In less than a decade utility scale solar grew from a negligible part of the US energy mix to the leading source of new generating capacity in 2016. With scale comes more mature technology and cost reductions, but as a whole, solar is a very young industry when compared to coal, gas, hydro, and even wind. Solar PV tracking technology is even younger. Only in the last few years has the majority of new utility capacity shifted from fixed tilt to tracking systems. So while mounting systems today are more advanced than ever, we’ve only scratched the surface of what they might be capable of in the future.
Progression from passive to active
Looking at the evolution of technology across industries, there is often a progression from passive to active to dynamic systems. A passive design has no ability to react to changes in the environment, and can be reliable, cost effective, and elegant. An active design uses sensors to trigger an action that improves performance, and requires a control system. Finally a dynamic design is one in which a connected device responds in multiple dimensions, enabling rich interactions with the world, and often moving beyond its original function.
Wind turbines (for electricity generation) provide a clear example of this technology arc. A key consideration in the design of any wind turbine is the management of variable wind. At low wind speeds the turbine needs to maximize power generation. At high wind speeds where the available power exceeds the rating of the turbine, power needs to be shed or the turbine would tear itself apart. Through the 1990s the best wind turbines were fixed pitch, an example of passive design, where rotors with carefully designed blades harness aerodynamic stall to elegantly control power. Some manufacturers looked to improve performance with variable pitch rotors, an example of active design. As described by a wind energy industry information group, “superficially, this approach seemed to offer better control than stall regulation, but it emerged through experience that pitch control […] could be quite problematic” because the control system could be caught with the blades in the wrong position. Today’s wind turbines embrace dynamic design. Variable speed drives work synergistically with variable pitch rotors to yield the best energy production and power quality. An advanced control system manages not just energy generation, but also ancillary functions including reactive power, frequency control, blade defrost, and from at least one manufacturer, a “bat function” used to protect bats. Highly granular data from the wind turbines allow predictive O&M. For wind turbines, dynamic design has increased generation, improved uptime and enabled grid services.
Inverters provide a further example of the evolution from passive to active to dynamic systems. Solar inverter manufacturers quickly developed maximum power point tracking (MPPT), whereby the inverter varies string voltage in real time to maximize the power of solar modules. Consistent with an active design, the inverter uses sensors to trigger an action that improves performance. Newer, dynamically designed smart inverters take a great leap forward in both capability and complexity. With few changes to inverter hardware, connected control systems unlock system level benefits. Nearly two dozen smart inverter functions are described by EPRI. Smart inverters embody dynamic design because they have rich, system level interactions that move well beyond the original function of the inverter.
Dynamic design in mounting systems
The evolution to dynamic design is now apparent in PV mounting systems. For background, the purpose of a mounting system is to hold solar modules in place against the elements and in a position advantageous for energy production. A suitably designed mounting system will maintain integrity for the life of the power plant despite corrosion, snow, ice, earthquakes, and hurricane force wind while keeping installation and maintenance costs to a minimum.
The first generation of mounting systems were fixed tilt and as the name implies, passive. Such systems have an elegant simplicity. The best fixed tilt designs are highly efficient from structural, supply chain, and installation perspectives. Because there are no moving parts, maintenance is kept to an absolute minimum. Some fixed tilt designs can handle slopes up to 20% grade and have a variety of foundation designs to accommodate even the worst geotechnical conditions. By pointing the modules south, energy generation is maximized (compared to facing any other single direction). In many conditions, a well-designed fixed tilt system remains the best choice -- especially as module costs decline and in locations where clouds or haze are frequent.
Trackers ushered in the era of active mounting system design. The most popular technology, horizontal single axis, points modules to the east at sunrise and follows the sun across the sky to the west. Depending on location, this configuration increases generation 15-25% relative to a fixed tilt system. A tilt sensor monitors the position of the array, and a control system makes calculations based on time and location to command the tracker to move to the optimum tilt. This comes at a cost, of course, in the cost of the metal and the cost of implementing an active control system. Trackers are an order of magnitude more complicated than fixed tilt systems and need a long-term maintenance plan, but trackers are now well accepted to provide a significant levelized cost of energy benefit when installed in the right location. The most cost-effective designs position up to 90 modules per table. A typical utility tracker table is nearly 100 yards long and 2 yards wide with the table along a North-South axis. The backbone of the tracker is called a torque tube because it carries significant torque from the modules to the actuator at the center of the table.
Addressing wind stow
Over time, manufacturers developed the concept of wind stow. By monitoring wind speed and moving the table to a favorable position during a wind event, the tracker may be designed for lower forces, ultimately reducing cost. Wind stow was a significant improvement. However, analogous to the first generation of active wind turbine designs, field experience revealed some issues. Intuitively stowing at a low angle (i.e., modules horizontal) minimizes wind load. What was discovered was that trackers with long tables in a low angle stow could be vulnerable to dynamic torsional forces caused by difficult to model phenomena such as torsional galloping. Existing systems with issues were retrofitted with dampers to reduce movement, and several manufacturers adopted a high angle stow, e.g., 30 degrees for NexTracker, and included dampers. A high angle stow helps control torsion but increases lateral wind load on the modules and mounting system. Other manufacturers took a different approach. Exosun’s unique system has short tables that limit torsional buildup. Array Technologies (ATI) eschews active elements; their newest tracker introduced a passive stow. A clutch in the gearbox allows the tables to slip to max tilt during a wind event, and the bearings provide torsional support to the torque tube. While reliable, the table must be designed to resist the full force of the wind at any angle.
Having developed control systems to respond to wind events, the progression to dynamic design became inevitable. Incorporating a number of sensors and control elements into an intelligent control system enables countless operational modes. In other words, trackers today can do a lot more than point at the sun. For example, a snow sensor can trigger a snow mode that moves trackers to maximum tilt -- shedding snow, reducing load by over 80%, and clearing the modules for energy production. O&M mode puts the tables in a convenient position for module cleaning. Various fault modes protect the system.
Tilt is the fundamental control element in a tracker. Less visible control elements include battery management and climate control in the tracker controllers. In 2016, SunLink introduced dynamic stabilization — a method for changing the tracker’s structural characteristics depending on real-time environmental conditions. The design adds a new dimension of control over the damping and stiffness of the array, opening a completely new solution space for increasing energy output, maintaining structural integrity, and lowering cost.
When activated, dynamic stabilization redirects forces into the foundation rather than allowing torque to build along the length of the torque tube. The enhanced control safely permits a low angle stow, minimizing wind loads on the structure. In sum, the structure can be made more modular and efficient. Minimizing wind loads has benefits for the modules, too. While not yet quantified, low angle stow is expected to reduce microcracking and degradation of the modules over the life of the solar plant.
Data from individual trackers are sent via wireless network to local SCADA and remote data monitoring systems. Leveraging communication and data security standards developed in tech and finance makes it possible to build secure yet accessible databases of project data. Innovations like APIs reduce the friction of connecting systems. Data accessibility will be a major shift in the utility industry where siloed systems have been the gold standard. The benefits of more accessible data are numerous. Careful analysis and presentation of the data is used for product development, early detection of issues, and predictive O&M. Going forward, project outcomes data will inform system design and quantify risk, reducing financing costs and accelerating the deployment of solar.
Ultimately dynamic design is about using intelligent, connected devices to provide system level benefits. As solar mounting systems mature, expect to see manufacturers investing in remote monitoring and control, inverter and storage integration, and avenues to grid services.
Written by Patrick Keelin, Director of Product Management, SunLink Corp.