Today's market offers a wide variety of trackers with different mounting options, driving mechanisms, power source, control and stow strategies. The present article provide a brief summary on important characteristics of trackers.
The core of a solar tracker is its driving mechanism, which is the motor that drives the tracker and is a very important integral part. Either alternate-current (AC), or direct-current (DC) electric motors are employed. AC induction motors can draw power directly from the grid. Stepper motors are offered at low prices, but the motor speed is limited to around 600 rpm and the devices are also sensitive to high temperature differences between different parts of the motor. These motors are disadvantageous in harsh storms when trackers have to quickly reach the stowing position. Brushed DC motors use a configuration of wound wire coils (armature), which act as a two-pole electromagnet. The directionality of the current is reversed by a mechanical rotary switch (commutator). Brushed DC motors are offered at a low cost with simple and inexpensive controllers and are ideal for extreme operating environments. A drawback is that the brushes are prone to wear and tear, needing periodic replacement to extend service life. In contrast, brushless (BL) motors are equipped with permanent magnets at the external rotor. Furthermore, brushless DC motors (BLDC) utilize three phases of driving coils and sensors to track the rotor position. They need less overall maintenance due to lack of brushes. BLDC motors are offered in a reduced size with much better thermal characteristics, lower electric noise generation and higher speed range, which is beneficial in stowing conditions. The high-end solution motors for trackers are intelligent BLDC motors with embedded control electronic units. However, according to Antai, the most widely followed method is DC brushed, mainly due to the cost-performance ratio. Clenergy is in the process of developing a BLDC motor to be launched in 2023 (see Improvements At Glance).
As for the power source, taking power from the grid is one way, while making the tracker autonomous using PV and storage is an interesting option, especially in the case of choice of a DC motor. Such self-powered systems are offered by the majority of companies listed in our survey.
Row architecture – independent or dual (or multi) – is an important design aspect of trackers. In case of independent-row architecture, each tracker is driven and controlled independently, while in dual-or multi-row HSAT systems two or more trackers are simultaneously moved with a single actuation system. Antai says that the dual-row trackers are not easy to implement, as they require a very specific terrain orography, and it is not clear if the link system is less expensive than one motor and one control system. Also, very high mounting tolerances are necessary. Thus, independent rows are more widespread, which is also reflected in our survey.
All drive system components are activated by a control and communication system. Solar trackers are mostly equipped with a programmable logic controller (PLC), which is driving the motors of the solar tracker to an optimal position depending on the actual irradiation and weather conditions. The PLC can use inputs from several sensors such as a GPS receiver, an inclinometer, a GMT clock, irradiation and wind sensors, and PV inverters to determine the optimal position in closed-loop or open-loop control algorithm.
Closed-loop controls, also called feedback controllers, use solar sensors or the inverter output to determine the sun's position. The input data from the sensors come into the controller unit, which drives the motors and actuators to position the tracker. However, the approach implies the use of expensive sensors. The majority of the participants in our market survey offer closed-loop control systems.
Open-loop control systems use a microcontroller and do not need any sensors to detect the sun's position. The sun's path is predicted by astronomic relationships programmed in a microprocessor which calculates the sun's position at any time. Some devices use a solar map for accurate tracking which, depending on the location, gives information on where the sun is at different times of the day throughout the year. This solution is used by Ideematec, Mechatron, BIG SUN Energy, Nexans and STI. Furthermore, some tracker suppliers, such as Schletter, use GPS signals to determine the tracker's latitude and longitude, as well as the date and time. With this information, the tracker will know the position of the sun for any given time and orient itself to face the sun. The tracker will be facing the sun even during cloudy periods; and when the clouds part, the tracker will already be positioned to maximize power production without any delays to reposition itself.
Some trackers use both, sensors and a solar map. When the weather is sunny, the sensors are used to track the sun; and during cloudy days, the information from the solar map is used. According to Antai, the communication between the control system and the inverter can ease the energy yield in a simple way.
In order to maximize the energy yield, shading should be obviously avoided. However, sometimes a shadow-free module position might not be optimal in terms of module-sun alignment. Some solar tracker manufacturers use smart control algorithms to detect when tracker self-shading between the module rows occurs and update the rotation angle to eliminate shading by module alignment for maximized solar generated electricity yield. This is the so-called backtracking, which is implemented by almost all participants in our survey.
Solar trackers integrate either active or passive mechanisms to reduce the impact of high wind and snow loads as well as hailstorms. In such extreme weather conditions, the trackers move from the operational to a more favorable stow position. Ideally, the stow position is not the same for all extreme weather conditions. As an example, 0° stow position, which is advantageous for reducing the sailing effect at high-wind speeds, might lead to module damages in case of hailstorms. Just a few tracker suppliers offer different stowing strategies for different potential weather risks. And the manufacturers define the best stow position based on the results from aeroelastic wind tunnel studies guide.
The text is an excerpt from TaiyangNews 2nd Market Survey on Solar Trackers, which was published in Dec. 2022 and can be downloaded for free here.
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