Guidelines for design and selection of module mounting structures
Module mounting structures play a very important role in efficient working of a solar power plant. While most of the components of balance of system (BOS) such as inverter DC cables junction boxes transformers, etc. are readily bought from the equipment suppliers the workmanship of an EPC contractor is reflected mainly through module mounting structures and wiring management
Recent installation of solar project in India has adopted various module mounting structure design which include the structures made of galvanized iron as well as aluminium structures. Some of the projects have adopted a combination of steal structure and aluminium structure where in vertical legs are made off hot dipped galvanized steel and rest of the members are made off extruded aluminium structure. Typically module mount structure comprises of following components
- Vertical posts
- Vertical rail (Rafter)
- Purlin (Horizontal rail)
- Tilt bracket
- Mounting clips
- Cable tray
The design of module mounting structure should consider local site conditions such as wind speed, rain fall & temp. While selecting module mounting structures. As the solar plant is expected to be design for a 25 years life time, it is important for project developer to ensure that the mounting structure remain until the life time of the project. Mounting structure life is high effected by corrosion and it is important that structure galvanization should be sufficient to protect it from corrosion. The typical structure galvanization thickness is kept of the order of 100-120 microns. In case of purlin the structure can be old formed steal with a lesser galvanization thickness of the order of 80 microns.
Typically the vertical posts are of C section, however recent design use sigma section as well in order to add strength and durability. Some of the structure design use tubular/square section which is good to provide connectivity with a rafter. The tubular section/ square section allow a U bolt and saddle connection to hold rafter. The purlin are lighter members in the structure on which modules are mounted. Many structure design use Z section purlin so that the bottom flinch of the Z section can be use for carrying the DC cables along with it. The purlin is typically rate of 2 mm thickness pre galvanized sheet to protect the purlin from corrosion. It is advisable to use snake tray to carry DC cables rather than carrying cable in the purlin the nuts & bolts use to connect structure member should be made off stainless steel in order to avoid corrosion. The modules are mounted on the structure with the help of module clips which is typically design for a panel thickness of about 50 mm. spring loaded hardware allows clips to be installed before panel is installed. The module structure should be grounded properly and the structure leg should have a provision of grounding strake.
A good structure design can significantly reduce structure weight per MW. It should have an integrated wiring management system so that cables can run through perforated sections it’s important to carry out on site pull-out test in order to decide the structure foundation death.
Many times the structures is design to accommodate single axis/double axis tracking system which make the structure design complicated. Mounting of modules of the structure should be done by torque wrenches so that it does not put excessive pressure on modules. Finally structure design should have adequate provisions to accommodate thermal expansion so that sagging in structure do not occur.
Dr Sanjay Vashishtha & Kapil Dev Sharma