For solar arrays, tilt angle is money. For sun-tracking arrays, heroic effort goes into optimizing tracking algorithms to tilt the panels for maximum power generation. But the industry is overlooking a valuable optimization parameter: the stow wind speed.
Ground-mounted, single-axis tracking arrays are designed to move into a neutral or stow position during high winds. A key design criterion is the wind speed that triggers this stow regime: too conservative and we lose generating time, too aggressive and we increase the risk of wind damage.
There is currently no industry standard when it comes to designing a stow system. Most designs are based on the designer’s previous experiences. There’s another option, though. We can look in more detail at exactly how the wind behaves at the array site—the local wind climate.
The key to optimizing a stow system is the fact that maximum wind events are not created equal. The issue is not so much what the wind’s peak speed is, but how it gets there. A regional, synoptic storm and a localized thunderstorm have instantly recognizable signatures when wind speed is plotted over time.
In the figure, the maximum gust wind speed for both events is about 50 mph (80 kph). But note the time scale of the event. A synoptic storm takes one days to reach peak speeds; a thunderstorm, under one hour. If you want your array to survive a violent thunderstorm, you’d better be able to put it into stow position on very short notice!
To understand how to adapt a stow regime to a site or region’s wind climate, we need to answer a series of questions. First, these two:
- How long does the array take to get to stow position?
- What wind speed should trigger the transition to stow position (stow wind speed)?
For existing arrays, our only choice is to set the trigger correctly. For new designs, there may be freedom to design mechanical systems to achieve a more desirable regime.
This is where understanding the wind climate can truly pay off. Money spent on high-speed drive system might be wasted on a system destined to operate in the synoptic storms of the Pacific Northwest. The same drive system could be critical to success for a system destined for the thunderstorm-prone Great Plains.
To continue with our questions, let’s assume an existing system that takes 10 minutes to reach the stow position. Now the question is this:
- What is the risk that the wind load will exceed the design wind load during those 10 minutes of the transition to stow position?
And that leads us to our final questions:
- What is biggest change from the mean wind speed during a period of 10 minutes?
- How often does that happen?
- Does the mean wind speed plus the maximum change exceed the design wind loads?
Until recently these questions would have been very difficult to answer. However, RWDI has developed advanced models of historical wind data that are detailed enough to provide answers on this time scale. We apply extreme value analysis to this historical data to detect the maximum change and predict how often that change will occur. We may also adjust the stow wind speed for weather conditions, such as overcast skies or dusk/dawn operation.
With this optimized stow wind speed, manufacturers may adjust the design of tracking motors, and operators may develop stow regimes that consider market pricing. In general, when the stow wind speed is more accurately adapted to local or regional conditions, manufacturers and operators have more latitude in their other optimization choices.
- RWDI Service: Wind Effects on Solar Energy Systems
We help manufacturers optimize the overall wind stability of photovoltaic racking.
- RWDI Service: Climate Analytics and Statistics
Good design—of buildings, infrastructure, communities—hinges on an accurate understanding of climate.
- Gibbons, Mike, Valerie Sifton, John Kilpatrick, Shehroz Hussain, and Kevin McNamara, "Effect of Selective Sampling of Wind Speeds on Parent and Extreme Distributions," 14th International Conference on Wind Engineering, Porto Alegre, Brazil, June 21–26, 2015. This paper reviews the methodology underlying the extreme value analysis used in the application described here.
- DNV GL, “Tracker Bankability Reviews: Guidelines for Stakeholders,” Document No. ENA-WP-19 Issue: C, 17 July 2018, p. 12. The firm DNV GL, as part of their work in quality assurance and risk management, has developed guidelines for independent engineering assessment (“bankability review”) of new solar trackers. It includes a good discussion of the broad issues that need to be considered in designing and reviewing a stow regime.