Smart PV design for tough terrains and wicked weather
by Michael Faraone Ph.D., P.E.
Featured in North American Clean Energy, May/June 2020 issue, Page 18 – 19
Tough climates and ground conditions should never hinder your ultimate solar success. Deploying durable, economical solar plants in extreme conditions means knowing the environmental and geotechnical challenges presented by strong winds, extreme frost, and steeply-sloped topography. Developers and EPCs can harness versatile racking products to ensure project reliability through the most arduous environments, be it a blizzard or a rock-strewn hillside.
Standing Up to Snow
Dense mounds of snow can pile up and place potentially damaging stress on solar racking structures, particularly on hilly or sloped terrain. Snow loads can be determined from snow maps provided by the American Society of Civil Engineers (ASCE), and certain locations may require data from their own case study due to extreme variation in elevation and climate. Projects in northern environments are built with front edge heights of 30 inches or higher to mitigate snow banking on the racking structure.
In certain regions of the country (like the foot of the Rocky Mountains), withstanding “high winds” refers to accounting for wind speeds in excess of 170 MPH. Depending on the tilt angle of the racking structure, significant lateral and uplift force can be imparted on the foundations. Make sure your system is designed in accordance to ASCE guidelines, as well as local design criteria, where terrain can have a big impact on wind speed.
Frost heave is another element that wreaks havoc below ground. When freezing temperatures penetrate soil, they cause an upward swelling that warps foundations, which can lead to costly damages. PV plant’s foundations are subject to adfreeze, in which the frozen soil adheres to the steel surface of the foundation, resulting in an uplift force known as frost jacking.
The key to selecting the right solar racking structure starts from the ground up. For sites with high wind speeds, you must ensure that the foundation has adequate uplift and lateral capacity (this can be confirmed through load testing of the project site). When combating against frost heave, embedment below the frost depth line is key. Longer driven piles can be specified with lengths at three times the frost depth to prevent damage from frost heave. Unfortunately, in addition to increasing costs, driving 18- to 30-foot piles is unwieldy and increases the likelihood of refusal upon striking bedrock and other impediments. Foundations like ground screws are a cost-effective means of resisting frost heave. Ground screws also mobilize more tension with a smaller foundation verses a pile, because the threaded portion is embedded below the frost depth line.
In the case of high wind and snow loads, be sure to select a racking solution using the latest steel design codes with load testing, to confirm that it can accommodate extensive amounts of snow and wind. Furthermore, consider choosing racking with flexible sizing, which will reduce the number of modules per foundation, and decrease loading on the system. If foundation counts become too high, look at increasing the size of the structural support members to provide a rack that is both structurally efficient and cost-effective.
When contending with high load environments, confirm that the module selected has the appropriate capacity to meet the applied downward and upward pressure. Various mounting methods (clamps, bolts, etc.), and mounting locations for PV modules can have their own individual load ratings; these need to be checked to ensure they meet the project’s loading criteria.
Trackers – In terms of tracking solutions, look for products with intelligent controls that secure systems with weather stow functionality. Onsite weather stations monitor severe conditions, automatically stowing the site when certain thresholds are crossed. Find systems integrated with a weather API to proactively stow your sites before a big storm hits. Also, make sure the tracker has undergone extensive Wind Tunnel Analysis to confirm additional loading due to the dynamic stability of the tracker.
Below the Surface
Rock, clay, sand, and other tenuous ground will impact the foundation that should be used. Knowing the correct solution to choose will help mitigate any subsurface risk. When working with bedrock, caliche, volcanic rock and glacial till, for example, solutions include pilot hole drilling with a ground screw foundation, or pre-drilling for piles using a concrete encasing. Unlike piles, screws can be embedded without ground modification, and efficiently drill past cobble and buried rocks without refusal — reducing upfront construction costs and eliminating subsurface risks. For extremely soft soils such as clay, helical piles are a common choice given their large single flight, which helps secure the ground anchor within the soil.
Run for The Hills
Hilly sites require racking structures that are highly adjustable to accommodate undulating ground with relative simplicity. This adjustability can result in huge savings to help minimize civil work and grading on a project.
Most racking vendors are limited to working with slopes up to 20 or 25 percent. Assess your racking to suit every terrain option with an eye toward nimble design, with connections for slope adjustability up to 36 percent. Trackers are more limited when it comes to high sloping terrain, but can be configured to work up to 20 percent.
Getting to Work
Uneven solar locations often require civil work to level your site for mounting. Civil work isn’t feasible for every project, as operations like grading can raise costs and/or alter precipitation runoff patterns that may not be acceptable to local code. Make sure to run a geotechnical report to assess rugged ground conditions via in-situ field tests. Worker safety is paramount when installing systems on rugged sites. As large equipment often can’t be used on steep slopes, consider smaller or more agile equipment to clamber up bumpy terrain.
Ultimately, careful evaluation of weather and terrain conditions – along with appropriate sourcing of products suitable to those conditions – helps safeguard plant integrity and performance before the first support structure is put in place.
Michael Faraone Ph.D., P.E. is Director of Engineering at TerraSmart, which has built over 3 GW’s of ground mounted, utility-scale solar projects across the United States. Dr. Faraone joined TerraSmart in 2016, and has been an instrumental leader of the engineering department to develop, analyze, and test the company’s innovative solar-racking solutions. His experience in deep foundations and extended studies in geotechnical engineering has earned him the nickname “Dr. Dirt.”