. For the previous non-self-recording timed observation data, it should be used after appropriate correction;
(b) The standard height of the anemometer should be 10m. When the observed height of the anemometer is quite different from the standard height, it can be converted to the wind speed v at the standard height as follows (Figure 3):
Where: vz——wind speed observed by anemometer (m/s);
a——The ground roughness index in the open and flat area, take 0.15.
(c) When using the cup type anemometer, the correction of air density affected by temperature and air pressure must be considered.
3) When selecting the maximum annual wind speed data, generally more than 25 years of wind speed data should be available; when it cannot be satisfied, the wind speed data should not be less than 10 years. The homogeneity of the observation data should be considered, and the non-uniform data should be reasonably corrected in combination with the conditions of the surrounding weather stations.
4) The statistical samples of wind speed and snow pressure should use the annual maximum value, and use the probability distribution of extreme value type I, and its distribution function is (Figure 4):
In the formula: x——the sample of the annual maximum wind speed or the annual maximum snow pressure;
u – the location parameter of the distribution, that is, the mode of its distribution;
a – the scale parameter of the distribution;
- the standard value of the sample;
μ – the mean value of the sample.
5) The maximum wind speed and maximum snow pressure xR with a return period R can be calculated as follows (Figure 5):
The wind pressure and snow pressure values with return periods of 10, 50 and 100 years can also be selected for cities across the country according to the appendix list of “Code for Building Structural Loads” (GB50009-2012). The corresponding values of other return periods R can be Based on the 10-year and 100-year wind and snow pressure values, it is determined as follows (Figure 6):
6) Basic wind pressure W. It should be calculated according to the following formula according to the basic wind speed (Figure 7):
In the formula: p——air density (t/m³);
v0 – basic wind speed.
The basic wind pressure W0 can be determined according to steps 1) to 6).
The combination coefficient, frequent value coefficient and quasi-permanent value coefficient of wind load may be taken as 0.6, 0.4 and 0.0 respectively.
- Snow load
The standard value of snow load in the design of photovoltaic module bracket structure mainly refers to the relevant specifications of building structure design. The calculation formula is as follows (Figure 8):
where: — standard value of snow load (kN/m²);
μr——the distribution coefficient of snow cover on the roof;
- Basic snow pressure (kN/m²).
The basic snow pressure value of each city in the country is based on the snow pressure with a 50-year return period according to the “Code for Loading of Building Structures” (GB50009-2012). When the basic snow pressure value of the construction site or city is not given in the code, the basic snow pressure value should be determined according to the building structure code, according to the local annual maximum snow pressure or snow depth data, according to the basic snow pressure definition, through statistical analysis The analysis is determined, and the effect of the sample should be considered in the analysis. When there is no local snow pressure or snow depth data, it can be determined based on the basic snow pressure or long-term data specified in the nearby area, through comparative analysis of meteorological and topographic conditions, or it can be approximately determined by referring to the national basic snow pressure distribution map in relevant regulations. The snow load in mountain areas should be determined after actual investigation. When there is no measured data, it can be used by multiplying the snow load value of the local adjacent open and flat ground by 1.2.
The combined value coefficient of snow load may be 0.7, the frequent value coefficient may be 0.6, and the quasi-permanent value coefficient should be 0.5, 0.2 and 0 according to the different snow load divisions I, II and III; use.
(1) Self-weight of photovoltaic modules and installation accessories
Photovoltaic modules are divided into traditional backplanes and glass backplanes according to the backplane material. Traditional backplanes are generally made of TPT, PET, or KPE. Photovoltaic modules are generally divided into framed modules and frameless modules according to different backplane materials. Framed modules mainly refer to modules with aluminum alloy frames. The aluminum alloy frame is used as a protective laminate to play a certain role in sealing and supporting; no The frame components are mainly double glass components.
Taking the general peak power 260~270W polysilicon module as an example, a module string is set to consist of 22 photovoltaic modules, the weight of a single photovoltaic module is g1, and the weight of the solar panel itself of a single module string is 22g1. In order to facilitate the calculation, the weight of the waterproof junction box, bird repellent, other auxiliary materials and cables required for the installation of photovoltaic modules related to a single module string photovoltaic module is converted to the self-weight of the photovoltaic module, that is, the self-weight of the photovoltaic modules and installation accessories of a single module string G1 = battery board itself ten weight of bird repellent + junction box self weight ten auxiliary materials self weight ten cable weight and so on. According to the different component forms and layouts selected, it is estimated that the dead weight Gi of the photovoltaic modules and installation accessories of a single module string is 5.5~7.5kN, and the standard weight of photovoltaic modules and installation accessories is converted to 0.15~0.21kN/m² per square meter.
When designing and checking the foundation of the photovoltaic module support, the self-weight of the photovoltaic module and installation accessories should be within the range of 0.15~0.21kN/m², which can generally be estimated by the value of 0.2kN/m².
(2) Self-weight of photovoltaic supports and installation accessories
The photovoltaic support of photovoltaic power generation system generally adopts hot-dip galvanized thin-walled steel or aluminum alloy. Wait.
The steel bracket of photovoltaic modules is mainly composed of front columns, rear columns, beams, diagonal braces, longitudinal beams (locust strips) and accessories at the connecting part. The front column, rear column, beam and diagonal brace are generally cold-formed thin-walled rectangular steel pipes or round steel pipes; longitudinal beams (die bars) are generally cold-formed thin-walled C-shaped channel steel with inner crimping; between columns, tie rods, and beams Node connectors and bolts are used for active connection; the material is generally Q235B, and the surface is hot-dipped with a certain thickness of zinc layer.
Combined with the characteristics of different regions, the boundary conditions of the bracket design mainly focus on wind load and snow load. According to the actual engineering statistics of multiple ground photovoltaic power stations, when crystalline silicon modules are used, the brackets and installation accessories required for 1MW are generally 42~60t. The amount of brackets and installation accessories required in places with large wind loads and snow loads will increase on this basis; when thin-film modules are used, since the power of each module is smaller than that of crystalline silicon modules, 1MW requires brackets and installation accessories. It is approximately higher than that of crystalline silicon components, and the difference is positively related to the power between the two components.
When designing and checking the foundation of the photovoltaic module support, the self-weight of the photovoltaic support and installation accessories is within the range of 0.042~0.06kN/w, which can generally be estimated by the value of 0.05kN/W.
- Load combination
The load combination mainly considers the dead load and variable load of the structure, among which the wind load considers two working conditions, the working condition I is the wind suction, and the working condition II is the wind pressure. Referring to relevant codes, the combined value coefficient of snow load can be taken as 0.7; the combined value coefficient of wind load can be taken as 0.6.
(1) Partial coefficient of dead load
1) When its effect is unfavorable to the structure:
For the combination controlled by the variable load effect, the dead load factor should be 1.2; for the combination controlled by the dead load effect, the dead load factor should be 1.35.
2) The combination when its effect is strong on the structure should be taken as 1.0.
(2) Partial coefficient of variable load
In general, take 1.4; take 1.3 for the live load of the industrial building floor structure with a standard value greater than 4kN/m². According to the actual situation of the project, the load combination can be divided into the following seven working conditions. During the design, calculation and analysis process of the support foundation, suitable working conditions can be selected for analysis, as follows:
1) 1.2 dead load;
2) 1.2 constant load + 1.4 wind load condition I + 1.4×0.7 snow pressure;
3) 1.2 dead load + 1.4 wind load condition II + 1.4×0.7 snow pressure;
4) 1.0 dead load + 1.4 wind load condition I;
5) 1.0 dead load + 1.4 wind load condition II;
6) 1.35 dead load + 1.4×0.6 wind load condition I;
7) 1.35 dead load + 1.4×0.6 wind load condition II.
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