With high precision solar trackers capable of boosting plant output by thirty percent, solar tracking technology should be at the heart of developers’ plant layout strategy.

By Toby Price in Barcelona

Offering positioning accuracy down to 0.1º and boosting output by up to 30%, it is no wonder some players in the sector are tweaking solar tracking control systems to squeeze that bit more juice out of each solar field.

“Solar tracking control is one of our key lines of research,” reports Luis Yebra, CIEMAT researcher at the sector-leading Almeria Solar Platform, revealing that several researchers and engineers at the site are involved in this work.

Put simply, a solar tracking control system consists of a micro-controller based system that either operates in an open loop, employing algorithms to plot the sun’s position at any given time; or in a closed loop, using a solar sensor to locate the position of the sun. These control systems then send signals to a solar tracker’s actuator telling it where to point.

While controllers that rely on sun sensing perform well, systems using algorithms to calculate sun position are the preferred choice in CSP plants. This is due to the fact that if the sun goes behind a cloud, a sun-sensing system does not know where to go.

This is not a major issue for photovoltaic systems, but where high-concentration solar thermal electric systems are concerned, the results can be damaging. Roger Davenport, Research Engineer with the technology company SAIC, warns: “If the aim point is off when the sun clears the cloud, you can burn things”.

A calculated sun position tracker, on the other hand, continues to track the sun even when it is obscured by cloud, so that once the cloud has passed, the tracker is still on target to accurately direct the energy.

“Algorithms for sun position are pretty good these days,” says Davenport.  “You can use a microprocessor to calculate where the sun is at any given time and location”.

Covering all the bases

These algorithms are then tweaked to take into account a wide spectrum of variables to ensure tracking accuracy under any conditions. Davenport’s team have produced algorithms that account for variables such as sag of the engine support arm, tilt of the pedestal or ground shifting, while maximising the output of the system in real time.

Meanwhile, Francisco Rodríguez, Systems Automation Engineer at the Seville School of Engineering, explains that algorithms are also designed to take into account different errors that can arise in these systems. For example, they can compensate for anomalies in the mounting of the receiver, or errors in the mechanical structure, to ensure that the receivers track the sun, irrespective.

Yebra says a main objective is to develop control algorithms that optimise tracking performance under different physical conditions. This, he says, can ensure that perturbations, due to wind for example, are ironed out.

Suntrack has gone one stage further, developing a “backtracking algorithm”, which analyses actual past events in a solar field to optimise receiver positioning. In doing so, shadowing among panels/trackers can be avoided and plant layout (land area/watt) improved. According to Suntrack, efficiency improvements of more than 23% are possible with its system.

Boosting plant efficiency

Improvements to controllers are also helping to reduce the energy demand of plants by ensuring receivers move as little as possible in their search for perfect alignment. One way of achieving this is to program controllers so that they do not send signals to certain trackers to constantly track the sun with the same accuracy when they are not needed.

Researchers at the Applied Research Centre for Intelligent Control and Automation in Mexico City are developing novel artificial intelligence-based advanced control algorithms with a goal of tracking the sun as precisely as possible to reduce unnecessary movements by receivers in order to minimise energy consumption.

There is scope for further efficiency gains. Yebra cites improvements in the energy efficiency of the controller and actuator, reductions in the cost of hardware and communications networks in the solar field, and the optimal integration of global control schematics for CSP plants as key areas for further investigation.

“One field still to be explored is to combine the tracker control system with the inverter control to achieve an improvement in the performance of the entire system,” adds Rodríguez; while Aitor Alapont, Export Director of Suntrack, suggests: “Maintenance-free and plug-and-play systems at a very reduced price (less than €600; US$810) should be the target for new control systems”.

Alapont is also attracted by solar tracker control innovations such as new angle sensors for closed-loop feedback, which he says are “easy to install with absolute and accurate angle measuring”, and new solar sensors for safety loops.

Finally, Davenport highlights that, particularly for central tower plants, more work needs to be done to develop systems that control entire solar fields. He is keen to optimise the aiming of heliostats in real time – moving their individual aim points around on the receiver to minimise temperature differences.

“With the large number of heliostats available, it seems to me that you could do very fine tuning of the receiver temperature distribution in real time and that this would be an interesting artificial intelligence problem,” he says.

Whatever the future brings, the little black boxes guiding solar trackers around the sky will continue to be the brains behind the brawn, bringing down the energy consumption of CSP plants and boosting performance.

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Toby Price: tprice.csptoday@gmail.com

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