Built to withstand the combination of powerful wind, snow, seismic and dead load, fixed-tilt and tracking ground-mount systems must be engineered to deal with two separate but equally powerful forces: bending and torsion.
Bending occurs when external loads are applied perpendicularly to the horizontal length of the mounting system. Torsional forces occur when a vortex forms above the leading edge of the mounting system, creating significant movement about the center chord, building up at even relatively low wind speeds.
Fixed-tilt systems effectively mitigate bending forces through c-channels, which can be perfectly optimized to handle the loads of any given project through changes in profile and gauge. These systems have an even greater advantage when it comes to torsion. With a strut at every post, rotational forces can be transmitted into the strut, then the post and ultimately the ground. The torsional forces don’t accumulate along the length of the structure.
In the case of tracker systems, dealing with bending and torsion is more difficult and complex. Typical tracker designs have relied on torque tubes to serve dual duty and handle both types of forces. As with fixed-tilt systems, trackers are engineered so that bending forces are transmitted through the posts. The torsional forces, however, accumulate along the length of the torque tube resulting in large forces at the center of the row—typically at the point of the slew drive or actuator.
As the torsional moments add up along the length of the row, the row lengths become limited as there is minimal support at each row end. This limitation creates design constraints and does not allow designers to fully optimize site layouts without compromising on cost by either creating a lot of variations in the torque tube sizes or standardizing the size but inherently having waste material. With more and more tracker projects being deployed in increasingly varied environments, manufacturers are re-evaluating traditional tracker design and looking beyond the torque tube approach for more effective and efficient ways to lower costs, mitigate dynamic torsional forces and create a more optimized project.
Common Approaches to Dealing with Dynamic Forces
The general industry term “dynamic forces” is really two separate items. The first is the “resonant vibration” and the second is “aeroelastic instability.” The resonant vibration effect shows up in bending, swaying and torsional modes. Torsional effects are not present in fixed-tilt systems, but are a concern on trackers through the posts that allow the torque tubes to rotate freely with the only backstop in the middle of the row at the motor.
The amount of additional force present due to torsional dynamics depends on two factors. The first is the natural frequency of the system. The second factor is the dampening of the system. The higher the frequency or dampening, the less pronounced the effect.
The solutions to minimize resonant vibration are fairly straightforward. A manufacturer can add material to increase the natural frequency or add friction and/or dampeners to increase the dampening effect. Both of these are expensive with significant failure risk. However, even if a manufacturer minimizes the effect of the resonant vibration, the additional loads and the capacity of the system will increase by 20 to 50%.
Aeroelastic instability or torsional galloping is different from resonant vibration and occurs when the wind vortices create a torsional load at the ends of the tracker row (where the torsional stiffness of the torque tube is reduced) that alternate between the upper and lower surface of the module. The torque tube acts as a spring and can potentially be a runaway reaction that completely destroys the structure. The solution here is to stow the tracker at a different tilt angle around 20° to minimize this effect. Doing so, however, will increase the loads on the tracker and will require additional materials to increase capacity of the structure as seen in the chart above.
Trackers have historically used several common design strategies to deal with the instability resulting from the vortices forming on and shedding from the leading edge as it twists up and down:
- Array Design – To mitigate torsional loads, row lengths have been limited to approximately 90 modules, and arrays have required a large center foundation. Though effective, these designs require additional material and installation costs.
- Stow Strategy – Moving a tracker’s position to stow as wind speeds increase can mitigate torsional loads on the structure. While this approach effectively minimizes dynamic loads, it also simultaneously increases other forces. Effectively managing loads in stow position requires more material, which translates into higher costs.
- Module Mounting – Tracker manufacturers have turned to design approaches such as short module mounting rails on torque tubes to reduce bending and torsional forces on the tracker structure. Similarly, a different stow strategy increases forces on modules especially with short rails. However, these approaches simultaneously potentially increase loads on the modules and the associated risk of module degradation due to cell microcracking.
Dynamic Force Design Advances
The good news is that by mimicking fixed-tilt systems, the latest tracker designs are introducing new ways to manage both bending and torsional forces. This design approach optimizes separate components to deal with each type of load and eliminates additive row-length forces by distributing loads at each post.
The introduction of locking dampers and sophisticated gear mechanisms/bearings at multiple points along the row length, for example, approximates the function of a kicker and reduces (but doesn’t eliminate) the amount of torsion that the torque tube must handle. An even more advanced solution is to do away with the torque tube altogether in favor of proven fixed-tilt structural elements that can be paired with responsive drive technology.
Advantages of a Tracking System without Torque Tubes
- Dynamic Forces – When torsional forces are distributed along the row, excessive center-row loads and unsupported row ends are eliminated and associated material costs are reduced. Row lengths can also be extended.
- Stow Strategy – When torsional forces are distributed, the tracker can be stowed at 0°, which reduces loads and is the optimal position for minimizing bending force stress on the structure.
- Dampeners – Dampeners are no longer required, saving money from installation through long-term O&M.
- Module Integrity – When trackers are stowed at 0° and mounting rails can be longer, there is less pulling and pushing on the modules, reducing the risk of microcracking and prolonging the longevity of the modules.
Axsus single-axis trackers demonstrate how adopting proven fixed-tilt strategies can successfully mitigate both bending and torsional loads for a more enduring and cost-effective tracker project solution.
To enable best manage bending loads, Axsus trackers use c-channels that vary in gauge depending on the loads of a unique project site.
In place of torque tubes, Axsus trackers employ drive units at every post that lock and function like a kicker. These drive units are in turn connected by a steel wire drive shaft—which effectively performs the action of row rotation at a much lower material cost. The system can be stowed at 0° and requires no dampers. Rows can be longer and row gaps can be smaller. These design advances translate into material costs that are 25% lower than leading trackers, as well as significant savings in shipping and transport with only two trucks required per megawatt.
Installation is also made simpler with lightweight roll formed sections that do not require heavy or specialized equipment like that used for unwieldy torque tubes and large foundations.
The solar industry is all about collaboration, and the tracking market will continue to grow by using tips learned in fixed-tilt mounting.
Axsus presented the Solar Power World webinar “PV Tracker Innovation: The Trouble with Torque Tubes” earlier this year. Watch the webinar on demand here.