Solar Tracking Makes Use of Industrial Control

DN Staff

January 20, 2011

10 Min Read
Solar Tracking Makes Use of Industrial Control

Solartracking applications present a series of tough challenges for automation andcontrol. Even though these systems don't require high-speed operation, systemintegration is a major issue with the need to network thousands of devicestogether, and ensure precise sun tracking and data logging. Mirroring the riseof large systems and concentratedphotovoltaics (CPV), control solutions are adapting along with the majoropportunity that the solar power industry represents.

"Our goal is toaddress the technology needs for solar power generation using automationsolutions that enhance efficiency," says Paul Ruland, product marketing -automation systems for SiemensIndustry Inc. "One way we are doing that is through a new algorithm fromthe National Renewable Energy Labs (NREL) that we put into a PLC functionblock.

"We took thatvery complex calculation, a euro Ssystemized it into our code andmade a usable function block that customers can parameterize themselves and useit with their particular solar technology to track the sun in the mostefficient manner."

Quite often, higher-end algorithms for solar positioning (SPAs) must runon an expensive embedded controller or a higher-end industrial PC. But theSiemens solution is able to use the NREL algorithm on its newest and smallestS7-1200 controller that sells for around $300.


"This has beenvery attractive to larger solar installations that have many nodes and notnecessarily a lot of I/O, but need this powerful computation capability andadvanced algorithm," Ruland says. "It accepts computations that very fewtraditional PLCs can handle."

Siemens' latestS7-1200 compact controller, which would be considered by most to be inthe micro PLC class, supports both 64-bit floating point math and the long realdata type. For two-axis solar tracking applications, this capability enables use of complex math calculations such as tracking the sun's azimuth(vertical) and zenith (horizon) angles.

The advantage of two-axis tracking is that, as the months andseasons pass, the azimuth and zenith angles change with the rotation of theearth and the angle of rotation of the earth around the sun. These two outputs, coupled with additionalenvironment-related inputs into the function block, allow the system to trackthe actual position of the node (latitude and longitude), the averagebarometric pressure, temperature, elevation, atmospheric refraction and otherparameters that the NREL algorithm (endorsed by the U.S. Dept. of Energy)supports.

All of these input parameters, as well as connecting the PLC toNTP protocol (National Time Protocol), assures that the system maintains theexact time of day at a particular location. a euro ...The combination of input parametersprovides the intelligence to move the solar collection technology exactly at theangle of the sun to get maximum efficiency.

"Compared to traditional technologies, it provides an increase inaccuracy of 30 percent or more," says Ruland.

The NREL algorithm alsohandles calculations up to the year 6000, where the previous version of thealgorithm was limited to 15 years and created an uncertainty in the calculationas well as a long-term sustainability question. Use of the PLC function blockmeans customers don't have to implement the algorithm as C source code anddevelop a runtime engine.

Another advantage of the systemis that an automation controller at each solar tracking node provides multiplediagnostics, data logging and functions such as a local HMI for service.Technicians can open the control cabinet and access predictive or preventivemaintenance features normally found in a PLC used in the manufacturingindustries. The S7-1200 also provides a built-in Ethernet port, internal data logging and the ability to connect to an NTPserver to keep its real-time clock accurate.

System Integration Challenges

"The biggest challenge that solar tracking systems faceis deployment of a huge number of solar devices, such as concentrators orphotovoltaic technology, so communication to the different devices is aproblem," says Ed Schultz, renewable energy business development manager for Beckhoff Automation.

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"The market is also highly cost driven, so there is a struggle tounderstand what technology works best in the field. Many companies don't haveyears of experience to draw on, so they go with what they know best," he says."A recent customer reported use of Ethernet to connect multiplexing (MUX)boxes, which then communicate via serial to the 17,000 heliostat mirrors in thesystem. We see interest in networking technology such as EtherCAT that offersalternative solutions."

Schultz says EtherCAT technology lends itself to solarapplications because the technology can use one master to communicate to asmany as 65,000 different nodes. But there also needs to be a business case andrisk management decision on how many nodes to address using a single master.Some customers decide to use one master to communicate to 400-500 slaves, andleverage built-in features such as media redundancy, diagnostics and theability to use Ethernet cabling.

"The combination of EtherCAT hardware architecture and advancedsoftware features provides networking technology that is well-suited for solartracking applications," says Schultz. "The networks are not high speed, soperformance is an advantage but not a requirement, but topology is veryimportant."

Another advantage withEtherCAT for the burgeoning solar industry is that the communication technologydoesn't require switches, routers or hubs, and the user doesn't need to assignIP addresses. When an application incorporates 17,000 heliostats and3,000-4,000 stations for the troughs, there is a clear benefit in eliminatingnetworking switchgear which is both cumbersome and expensive.

With a system architecture like that featured by EtherCAT, all ofthe controls are running through one main controller - a PC back at the mainfacility - rather than through distributed controls in the field and havingthem communicate back to the host. Schultz says that many users often havemultiple embedded PCs deployed in the field to control a couple hundred panels.


In these distributed applications, "the more you can eliminatehardware makes an impact on reliability and uptime," he says.

One trend Schultz has seen play out in the development of solarplants is the move toward larger systems.

"There has been a trend toward huge 850 MW to 1 GW solar powerplants. But with 7,000 acres of solar dishes or troughs, there is also anenvironmental concern," he says.

In recognition of the environmental issues with such large deployments,Schultz says Beckhoff is seeing increased interest in developing smaller 50-100MW plants that can be done in a 700 to 1,000 acre field where combining the system together makes more sense rather than attempting to overcome theproblems with permitting for a larger system and the need for an environmentalimpact statement.

Concentrated Photovoltaics

The increased use of concentrated photovoltaic technology(CPV) is considered a significant trend that shows the solar industry isstarting to reach commercial maturity. Some companies are predicting that itcould become one of the lowest cost forms of solar power.

"CPV systems require very precise tracking because they use ahigh level of solar concentration," says Brian MacCleery, principal product managerfor Clean Energy at National Instruments."Typically a Fresnel lens in front of the solar cell, similar to the effect oflooking through binoculars at the sun, creates a tiny beam of light that ishigh intensity and can be 300 times more intense than ambient sunlight."

He says that the sun movingeven a small amount in the sky can cause the highly concentrated beam, eventhough it is only travelling a few millimeters, to move off the tiny solar cellif it isn't tracked
properly. To account for this, CPVsystems must track in two dimensions to provide sub-degree accuracies andmaintain maximum power output. Encoders are key to helping the CPV systemsmaintain arc-seconds of precision.

The main benefit of CPV isthat it reduces costs by minimizing the amount of PV material required, usingglass or plastic for the lenses and a metal backing similar to a headlamp.

National Instruments has worked with a company developingstandards for characterizing the performance of concentrated photovoltaics.Testing standards for flat-panel modules have become very well-defined. Butsince CPV is a new area, the standards body has been working to define thestandards for its characterization, as well.

"It's an interesting challenge because to characterize theperformance, you need a control system that tracks the sun. Unlike a flat panelsystem, the sun can move enough in a few minutes to affect the power output ofa CPV system," says MacCleery. "The testing systemuses advanced motion control algorithms that automatically scan to determinethe performance of the cells."

A key performance measurementis the sensitivity of both the cell and lens design to sun tracking error. Ifthe user wants to get 90 percent of the maximum theoretical performance output,the goal might be to maintain tracking accuracy to within 0.5 degrees,depending on the system design.

The system is an interestingexample of using advanced analysis of measurement data, and feeding thatinformation directly into the control system. It automatically moves in amotion pattern to characterize the "acceptance angle" performance of the cells,based on tracking error, and usesactual performance data to develop an I-V characterization of the cell.

The theory of I-Vcharacterization is that PV cells can be modeled as a current source inparallel with a diode. When there is no light present to generate any current,the PV cell behaves like a diode. Asthe intensity of incident light increases, current is generated by the PV cell.In an ideal cell, the total current I is equal to the current I? generated bythe photoelectric effect minus the diode current ID.

To do the analysis, the systemcaptures voltage and current information as it sweeps through the cell'sperformance range. Based on that information, the application calculates theresistance of the cell, its efficiency in converting sunlight, as well as afull analysis of cell performance. Historically, MacCleery says this was doneonly in the testing lab as the panels were being manufactured but now, with theavailability of advanced instrumentation capabilities, more analysis andcharacterization is moving into the field and fusing with the control systems.The reason for this is because instrumentation provides valuable data for optimizingthe control and achieving the goal of getting peak power out of the solar farm.

"This combination of instrumentation and analytics, combined withthe control system, is a general trend we see in many applications," saysMacCleery. "Higher quality measurement and advanced analysis are creating moresophisticated control systems, enabling automationsystems to be optimized. This isn't just something happening in factories likewe typically hear about; it's also happening in field applications such as suntracking."

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