Page 34 - North American Clean Energy May June 2018 Issue

solar power
Cost-E ective PV in Northern Climates
Getting it done right from the start
by David Pichard, Rob Souliere and Eugene Koval
The right engineering
Making the numbers work is one thing, but making sure you’re using the right numbers is something else entirely. Use the standard engineering
ASCE 7 guidelines, but research local standards carefully. In areas where PV development is new, engineers need to consider regional building codes for other types of projects, to help them learn about local snow loads, wind shear, and frost heave potential.
Foundation designs in particular must take geotechnical forces into account. When water freezes and expands in subsurface soils, it can create ice lenses that shift and push upward, carrying solar rack foundations along with them. Unfortunately, the rate of this frost heave is dynamic and can occur at over
an inch per day, depending on local conditions, including the soil’s moisture depths. Heaving foundations can ex structures, crack modules, and cause catastrophic damage to conductors and grounding.
Check what historical records show for the depth of subsurface frost. Designs that stipulate a range of three to nine feet, for example, are not speci c enough to counter frost heave. Pile lengths must be long enough to meet the deepest frost numbers. How much snow accumulates in the region? Di erent foundations and designs can accommodate varying weights. What are the region’s maximum wind speeds? Mounting designs can handle the stresses if local data are taken into account.
Key design features and decisions should include:
• Install as much pile material as necessary (size, depth) after running a proper
pre-production test program. Reduce frost heave risk by installing accurately engineered helical piles, which minimizes the true cost per kWh produced, without compromising on the ground anchoring of the PV system and increasing maintenance costs. Driven piles aren’t suited to areas with deep subsurface frost.
• Reduce mechanical connections. Avoid systems that require lubrication, and use bearings, which can fail during the natural expansion and contraction cycles in areas with extreme temperature ranges.
• Select enclosures, cabling, conduits, wire management etc. adequately rated for a wide range of temperature variance and exposure to harsh conditions - which most people do. But also ensure the installation and placement of BOS fully takes advantage of the higher spec products: installed right with easy access, wherever preventative or curative maintenance is needed.
• Limit cut-and- ll operations to preserve existing grades. Precipitation and wide temperature swings destabilize disturbed soil more readily than untouched ground. Look for state-of-the-art top of pile engineering, combined with advanced racking, that adapts to challenging topography.
The right products
Resist the temptation to go with the least costly options; seek out quality components that have withstood a range of third-party testing and certi cation processes. Choose parts
built to tolerate the maximum stresses recorded in the region. Wind tunnel checks, accelerated life-cycle testing, and cyclic corrosion studies show proof of quality.
Temperature range studies are a vital tool. Inverters, cabling, and mounts should show both lab-based and in-situ performance measures. Panel stringing components and designs should take both snow load and temperature extremes into consideration.
Select racking structures or trackers with the right corrosion resistance,
from galvanizing applied to the steel components. For single axis trackers, speci cations should include how well the system holds up in sustained winds in any operating position (ideally over 40 kilometers per hour) and in stow position (a base design rate over 140 kilometers
• LowProfile Construction
• LargeO set Adjustment
• Unique Cutting Swing Arm Prevents Damage
• HeavyDuty
CAL-AG ENTERPRISES
2020 S. Golden State #106, Fowler CA 93625 Cell: 559-779-8703
Email: [email protected] Contact: Jose Sanchez
SUPERB HORTICULTURE
2811 Michigan Rd, Plymouth IN 46563 Phone: 800-567-8264
Email: [email protected] Contact: Keith Norman
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MAY•JUNE2018 /// www.nacleanenergy.com
What works in the desert won’t last on the northern prairie. As utility-scale PV grows beyond its traditional sun-soaked environments, developers and EPCs have faced a steep learning curve. Experience gained in the desert doesn’t necessarily translate into designing for the geotechnical and environmental challenges of northern frost, snow loads, and strong winds. What is accepted as cost-e ective in traditional settings, can prove disastrous under extreme conditions.
2.7MW Project in Lake Waconia
This fixed-tilt system in Minnesota was constructed during winter 2017. Winter construction poses many challenges, including poor irradiance, cold temperatures, snow load on the modules, low PV production, and impaired road access to the site. Ideally, all civil work should be concluded before the seasonal ground freeze.