According to relevant statistics, in a large-scale solar power station project, the construction and installation cost accounts for about 21% of the total investment in photovoltaic projects, while the investment in solar photovoltaic brackets only accounts for about 3% of the total cost. Therefore, compared with the high investment of solar power plants, the fluctuation of the bracket cost is not a sensitive factor. The cost of selecting high-end brackets is only increased by less than 1%. However, if the selected brackets are not suitable, the later maintenance costs will be greatly increased. The overall consideration is not Cost-effective.
One of the most important characteristics of any type of solar PV module assembly part is weather resistance. It is necessary to ensure that the structure must be strong and reliable for 25 years, and can withstand such as environmental erosion, wind, snow loads and other external effects. Safe and reliable installation, maximum use effect with minimum installation cost, almost maintenance-free, reliable repair, and recyclability are all important factors to be considered when making a choice.
At present, some bracket companies have applied high wear-resistant materials to resist wind and snow loads and other corrosive effects, and comprehensively utilized technical processes such as aluminum alloy anodizing, ultra-thick hot-dip galvanizing, stainless steel, and anti-UV aging to ensure solar brackets and solar tracking. service life.
1. Common forms of photovoltaic brackets
Photovoltaic brackets have a variety of classification methods, such as welding type and assembly type according to the connection method, fixed type and daily type according to the installation structure, and ground type and roof type according to the installation location. No matter what kind of photovoltaic system, the structure of the bracket is generally similar, including connecting parts, columns, keels, beams, auxiliary parts and other parts.
1.1 Fixed photovoltaic support
Fixed photovoltaic support, as the name suggests, refers to a support system that maintains the same orientation and angle after installation. The fixed installation method directly places the solar photovoltaic modules toward the low-latitude area (at a certain angle to the ground), and forms a solar photovoltaic array in series and parallel, so as to achieve the purpose of solar photovoltaic power generation. There are various fixing methods, such as the ground fixing method, such as the pile foundation method (direct burying method), the concrete block counterweight method, the pre-embedding method, the ground anchor method, etc. The roof fixing method has different solutions depending on the roof material. .
The support of the solar cell array is usually fixed by hot-dip galvanized steel or stainless steel anchor bolts protruding from the reinforced concrete foundation. When a concrete foundation is used on the roof of a house, part of the waterproof layer of the house is peeled off, and the concrete surface is peeled off. Weld together the reinforcement of the concrete base for the array on the reinforcement of the patio. When the steel bar cannot be welded, in order to use the adhesion and self-weight of the concrete to resist the wind pressure, the surface of the concrete base is uneven to increase the adhesion. After that, a secondary waterproofing treatment is performed with a waterproofing filler.
If the above method cannot be implemented, a relatively expensive weather-resistant buffer material such as silica gel can be laid on the waterproof layer, a hot-dip galvanized heavy steel frame is installed on it, and then the array bracket is fixed on the steel frame. The steel skeleton is connected to the protruding eaves wall around the house with plastic bolts. The purpose is that the wind pressure will not cause the array and the steel frame to move. Auxiliary reinforcement.
1.1.1 Roof photovoltaic system bracket
The installation environment of the roof photovoltaic bracket includes sloping roof and flat roof. The installation should conform to the roof environment without damaging the inherent structure and self-waterproof system. The roofing materials include glazed tiles, color steel tiles, linoleum tiles, concrete surfaces, etc. Different support solutions are used for different roofing materials.
The roof is divided into two types: slope and plane according to the inclination angle, so there are many choices for the inclination angle of the roof photovoltaic system. For the sloping roof, the tiling method is usually used to conform to the roof slope, or it can be arranged at a certain inclination angle with the roof. , but this approach is relatively complicated, and there are few cases; for flat roofs, there are two options: tiling and tilting at a certain angle.
For different roofing materials, there will be different support systems.
1) Glazed tile roof bracket
Glazed tile is a building material made of soft and hard raw materials such as alkaline earth and purple sand after extrusion and plastic pressing. The material is brittle and has poor load-bearing capacity. When installing the bracket, a specially designed main support member is generally used to fix the lower roof of the glazed tile to support the main beam and beam of the bracket. The support members such as connecting plates are usually designed with multiple openings, which can flexibly and effectively realize the position adjustment of the bracket. Aluminum alloy pressure blocks are used for crimping between the components and the beam.
2) Color steel tile roof support
Color steel plate is a steel formed by cold pressing or cold rolling of thin steel plate. The steel sheet is made of organic coated sheet steel (or color steel sheet), galvanized sheet steel, anti-corrosion sheet steel (containing asbestos asphalt layer) or other sheet steel.
Profiled steel plate has the advantages of light unit weight, high strength, good seismic performance, fast construction, beautiful appearance, etc. It is a good building material and component, mainly used for enclosure structures, floor slabs, and can also be used for other structures.
Roof color steel tiles are generally divided into: upright seam type, bite type (angle chisel type) type, snap type (dark buckle type) type, and fixed part connection (open nail type) type.
Standing Seam Type Bite Type (Angle Type)
Bayonet (Concealed) Type Fixture Connection (Open Nail) Type
When installing the photovoltaic system on the roof of the color steel tile, the shape of the color steel tile and its load-bearing capacity should be fully considered to determine the fixing method of the bracket. The fixing method of the color steel tile roof bracket is mainly determined according to the shape of the color steel tile, as shown in Figure 4: Ground bracket fixing method
3) Concrete roof support
Concrete roof photovoltaic supports are generally fixed with a fixed inclination angle, and can also be arranged in a tiled way. This type of roof fixing method is mainly fixed by concrete foundation and standardized fixed connectors, and it is divided into two types: cast-in-place type and pre-cast type.
The cast-in-place rectangular foundation on the concrete roof is suitable for areas and roofs with small roof bearing and high wind load; as shown in the following figure: 1. The rectangular foundation is connected to the roof with chemical anchor bolts; 2. Standardized fixed connectors are installed on the rectangular foundation; 3. Bracket and component assembly.
A precast rectangular foundation is placed on the concrete roof, which is suitable for areas and roofs with small roof bearing and small wind load; prefabricated standardized fixed connectors on the rectangular foundation.
1.1.2 Ground photovoltaic system
The ground photovoltaic system refers to the photovoltaic system whose installation site is selected on the open ground outdoors. The bracket fixing method of common large ground photovoltaic systems varies with factors such as geology, environment, and climate. Concrete strip (block) foundation form is generally used (special foundation needs to be consulted by professional soil mechanics designers), and pile foundation type, ground anchor type and other methods can also be used.
There are four different foundation forms that can be selected according to the actual situation. Among them, the method of concrete block counterweight and embedded parts is often used in rooftop solar construction or renovation, which can effectively avoid damage to the roof waterproof layer and other structures; ground anchor method and direct buried type It is often used in the construction of solar power plants and has the characteristics of stability and high reliability.
According to the construction experience, the ground anchor method has the strongest construction foundation and the highest safety, but the connection between the ground anchor and the solar photovoltaic bracket needs to be customized, and the cost is very high. In contrast, the direct burial construction is simple and convenient. It only needs to use a drilling machine to open holes and pour concrete on site, and insert the channel steel directly into the hole before the concrete is solidified. However, compared with the ground anchor method, the direct burial The foundation has high requirements on the self-sustainability of the soil on site, and a preliminary geological survey test is required. Of course, if the geological conditions are very secure, preliminary geological exploration can also be omitted.
The arrangement of the primary and secondary beams of the solar photovoltaic support mainly depends on the placement method of the solar panels. In general, the direct burial method is obviously superior to the ground anchor method when the electrical conditions permit.
1.2 Tracking photovoltaic support
When the sun's rays are perpendicular to the panel, the solar energy receives the most solar energy and generates the highest power. But the earth is revolving and rotating all the time, so the angle of the sun's rays is changing all the time. As for the fixed bracket, because the battery panel is fixed, it cannot guarantee that the sun's rays are perpendicular to the battery panel as much as possible, and the solar energy cannot be fully utilized.
Therefore, the tracking system is aimed at the sun as much as possible, so that the sun rays receive more sunlight per unit area of the battery panel, thereby increasing the power generation. At present, tracking systems include single-axis tracking systems and dual-axis tracking systems. Single-axis tracking systems are further divided into horizontal single-axis tracking systems and oblique single-axis tracking systems.
1.2.1 Horizontal single-axis tracking
Horizontal single-axis tracking is suitable for low latitudes, usually tracking the sun's altitude angle to increase the vertical component of the sun's rays in the panel to increase its power generation. The horizontal single-axis tracking system does not simply track the sun's elevation angle, but adopts a complex calculation algorithm to maximize the vertical component of the sun's rays on the battery panel to control its movement angle. This maximizes the photovoltaic power generation. Horizontal uniaxial tracking can generally be increased by between 20% and 30% relative to fixed brackets.
1.2.2 Inclined single axis tracking
The inclined single axis is suitable for the latitude higher than 30 degrees. The latitude angle is compensated by the inclination angle of the rotating axis, and then the sun altitude angle is tracked in the direction of the rotating axis, so as to achieve a better increase of photovoltaic power generation. Generally, the power generation can be increased by 25% to 35% relative to the fixed bracket.
1.2.3 Dual-axis tracking
Dual-axis tracking, with two rotating axes moving at the same time, ensures that the solar panel is always perpendicular to the sun's rays, so dual-axis tracking improves the efficiency of solar power generation. It can be increased by 35%-45%.
2. Load calculation
When installing the solar cell array on the ground or on the roof of the house, as well as on the flat roof of the house, first lay a solid foundation, and then design the bracket. The brackets (supports) are mostly steel structures.
The bracket is used when installing a solar cell array with a height of 4m or less from the lower end to the upper end. When designing the structure, the allowable stress design is taken as the basis, and the design load is based on the equivalent static load. Up to now, there is no design standard for the support of solar cell arrays. If it is considered as an electrical equipment, it should follow the design standard of the power transmission support, and if it is considered as a building, it should follow the construction law and building load. However, these standards have some differences in the consideration of design objects and design methods, and are not suitable for design standards called solar cell arrays.
2.1 Imaginary load
As the imaginary load in the design of the support structure for the solar cell array, there are fixed load with permanent effect, wind pressure load, snow load and earthquake load due to natural external forces. In addition, there are "temperature loads" due to temperature changes, but in supports other than long members of welded structures, they are small compared to other loads, so they are ignored.
①Fixed load (G). The sum of the component mass ( M G ) and the support equivalent mass ( K G ).
②Wind pressure load (W). The sum (vector sum) of the wind pressure (M W ) applied to the assembly and the wind pressure (K W ) applied to the support.
③Snow load (S). Snow load perpendicular to the component face.
④ Earthquake load (K). The horizontal seismic force applied to the support (the seismic load is generally less important than the wind pressure load in the steel structure support)
The load conditions and load combinations are shown in Table 1. For the load combination in snowy areas, set the snow load to 70% of the normal load, and set it to 35% in the event of a storm and an earthquake.
2.2 Wind pressure load
When designing a support structure for installing a solar cell array, the largest load among the imaginary loads is generally the wind pressure load. The majority of wind-induced damage in battery arrays occurs during high winds. The wind pressure load specified here is only applicable to the design for the purpose of preventing damage caused by strong wind.
(1) Wind pressure load at design time
Wind pressure load acting on the array: W = CW×q ×AW where W is the wind pressure load (N); C W is the wind coefficient; q is the design velocity pressure (N/m2); A W is the wind-receiving area (m2) .
(2) Speed pressure at design time
Speed pressure during design: q = q0×α×I×J where q is the design speed pressure (N/m2); q0 is the reference speed pressure (N/m2); α is the height compensation coefficient; I is the application coefficient; J is the environmental coefficient.
For the design speed pressure q, it should generally be calculated according to the following criteria: For the occasions below 16m above the ground and above 16m, the velocity pressure formula should be calculated according to the following principles: the occasions below 16m above the ground: 60; the occasions above 16m above the ground: 1204. Here, h is the height above the ground. In the case of installation 31m above the ground, the wind coefficient is specified as 1.5 or more.
① Reference speed pressure q0. Set the reference height to 10m and calculate it by the following formula: q0= 0.5ρ×V 02 where q0 is the reference speed pressure (N/m2); ρ is the air density and wind speed (N s2/m4); V0 is the design reference ( m/s). The density of air is different in summer and winter. From a safety point of view, the larger value in winter is 1.274N·s2/m4. The reference wind speed for design is taken at the installation site of the solar cell array, at a height of 10m above the ground, and the maximum instantaneous wind speed reproduced within 50 years.
②Height correction coefficient α. The velocity pressure varies with the altitude above the ground, so altitude correction is required. The height correction coefficient is calculated by the following formula: α= , where α is the height correction coefficient; h is the height of the array above the ground; h0 is the height above the reference ground l0m; n is the degree of incremental change due to height, and 5 is the standard.
③Use factor I . is a coefficient corresponding to the importance of the use of the solar photovoltaic power generation system (see Table 2). Generally, the design reproduction period of the wind speed of a solar photovoltaic power generation system is set to 50 years, which corresponds to a use factor of 1.0.