Page 18 - North American Clean Energy May/June 2020 Issue

solar energy
Hardcore Solar
Smart PV design for tough terrains and wicked weather
by Michael Faraone Ph.D., P.E.
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.
EXTREME WEATHER
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.
Withstanding Wind
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. Wherever terrain can have a big impact on wind speed, make sure your system is designed in accordance to ASCE guidelines, as well as local design criteria.
Fighting Freeze
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 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.
Foundations
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.
Ground Mount
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.
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MAY•JUNE 2020 /// www.nacleanenergy.com