Solar Tracking Makes Use of Industrial Control

Solar
tracking applications present a series of tough challenges for automation and
control. Even though these systems don't require high-speed operation, system
integration is a major issue with the need to network thousands of devices
together, and ensure precise sun tracking and data logging. Mirroring the rise
of large systems and concentrated
photovoltaics
(CPV), control solutions are adapting along with the major
opportunity that the solar power industry represents.

"Our goal is to
address the technology needs for solar power generation using automation
solutions that enhance efficiency," says Paul Ruland, product marketing -
automation systems for Siemens
Industry Inc
. "One way we are doing that is through a new algorithm from
the National Renewable Energy Labs (NREL) that we put into a PLC function
block.

"We took that
very complex calculation, a euro Ssystemized it into our code and
made a usable function block that customers can parameterize themselves and use
it with their particular solar technology to track the sun in the most
efficient manner."

Quite often, higher-end algorithms for solar positioning (SPAs) must run
on an expensive embedded controller or a higher-end industrial PC. But the
Siemens solution is able to use the NREL algorithm on its newest and smallest
S7-1200 controller that sells for around $300.

Solar Tracking Makes Use of Industrial Control

"This has been
very attractive to larger solar installations that have many nodes and not
necessarily a lot of I/O, but need this powerful computation capability and
advanced algorithm," Ruland says. "It accepts computations that very few
traditional PLCs can handle."

Siemens' latest
S7-1200 compact controller, which would be considered by most to be in
the micro PLC class, supports both 64-bit floating point math and the long real
data 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 and
seasons pass, the azimuth and zenith angles change with the rotation of the
earth and the angle of rotation of the earth around the sun. These two outputs, coupled with additional
environment-related inputs into the function block, allow the system to track
the actual position of the node (latitude and longitude), the average
barometric pressure, temperature, elevation, atmospheric refraction and other
parameters that the NREL algorithm (endorsed by the U.S. Dept. of Energy)
supports.

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

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

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

Another advantage of the system
is that an automation controller at each solar tracking node provides multiple
diagnostics, data logging and functions such as a local HMI for service.
Technicians can open the control cabinet and access predictive or preventive
maintenance features normally found in a PLC used in the manufacturing
industries. The S7-1200 also provides a built-in Ethernet port, internal data logging and the ability to connect to an NTP
server to keep its real-time clock accurate.

System Integration Challenges

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

Solar Tracking Makes Use of Industrial Control

Click here for larger image

"The market is also highly cost driven, so there is a struggle to
understand what technology works best in the field. Many companies don't have
years 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 the
system. We see interest in networking technology such as EtherCAT that offers
alternative solutions."

Schultz says EtherCAT technology lends itself to solar
applications because the technology can use one master to communicate to as
many as 65,000 different nodes. But there also needs to be a business case and
risk 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, and
leverage built-in features such as media redundancy, diagnostics and the
ability to use Ethernet cabling.

"The combination of EtherCAT hardware architecture and advanced
software features provides networking technology that is well-suited for solar
tracking applications," says Schultz. "The networks are not high speed, so
performance is an advantage but not a requirement, but topology is very
important."

Another advantage with
EtherCAT for the burgeoning solar industry is that the communication technology
doesn't require switches, routers or hubs, and the user doesn't need to assign
IP addresses. When an application incorporates 17,000 heliostats and
3,000-4,000 stations for the troughs, there is a clear benefit in eliminating
networking switchgear which is both cumbersome and expensive.

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

Solar Tracking Makes Use of Industrial Control

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

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

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

In recognition of the environmental issues with such large deployments,
Schultz says Beckhoff is seeing increased interest in developing smaller 50-100
MW 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 the
problems with permitting for a larger system and the need for an environmental
impact statement.

Concentrated Photovoltaics

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

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

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

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

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

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

A key performance measurement
is the sensitivity of both the cell and lens design to sun tracking error. If
the 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 interesting
example of using advanced analysis of measurement data, and feeding that
information directly into the control system. It automatically moves in a
motion pattern to characterize the "acceptance angle" performance of the cells,
based on tracking error, and uses
actual performance data to develop an I-V characterization of the cell.

The theory of I-V
characterization is that PV cells can be modeled as a current source in
parallel with a diode. When there is no light present to generate any current,
the PV cell behaves like a diode. As
the 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 by
the photoelectric effect minus the diode current ID.

To do the analysis, the system
captures voltage and current information as it sweeps through the cell's
performance range. Based on that information, the application calculates the
resistance of the cell, its efficiency in converting sunlight, as well as a
full analysis of cell performance. Historically, MacCleery says this was done
only in the testing lab as the panels were being manufactured but now, with the
availability of advanced instrumentation capabilities, more analysis and
characterization is moving into the field and fusing with the control systems.
The reason for this is because instrumentation provides valuable data for optimizing
the control and achieving the goal of getting peak power out of the solar farm.

"This combination of instrumentation and analytics, combined with
the control system, is a general trend we see in many applications," says
MacCleery. "Higher quality measurement and advanced analysis are creating more
sophisticated control systems, enabling automation
systems to be optimized. This isn't just something happening in factories like
we typically hear about; it's also happening in field applications such as sun
tracking."

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