1. Ground installation system
There are two main application scenarios for ground-mounted photovoltaic systems: usually in residential or commercial parks where rooftop installation is not feasible and there is a lot of ground space, or in super-large photovoltaic systems. Ground-mounted systems have many advantages: First, the installation inclination and orientation are not restricted by the shape and direction of the roof or building surface, so the photovoltaic array can be installed according to the optimal inclination and azimuth; the installer can install the photovoltaic system on the ground plane, thereby Avoid working at heights. Ladder and elevator are no longer used during the installation process, which also saves time for some system installation projects, but the ground installation system may also require civil construction, trenching, and wind and snow protection. The material, labor, and engineering costs associated with installing photovoltaic arrays on the ground will increase, which may not be economical for many small and medium-sized grid-connected north-volt systems.
The installation of this type of system should comply with local building laws and planning regulations, and may also need to submit a planning application for the local government. A certain distance should be maintained between the system installation location and the boundary. This requires a surveyor. It is also important to avoid damage to groundwater facilities and electrical and telecommunications cables. By contacting local water authorities and power companies, mastering a map showing potential hazards to identify these risks. Many places already have services such as “call before excavation”, which are dedicated to this purpose and the construction industry.
According to local regulations, it may be necessary to install a fence to prevent authorized personnel from entering the photovoltaic array area. For ground-mounted photovoltaic arrays that are easy to see, it is usually recommended to install fences to prevent theft and looting. Large-scale photovoltaic systems may even include security cameras, motion sensors, and advanced monitoring devices to reduce spying.
①Floor bracket installation method
The ground bracket installation method is similar to the roof bracket installation method, and it is more flexible in the orientation and inclination of the photovoltaic array. The ground bracket installation method is a fixed inclination installation system, usually using prefabricated steel or aluminum array plate racks to firmly fix the photovoltaic modules in place. Photovoltaic modules are usually clamped or bolted to the frame, and the frame is fixed in the concrete foundation. In locations with suitable wind and soil conditions, screw piles or cement piers can be used to reasonably replace concrete foundations.
Generally, large-scale ground-mounted photovoltaic systems require photovoltaic arrays to be installed in rows to conform to a given space. The disadvantage of this method is that the photovoltaic array may block its rear modules. As described in Chapter 3, even a small block on the photovoltaic assembly will greatly reduce the output power, especially in the morning or evening when the sun is low. , The photovoltaic array projection is very long. The installer must choose the appropriate row spacing to avoid blocking during most of the year: this needs to be calculated with a triangular relationship. For most areas outside the tropics, the general rule of thumb is that the minimum distance between rows should be It is 3 times the height of the installation structure. The provider of the installation hardware usually provides the necessary computing software. The closer to the tropics, the array spacing may decrease. However, since shading is an important concern, a complete shading analysis should be performed before the system is installed. Ground-mounted systems should also consider maintenance requirements. For example, if the array is installed in a field, the cost of mowing must be considered in the maintenance cost.
②Pole installation method
For systems with fewer photovoltaic modules, pole-mounted installations are more popular. The main advantage of this type of system is that it is cheap, because it does not require a lot of installation materials and is usually adjustable. The installer can adjust the tilt angle according to the season, so as to ensure that the photovoltaic array is at the optimal tilt angle for most of the year.
③Sun tracking system
The sun tracking system is a mechanism that rotates the photovoltaic array to ensure that it faces the sun, so that the photovoltaic module can maintain the peak power output for a long time in a day. This type of system is much more expensive. However, in places where space is limited or where the tracking system can increase a lot of power, the increase in the cost of the tracking system is reasonable in the entire life cycle of the project.
Most tracking systems use sun position detection sensors or single-chip microcomputers to calculate the sun’s position, and then use motors and gears to rotate the photovoltaic array. There are two main types of tracking systems: single-axis trackers and dual-axis trackers. The single-axis tracking system uses a rotating shaft to rotate the photovoltaic module to track the sun’s trajectory in a day from east to west. However, the dual-axis tracking system tracks the sun’s trajectory along two axes: similar to a single-axis tracking system, it tracks the sun’s trajectory in the sky; it also adjusts the inclination to account for changes in the sun’s altitude during the year (that is, when the sun is in winter). The position in the sky is low, so the inclination angle of the photovoltaic module is relatively steep).
The disadvantage of the tracking system is that there are many rotating parts, which increase the maintenance cost during the life cycle of the system. Due to the addition of these rotating components, the risk of mechanical failure of the tracking system is higher than that of the fixed inclination system. For single-axis tracking or dual-axis tracking, the increase in output power of the photovoltaic array should be calculated, so that the economic benefits of the tracking system can be used to subtract the cost of materials, installation and maintenance. The tracking system can also use case light to improve tracking efficiency. Concentrator systems are still rare for practical engineering and academic research.
2. Wind load
Wind load is used to describe the wind force that the photovoltaic module bears, including the suction or lift force generated on the photovoltaic module when the wind blows through the photovoltaic array, and the downward pressure or side shear force generated by the strong wind on the module. Unless the photovoltaic array can withstand the wind load that may occur on site, the array is not installed safely. The wind speed varies greatly around the world, so it is very important to ensure that the selected components and installation systems are suitable for on-site use. The module installation manual or data manual specifies the maximum load rating of the module, and these information should be consulted when selecting the module.
The influence of wind load also depends on the height of the photovoltaic array, the inclination angle and the exposure of the photovoltaic modules to the sun. In order to minimize the wind load, the photovoltaic array should be installed parallel to the known wind direction. If it is installed on a roof, it should be kept at a distance from the corners and the edge of the roof. The design and installation of the photovoltaic array structure must meet the on-site wind speed requirements, so the array will not bring up the roof. This should be closely integrated with local standards (see Resources section), because the local markings reflect local design conditions.
Under normal circumstances, the structural engineers of the manufacturer and supplier of the photovoltaic array frame or structure will conduct a ground survey to make it meet the specific wind load conditions of the country or region of sale. This kind of design scheme should ensure that the strength of the metal parts in the frame is sufficient to withstand the wind load, so the wind load calculation of the structure is generally not a problem. However, the key factor is the attachment point where the mounting structure is connected to the roof. The key considerations of the design plan are to determine the strength of the attachment and calculate how many attachment points are needed to fix the installation structure on the roof. The installation system sold by the manufacturer usually includes guide rails and fixed brackets, which are accessories required for installing photovoltaic arrays. For example, hurricanes, tornadoes or typhoons may occur in tropical areas, so more roof fixings and mounting brackets are required. The installation of knots should comply with local wind exposure regulations, and it may be necessary to seek qualified engineers to help obtain structural certification to meet corresponding requirements.
3. Lightning protection
The component frame and mounting structure are almost all made of metal, which is a good electrical conductor. The installation structure generally needs to be grounded, and relevant national electrical codes and standards need to be reviewed, because such codes vary greatly from country to country. Incorrect lightning protection can increase the risk of damage to the photovoltaic array or building under lightning conditions.