When Albuquerque, N.M.-based Array Technologies Inc. (ATI) unveiled its DuraTrack HZ v3 single-axis tracker last year, it was the culmination of an engineering process that sought to make the systems more survivable in the field – particularly under conditions of high wind and snow loads.
John Williamson, ATI’s engineering manager, says both the new and previous versions of the system, which the company still installs and supports, have been designed so that the trackers will withstand wind and snow loads as required by code in whichever position they happen to be in. The main difference in the latest version is a torsion mitigation system.
“Whenever you have a very long row of trackers, if you are holding them rigid at just one point, as with a drive train or a gearbox, all of the torsional force on the row is concentrated onto those components,” Williamson says. “With the torsion relief system, we have alleviated that while still designing the system for the lateral loads that are caused by wind and snow in any position to prevent damage.”
The torsion mitigation system incorporates stops into the column of each unit that distributes those torsional loads instead of concentrating them in the center. Moreover, the feature operates without power, so the torsion mitigation system functions in the event of a power outage. This, plus the design requirements to withstand loads in any position, is intended to prevent damage over the system’s lifetime.
According to Williamson, the design philosophy of the HZ v3 was the result of careful analysis of how the HZ v2-series installations performed under loads. One interesting observation regarding the aerodynamics of tracker systems is that panels at a higher angle tend to be under higher lateral wind loads. A commonly used counter to this is for trackers to have a stow mode for conditions of high wind. Such a stow mode typically puts the tracker into a horizontal position. Williamson says that this can produce troubles of its own.
“When you are at very low angles, you actually have higher torsional loads,” he says. “So, it is not exactly true that putting something at a low angle is going to protect it. We’ve had several tracker systems in wind storms. We have observed that the installations that, ironically, were at low angles produced more issues than the ones at high angles. Since we design for all positions, the structures were still standing either way. But, we had some minor damage on some of the systems that were at low angles.”
In trying to understand why, ATI’s analysis showed that the problem was related to the concentration of the torsional forces combined with torsional instabilities in the systems. Engineers focused on protecting the systems on the HZ v2 series with dampers to prevent this type of damage in wind events if the tracker is at a low angle.
For the HZ v3 system, instead of going with a flat stow position, the wind naturally wants to push the array into a vertical position. As the array gets more vertical, the system actually becomes more stable against the wind, Williamson says. Through the aerodynamic characteristics of the system, it naturally will want to move into the stops, which are the system’s normal limit for operation – plus or minus 52 degrees from horizontal.
In addition to enabling the trackers’ aerodynamic properties to move the system into the stops under wind loading, Williamson says the arrangement serves to counter the effects of snow accumulation on the panels. The 52-degree angle typically dumps snow that accumulates during the course of a snowfall.
One observed phenomenon related to this is that repeated snow dumps tend to produce a pile of snow underneath the edges of the module frame. When such a pile melts and then refreezes, it can turn into a serious obstruction. If the new system runs into such an obstacle, Williamson says, the torsion limiter will prevent the tracker from continuing to push it, which should mitigate damage, as well.
“One thing that is normal for any array is change,” he says. “We are constantly evolving our systems as we are installing them – learning about what is working and not working so well. That is the role of observation and engineering. We make little tweaks to the product to make it better, cheaper and easier to put together.”
Michael Puttre is a freelance writer based in New York.