Wednesday, 23 November 2016

Top 3 Learnings for Solar Mounting Structures Design

AGAMI has been fortunate to have worked in a broad range of areas of module mounting solutions for Solar Utility Projects. We have done designs of mounting systems as well as peer reviews. AGAMI might be one of the few , if not the only consultant in India currently, to have done a full-fledged structural design for a Tracker Structure. This provided very deep insights into wind tunnel studies, dynamic failure modes of structures in wind and quantification of complex wind effects.

As this is a nascent field, and the engineering consulting industry is taking its first steps, there are many critical errors in the design of these seemingly simple structures. The below is a round-up of common errors that consultants/EPC design teams need to watch out for, which can save your project in extreme wind events.

Learning 1: Forgetting the “NOTE 1” in Wind Load Application from IS 875:3

IS 875:3 Table 11: Monoslope roof coefficients, which is the wind table referred for all solar MMS design, has hidden the most important criteria which has the ability to govern design of your members as a footnote, which reads thus -
..the center of pressure should be taken to act at 0.3*w from the windward edge.  
In Layman terms, what this means is, the wind pressure is not symmetric but is BIASED towards the windward end. This is to be expected , for why would something as complex as wind, be completely uniform across your panel?

Applying this criteria results in lot of bending moments or torsions in most systems, and has a significant impact. This has been caught time and again in our peer reviews with large EPCs, as also with reviewing designs of other consultants, and is indeed a “rookie” mistake many commit.

Learning 2: Providing Specs to the Geotechnical Agencies

The geotechnical agencies investigating the parcel are often given no idea of what kind of structures would be coming , and what their output needs to be. The report often comes up with recommendations that are more aligned towards normal Building structures.

For e.g. the recommended “type” of foundation and example calculation in the report would be of a PAD FOOTING, at 3m depth below ground. Whereas a pad footing is never used in Solar MMS.
This creates various problems, say if the first 2 metres of soil were problematic, the geotechnical consultant solved the problem by saying give pad footings at 3m, visualizing a building coming up in the parcels. Other times, the report would have no testing data about first 1-1.5 m of soil.This results in bad foundation design, based on interpolations and speculations where none is needed.

Hence, detailed specifications are necessary to prime a Geotechnical agency into what needs to be done w.r.t. Solar Projects. The geotechnical specs should provide info about the type of structures, and request for calculation of capacities of a bored pile 1.5 or 2m deep.

Learning No 3: Not Neglecting Dynamic Effects for low tonnage designs

It is seen that many large EPCs are doing some interesting experiments in module mounting to reduce tonnage per MW. Already most EPCs are in the 30 tons range, and still aggressively pushing further down. The approaches are based mostly on using very high strength steel or galvalume, which has twice the strength of usual steel. This creates very thin structures that pass all the static tests of design.

What is often neglected is that, as the structures become more slender, dynamic effects become more powerful. Wind creates vortices at a particular periodicity, and if you are unlucky and the period of oscillation of your structure matches the vortices, resonance happens. The structure will then most probably fly off or damage a lot of mounted equipment. A more comprehensive analysis of the adequacy of the structure is required at such competitive levels of tonnage.
Nimish Prabhukhanolkar, B.Tech/M.Tech IIT Bombay is Partner/Founder at AGAMI ENGINEERING CONSULTANTS where he heads the overall technical solutions by AGAMI.

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