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Commercial Ground Mount Solar Systems: The Basics
Wednesday, September 9, 2020
Ground mount: Mother earth
As roof real estate for solar is slowly dwindling the commercial solar designer and installer is peering down. No, not at their feet but terra firma. Mother Earth.
Commercial solar ground mount systems compared to roof mount offer another layer of complexity on top of existing installation considerations not to mention the extra documentation that is not required for roof-based solar energy systems!
What is a ground mount solar system?
So what constitutes a ground mount system? Basically a ground mount system involves the construction of a free standing steel and/or aluminium structure that is firmly anchored to the ground, in most cases, and supports a photovoltaic array.
The whole system is supported by a series of posts that interface with the earth in a variety of ways.
Ground mount methods
With smaller off-grid systems from yesteryear, holes were dug and hardwood posts with correct durability ratings were used. With the larger solar farms the posts in most cases are pile driven. Lengths of galvanized posts are systematically positioned and then with specially designed machinery, effectively smashed into the ground, similar to the way a star picket is driven in.
Pile drive, super foundation earth screw
Other methods include earth screws, cable tensioning systems and products such as the SuperFoundation which is a metal base plate securely anchored to the ground by a series of pins driven in the earth diagonally. There are also ballast-based systems where the massive weight of the base provides the required structural stability.
Regardless of the method of interface, the commercial solar designers of such systems encounter something that roof mount system designers don’t, and that is soil conditions.
Different types of soil have differing levels of clay, silt, sand, rock and topsoil and even within clays there are differences; reactive and non reactive. Some of the earlier solar farms that utilized pile driving techniques are experiencing severe post leaning due to a variety of factors that include the soil’s reactive clay composition. In some of these circumstances concrete has been added in an attempt to stabilise the array.
Roof mount versus ground mount
Effectively the soil is the ground mount equivalent of the structure of the roof. So, instead of a roof structural engineering document there is a geotech report. And both are subject to wind loading based on the geographic location of course.
What about documentation? With all commercial solar projects there is a considerable amount of documentation when dealing with the distributor and the retailer in addition to all the electrical form requirements but with ground mount systems there is another layer again.
Overlays, plans, regulations
Some of these may include: vegetation overlays, aboriginal heritage overlays, traffic management plans, approval to access Crown land, rare and endangered plant surveys. All of these relate back to the relevant acts, regulations, clauses.
This results in a considerable increase in planning and project management timelines as well as potentially increasing the overall cost of the project. Be careful! Most companies venturing into the area of ground mount commercial systems for the first time learnt the hard way.
When it comes to a roof installation, commercial solar companies will design for the panels to be FTR ( Flat to Roof) or on a tilt. This decision is based on the physical nature of the roof and is weighed against a cost benefit analysis around increased labour costs versus more output etc (see blog on Commercial Businesses and renewables), but with ground mount systems, in nearly all cases, panels are tilted at their optimum for the location.
Where this is not always the case, there is the question of energy density where every square metre of area has a value attached to it. In addition the discrepancy between the feed in tariff dollar value (excess solar production flowing out to the grid) and the price of electricity flowing into the site, coupled with actual site consumption load profiles, has the potential to modify the design.
The basics of ground mount solar are still the same as any roof mount system but there are additional considerations to take into account. Overall project times from inception through to completion are usually greater than with roof mount commercial solar systems and this is where inexperienced solar companies can come unstuck. Like with everything, planning and documentation is of paramount importance.