Issues in designing and layouts of solar mounting the structure
Large scale MW class solar PV system spread over 25 to 35 Acer area for a typical 5 MW plant. Appropriate layout of arrays becomes very important for optimized cable length as well as optimized energy production. A typical solar plant of 5 MW capacities uses about 250 to 300 Tons of steel structure and about 22 KM of DC cables. While designing the plant layout it is equally optimizing the cable size, length and as well as structure used for the plant. Over the year, module structures are available in pre engineered form in term of steel as well as aluminum structures. Although aluminum section are costlier but light weight and can provide good finishing to a solar installation. The strength of aluminum structure is also a concern and hence, most of the solar plant installed in India has selected steel structure for module mounting. Most of the structures are pre fabricated structures and don’t require field modification such as cutting, drilling and welding. The typical steal structures used are of hot dipped galvanized steel or pre galvanized cold rolled steel. It is important to take care of the galvanization and recommended that steel sheet section should be hot dipped galvanized and has a minimum galvanization level of 100 – 120 Microns in order to avoid correction during the project life of 20 years. Module mounting structure are long sections running from east to west for about 20 m and thus middle support through legs at a distance of about 3-4 m each. Steel mounting structures are prone to thermal expansion and the co-efficient of thermal expansion of steel is 15*10-6 m/ deg C. Hence, the provision should be made for sliding of 1 member over another member through a proper sliding joint in the mounting structures. The section should also be selected in such a way that they are capable of incorporating better DC cable run management to protect the conductors and help installers for providing proper support to the array wiring.
Many times the solar field terrain is uneven and requires variable leg length to achieve the level installation and avoid the shading of one table over another table. It becomes critical if the liner shading is not taken into consideration while designing the differential heights of the table. Selection of foundation is also one of the important factors for the structure design and layout. Where ever excavation/ drilling is possible, concrete structure are recommended, however, some of the project developer use ramming rather than concrete structure to save time and money. Rammed structures are more prone to corrosion and life expectancy for 20 years becomes critical. The structures vertical member should be designed in such a way that they can accommodate variability in height from one table to another table.
Apart from the structure member clamping of module is also one of the critical consideration to mount the module on the structure, structure frame, there are various top down clams are available which can hold the with frame and frameless module efficiently. In case these considerations are not made during array installation the structure faces sagging and the tilt angle get disturbed. If the thermal expansion is not accommodated properly during the design phase in can create lot of problems to hold the module at the frame. Especially in case of frame less module, if the installation is not proper thermal expansion is leading to sagging. Modules breaking rate will be very high in solarplant. In a typical installation, we have observed that one of the developer has got module failure of the order of 1/3 of the total installation of the plant just because the mounting structure were not properly designed during the designed and installation phase/. Some the photographs during our projects have been highlighted in this article for better understanding of module mounting and installation defects and best practices in mounting systems.