India’s competitive solar tariffs call for design optimization by trading off cost, reliability and durability. Module mounting structure is one of the important components of Balance of System (BoS). India is a vast country and has varied atmospheric and soil conditions. In view of this single design of structure for different geographies is neither cost-effective nor advisable. Hot dipped galvanized steel having appropriate coating thickness, depending on the environment corrosivity of the region, as column post and pre-galvanized steel/Al-Zn coated steel sheets for Rafter and Purlins have de facto become industry standards. The forces acting on the Module Mounting Structure (MMS) are the module and structure dead load and wind load. There is no separate Indian design code for MMS. IS 875 Part III- 1987 revised in 2015 is used for estimating the wind load. The wind speed in India varies from 33-50 m/s depending on the plant location. The various loads are input to the stress analysis Finite Element Model (FEM) of a basic building block of a MMS. Assumptions are intrinsic to all finite element analysis. The loads and displacements at different locations of structure are valid so long as the assumptions hold. Based on the calculated values of stress and displacement, the safety of the designed structures is ascertained using IS 8002007 code meant for Hot Rolled Steel Structures and 801-1975 for Cold Rolled Steel Structures. MMS have been reported to fail in field despite being design code compliant. Failures are more prevalent in the recent structures ever since the tonnage per MW has been optimized. Interestingly, structural failures have occurred at wind speeds much lower than what they were designed for.
There are several assumptions in the FEM which need closer examination. For instance, because of poor alignment in field the purlins are found not to sit squarely on rafter. Similarly, if there are four clamps holding the module all are not tightened to same torque levels do not have the same overlap with the module frames and hence the joints assumed to be rigid in FEM are no longer valid. The wind pattern inside the solar field is altered because of the array layout and minor details such as gap between Finished Ground Level (FGL) and lowest point of module and gap between two modules have a bearing on the wind load which does not remain uniform as per the initial assumption. This may, therefore, call for coupled analysis involving Computational Fluid Dynamics (CFD) and Stress Analysis which need to be validated by the wind tunnel test. However, given the time constraints, expenses involved and the fact that it does not cause fatality such elaborate study and analyses are seldom done. The design does not take Gust into account but always considers average wind speed. A 3 sec gust may not impact a large structure or building as it will not be able to load all parts of building simultaneously but solar structures being small will be impacted by the short duration gust and are vulnerable to such loads. Also, we treat structures to be dynamically rigid which they are not and hence the resonance effects have to be considered. Solar structure is not considered to be a streamlined body and hence there is a pressure difference on the top and rear sides of modules which leads to vortex shedding. Vortex shedding is an oscillating flow and in case its frequency matches with the natural frequency of the structure then resonance may lead to dynamic excitation. It has been shown and also observed that interior rows of panels are more susceptible to damage than the perimeter rows of panels. Given these design nuances structures have been observed to fail at lower wind speed despite meeting the design codes.
Indian climate challenges solar power plant installers to design plants which can withstand the force of nature for decades. The module mounting structure of a solar power plant has to be designed keeping in mind the expected climate conditions and also the extreme weather conditions. For example, module mounting structure in places with very high temperatures has to be designed in such a way that the metal expansion caused by high temperature can be accommodated. Clearances must be given in the structure so that stresses such as bending stress, tensile stress etc can be relieved by itself.
Corrosion is a common denominator in every climate and biggest enemy of module mounting structure. Using hot dipped galvanized structure with 80-120 micron galvanization is a proven way to ensure a corrosion free structure. The installer needs to ensure that galvanizing coating is intact during the transportation and erection of the structure. Visual inspection of the structure should be carried out post installation to ensure no tears in the coating. Zinc sprays can be used to cover small spots of coating tears. The nut bolts used in the structure should also be stainless steel to ensure maximum strength and corrosion resistance. SS316 grade nuts, washers and bolts offer corrosion resistance for years to come.
Extreme weather conditions should always be taken into consideration while designing the solar power plants. For example, Rooftop solar power plants are installed on high rises and wind loads are considerably high at such altitudes. Rooftops have a lot of structures such as tanks, walls etc which obstruct wind. A rooftop solar power plant can create conditions which can obstruct the wind, trapping the air under the panels. If the module mounting structure is not designed to allow the trapped air to escape, an uplift can be created on the structure. Normal wind conditions don’t affect the structure but during extreme winds the uplift can be considerable enough to damage the structure. Designing proper counter weights, distributing load equally on all supports and leaving enough space to allow the winds to pass on are some of the factors that can be incorporated into the structure design.