Solar tracker controls boost performance

As the size of utility-scale PV plants continues to grow, control systems for solar tracker farms are becoming a more crucial component of the overall array. Robust control systems have become critical for managing multiple inverters, for meeting the demands of the grid operator, and for eking out increments to performance. “In the future, the U.S. solar industry will likely follow a path where there will be continued innovation in control systems that could bump up performance; even a 1% gain for these large plants is substantial,” says Matt Campbell, Senior Director of Power Plant Products for SunPower, based in California.

Programmable logic units dock

Small tracker arrays typically utilize programmable logic controllers (PLCs) to connect to individual sensors and to convert the acquired signals into digital data for operator interaction through a human machine interface, like a touch screen.
A variety of companies offer off-the-shelf PLCs along with various proprietary software routines – including astronomical, power analytics and stowage – that a small PV facility requires. However, once a PV farm size is increased to several megawatts, the multiple inverters needed to serve the longer tracker strings require another layer of computer control to manage the combined flows of power from the sun into the grid.
These larger solar arrays use so-called distributed control systems (DCSs) that may or may not be centralized by design. And the DCSs typically employ multiple PLCs on one or more trackers throughout an array, along with separate controllers for distinct functional subsystems – like data monitoring, weather and security.
Larger PV arrays typically are planned with 1 to 2 MW of capacity per inverter, one source says, so a 200 MW field may have 200 PLCs. However, “a large field might have a combination of a high endcentral processor and multiple mid-scale compact logic processors, which provides the flexibility of a highly available system, redundant controller capability, and easily scalable telecom within the same [PLC] chassis; it can be very flexible,” says Dave Schaetz, a global industry technical consultant for Rockwell Automation, based in Portland.
While some array architecture designs use a simple PLC on each tracker, other designs use one more sophisticated PLC on many trackers. The Patriot Solar Group, based in Albion, Michigan, U.S., for example, offers an Allen-Bradley MicroLogix 1400 microprocessor from Rockwell Automation that can handle 500 feet of panels with a single PLC unit. Their system, like many U.S. tracker controllers, relies on U.S. National Renewable Energy Laboratory (NREL) software for astronomic positioning of the trackers.
Other controller systems utilize light sensors for positioning, like Germany-based DEGERenergie’s D100 tracker system with its patented “Maximum Light Detection (MLD).” According to a Fraunhofer Institute study, the Deger MLD light tracking performs up to 6% better than astronomical-based software tracking, in part because of its ability to deal with cloud cover or snow reflections, says Adam Glapiak, the International Sales Manager for the company. “Since the system is sensor-based and no computer or software is involved, there is no need at all to collect data,” he suggests.
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Tracking systems with Deger MLD technology have the highest energy gains in the winter months, Glapiak points out. The generated additional yield from the Deger MLD tracker is visible both during low level diffuse light conditions and high level diffuse light conditions, according to the Fraunhofer study. “Those two points are very important, especially for self-consumption of generated energy or net metering,” he adds.
The use of PLCs without a master controller are nominally a less expensive solution for a solar farm, but the higher cost of the DSC can often be offset through more rigorous performance analysis, preventative maintenance savings and incrementally greater energy production. In general, a PV solar tracking control system can cost 2 – 4% of the total capital investment in a power plant, several sources agree. But where the capability and scale of a developer’s business warrants it, building a modular control system in house – like SunPower’s TMAC – can reduce the relative cost of the system as it rolls out to multiple plants, reckons Campbell.

Distributed architecture

One provider of distributed control systems is the Pittsburgh-based Power & Water Solutions subsidiary of Emerson Process Management – based in Round Rock, Texas – which produces the Ovation control system, adaptable to widely varying solutions. “Integration of other (sub)systems in the plant, such as inverters, meteorological stations, revenue meters, switch gear, and protection systems, is much easier when the development and implementation is being done on a DCS that has a common database platform,” says Tom Snowdon, Emerson’s Business Development Manager for Renewable Energy. “We can communicate as frequently as once per second between the CPU [central processing unit] and each inverter. The DCS is used to coordinate the outputs of all the inverters to result in the desired plant output.” In December 2012, Emerson was selected to supply control technology for the 110 MW Catalina Solar project by Bechtel. The plant was developed by EDF Renewable Energy, the San Diego-based U.S. subsidiary of EDF Energies Nouvelles, of Paris. The Ovation system controls real and reactive power at the plant, the interconnection breakers to the grid, and implementation of directives from the California Independent System Operator through a Remote Intelligent Gateway, as the plant serves San Diego Gas & Electric’s power purchase agreement.
Similarly, Lauren Engineers and Constructors of Texas has installed concentrating solar control systems based on Rockwell Automation equipment at plants in both the United States: at the 64 MW Nevada Solar One plant for Acciona Energy; and the 75 MW Martin Next Generation Solar Energy Center for Florida Power & Light; and more recently in India at the 50 MW Godawari Green Energy plant in Rajasthan, for the Hira Group. While the company is still seeking its first utility-scale PV installation project, the control system it has used for the three CSP projects is adaptable to PV, says Robert Heard, a senior engineer at Lauren.
Operational controls are at times challenged by more than mere architecture. “Apart from heavy dust and heavy rain in India during construction, the temperature measured over 132°F this summer and the Rockwell system did not fail; it is very robust,” he observes. Beyond daunting operational tasks, the bottom line calculation is always a challenge, as well. “Performance analysis is designed by us and the owner on a plant-by-plant basis, then Rockwell writes a routine for it – everybody likes something different,” Heard says.

The in-house approach

SunPower began to develop a proprietary DSC about four years ago, which has culminated in their TMAC tracker control system, now installed at some 1.5 GW worth of PV plants. The 250 MW California Valley Solar Ranch was “the first utility-scale solar energy project in the United States to use wireless tracking monitors and control systems,” the company says. The plant uses a controller for each tracker motor (serving 300 kW worth of panels each), a monitor for each combiner box for the 80,000 strings on the site, and a master server. “The web-based TMAC server allows statistical analysis, diagnostics, learning mechanisms, and stow regime decisions – based on an interconnection with NOAA [the National Oceanic and Atmospheric Administration],” says Campbell.
“The TMAC tracker software is custom, the control circuit board is custom, and the SCADA [supervisory control and data acquisition], maintenance and data warehousing software is off the shelf,” continues Campbell. “The software and controls are standard for our 1.5 MW Oasis modular power block architecture, which we use for our own EPC [engineering, procurement and construction] and also sell as a kit,” he says.
Another fresh approach to solar control systems has come from QBotix, which moves its monorail-riding PLC-plus-actuator robot from tracker totracker in a continuous 40 minute loop around a 340 kW array.
“Our use of a robot brings a new kind of controller to the mix,” says Wasiq Bokhari, CEO and Founder of the Menlo Park, California-based start up. “Some people would call it a mobile controller, so it’s a new kind of animal, fundamentally different from all the other control systems out there,” he says. “The benefit is that it can optimize cost and control, in part, because we have taken out a lot of steel,” he says. Having a more sophisticated computer present at each tracker, if only for a few minutes, means more types and more detailed data can be recorded, says Paul Breslow, the Associate Director of Marketing at QBotix. “The robot keeps detailed data on each tracker, not available at such detail on any other system, and we can make that data available to customers, where it is desirable, and possibly to the industry at large,” he adds.

Future controller developments

System automation is historically an evolving goal, and in the PV industry it is perhaps particularly so, given the early stage of U.S. smart grid development. “We are seeing a greater interface of plant control with the utility – driving down to distributed systems at a local level – and that requires a lot of communications capacity,” says Schaetz. Smart grid connections might allow PV plant control systems to play a more direct role in ISO load balancing, in frequency response and in voltage regulation,” Snowdon explains.
And as PV control systems become more broadly capable, more non-traditional functions are expected to be included. SunPower, for example, has begun commercial use of acquired technology for a robotic panel cleaner that will tackle its first 25 MW plant during 2014, Campbell notes.
The volume of plant-specific control data, which is already staggering, is only expected to grow. Lauren found that in one plant their controller communicated 22,000 different readings every five seconds. “We might even have gone faster, but we found that a pace of five seconds was good in order to keep communications links from running into one another. The common internet has lots of collisions,” explains Heard.
Wireless technology is also bound to improve at PV plants, since it replaces miles and miles of hard copper or fiber optic communications wire. Indeed, one of Vermont-based AllEarth Renewable’s AllSun tracker installations in South Burlington, U.S., was inaugurated by the state governor Pete Shumlin using just an iPhone. Such is the sophisticated future awaiting the solar industry’s tracker sector.