The foundations of successful large-scale solar farms
On any large-scale renewable energy project, every opportunity to reduce costs counts.
Utility-scale solar farm projects require significant investment. So to keep total project costs under control and ensure success, it is vital that every step in design, construction and operation explores the most economical ways to create an efficient and safe project for the long term.
Although the civil engineering components of site development for a large-scale solar project may be smaller than the structural or electrical packages, civil costs during construction could add up to 8-10% of the total project cost – and that’s nothing to sneeze at.
Upfront consideration of costs and risks – both short-term and long-term – in the civil design of the plant can make a sizeable difference to the project’s total cost of construction, cost of long-term maintenance, and long-term financial returns.
Achieving the best and most economical civil works design means thinking carefully about the specific conditions of the site and the expected operating conditions of the project over 25 to 30 years, taking into account the potential for these conditions to change over time.
Getting the foundations right
Utility-scale solar farms need ground mounting systems and layout that suit the specific topography and geology of the site. Identifying the optimum solution in these areas is a key part of achieving a cost-effective design.
The mounting system functions as a foundation structure, and can also set the orientation and elevation of the PV arrays to maximise energy performance. Orientation, tilt and elevation need to be addressed in the civil design, for example to work out the PV panels’ ground clearance or likelihood of collision. As PV panels are lightweight structures, design of a stable foundation must also attend to vertical uplift and lateral resistance, in accordance with the relevant standards.
The various brands of tracking systems all compete to provide cost-effective solutions, particularly to address the challenges of undulating sites – so research the range of options to find the best fit for your project.
A lack of proper geotechnical ground investigation can lead to selection of inadequate foundation types, resulting in costly variations and delays. On-site pull tests will help support and augment the recommendations of the geotechnical study to optimise selection of a cost-effective foundation solution.
Installations that follow the topography are likely to allow a reduction in earthworks (cut/fill) and allow easier and more consistent choice of pier height and length per row. Shorter rows can follow the terrain better than long rows, but this is not the only criteria for selection. You should also consider aspects such as cost of cable management, extra motors and installation time.
The height/length, material and foundation options for piers become very significant when dealing with thousands of piers per solar farm. This means that you need adequate representation and quality assurance for design outputs.
Each individual pier height should be determined from detailed topographic surveys and computer modelling, taking into account the different minimum heights required to avoid ground collision, and the maximum heights that will be most economical for long-term operational and maintenance.
Another consideration in the search for cost savings is balancing the distribution of piers against the cost of earthworks, which will vary depending on the depth of cut and fill required.
There are a number of different foundation options, such as driven steel piers, screw piers, and precast concrete footings. But determining the appropriate choice must be based on a comprehensive understanding of the geology and geotechnical model of the entire site – which may vary across the large area of land occupied by the solar farm.
Rocky sites may result in driven piers reaching premature refusal. Sandy sites will vary in cementation and looseness, influencing any attempt to have a constant pier ground embedment. And clayey sites are prone to shrink-swell changes depending on changes in the level of moisture due to flooding or drought.
Planning for a changing future
For prudent design, you need to consider how the site conditions could change over the whole life of the solar farm, as early pull tests won’t predict future conditions. For example, how could the site be affected by flooding or by erosion – both now and under future climate conditions?
Thorough stormwater assessments (hydrological and hydraulic) are required to develop a good understanding of the existing (before development) and proposed (after development) drainage patterns of the site. If the site development alters the pre-development conditions, sediment control measures will need to be put in place.
The stormwater modelling will help to quantify the extent of flooding and potential for erosion.
For flat sites, allow for inundation (with major impacts on construction and potential deterioration of the site, trenches and foundations); for undulating sites, expect flash flooding and potential erosion and transfer of sediment.
Floodwater heights might affect solar farm infrastructure such as tracker motors or combiner boxes, so these will need to be installed at safe heights to minimise major potential repair or replacement costs.
Stormwater assessments will also quantify erosion potential around foundations, so that you can put suitable control measures in place, such as allowing for internal and external discharge points and armouring erosion-prone areas with riprap or erosion protection geotextiles.
Beyond the solar array
A number of other factors need to be considered in the civil works package, all of which have potential for efficiencies to reduce cost and avoid delays.
Roading alignments (horizontal and vertical) must consider the different needs of the construction phase and the longer-term operational period. Design of earthworks and road surface should take account of potential impacts on drainage patterns and potential erosion at low points.
Cable trenching should address interfaces and collisions with other infrastructure, and the need for backfill to meet requirements for thermal resistivity and strength.
Fencing is required for adequate site security specifically during operational life, but must be considered along with the need for access at various stages of the project. Fencing insulation and earthing needs must also be assessed in the design.
Civil design will also address the requirements of hardstand and laydown areas, particularly for the demanding construction phase.
Designing the risks out of your project
Based on our experience, we recommend that the civil design of a utility-scale solar farm should go through three phases: concept design, optimisation, and detailed design. If you rush from the concept design phase into detailed design, you’re opening your project to risks.
Significant optimisation will then be needed during the detailed design phase, which could potentially alter the concept design. That means rework, costing you time and money.
To achieve a civil design that avoids project risk and sets the right conditions for cost-effective delivery, be sure that you have timely, accurate and comprehensive topographic and geotechnical data about the site, that you carefully consider your earthworks material and handling, and that your systems and equipment interface all infrastructure effectively.
Keeping construction budgets under control and schedules on track depends on all the careful, detailed work done across all the aspects of the project at the design stages. Get your civil works right, and you’re tilting your solar farm towards success, whatever the weather.
About the author
Al Ahmed-Zeki is a Specialist Civil Engineer with more than 30 years of civil engineering experience on renewable energy assets, including dams, solar farms and wind farms. He had led the civil design for a number of major solar projects from pre-bidding assessments through to due diligence and detailed design. Risk management is one of his prime interests. Al has published a number of technical papers and presented in the fields of geotechnics and dams engineering.
September 12, 2017